Hydrogeological characterisation and prospect of basement Aquifers of Ibarapa region, southwestern Nigeria

Hydrogeological characterisation and prospect of basement Aquifers of Ibarapa region,... The present study involved the use of 82 geo-electric soundings, and the measurement of well inventory and conduct of yield tests in 19 wells across the various bedrock terrains of Ibarapa region of southwestern Nigeria. The aim is to proffer solution to the unsustainable yield of the available boreholes in order to effectively exploit the existing groundwater resource in the area. From the geological reports, the area is underlain by four principal crystalline rocks that include porphyritic granite, gneisses, amphibolite and migmatite. The geo-electric studies revealed that the degree and extent of development of the weathered–fractured component varied, leading to diversity in groundwater yield and in aquifer vulnerability to contamina- tion. The thickness of the weathered layer is greater than 18 m in areas underlain by amphibolite and gneisses and less than 13 m within migmatite and porphyritic granite terrains. High groundwater yield greater than 70 m /day was recorded in wells within the zones of rock contacts and in areas with large concentration of bedrock fractures and elevated locations across the various bedrock terrains. Aquifer vulnerability is low in amphibolite, high in granitic terrains, low to moderate in gneisses and high to moderate in migmatite. Also, wells’ depths and terrain elevation have a moderate to strong indirect relationship with groundwater yield in most bedrock terrains, except in high topographic areas underlain by porphyritic granite. Therefore, there is need for modification of well depth in accordance with the terrain elevation and hydrogeological complexity of the weathered–fractured components of the variuos bedrock terrains, so as to ensure a sustainable groundwater yield. Keywords Geo-electric studies · Borehole topography and depth · Groundwater yield · Basement aquifers Introduction of Ibarapa areas rely on groundwater supply tapped from hundreds of hand-pump boreholes that penetrated various Large parts of southwestern (SW) Nigeria are completely intrusive crystalline bedrocks of Precambrian age. Yet, even underlain by intrusive crystalline rocks that can neither with these provisions, availability of potable water is still store nor transmit water, except when fractured, or where inadequate, largely due to the unsustainable yield of many there is significant regolith thickness over the basement. The of the functioning boreholes (Akanbi 2017). This scenario study area, which lies within Ibarapa region of SW Nigeria, is further complicated due to rise in human population in has no functioning town water supply scheme, and human the area as typical of many developing nations including settlements are not connected with piped water facilities Nigeria. This has led to increase in demand for fresh water (Fashae et al. 2014; Akanbi 2016). Therefore, for domestic supply in Ibarapa area. However, groundwater development water needs and even for agricultural purposes, residents is hampered in this region as a result of lack of pre-drilling and drilling data and little is known about the hydrogeology of the present study area. This has not helped in providing information needed for further groundwater development in Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1320 1-018-0731-9) contains the area. Also, considering the fact that the hydrogeology of supplementary material, which is available to authorized users. weathered–fractured aquifers is highly heterogeneous due to complexity in geological, structural and geomorphological * Olanrewaju Akinfemiwa Akanbi features (Akanbi 2017), expansion of the existing ground- olanrewajuakanbi@yahoo.com water supply in Ibarapa is quite challenging. These high Department of Earth Sciences, Ajayi Crowther University, demand for groundwater resource and the variability of its Oyo, Oyo State, Nigeria Vol.:(0123456789) 1 3 89 Page 2 of 22 Applied Water Science (2018) 8:89 occurrence in basement terrains have made it mandatory to state in the SW Nigeria and covers a total landmass of about investigate the hydrogeological setting and to validate the 750 km . The area encompasses most township areas that groundwater yield of the basement aquifers across the vari- include Igboora, Idere, Ayete and Tapa and their adjoin- ous bedrock terrains of the study area within Ibarapa region ing human communities within Ibarapa region (Fig. 1). SW of SW Nigeria. Nigeria is bounded at the west by the Republic of Benin and For hydrogeological characterisation of the basement by Atlantic Ocean at the south. aquifers in Nigeria, the applications of electrical resistiv- Ibarapa region lies within the tropical climatic zone, ity methods have been used for decades (Olorunfemi and marked by two distinct seasons of wet and dry periods. Olorunniwo 1985, 1990; Ako et al. 1990; Malomo et al. The dry season extends from November to March with an 1991; Olayinka and Mbachi 1992; Olorunfemi and Okhue average monthly precipitation of less than 25 mm. The wet 1992; Olayinka and Olayiwola 2001; Oseji et al. 2005; Alile season peaks around June and September with monthly rain- et al. 2008). Most of these investigations used the primary fall exceeding 100 mm. The total annual rainfall in the area geo-electric parameters, which are the layer resistivity ranges from 813 to 1853 mm, while the annual temperature and the corresponding thickness for interpretations of the ranges from 24 to 29 °C. When compared to an average hydrogeological settings of the crystalline basement aqui- rainfall of 1541 mm/year for the entire SW Nigeria, the study fers. These studies described the lithology and deduced the area has lower average precipitation and higher average tem- aquifer characteristics and water saturation of the residual perature (Munaserei 1979; Daly et al. 1981; David 1988; overburden and the underlying fractured bedrocks. However, Tijani 2016). Hence, it is expected that the vegetal cover more recent research works (Oladapo and Akintorinwa 2007; will not be as thick, compared to other parts of SW Nigeria. Abiola et al. 2009; Aweto 2011; Jayeoba and Oladunjoye, The vegetal cover is more of derived savannah forested land 2013; Akanbi 2016) employed Dar-Zarrouk variables in marked by scattered-scanty small trees and shrubs, sand- conjunction with the primary geo-electric parameters to wiched by tall grasses. It lies between the transition zone demarcate areas into various groundwater potential zones of lowland rain forest of southern areas of Nigeria and the and for estimation of the groundwater vulnerability assess- savannah zone of the further northern areas of Oyo state. ment and protective capacity of the overburden unit. The Dar-Zarrouk variables are the secondary geo-electric param- Geology, geomorphology and drainage eters that included the longitudinal conductance, the traverse resistance and the coefficient of anisotropy first introduced The study area is underlain by igneous and metamorphic by Maillet (1947). These variables were derived from the rocks of Precambrian age (David 1988; NGSA 2009; Weera- integration of primary geo-electric parameters (i.e. the layer warnakula 1986) and the four principal crystalline rocks, thicknesses and their corresponding true resistivities) and namely porphyritic and homogeneous medium-grained gran- comparison of the passage of electric current across and par- ite; banded and augen gneisses; amphibolite; and migma- allel to the geo-electric boundaries to water passage through tite (Akanbi 2017). The distribution of the major rock units subsurface layers was made. in the study area is presented in Fig. 2. The outcropping However, for further understanding of groundwater sus- exposures of porphyritic biotite granite are the dominant tainability at Ibarapa region of SW Nigeria basement ter- bedrock at the middle section and occupy about one-third rains, measurements of groundwater yield and evaluations of the entire study area. The landforms formed by granitic of the well inventory of the basement aquifers also have to outcrops are ridges and inselbergs with intermediate low- be incorporated. This is due to erratic and localised nature of lying areas, and porphyritic granite is the major rock that groundwater occurrence in subsurface rock discontinuities underlain Idere, Ayete and Tapa township areas. Porphyritic and within the weathered unit (Offodile 2002). Therefore, granite is composed mainly of quartz, biotite and larger- the present study used vertical electrical sounding (VES) grained potassic feldspar. The porphyritic biotite granite is geophysical method, well inventory and yield to characterise a felsic rock as a result of the presence of larger amount of the hydrogeological settings and to attest the groundwater leucocratic minerals that include the quartz and the feld- prospect of the basement aquifers of Ibarapa region. spars. The medium-grained granite occurs as major map- pable intrusions in other principal rock units and is confined to a small section at the eastern part of the study area. The The study area mineral grains are massive, medium-grained and homogene- ous and massive. Location, climate and vegetation The amphibolite outcrops are found as disjointed low- lying boulder-sized units, and it is the major rock unit The study area lies between longitude 3°07′E to 3°21′E and that underlies Igboora Township. In respect of textural latitudes 7°21′N to 7°37′N (Fig. 1) of Ibarapa region of Oyo attribute, the amphibolite is fine to medium-grained 1 3 Applied Water Science (2018) 8:89 Page 3 of 22 89 Fig. 1 Location and accessibility map of the study area dark-coloured rock. Two varieties of amphibolite are found post-crystallisation structural events like folds and frac- at Igboora due to textural difference. These are the massive tures in the areas. and the schistose varieties. Lineation trend on the schis- The geomorphology of the area widely varies and can tose variety ranges from 093° to 124°, and the foliation be categorised into two main hydro-geomorphic units, plane dips between 38° and 44°W. Gneisses are mainly namely the high-lying and low-lying areas. The highlands restricted to the western part, though small area is under- are characterised by granitic hills, ridges and inselbergs that lain by augen gneiss at the south-east section of the study traversed the upper central section and extending from the area. The gneissic rock in the area is of the banded and the north of Tapa to Idere, and small parts at the north-east and augen varieties. Mineral foliation strikes between 339° and north-west regions of the study area. The elevations of some 360° and dips between 36°E and 40°E. However, at the of the hills are above 200 m above the mean sea level. The north-west region around Alagba area, the dip of the folia- ridges and inselbergs are granitic, and the texture is often tion is vertical (Fig. 2). Lastly, the migmatite terrains are porphyritic. However, major sections of the terrains under- mostly restricted to the adjoining villages such as Kondo, lain by migmatite are also highlands. The southern areas are Obatade, Tobalogbo and Alabi Oja at the northeastern largely lowlands and gently high rising landforms underlain part, and it is intruded upon at Idere by porphyritic gran- by gneisses and amphibolite. The major floodplain in the ite. The outcrops of migmatite preserved a lot of structural 1 3 89 Page 4 of 22 Applied Water Science (2018) 8:89 Fig. 2 Geological map of the study area (updated after Weerawarnakula 1986) area is found at the south-west part underlain by banded connected to the primary rivers forming in orthogonal and gneiss. oblique manners. This drainage pattern is more or less a The drainage pattern is dendritic as common in base- design that separates the entire study area into interfluvial ment areas characterised by high contrast in relief. In a regions. dendritic drainage pattern, tributary streams generally join at an acute angle forming Y-shaped junctions. The river network consists of primary rivers, namely Iworo, Ayin, Afo-Ape, Opeki and Aboluku sourced from the highlands Materials and methods terrain underlain by crystalline rocks. River Ofiki is the major river that drains the gneissic and granitic terrains, Existing topographical and geological maps of the study and it runs into River Ayin (secondary river) at the SW area were processed using ArcGIS 10.0. Conventional geo- region. It is remarkable to note that the primary rivers in logical mappings were also conducted to update the exist- the area run along the rock boundaries and coincide with ing map for accurate fixing of rock boundary and demar - the major structural NNW–SSE and NNE–SSW trends cating the area into various bedrock settings. A total of 82 development in the area. Other streams are intermittently geo-electric soundings, groundwater yield tests and taking 1 3 Applied Water Science (2018) 8:89 Page 5 of 22 89 of well inventory data were conducted in 19 boreholes in Qualitative methods a systematic approach across the various bedrock settings. True layer’s resistivities and corresponding thicknesses were Geo‑electrical surveys and groundwater prospect employed for qualitative interpretation. This involved char- acterising the extent and the degree of rock decomposition The spread of the 82 electrical soundings across the study from layers’ thicknesses and lithologic description of the area included a total of 25 soundings within terrains under- geo-electric layers, particularly the saprolite and the bed- lain by amphibolite, 19 on gneisses, 18 on migmatite and 20 rock using the corresponding layers’ resistivities. The sap- on porphyritic granite using Schlumberger electrode con- rolite is the weathered layer and one of the water-bearing figuration. The apparent resistivity field data were acquired zones in basement areas, characterised by high porosity, with Campus Omega resistivity meter, and the range of cur- and when the bedrocks are not fractured the saprolite is the rent electrode (AB) separation was between 200 and 266 m. only alternative water-bearing zones in basement area. In This range of AB separation was considered appropriate for this regard, successful groundwater prospect of the saprolite groundwater investigation in typical crystalline basement depends on its resistivity and its thickness and availability of Nigeria, where groundwater occurs at shallow depths of regular source of recharge (Singhal and Gupta 1999). (Offodile 2002). The systematic approach to VES surveys The range of resistivity for