− − This paper reports the results of higher F and HCO concentrations and its response to high pH level in a hard rock terrain in Tamil Nadu, India. About 400 groundwater samples from the study area were collected from a period of four different seasons − − and analysed for F , HCO and other major cations and anions. The key rationale for the higher fluoride and bicarbonate in the study area is the soaring rate of the leaching fluoride-bearing minerals and weathering processes. Fluoride and HCO −1 −1 ranges from BDL to 3.30 mgl and 12 to 940 mgl , its concentrations are lower for the period of SWM and it increases during POM and reaches to a maximum in PRM. Higher dissolution is observed in the NEM season due to rainfall impact. − − Spatial distribution and factor score show that the higher concentrations of F and HCO are eminent in the northern and central zone of the study area due to the impact of lithology. The higher values in pC O versus HCO plot indicate higher residence time which favours more water–rock interactions, which further increase the F concentrations in groundwater. − − HCO is linearly correlated with F which indicates that these ions were consequent from the weathering influences. At the same time, poor correlation of F with pH could possibly be due to the increase of alkalinity follow-on from the swell of 2+ − bicarbonate level with very low Ca that promotes increase in F concentration in the groundwater. Keywords Fluoride · Bicarbonate · Water–rock interactions · Weathering processes · Lithology · India Introduction poor quality of groundwater is attributable to variety of reasons including interaction between water, soluble min- Growing groundwater contamination causes not only the erals, salts, and anthropogenic pollution (Codling et al. deterioration of water quality but also make threats to 2014; Subba Rao et al. 2016). Despite the intricate hydro human health, the stability of aquatic ecosystems, eco- and biogeochemical issues, the quantity of fresh ground nomic improvement and social wealth. The cause for the water is limited. Besides the major components in water * C. Singaraja K. Balamurugan email@example.com firstname.lastname@example.org S. Chidambaram K. Tamizharasan email@example.com firstname.lastname@example.org Noble Jacob Department of Geology, Presidency College (Autonomous), email@example.com Chennai, Tamil Nadu 600005, India G. Johnson Babu Department of Earth Sciences, Annamalai University, firstname.lastname@example.org Annamalai Nagar, Tamil Nadu 608002, India S. Selvam Isotope and Radiation Application Division, Bhabha Atomic email@example.com Research Centre, Mumbai 400 085, India P. Anandhan Department of Geology, V.O. Chidambaram College, firstname.lastname@example.org Thoothukudi, Tamil Nadu 628 008, India E. Rajeevkumar email@example.com Vol.:(0123456789) 1 3 54 Page 2 of 14 Applied Water Science (2018) 8:54 Fig. 1 Location and geology map of the study area + 2+ 2+ 2− − − Generally, fluoride show negative correlations with cal- namely Na, Mg, Ca , SO , HCO , and Cl , the sec- 4 3 – + 3− − 2− cium and magnesium concentrations and positive correlation ondary components include F, K , PO , NO and CO 4 3 3 with bicarbonate concentrations in groundwater (Chae et al. (Chidambaram et al. 2013). Fluoride and bicarbonate 2006; Gao et al. 2011; Subba Rao et al. 2016). Higher levels becomes toxic if it occurs in drinking water, and the maxi- − − of dissolved fluoride are usually related with high pH and mum acceptable limit of F and HCO are 1.5 and 125 to + − −1 Na –HCO type waters, while the temperature and depth of 350 mgl , respectively, as per WHO standards (2004). groundwater can also play vital roles (Chae et al. 2007). Ion Fluorine has unenthusiastic effects on human health when exchange has been found to influence fluoride levels through its intake levels are either too high or too low. High flu - both base exchange, which reduces calcium concentrations oride level can cause negative