A hydrochemical approach to estimate mountain front recharge in an aquifer system in Tamilnadu,India

Banaja Rani Panda?S.Chidambaram?N.Ganesh?V.S.Adithya?M.V.Prasanna?K.Pradeep?U.Vasudevan

1 Introduction

The MFR is the contribution of mountains to recharging aquifers in adjacent basins along a MF.There are several techniques for the successful determination of MFR in different regions.Mohrbacher(1983)used stable isotopes and groundwater chemistry as indicators of MFR.Andrew et al.(2003)used noble gases to investigate MFR.Ping et al.(2014)have quantified groundwater recharge using the chloride mass balance method in a semi-arid mountain terrain,in South Interior British Columbia,Canada.Kambhammettu et al.(2011)evaluated three MFR estimation techniques using a numerical modeling in the Southern Jornada Del Muerto Basin,New Mexico.Table 1 lists the various studies that trialed a hydrogeochemical approach to understanding MFR.

Several previous investigations that have been undertaken in the Courtallam region but only on groundwater potentiality and fluctuation(Balasubramanian and Sastri 1994;Public Works Department(PWD)2002),quality and suitability of groundwater for domestic and irrigation purpose(Subramaniet al.2005a),and mineral composition and occurrence of various rock formation(Subramani et al.2005b).But there has been no attempt to understand the MFR either by using hydrogeochemistry or by other techniques in this region.This is the first attempt on groundwater recharge estimation using hydrogeochemistry and geostatistical tools along the foothills of Courtallam.

The main objective of our study is to estimate MFR by interpreting relevant hydrogeochemical data. Thisincludes interrelationships among a set of ions,which may vary with land-use pattern and lithology,the paper also focuses on the use of factor scores to identify hydrogeochemically active regimes represented by the major factors.This study further aims at geochemical evaluation of the aquifer system based on ionic ratios,water chemistry,hydrochemical facies,and factors controlling groundwater quality.

Table 1 Various researches that trialed a hydrogeochemical approach to understand MFR

2 Study area

Courtallam is located in the northwestern part of Tirunelveli District,Tamilnadu,with an aerial coverage of 1789 km2(Fig.1).The area forms a pedimented plain of the Chittar River,which is a major tributary of the Tamiraparani River,and extends south and north of Courtallam forming a part of the Chittar,Vaippar,and Tamiraparani rivers basins.The Chittar River arises in the Courtallam hills in the study area,moves along the Tenkasi and Tirunelvelli to a distance of 60 km,and then joins with the Tambramparni River at Sivalaperi.The Jambunadi River arises in the Kulratti hills and flows in the southern part of the study area.There are several waterfalls in this region,which attract tourists and results in urbanization and increased water demand.

The area is drained by river Chittar,which originates near Courtallam with fivebeautiful fallsin Poigaimalai hill ranges of Western Ghats(Subramani et al.2005a).There are nine waterfalls in Courtallam viz.,Main falls,Chitraruvi,Shenbagadevi falls,Honey Falls,Five Falls,Tiger Falls,Old Falls,Orchard Falls,and the New Falls.Its bracing season extends particularly from June to September and contributes water to respective streams/river flowing in this region.Chittar River is considered to be non-perennial since it carries water only during the months of November and December(Subramani et al.2005a),and during thenon-monsoon periods,thegroundwater servesas the major source.A thin layer of alluvial floodplain deposits,mainly sand,is found along the banks of Chittar River.Borehole Lithology recordsreveal that the thickness of alluvial depositsismorein bajada and valley fills(about 10–15 m)(Subramani 2005),which provides more chance of recharge.

Fig.1 Index map of the study area

Geologically,the area is occupied with hornblende biotite gneisses and charnockite,composed of feldspar with quartz grains,biotite,and hornblende(Fig.2).These formations are crossed by sets of joints and fractures,which have also resulted in the weathering of the coarser rocks.Weathering occurs due to mechanical and chemical processes that take place while water in the fractures interacts with the formation(Singh et al.2003).The distribution of the weathered zone varies from place to place within the basin (Subramani 2005).The weathered zone facilitates the movement and storage of groundwater through a network of joints,faults,and lineaments,which form conspicuous structural features.Another dominant formation along the foothill is charnockite.This formation is less weathered,jointed or fractured compared to the previous one(Subramani 2005)and could,therefore,be considered as impermeable and increases the residence time of water resulting in higher rock-water interaction.