lithological characterisation and involved selective appropriation of suitable locations for groundwater prospect of the saprolite is presented in Table 1. sounding in order to cover the various bedrock settings and Groundwater prospect of saprolite is poor if the resistivity to avoid sounding close to outcropping rock exposures. For is between 1 and 60 Ωm for predominantly clayey lithol- interpretation of VES data for hydrogeological characterisa- ogy and when it is more resistive and greater than 400 Ωm tion, quantitative and qualitative methods of interpretation for ‘hardpan’ or compacted lateritic clays. This is because of the field apparent resistivity data were employed (Murali clayey saprolite are characterised by low to negligible water and Patangay 2006). transmission attributes. The prospect is adjudged moderate when the resistivity lies within the range 61–150 Ωm for Quantitative methods less clayey and sandy soils and good when the saprolite is more sandy to gravelly and resistivity lies between 151 and The VES quantitative interpretation method involved 400 Ωm. However, sustainability of groundwater supply is originating primary geo-electric parameters that included guaranteed when the bedrock is fractured and there are good the layers’ resistivities and thicknesses from partial curve connections between the fractures and the weathered layer matching and computer iteration techniques. This was done (Akanbi 2016). Hence, for bedrock, groundwater prospect by plotting the apparent resistivities obtained from the field is good when fractured and the resistivity is below 600 Ωm. against half electrode spacing (AB/2) on a log–log chart. The prospect is moderate when the bedrocks are partially The plotted field curves were then matched with standard or slightly weathered and the resistivity is between 601 and auxiliary curves of Orellana and Mooney (1966) to derive 1800 Ωm. Fresh bedrocks are marked with high resistivity the initial layer resistivity and corresponding thickness for in excess of 1800 Ωm and have little or no groundwater each geo-electric layer. These initial geo-electric parameters potential (Table 1). were used in modelling in Vander (1988) modelling pack- Also, the variation in weathering with depth was estab- age to generate the true resistivity and thickness that are the lished by contouring the apparent resistivities for depths cor- primary geo-electric parameters. responding to half of the current electrode distances (AB/2) of 10, 24, 42 and 75 m. Additionally, the true resistivities of the saprolite and the bedrock were equally contoured to interprete the groundwater prospect of the subsurface Table 1 Range of resistivity for lithological characterisation and groundwater prospect of saprolite and bedrock Modified after Olorunfemi and Olorunniwo (1985), David (1988), and Akanbi (2017) S/n Resistivity Lithological description of the saprolite Groundwater Bedrock Description of the bedrock Groundwater range (Ωm) prospect of resistivity prospect of saprolite (Ωm) bedrock 1 0–50 Predominantly clayey Poor >1800 Fresh Negligible > 400 Compacted clay/hardpan 2 51–150 Sand and clay mixture Moderate 601–1800 Weak/slightly weathered Moderate 3 151–400 Predominantly sandy to gravelly Good < 600 Fractured Good 1 3 89 Page 6 of 22 Applied Water Science (2018) 8:89 environments. From the bedrock iso-resistivity map, local concept for longitudinal conductance assumes that the earth fracture zones that ensure continuity of groundwater pros- is parallel with n layer and the flow of current through and pecting in the underlying rocks were demarcated. Further- across such sequence is controlled by individual layer’s more, aside the lithology of the saprolite units, the thickness resistivity and its respective thicknesses. The longitudinal of the saprolite is another crucial factor that is considered conductance of a geo-electric layer is the ratio of individ- in groundwater prospecting of the weathered layer in base- ual thickness and the corresponding resistivity. The unit ment terrain. Hence, qualitative method also involved gen- of measurement is in mhos. However, for a geo-electric erating the thickness map of the saprolite and the resistivity sequence with more than one layer, the total longitudinal maps of both the bedrock and that of the former to classify conductance (S) is the summation of the ratios of individual groundwater prospect of the various bedrock terrains across layer’s thickness and corresponding resistivity; that is for the study area. a geo-electric sequence with nth layers above the bedrock, Likewise, qualitative interpretation included the estima- S = ∑hi/ρi, where i = 1st, 2nd, 3rd,…, nth layer above the tion of the longitudinal conductance, which is one of the bedrock. S is crucial in relating the ratios of individual layer Dar-Zarrouk variables. The Dar-Zarrouk variables are sec- thickness to its lithology and is applicable for measuring ondary geo-electric parameters derived from true layer’s aquifer vulnerability to surface environment contamina- resistivity and corresponding thickness. The background tion. Therefore, locations with thicker and more clayey Fig. 3 Locations of VES points on geological map 1 3 Applied Water Science (2018) 8:89 Page 7 of 22 89 weathered-regolith will be characterised by higher S, and Groundwater table and depth of each borehole were consequently, higher protective capacity than areas with measured in metre in the field using automatic water level thinner and more sandy regolith. The spread of the VES indicator. Groundwater discharge was carried out with half points across the area is presented in Fig. 3. horse power submersible pump, and the yield was meas- ured in m /day using ISO 4064 DN20 water flow meter Measurement of groundwater yield and well with accuracy of 0.1 l. The pipe was extended to nearby inventory concrete surface water drains to expel the discharge water beyond the vicinity of the pumped well so as to avoid Taking of well inventory including well topography (WT), artificial recharge of the pumped well. well depth (WD), groundwater table (GWT) and meas- A portable etrex Garmin Global positioning System urement of groundwater discharge (Q) was conducted on (GPS) was used to record the coordinates of all sound- nineteen drilled wells that spread through the principal bed- ings and wells locations across the area. Elevations were rock terrains. The numerical distribution of the boreholes measured relative to the mean sea level. These coordinates included five each in locations underlain by amphibolite, are geo-referenced and used for contouring of geo-elec- migmatite and gneissic bedrocks, and the last four on gra- tric parameters and siting of survey points in maps using nitic terrain (Fig. 4). Surfer 10 software. The generated maps were applicable Fig. 4 Borehole locations on geological map 1 3 89 Page 8 of 22 Applied Water Science (2018) 8:89 for visual descriptions of vertical and spatial variations of General profile of weathering and bedrock geo-parameters across the study area. Window 10 Excel fracturing software was used for statistical evaluation of data such as for estimation of mean and correlation analyses of Apart from fractures, the regolith thickness and its litho- variables. logical attributes are crucial to groundwater occurrence. The summary of the primary geo-electric parameters is presented in Table 2. The general weathering profile in the study area Results and discussion irrespective of bedrock type is mainly three (3)-layered sequence, namely the topsoil, the saprolite and the bedrock. The geo‑electric curves The resistivity of the topsoil varies, ranging from as low as 38.4 Ωm in amphibolite to as high as 4478.2 Ωm at Alaraba The generated VES curves are dominantly 3-layer H type, underlain by gneissic bedrock. The mean value of 658.5 characterised by relatively more conductive middle layer that Ωm obtained over amphibolite is typically lower as com- terminates on more resistive inn fi ite layer. Only seven curves pared to the range of 1152–1344 Ωm as average values for that represented just 8.5% exceeded 3-layer geo-electric other bedrocks. Topsoil is commonly made up of alluvium sequence, and this included four-layer KH-type curve that particles, characterised by wide resistivity variations as a are four in number, one each of HK and HA sequence, and result of land use pattern. The thickness of the top soil is two 5-layer HKH VES curves. however an important factor when considering contamina- tion through direct groundwater recharge. The thickness of the top soil exceeded 2.0 m only in 20 locations, and this is Weathering characterisation more frequent in areas underlain by amphibolite and por- phyritic granite. Nonetheless, on the average, the thickness Iso‑apparent resistivity map of the top soil is generally less than two metres for all the bedrocks (Fig. 6). The apparent iso-resistivity maps (Fig.  5a–d) revealed The iso-resistivity and isopach (thickness) map of the the weathering attributes at various depths correspond- saprolite layer are presented in Fig. 7a, b, respectively. The ing to half AB separations of 10, 24, 42 and 75 m. These general spread of the resistivity of the saprolite across the diagrams reflected weathering heterogeneities typified by study area was between 9.1 and 1903 Ωm (Table 2). The increase in resistivity with depth. At corresponding depth minimum resistivity obtained for the bedrocks is lowest in of AB/2 separation of 10 m (Fig. 5a), highly to moderately amphibolite with 9.1 Ωm and highest in migmatite with 28.1 weathered zones with resistivities below 300 Ωm are pre- Ωm. The range of the maximum resistivity is 225.2–810.6 dominant and soil development is at the peak. Localised Ωm. The mean value of the resistivity of the saprolite is zones with < 100 Ωm are more dominant within amphibolite also lowest in areas underlain by amphibolite with 53.1 Ωm, and terrains underlain by gneisses. At half AB separation while average resistivities for areas underlain by other bed- of 24 m, there is reduction in area coverage of the highly rocks are higher with 93, 118 and 204 Ωm in porphyritic weathered zones at the corresponding depth. This resulted granite, gneisses and migmatite, respectively. Based on the in the thinning out of highly weathered lithology on areas most widespread resistivity range of 61–150 Ωm (Fig. 7a underlain by gneisses and amphibolite, while the spatial and Table 2), the saprolite is dominated by composite of coverage of moderately weathered lithology with resistiv- sand and clay across the area. Occasionally, there is occur- ity 100–600  Ωm (Fig.  5b) expanded. At higher half AB rence of compacted lateritic clay layer, otherwise known as separations of 42 m, poorly weathered and bedrock subsur- ‘hardpan’ associated with very high resistivity that ranged face horizons with resistivity > 600 Ωm are conspicuously between 433.7 and 4412.9. represented (Fig. 5c). Lastly, at AB/2 separations of 75 m, The thickness of the saprolite ranges from 3.80 to highly weathered zones have virtually disappeared and the 38.70 m at an average (av.) of 16.56 m, 4.40–57.70 m (av. weathering is poor, and bedrock horizon are now becoming 15.83 m), 3.40–22.40 m (av. 11.31 m) and 1.80–19.40 m (av. dominant (Fig.  5d). However, even at AB/2 = 75  m, large 9.52 m) correspondingly on amphibolite, gneisses, migma- part of the study area is still categorised as being moderately tite and porphyritic granite bedrocks. On the average, areas weathered. This is an indication of a fairly deep weather- underlain by gneisses and amphibolite are characterised ing development with resistivities 100–600 Ωm across the by deeper weathered layer. The total regolith thicknesses study area. were correspondingly 18.49 and 18.23, compared to 12.77 and 11.29 for migmatite and granitic terrains, respectively (Table 2). The most widespread thickness from the isopach map (Fig.  7b) is within the range of 7–17  m. However, 1 3 Applied Water Science (2018) 8:89 Page 9 of 22 89 Fig. 5 Indication of weathering development at corresponding depths to half AB separations of a 10 m, b 24 m, c 42 m, and d 75 m 1 3 89 Page 10 of 22 Applied Water Science (2018) 8:89 Fig. 5 (continued) 1 3 Applied Water Science (2018) 8:89 Page 11 of 22 89 Table 2 Statistics of the geo- Geo-electric Explanation Min Max Mean Median electric parameters by bedrocks parameters Amphibolite (n = 25)  ρ1 (Ωm) Resistivity of the topsoil 38.40 3478.00 658.47 407.60  ρ2 (Ωm) Resistivity of saprolite layer 9.10 225.20 53.10 36.35  ρ3 (Ωm) Resistivity of bedrock 105.70 5472.70 1515.59 927.00  h1 (m) Thickness of top soil 0.40 5.80 1.68 1.45  h2 (m) Thickness of saprolite layer 3.80 38.70 16.56 16.00  H (m) Total regolith thickness 4.40 39.40 18.06 17.20  S (mhos) Total longitudinal conductance 0.02 1.42 0.45 0.40 Gneisses (n = 19)  ρ1 (Ωm) Resistivity of the topsoil 176.90 4478.20 1343.59 984.30  ρ2 (Ωm) Resistivity of saprolite layer 18.80 810.60 117.95 69.00  ρ3 (Ωm) Resistivity of bedrock 196.60 9601.40 1814.92 934.50  h1 (m) Thickness of top soil 0.80 2.30 1.51 1.40  h2 (m) Thickness of saprolite layer 4.40 57.70 15.83 12.50  H (m) Total regolith thickness 6.30 59.00 18.49 16.60  S (mhos) Total longitudinal conductance 0.01 1.06 0.28 0.20 Migmatite (n = 18)  ρ1 (Ωm) Resistivity of the topsoil 237.20 2331.10 1194.21 965.70  ρ2 (Ωm) Resistivity of saprolite layer 28.10 565.50 204.39 61.00  ρ3 (Ωm) Resistivity of bedrock 191.30 6863.30 1488.25 989.15  h1 (m) Thickness of top soil 0.50 2.80 1.56 1.40  h2 (m) Thickness of saprolite layer 3.40 22.40 11.31 11.85  H (m) Total regolith thickness 4.60 23.60 12.77 12.95  S (mhos) Total longitudinal conductance 0.02 0.60 0.23 0.18 Porphyritic granite (n = 20)  ρ1 (Ωm) Resistivity of the topsoil 110.50 3271.90 1151.57 995.40  ρ2 (Ωm) Resistivity of saprolite layer 26.50 294.00 93.03 65.55  ρ3 (Ωm) Resistivity of bedrock 547.20 29,903.80 4601.23 2605.35  h1 (m) Thickness of top soil 0.60 4.50 1.76 1.25  h2 (m) Thickness of saprolite layer 1.80 19.40 9.52 8.30  H (m) Total regolith thickness 2.40 20.80 11.29 10.65  S (mhos) Total longitudinal conductance 0.05 0.38 0.15 0.12 localised deeper weathered zones with thickness exceeding Exactly two-thirds of the bedrock fractures are restricted 17 m are found on gneisses and to a lesser extent on amphi- to the south-east part in areas close to rock contact zones bolite (Fig. 7b). around Sekere and Igboora. These fractures are also aligned The bedrock iso-resistivity contour map is presented in along the same direction of NW–SE trend at which the rock Fig. 8 with resistivity range of 105–29,903 Ωm. Bedrock contacts are aligned (Fig. 9). The depth to fractured zones resistivity of < 600 Ωm are indicative of basement fractures varies and are found at 7.9–29.9 m (av. 18.3 m) below the and this occurred at 24 locations, out of which eleven were earth surface on amphibolite terrains and at depth range of within amphibolite, five in gneissic, seven in migmatite ter - 16.6–27.3 m (av. 22.3 m) on gneisses. Fracture bedrocks rains and only one location at Idere on porphyritic granite. were found at relatively shallower depths of 6.7–19.5 m (av. The most promising fracture zones with < 600 Ωm bed- 13.6 m) on migmatite and 5.8 m on porphyritic granite at rock resistivity are localised in areas underlain by banded Idere. gneiss at the NW and SW regions, and SE zone underlain by migmatite and amphibolite (Fig. 8). Porphyritic granitic terrains are characterised by higher resistivity at an aver- age of 4601 Ωm. Bedrock fracturing is least within granitic terrains. 