Impacts of hydrogeochemical processes and anthropogenic activities on groundwater quality in the Upper Precambrian sedimentary aquifer of northwestern Burkina Faso

Impacts of hydrogeochemical processes and anthropogenic activities on groundwater quality in the... This study investigates the hydrogeochemical and anthropogenic factors that control groundwater quality in an Upper Pre- cambrian sedimentary aquifer in the northwestern Burkina Faso. The raw data and statistical and geochemical modeling results were used to identify the sources of major ions in dug well, private borewell and tap water samples. Tap waters were classified as Ca–HCO and Ca–Mg–HCO types, reflecting the weathering of the local dolomitic limestones and silicate 3 3 minerals. Dug well waters, with a direct contact with various sources of contamination, were classified as Ca–Na–K–HCO type. Two factors that explain 94% of the total variance suggested that water–rock interaction was the most important factor 2+ 2+ − 2− controlling the groundwater chemistry. Factor 1 had high loadings on pH, Ca, Mg, HCO, SO and TDS. These vari- 3 4 − − 2− ables were also strongly correlated indicating their common geogenic sources. Based on the HCO /(HCO + SO ) ratios 3 3 4 2+ 2+ − 2− (0.8–0.99), carbonic acid weathering appeared to control Ca, Mg, HCO and SO acquisition in the groundwater. 3 4 2+ 2+ With relatively lower Ca and Mg concentrations, the majority of dug well and borewell waters were soft to moderately hard, whereas tap waters were considered very hard. Thus, the dug well and, to a lesser extent, borewell waters are likely to + − − + have a low buffering capacity. Factor 2 had high loadings on N a, NO and Cl . The strong correlation between Na and − − + NO and Cl implied that factor 2 represented the anthropogenic contribution to the groundwater chemistry. In contrast, K had moderate loadings on factors 1 and 2, consistent with its geogenic and anthropogenic sources. The study demonstrated that waters from dug wells and borewells were bacteriologically unsafe for human consumption, and their low buffering capacity may favor mobility of potentially toxic heavy metals in the aquifer. Not only very hard tap waters have aesthetic inconvenient, but their consumption may also pose health problems. Keywords Sedimentary aquifer · Tap water · Dug wells · Borewells · Water–rock interaction Introduction seasonal surface water flow, and thus, surface water becomes an unreliable source for water supply. As a result, people Following severe droughts in 1970s, a massive internal have been heavily relying on groundwater for domestic water migration from drier central plateau and northern regions supply and livestock watering (Derouane and Dakoure 2006; toward a more humid northwestern Burkina Faso has Courtois et al. 2010; Huneau et al. 2011). Traditional hand- put a tremendous pressure on the regional surface water dug wells are the main sources of groundwater in the region. resources (Kessler and Greerling 1994). The northwestern In order to meet the ever-increasing demands for water, hun- Burkina Faso has been also subject to adverse effects of cli - dreds of borewells, equipped with hand pumps, were drilled mate changes such as erratic precipitations and decrease in in the Kossi Province one of the four provinces in the north- western Burkina Faso (Barry et al. 2005) and the site of the present study. The borewells draw groundwater from deep * A. Sako fractured sedimentary rocks, whereas the dug wells abstract aboubakar.sako@gmail.com shallow groundwater within weathered mantle layers (Col- lectif 1990). Université de Dédougou, BP. 139, Dédougou, Burkina Faso 2 Although groundwater constitutes an important asset for Département des Sciences de la Terre, Université Ouaga 1 Pr socioeconomic development of the northwestern Burkina Joseph Ki-Zerbo, 09 BP 848, Ouagadougou 09, Burkina Faso Vol.:(0123456789) 1 3 88 Page 2 of 14 Applied Water Science (2018) 8:88 1 3 Applied Water Science (2018) 8:88 Page 3 of 14 88 ◂Fig. 1 a Geographical map of Burkina Faso; b geomorphological formations shared by Mali and Burkina Faso. These forma- map of the Kossi floodplain, showing the study area; c groundwater tions are essentially made of an alternation of pink siltstones sampling points superimposed on the simplified local lithological and argillites with glauconite and dolomitic limestone lenses units. The lithology of tap water from the public water supply points capped with silexite (Ouédraogo 1998). As in the crystalline may not correspond to their sampling lithology basement areas that make up 80% of Burkina Faso, two types of discontinuous aquifers are encountered in the study area. Faso, the hydrogeochemical studies pertaining groundwater A shallow (5–20 m) aquifer located in the weathered lateritic quality in this large transboundary aquifer are scanty. The layer, which is superimposed on a deep aquifer within the local groundwater quality is likely to be controlled by both joined sandstone layers in the sedimentary sequence (CIEH natural and anthropogenic factors. Water–rock interaction 1976; BILAN D’EAU 1993). The thickness of the deep (i.e., chemical weathering and cation exchange processes) aquifer is poorly known, and it varies according to the lithol- can be the most important natural factor that controls the ogy. In contrast to crystalline basement aquifers, the high groundwater quality (Fetter 1994; Appelo and Postma 2005; per meability (1.8 × 10  m/s) of sedimentary rocks makes Li et al. 2016). In contrast, excessive use of fertilizer, non- the southeast Taoudeni sedimentary formations excellent −4 protection of wells and poor sanitary conditions are poten- aquifers, with an estimated storage coefficient of 1 × 10 tial sources of anthropogenic pollution (Groen et al. 1988; and significant yields up to 100 m /h (Gombert 1998). That Li et al. 2017; Yameogo and Savadogo 2002; Huneau et al. is, the only two permanent watercourses in the country (i.e., 2011; Wu et al. 2017). The monitoring of the physicochemi- the Mouhoun and Comoé rivers) are directly fed by springs cal and biological conditions of groundwater is necessary for originated from sedimentary aquifers (Talbaoui 2009). an efficient water resource management and development The local groundwater recharge occurs through direct of aquifer protection strategies. Therefore, the objectives of infiltration of rainwater and indirect infiltration of runoff via the present study were (1) to identify the hydrogeochemi- depressions, streams and alluvial valleys (Groen et al. 1988; cal processes and anthropogenic activities that govern the Barry et al. 2005). The regional water table shows a seasonal chemical composition of dug wells, private borewells and variation of 1–2 m. The estimated total volume of groundwa- tap water provided by the public water supply system of an ter in the Nouna commune is 0.4 million m /year, whereas Upper Precambrian sedimentary aquifer, and (2) to evaluate the renewable resource is about 0.5 million m /year (MEE the suitability of the groundwater for human consumption. 2001). Consequently, groundwater resource development in The findings of this study will contribute to bridging the the commune is very limited compared to the resource avail- gap between anthropogenic factors and hydrogeochemical ability. More than half of the resources are used for domestic processes that control groundwater quality in a sedimentary water supply and the remaining for livestock watering (MEE and semi-urban setting. 2001). Poor sanitation, lack of an effective management of domestic wastes, inadequate protection of dug wells from surface runoff and animal droppings make the groundwater Site description highly vulnerable to anthropogenic pollution. The study area is located in the town of Nouna, the Kossi Province (Northwestern), 306  km of Ouagadougou the Materials and methods capital city of Burkina Faso (Fig. 1a). The area is part of a floodplain of the ephemeral Kossi River basin (Fig.  1b). This Twenty groundwater samples were collected from six major plain contains several ponds of variable sizes, separated by wards of Nouna in dry season 2017 (Fig.  1c). Five sam- elevated zones (200–300 m a.s.l). The local climate is of the ples were collected from representative private borewells north-Sudanian type, characterized by a dry season (Octo- (B1–B5), five from shallow hand-dug wells with large ber–May) and a wet season (June–September). With an aver- diameters (W1–W2), whereas 10 samples were collected age annual rainfall of 887 mm, the Nouna commune falls in from the public water supply system (P1–P10; Table 1). In the so-called the Bread Basket of Burkina Faso, where sub- order to obtain high water flow rates, the groundwater sup- sistence and cotton farming and livestock bring a substantial plied by the public water supply system is abstracted from income to the populations. As in the whole country, the plain relatively deeper aquifers. The hand-dug well samples were has undergone a marked decrease in rainfall since the 1970s drawn using a