Rainfallplaysanimportantroleinrechargeprocess.Dueto topographic variations,mountainsreceivemoreprecipitation than the basin floor with asignificant fraction in theform of rain.Slopesof thisregion vary fromflatland to vertical cliffs.Courtallam isa hilly region whose height variesfrom 150 to 1500 m with narrow valleysendowed with steep slopes.The surface water from a rain storm flows rapidly down a steep hillside and have small rates of infiltration into the soil or bedrock.On theother hand,with gentlesloping or flat areas,thesurfacewater doesnot flow rapidly and thereisagreater opportunity for the water to infiltrate downward towardsthe water table(Rushton and Ward 1979)in thepediplain region and along the riparian region.Moreover,the recharge in the foothill also moves laterally along the groundwater flow direction,which adds to the increase of water level in the riparian region.The region receives an average rainfall of 917.88 mm/year as reported by Indian Meteorological Department,(IMD),which is normal.About 70%of total rainfalliscontributedbythenortheastmonsoonandrestbythe southwestmonsoonandrainfall duringthetransitionalperiod.The geomorphology,geology and structural geology of the region control the recharge and discharge of groundwater in thisregion(Salamaet al.1994).

Fig.2 Lithology with sample location points

The groundwater is contained in different aquifers of this region and their characteristics are summarized in Fig.3.The transmissivity,hydraulic conductivity,specific yield,and storativity,are maximum in porous formations followed by fractured and weathered formations.Minimum valuesalso show thesametrend irrespectiveof all thethree formations.Aquifer characteristics vary considerably depending upon lithology,topography,and nature of weathering(CGWB 2009).

The depth to water table is more at MF as compared to riparian zone(Fig.4).The AMSL map shows that the gradient is towards the southeastern part of the study area.Mountainous regions act as runoff zones due to steep gradient whereas slope of peneplain is comparatively gentle thus water is recharged,and depending upon this recharging process with respect to seasonal rainfall events,there is variation in water level(Bhagyashri et al.2011).Pediplains are very good for groundwater prospecting because they reduce the velocity of surface runoff and thus provide more chance of water accumulation(Yousef et al.2015).Thus,when the velocity of the flowing water is reduced asit reachesthe foot of mountain slopeand it starts recharging.The extent of vertical and lateral recharge depends on the thickness and extent of the aquifer(Theis 1940).

Fig.3 Aquifer parameters(CGWB 2009)

Fig.4 Groundwater flow direction of the region

Groundwater generally occurs under phreatic conditions in the weathered mantle and under semi-confined conditions in the fissured and fractured zones at deeper levels(CGWB 2009).The thicknessof theweathered zonein this region isnot uniform and variesbetween 8 and 34 m below ground level(Subramani et al.2009).In the study area,depth to water table varies between 2.1 and 16 m below ground level during pre-monsoon and between 0.91 and 16 m below ground level during post-monsoon.The water table rises during November and December,attains its maximum at January,and starts declining from February onwards and attains its deepest level during September or October(Subramani 2005).A long-term fluctuation is observed by CGWB(2009)for the period of 1991–2007,which shows that water level rises and falls within a range of 0.0021–1.1284 and 0.016–0.689 m/year,respectively.

3 Methodology

To understand the process of MFR by hydrogeochemical studies,53 groundwater sampleswerecollected during premonsoon period from hand pumps to cover different lithounits of the study area.As the study area was comprised of hilly terrains,it was difficult to collect samples for the entire area.Thus,some places were left without sampling.The water sampleswere measured for major ions such as Ca,Mg,Cl,HCO3by titration method,Na and K by flame photometry(CL 378),and SO4,PO4,NO3and H4SiO4by spectrophotometer(DR 6000).Physical parameters like pH,total dissolved solids(TDS),temperature,humidity,and conductivity were analyzed using a portable water analysis kit.The analytical precision of the measurements of cations and anions were determined by calculating the ionic balance error that varies by about 5%–10%(Freeze and Cherry 1979).

Geostatistical techniques simplify and organize large geochemical datasets into meaningful information.Multivariate statistical analysisisa quantitative and independent approach of groundwater classification,and it allows grouping of groundwater samples using correlations between chemical parameters in the samples.In this study,the Statistical Package of Social Sciences(SPSS)version 10 was used for correlation analysis and factor analysis.