1 3 89 Page 12 of 22 Applied Water Science (2018) 8:89 Fig. 6 Topsoil thickness map across the study area 0.20 mhos but the vulnerability is even higher at Jagunode Total longitudinal conductance (S) and aquifer and Sekere in the SE region (Fig. 10). Within amphibolite vulnerability terrains, S largely exceeds 0.30 mhos and aquifers are less vulnerable to surface contamination. From Table 2, the total longitudinal conductance (in mhos) of the regolith ranged from 0.01 to 1.42 across the study Well inventory data and groundwater yield area. The conductance is however relatively higher in ter- rains underlain by amphibolite and gneisses with corre- The results of groundwater yield and well inventory data sponding values of 0.02–1.42 at an average of 0.45 (av. 0.45) of the nineteen boreholes across the four principal bed- and 0.01–1.06 (av. 0.28), compared to 0.02–0.60 (av. 0.23) rock terrains are presented in Table 3. Boreholes BH01 to in migmatite and 0.05–0.38 mhos (av. 0.15) for porphyritic BH05 were those on amphibolite terrains, while BH06 to granite. Aquifer is more vulnerable in areas with lower con- BH10 on gneisses, BH11 to BH15 on migmatite and BH16 ductance, most especially within porphyritic granitic ter- to BH19 penetrated porphyritic granite (Fig. 4). Form the rains and other adjoining areas with < 0.20 mhos (Fig. 10). general statistics, borehole elevation was between 139 m and The conductance in areas underlain by gneisses varies 215 m above mean sea level, while borehole depths were extensively, but large area lies within the low to moderate between 15.9 and 43 m. The depth to water table ranges catchment. Locations with low vulnerability occur in three from 1.5 to 14.5  m and groundwater yield was between pockets/localised zones at the mid-north-west and south- 32.8 and 99.8 m /day (Table 3), which was approximately west. However, the latter zone occupies more extensive 32,800–99,800 l/day. area than the two localised portions at the north-west area. However, to show the influence of bedrock on ground- Nonetheless, the vulnerability is high for the regolith overly- water yield and on well inventory data, statistical summary ing Alagba area at the north-west zone. For areas underlain of the parameters by bedrocks are presented in Table 4. The by migmatite, the vulnerability lies between moderate to well depths range about 20–38 m on amphibolite, 31–43 m high. For most terrains underlain by migmatite, S exceeds 1 3 Applied Water Science (2018) 8:89 Page 13 of 22 89 Fig. 7 a Resistivity map of the study area. b Saprolite thickness map of the area 1 3 89 Page 14 of 22 Applied Water Science (2018) 8:89 Fig. 8 Bedrock iso-resistivity map of the study area on gneisses, and 18–36 m on migmatite and 16–37 m on por- collaborated by Q/GWT (Fig. 11b) relationships, whereby phyritic granite. Also, GWT occurred in the range 1.5–10.7, shallow groundwater table will guarantee high groundwater 4.2–14.5, 3.3–11.8 and 2.0–6.5 m, while Q in m /day was yield in amphibolite and gneisses terrains (where R = −0.83 56–93.6, 32.8–78.9, 41.9–99.8 and 56.8–91.1, respectively. and − 0.50, respectively) and to a lower extent within mig- Based on the average values, terrains underlain by gneisses matite terrains where the relationship is weak (R = −0.21). were characterised by deeper wells with 36.1 m compared to However, within granitic terrains, the relationship between Q those measured in other terrains with corresponding mean and GWT is positive and strong (R = 0.52). This showed that values of 29.3, 27.7 and 25.2 m for wells within amphibolite, shallow groundwater table does not guarantee high ground- migmatite and granite. Equally, GWTs are found at shal- water yield within granitic terrains. On the other hand, the lower average depths of 4.0–6.0 m within these bedrocks relationships between depth to water table and well depth are in comparison with those within gneisses with a mean of diverse across the bedrock terrains (Fig. 11c). It is strong and 8.2 m. On the other hand, groundwater yield, Q, was low- negative within granitic terrains (R = −0.92), moderate and est in gneissic bedrock with 53.2 m /day, compared to the negative in gneissic terrains (R = −0.38), strong and positive highest yield of 72.5 m /day obtained within amphibolite within migmatite (R = 0.78) and weak and amphibolite with 3 3 terrains, and 69.0 m /day and 68.3 m /day correspondingly R = 0.29. This indicated that deep wells are characterised by in wells that penetrated granitic and migmatite bedrocks. shallower water table in granitic and to a lesser extent within Results of statistical correlation of well data and ground- gneissic terrains. However, this relationship is direct in both water yield by bedrock affiliations are shown in Fig.  11. migmatite and amphibolite aquifers, though the significance The coefficients of correlation, R, is strong and negative is lower within terrains underlain by the former bedrock. (i.e. R < −0.5) between groundwater yield (Q), well depth For the relationships occurring between groundwater dis- (WD) and groundwater table (GWT) in the bedrocks, except charge, Q, and well topography, the associations are negative within migmatite, where R = −0.04 for Q/WD and − 0.21 for across the bedrocks, with the exception of granitic terrains. Q/GWT and the relationships are insignificant and weak, In respect to significance of relationships, it is fairly strong in respectively (Fig. 11a, b). This implies that deep wells do not amphibolite (R = −0.50), moderate in migmatite (R = −0.39) guarantee high groundwater yield, particularly in aquifers and weak in gneisses with R = −0.21 (Fig. 11d). However, within gneisses, amphibolite and granitic terrains. This is the relationship is very strong and positive in granitic terrain 1 3 Applied Water Science (2018) 8:89 Page 15 of 22 89 Fig. 9 Fractured bedrock locations on geological map of the study area with R = 0.88 (Fig. 11d). This implied that locations with influences well depth within locations underlain by gneisses high topography are characterised by high groundwater yield and migmatite. However, the relief of the area has a direct in granitic terrains, whereas the yield is low in elevated areas association with the depth of the wells in amphibolite and an underlain by other bedrocks. indirect relationship in granitic terrains. Equally, there is no The study has shown that there is no viable significant rela- significant relationship between groundwater table and well tionship existing between well topography and well depths topography also within gneisses and migmatite terrains, while within terrains underlain by both gneisses and migmatite, the associations are perfect and negative in granitic terrains whereas these associations are strong in amphibolite and (R = −0.95) and positive and moderate within amphibolite granitic terrains. Nevertheless, it is indirect in granite with with R = 0.43 (Fig.  11f). The perfect negative relationship R = −0.82 and positive in amphibolite with R = 0.56 (Fig. 11e). between these parameters in granitic terrains indicated that This means that the topography of the area is not a factor that 1 3 89 Page 16 of 22 Applied Water Science (2018) 8:89 Fig. 10 Aquifer vulnerability map of the area Table 3 Results of groundwater discharge and well inventory data Hydrogeological characterisation and groundwater prospects of the weathered–fractured aquifers S/n Borehole no. WT (m) WD (m) GWT (m) Q (m /day) by bedrock terrains 1 BH01 172 27.7 10.7 56.0 2 BH02 187 31.6 6.4 73.2 Amphibolite terrains 3 BH03 170 38.0 5.0 71.7 4 BH04 175 30.4 3.2 67.9 The total thickness of the regolith units of areas that were 5 BH05 159 18.9 1.5 93.6 mainly underlain by amphibolite were between 4.40 and 6 BH06 174 43.0 4.2 40.4 39.40 m (av. 18.23 m). The middle layer (or the saprolite 7 BH07 169 30.9 6.7 75.9 unit) alone has thickness range of 3.80–38.70 m at an aver- 8 BH08 143 34.6 6.4 78.9 age of 16.56 m. The saprolite units have an average resistiv- 9 BH09 171 34.1 14.5 32.8 ity value of 53.10 Ωm, and localised zones of high longitu- 10 BH10 139 38.0 9.4 37.9 dinal conductance (S) occurred within amphibolite terrains 11 BH11 188 26.4 3.3 41.9 (Fig. 10). The high S values obtained within amphibolite ter- 12 BH12 142 29.1 7.1 77.0 rains also suggest that the degree and extent of rock weather- 13 BH13 182 29.7 4.2 99.8 ing is high (Fig. 7). The development of fine-grained regolith 14 BH14 188 17.8 3.5 69.5 is attributable to the high susceptibility of the amphibolite 15 BH15 199 35.6 11.8 53.2 bedrock to weathering due to large composition of dark- 16 BH16 155 36.5 2.1 57.7 coloured ferromagnesian minerals such as hornblende and 17 BH17 215 20.3 5.3 91.1 biotite in the mineral assemblage (Akanbi, 2016). These 18 BH18 209 15.9 6.5 70.4 minerals (i.e. ferromagnesian) are largely unstable at surface 19 BH19 141 28.0 2.0 56.8 environment, since they crystallise at higher melting points. Additionally, the schistose textural attributes of Igboora amphibolite facilitates easy breakdown of rock by physical terrain elevation has a very strong indirect influence on the processes and rock mass decomposition by chemical pro- groundwater table of aquifer zone. cesses. Hence, aside the fact that the weathering develop- ment is well pronounced in areas underlain by amphibolite, 1 3 Applied Water Science (2018) 8:89 Page 17 of 22 89 Table 4 Statistical summary of 3 Bedrocks Statistics WT (m) WD (m) GWT (m) Q (m /day) geo-electric parameters and well inventory data by bedrocks Amphibolite, n = 5 Minimum 159.0 18.9 1.5 56.0 Maximum 187.0 38.0 10.7 93.6 Mean 172.6 29.3 5.4 72.5 Gneisses, n = 5 Minimum 139.0 30.9 4.2 32.8 Maximum 174.0 43.0 14.5 78.9 Mean 159.2 36.1 8.2 53.2 Migmatite, n = 5 Minimum 142.0 17.8 3.3 41.9 Maximum 199.0 35.6 11.8 99.8 Mean 179.8 27.7 6.0 68.3 Por. granite, n = 4 Minimum 141.0 15.9 2.0 56.8 Maximum 215.0 36.5 6.5 91.1 Mean 180.0 25.2 4.0 69.0 this terrain also has the highest number of bedrock fractures groundwater prospect within areas underlain by amphibolite compared to other terrains. Eleven (11) locations out of a is high and largely from the fractured zones. This is also sup- total of 24 locations with < 600 Ωm bedrock resistivities are ported by the higher yields that is > 67 m /day for most wells found within amphibolite terrains. This represented 46% (Table 3) within amphibolite. However, from hydrological of all fractured bedrock locations. Also, the relative per- relationships, well elevation has a moderate and an indirect centage of fractured bedrocks occurrences to those that are effect on groundwater yield, and higher groundwater yields unaltered is larger (Table 5, Figs. 9, 12). Furthermore, aside are more associated with lowland areas (Fig. 11b, d). the fractured bedrocks, another 12% amphibolite bedrocks are partially weathered. The partially altered bedrocks are Gneisses terrains those that are slightly weathered and are also liable to hav- ing moderate groundwater prospect with resistivity range of Just like terrains underlain by amphibolite, areas underlain 600–1800 Ωm. Generally, from the 25 soundings conducted by gneisses are also characterised by occurrence of thick within amphibolite terrains, the bedrocks of about 44% loca- regolith units of 6.30–59.00 m with an average thickness of tions are fractured, 12% are regarded as weak or partially 18.49 m. The thickness of the saprolite was between 4.40 weathered while 44% bedrocks are unaltered (Fig.  12). and 57.70 m and average of 15.83 m (Table 2). Localised Depths to fractured bedrocks are within 7.9–29.9 at an aver- zones of deeper weathering also exist at the NW area and at age of 18.3 m (Table 5). Also, fresh bedrock resistivity was the SW floodplain regions where saprolite thickness exceeds less than 2500 Ωm, except at the southwestern section of 22.0 m (Fig. 7b). Possible factors that may favour the devel- Igboora where bedrock resistivities range between 4500 and opment of thick regolith in areas underlain by gneisses are 5500 Ωm. mineralogical content and foliation features of the gneisses. The hydrogeological setting of terrains underlain by The resistivity of the saprolite ranged extensively from amphibolite, as described above, hereby supports generation 18.80 to 810.60 Ωm. Based on the average regolith resistiv- of artesian aquifer. This is supported by very strong indirect ity of 117.95 Ωm, the regolith units are coarser compared relationship occurring between yield and groundwater table to amphibolite terrains. The total longitudinal conductance (Fig. 11b). This means that the water table occurs at shallow ranges from 0.01 to 1.06 with an average of 0.28 mhos depth in more prolific wells at Igboora, which emphasised (Table  2). This is markedly lower than those obtained in that the groundwater is under pressure and rises above the amphibolite (0.45 mhos), which means that the aquifer vul- fractured bedrock aquifers. This is as a result of widespread nerability in gneisses is relatively higher compared to those occurrence of thick and largely fine-grained regolith over - in amphibolite terrain. Accordingly, the potential for water lying the main groundwater-bearing zones of fractured and infiltration through the regolith, inferred from the total lon- weak amphibolitic bedrocks. Additionally, groundwater gitudinal conductance, is better in gneiss terrains than in quality is expected to be fairly good as a result of the over- amphibolite. Notwithstanding, bedrock resistivities of only lying fine-grained regolith that prevents direct groundwater five out of nineteen (19) locations were fractured (Table  5, recharge of the bedrock aquifers (Fig. 10). Furthermore, the Fig. 12). However, the percentage of occurrences of partially occurrences of fractured bedrocks at relatively deeper zones weathered basement was 48% and quite enormous (Fig. 12), at an average of 18.3 m will further enhance the protection and with this, the degree of rock decomposition can also of the enclosed groundwater resource in the area. Hence, be said to be equally high. Bedrock fractures occurred at 1 3 89 Page 18 of 22 Applied Water Science (2018) 8:89 Yield Vs. Groundwater table (a) (b) Yield Vs. Well Depth 50 50 40 40 10.0 20.0 30.0 40.0 0.05.0 10.0 15.0 Well Depth (m) Groundwater table (m) Amphibolite, R = - 0.54 Gneisses, R = - 0.57 Amphibolite, R = - 0.83 Gneisses, R = - 0.50 Migmatite, R = - 0.04 P. Granite, R = - 0.65 Migmatite, R = - 0.21 P. Granite, R = 0.52 (c) (d) Yield Vs. Well Elevation Well Depth Vs. Groundwater table 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 0.0 5.0 10.0 15.0 120 140 160 180 200 220 Groundwater table (m) Well Elevation (m) Amphibolite, R = - 0.50 Gneisses, R = - 0.21 Amphibolite, R = 0.29 Gneisses, R = - 0.38 Migmatite, R = - 0.39 P. Granite, R = 0.88 Migmatite, R = 0.78 P. Granite, R = - 0.92 (e) (f) Water Table Vs. Well Elevation Well Depth Vs. Well Elevation 20.0 45.0 18.0 40.0 16.0 14.0 35.0 12.0 30.0 10.0 25.0 8.0 6.0 20.0 4.0 15.0 2.0 10.0 0.0 120 140 160 180 200 220 120 140 160 180 200 220 Well Elevation (m) Well Elevation (m) Amphibolite, R = 0.56 Gneisses, R = 0.04 Amphibolite, R = 0.43 Gneisses, R = 0.01 Migmatite, R = 0.02 P. Granite, R = - 0.82 Migmatite, R = 0.08 P. Granite, R = - 0.95 Fig. 11 a Plot of groundwater yield against well depth. b Plot of e Plot of well depth against well depth. f Plot of groundwater table groundwater yield against water table. c Plot of well depth against against well elevation groundwater table. d Plot of groundwater yield against well elevation. 