sterilized bucket and filtered through Milli- (~ 200 mm), putting a great pressure on water resources. pore membrane (0.45 µm) into two sets of new high-density Currently, rainfall is characterized by a great intra- and inter- polyethylene bottles (HDP), whereas those of borewells and annual irregularity (Frappart et al. 2009). tap water were directly pumped through filter capsules into The area is underlain by Upper Precambrian sedimen- two sets of HDP. One set of the samples was acidified with tary rocks known as the southeast Taoudeni sedimentary ultrapure HNO (pH > 3), whereas the other set was left 1 3 88 Page 4 of 14 Applied Water Science (2018) 8:88 1 3 Table 1 Physicochemical and bacteriological parameters of dug well (W1–W5), borewell (B3–B5) and tap water (P1–P10) samples Sample Physicochemical parameters Bacteriological counts 2+ 2+ + + − 2− − − 1 2 3 pH Temp EC TDS TH Ca Mg Na K HCO SO NO Cl FC TC FS 3 4 3 °C (µs/C) (mg/L)(mg CaCO /L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 1UFC/100 mL 1UFC/100 mL 1UFC/100 mL W1 6.1 31.6 75 51 51 16 2.82 11 4.8 49 2.9 27 4 18 144 > 1000 W2 6.1 32.3 134 84 43 10 4.38 23 6.5 40 3.5 25 7 7 > 1000 > 1000 W3 6.2 30.1 145 67 38 10 2.97 17 3.6 34 4.6 31 14 28 > 1000 64 W4 6.1 32.9 86 48 33 10 1.85 13 5.1 33 1.9 29 8 34 > 1000 > 1000 W5 6.5 33.0 524 201 96 23 9.34 66 15 33 2.1 77 55 57 > 1000 > 1000 B3 6.7 34.3 121 72 64 9.8 9.68 4 7.6 104 4.8 9.2 1 4 43 5 B4 7.4 33.5 501 168 204 38 26.8 19 14 287 14 11 14 0 98 24 B5 5.2 32.1 34 7 32 6.6 3.79 2.4 3.3 76 3.1 12 1 0 45 12 P1 7.4 33.2 514 190 247 45 32.8 18 11 288 22 6 3 0 5 0 P2 7.9 32.9 412 151 211 35 29.9 2 9.1 268 2.1 5.3 5 0 0 0 P3 7.7 33.3 426 168 244 46 31.7 3.9 8.8 273 4.8 5.8 5 0 2 0 P4 7.8 33.2 502 169 263 44 37.3 3.5 9.3 260 20 5.7 2 0 0 0 P5 7.5 32.8 506 178 258 45 35.5 3 9 296 23 5.2 2 0 0 0 P6 7.8 32.9 430 166 242 44 31.9 3.7 8.4 281 22 6.2 5 0 7 0 P7 7.7 32.3 422 176 260 47 35 4.5 9.3 295 22 6 4 0 11 0 P8 7.8 33.3 453 169 261 45 36.3 3.9 8.6 271 23 4.7 6 0 0 0 P9 7.8 33.2 503 176 259 44 36.1 3.9 9.1 299 22 6.5 2 0 0 0 P10 8.0 34.1 516 179 260 41 38.3 5.8 8.2 299 22 4 5 23 0 0 WHO 6.5 ≤ pH ≤ 8.5 25.0 400 1000 200 100 50 150 12 100 250 50 200 0UFC/100 mL 0UFC/100 mL 0UFC/100 mL 1. Fecal coliforms 2. Total coliforms 3. Fecal streptococci 4. World Health Organization Applied Water Science (2018) 8:88 Page 5 of 14 88 non-acidified. A third set of samples was collected and kept unfiltered and non-acidified in glass bottles for bacteriologi- cal counts. Electrical conductivity (EC), pH and total dis- solved solids (TDS) were measured in the field using cali- brated meters with standard solutions. The samples were put in ice box and taken to laboratory for major cation and anion analysis. 2+ 2+ In the laboratory, concentrations of Ca and Mg were estimated titrimetrically using 0.05 N EDTA and 0.01 N, − − whereas those of HCO and Cl by H SO and AgNO 3 2 4 3 titration, respectively. Sodium and K concentrations were determined by flame photometric method (APHA 1995), and 2− − those of SO and NO by UV–Vis spectrophotometric 4 3 technique. Total hardness (TH) was determined by EDTA complexometric titration method (WHO 1999). Analytical reagent grades and milli-Q water were used for the analy- ses. Two borewell samples (B1 and B2) had large charge balance errors (> ± 10%) and were not included in the data interpretation. Nutrient MacConkey agar was used for total coliform bacterial count and Eosin for total fecal coliform. The petri dishes containing agar and diluted groundwater samples were incubated under appropriate conditions (time and temperature). The bacteriological counts per 100 mL were estimated from the MPN table (APHA9221D). − + Fig. 2 a Relationship of total anions (TZ ) to total cations (TZ ) of R-mode factor analysis (Wu et  al. 2014) was used to the groundwater samples; b relationship of electrical conductivity (EC) to total dissolved solids (TDS) assess the relationships between the physicochemical param- eters of the groundwater, using SPSS package (version 20), whereas Visual MINTEQ (version 3.1) was used to calculate drawn from the weathered mantle aquifer appeared to be saturation indices (SI) of carbonate and evaporite minerals as well as partial CO pressure of the groundwater. less mineralized compared to those from the deep fractured + − aquifer. As result, ZT and TZ were higher in tap waters (medians = 172 and 336 µeq/L; Table 2) from the deep aqui- fer than in dug wells from the weathered mantle aquifer. The Results and discussion differences in recharge flow paths could also be an explana- tion for the observed mineralization trends. Thus, weakly Groundwater constituents mineralized groundwaters are often associated with rapid recharge (i.e., younger residence time) of the shallow aqui- The physicochemical data of groundwater highlighted dis- tinct differences between shallow dug well, borewell and fers, whereas highly mineralized groundwaters (i.e., older residence time) have been attributed to paleo-recharge or tap waters. A strong relationship (R = 0.96) between total + − cations TZ and total anions TZ (Fig. 2a) implied that con- slow circulation processes in deep aquifers (Fritz 1997; Sto- ber and Bucher 1999; Cook et al. 2005; Bucher and Stober tribution of non-measured ions to charge balance was not significant. Furthermore, the relationship between EC and 2010; Armandine Les Lands et al. 2014). The high coefficients of variance (CV > 50%; Table 2) and TDS (R = 0.96; Fig. 2b) suggested that the groundwaters were unlikely to contain substantial amounts of uncharged spatial distribution, illustrated by boxplots (Fig. 3), showed a heterogeneous abundance of most physicochemical parame- soluble compounds (e.g., silica, manganese, aluminum and iron) that may contribute to TDS contents (Datta and Tyagi ters in dug wells. This is probably due to the sources and the nature of the recharge, the host rock geology, and the short 1996; Prasanna et al. 2011). In overall, EC and TDS were low in the groundwaters residence time of the groundwater in the weathered mantle aquifer (Back and Hanshaw 1971). On contrary, groundwa- (Table 1). This suggests the absence of salt in the recharge water and limited groundwater mineralization (Han and ter composition of tap waters was remarkably homogene- ous (CV < 50%; except N a ) and most variables had similar Liu 2004; Smedley et al. 2007; Huneau et al. 2011; Jean- nin et al. 2016). Because of intense leaching, groundwaters values for mean and median, reflecting primarily the long 1 3 88 Page 6 of 14 Applied Water Science (2018) 8:88 o fl w lines and dispersive mixing that may have smoothed out any temporal fluctuations in the groundwater composition (Mazor et al. 1993; Dhar et al. 2008). Although the major- ity of the samples had pH values within the World Health Organization (WHO 2006) guideline limit for drinking water (pH = 6.5–8.5), the dug well and borewell waters had lower pH (medians = 6.2 ± 0.2 and 6.4 ± 0.9) relative to those of tap waters (median = 7.8 ± 0.2). The high pH in tap waters relative to dug well waters is consistent with positive cor- relations between pH and the resident time usually observed in deeper aquifers (Morgenstern and Daughney 2012). Total hardness (TH) in the well waters had distribution patterns similar to those of pH, TDS and EC with TH ranging from 33 to 236 mg CaCO /L. The dug well waters exhibited the lowest TH (median = 47 ± 24.6 mg CaCO /L and CV = 44%), whereas the highest concentrations were observed in tap waters (median = 258 ± 18 mg CaCO /L and CV = 18%). Again, the high TH in tap waters can be attributed to long residence time of groundwater in the deep fractured aquifer, leading to extended chemical weathering of dolo- mitic limestones (Frape et al. 1984). With hardness values largely exceeding the WHO guideline value for drinking water, the tap waters were categorized as very hard, while those of dug wells as soft to moderately hard. Soft waters, with low alkalinity and buffering capacity, may favor the mobility of potentially toxic heavy metals in the aquifer (De Schamphelaere and Janssen 2004; Kirby and Cravotta 2005). In contrast, hard waters require more soap to produce lather, and thus, it is unsuitable for domestic use (Srinivasa Rao and Jugran 2003). Some evidence has also indicated the role played by hard waters in heart diseases and prenatal mortal- ity (Schroeder 1960; Agarwal and Jagetai 1997). Although such cases have not been reported in the present study area, the desirability of softer drinking water is evident among the local population. As a result, the water provided by the public water supply system should be treated before it gets to the consumers. Sodium was the dominant cation in dug well waters 2+ + 2+ followed by C a, K and M g , whereas cation abun- dance in borewell and tap waters was in decreasing order 2+ 2+ + + of Ca > Mg > K >Na (Table  1). The low EC, TDS, HCO and TH contents observed in dug well and borewell waters suggest short contact times between groundwater and the aquifer minerals. This is consistent with the low