effects on human health, (Edmunds and Smedley 2005; Chae et al. 2006) and anion such as dental fluorosis, skeletal fluorosis, impaired thy - − − exchange, in which OH in groundwater replaces F on cer- roid function, and lower intelligence in children (Edmunds tain clay minerals or weathered micas (Guo et al. 2007). and Smedley 2013). Alkalinity in natural groundwater is Thermodynamics computation indicated that some of this mainly derived from the dissolution of carbonate min- high fluoride groundwater is supersaturated with respect to erals and CO present in the atmosphere and in the soil fluorite, fluorapatite and hydroxyl-apatite (Singaraja et al. above the water table (Gao et al. 2016a, b). The fluoride 2012; Chidambaram et al. 2012; Li et al. 2015). The overuse in groundwater shows variation with respect to the lithou- of fertiliser on the soil for agricultural activities would also nits of the study area. High concentration of F is seen be the possible source for the high fluoride and bicarbonate in groundwater from hard rock, fluvial as well as alluvial content in groundwater (Singaraja et al. 2012; Gao et al. aquifers, which might be derived from minerals such as 2016a, b). It is also implicit that the similar lithology from micas, apatite, fluorite, soil dust, chemical stimulant, clay the nearby district and furthermore hard rock terrains are and shale (Naseem et al. 2010; Manikandan et al. 2014; usually reported to have high concentrations of fluoride and Li et al. 2015). Hence, fluoride problems are predisposed bicarbonate in groundwater (Chidambaram et al. 2012; Man- to arise in hard rock and sedimentary aquifer where these ikandan et al. 2014). In addition, higher concentrations of minerals are rich in the country rocks. − − F and HCO could possibly come from hornblende biotite 1 3 Applied Water Science (2018) 8:54 Page 3 of 14 54 gneiss (HBG) and charnockites subjected to weathering in hard rock terrain (Singaraja et al. 2012; Manivannan et al. 2010; Subba Rao et al. 2016). In India, several states face problems due to the high − − F and HCO content in groundwater. It is well-known that F and HCO contaminations are present in the ground- water in the various districts of Tamil Nadu State, India. Tuticorin district in Tamil Nadu is one such big area where high concentrations of fluoride and bicarbonate are present in groundwater. However, no major studies have been con- ducted till date in this area. In the present study, an attempt is made to correlate the chemical ions and geological sequence to identify the origin and geochemical processes prevailing in high fluoride and bicarbonate groundwater in Tuticorin district of Tamil Nadu. Another objective of the study is to understand the spatial variations and saturation index of fluoride and bicarbonate minerals in groundwater. Fig. 2 Box plot for the maximum, minimum, and average of the chemical constituents in groundwater during PRM and POM (all val- −1 ues in mg l except pH) 1 3 −1 Table 1 Maximum, minimum and average of the chemical constituents in groundwater representing all four seasons (all values in mg l except EC in µS/cm and pH) 2+ 2+ + + − − − − − 2− Temp °C pH TDS EC Ca Mg Na K F Cl HCO NO PO H SiO SO 4 4 3 3 4 Pre-monsoon Avg 32.35 7.57 1843.33 2878.23 100.54 78.62 424.56 45.55 0.53 922.83 318.88 7.02 0.52 72.55 61.75 Max 35.90 9.20 16731.21 26240.00 1600.00 1248.00 3980.00 520.50 3.30 10812.25 940.20 148.00 12.00 456.00 125.00 Min 27.60 6.80 204.62 308.80 4.00 4.80 14.80 0.50 BDL 35.45 36.60 0.51 BDL 1.00 1.25 Southwest monsoon Avg 29.82 7.83 1587.91 2504.24 109.68 96.31 308.72 32.29 0.45 705.84 160.95 7.22 0.14 70.95 42.01 Max 31.61 9.40 13906.94 21794.00 500.00 895.00 4250.00 293.00 2.70 9052.50 536.80 148.20 1.02 274.00 75.76 Min 28.22 6.80 312.25 461.00 29.00 9.00 10.00 2.00 BDL 35.45 12.20 0.71 BDL 2.00 3.67 Northeast monsoon Avg 28.45 7.33 1369.40 2141.60 86.38 54.99 283.03 38.09 0.47 545.15 316.46 6.74 0.27 48.94 52.19 Max 29.60 8.60 9283.70 14598.00 