Major cation and anion data obtained from the laboratory analysis were used as input data for factor analysis.New groups of variables were generated from the initial dataset,which isalinear combination of original variables.An Eigenvalue of 1 was taken as a minimum for factor extraction(Kaiser 1958;Harmann 1960).The first factor obtained describes the largest part of the variance.The following factors explain smaller partsof the data variance.The factor loadings matrix was rotated to an orthogonal simple structure according to the Varimax rotation technique(Davis 1973).Higher factor loadings(close to 1 or-1)describes strong relationships(positive or negative)among variables.Factor scores are calculated for each sample and are plotted in spatial diagrams.Spatial diagrams were drawn using Map Info software(Professional 8.5)combined with Vertical Mapper.Extreme positive factor scores(＞0)reflect where the sample ismost affected whereas negative factor scores reflect the samples least affected by the chemical processes.

4 Results and discussion

4.1 Hydrogeochemistry

Table 2 lists the maximum,minimum,and average values of different parametersin the groundwater as well as WHO(2004)standards for drinking water.In general,most of the samples of this region have high EC values which cover a major portion of the area.High EC values are noted in the western,southern,and central part of the study area.A sharp increase of ECin this region could have been caused by the factorslike land use patternssuch asagricultureand waste.High chloride,nitrate,and phosphate contentscould have been due to the association with agricultural activities,which are known to increase EC(Panabokke et al.2002;Jeyaruba and Thushyanthy 2009).

The groundwater in this region is slightly alkaline(Table 2).Thepermissiblerangefor thepHin groundwater is 6.5–8.5(WHO 2004)and all sample values fall within this range(Table 3).TDS gives the general nature of groundwater quality and extent of contamination(Davis and de Wiest 1966).The maximum concentration of TDS was found in the location named Panaiyur,located in the northernmost part of the study area.Almost all sample values fall below the permissible limit for the TDS(WHO 2004).Only four sample values have a higher concentration of TDSabove the permissible TDSlimit for drinking water(Table 3).

The highest concentration of HCO3is also noted in Panaiyur(Table 4).This may be due to contributions from carbonatereleased during theprocessof weathering.About 21 sample values fall above the permissible limit(WHO 2004).In the central part of the region,samples have the highest concentration of Cl.Almost 37 samples fall above the permissible limit(WHO 2004),signifying the anthropogenic activities like infiltration of municipal and domestic sewage into the aquifers and other problems like sewer leakage,faulty septic tank operation,and land fillleachates(Panno et al.1999a)could be the major sources of chloride.The highest concentration of SO4is found at Panaiyur(Table 4).All the sample values fall below the permissible limit of drinking water.The sources of this ion may be due to the impact of fertilizersasthe study areawas near the agricultural region(Chidambaram 2000).

Table 2 Maximum,minimum and average of thechemical constituents in groundwater(All valuesarein mg/L except ECin μs/cm,Temp in °C and pH)

NO3in water is mainly due to organic sources or from industrial and agricultural chemicals(Feth 1966).Almost 99%of the sample values exceeds the permissible limit of NO3(WHO 2004)and themaximum concentration isnoted at Virasikamani(Table 4).Elevated concentration of NO3is noted in the northern and southwestern part of the study area,which may be due to fertilizer used in agricultural fields(Prasanna 2008),organic effluents of nitrogen fixing bacteria,leaching of animal dung,and sewage and septic tank leakage through soil and water matrix to groundwater(Chidambaram 2000).

The highest concentration of H4SiO4is found at VeddakkuPudur(Table 4).Likewise,the highest phosphate concentration of groundwater samples at 0.52 mg/L was noted at Tenkasi.The maximum concentration of Na+is noted at Sendamaram(Table 4)and 14 samples fall above the permissible limit.The high concentration of sodium in the study area indicatesthe influence of weathering.As the major rock types present in this area are charnockite and hornblende–biotite-gneiss,the Na in groundwater is derived from the plagioclase feldspar present in the underlying rock type.(Drever 1988;Chidambaram et al.2008).

Calcium(Ca)is maximum at Pattanadinpatti.22 sample values exceed the permissible limit.The area is underlain by charnockite and hornblende-biotite-gneiss,which could be the major source to contribute Ca in the groundwater (Thivya et al. 2015). The highest concentration of Mg is noted at Panaiyur(Table 4)which is the northernmost part of the study area where charnockite is the major rock type and mostly contains Mg-rich pyroxene.Thus,it contributes Mg to the groundwater(Thivya et al.2015)and 19 samples fall above the permissible limit(WHO 2004).The maximum value of K was located around Velayudapuram,35 samples fall above the permissible limit.K concentration in this area may be derived from the weathering of K-Feldspar present in charnockite and hornblende–biotitegneiss formation(Thivya et al.2015).