K (except W5 and B4) concentrations in dug well and bore- well waters relative to tap waters (8–11 mg/L). Potassium concentrations in groundwater up to 10 mg/L are attributed to orthoclase or clay weathering, whereas concentrations above 10 mg/L may indicate external sources of K abun- 2− − dance (Rail 2000). Bicarbonate, SO and N O were the 4 3 dominant anions in the wells with the highest HC O and 2− SO concentrations observed in tap waters. Although these ion concentrations in the groundwater were within the WHO 1 3 Table 2 Means medians, standard deviations (SD) and coefficients of variance (CV) of physicochemical parameters of the groundwater samples Parameter Dug wells Borewells Public water supply points Min Max Mean Median SD % CV Min Max Mean Median SD % CV Min Max Mean Median SD % CV pH 6.1 6.5 6.2 6.2 0.2 3 5.2 7.4 6.4 6.5 0.9 14 7.4 8 7.7 7.8 0.2 2 EC (µS/cm) 75 524 223 140 181.7 81 34 501 238 180 199 82 412 516 468 468 41 9 TDS (mg/L) 48 201 100 76 61.3 61 7 168 84 78 66 83 151 190 172 172 11 7 TH (mg/L) 33 96 56 47 24.6 44 32 204 107 86 73 79 211 263 248 258 18 7 2+ Ca (mg/L) 10 22.96 15 13 5.3 36 6.64 38 20 15 14 68 35.2 47 43 44 4 8 2+ Mg (mg/L) 2 9.34 5 4 2.9 62 3.79 27 14 12 10 88 29.9 38 34 35 3 8 Na (mg/L) 11 66.49 30 20 22.3 75 2.37 19 9 7 7 86 2 18 6 4 5 86 K (mg/L) 4 15.05 8 6 4.5 59 3.27 14 9 8 4 58 8 11 9 9 1 9 HCO (mg/L) 33 48.8 39 36 6.4 17 76.25 287 166 135 91 71 260 299 283 283 14 5 2− SO (mg/L) 1.9 4.6 3 3 1.0 34 3.1 14 8 6 5 112 2.1 23 17 22 8 47 NO (mg/L) 25 77 42 30 21.4 51 9.2 12 11 11 1 53 4 6.5 5 6 1 15 Cl (mg/L) 3.5 55.4 21 11 20.9 100 0.8 14 6 4 6 114 2 6.1 4 4 2 41 TZ (µeq/L) 42.1 146.1 76 56 42.1 55 26.5 162 85 68 58 73 141 184 169 172 13 8 TZ (µeq/L) 73.5 169.5 105 87 38.7 37 95.45 340 199 161 107 48 283 352 327 336 26 8 Applied Water Science (2018) 8:88 Page 7 of 14 88 Fig. 3 a Boxplots of naturally affected physicochemical parameters in the Upper Pre- cambrian sedimentary aquifer of the northwestern Burkina Faso. The tops and bottoms of the boxes represent the 75th and 25th percentiles, respectively. The horizontal line across the boxes indicates the median. The vertical lines from the tops and bottoms of the boxes extend to 90th and 10th percentiles, respectively. b Boxplots of anthropogenically affected phys- icochemical parameters in the Upper Precambrian sedimentary aquifer of the northwestern Burkina Faso 1 3 88 Page 8 of 14 Applied Water Science (2018) 8:88 Fig. 4 R-mode factor scores for factors 1 and 2 of the groundwa- ter samples. Potassium is projected half way between geogenic and anthropogenic factors permissible limits for drinking water, NO concentrations in dug well waters exceeded the natural nitrate concentra- tions (5–7 mg/L; Appelo and Postma 1999). None of dug well and borewell samples complied with the WHO guide- line values for coliforms (Table 1), and hence, water from these wells requires treatment before human consumption. Processes controlling groundwater chemistry Fig. 5 Mixing diagrams of Ca/Na versus HCO /Na and Ca/Na versus Mg/Na for silicate and carbonate minerals in the Upper Precambrian Chemical weathering, cation exchange, evaporation and sedimentary aquifer of the northwestern Burkina Faso antropogenical activities are the common hydrogeochemi- cal processes that control groundwater chemistry. In order to shed light on these complex processes, statistical and carbonate end members. According to these authors, the geochemical techniques were used. Thus, the R-mode factor carbonate end member is characterized by Ca/Na, Mg/Na analysis, after varimax rotation (Kaiser 1960), produced two and HCO /Na ratios of 45 ± 25, 15 ± 10 and 90 ± 40 mg/L, factors (with eigenvalues > 1) that explain 94% of the total respectively, whereas the chemistry of water draining variance (Fig. 4). With 63.4% of the total variance, factor 1 silicate is characterized by Ca/Na = 0.3 ± 0.15 mg/L, Mg/ is the most important factor that influences the groundwater Na = 0.24 ± 0.12  mg/L and HCO /Na = 2 ± 1  mg/L. In the 2+ chemistry. This factor had high absolute loadings on Ca , present study, the bivariate plots identified silicate weather - 2+ − 2−, Mg , TH, HCO , pH, SO EC and TDS and a moder- ing and carbonate dissolution as the two hydrogeochemical 3 4 ate loading on K . As expected, there were strong positive processes controlling the groundwater chemistry. Dug well 2+ 2+ correlations between Mg and Ca (r = 0.96) and between and, to a lesser degree, borewell samples plotted closer to the 2+ 2+ TH and Ca and Mg (r = 097 and 0.99, respectively). The silicate end member, while those of tap water tended toward 2+ 2+ pH was also positively correlated with TH, Ca , Mg and the carbonate end member (Fig.  5a, b). Because of their − 2+ 2+ HCO (Table 3). That is, an increase in Ca , Mg and proximity to the surface, dug well waters were closer to the HCO concentrations through chemical weathering will evaporite dissolution end member than those of borewells increase the groundwater pH. Therefore, it can be suggested and tap waters (Fig. 5). that the factor 1 reflects water–rock interaction within the The extent of water–rock interaction was further assessed 2+ 2+ aquifer. through the molar ratios of Mg /Ca of the samples. All 2+ 2+ The influence of water–rock interaction on the groundwa- samples had Mg /Ca ratios less than 2 (Table 4), indicating ter chemistry was examined through bivariate mixing plots silicate weathering (Weaver et al. 1995). The average molar + 2+ + 2+ 2+ 2+ + of Na-normalized Ca versus Na-normalized Mg and ratios of (Ca + Mg )/TZ (0.44 and 0.48) in the borewells + − + + + Na-normalized HCO on log–log scale (Fig. 5; Gaillar- also exceeded those of tap waters (Na + K )/TZ (0.13 and det et al. 1999). Gaillardet et al. (1999) used published data 0.04). This reflects weathering of dolomitic limestones in the + + + of well-characterized lithologies to determine silicate and source aquifer. In contrast, the average (Na + K )/TZ ratio 1 3 Applied Water Science (2018) 8:88 Page 9 of 14 88 Table 3 Pearson’s correlation 2+ 2+ + + − 2− − − PH EC TDS TH Ca Mg Na K HCO SO NO Cl 3 4 3 matrix for selected physicochemical parameters pH 1.00 of the groundwater samples EC 0.85 1.00 (correlation coefficients ≥ 0.60 TDS 0.85 0.98 1.00 are in bold) TH 0.94 0.89 0.86 1.00 2+ Ca 0.92 0.9 0.88 0.99 1.00 2+ Mg 0.95 0.86 0.83 0.99 0.97 1.00 Na − 0.30 0.1 0.16 − 0.34 − 0.27 − 0.40 1.00 K 0.54 0.80 0.82 0.52 0.56 0.48 0.49 1.00 HCO 0.91 0.80 0.76 0.97 0.94 0.98 − 0.48 0.45 1.00 2− SO 0.72 0.69 0.66 0.83 0.80 0.83 − 0.34 0.31 0.80 1.00 NO − 0.50 − 0.19 − 0.14 − 0.59 − 0.52 − 0.64 0.91 0.18 − 0.72 − 0.56 1.00 Cl − 0.20 0.20 0.24 − 0.25 − 0.18 − 0.30 0.94 0.53 − 0.39 − 0.33 0.90 1.00 Table 4 Saturation indices Sample PCO Saturation indices Hydrochemical and partial pressures of CO ratios of carbonate and evaporite minerals of the groundwater atm Calcite Dolomite Aragonite Anhydrite Gypsum Halite Mg/Ca SO /Cl samples (saturated and −2 W1 3.9 × 10 − 2.4 − 10.5 − 2.5 – – – 0.3 0.6 supersaturated indices are in −2 bold) W2 3.2 × 10 − 2.7 − 5.4 − 2.8 − 3.4 − 3.2 − 8.3 0.7 0.4 −2 W3 2.2 × 10 − 2.6 − 5.5 − 2.8 − 3.3 − 3.1 − 8.1 0.5 0.2 −2 W4 2.7 × 10 − 2.7 − 5.9 − 2.8 − 3.7 − 3.4 − 8.5 0.3 0.2 −2 W5 1.0 × 10 − 2.1 − 4.2 − 2.2 − 3.4 − 3.2 − 6.9 0.7 0.0 −3 B3 2.9 × 10 − 17 − 3.01 − 1.8 − 3.3 − 3.0 − 10.0 1.6 4.4 −3 B4 8.1 × 10 − 0.055 0.09 − 0.2 − 2.5 − 2.2 − 8.1 1.2 0.7 −3 B5 2.0 × 10 − 3.5 − 6.8 − 3.6 − 3.6 − 3.4 − 10.1 0.9 2.3 −2 P1 1.1 × 10 0.008 0.2 − 0.1 − 2.2 − 2.0 − 8.8 1.2 5.2 −3 P2 3.2 × 10 0.1 1.1 0.3 − 3.3 − 3.0 − 9.5 1.4 0.3 −3 P3 5.2 × 10 0.3 0.8 0.2 − 2.8 − 2.6 − 9.3 1.1 0.7 −3 P4 3.9 × 10 0.3 1.0 0.2 − 2.3 − 2.0 − 9.7 1.4 8.7 −3 P5 8.9 × 10 0.1 0.5 − 0.03 − 2.2 − 1.9 − 9.7 1.3 7.1 −3 P6 4.2 × 10 0.4 1.0 0.2 − 2.2 − 1.9 − 9.3 1.2 3.5 −3 P7 5.6 × 10 0.3 0.8 0.2 − 2.2 − 1.9 − 9.3 1.2 4.3 −3 P8 4.1 × 10 0.4 1.0 0.2 − 2.2 − 1.9 − 9.2 1.3 2.8 −3 P9 4.5 × 10 0.4 1.1 0.3 − 2.2 − 1.9 − 9.8 1.3 10.8 −3 P10 2.3 × 10 0.6 1.5 0.4 − 2.3 − 2.0 − 9.1 1.5 3.3 2+ 2+ + was slightly higher than that of (Ca + Mg )/TZ in dug − + + Cl − Na + K wells. Thus, the behavior of alkali and alkaline earth ions in CAI − 2 = (2) − 2− 2− − the dug wells may be controlled by cation exchange between HCO + SO + CO + NO 3 4 3 3 the groundwater and the clay minerals often encountered in 2+ 2+ the lateritic layers. The chloro-alkaline indices (CAI-1 and If both CAI-1 and CAI-2 are negative, Ca and Mg CAI-2) were used to study a possible ion exchange between have been adsorbed onto the aquifer materials and Na or/ the groundwater and the aquifer materials during the resi- and K are released in the groundwater (i.e., reverse ion dence time and movement (Schoeller 1965; Marghade et al. exchange). In contrast, if the indices are positive, alka- 2+ 2+ 2012). The chloro-alkaline indices (all the ions are expressed line earth ions (Ca and Mg ) have been released in the in meq/L) were calculated as follows (Eqs. 1, 2): groundwater and alkalis retained by the aquifer materi- als (i.e., direct ion exchange; Schoeller 1967). The Sch- − + + Cl − Na + K (1) oeller