560.00 296.00 3400.00 196.00 2.90 5276.00 915.00 129.00 4.35 248.00 99.78 Min 27.00 6.00 261.81 417.00 8.00 2.40 20.00 1.00 BDL 35.45 61.00 0.15 BDL 1.43 2.00 Post-monsoon Avg 31.82 7.67 1484.23 2357.86 81.37 50.04 365.27 25.42 0.46 636.04 267.86 4.34 0.21 37.78 12.21 Max 35.80 8.80 19365.68 30200.00 608.00 559.20 6812.00 218.50 2.43 10989.50 683.20 57.00 3.21 324.00 15.20 Min 29.30 6.95 253.92 372.00 4.00 2.40 14.70 0.83 BDL 35.45 50.40 0.23 BDL 1.00 2.00 54 Page 4 of 14 Applied Water Science (2018) 8:54 Fig. 3 Major ion chemistry showing nature of water (fields after Gibbs 1970) entire district during pre-monsoon (PRM), southwest mon- Description of study area soon (SWM), northeast monsoon (NEM) and post-monsoon (POM). The physico-chemical parameters like pH, electrical The study area, lying between latitude 8°19′ to 9°22′N and longitude 77°40′ to 78°23′E with a total area of 4620 Km conductivity (EC), temperature of groundwater were meas- ured in situ using hand-held instruments (Eutech make). The (Fig. 1) is situated in southeast part of Tamil Nadu, India. Geologically, five most important lithounits are present in this F was analysed using the Orion Ion selective electrodes and − − 2− − − HCO by titration method; Cl , SO , PO , NO , H SiO , area, hornblende biotite gneiss, charnockite, quartzite, granite, 4 4 3 4 4 3 2+ 2+ + + and alluvial and fluvial plains (Fig. 1). The sandstone and Ca, Mg, Na and K were analysed using the ion chro- matograph (IC, Metrohm 861). The accuracy of analysis of teri sands form small patches in the study area. Three major rivers such as Tambraparani, Vaiparand and Karamanaiyar major ions was checked by error percentage computation and it was found to be less than 10% (Hem, 1985). In this are draining within the study area. The area experiences a tropical climate with a yearly rainfall that ranges between 570 study, saturation indices (SI) of fluoride and carbonate min- erals were calculated by WATEQ4F (Hammarstrom et al. and 740 mm (CGWB 2009). The V.O. Chidambaram port and hurriedly rising industrial region consisting of major and 2005) software. The correlation and factor analyses were computed using Statistical Package of Social Studies (SPSS) small-scale industries (SIPCOT) are developing in the fluvial region of the study area (Singaraja et al. 2012). version 17 and spatial diagram were constructed using Arc GIS software. Materials and method Results and discussion Sample collection and analysis The maximum, minimum, and average values of vari- ous chemical constituents in the groundwater sam- A total of about 400 groundwater samples were collected covering most of the geological formations representing the ples collected from the study area is shown in Table 1. 1 3 Applied Water Science (2018) 8:54 Page 5 of 14 54 The ascending order of cations and anions concen- in PRM. The concentration of silica ( H SiO ) ranges from 4 4 + 2+ 2+ + −1 trations areas follows: N a > Ca > Mg > K and 1.2 to 125 mgl , higher values are observed during PRM − − 2− − − 3− Cl > HCO > SO > H SiO > NO > F > PO during compared to other seasons. Higher concentration of H SiO 4 4 3 4 4 3 4 4 all the seasons. The pH ranges from 6 to 9.40 indicating in groundwater is chiefly attributable to the weathering of 3− that the groundwater is changing from slightly acidic to alka- silicate minerals (Prasanna et al. 2010). PO concentration −1 line nature. The lowest pH is observed in NEM but high- ranges from BDL to 12 mgl ; higher values are seen dur- 2− 3− est in PRM. EC ranges from 310 to 30,200 μS/cm in the ing PRM. SO and PO in the groundwater may also arise 4 4 study area. In all seasons, higher electrical conductivities from the use of fertilisers. In addition, it can also come from were noted along the coastal region which was possibly due apatite mineral found in HBG and charnockite (Brindha to seawater intrusion (Singaraja et al. 2012). TDS in the et al. 2012; Chidambaram et al. 2012). One of the reasons −1 groundwater samples