From the Fig.5,it was observed that the NO3concentration is more along the foothills.That means aquifers were recharged by the sewage from anthropogenic activities(Thivya et al.2013).In the southwestern part,weaker ionic strength with higher NO3and in northwestern part both high ionic strength and NO3was noted.High ionic strength indicates the lesser rate of water flow through the pore spaces in the hard rock terrain and higher residence time(Mohrbacher 1983).Weaker ionic strength indicates more fresh water recharge through the pediplains.It can be seen that river flows in this region are reflected in the varied ionic strength and recharge events.Thus,the maximum contours of all the ions fall along the foothills with high ionic strength(Fig.5)which is an indirect indication of MFR.High HCO3along foothills also indicates predominant weathering as well as recharge(Stumm and Morgan 1996).

4.2 Hydrogeochemical facies

The hydrogeochemical facies concept was developed for identification of water type and visualization of trends of hydrochemistry(Smoor 1967;Schreiber et al.1999).Thus,the interpretation of hydrogeochemical facies is used to determine the flow pattern and chemical history of groundwater(Piper 1944;Nwankwoala and Udom 2011).The study shows the existence of four groundwater facies(Fig.6)i.e.,(i)Na–Ca–Cl–HCO3,(ii)Na–Cl–HCO3,(iii)Ca–Na–Cl–HCO3,(iv)Na–Ca–HCO3–Cl.

Table 3 Analytical values of all chemical constituents in groundwater(All values are in mg/L except EC in μs/cm,Temp in °C and pH)

Table 3 continued

Table 4 Locations with highest concentration of geochemical parameters

Fig.5 Spatial distribution of ionic strength along with integrated contour line of higher concentration of(Na+,HCO3,Ca2+,Mg2+,H4SiO4,K+and NO3-ions)

The Na–Ca–Cl–HCO3facies is predominant along the foothills,where the underlying rocks are known to contain hornblende[Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2],biotite,quartz(SiO2),and plagioclase feldspar(NaAlSi3O8–CaAl2Si2O8).The water type must have evolved from the interaction of these minerals.This water type is also notedin residential areas and the area of intense agricultural activities(Fig.7).Thus,it may have evolved either due to weathering or by anthropogenic activities.

Fig.6 Piper plot

Infiltration of aquifers by municipal and domestic sewage from urbanized area as well as chemicals used in agricultural lands leads to preferential leaching of high soluble surface and near surface soil Na–Cl salts,which are completely dissolved during resolution(Akiti 1980).Subsequent evaporative concentration may also cause the less soluble salts to precipitate out of solution resulting in the percolating water becoming enriched with Cl(Drever and Smith 1978).The concentration of Ca decreases,however Naincreasesdueto theionic substitution of Ca+by Na+as shown in the following equation:

Here X is considered to be a soil exchanger,and when groundwater gets mixed with Na(derived from plagioclase feldspar),it isprecipitated and the Ca+ion ispushed away by Na+ion,thus Na+becomes dominant in the groundwater by ion exchange process(Kumar and Allapat 2005),thus the influence of anthropogenic and ion exchange process leads to the Na–Cl water type.

The complete dissolution of Na–Cl may also cause the Ca–HCO3to precipitate out of solution and resulting in the percolating water becoming enriched with Ca–HCO3found in near stagnant water bodies(Wotany et al.2013).Considering the lithologic formations present in the study area,hornblende[Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2],biotite,quartz(SiO2),and plagioclase feldspar (NaAlSi3O8–CaAl2Si2O8)arethechief minerals.Thus,Caconcentration may increase in groundwater either due to the addition of more Ca ions,leached out from the main rock types(i.e.charnockite and gneiss),or when the Na ion is substituted by the Ca ion on the solid surface by ion exchange process.

Na–Ca–HCO3–Cl water type generally dominates in the northeastern and southeastern side of the study area and may be due to water–rock interaction or weathering process.The weathering of minerals results in higher concentration of HCO3ions.Thus,it is interpreted that this water type is dominated by the weathering process.The four water types define three evolutionary pathways of dominant facies occurring along the groundwater flow direction of the study area:(1)Ca–Na ion exchange,(2)increase of Ca content either by the addition of Ca ion or Na–Ca ion exchange process and(3)predominate weathering process(Fig.8).Thus,the weathering process dominates over ion exchange as well as anthropogenic processes.The predominance of weathering process indicates significant recharge along the foothills(Ophori and Toth 1989;Subba et al.2007;Srinivasamoorthy et al.2008).