1 3 Well depth (m) Well depth (m) Yield (m /day) Water Table (m) 3 3 Yield (m /day) Yield (m /day) Applied Water Science (2018) 8:89 Page 19 of 22 89 Table 5 Frequency and depths S/n Rock units Total no of Frequency and relative percentage of occur- Depth of occur- of occurrence of fractured VES points rences of fractured bedrock in each bedrock rence (m) bedrocks n (%) Min Max Mean 1 Amphibolite 25 11 (44%) 7.9 29.9 18.3 2 Gneisses 19 5 (26%) 16.6 27.3 22.5 3 Migmatite 18 7 (39%) 6.7 19.5 13.7 4 Porphyritic granite 20 1 (5%) – – – Migmatite terrains The thickness of regolith development in areas underlain 60 by migmatite is shallower when compared with terrains underlain by amphibolite and gneisses. The thickness of the 44 44 regolith was 4.60–23.6 m and average 12.77 m, while that of the saprolite was 3.40 –22.40 at an average of 11.31 m. 26 26 The resistivity of the regolith was 28.10–565.50 Ωm with 15 mean value of 204.39 Ωm. Based on the mean resistivity of 204.39 Ωm, the regolith units are sandy and coarser. Hence, migmatite terrains are characterised by lower longitudinal AMPHIBOLITE GNEISSES MIGMATITE POR.GRANITE conductance of 0.02–0.60 (av. 0.23  mhos), compared to those within amphibolite and gneisses. Additionally, seven Bedrocks bedrocks out of eighteen were fractured (Table 5, Fig. 12). Fractured Bedrock Weak Bedrock Fresh Bedrock However, most of the fractured bedrock were found localised within the SE region and aligned in NW direction, while Fig. 12 Comparative percentage frequencies of bedrock conditions other fractured zones are widely spaced from one another. Notwithstanding, 33% of bedrock were classified as weak, which is higher than those found in porphyritic granite and the depth of 16.6–27.3 m (av. 22.5 m). This is compara- even in amphibolite terrains (Fig. 12). However, the risk of tively deeper than other bedrocks (Table 5). Nonetheless, groundwater contamination is higher in migmatite terrains areas underlain by gneisses are expected to have better water than in gneisses and much higher in comparison with amphi- transmission ability owing to the thick and the fairly coarser bolite terrains. From these results, groundwater occurrences regolith. Additionally, the foliation structures in gneisses in terrains underlain by migmatite exist in unconfined state, will also serve as subsurface water conduits apart from aid- and the water table is exposed to atmospheric conditions. ing rock weathering. Additionally, as a result of occurrences of bedrock fractures The hydrogeological setting in gneissic terrains will gen- at shallower depths of 6.7–19.5 m (av. 13.7 m) (Table 5) and erate a semi-confined aquifer system due to occurrence of the overlying coarse regolith, aquifer vulnerability to con- coarser regolith that is overlying the largely partially altered tamination is high to moderate in location will deeper and bedrocks. The thick weathered units will serve as both water- less coarse regolith. Also, well depth has little or no signifi- bearing zone and a semi-confining unit to the largely par - cant relationship with the groundwater yield, whereas there tially weathered bedrocks that are expected to have good is a moderate and indirect relationship existing between the water-bearing capacity. Water quality may be poor in these well discharge and the topography of the area (Fig. 11a, d). terrains when tapped from the regolith, whereas water from wells that tap into the relatively deeper fractured bedrocks at Porphyritic granite terrains an average depth of 22.5 m will be safer. Well topography in gneisses has little effect on the groundwater yield, whereas Terrains underlain by porphyritic granite are the least well depth has moderate indirect significance on yield, and weathered and the least fractured (Figs.  7, 8, 9, 12, and deep wells do not support high groundwater yield (Fig. 11a). Table 5) across the study area. The overburden thickness was 2.4–20.8 m (11.3 m) compared to the averages of over 18 m in both amphibolite and gneisses and about 13 m in migmatite terrains. Also, the average thickness of the sap- rolite was just 9.5 m, whereas it was 16.6, 15.8 and 11.3 m 1 3 Bedrock condition (%) 89 Page 20 of 22 Applied Water Science (2018) 8:89 correspondingly in other bedrocks (Table 2). Weathering fractures (Fig. 9). Aquifers within this terrain are charac- 3 3 intensity was not only comparatively shallower in granitic terised by higher yields of 56.0–93.6 m /day (av. 72.5 m / terrains, but the weathered layer also terminates mainly on day). This is despite poor groundwater prospect of the thick unaltered or fresh bedrocks. The porphyritic bedrock resis- and largely fine-grained overlying weathered-regolith. This tivity was 547–29,903 Ωm at an average of 4601 Ωm, while showed that fractured bedrocks are able to provide better in other terrains, the averages were 1516, 1815, 1488 Ωm, groundwater yield more than the overlying weathered-reg- respectively. Additionally, bedrock of just one location is olith that is underlain by impermeable crystalline bedrocks. fractured, and this is at Idere, and the fractured zone occurs For areas underlain by gneisses, though, weathering was at shallow depth of 5.8 m (Table 5, Fig. 9). The bedrocks of more intensive, and the recorded yield of 32.8–78.9 m / just three locations can be said to be slightly weathered, and day (av. 53.2 m /day) was the lowest in all bedrock setting the rest (that is 80%) are fresh basement (Fig. 12). Therefore, in the area. This is as a result of fewer developments of fresh granitic bedrock occurs at shallow depth. However, bedrock fractures in comparison with those within amphi- areas with fairly thick regolith > 10 m are found in areas bolite and migmatite regions. Apart from this, the few close to rock contact zones at Idere and other few locations. fractured zones were isolated and not closely packed as in Notwithstanding, the average S for regolith within the gra- the other two terrains (Fig. 9). Also, the vulnerability of nitic bedrock terrains was 0.15 mhos. This is much lower in the weathered layer, which is the alternative water-bearing comparison with 0.45, 0.28 and 0.23 mhos as the average zone is low to moderate (Fig.  10); hence, groundwater total longitudinal conductance correspondingly for the rego- occurrence within gneissic terrains is modest. However, lith layer within other bedrock terrains. groundwater yield of 75.9 m /day was obtained in well Generally, due to the shallow extent of rock weather- BH07 in a location close to Alagba where prominent frac- ing, aquifers are unconfined and occur at shallower depths tures are found. Within migmatite terrain, groundwater compared to those within other bedrock terrains. Also, the yield is higher with 41.9–99.8 (av. 68.3) m /day, compared 3 3 average yield of 69 m /day, which is good, may not be sus- to those in gneisses (i.e. av. 53.2 m /day). The more pro- tainable for longer period. This is as a result of the fact that lific yield in migmatite is attributable to localised higher the underlying granitic bedrocks are typically impermeable occurrences of bedrock fractures. This is supported by the and are found at shallow depths. Thereby, the only water- groundwater yield > 70 m /day that was obtained in wells bearing zone is the overlying regolith in granitic terrain, BH12 and BH13 located within rock contact zones char- and as a consequence, the groundwater system is exposed acterised by numerous fractures that are closely spaced to seasonal fluctuations as well as pollution from surface (Figs. 4, 9). This also showed that rock contacts promote environment. Therefore, the aquifer vulnerability is high bedrock fractures, and wells sited within these subsurface (Fig. 10). Additionally, deep wells would not guarantee high discontinuities are most likely to be more productive than yield in granitic terrains as revealed by the strong indirect those within any typical bedrock terrains. relationship that existed between groundwater yields and The granitic aquifers have the second highest yield of well depths (Fig. 11a). However, area topography has a very 56.8–91.1 (av. 69.0) m /day after those of amphibolite. The strong positive influence on the groundwater yield and high- high yields were from the two wells at Idere, i.e. BH17 and lands areas are characterised by higher yield than lowland BH18 with yields of 91.1 and 70.4 m /day, respectively. areas unlike other bedrocks terrains (Fig. 11d). The other two wells further north at Ayete and Tapa were The summary of hydrogeological attributes under each characterised by lower yield that is < 60  m /day. High bedrock terrain is highlighted in Table 6. groundwater yield at Idere is attributable to topographic Groundwater prospect is good in areas underlain by effect (Fig.  11d) and nearness to fracture and rock con- amphibolite mainly due to high frequency of bedrock tact zones (Fig.  9). The elevations of wells at Idere are Table 6 Summary of hydrogeological characterisations of bedrock terrains at Ibarapa region S/n Bedrock Extent and Predominant Bedrock conditions Aquifer vulnerability Groundwater yield Ground- degree of saprolite lithol- water weathering ogy prospect 1 Amphibolite High Clayey Fractured to unaltered Low High Good 2 Gneisses High Sandy and clay Slightly altered Low to moderate Low Fair 3 Migmatite Fair Sandy Fractured to partly High to moderate Fairly high Moderate altered 4 Porphyritic granite Low Sands and clay Mainly unaltered High High at elevated ter- Fair rains 1 3 Applied Water Science (2018) 8:89 Page 21 of 22 89 both > 200 m, whereas the wells’ elevations at Tapa and demands for a systematic siting of wells for future ground- Ayete are < 160 m (Table 3). Conversely, the possibilities water development in the area. of siting bedrock aquifers are rare on granitic terrain due Lastly, variance in the intensity of weathering, discrep- to paucity of fractures. Nonetheless, from the good yield ancies in bedrock fractures, irregularity in area topography of the mainly regolith aquifers at elevated granitic terrains, and well depth have profound effects on the prospect for the groundwater prospect is fair. However, the yield will groundwater in the area. Methodical siting of wells in line be more sustainable, if there is a source of regular recharge with the present findings will ensure better and more sus- of the groundwater system. Therefore, the sustainability tainable groundwater yield across the study area. This will of the weathered-regolith aquifers in granitic areas to a also limit the impact of hydrogeological complexity brought large extent depends on the climatic conditions. Also, the about by geological and other natural factors that are beyond likelihood of contamination of groundwater from surface human regulations. effluent is high in granitic terrains, since the aquifer sys- Open Access This article is distributed under the terms of the Crea- tem is mainly the saprolite. In addition to this, because tive Commons Attribution 4.0 International License (http://creat iveco weathering is less developed, impermeable granitic bed- mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- rock is found at shallow depth and aquifer vulnerability tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the is quite high. Creative Commons license, and indicate if changes were made. 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Geophysics 2:529–556 ticularly within high topographic areas underlain by granite. Malomo S, Okufarasin VA, Olorunniwo MA, Omode AA (1991) For other areas, lowland aquifers are more prolific in respect Groundwater chemistry of weathered zone aquifers of an area of groundwater yield. This hydrogeological complexity underlain by basement complex rocks. J Afr Earth Sci 11:357–371 1 3 89 Page 22 of 22 Applied Water Science (2018) 8:89 Munaserei D (1979) A comparative analysis of the streamflow for Olorunfemi MO, Olorunniwo MA (1990) The determination of the instrumented basins of rivers Ogun and Ofiki. B.Sc. Project, geo-electric parameters of some Nigerian residual and detrital Department of Geography, University of Ibadan clays. J Min Geol 26(1):81–85 Murali S, Patangay NS (2006) Principles and applications of ground- Orellana E, Mooney HM (1966) Master Tables and curves for vertical water geophysics, 3rd edn. Association of Exploration Geophysi- electrical sounding over layered structures: Madrid Interciecia. cists, Hyderabad Geophysics 28:99–110 NGSA (2009) Geological and mineral resources map of south-western Oseji JO, Atakpo EA, Okolie EC (2005) Geo-electric investigation of zone, Nigeria. NGSA, Abuja the aquifer characteristics and groundwater potential in Kwale, Offodile ME (2002) Ground water study and development in Nigeria, Delta state, Nigeria. J Appl Sci Environ Manag 9(1):157–160 2nd edn. Mecon Geology and Engineering Services Limited, Jos Singhal BBS, Gupta RP (1999) Applied hydrogeology of fractured Oladapo MI, Akintorinwa OJ (2007) Hydrogeophysical study of rocks. Kluwer, Dordrecht Ogbese southwestern Nigeria. Glob J Pure Appl Sci 13(1):55–61 Tijani MN (2016) Groundwater: the buried vulnerable treasure. Inau- Olayinka AI, Mbachi CNC (1992) A technique for the interpretation of gural lecture. University of Ibadan, Ibadan electrical soundings from crystalline basement areas of Nigeria. J Vander VBPA (1988) Resist software version 1.0: M.Sc. Research Min Geol 28(2):273–281 Project. ITC, Delft, Netherlands. Copyright @2004 ITC.IT-RSG/ Olayinka AI, Olayiwola MA (2001) Integrated use of geo-electrical DSG imaging and hydrochemical methods in delineating limits of pol- Weerawarnakula S (1986) Petrology and geochemistry of Precam- luted surface and ground-water at landfill site in Ibadan areas, brian rocks in Igboora area, southwestern Nigeria. M.Phil. thesis, soutwestern Nigeria. J Min Geol 37(1):55–68 Department of Geology, University of Ibadan Olorunfemi MO, Okhue ET (1992) Hydrogeologic and geologic sig- nificance of a geo-electric survey at Ile-Ife, Nigeria. J Min Geol Publisher’s Note Springer Nature remains neutral with regard to 28(2):221–229 jurisdictional claims in published maps and institutional affiliations. OlorunfemI MO, Olorunniwo MA (1985) Geo-electric parameters and aquifer characteristics of some parts of southwestern Nigeria. Geologia Applicata E. Idrogeologia 20:99–109 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Water Science Springer Journals