varies between 205 and 19,370 mgl for the high bicarbonate in the study area is the high rate of and higher concentrations are found during PRM followed the dissociation of H CO (Rafique et al. 2015). HCO con- 2 3 −1 by SWM, POM and NEM. Water temperatures have a wide centration ranges from 1 to 940 mgl , with higher values distribution in all the seasons which ranges from 28 to 36 °C. in PRM samples followed by NEM, POM and SWM. It is 2+ The Ca concentration in the groundwater ranges from 4 attributed to carbonate chemical weathering process (Mon- −1 to 1600 mgl . It is higher in SWM and lower during POM. dal and Singh 2004; Li et al. 2015). It is also noted that the 2+ − The Ca may be derived from calcite, hornblende and pla- higher values of HCO in the groundwater controls the min- 2+ gioclase minerals. Mg concentration ranges from 2.4 to eral dissolution processes (Thivya et al. 2015). Fluoride con- −1 −1 1248 mgl in the groundwater samples of the study area. centration ranges from BDL to 3.3 mgl , with higher values 2+ Relatively higher concentration of Mg is observed in the in PRM season followed by NEM, POM and SWM. Horn- SWM season and lesser during POM season. Higher levels blende biotite gneiss is one of the most important sources 2+ of Mg in groundwater is seen in HBG and charnockite region located along the western part of the study area that 2+ have Mg affluent pyroxene as the predominant mineral 2+ (Chidambaram et al. 2012). Also, the higher level of Mg seen along the coastal zone perhaps may be because of the influence of seawater intrusion (Subba Rao et al. 2012; Sin- garaja et al. 2014). Na concentration ranges from 10 to −1 6212 mgl which is higher in the PRM and lesser in NEM. The main sources of N a in groundwater are hornblende biotite gneiss, charnockite, plagioclase feldspar rock types seen along the western part of the region (Srinivasamoor- thy et al. 2009; Thivya et al. 2013; Venkatramanan et al. 2013) as well as seawater intrusion along the coastal region (Thilagavathi et al. 2014). K concentration ranges between −1 0.5 and 520 mgl with higher values observed in POM. At a few locations, abnormalities are seen irrespective of sea- sons possibly due to weathering and urban landfill leaching (Thivya et al. 2015; Selvam et al. 2016). Cl is the leading ion among anions during all the sea- sons, and its higher values are noted in the coastal zone, may be because of seawater intrusion (Srinivasamoorthy et al. 2008; Chidambaram et al. 2013). Its concentration −1 ranges from 35.5 to 10,990 mgl ; higher values are promi- 2− nent in PRM groundwater samples. SO concentration −1 ranges from 1 to 456 mgl , and the highest concentration is observed in PRM and lesser in POM. Higher concentra- 2− tions of SO mainly derived from evaporate minerals such as gypsum and anhydrite, which are sulphates of magnesium and sodium. It would also possibly migrate to groundwater from salt pan activities happening along the eastern part − − − of the study area (Chandrasekharan et al. 1997). Nitrate Fig. 4 a Variation of F concentrations with HCO and b F concen- −1 ranges from 0.2 to 148 mgl , which is noted to be higher trations with pH in groundwater samples from Tuticorin district 1 3 54 Page 6 of 14 Applied Water Science (2018) 8:54 Fig. 5 Variation of saturation index of different fluoride minerals with dissolved F for samples representing different seasons of F in the groundwater (Chidambaram et al. 2013). The The origin of high fluoride in groundwater lithological influences in the dissolution of fluoride ions in groundwater are studied by Manikandan et al. (2014). The In all seasons, F is found to be positively correlated with higher concentrations of fluoride in groundwater could be HCO with significant correlation coefficient (Fig. 4). due to the weathering as well as leaching of biotite, horn- Maximum values are observed in PRM. Figure 4 shows blende, apatite and mica minerals (Srinivasamoorthy et al. that high concentration