Fig.7 Land use map along with hydrogeochemical facies points of the study area

Fig.8 Three evolutionary pathways of dominant facies

The spatial distribution of ionic strength along with hydrogeochemical facies and lithology(Fig.9)shows that almost all the water collected from the wells along the foothills are of the Na–Ca–Cl–HCO3type.However,the sample values along the northwestern side have weaker ionic strength compared to those on the northern side,which states that water type remains the same but the ionic strength varies because the Piper plot only describes the percentage of major ions,but other minor ions dissolved in solution may be the reason of this varied ionic strength.

The Na–Cl–HCO3water type,noted along the contact between charnockite and gneiss(Fig.9)and near the river flood plain,indicates that it may have evolved from interaction with the albite-rich gneissic rock or with charnockite as a high amount of Na+is equally present in both rock types.At this stage,it is too early to infer whether Na+is derived from gneiss or charnockite.But higher ionic strength indicates the leaching of ions and river water recharge(Thilagavathi et al.2012;Prasanna et al.2010).

Lithology of the study area is mainly represented by feldspar(CaAl2Si3O8),hornblende[Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2],but biotite,pyroxene,and quartz(SiO2)are also abundantly present in the rock types.Ca–Na–Cl–HCO3water type found distributed along the higher ionic strength region indicating the longer flow path(Mohrbacher 1983),but it may be derived from the gneissic rock or charnockite.The Ca content is higher in gneissic rocks as compared to charnockite(Rajesh and Santosh 2012).The charnockite consists predominantly of minerals like quartz(SiO2),microcline(potash-rich),and Mg-rich pyroxene(hypersthene),but its Ca-bearing minerals are minor.Thus,this Ca-rich water type may have been derived from the interaction with a gneissic rock.

The spatial distribution of Ca(Fig.10)shows that Ca is high in the southern part of the study area,which isunderlain by gneissic rock only.Thus,this could be the reason for Ca-rich water in this region.Also,the high ionic strength suggests that water may have interacted with the hornblende–biotite-gneiss for a long time.Long flow path(Mohrbacher 1983)from high elevations affords more time for water to equilibrate with gneiss than a short flow path from lower elevations and weaker ionic strength.

Fig.9 Spatial distribution of Ionic Strength along with Facies points and lithology

Fig.10 Spatial distribution of Calcium in the study area

Fig.11 Spatial distribution of log pCO2 with hydrogeochemical facies

Figure 11 shows a sharp increase of pCO2values towards the foothills region,which reveals that the atmospheric interaction has become lesser and recharge predominant.High values of pCO2indicate high residence time(Prasanna 2008).It suggests that the water type has been recharged by acquiring longer flow path during its infiltration into deeper level near the foothills.A sharp decrease of pCO2values noted along the other regions reveals that there is more interaction of the groundwater system with the atmosphere(Chidambaram et al.2011).Thus,it can be inferred that the water type has been acquired a shorter flow path,and recharged from a lower elevation.

Figure 12 infers that,in most of the samples,the groundwater has an excess of HCO3and falls between 0.1 and 0.5.The HCO3is derived mainly from the CO2of the soil zone,during weathering of minerals in country rocks The decay of organic matter and root respiration contributes high CO2pressure to the sub-surface soil zone,when it combined with rain water(H2O),it forms HCO3(Subba and Surya 2009)as shown in the equation:

The HCO3can be derived from the silicate minerals dissolved by H2CO3as shown in the following equation

The value of Ca2++Mg2+versus HCO3(Fig.12)depicts that HCO3ion found at an elevated amount in the groundwater,which is balanced by Na+ion,as a silicate weathering process is mainly responsible for releasing the Na+and HCO3ions into the groundwater system(Subba and Surya 2009).In Fig.12(group A),few sample values also show Ca and Mg isthe dominant cations either due to the preferential release of Ca,Mg from the mineral weathering of exposed bed rock or cation exchange reaction(Thilagavathi et al.2013).In this case,a high concentration of alkalis(Na+and K+)in their halide solution replaces Ca2+and Mg2+on the surface of clay minerals in aquifer matrix.Due to ion exchange process,Na+concentration decreases which is accompanied by an increase of Ca2+and Mg2+concentration in solution(El-Sayed et al.2012).