Hydrogeological characterisation and prospect of basement Aquifers of Ibarapa region, southwestern Nigeria

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Abstract

The present study involved the use of 82 geo-electric soundings, and the measurement of well inventory and conduct of yield tests in 19 wells across the various bedrock terrains of Ibarapa region of southwestern Nigeria. The aim is to proffer solution to the unsustainable yield of the available boreholes in order to effectively exploit the existing groundwater resource in the area. From the geological reports, the area is underlain by four principal crystalline rocks that include porphyritic granite, gneisses, amphibolite and migmatite. The geo-electric studies revealed that the degree and extent of development of the weathered–fractured component varied, leading to diversity in groundwater yield and in aquifer vulnerability to contamina- tion. The thickness of the weathered layer is greater than 18 m in areas underlain by amphibolite and gneisses and less than 13 m within migmatite and porphyritic granite terrains. High groundwater yield greater than 70 m /day was recorded in wells within the zones of rock contacts and in areas with large concentration of bedrock fractures and elevated locations across the various bedrock terrains. Aquifer vulnerability is low in amphibolite, high in granitic terrains, low to moderate in gneisses and high to moderate in migmatite. Also, wells’ depths and terrain elevation have a moderate to strong indirect relationship with groundwater yield in most bedrock terrains, except in high topographic areas underlain by porphyritic granite. Therefore, there is need for modification of well depth in accordance with the terrain elevation and hydrogeological complexity of the weathered–fractured components of the variuos bedrock terrains, so as to ensure a sustainable groundwater yield. Keywords Geo-electric studies · Borehole topography and depth · Groundwater yield · Basement aquifers Introduction of Ibarapa areas rely on groundwater supply tapped from hundreds of hand-pump boreholes that penetrated various Large parts of southwestern (SW) Nigeria are completely intrusive crystalline bedrocks of Precambrian age. Yet, even underlain by intrusive crystalline rocks that can neither with these provisions, availability of potable water is still store nor transmit water, except when fractured, or where inadequate, largely due to the unsustainable yield of many there is significant regolith thickness over the basement. The of the functioning boreholes (Akanbi 2017). This scenario study area, which lies within Ibarapa region of SW Nigeria, is further complicated due to rise in human population in has no functioning town water supply scheme, and human the area as typical of many developing nations including settlements are not connected with piped water facilities Nigeria. This has led to increase in demand for fresh water (Fashae et al. 2014; Akanbi 2016). Therefore, for domestic supply in Ibarapa area. However, groundwater development water needs and even for agricultural purposes, residents is hampered in this region as a result of lack of pre-drilling and drilling data and little is known about the hydrogeology of the present study area. This has not helped in providing information needed for further groundwater development in Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1320 1-018-0731-9) contains the area. Also, considering the fact that the hydrogeology of supplementary material, which is available to authorized users. weathered–fractured aquifers is highly heterogeneous due to complexity in geological, structural and geomorphological * Olanrewaju Akinfemiwa Akanbi features (Akanbi 2017), expansion of the existing ground- olanrewajuakanbi@yahoo.com water supply in Ibarapa is quite challenging. These high Department of Earth Sciences, Ajayi Crowther University, demand for groundwater resource and the variability of its Oyo, Oyo State, Nigeria Vol.:(0123456789) 1 3 89 Page 2 of 22 Applied Water Science (2018) 8:89 occurrence in basement terrains have made it mandatory to state in the SW Nigeria and covers a total landmass of about investigate the hydrogeological setting and to validate the 750 km . The area encompasses most township areas that groundwater yield of the basement aquifers across the vari- include Igboora, Idere, Ayete and Tapa and their adjoin- ous bedrock terrains of the study area within Ibarapa region ing human communities within Ibarapa region (Fig. 1). SW of SW Nigeria. Nigeria is bounded at the west by the Republic of Benin and For hydrogeological characterisation of the basement by Atlantic Ocean at the south. aquifers in Nigeria, the applications of electrical resistiv- Ibarapa region lies within the tropical climatic zone, ity methods have been used for decades (Olorunfemi and marked by two distinct seasons of wet and dry periods. Olorunniwo 1985, 1990; Ako et al. 1990; Malomo et al. The dry season extends from November to March with an 1991; Olayinka and Mbachi 1992; Olorunfemi and Okhue average monthly precipitation of less than 25 mm. The wet 1992; Olayinka and Olayiwola 2001; Oseji et al. 2005; Alile season peaks around June and September with monthly rain- et al. 2008). Most of these investigations used the primary fall exceeding 100 mm. The total annual rainfall in the area geo-electric parameters, which are the layer resistivity ranges from 813 to 1853 mm, while the annual temperature and the corresponding thickness for interpretations of the ranges from 24 to 29 °C. When compared to an average hydrogeological settings of the crystalline basement aqui- rainfall of 1541 mm/year for the entire SW Nigeria, the study fers. These studies described the lithology and deduced the area has lower average precipitation and higher average tem- aquifer characteristics and water saturation of the residual perature (Munaserei 1979; Daly et al. 1981; David 1988; overburden and the underlying fractured bedrocks. However, Tijani 2016). Hence, it is expected that the vegetal cover more recent research works (Oladapo and Akintorinwa 2007; will not be as thick, compared to other parts of SW Nigeria. Abiola et al. 2009; Aweto 2011; Jayeoba and Oladunjoye, The vegetal cover is more of derived savannah forested land 2013; Akanbi 2016) employed Dar-Zarrouk variables in marked by scattered-scanty small trees and shrubs, sand- conjunction with the primary geo-electric parameters to wiched by tall grasses. It lies between the transition zone demarcate areas into various groundwater potential zones of lowland rain forest of southern areas of Nigeria and the and for estimation of the groundwater vulnerability assess- savannah zone of the further northern areas of Oyo state. ment and protective capacity of the overburden unit. The Dar-Zarrouk variables are the secondary geo-electric param- Geology, geomorphology and drainage eters that included the longitudinal conductance, the traverse resistance and the coefficient of anisotropy first introduced The study area is underlain by igneous and metamorphic by Maillet (1947). These variables were derived from the rocks of Precambrian age (David 1988; NGSA 2009; Weera- integration of primary geo-electric parameters (i.e. the layer warnakula 1986) and the four principal crystalline rocks, thicknesses and their corresponding true resistivities) and namely porphyritic and homogeneous medium-grained gran- comparison of the passage of electric current across and par- ite; banded and augen gneisses; amphibolite; and migma- allel to the geo-electric boundaries to water passage through tite (Akanbi 2017). The distribution of the major rock units subsurface layers was made. in the study area is presented in Fig. 2. The outcropping However, for further understanding of groundwater sus- exposures of porphyritic biotite granite are the dominant tainability at Ibarapa region of SW Nigeria basement ter- bedrock at the middle section and occupy about one-third rains, measurements of groundwater yield and evaluations of the entire study area. The landforms formed by granitic of the well inventory of the basement aquifers also have to outcrops are ridges and inselbergs with intermediate low- be incorporated. This is due to erratic and localised nature of lying areas, and porphyritic granite is the major rock that groundwater occurrence in subsurface rock discontinuities underlain Idere, Ayete and Tapa township areas. Porphyritic and within the weathered unit (Offodile 2002). Therefore, granite is composed mainly of quartz, biotite and larger- the present study used vertical electrical sounding (VES) grained potassic feldspar. The porphyritic biotite granite is geophysical method, well inventory and yield to characterise a felsic rock as a result of the presence of larger amount of the hydrogeological settings and to attest the groundwater leucocratic minerals that include the quartz and the feld- prospect of the basement aquifers of Ibarapa region. spars. The medium-grained granite occurs as major map- pable intrusions in other principal rock units and is confined to a small section at the eastern part of the study area. The The study area mineral grains are massive, medium-grained and homogene- ous and massive. Location, climate and vegetation The amphibolite outcrops are found as disjointed low- lying boulder-sized units, and it is the major rock unit The study area lies between longitude 3°07′E to 3°21′E and that underlies Igboora Township. In respect of textural latitudes 7°21′N to 7°37′N (Fig. 1) of Ibarapa region of Oyo attribute, the amphibolite is fine to medium-grained 1 3 Applied Water Science (2018) 8:89 Page 3 of 22 89 Fig. 1 Location and accessibility map of the study area dark-coloured rock. Two varieties of amphibolite are found post-crystallisation structural events like folds and frac- at Igboora due to textural difference. These are the massive tures in the areas. and the schistose varieties. Lineation trend on the schis- The geomorphology of the area widely varies and can tose variety ranges from 093° to 124°, and the foliation be categorised into two main hydro-geomorphic units, plane dips between 38° and 44°W. Gneisses are mainly namely the high-lying and low-lying areas. The highlands restricted to the western part, though small area is under- are characterised by granitic hills, ridges and inselbergs that lain by augen gneiss at the south-east section of the study traversed the upper central section and extending from the area. The gneissic rock in the area is of the banded and the north of Tapa to Idere, and small parts at the north-east and augen varieties. Mineral foliation strikes between 339° and north-west regions of the study area. The elevations of some 360° and dips between 36°E and 40°E. However, at the of the hills are above 200 m above the mean sea level. The north-west region around Alagba area, the dip of the folia- ridges and inselbergs are granitic, and the texture is often tion is vertical (Fig. 2). Lastly, the migmatite terrains are porphyritic. However, major sections of the terrains under- mostly restricted to the adjoining villages such as Kondo, lain by migmatite are also highlands. The southern areas are Obatade, Tobalogbo and Alabi Oja at the northeastern largely lowlands and gently high rising landforms underlain part, and it is intruded upon at Idere by porphyritic gran- by gneisses and amphibolite. The major floodplain in the ite. The outcrops of migmatite preserved a lot of structural 1 3 89 Page 4 of 22 Applied Water Science (2018) 8:89 Fig. 2 Geological map of the study area (updated after Weerawarnakula 1986) area is found at the south-west part underlain by banded connected to the primary rivers forming in orthogonal and gneiss. oblique manners. This drainage pattern is more or less a The drainage pattern is dendritic as common in base- design that separates the entire study area into interfluvial ment areas characterised by high contrast in relief. In a regions. dendritic drainage pattern, tributary streams generally join at an acute angle forming Y-shaped junctions. The river network consists of primary rivers, namely Iworo, Ayin, Afo-Ape, Opeki and Aboluku sourced from the highlands Materials and methods terrain underlain by crystalline rocks. River Ofiki is the major river that drains the gneissic and granitic terrains, Existing topographical and geological maps of the study and it runs into River Ayin (secondary river) at the SW area were processed using ArcGIS 10.0. Conventional geo- region. It is remarkable to note that the primary rivers in logical mappings were also conducted to update the exist- the area run along the rock boundaries and coincide with ing map for accurate fixing of rock boundary and demar - the major structural NNW–SSE and NNE–SSW trends cating the area into various bedrock settings. A total of 82 development in the area. Other streams are intermittently geo-electric soundings, groundwater yield tests and taking 1 3 Applied Water Science (2018) 8:89 Page 5 of 22 89 of well inventory data were conducted in 19 boreholes in Qualitative methods a systematic approach across the various bedrock settings. True layer’s resistivities and corresponding thicknesses