of F are observed in both low and 2008; Chidambaram et al. 2013). The seasonal variation high pH conditions that may be attributed to weathering of of fluoride and bicarbonate is shown in Fig. 2. It is seen biotite minerals from HBG. In addition, alkaline pH condi- that most of the samples fall above the median value during tions are favourable for the dissolution of F into ground- PRM, NEM and POM, but there is not much variation dur- water (Chidambaram et al. 2012; Subba Rao et al. 2012; ing SWM. Fluoride concentration beyond the WHO drinking Singaraja et al. 2014). At lower pH conditions, groundwa- water limit is observed during PRM (9%), SWM (7%) and ter containing dissolved HCO in the form of H CO reacts 2 3 + − − NEM and POM (6%). In the case of bicarbonate, majority with the minerals and releases H, F , cations and HCO , of the samples are above the WHO permissible limit like according to the equation given below. PRM (33%), SWM (4%), NEM (37%) and POM (22%). For (Na) (Ca, Mg, Fe) Al Si O (OH) 2 3 2 6 20 4 inferring the dominant controls on water quality, the data + H O + H CO are plotted on Gibbs diagrams (Gibbs1970) (Fig. 3). These 2 2 3 plots suggest that groundwater in the study area is closely → Ca, Mg Fe Al Si O (OH) 3 3 3 5 20 influenced by weathering, although some samples appear to − + + − + F + Na + H + HCO reflect only the influence of evaporation. This finding sug - At higher pH conditions, hydrolysis takes place which gests that the groundwater chemistry is largely controlled by releases (OH) along with cations and fluoride according weathering and precipitation of lower solubility minerals, as to the equation given below. evaporation causes salinity to increase. 1 3 20 Applied Water Science (2018) 8:54 Page 7 of 14 54 Legend Legend PRM HCO3 G SWM P NEM JH P PP PPP JPH J JJPJJ PGJH HH PP JPPJHPG P GPPJJP POM PHHJJ JHPPJHPHP HJPHJJ JHPPPJPPJPH HH HHGJ HP PPP JH HPHHHG PJP HJ PPHHP HJ PPGJJJ H HHP JJGHJPP H PHPGHG PH J GGG HHHPHPG H HJGP P GJ JP H HP G G HPHJJHP J HPHGP JG PHP JP H J J G PGG JGH H PP HHP PGG HJ 80 HGHJ J P HGHH H P G HP J G J P JP PJJH P PJP G HJJ PH G J J PG PGJ J GHH G HGP JG G HGJ G G PH J J PH P G JP J PH J GJ JHG HP G J H GG HP G H H H HGPJ H HHP H JJ J J J HG H P P H G H GP G GJ P G J J J 60 P H H J J G J H GHG JGG GPJ H G P H G H P J H G G G G GP G G J HJJ H JG G G G G G G G J JG J G 40 J G G G J J G K SO4 − + 2− Fig. 7 Relative HCO , K and SO contents of waters of the study 3 4 area higher dissolution and dispersivity of F is observed in the NEM season due to the impact of rainfall. Fluoride‑bearing minerals The association of F ion to the saturation index (SI) of dif- Fig. 6 a Log pCO versus HCO and b Log pCO versus pH relation- 2 2 ferent F bearing minerals like fluorite [CaF ], fluorapatite ship of groundwater samples for different seasons [Ca (PO ) (OH)] and hydroxyl-apatite [Ca (PO ) F] are 5 4 3 5 4 3 studied. The calculated saturation indices (SI) values for flu- orite minerals and log of dissolved concentration of fluoride (Na) (Ca, Mg, Fe) Al Si O (OH) + H O 2 3 2 6 20 4 2 are shown in Fig. 5. Most of the groundwater samples were → Mg Fe Al Si O (OH) 3 3 3 5 20 saturated or over-saturated with respect to the dissolved con- − − + 2+ 2+ − + OH + F + Na + Ca + Mg + HCO centration of fluoride. Initially, SI increases simultaneously − − with the F concentration, until F concentration reached a These equations indicate that elevated HCO contents value of 6 due to long residence time of water–rock interac- favour F release from the hard rock aquifer matrix into tion which lead to the release of F from fluorine-bearing the groundwater. The HBG and charnockites present in the minerals (Rafique et al. 2015; Li et al. 2015). Saturations study area contains mica, quartz, calcite and feldspar. The show fluorapatite dominance followed by hydroxyl-apatite F present in the mineral will be desorbed into the ground- and fluorite. F when released into the system, it readily water ,when the HCO concentrations