indices of the groundwater samples of the present CAI − 1 = Cl 1 3 88 Page 10 of 14 Applied Water Science (2018) 8:88 2 2+ 2 − 2 study were negative (Fig.  6a), suggesting that reverse (R = 0.61), Mg (R = 0.78) and HCO (R = 0.73) and + + ion exchange could contribute to N a and K abundance dolomite saturation indices. Similarly, gypsum saturation in the wells. However, the linear plot (Fig.  6b) between indices correlated well with TDS (R = 0.62; Fig. 7). This + + − 2+ 2+ 2− − Na + K –Cl and (Ca + Mg )–(SO + HCO ) showed indicates that the groundwaters have the capacity to dis- 4 3 2 2+ 2+ a weak relationship (R = 0.132) and a slope of 1.214. This solve dolomite and gypsum, and the bulk of Ca , Mg 2 2− is far from the theoretical correlation (R > 90%) coefficient and SO concentrations is assumed to be from dissolu- and slope of about − 1 (Fisher and Mullican 1997; Wen tion of these minerals. et al. 2005; Yidana and Yidana 2010). Therefore, it can be Only moderate positive correlations were observed + 2+ 2+ assumed that chemical weathering is the single most impor-between K , pH, TDS, Ca and Mg , which suggested tant hydrogeochemical process that controls distribution of that K were only partially influenced by chemical weather - 2+ 2+ 2− − Ca, Mg, SO and HC O in the groundwaters. The ing. In addition to orthoclase dissolution, excessive applica- 4 3 abundance of these ions in the groundwaters is a function of tion of KCl as a fertilizer may have contributed to K and carbonate mineral distribution in the host aquifer materials. Cl loadings in the groundwater (Lee et al. 2005). Because 2+ 2+ 2− Thus, the relative high Ca , Mg, SO and the groundwaters were under-saturated with respect to gyp- − −2 HCO concentrations in tap waters corroborates the avail- sum, the low SO concentrations, particularly in dug wells 3 4 ability of carbonate minerals in deeper aquifers as well as (SO /Cl > 1), indicated a possible sulfate reduction by micro- longer residence times of the groundwater. As a result, organisms (Lavitt et al. 1977; Datta and Tyagi 1996). Further samples from tap water were saturated with carbonate min- evidence to the microbial activities is highlighted by high erals (Table 4). Nevertheless, the calcite saturation indices bacterial counts and high partial pressures of CO (pCO ) in 2 2 2+ − did not correlate with TDS, Ca and H CO suggesting the dug wells (Tables 1, 4). That is, the calculated pC O of 3 2 that calcite did not continue to dissolve in the aquifer the groundwater were greater than that of the atmospheric −3.4 following its saturation (Fig.  7). In contrast, strong lin- pCO (10 atm) with the highest values observed in the 2+ 2 ear relationships existed between Ca (R = 0.76), TDS dug wells. This suggests that infiltrating water into the aqui- fer via soil tends to have higher dissolved C O produced by organic matter decomposition and root respiration (Eq. 3). This biogeochemical process is likely to produce carbonic acid (H CO ) in the groundwater (Eq. 4), which is responsi- 2 3 ble for mineral weathering (Eqs. 5, 6; Drever 1988). CH O(aq) + O (aq) →  () + H O (3) 2 2  2 CO (g) + H O = (4) 2 2 H CO = H + HCO (5) 2 3 3 CaMg CO + 2  = Ca + Mg + 4HCO (6) 3   3 The substantial decline in pCO followed by an increase in pH in tap water could be attributed to CO outgassing in deep aquifers (Subba et al. 2006). Another source of pro- ton in the groundwater could be sulfide mineral oxidation (Eq. 7; Berner and Berner 1987; Sarin et al. 1989; Singh and Hasnain 2002). 2+ 2− + 2FeS + 7O + 2H O = 2Fe + 4SO + 4 (7) 2 2 2 Carbonic acid and sulfide mineral oxidation weathering − − 2− can be distinguished by the HCO /(HCO + SO ) ratios 3 3 4 − − 2− (Pandey et al. 2001). The HCO /(HCO + SO ) ratio 3 3 4 equal to 1 indicates that carbonic acid is the main proton source for chemical weathering, whereas a ratio of 0.5 sug- gests that both carbonic acid and the proton from pyrite oxi- dation were responsible for the groundwater ion acquisition. Fig. 6 a CAI-1 versus CAI-2 bivariate diagram and b weak linear − − 2− In the present groundwater samples, HCO /(HO + SO ) 3 3 4 relationship between Na + K–Cl and (Ca + Mg)–(SO + HCO ) and 4 3 varied from 0.8 to 0.99, suggesting that carbonic acid the groundwater samples 1 3 Applied Water Science (2018) 8:88 Page 11 of 14 88 Fig. 7 a Relationship of TDS to dolomite saturation indices; b 2+ relationship of Ca to dolomite saturation indices; c, d relation- − 2+ ships of HCO and Mg to dolomite saturation indices; e relationship of TDS to gypsum saturation indices weathering of carbonate, dolomite and gypsum controlled to halite dissolution as all samples were under-saturated 2+ 2+ − 2− the abundance of Ca, Mg, HCO and SO in the with respect to halite. However, if there were halite deposits 3 4 groundwater. within the aquifer sediments, one could expect to find local - + − − Factor 2 had high loadings on Na, Cl and NO . ized saline waters (high TDS) in the groundwater. Instead, Although Na may derived from silicate weathering (Mey- dug well waters, with relatively low TDS, exhibited the high- beck 1987), halite dissolution, a strong positive correlation est Cl concentrations. Halite dissolution cannot therefore + − − between Na and NO , an index of anthropogenic activi- be the main source of Cl in the groundwater. Furthermore, ties (David and Gentry 2000), implied that anthropogenic Cl concentration in rock-forming minerals (biotite) com- sources such as untreated sewage effluent had greatly monly found in the study area is thought to be very low, + − contributed to Na loading into the groundwater system and that weathering is unlikely to be the source of Cl in (Patterson 1997). According to Patterson (1997), laundry the groundwater. Atmospheric deposition (dust and rainfall) detergent powders provide up to 40% of Na in wastewater. and decomposition of organic matter may be the primarily + − The anthropogenic contribution to Na loading is further source of Cl abundance in the present groundwater (Freeze corroborated by its relative high concentrations in dug well and Cherry 1979). The atmospheric origin of Cl was fur- waters, directly influenced by surface pollution, compared ther supported by the low Cl/TZ (< 1) of the groundwater, to tap waters from deeper aquifer. The strong relationship and hence, Cl would be present as NaCl (Kortatsi et al. + − observed between Na and Cl (r = 0.94) could be attributed 1 3 88 Page 12 of 14 Applied Water Science (2018) 8:88 Fig. 8 a Piper diagram display- ing the dominant water types of the groundwaters; b Schöeller diagram showing major ion distribution patterns of the groundwaters. Tap waters are 2+ 2+ enriched in Mg and Ca , while dug well waters tend to + + have high N a and K content. Bicarbonate is the dominant anion in the samples 2008). Thus, factor 2 reflects anthropogenic influence on the human consumption. In addition to urgent need to improve groundwater quality. the general sanitation conditions in Nouna, the dug wells A moderate positive correlation between K and require special care so that the pollutants from various − + Cl (r = 0.53) and between and N a (r = 0.49) implied that sources can be stopped. Future investigation that includes both geogenic and anthropogenic sources had contributed seasonal variations and heavy metal concentrations is to K loading in the groundwater. Based on the water–rock planned. interaction types, Piper triplot (Piper 1944; Fig. 8a) clas- Acknowledgements We thank the Director general and laboratory sified tap waters and the majority of borewell waters as staff of the Office National de l’Eau et de l’Assainissement (ONEA) Ca–HCO or Ca–Mg–HCO type, consistent with dissolu- 3 3 in Ouagadougou for major cation and anion concentration analyses of tion of dolomitic limestone and silicate (i.e., amphiboles, the groundwater. We would like to thank Dr. Saga S. Sawadogo for pyroxenes, olivine and biotite) minerals. The groundwaters producing Geological and groundwater sampling maps. Comments and suggestions from two anonymous reviewers greatly improved the from dug wells were characterized by weathering of alumi- manuscript. nosilicate minerals and human activities (Ca–Na–K–HCO ). Furthermore, the Schoeller semi-logarithmic diagram (Sch- Open Access This article is distributed under the terms of the Crea- oeller 1962; Fig. 8b) discriminated samples with similar dis- tive Commons Attribution 4.0 International License (http://creat iveco tribution patterns. With longer water–rock interaction, tap mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- 2+ 2+ 2− − tion, and reproduction in any medium, provided you give appropriate waters had higher Mg and Ca, SO and HCO con- 4 3 credit to the original author(s) and the source, provide a link to the centrations relative to dug well and borewell waters. Creative Commons license, and indicate if changes were made. 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Impacts of hydrogeochemical processes and anthropogenic activities on groundwater quality in the Upper Precambrian sedimentary aquifer of northwestern Burkina Faso