were Geo‑electrical surveys and groundwater prospect employed for qualitative interpretation. This involved char- acterising the extent and the degree of rock decomposition The spread of the 82 electrical soundings across the study from layers’ thicknesses and lithologic description of the area included a total of 25 soundings within terrains under- geo-electric layers, particularly the saprolite and the bed- lain by amphibolite, 19 on gneisses, 18 on migmatite and 20 rock using the corresponding layers’ resistivities. The sap- on porphyritic granite using Schlumberger electrode con- rolite is the weathered layer and one of the water-bearing figuration. The apparent resistivity field data were acquired zones in basement areas, characterised by high porosity, with Campus Omega resistivity meter, and the range of cur- and when the bedrocks are not fractured the saprolite is the rent electrode (AB) separation was between 200 and 266 m. only alternative water-bearing zones in basement area. In This range of AB separation was considered appropriate for this regard, successful groundwater prospect of the saprolite groundwater investigation in typical crystalline basement depends on its resistivity and its thickness and availability of Nigeria, where groundwater occurs at shallow depths of regular source of recharge (Singhal and Gupta 1999). (Offodile 2002). The systematic approach to VES surveys The range of resistivity for lithological characterisation and involved selective appropriation of suitable locations for groundwater prospect of the saprolite is presented in Table 1. sounding in order to cover the various bedrock settings and Groundwater prospect of saprolite is poor if the resistivity to avoid sounding close to outcropping rock exposures. For is between 1 and 60 Ωm for predominantly clayey lithol- interpretation of VES data for hydrogeological characterisa- ogy and when it is more resistive and greater than 400 Ωm tion, quantitative and qualitative methods of interpretation for ‘hardpan’ or compacted lateritic clays. This is because of the field apparent resistivity data were employed (Murali clayey saprolite are characterised by low to negligible water and Patangay 2006). transmission attributes. The prospect is adjudged moderate when the resistivity lies within the range 61–150 Ωm for Quantitative methods less clayey and sandy soils and good when the saprolite is more sandy to gravelly and resistivity lies between 151 and The VES quantitative interpretation method involved 400 Ωm. However, sustainability of groundwater supply is originating primary geo-electric parameters that included guaranteed when the bedrock is fractured and there are good the layers’ resistivities and thicknesses from partial curve connections between the fractures and the weathered layer matching and computer iteration techniques. This was done (Akanbi 2016). Hence, for bedrock, groundwater prospect by plotting the apparent resistivities obtained from the field is good when fractured and the resistivity is below 600 Ωm. against half electrode spacing (AB/2) on a log–log chart. The prospect is moderate when the bedrocks are partially The plotted field curves were then matched with standard or slightly weathered and the resistivity is between 601 and auxiliary curves of Orellana and Mooney (1966) to derive 1800 Ωm. Fresh bedrocks are marked with high resistivity the initial layer resistivity and corresponding thickness for in excess of 1800 Ωm and have little or no groundwater each geo-electric layer. These initial geo-electric parameters potential (Table 1). were used in modelling in Vander (1988) modelling pack- Also, the variation in weathering with depth was estab- age to generate the true resistivity and thickness that are the lished by contouring the apparent resistivities for depths cor- primary geo-electric parameters. responding to half of the current electrode distances (AB/2) of 10, 24, 42 and 75 m. Additionally, the true resistivities of the saprolite and the bedrock were equally contoured to interprete the groundwater prospect of the subsurface Table 1 Range of resistivity for lithological characterisation and groundwater prospect of saprolite and bedrock Modified after Olorunfemi and Olorunniwo (1985), David (1988), and Akanbi (2017) S/n Resistivity Lithological description of the saprolite Groundwater Bedrock Description of the bedrock Groundwater range (Ωm) prospect of resistivity prospect of saprolite (Ωm) bedrock 1 0–50 Predominantly clayey Poor >1800 Fresh Negligible > 400 Compacted clay/hardpan 2 51–150 Sand and clay mixture Moderate 601–1800 Weak/slightly weathered Moderate 3 151–400 Predominantly sandy to gravelly Good < 600 Fractured Good 1 3 89 Page 6 of 22 Applied Water Science (2018) 8:89 environments. From the bedrock iso-resistivity map, local concept for longitudinal conductance assumes that the earth fracture zones that ensure continuity of groundwater pros- is parallel with n layer and the flow of current through and pecting in the underlying rocks were demarcated. Further- across such sequence is controlled by individual layer’s more, aside the lithology of the saprolite units, the thickness resistivity and its respective thicknesses. The longitudinal of the saprolite is another crucial factor that is considered conductance of a geo-electric layer is the ratio of individ- in groundwater prospecting of the weathered layer in base- ual thickness and the corresponding resistivity. The unit ment terrain. Hence, qualitative method also involved gen- of measurement is in mhos. However, for a geo-electric erating the thickness map of the saprolite and the resistivity sequence with more than one layer, the total longitudinal maps of both the bedrock and that of the former to classify conductance (S) is the summation of the ratios of individual groundwater prospect of the various bedrock terrains across layer’s thickness and corresponding resistivity; that is for the study area. a geo-electric sequence with nth layers above the bedrock, Likewise, qualitative interpretation included the estima- S = ∑hi/ρi, where i = 1st, 2nd, 3rd,…, nth layer above the tion of the longitudinal conductance, which is one of the bedrock. S is crucial in relating the ratios of individual layer Dar-Zarrouk variables. The Dar-Zarrouk variables are sec- thickness to its lithology and is applicable for measuring ondary geo-electric parameters derived from true layer’s aquifer vulnerability to surface environment contamina- resistivity and corresponding thickness. The background tion. Therefore, locations with thicker and more clayey Fig. 3 Locations of VES points on geological map 1 3 Applied Water Science (2018) 8:89 Page 7 of 22 89 weathered-regolith will be characterised by higher S, and Groundwater table and depth of each borehole were consequently, higher protective capacity than areas with measured in metre in the field using automatic water level thinner and more sandy regolith. The spread of the VES indicator. Groundwater discharge was carried out with half points across the area is presented in Fig. 3. horse power submersible pump, and the yield was meas- ured in m /day using ISO 4064 DN20 water flow meter Measurement of groundwater yield and well with accuracy of 0.1 l. The pipe was extended to nearby inventory concrete surface water drains to expel the discharge water beyond the vicinity of the pumped well so as to avoid Taking of well inventory including well topography (WT), artificial recharge of the pumped well. well depth (WD), groundwater table (GWT) and meas- A portable etrex Garmin Global positioning System urement of groundwater discharge (Q) was conducted on (GPS) was used to record the coordinates of all sound- nineteen drilled wells that spread through the principal bed- ings and wells locations across the area. Elevations were rock terrains. The numerical distribution of the boreholes measured relative to the mean sea level. These coordinates included five each in locations underlain by amphibolite, are geo-referenced and used for contouring of geo-elec- migmatite and gneissic bedrocks, and the last four on gra- tric parameters and siting of survey points in maps using nitic terrain (Fig. 4). Surfer 10 software. The generated maps were applicable Fig. 4 Borehole locations on geological map 1 3 89 Page 8 of 22 Applied Water Science (2018) 8:89 for visual descriptions of vertical and spatial variations of General profile of weathering and bedrock geo-parameters across the study area. Window 10 Excel fracturing software was used for statistical evaluation of data such as for estimation of mean and correlation analyses of Apart from fractures, the regolith thickness and its litho- variables. logical attributes are crucial to groundwater occurrence. The summary of the primary geo-electric parameters is presented in Table 2. The general weathering profile in the study area Results and discussion irrespective of bedrock type is mainly three (3)-layered sequence, namely the topsoil, the saprolite and the bedrock. The geo‑electric curves The resistivity of the topsoil varies, ranging from as low as 38.4 Ωm in amphibolite to as high as 4478.2 Ωm at Alaraba The generated VES curves are dominantly 3-layer H type, underlain by gneissic bedrock. The mean value of 658.5 characterised by relatively more conductive middle layer that Ωm obtained over amphibolite is typically lower as com- terminates on more resistive inn fi ite layer. Only seven curves pared to the range of 1152–1344 Ωm as average values for that represented just 8.5% exceeded 3-layer geo-electric other bedrocks. Topsoil is commonly made up of alluvium sequence, and this included four-layer KH-type curve that particles, characterised by wide resistivity variations as a are four in number, one each of HK and HA sequence, and result of land use pattern. The thickness of the top soil is two 5-layer HKH VES curves. however an important factor when considering contamina- tion through direct groundwater recharge. The thickness of the top soil exceeded 2.0 m only in 20 locations, and this is Weathering characterisation more frequent in areas underlain by amphibolite and por- phyritic granite. Nonetheless, on the average, the thickness Iso‑apparent resistivity map of the top soil is generally less than two metres for all the bedrocks (Fig. 6). The apparent iso-resistivity maps (Fig.  5a–d) revealed The iso-resistivity and isopach (thickness) map of the the weathering attributes at various depths correspond- saprolite layer are presented in Fig. 7a, b, respectively. The ing to half AB separations of 10, 24, 42 and 75 m. These general spread of the resistivity of the saprolite across the diagrams reflected weathering heterogeneities typified by study area was between 9.1 and 1903 Ωm (Table 2). The increase in resistivity with depth. At corresponding depth minimum resistivity obtained for the bedrocks is lowest in of AB/2 separation of 10 m (Fig. 5a), highly to moderately amphibolite with 9.1 Ωm and highest in migmatite with 28.1 weathered zones with resistivities below 300 Ωm are pre- Ωm. The range of the maximum resistivity is 225.2–810.6 dominant and soil development is at the peak. Localised Ωm. The mean value of the resistivity of the saprolite is zones with < 100 Ωm are more dominant within amphibolite also lowest in areas underlain by amphibolite with 53.1 Ωm, and terrains underlain by gneisses. At half AB separation while average resistivities for areas underlain by other bed- of 24 m, there is reduction in area coverage of the highly rocks are higher with 93, 118 and 204 Ωm in porphyritic weathered zones at the corresponding depth. This resulted granite, gneisses and migmatite, respectively. Based on the in the thinning out of highly weathered lithology on areas most widespread resistivity range of 61–150 Ωm (Fig. 7a underlain by gneisses and amphibolite, while the spatial and Table 2), the saprolite is dominated by composite of coverage of moderately weathered lithology with resistiv- sand and clay across the area. Occasionally, there is occur- ity 100–600  Ωm (Fig.  5b) expanded. At higher half AB rence of compacted lateritic clay layer, otherwise known as separations of 42 m, poorly weathered and bedrock subsur- ‘hardpan’ associated with very high resistivity that ranged face horizons with resistivity > 600 Ωm are conspicuously between 433.7 and 4412.9. represented (Fig. 5c). Lastly, at AB/2 separations of 75 m, The thickness of the saprolite ranges from 3.80 to highly weathered zones have virtually disappeared and the 38.70 m at an average (av.) of 16.56 m, 4.40–57.70 m (av. weathering is poor, and bedrock horizon are now becoming 15.83 m), 3.40–22.40 m (av. 11.31 m) and 1.80–19.40 m (av. dominant (Fig.  5d). However, even at AB/2 = 75  m, large 9.52 m) correspondingly on amphibolite, gneisses, migma- part of the study area is still categorised as being moderately tite and porphyritic granite bedrocks. On the average, areas weathered. This is an indication of a fairly deep weather- underlain by gneisses and amphibolite are characterised ing development with resistivities 100–600 Ωm across the by deeper weathered layer. The total regolith thicknesses study area. were correspondingly 18.49 and 18.23, compared to 12.77 and 11.29 for migmatite and granitic terrains, respectively (Table 2). The most widespread thickness from the isopach map (Fig.  7b) is within the range of 7–17  m. However, 1 3 Applied Water Science (2018) 8:89 Page 9 of 22 89 Fig. 5 Indication of weathering development at corresponding depths to half AB separations of a 10 m, b 24 m, c 42 m, and d 75 m 1 3 89 Page 10 of 22 Applied Water Science (2018) 8:89 Fig. 5 (continued) 1 3 Applied Water Science (2018) 8:89 Page 11 of 22 89 Table 2 Statistics of the geo- Geo-electric Explanation Min Max Mean Median electric parameters by bedrocks parameters Amphibolite (n = 25)  ρ1 (Ωm) Resistivity of the topsoil 38.40 3478.00 658.47 407.60  ρ2 (Ωm) Resistivity of saprolite layer 9.10 225.20 53.10 36.35  ρ3 (Ωm) Resistivity of bedrock 105.70 5472.70 1515.59 927.00  h1 (m) Thickness of top soil 0.40 5.80 1.68 1.45  h2 (m) Thickness of saprolite layer 3.80 38.70 16.56 16.00  H (m) Total regolith thickness 4.40 39.40 18.06 17.20  S (mhos) Total longitudinal conductance 0.02 1.42 0.45 0.40 Gneisses (n = 19)  ρ1 (Ωm) Resistivity of the topsoil 176.90 4478.20 1343.59 984.30  ρ2 (Ωm) Resistivity of saprolite layer 18.80 810.60 117.95 69.00  ρ3 (Ωm) Resistivity of bedrock 196.60 9601.40 1814.92 934.50  h1 (m) Thickness of top soil 0.80 2.30 1.51 1.40  h2 (m) Thickness of saprolite layer 4.40 57.70 15.83 12.50  H (m) Total regolith thickness 6.30 59.00 18.49 16.60  S (mhos) Total longitudinal conductance 