are high because adsorbs on to the apatite and makes it fluorapatite, later the of HCO which is a strong competitor for sorption (Gao − − (OH) in the hydroxyl-apatite are preferred by F and at the et al. 2011). In few samples at high pH, OH could release − − end, a separate compound namely fluorite is formed. There F ions and result in the sorption of F into the aquifers is a clear linear relationship between F concentration and SI (Jacks et al. 2005). The release of high concentration of of fluorite irrespective of the seasons (Edmunds and Smed- fluoride into groundwater at higher pH ranges may also be ley 2005; Rafique et al. 2015). due to anthropogenic sources. − − Generally, it is seen that the HCO and F concentrations Relationship between pCO and HCO are lesser during SWM and it increases during POM and 2 reaches to a maximum in PRM and then they get diluted The pCO values in the groundwater ranges from − 4.19 to during NEM (Fig. 4). This leads to the interpretation that 2 − 1.43 during PRM, − 4.43 to − 1.43 during SWM, − 3.14 to 1 3 80 54 Page 8 of 14 Applied Water Science (2018) 8:54 Fig. 8 Variation of saturation index of different carbonate minerals with HCO for samples collected during different seasons − 0.38 and − 3.53 to − 0.89 during NEM and POM seasons, respectively, indicating an increase in pCO with increase in bicarbonate (Fig. 6). The increase is found in all the ground- water samples irrespective of season. The higher values of pCO also indicate longer residence times (Prasanna et al. 2010). Water with a high pC O of around − 2 indicates deep circulation of groundwater with a lesser amount of atmos- pheric contact or higher saturation of carbonate minerals. This results from the interaction of minerals present in the rocks through which it flows (Ayoob and Gupta 2006; Chid- ambaram et al. 2012). It is also evident that log pC O values linearly decreases with pH indicating fresh water recharge or less atmospheric interaction with the system (Chidambaram et al. 2013). The higher values of log pC O coincide with the lesser values of pH indicating rock–water interactions. The pH increases from 6 to 9.4 (Fig. 6), this progressive increase of H concentration and log pCO values indicate less inter- action of the groundwater system with the atmosphere. The ion exchange reactions occur by the exchange of H ions in the host rock. In all seasons, the groundwater remains as HCO type. From the Fig. 7, it is clear that there is a shift + + 2+ − − from Na + K to Ca and Cl to HCO type (Manivannan et al. 2010; Singaraja et al. 2014). These imply that higher pCO leads to higher bicarbonate concentrations in the groundwater, which in turn enhances the release of F due − − Fig. 9 a F and b HCO versus geology for different seasons in the to higher rock–water interactions under long residence time. study area 1 3 Applied Water Science (2018) 8:54 Page 9 of 14 54 Fig. 10 Spatial distribution of F during different seasons SI of carbonate minerals of all carbonate minerals increases with increase in HCO during all seasons, due to increased groundwater recharge Saturation index for minerals such as magnesite and subsequent dilution of chemical constituents present in groundwater. They form near saturation to saturation state. (MgCO ), aragonite (CaCO ), calcite (CaCO ), dolomite 3 3 3 (CaMg(CO ) ) were calculated. Saturation index of carbon- Excess of HCO may result in stoichiometric dissolution 3 2 3 of calcite and ion exchange (Gomez et al. 2006). A few ate minerals are plotted against the concentration of HCO −1 (in mgl ) (Fig. 8). The saturation state of carbonate miner- samples show under saturation, which may be due to non- 2+ 2+ availability of cations (Ca and Mg ) which might have als are in the following orders: SI > SI > SI > SI during C A D M PRM and NEM. SI > SI > SI > SI during SWM and been removed from the aqueous system due to the process D C D M of cation exchange (McNab et al. 2009; Li et al. 2015). The SI > SI > SI > SI during POM season, respectively. SI C D A M 1 3 54 Page 10 of 14 Applied Water Science (2018) 8:54 Fig. 11 Spatial distribution of HCO during different seasons carbonate minerals generally attain saturation when the con- Ca Mg(CO ) , which results in the increase of SI in the 3 2 Do − −1 centration of HCO reaches nearly 300 mgl . It is interest- saturation state than the other carbonate minerals. The satu- ing to note that the majority of samples attain saturation ration of magnesite requires 2 Mg atoms to form an MgC O after this concentration, which may be due to the low flow compound. So, the first available free magnesium tends to conditions with high pCO value and extent of weathering join with existing calcite to form dolomite or it forms new by the parent material (Rosa Cidu and Luca Mereu 2007). dolomite. It may due to overall precipitation reaction that It is also appealing to the order of dominance that are under proceeds as below (Davis et al. 1997). saturation is SI > SI and Si < SI but SI < SI and Mg Do C Ar Mg Do 2+ Si > SI are the state of saturation. It may be due to the 1. CaCO + Mg → CaMg CO C Ar 3 3 preferential addition of Mg to CaCO and formation of new 1 3 Applied Water Science (2018) 8:54 Page 11 of 14 54 Relationship in support of fluoride 2+ 2+ 2− 2. Ca + Mg + 2CO → CaMg CO and bicarbonate concentration Factor analysis − − Using factor analysis, four factors were taken for PRM, SWM Occurrence of high F and HCO groundwater and POM seasons and five factors for the NEM (Table 2). Liu et al. (2003) classified the absolute factor loadings values The order of dominance of average fluoride concentration in as 0.3–0.5 (weak), 0.5–0.75 (moderate) and > 0.75 (strong), various lithounits in all the seasons is as follows: hornblende respectively. Fluoride concentration signifies as the third biotite gneiss > char nockite > alluvial plain (r iver depos- factor during PRM with 10.82 percentage of variance (PV). ited) > fluvial plain (coastal deposited) > quartzite > granite SWM results reveal that the second factor with 12.93 PV is (Fig. 9). The bicarbonate dominance during all the seasons − − represented by F . The F factor is represented as the fourth is as follows: hornblende biotite gneiss > charnockite > flu- factor with 9.38 PV during NEM and again as second factor vial plain (coast deposited) > alluvial plain (river depos- − − by 12.16 PV during POM. High F and HCO groundwater ited) > quartzite > granite (Fig. 9). Lithologically, the highest − − at Tuticorin District displayed distinctive major ion chemis- F and HCO contents in the groundwater occurred in the − − try. Generally, higher loading of F and HCO with relatively hornblende biotite gneiss areas of the study area. The spatial − − negative or low loading of other ions (Table 2), particularly variations of the F and HCO concentrations are presented 2+ 2+ 2+ 2+ 2− Ca, Mg, Na, K , pH and SO concentrations may in Figs. 10 and 11. Physiographically, the highest F and be due to increasing weathering process with dissolution of HCO contents in groundwater occur in the northern part of fluorine-bearing minerals and the desorption of exchangeable the Tuticorin District due to HBG and charnockites geology F from the loess. Long-term water–rock interactions may lead (Manivannan et al. 2010; Manikandan et al. 2014). In the to the release of F from fluorine-bearing minerals (Rafique northern part of the Tuticorin District, the groundwater flow et al. 2015; Gao et al. 2016a, b). In this area, hard rocks, such paths are longer and hence will have longer residence time as HBG and charnockite, contained abundant fluorine-bearing and higher degree of rock–water interactions. This leads to minerals, including fluorite, biotite, hornblende and apatite higher concentrations of most of the elements in the ground- − − (Singaraja et al. 2014). As one of the major u fl orine-bearing water including F and HCO . In other parts, the groundwa- natural minerals, fluorite dissolves