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Earth Sciences; Hydrogeology; Water Industry/Water Technologies; Industrial and Production Engineering; Waste Water Technology / Water Pollution Control / Water Management / Aquatic Pollution; Nanotechnology; Private International Law, International & Foreign Law, Comparative Law
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Abstract

This study investigates the hydrogeochemical and anthropogenic factors that control groundwater quality in an Upper Pre- cambrian sedimentary aquifer in the northwestern Burkina Faso. The raw data and statistical and geochemical modeling results were used to identify the sources of major ions in dug well, private borewell and tap water samples. Tap waters were classified as Ca–HCO and Ca–Mg–HCO types, reflecting the weathering of the local dolomitic limestones and silicate 3 3 minerals. Dug well waters, with a direct contact with various sources of contamination, were classified as Ca–Na–K–HCO type. Two factors that explain 94% of the total variance suggested that water–rock interaction was the most important factor 2+ 2+ − 2− controlling the groundwater chemistry. Factor 1 had high loadings on pH, Ca, Mg, HCO, SO and TDS. These vari- 3 4 − − 2− ables were also strongly correlated indicating their common geogenic sources. Based on the HCO /(HCO + SO ) ratios 3 3 4 2+ 2+ − 2− (0.8–0.99), carbonic acid weathering appeared to control Ca, Mg, HCO and SO acquisition in the groundwater. 3 4 2+ 2+ With relatively lower Ca and Mg concentrations, the majority of dug well and borewell waters were soft to moderately hard, whereas tap waters were considered very hard. Thus, the dug well and, to a lesser extent, borewell waters are likely to + − − + have a low buffering capacity. Factor 2 had high loadings on N a, NO and Cl . The strong correlation between Na and − − + NO and Cl implied that factor 2 represented the anthropogenic contribution to the groundwater chemistry. In contrast, K had moderate loadings on factors 1 and 2, consistent with its geogenic and anthropogenic sources. The study demonstrated that waters from dug wells and borewells were bacteriologically unsafe for human consumption, and their low buffering capacity may favor mobility of potentially toxic heavy metals in the aquifer. Not only very hard tap waters have aesthetic inconvenient, but their consumption may also pose health problems. Keywords Sedimentary aquifer · Tap water · Dug wells · Borewells · Water–rock interaction Introduction seasonal surface water flow, and thus, surface water becomes an unreliable source for water supply. As a result, people Following severe droughts in 1970s, a massive internal have been heavily relying on groundwater for domestic water migration from drier central plateau and northern regions supply and livestock watering (Derouane and Dakoure 2006; toward a more humid northwestern Burkina Faso has Courtois et al. 2010; Huneau et al. 2011). Traditional hand- put a tremendous pressure on the regional surface water dug wells are the main sources of groundwater in the region. resources (Kessler and Greerling 1994). The northwestern In order to meet the ever-increasing demands for water, hun- Burkina Faso has been also subject to adverse effects of cli - dreds of borewells, equipped with hand pumps, were drilled mate changes such as erratic precipitations and decrease in in the Kossi Province one of the four provinces in the north- western Burkina Faso (Barry et al. 2005) and the site of the present study. The borewells draw groundwater from deep * A. Sako fractured sedimentary rocks, whereas the dug wells abstract aboubakar.sako@gmail.com shallow groundwater within weathered mantle layers (Col- lectif 1990). Université de Dédougou, BP. 139, Dédougou, Burkina Faso 2 Although groundwater constitutes an important asset for Département des Sciences de la Terre, Université Ouaga 1 Pr socioeconomic development of the northwestern Burkina Joseph Ki-Zerbo, 09 BP 848, Ouagadougou 09, Burkina Faso Vol.:(0123456789) 1 3 88 Page 2 of 14 Applied Water Science (2018) 8:88 1 3 Applied Water Science (2018) 8:88 Page 3 of 14 88 ◂Fig. 1 a Geographical map of Burkina Faso; b geomorphological formations shared by Mali and Burkina Faso. These forma- map of the Kossi floodplain, showing the study area; c groundwater tions are essentially made of an alternation of pink siltstones sampling points superimposed on the simplified local lithological and argillites with glauconite and dolomitic limestone lenses units. The lithology of tap water from the public water supply points capped with silexite (Ouédraogo 1998). As in the crystalline may not correspond to their sampling lithology basement areas that make up 80% of Burkina Faso, two types of discontinuous aquifers are encountered in the study area. Faso, the hydrogeochemical studies pertaining groundwater A shallow (5–20 m) aquifer located in the weathered lateritic quality in this large transboundary aquifer are scanty. The layer, which is superimposed on a deep aquifer within the local groundwater quality is likely to be controlled by both joined sandstone layers in the sedimentary sequence (CIEH natural and anthropogenic factors. Water–rock interaction 1976; BILAN D’EAU 1993). The thickness of the deep (i.e., chemical weathering and cation exchange processes) aquifer is poorly known, and it varies according to the lithol- can be the most important natural factor that controls the ogy. In contrast to crystalline basement aquifers, the high groundwater quality (Fetter 1994; Appelo and Postma 2005; per meability (1.8 × 10  m/s) of sedimentary rocks makes Li et al. 2016). In contrast, excessive use of fertilizer, non- the southeast Taoudeni sedimentary formations excellent −4 protection of wells and poor sanitary conditions are poten- aquifers, with an estimated storage coefficient of 1 × 10 tial sources of anthropogenic pollution (Groen et al. 1988; and significant yields up to 100 m /h (Gombert 1998). That Li et al. 2017; Yameogo and Savadogo 2002; Huneau et al. is, the only two permanent watercourses in the country (i.e., 2011; Wu et al. 2017). The monitoring of the physicochemi- the Mouhoun and Comoé rivers) are directly fed by springs cal and biological conditions of groundwater is necessary for originated from sedimentary aquifers (Talbaoui 2009). an efficient water resource management and development The local groundwater recharge occurs through direct of aquifer protection strategies. Therefore, the objectives of infiltration of rainwater and indirect infiltration of runoff via the present study were (1) to identify the hydrogeochemi- depressions, streams and alluvial valleys (Groen et al. 1988; cal processes and anthropogenic activities that govern the Barry et al. 2005). The regional water table shows a seasonal chemical composition of dug wells, private borewells and variation of 1–2 m. The estimated total volume of groundwa- tap water provided by the public water supply system of an ter in the Nouna commune is 0.4 million m /year, whereas Upper Precambrian sedimentary aquifer, and (2) to evaluate the renewable resource is about 0.5 million m /year (MEE the suitability of the groundwater for human consumption. 2001). Consequently, groundwater resource development in The findings of this study will contribute to bridging the the commune is very limited compared to the resource avail- gap between anthropogenic factors and hydrogeochemical ability. More than half of the resources are used for domestic processes that control groundwater quality in a sedimentary water supply and the remaining for livestock watering (MEE and semi-urban setting. 2001). Poor sanitation, lack of an effective management of domestic wastes, inadequate protection of dug wells from surface runoff and animal droppings make the groundwater Site description highly vulnerable to anthropogenic pollution. The study area is located in the town of Nouna, the Kossi Province (Northwestern), 306  km of Ouagadougou the Materials and methods capital city of Burkina Faso (Fig. 1a). The area is part of a floodplain of the ephemeral Kossi River basin (Fig.  1b). This Twenty groundwater samples were collected from six major plain contains several ponds of variable sizes, separated by wards of Nouna in dry season 2017 (Fig.  1c). Five sam- elevated zones (200–300 m a.s.l). The local climate is of the ples were collected from representative private borewells north-Sudanian type, characterized by a dry season (Octo- (B1–B5), five from shallow hand-dug wells with large ber–May) and a wet season (June–September). With an aver- diameters (W1–W2), whereas 10 samples were collected age annual rainfall of 887 mm, the Nouna commune falls in from the public water supply system (P1–P10; Table 1). In the so-called the Bread Basket of Burkina Faso, where sub- order to obtain high water flow rates, the groundwater sup- sistence and cotton farming and livestock bring a substantial plied by the public water supply system is abstracted from income to the populations. As in the whole