0.01 1.06 0.28 0.20 Migmatite (n = 18)  ρ1 (Ωm) Resistivity of the topsoil 237.20 2331.10 1194.21 965.70  ρ2 (Ωm) Resistivity of saprolite layer 28.10 565.50 204.39 61.00  ρ3 (Ωm) Resistivity of bedrock 191.30 6863.30 1488.25 989.15  h1 (m) Thickness of top soil 0.50 2.80 1.56 1.40  h2 (m) Thickness of saprolite layer 3.40 22.40 11.31 11.85  H (m) Total regolith thickness 4.60 23.60 12.77 12.95  S (mhos) Total longitudinal conductance 0.02 0.60 0.23 0.18 Porphyritic granite (n = 20)  ρ1 (Ωm) Resistivity of the topsoil 110.50 3271.90 1151.57 995.40  ρ2 (Ωm) Resistivity of saprolite layer 26.50 294.00 93.03 65.55  ρ3 (Ωm) Resistivity of bedrock 547.20 29,903.80 4601.23 2605.35  h1 (m) Thickness of top soil 0.60 4.50 1.76 1.25  h2 (m) Thickness of saprolite layer 1.80 19.40 9.52 8.30  H (m) Total regolith thickness 2.40 20.80 11.29 10.65  S (mhos) Total longitudinal conductance 0.05 0.38 0.15 0.12 localised deeper weathered zones with thickness exceeding Exactly two-thirds of the bedrock fractures are restricted 17 m are found on gneisses and to a lesser extent on amphi- to the south-east part in areas close to rock contact zones bolite (Fig. 7b). around Sekere and Igboora. These fractures are also aligned The bedrock iso-resistivity contour map is presented in along the same direction of NW–SE trend at which the rock Fig. 8 with resistivity range of 105–29,903 Ωm. Bedrock contacts are aligned (Fig. 9). The depth to fractured zones resistivity of < 600 Ωm are indicative of basement fractures varies and are found at 7.9–29.9 m (av. 18.3 m) below the and this occurred at 24 locations, out of which eleven were earth surface on amphibolite terrains and at depth range of within amphibolite, five in gneissic, seven in migmatite ter - 16.6–27.3 m (av. 22.3 m) on gneisses. Fracture bedrocks rains and only one location at Idere on porphyritic granite. were found at relatively shallower depths of 6.7–19.5 m (av. The most promising fracture zones with < 600 Ωm bed- 13.6 m) on migmatite and 5.8 m on porphyritic granite at rock resistivity are localised in areas underlain by banded Idere. gneiss at the NW and SW regions, and SE zone underlain by migmatite and amphibolite (Fig. 8). Porphyritic granitic terrains are characterised by higher resistivity at an aver- age of 4601 Ωm. Bedrock fracturing is least within granitic terrains. 1 3 89 Page 12 of 22 Applied Water Science (2018) 8:89 Fig. 6 Topsoil thickness map across the study area 0.20 mhos but the vulnerability is even higher at Jagunode Total longitudinal conductance (S) and aquifer and Sekere in the SE region (Fig. 10). Within amphibolite vulnerability terrains, S largely exceeds 0.30 mhos and aquifers are less vulnerable to surface contamination. From Table 2, the total longitudinal conductance (in mhos) of the regolith ranged from 0.01 to 1.42 across the study Well inventory data and groundwater yield area. The conductance is however relatively higher in ter- rains underlain by amphibolite and gneisses with corre- The results of groundwater yield and well inventory data sponding values of 0.02–1.42 at an average of 0.45 (av. 0.45) of the nineteen boreholes across the four principal bed- and 0.01–1.06 (av. 0.28), compared to 0.02–0.60 (av. 0.23) rock terrains are presented in Table 3. Boreholes BH01 to in migmatite and 0.05–0.38 mhos (av. 0.15) for porphyritic BH05 were those on amphibolite terrains, while BH06 to granite. Aquifer is more vulnerable in areas with lower con- BH10 on gneisses, BH11 to BH15 on migmatite and BH16 ductance, most especially within porphyritic granitic ter- to BH19 penetrated porphyritic granite (Fig. 4). Form the rains and other adjoining areas with < 0.20 mhos (Fig. 10). general statistics, borehole elevation was between 139 m and The conductance in areas underlain by gneisses varies 215 m above mean sea level, while borehole depths were extensively, but large area lies within the low to moderate between 15.9 and 43 m. The depth to water table ranges catchment. Locations with low vulnerability occur in three from 1.5 to 14.5  m and groundwater yield was between pockets/localised zones at the mid-north-west and south- 32.8 and 99.8 m /day (Table 3), which was approximately west. However, the latter zone occupies more extensive 32,800–99,800 l/day. area than the two localised portions at the north-west area. However, to show the influence of bedrock on ground- Nonetheless, the vulnerability is high for the regolith overly- water yield and on well inventory data, statistical summary ing Alagba area at the north-west zone. For areas underlain of the parameters by bedrocks are presented in Table 4. The by migmatite, the vulnerability lies between moderate to well depths range about 20–38 m on amphibolite, 31–43 m high. For most terrains underlain by migmatite, S exceeds 1 3 Applied Water Science (2018) 8:89 Page 13 of 22 89 Fig. 7 a Resistivity map of the study area. b Saprolite thickness map of the area 1 3 89 Page 14 of 22 Applied Water Science (2018) 8:89 Fig. 8 Bedrock iso-resistivity map of the study area on gneisses, and 18–36 m on migmatite and 16–37 m on por- collaborated by Q/GWT (Fig. 11b) relationships, whereby phyritic granite. Also, GWT occurred in the range 1.5–10.7, shallow groundwater table will guarantee high groundwater 4.2–14.5, 3.3–11.8 and 2.0–6.5 m, while Q in m /day was yield in amphibolite and gneisses terrains (where R = −0.83 56–93.6, 32.8–78.9, 41.9–99.8 and 56.8–91.1, respectively. and − 0.50, respectively) and to a lower extent within mig- Based on the average values, terrains underlain by gneisses matite terrains where the relationship is weak (R = −0.21). were characterised by deeper wells with 36.1 m compared to However, within granitic terrains, the relationship between Q those measured in other terrains with corresponding mean and GWT is positive and strong (R = 0.52). This showed that values of 29.3, 27.7 and 25.2 m for wells within amphibolite, shallow groundwater table does not guarantee high ground- migmatite and granite. Equally, GWTs are found at shal- water yield within granitic terrains. On the other hand, the lower average depths of 4.0–6.0 m within these bedrocks relationships between depth to water table and well depth are in comparison with those within gneisses with a mean of diverse across the bedrock terrains (Fig. 11c). It is strong and 8.2 m. On the other hand, groundwater yield, Q, was low- negative within granitic terrains (R = −0.92), moderate and est in gneissic bedrock with 53.2 m /day, compared to the negative in gneissic terrains (R = −0.38), strong and positive highest yield of 72.5 m /day obtained within amphibolite within migmatite (R = 0.78) and weak and amphibolite with 3 3 terrains, and 69.0 m /day and 68.3 m /day correspondingly R = 0.29. This indicated that deep wells are characterised by in wells that penetrated granitic and migmatite bedrocks. shallower water table in granitic and to a lesser extent within Results of statistical correlation of well data and ground- gneissic terrains. However, this relationship is direct in both water yield by bedrock affiliations are shown in Fig.  11. migmatite and amphibolite aquifers, though the significance The coefficients of correlation, R, is strong and negative is lower within terrains underlain by the former bedrock. (i.e. R < −0.5) between groundwater yield (Q), well depth For the relationships occurring between groundwater dis- (WD) and groundwater table (GWT) in the bedrocks, except charge, Q, and well topography, the associations are negative within migmatite, where R = −0.04 for Q/WD and − 0.21 for across the bedrocks, with the exception of granitic terrains. Q/GWT and the relationships are insignificant and weak, In respect to significance of relationships, it is fairly strong in respectively (Fig. 11a, b). This implies that deep wells do not amphibolite (R = −0.50), moderate in migmatite (R = −0.39) guarantee high groundwater yield, particularly in aquifers and weak in gneisses with R = −0.21 (Fig. 11d). However, within gneisses, amphibolite and granitic terrains. This is the relationship is very strong and positive in granitic terrain 1 3 Applied Water Science (2018) 8:89 Page 15 of 22 89 Fig. 9 Fractured bedrock locations on geological map of the study area with R = 0.88 (Fig. 11d). This implied that locations with influences well depth within locations underlain by gneisses high topography are characterised by high groundwater yield and migmatite. However, the relief of the area has a direct in granitic terrains, whereas the yield is low in elevated areas association with the depth of the wells in amphibolite and an underlain by other bedrocks. indirect relationship in granitic terrains. Equally, there is no The study has shown that there is no viable significant rela- significant relationship between groundwater table and well tionship existing between well topography and well depths topography also within gneisses and migmatite terrains, while within terrains underlain by both gneisses and migmatite, the associations are perfect and negative in granitic terrains whereas these associations are strong in amphibolite and (R = −0.95) and positive and moderate within amphibolite granitic terrains. Nevertheless, it is indirect in granite with with R = 0.43 (Fig.  11f). The perfect negative relationship R = −0.82 and positive in amphibolite with R = 0.56 (Fig. 11e). between these parameters in granitic terrains indicated that This means that the topography of the area is not a factor that 1 3 89 Page 16 of 22 Applied Water Science (2018) 8:89 Fig. 10 Aquifer vulnerability map of the area Table 3 Results of groundwater discharge and well inventory data Hydrogeological characterisation and groundwater prospects of the weathered–fractured aquifers S/n Borehole no. WT (m) WD (m) GWT (m) Q (m /day) by bedrock terrains 1 BH01 172 27.7 10.7 56.0 2 BH02 187 31.6 6.4 73.2 Amphibolite terrains 3 BH03 170 38.0 5.0 71.7 4 BH04 175 30.4 3.2 67.9 The total thickness of the regolith units of areas that were 5 BH05 159 18.9 1.5 93.6 mainly underlain by amphibolite were between 4.40 and 6 BH06 174 43.0 4.2 40.4 39.40 m (av. 18.23 m). The middle layer (or the saprolite 7 BH07 169 30.9 6.7 75.9 unit) alone has thickness range of 3.80–38.70 m at an aver- 8 BH08 143 34.6 6.4 78.9 age of 16.56 m. The saprolite units have an average resistiv- 9 BH09 171 34.1 14.5 32.8 ity value of 53.10 Ωm, and localised zones of high longitu- 10 BH10 139 38.0 9.4 37.9 dinal conductance (S) occurred within amphibolite terrains 11 BH11 188 26.4 3.3 41.9 (Fig. 10). The high S values obtained within amphibolite ter- 12 BH12 142 29.1 7.1 77.0 rains also suggest that the degree and extent of rock weather- 13 BH13 182 29.7 4.2 99.8 ing is high (Fig. 7). The development of fine-grained regolith 14 BH14 188 17.8 3.5 69.5 is attributable to the high susceptibility of the amphibolite 15 BH15 199 35.6 11.8 53.2 bedrock to weathering due to large composition of dark- 16 BH16 155 36.5 2.1 57.7 coloured ferromagnesian minerals such as hornblende and 17 BH17 215 20.3 5.3 91.1 biotite in the mineral assemblage (Akanbi, 2016). These 18 BH18 209 15.9 6.5 70.4 minerals (i.e. ferromagnesian) are largely unstable at surface 19 BH19 141 28.0 2.0 56.8 environment, since they crystallise at higher melting points. Additionally, the schistose textural attributes of Igboora amphibolite facilitates easy breakdown of rock by physical terrain elevation has a very strong indirect influence on the processes and rock mass decomposition by chemical pro- groundwater table of aquifer zone. cesses. Hence, aside the fact that the weathering develop- ment is well pronounced in areas underlain by amphibolite, 1 3 Applied Water Science (2018) 8:89 Page 17 of 22 89 Table 4 Statistical summary of 3 Bedrocks Statistics WT (m) WD (m) GWT (m) Q (m /day) geo-electric parameters and well inventory data by bedrocks Amphibolite, n = 5 Minimum 159.0 18.9 1.5 56.0 Maximum 187.0 38.0 10.7 93.6 Mean 172.6 29.3 5.4 72.5 Gneisses, n = 5 Minimum 139.0 30.9 4.2 32.8 Maximum 174.0 43.0 14.5 78.9 Mean 159.2 36.1 8.2 53.2 Migmatite, n = 5 Minimum 142.0 17.8 3.3 41.9 Maximum 199.0 35.6 11.8 99.8 Mean 179.8 27.7 6.0 68.3 Por. granite, n = 4 Minimum 141.0 15.9 2.0 56.8 Maximum 215.0 36.5 6.5 91.1 Mean 180.0 25.2 4.0 69.0 this terrain also has the highest number of bedrock fractures groundwater prospect within areas underlain by amphibolite compared to other terrains. Eleven (11) locations out of a is high and largely from the fractured zones. This is also sup- total of 24 locations with < 600 Ωm bedrock resistivities are ported by the higher yields that is > 67 m /day for most wells found within amphibolite terrains. This represented 46% (Table 3) within amphibolite. However, from hydrological of all fractured bedrock locations. Also, the relative per- relationships, well elevation has a moderate and an indirect centage of fractured bedrocks occurrences to those that are effect on groundwater yield, and higher groundwater yields unaltered is larger (Table 5, Figs. 9, 12). Furthermore, aside are more associated with lowland areas (Fig. 11b, d). the fractured bedrocks, another 12% amphibolite bedrocks are partially weathered. The partially altered bedrocks are Gneisses terrains those that are slightly weathered and are also liable to hav- ing moderate groundwater prospect with resistivity range of Just like terrains underlain by amphibolite, areas underlain 600–1800 Ωm. Generally, from the 25 soundings conducted by gneisses are also characterised by occurrence of thick within amphibolite terrains, the bedrocks of about 44% loca- regolith units of 6.30–59.00 m with an average thickness of tions are fractured, 12% are regarded as weak or partially 18.49 m. The thickness of the saprolite was between 4.40 weathered while 44% bedrocks are unaltered (Fig.  12). and 57.70 m and average of 15.83 m (Table 2). Localised Depths to fractured bedrocks are within 7.9–29.9 at an aver- zones of deeper weathering also exist at the NW area and at age of 18.3 m (Table 5). Also, fresh bedrock resistivity was the SW floodplain regions where saprolite thickness exceeds less than 2500 Ωm, except at the southwestern section of 22.0 m (Fig. 7b). Possible factors that may favour the devel- Igboora where bedrock resistivities range between 4500 and opment of thick regolith in areas underlain by gneisses are 5500 Ωm. mineralogical content and foliation features of the gneisses. The hydrogeological setting of terrains underlain by The resistivity of the saprolite ranged extensively from amphibolite, as described above, hereby supports generation 18.80 to 810.60 Ωm. Based on the average regolith resistiv- of artesian aquifer. This is supported by very strong