rapidly under natural condi- ter flow paths are comparatively shorter, residence time is tions (Fig. 5). Because the fluorine-bearing minerals are spar - lower and there is a less time of contact between water and − − ingly soluble, the presence of high F concentrations in the minerals. It is also noted that F and HCO concentrations in groundwater require a long residence time, which is possible in alluvial and fluvial plain are typically low. But, higher con- the northern parts of Tuticorin District (Singaraja et al. 2015). centrations of F observed in these parts may be attributable to the mud, dust, industrial discharge, chemical fertilisers as well as the sources of clays, biotite and apatite at higher rate of evaporation (Subba Rao 2009). Significant HCO concen- Table 2 Summary of fluoride represented factors during different trations are noted in the western and the eastern parts along periods of the study the alluvial and fluvial region. The source of HCO in non- Seasons PRM SWM NEM POM calcareous aquifers are derived by two different processes, one is when CO mixes with water, it forms carbonic acid 2 Total number of factors 4 4 5 4 which decreases the pH of groundwater Fluoride Factor 3 2 4 2 pH 0.17 0.29 0.10 0.04 CO + H O ↔ H CO ↔ HCO . 2 2 2 3 2+ Ca 0.02 − 0.06 − 0.01 − 0.08 2+ Mg − 0.01 0.04 0.06 − 0.05 Second, the bicarbonate is derived when CO dissolves Na 0.02 − 0.09 0.00 0.08 in water. This forms HCO , which is a pH buffer. Hence, K 0.08 0.08 − 0.07 − 0.02 groundwater with high HCO concentrations will have rela- Cl − 0.02 − 0.09 − 0.06 0.00 tively high pH HCO 0.80 0.84 0.55 0.81 − + − NO − 0.03 − 0.04 − 0.10 0.07 CO + H O ↔ H CO ↔ HCO + H . 2 2 2 3 PO 0.02 0.14 − 0.05 − 0.07 Also, it is noted that the spatial distribution of HCO 3 − F 0.85 0.80 0.92 0.81 coincides with that of F concentration in the study area. 2− 0.17 0.20 0.20 0.10 SO H SiO − 0.06 − 0.06 0.08 0.22 4 4 1 3 54 Page 12 of 14 Applied Water Science (2018) 8:54 − − Fig. 12 Spatial distribution of F and HCO factor score during different seasons concentrations allowed additional fluorite to dissolve, further Particularly, in all the seasons, HCO is positively correlated − − with F and higher factor scores are established in northern increasing F concentrations in groundwater. Hence, cation exchange promotes the release of F from the groundwater at part of the study area suggesting that the minerals dissolve in the groundwater (Chidambaram et al. 2012; Singaraja et al. Tuticorin District (Singaraja et al. 2016). Agricultural activity is unlikely a major source of F in the study area, given the 2013). In addition, pH and F is weak correlation as increase − − of alkalinity is attributable to the swell of bicarbonate ions, insignificant correlation between F and NO concentrations in groundwater (Chatterjee and Mohabey 1998; Singaraja these ions control on pH values ,Hence, the alkalinity inversely relationship of the pH level during all the seasons (Subba Rao et al. 2015). Figures 10 and 11 clearly indicate that elevated − − − HCO contents favoured F release from the aquifer matrix et al. 2003; Singaraja et al. 2014). F and HCO concentrations 3 3 2+ − − have negative loading with respect to calcite and lower Ca into groundwater. Spatial distribution of F and HCO values 1 3 Applied Water Science (2018) 8:54 Page 13 of 14 54 Open Access This article is distributed under the terms of the Crea- corresponds to the hydrogeochemical active zone (Fig. 12) tive Commons Attribution 4.0 International License (http://creat iveco resulting from the factor scores, possibly due to the migration mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- of these ions from the similar source. tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Conclusion The above study provides evidence on the major difference References in the chemical composition of groundwater with respect to F and HCO concentration during all the seasons. 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