country, the plain relatively deeper aquifers. The hand-dug well samples were has undergone a marked decrease in rainfall since the 1970s drawn using a sterilized bucket and filtered through Milli- (~ 200 mm), putting a great pressure on water resources. pore membrane (0.45 µm) into two sets of new high-density Currently, rainfall is characterized by a great intra- and inter- polyethylene bottles (HDP), whereas those of borewells and annual irregularity (Frappart et al. 2009). tap water were directly pumped through filter capsules into The area is underlain by Upper Precambrian sedimen- two sets of HDP. One set of the samples was acidified with tary rocks known as the southeast Taoudeni sedimentary ultrapure HNO (pH > 3), whereas the other set was left 1 3 88 Page 4 of 14 Applied Water Science (2018) 8:88 1 3 Table 1 Physicochemical and bacteriological parameters of dug well (W1–W5), borewell (B3–B5) and tap water (P1–P10) samples Sample Physicochemical parameters Bacteriological counts 2+ 2+ + + − 2− − − 1 2 3 pH Temp EC TDS TH Ca Mg Na K HCO SO NO Cl FC TC FS 3 4 3 °C (µs/C) (mg/L)(mg CaCO /L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 1UFC/100 mL 1UFC/100 mL 1UFC/100 mL W1 6.1 31.6 75 51 51 16 2.82 11 4.8 49 2.9 27 4 18 144 > 1000 W2 6.1 32.3 134 84 43 10 4.38 23 6.5 40 3.5 25 7 7 > 1000 > 1000 W3 6.2 30.1 145 67 38 10 2.97 17 3.6 34 4.6 31 14 28 > 1000 64 W4 6.1 32.9 86 48 33 10 1.85 13 5.1 33 1.9 29 8 34 > 1000 > 1000 W5 6.5 33.0 524 201 96 23 9.34 66 15 33 2.1 77 55 57 > 1000 > 1000 B3 6.7 34.3 121 72 64 9.8 9.68 4 7.6 104 4.8 9.2 1 4 43 5 B4 7.4 33.5 501 168 204 38 26.8 19 14 287 14 11 14 0 98 24 B5 5.2 32.1 34 7 32 6.6 3.79 2.4 3.3 76 3.1 12 1 0 45 12 P1 7.4 33.2 514 190 247 45 32.8 18 11 288 22 6 3 0 5 0 P2 7.9 32.9 412 151 211 35 29.9 2 9.1 268 2.1 5.3 5 0 0 0 P3 7.7 33.3 426 168 244 46 31.7 3.9 8.8 273 4.8 5.8 5 0 2 0 P4 7.8 33.2 502 169 263 44 37.3 3.5 9.3 260 20 5.7 2 0 0 0 P5 7.5 32.8 506 178 258 45 35.5 3 9 296 23 5.2 2 0 0 0 P6 7.8 32.9 430 166 242 44 31.9 3.7 8.4 281 22 6.2 5 0 7 0 P7 7.7 32.3 422 176 260 47 35 4.5 9.3 295 22 6 4 0 11 0 P8 7.8 33.3 453 169 261 45 36.3 3.9 8.6 271 23 4.7 6 0 0 0 P9 7.8 33.2 503 176 259 44 36.1 3.9 9.1 299 22 6.5 2 0 0 0 P10 8.0 34.1 516 179 260 41 38.3 5.8 8.2 299 22 4 5 23 0 0 WHO 6.5 ≤ pH ≤ 8.5 25.0 400 1000 200 100 50 150 12 100 250 50 200 0UFC/100 mL 0UFC/100 mL 0UFC/100 mL 1. Fecal coliforms 2. Total coliforms 3. Fecal streptococci 4. World Health Organization Applied Water Science (2018) 8:88 Page 5 of 14 88 non-acidified. A third set of samples was collected and kept unfiltered and non-acidified in glass bottles for bacteriologi- cal counts. Electrical conductivity (EC), pH and total dis- solved solids (TDS) were measured in the field using cali- brated meters with standard solutions. The samples were put in ice box and taken to laboratory for major cation and anion analysis. 2+ 2+ In the laboratory, concentrations of Ca and Mg were estimated titrimetrically using 0.05 N EDTA and 0.01 N, − − whereas those of HCO and Cl by H SO and AgNO 3 2 4 3 titration, respectively. Sodium and K concentrations were determined by flame photometric method (APHA 1995), and 2− − those of SO and NO by UV–Vis spectrophotometric 4 3 technique. Total hardness (TH) was determined by EDTA complexometric titration method (WHO 1999). Analytical reagent grades and milli-Q water were used for the analy- ses. Two borewell samples (B1 and B2) had large charge balance errors (> ± 10%) and were not included in the data interpretation. Nutrient MacConkey agar was used for total coliform bacterial count and Eosin for total fecal coliform. The petri dishes containing agar and diluted groundwater samples were incubated under appropriate conditions (time and temperature). The bacteriological counts per 100 mL were estimated from the MPN table (APHA9221D). − + Fig. 2 a Relationship of total anions (TZ ) to total cations (TZ ) of R-mode factor analysis (Wu et  al. 2014) was used to the groundwater samples; b relationship of electrical conductivity (EC) to total dissolved solids (TDS) assess the relationships between the physicochemical param- eters of the groundwater, using SPSS package (version 20), whereas Visual MINTEQ (version 3.1) was used to calculate drawn from the weathered mantle aquifer appeared to be saturation indices (SI) of carbonate and evaporite minerals as well as partial CO pressure of the groundwater. less mineralized compared to those from the deep fractured + − aquifer. As result, ZT and TZ were higher in tap waters (medians = 172 and 336 µeq/L; Table 2) from the deep aqui- fer than in dug wells from the weathered mantle aquifer. The Results and discussion differences in recharge flow paths could also be an explana- tion for the observed mineralization trends. Thus, weakly Groundwater constituents mineralized groundwaters are often associated with rapid recharge (i.e., younger residence time) of the shallow aqui- The physicochemical data of groundwater highlighted dis- tinct differences between shallow dug well, borewell and fers, whereas highly mineralized groundwaters (i.e., older residence time) have been attributed to paleo-recharge or tap waters. A strong relationship (R = 0.96) between total + − cations TZ and total anions TZ (Fig. 2a) implied that con- slow circulation processes in deep aquifers (Fritz 1997; Sto- ber and Bucher 1999; Cook et al. 2005; Bucher and Stober tribution of non-measured ions to charge balance was not significant. Furthermore, the relationship between EC and 2010; Armandine Les Lands et al. 2014). The high coefficients of variance (CV > 50%; Table 2) and TDS (R = 0.96; Fig. 2b) suggested that the groundwaters were unlikely to contain substantial amounts of uncharged spatial distribution, illustrated by boxplots (Fig. 3), showed a heterogeneous abundance of most physicochemical parame- soluble compounds (e.g., silica, manganese, aluminum and iron) that may contribute to TDS contents (Datta and Tyagi ters in dug wells. This is probably due to the sources and the nature of the recharge, the host rock geology, and the short 1996; Prasanna et al. 2011). In overall, EC and TDS were low in the groundwaters residence time of the groundwater in the weathered mantle aquifer (Back and Hanshaw 1971). On contrary, groundwa- (Table 1). This suggests the absence of salt in the recharge water and limited groundwater mineralization (Han and ter composition of tap waters was remarkably homogene- ous (CV < 50%; except N a ) and most variables had similar Liu 2004; Smedley et al. 2007; Huneau et al. 2011; Jean- nin et al. 2016). Because of intense leaching, groundwaters values for mean and median, reflecting primarily the long 1 3 88 Page 6 of 14 Applied Water Science (2018) 8:88 o fl w lines and dispersive mixing that may have smoothed out any temporal fluctuations in the groundwater composition (Mazor et al. 1993; Dhar et al. 2008). Although the major- ity of the samples had pH values within the World Health Organization (WHO 2006) guideline limit for drinking water (pH = 6.5–8.5), the dug well and borewell waters had lower pH (medians = 6.2 ± 0.2 and 6.4 ± 0.9) relative to those of tap waters (median = 7.8 ± 0.2). The high pH in tap waters relative to dug well waters is consistent with positive cor- relations between pH and the resident time usually observed in deeper aquifers (Morgenstern and Daughney 2012). Total hardness (TH) in the well waters had distribution patterns similar to those of pH, TDS and EC with TH ranging from 33 to 236 mg CaCO /L. The dug well waters exhibited the lowest TH (median = 47 ± 24.6 mg CaCO /L and CV = 44%), whereas the highest concentrations were observed in tap waters (median = 258 ± 18 mg CaCO /L and CV = 18%). Again, the high TH in tap waters can be attributed to long residence time of groundwater in the deep fractured aquifer, leading to extended chemical weathering of dolo- mitic limestones (Frape et al. 1984). With hardness values largely exceeding the WHO guideline value for drinking water, the tap waters were categorized as very hard, while those of dug wells as soft to moderately hard. Soft waters, with low alkalinity and buffering capacity, may favor the mobility of potentially toxic heavy metals in the aquifer (De Schamphelaere and Janssen 2004; Kirby and Cravotta 2005). In contrast, hard waters require more soap to produce lather, and thus, it is unsuitable for domestic use (Srinivasa Rao and Jugran 2003). Some evidence has also indicated the role played by hard waters in heart diseases and prenatal mortal- ity (Schroeder 1960; Agarwal and Jagetai 1997). Although such cases have not been reported in the present study area, the desirability of softer drinking water is evident among the local population. As a result, the water provided by the public water supply system should be treated before it gets to the consumers. Sodium was the dominant cation in dug well waters 2+ + 2+ followed by C a, K and M g , whereas cation abun- dance in borewell and tap waters was in decreasing order 2+ 2+ + + of Ca > Mg > K >Na (Table  1). The low EC, TDS, HCO and TH contents