indirect ity of 117.95 Ωm, the regolith units are coarser compared relationship occurring between yield and groundwater table to amphibolite terrains. The total longitudinal conductance (Fig. 11b). This means that the water table occurs at shallow ranges from 0.01 to 1.06 with an average of 0.28 mhos depth in more prolific wells at Igboora, which emphasised (Table  2). This is markedly lower than those obtained in that the groundwater is under pressure and rises above the amphibolite (0.45 mhos), which means that the aquifer vul- fractured bedrock aquifers. This is as a result of widespread nerability in gneisses is relatively higher compared to those occurrence of thick and largely fine-grained regolith over - in amphibolite terrain. Accordingly, the potential for water lying the main groundwater-bearing zones of fractured and infiltration through the regolith, inferred from the total lon- weak amphibolitic bedrocks. Additionally, groundwater gitudinal conductance, is better in gneiss terrains than in quality is expected to be fairly good as a result of the over- amphibolite. Notwithstanding, bedrock resistivities of only lying fine-grained regolith that prevents direct groundwater five out of nineteen (19) locations were fractured (Table  5, recharge of the bedrock aquifers (Fig. 10). Furthermore, the Fig. 12). However, the percentage of occurrences of partially occurrences of fractured bedrocks at relatively deeper zones weathered basement was 48% and quite enormous (Fig. 12), at an average of 18.3 m will further enhance the protection and with this, the degree of rock decomposition can also of the enclosed groundwater resource in the area. Hence, be said to be equally high. Bedrock fractures occurred at 1 3 89 Page 18 of 22 Applied Water Science (2018) 8:89 Yield Vs. Groundwater table (a) (b) Yield Vs. Well Depth 50 50 40 40 10.0 20.0 30.0 40.0 0.05.0 10.0 15.0 Well Depth (m) Groundwater table (m) Amphibolite, R = - 0.54 Gneisses, R = - 0.57 Amphibolite, R = - 0.83 Gneisses, R = - 0.50 Migmatite, R = - 0.04 P. Granite, R = - 0.65 Migmatite, R = - 0.21 P. Granite, R = 0.52 (c) (d) Yield Vs. Well Elevation Well Depth Vs. Groundwater table 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 0.0 5.0 10.0 15.0 120 140 160 180 200 220 Groundwater table (m) Well Elevation (m) Amphibolite, R = - 0.50 Gneisses, R = - 0.21 Amphibolite, R = 0.29 Gneisses, R = - 0.38 Migmatite, R = - 0.39 P. Granite, R = 0.88 Migmatite, R = 0.78 P. Granite, R = - 0.92 (e) (f) Water Table Vs. Well Elevation Well Depth Vs. Well Elevation 20.0 45.0 18.0 40.0 16.0 14.0 35.0 12.0 30.0 10.0 25.0 8.0 6.0 20.0 4.0 15.0 2.0 10.0 0.0 120 140 160 180 200 220 120 140 160 180 200 220 Well Elevation (m) Well Elevation (m) Amphibolite, R = 0.56 Gneisses, R = 0.04 Amphibolite, R = 0.43 Gneisses, R = 0.01 Migmatite, R = 0.02 P. Granite, R = - 0.82 Migmatite, R = 0.08 P. Granite, R = - 0.95 Fig. 11 a Plot of groundwater yield against well depth. b Plot of e Plot of well depth against well depth. f Plot of groundwater table groundwater yield against water table. c Plot of well depth against against well elevation groundwater table. d Plot of groundwater yield against well elevation. 1 3 Well depth (m) Well depth (m) Yield (m /day) Water Table (m) 3 3 Yield (m /day) Yield (m /day) Applied Water Science (2018) 8:89 Page 19 of 22 89 Table 5 Frequency and depths S/n Rock units Total no of Frequency and relative percentage of occur- Depth of occur- of occurrence of fractured VES points rences of fractured bedrock in each bedrock rence (m) bedrocks n (%) Min Max Mean 1 Amphibolite 25 11 (44%) 7.9 29.9 18.3 2 Gneisses 19 5 (26%) 16.6 27.3 22.5 3 Migmatite 18 7 (39%) 6.7 19.5 13.7 4 Porphyritic granite 20 1 (5%) – – – Migmatite terrains The thickness of regolith development in areas underlain 60 by migmatite is shallower when compared with terrains underlain by amphibolite and gneisses. The thickness of the 44 44 regolith was 4.60–23.6 m and average 12.77 m, while that of the saprolite was 3.40 –22.40 at an average of 11.31 m. 26 26 The resistivity of the regolith was 28.10–565.50 Ωm with 15 mean value of 204.39 Ωm. Based on the mean resistivity of 204.39 Ωm, the regolith units are sandy and coarser. Hence, migmatite terrains are characterised by lower longitudinal AMPHIBOLITE GNEISSES MIGMATITE POR.GRANITE conductance of 0.02–0.60 (av. 0.23  mhos), compared to those within amphibolite and gneisses. Additionally, seven Bedrocks bedrocks out of eighteen were fractured (Table 5, Fig. 12). Fractured Bedrock Weak Bedrock Fresh Bedrock However, most of the fractured bedrock were found localised within the SE region and aligned in NW direction, while Fig. 12 Comparative percentage frequencies of bedrock conditions other fractured zones are widely spaced from one another. Notwithstanding, 33% of bedrock were classified as weak, which is higher than those found in porphyritic granite and the depth of 16.6–27.3 m (av. 22.5 m). This is compara- even in amphibolite terrains (Fig. 12). However, the risk of tively deeper than other bedrocks (Table 5). Nonetheless, groundwater contamination is higher in migmatite terrains areas underlain by gneisses are expected to have better water than in gneisses and much higher in comparison with amphi- transmission ability owing to the thick and the fairly coarser bolite terrains. From these results, groundwater occurrences regolith. Additionally, the foliation structures in gneisses in terrains underlain by migmatite exist in unconfined state, will also serve as subsurface water conduits apart from aid- and the water table is exposed to atmospheric conditions. ing rock weathering. Additionally, as a result of occurrences of bedrock fractures The hydrogeological setting in gneissic terrains will gen- at shallower depths of 6.7–19.5 m (av. 13.7 m) (Table 5) and erate a semi-confined aquifer system due to occurrence of the overlying coarse regolith, aquifer vulnerability to con- coarser regolith that is overlying the largely partially altered tamination is high to moderate in location will deeper and bedrocks. The thick weathered units will serve as both water- less coarse regolith. Also, well depth has little or no signifi- bearing zone and a semi-confining unit to the largely par - cant relationship with the groundwater yield, whereas there tially weathered bedrocks that are expected to have good is a moderate and indirect relationship existing between the water-bearing capacity. Water quality may be poor in these well discharge and the topography of the area (Fig. 11a, d). terrains when tapped from the regolith, whereas water from wells that tap into the relatively deeper fractured bedrocks at Porphyritic granite terrains an average depth of 22.5 m will be safer. Well topography in gneisses has little effect on the groundwater yield, whereas Terrains underlain by porphyritic granite are the least well depth has moderate indirect significance on yield, and weathered and the least fractured (Figs.  7, 8, 9, 12, and deep wells do not support high groundwater yield (Fig. 11a). Table 5) across the study area. The overburden thickness was 2.4–20.8 m (11.3 m) compared to the averages of over 18 m in both amphibolite and gneisses and about 13 m in migmatite terrains. Also, the average thickness of the sap- rolite was just 9.5 m, whereas it was 16.6, 15.8 and 11.3 m 1 3 Bedrock condition (%) 89 Page 20 of 22 Applied Water Science (2018) 8:89 correspondingly in other bedrocks (Table 2). Weathering fractures (Fig. 9). Aquifers within this terrain are charac- 3 3 intensity was not only comparatively shallower in granitic terised by higher yields of 56.0–93.6 m /day (av. 72.5 m / terrains, but the weathered layer also terminates mainly on day). This is despite poor groundwater prospect of the thick unaltered or fresh bedrocks. The porphyritic bedrock resis- and largely fine-grained overlying weathered-regolith. This tivity was 547–29,903 Ωm at an average of 4601 Ωm, while showed that fractured bedrocks are able to provide better in other terrains, the averages were 1516, 1815, 1488 Ωm, groundwater yield more than the overlying weathered-reg- respectively. Additionally, bedrock of just one location is olith that is underlain by impermeable crystalline bedrocks. fractured, and this is at Idere, and the fractured zone occurs For areas underlain by gneisses, though, weathering was at shallow depth of 5.8 m (Table 5, Fig. 9). The bedrocks of more intensive, and the recorded yield of 32.8–78.9 m / just three locations can be said to be slightly weathered, and day (av. 53.2 m /day) was the lowest in all bedrock setting the rest (that is 80%) are fresh basement (Fig. 12). Therefore, in the area. This is as a result of fewer developments of fresh granitic bedrock occurs at shallow depth. However, bedrock fractures in comparison with those within amphi- areas with fairly thick regolith > 10 m are found in areas bolite and migmatite regions. Apart from this, the few close to rock contact zones at Idere and other few locations. fractured zones were isolated and not closely packed as in Notwithstanding, the average S for regolith within the gra- the other two terrains (Fig. 9). Also, the vulnerability of nitic bedrock terrains was 0.15 mhos. This is much lower in the weathered layer, which is the alternative water-bearing comparison with 0.45, 0.28 and 0.23 mhos as the average zone is low to moderate (Fig.  10); hence, groundwater total longitudinal conductance correspondingly for the rego- occurrence within gneissic terrains is modest. However, lith layer within other bedrock terrains. groundwater yield of 75.9 m /day was obtained in well Generally, due to the shallow extent of rock weather- BH07 in a location close to Alagba where prominent frac- ing, aquifers are unconfined and occur at shallower depths tures are found. Within migmatite terrain, groundwater compared to those within other bedrock terrains. Also, the yield is higher with 41.9–99.8 (av. 68.3) m /day, compared 3 3 average yield of 69 m /day, which is good, may not be sus- to those in gneisses (i.e. av. 53.2 m /day). The more pro- tainable for longer period. This is as a result of the fact that lific yield in migmatite is attributable to localised higher the underlying granitic bedrocks are typically impermeable occurrences of bedrock fractures. This is supported by the and are found at shallow depths. Thereby, the only water- groundwater yield > 70 m /day that was obtained in wells bearing zone is the overlying regolith in granitic terrain, BH12 and BH13 located within rock contact zones char- and as a consequence, the groundwater system is exposed acterised by numerous fractures that are closely spaced to seasonal fluctuations as well as pollution from surface (Figs. 4, 9). This also showed that rock contacts promote environment. Therefore, the aquifer vulnerability is high bedrock fractures, and wells sited within these subsurface (Fig. 10). Additionally, deep wells would not guarantee high discontinuities are most likely to be more productive than yield in granitic terrains as revealed by the strong indirect those within any typical bedrock terrains. relationship that existed between groundwater yields and The granitic aquifers have the second highest yield of well depths (Fig. 11a). However, area topography has a very 56.8–91.1 (av. 69.0) m /day after those of amphibolite. The strong positive influence on the groundwater yield and high- high yields were from the two wells at Idere, i.e. BH17 and lands areas are characterised by higher yield than lowland BH18 with yields of 91.1 and 70.4 m /day, respectively. areas unlike other bedrocks terrains (Fig. 11d). The other two wells further north at Ayete and Tapa were The summary of hydrogeological attributes under each characterised by lower yield that is < 60  m /day. High bedrock terrain is highlighted in Table 6. groundwater yield at Idere is attributable to topographic Groundwater prospect is good in areas underlain by effect (Fig.  11d) and nearness to fracture and rock con- amphibolite mainly due to high frequency of bedrock tact zones (Fig.  9). The elevations of wells at Idere are Table 6 Summary of hydrogeological characterisations of bedrock terrains at Ibarapa region S/n Bedrock Extent and Predominant Bedrock conditions Aquifer vulnerability Groundwater yield Ground- degree of saprolite lithol- water weathering ogy prospect 1 Amphibolite High Clayey Fractured to unaltered Low High Good 2 Gneisses High Sandy and clay Slightly altered Low to moderate Low Fair 3 Migmatite Fair Sandy Fractured to partly High to moderate Fairly high Moderate altered 4 Porphyritic granite Low Sands and clay Mainly unaltered High High at elevated ter- Fair rains 1 3 Applied Water Science (2018) 8:89 Page 21 of 22 89 both > 200 m, whereas the wells’ elevations at Tapa and demands for a systematic siting of wells for future ground- Ayete are < 160 m (Table 3). Conversely, the possibilities water development in the area. of siting bedrock aquifers are rare on granitic terrain due Lastly, variance in the intensity of weathering, discrep- to paucity of fractures. Nonetheless, from the good yield ancies in bedrock fractures, irregularity in area topography of the mainly regolith aquifers at elevated granitic terrains, and well depth have profound effects on the prospect for the groundwater prospect is fair. However, the yield will groundwater in the area. Methodical siting of wells in line be more sustainable, if there is a source of regular recharge with the present findings will ensure better and more sus- of the groundwater system. Therefore, the sustainability tainable groundwater yield across the study area. This will of the weathered-regolith aquifers in granitic areas to a also limit the impact of hydrogeological complexity brought large extent depends on the climatic conditions. Also, the about by geological and other natural factors that are beyond likelihood of contamination of groundwater from surface human regulations. effluent is high in granitic terrains, since the aquifer sys- Open Access This article is distributed under the terms of the Crea- tem is mainly the saprolite. In addition to this, because tive Commons Attribution 4.0 International License (http://creat iveco weathering is less developed, impermeable granitic bed- mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- rock is found at shallow depth and aquifer vulnerability tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the is quite high. Creative Commons license, and indicate if changes were made. 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Applied Water ScienceSpringer Journals

Published: May 28, 2018

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