observed in dug well and borewell waters suggest short contact times between groundwater and the aquifer minerals. This is consistent with the low K (except W5 and B4) concentrations in dug well and bore- well waters relative to tap waters (8–11 mg/L). Potassium concentrations in groundwater up to 10 mg/L are attributed to orthoclase or clay weathering, whereas concentrations above 10 mg/L may indicate external sources of K abun- 2− − dance (Rail 2000). Bicarbonate, SO and N O were the 4 3 dominant anions in the wells with the highest HC O and 2− SO concentrations observed in tap waters. Although these ion concentrations in the groundwater were within the WHO 1 3 Table 2 Means medians, standard deviations (SD) and coefficients of variance (CV) of physicochemical parameters of the groundwater samples Parameter Dug wells Borewells Public water supply points Min Max Mean Median SD % CV Min Max Mean Median SD % CV Min Max Mean Median SD % CV pH 6.1 6.5 6.2 6.2 0.2 3 5.2 7.4 6.4 6.5 0.9 14 7.4 8 7.7 7.8 0.2 2 EC (µS/cm) 75 524 223 140 181.7 81 34 501 238 180 199 82 412 516 468 468 41 9 TDS (mg/L) 48 201 100 76 61.3 61 7 168 84 78 66 83 151 190 172 172 11 7 TH (mg/L) 33 96 56 47 24.6 44 32 204 107 86 73 79 211 263 248 258 18 7 2+ Ca (mg/L) 10 22.96 15 13 5.3 36 6.64 38 20 15 14 68 35.2 47 43 44 4 8 2+ Mg (mg/L) 2 9.34 5 4 2.9 62 3.79 27 14 12 10 88 29.9 38 34 35 3 8 Na (mg/L) 11 66.49 30 20 22.3 75 2.37 19 9 7 7 86 2 18 6 4 5 86 K (mg/L) 4 15.05 8 6 4.5 59 3.27 14 9 8 4 58 8 11 9 9 1 9 HCO (mg/L) 33 48.8 39 36 6.4 17 76.25 287 166 135 91 71 260 299 283 283 14 5 2− SO (mg/L) 1.9 4.6 3 3 1.0 34 3.1 14 8 6 5 112 2.1 23 17 22 8 47 NO (mg/L) 25 77 42 30 21.4 51 9.2 12 11 11 1 53 4 6.5 5 6 1 15 Cl (mg/L) 3.5 55.4 21 11 20.9 100 0.8 14 6 4 6 114 2 6.1 4 4 2 41 TZ (µeq/L) 42.1 146.1 76 56 42.1 55 26.5 162 85 68 58 73 141 184 169 172 13 8 TZ (µeq/L) 73.5 169.5 105 87 38.7 37 95.45 340 199 161 107 48 283 352 327 336 26 8 Applied Water Science (2018) 8:88 Page 7 of 14 88 Fig. 3 a Boxplots of naturally affected physicochemical parameters in the Upper Pre- cambrian sedimentary aquifer of the northwestern Burkina Faso. The tops and bottoms of the boxes represent the 75th and 25th percentiles, respectively. The horizontal line across the boxes indicates the median. The vertical lines from the tops and bottoms of the boxes extend to 90th and 10th percentiles, respectively. b Boxplots of anthropogenically affected phys- icochemical parameters in the Upper Precambrian sedimentary aquifer of the northwestern Burkina Faso 1 3 88 Page 8 of 14 Applied Water Science (2018) 8:88 Fig. 4 R-mode factor scores for factors 1 and 2 of the groundwa- ter samples. Potassium is projected half way between geogenic and anthropogenic factors permissible limits for drinking water, NO concentrations in dug well waters exceeded the natural nitrate concentra- tions (5–7 mg/L; Appelo and Postma 1999). None of dug well and borewell samples complied with the WHO guide- line values for coliforms (Table 1), and hence, water from these wells requires treatment before human consumption. Processes controlling groundwater chemistry Fig. 5 Mixing diagrams of Ca/Na versus HCO /Na and Ca/Na versus Mg/Na for silicate and carbonate minerals in the Upper Precambrian Chemical weathering, cation exchange, evaporation and sedimentary aquifer of the northwestern Burkina Faso antropogenical activities are the common hydrogeochemi- cal processes that control groundwater chemistry. In order to shed light on these complex processes, statistical and carbonate end members. According to these authors, the geochemical techniques were used. Thus, the R-mode factor carbonate end member is characterized by Ca/Na, Mg/Na analysis, after varimax rotation (Kaiser 1960), produced two and HCO /Na ratios of 45 ± 25, 15 ± 10 and 90 ± 40 mg/L, factors (with eigenvalues > 1) that explain 94% of the total respectively, whereas the chemistry of water draining variance (Fig. 4). With 63.4% of the total variance, factor 1 silicate is characterized by Ca/Na = 0.3 ± 0.15 mg/L, Mg/ is the most important factor that influences the groundwater Na = 0.24 ± 0.12  mg/L and HCO /Na = 2 ± 1  mg/L. In the 2+ chemistry. This factor had high absolute loadings on Ca , present study, the bivariate plots identified silicate weather - 2+ − 2−, Mg , TH, HCO , pH, SO EC and TDS and a moder- ing and carbonate dissolution as the two hydrogeochemical 3 4 ate loading on K . As expected, there were strong positive processes controlling the groundwater chemistry. Dug well 2+ 2+ correlations between Mg and Ca (r = 0.96) and between and, to a lesser degree, borewell samples plotted closer to the 2+ 2+ TH and Ca and Mg (r = 097 and 0.99, respectively). The silicate end member, while those of tap water tended toward 2+ 2+ pH was also positively correlated with TH, Ca , Mg and the carbonate end member (Fig.  5a, b). Because of their − 2+ 2+ HCO (Table 3). That is, an increase in Ca , Mg and proximity to the surface, dug well waters were closer to the HCO concentrations through chemical weathering will evaporite dissolution end member than those of borewells increase the groundwater pH. Therefore, it can be suggested and tap waters (Fig. 5). that the factor 1 reflects water–rock interaction within the The extent of water–rock interaction was further assessed 2+ 2+ aquifer. through the molar ratios of Mg /Ca of the samples. All 2+ 2+ The influence of water–rock interaction on the groundwa- samples had Mg /Ca ratios less than 2 (Table 4), indicating ter chemistry was examined through bivariate mixing plots silicate weathering (Weaver et al. 1995). The average molar + 2+ + 2+ 2+ 2+ + of Na-normalized Ca versus Na-normalized Mg and ratios of (Ca + Mg )/TZ (0.44 and 0.48) in the borewells + − + + + Na-normalized HCO on log–log scale (Fig. 5; Gaillar- also exceeded those of tap waters (Na + K )/TZ (0.13 and det et al. 1999). Gaillardet et al. (1999) used published data 0.04). This reflects weathering of dolomitic limestones in the + + + of well-characterized lithologies to determine silicate and source aquifer. In contrast, the average (Na + K )/TZ ratio 1 3 Applied Water Science (2018) 8:88 Page 9 of 14 88 Table 3 Pearson’s correlation 2+ 2+ + + − 2− − − PH EC TDS TH Ca Mg Na K HCO SO NO Cl 3 4 3 matrix for selected physicochemical parameters pH 1.00 of the groundwater samples EC 0.85 1.00 (correlation coefficients ≥ 0.60 TDS 0.85 0.98 1.00 are in bold) TH 0.94 0.89 0.86 1.00 2+ Ca 0.92 0.9 0.88 0.99 1.00 2+ Mg 0.95 0.86 0.83 0.99 0.97 1.00 Na − 0.30 0.1 0.16 − 0.34 − 0.27 − 0.40 1.00 K 0.54 0.80 0.82 0.52 0.56 0.48 0.49 1.00 HCO 0.91 0.80 0.76 0.97 0.94 0.98 − 0.48 0.45 1.00 2− SO 0.72 0.69 0.66 0.83 0.80 0.83 − 0.34 0.31 0.80 1.00 NO − 0.50 − 0.19 − 0.14 − 0.59 − 0.52 − 0.64 0.91 0.18 − 0.72 − 0.56 1.00 Cl − 0.20 0.20 0.24 − 0.25 − 0.18 − 0.30 0.94 0.53 − 0.39 − 0.33 0.90 1.00 Table 4 Saturation indices Sample PCO Saturation indices Hydrochemical and partial pressures of CO ratios of carbonate and evaporite minerals of the groundwater atm Calcite Dolomite Aragonite Anhydrite Gypsum Halite Mg/Ca SO /Cl samples (saturated and −2 W1 3.9 × 10 − 2.4 − 10.5 − 2.5 – – – 0.3 0.6 supersaturated indices are in −2 bold) W2 3.2 × 10 − 2.7 − 5.4 − 2.8 − 3.4 − 3.2 − 8.3 0.7 0.4 −2 W3 2.2 × 10 − 2.6 − 5.5 − 2.8 − 3.3 − 3.1 − 8.1 0.5 0.2 −2 W4 2.7 × 10 − 2.7 − 5.9 − 2.8 − 3.7 − 3.4 − 8.5 0.3 0.2 −2 W5 1.0 × 10 − 2.1 − 4.2 − 2.2 − 3.4 − 3.2 − 6.9 0.7 0.0 −3 B3 2.9 × 10 − 17 − 3.01 − 1.8 − 3.3 − 3.0 − 10.0 1.6 4.4 −3 B4 8.1 × 10 − 0.055 0.09 − 0.2 − 2.5 − 2.2 − 8.1 1.2 0.7 −3 B5 2.0 × 10 − 3.5 − 6.8 − 3.6 − 3.6 − 3.4 − 10.1 0.9 2.3 −2 P1 1.1 × 10 0.008 0.2 − 0.1 − 2.2 − 2.0 − 8.8 1.2 5.2 −3 P2 3.2 × 10 0.1 1.1 0.3 − 3.3 − 3.0 − 9.5 1.4 0.3 −3 P3 5.2 × 10 0.3 0.8 0.2 − 2.8 − 2.6 − 9.3 1.1 0.7 −3 P4 3.9 × 10 0.3 1.0 0.2 − 2.3 − 2.0 − 9.7 1.4 8.7 −3 P5 8.9 × 10 0.1 0.5 − 0.03 − 2.2 − 1.9 − 9.7 1.3 7.1 −3 P6 4.2 × 10 0.4 1.0 0.2 − 2.2 − 1.9 − 9.3 1.2 3.5 −3 P7 5.6 × 10 0.3 0.8 0.2 − 2.2 − 1.9 − 9.3 1.2 4.3 −3 P8 4.1 × 10 0.4 1.0 0.2 − 2.2 − 1.9 − 9.2 1.3 2.8 −3 P9 4.5 × 10 0.4 1.1 0.3 − 2.2 − 1.9 − 9.8 1.3 10.8 −3 P10 2.3 × 10 0.6 1.5 0.4 − 2.3 − 2.0 − 9.1 1.5 3.3 2+ 2+ + was slightly higher than that of (Ca + Mg )/TZ in dug − + + Cl − Na + K wells. Thus, the behavior of alkali and alkaline earth ions in CAI − 2 = (2) − 2− 2− − the dug wells may be controlled by cation exchange between HCO + SO + CO + NO 3 4 3 3 the groundwater and the clay minerals often encountered in 2+ 2+ the lateritic layers. The chloro-alkaline indices (CAI-1 and If both CAI-1 and CAI-2 are negative, Ca and Mg CAI-2) were used to study a possible ion exchange between have been adsorbed onto the aquifer materials and Na or/ the groundwater and the aquifer materials during the resi- and K are released in the groundwater (i.e., reverse ion dence time and movement (Schoeller 1965; Marghade et al. exchange). In contrast, if the indices are positive, alka- 2+ 2+ 2012). The chloro-alkaline indices (all the ions are expressed line earth ions (Ca and Mg ) have been released in the in meq/L) were calculated as follows (Eqs. 1, 2): groundwater and alkalis retained by the aquifer materi- als (i.e., direct ion exchange; Schoeller 1967). The Sch- − + + Cl − Na + K (1) oeller indices of the groundwater samples of the present CAI − 1 = Cl 1 3 88 Page 10 of 14 Applied Water Science (2018) 8:88 2 2+ 2 − 2 study were negative (Fig.  6a), suggesting that reverse (R = 0.61), Mg (R = 0.78) and HCO (R = 0.73) and + + ion exchange could contribute to N a and K abundance dolomite saturation indices. Similarly, gypsum saturation in the wells. However, the linear plot (Fig.  6b) between indices correlated well with TDS (R = 0.62; Fig. 7). This + + − 2+ 2+ 2− − Na + K –Cl and (Ca + Mg )–(SO + HCO ) showed indicates that the groundwaters have the capacity to dis- 4 3 2 2+ 2+ a weak relationship (R = 0.132) and a slope of 1.214. This solve dolomite and gypsum, and the bulk of Ca , Mg 2 2− is far from the theoretical correlation (R > 90%) coefficient and SO concentrations is assumed to be from dissolu- and slope of about − 1 (Fisher and Mullican 1997; Wen tion of these minerals. et al. 2005; Yidana and Yidana 2010). Therefore, it can be Only moderate positive correlations were observed + 2+ 2+ assumed that chemical weathering is the single most impor-between K , pH, TDS, Ca and Mg , which suggested tant hydrogeochemical process that controls distribution of that K were only partially influenced by chemical weather - 2+ 2+ 2− − Ca, Mg, SO and HC O in the groundwaters. The ing. In addition to orthoclase dissolution, excessive applica- 4 3 abundance of these ions in the groundwaters is a function of tion of KCl as a fertilizer may have contributed to K and carbonate mineral distribution in the host aquifer materials. Cl loadings in the groundwater (Lee et al. 2005). Because 2+ 2+ 2− Thus, the relative high Ca , Mg, SO and the groundwaters were under-saturated with respect to gyp- − −2 HCO concentrations in tap waters corroborates the avail- sum, the low SO concentrations, particularly in dug wells 3 4 ability of carbonate minerals in deeper aquifers as well as (SO /Cl > 1), indicated a possible sulfate reduction by micro- longer residence times of the groundwater. As a result, organisms (Lavitt et al. 1977; Datta and Tyagi 1996). Further samples from tap water were saturated with carbonate min- evidence to the microbial activities is highlighted by high erals (Table 4). Nevertheless, the calcite saturation indices bacterial counts and high partial pressures of CO (pCO ) in 2 2 2+ − did not correlate with TDS, Ca and H CO suggesting the dug wells (Tables 1, 4). That is, the calculated pC O of 3 2 that calcite did not continue to dissolve in the aquifer the groundwater were greater than that of the atmospheric −3.4 following its saturation (Fig.  7). In contrast, strong lin- pCO (10 atm) with the highest values observed in the 2+ 2 ear relationships existed between Ca (R = 0.76), TDS dug wells. This suggests that infiltrating water into the aqui- fer via soil tends to have higher dissolved C O produced by organic matter decomposition and root respiration (Eq. 3). This biogeochemical process is likely to produce carbonic acid (H CO ) in the groundwater (Eq. 4), which is responsi- 2 3 ble for mineral weathering (Eqs. 5, 6; Drever 1988). CH O(aq) + O (aq) →  () + H O (3) 2 2  2 CO (g) + H O = (4) 2 2 H CO = H + HCO (5) 2 3 3 CaMg CO + 2  = Ca + Mg + 4HCO (6) 3   3 The substantial decline in pCO followed by an increase in pH in tap water could be attributed to CO outgassing in deep aquifers (Subba et al. 2006). Another source of pro- ton in the groundwater could be sulfide mineral oxidation (Eq. 7; Berner and Berner 1987; Sarin et al. 1989; Singh and Hasnain 2002). 2+ 2− + 2FeS + 7O + 2H O = 2Fe + 4SO + 4 (7) 2 2 2 Carbonic acid and sulfide mineral oxidation weathering − − 2− can be distinguished by the HCO /(HCO + SO ) ratios 3 3 4 − − 2− (Pandey et al. 2001). The HCO /(HCO + SO ) ratio 3 3 4 equal to 1 indicates that carbonic acid is the main proton source for chemical weathering, whereas a ratio of 0.5 sug- gests that both carbonic acid and the proton from pyrite oxi- dation were responsible for the groundwater ion acquisition. Fig. 6 a CAI-1 versus CAI-2 bivariate diagram and b weak linear − − 2− In the present groundwater samples, HCO /(HO + SO ) 3 3 4 relationship between Na + K–Cl and (Ca + Mg)–(SO + HCO ) and 4 3 varied from 0.8 to 0.99, suggesting that carbonic acid the groundwater samples 1 3 Applied Water Science (2018) 8:88 Page 11 of 14 88 Fig. 7 a Relationship of TDS to dolomite saturation indices; b 2+ relationship of Ca to dolomite saturation indices; c, d relation- − 2+ ships of HCO and Mg to dolomite saturation indices; e relationship of TDS to gypsum saturation indices weathering of carbonate, dolomite and gypsum controlled to halite dissolution as all samples were under-saturated 2+ 2+ − 2− the abundance of Ca, Mg, HCO and SO in the with respect to halite. However, if there were halite deposits 3 4 groundwater. within the aquifer sediments, one could expect to find local - + − − Factor 2 had high loadings on Na, Cl and NO . ized saline waters (high TDS) in the groundwater. Instead, Although Na may derived from silicate weathering (Mey- dug well waters, with relatively low TDS, exhibited the high- beck 1987), halite dissolution, a strong positive correlation est Cl concentrations. Halite dissolution cannot therefore + − − between Na and NO , an index of anthropogenic activi- be the main source of Cl in the groundwater. Furthermore, ties (David and Gentry 2000), implied that anthropogenic Cl concentration in rock-forming minerals (biotite) com- sources such as untreated sewage effluent had greatly monly found in the study area is thought to be very low, + − contributed to Na loading into the groundwater system and that weathering is unlikely to be the source of Cl in (Patterson 1997). According to Patterson (1997), laundry the groundwater. Atmospheric deposition (dust and rainfall) detergent powders provide up to 40% of Na in wastewater. and decomposition of organic matter may be the primarily + − The anthropogenic contribution to Na loading is further source of Cl abundance in the present groundwater (Freeze corroborated by its relative high concentrations in dug well and Cherry 1979). The atmospheric origin of Cl was fur- waters, directly influenced by surface pollution, compared ther supported by the low Cl/TZ (< 1) of the groundwater, to tap waters from deeper aquifer. The strong relationship and hence, Cl would be present as NaCl (Kortatsi et al. + − observed between Na and Cl (r = 0.94) could be attributed 1 3 88 Page 12 of 14 Applied Water Science (2018) 8:88 Fig. 8 a Piper diagram display- ing the dominant water types of the groundwaters; b Schöeller diagram showing major ion distribution patterns of the groundwaters. Tap waters are 2+ 2+ enriched in Mg and Ca , while dug well waters tend to + + have high N a and K content. Bicarbonate is the dominant anion in the samples 2008). Thus, factor 2 reflects anthropogenic influence on the human consumption. In addition to urgent need to improve groundwater quality. the general sanitation conditions in Nouna, the dug wells A moderate positive correlation between K and require special care so that the pollutants from various − + Cl (r = 0.53) and between and N a (r = 0.49) implied that sources can be stopped. Future investigation that includes both geogenic and anthropogenic sources had contributed seasonal variations and heavy metal concentrations is to K loading in the groundwater. Based on the water–rock planned. interaction types, Piper triplot (Piper 1944; Fig. 8a) clas- Acknowledgements We thank the Director general and laboratory sified tap waters and the majority of borewell waters as staff of the Office National de l’Eau et de l’Assainissement (ONEA) Ca–HCO or Ca–Mg–HCO type, consistent with dissolu- 3 3 in Ouagadougou for major cation and anion concentration analyses of tion of dolomitic limestone and silicate (i.e., amphiboles, the groundwater. We would like to thank Dr. Saga S. Sawadogo for pyroxenes, olivine and biotite) minerals. The groundwaters producing Geological and groundwater sampling maps. Comments and suggestions from two anonymous reviewers greatly improved the from dug wells were characterized by weathering of alumi- manuscript. nosilicate minerals and human activities (Ca–Na–K–HCO ). Furthermore, the Schoeller semi-logarithmic diagram (Sch- Open Access This article is distributed under the terms of the Crea- oeller 1962; Fig. 8b) discriminated samples with similar dis- tive Commons Attribution 4.0 International License (http://creat iveco tribution patterns. With longer water–rock interaction, tap mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- 2+ 2+ 2− − tion, and reproduction in any medium, provided you give appropriate waters had higher Mg and Ca, SO and HCO con- 4 3 credit to the original author(s) and the source, provide a link to the centrations relative to dug well and borewell waters. Creative Commons license, and indicate if changes were made. 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