Fig.12 Scatter plot of(Ca+Mg)/HCO3 verses(Na+K)/HCO3

Alkaline earths(Ca2+and Mg2+)in their sulfates and part of their carbonates in solution replace the alkalis(Na+and K+)on the surface of clay minerals in the aquifer matrix.Thus,ion exchange process increases the Na+ion concentration and decreasesthe Ca2+and Mg2+in solution(El-Sayed et al.2012).This could be the reason for dominance of Na+and K+in few sample values(Fig.12,group B).Carbonate minerals get dissolved in water during its flow and increase the Ca2+concentration.This hike in Ca2+concentration reflects the release of carbonate during the process of weathering whereas few sample values show an increasing trend of Na+concentration in all the formation,which may be due to alkali mineral weathering or ion exchange process(Thilagavathi et al.2013).

Hence it can be inferred that weathering is the major process which controls the chemical composition of groundwater as most of the sample values have higher concentration of HCO3-(Stumm and Morgan 1996),which also indicates recharge(Ophori and Toth 1989;Subba et al.2007;Srinivasamoorthy et al.2008).

Figure 13 shows that the southwestern,northwestern,and central part of the area has high values of H4SiO4/HCO3,which indicate predominant silicate weathering.This part of the study area is underlain by charnockite.Quartz and feldspars(potash feldspar,soda feldspar)are two major minerals in charnockite.Thus,weathering of soda feldspar(albite)and potash feldspars(orthoclase and microcline)may contribute silicate to groundwater(Thilagavathi et al.2013).Feldspars are more susceptible to weathering and alteration than quartz in silicate rocks(Manish et al.2009).Carbonate dissolution also leads to silicate weathering(Ramesh and Gowri 2012),so less carbonate and more silica infiltrated into the aquifer results in silica-rich groundwater in this region.Besides these,other parts show dominant carbonateweathering due to the release of HCO3during the process of weathering(Thilagavathi et al.2013),which is explained in the following equations:

Fig.13 Spatial distribution of mole ratio of H4SiO4/HCO3

When plagioclase feldspar weathers to kaolinite,the HCO3content increases in water with increasing anorthite content as shown in the reaction:

The presence of CO2in the reactant side of the above equation signifies both bicarbonate and silica are yielded by same minerals in a different environment.The bicarbonate yielded is from dissolved CO2in groundwater(Thilagavathi et al.2013).Silicate minerals weathering also yield to HCO3in groundwater as a product irrespective of parent minerals composition.Opaline silica releases,in the same reaction,depend upon pH condition.Higher Ca2+and Mg2+values indicate the dissolution of silicate minerals,which contributes calcium and magnesium to groundwater(Thilagavathi et al.2013).Thus,all the abovereactionsrepresent the release of HCO3ionsinto groundwater,which predominantly reflects weathering process.This also describes the significant recharge along the foothills.Ophori and Toth 1989;Subba et al.2007;Srinivasamoorthy et al.2008).

The majority of sample values clustered around and above the 1:1 line(Fig.14)which shows a predominance of carbonate weathering.Our study supports the statement of Garrels(1976),Datta and Tyagi(1996)that carbonate weathering contribute Ca,Mg,SO4,and HCO3to the groundwater along with silicate weathering.Most of the samples along the foothills(Fig.14)fall in the equiline,which indicates predominance of both carbonate and silicate weathering,further it is also interesting to note that this cluster is between values of 2 to 8 and higher values representing the carbonate weathering is governed by the lithology,pH,and pCO2values(Chidambaram et al.2007;Thilagavathi et al.2013).

The Ca2+/Mg2+ratio determines the quality of groundwater by depicting the process of calcite,dolomite dissolution,and silicate weathering(Ramesh and Gowri 2012).Ratio 1 indicates dolomite dissolution,whereas higher ratio(＞1)shows calcite dissolution(Mayo and Loucks 1995).But higher ratio that is greater than 2 is an indication of silicate mineral dissolution,which is responsible for Ca and Mg in groundwater(Katz et al.1998).Most of the sample values have ratio ranges from 1 to 2(Fig.15)that means calcite dissolution is more whereas a few sample values have a ratio more than 2.Thus,both the processes i.e.silicate weathering as well as calcite dissolution have a contribution to the variation of the chemical composition of the groundwater in this area.A majority of sample values near the foothills have a ratio less than 2 which signifies that Mg2+is relatively higher along the foothills because the mountain is predominantly of Charnockites.

Fig.14 Ca+Mg versus SO4+HCO3 indicating predominance of carbonate weathering in the region

Fig.15 Ca2+/Mg2+ratio showing calcite and silicate weathering in the region

4.3 Correlation matrix

The correlation matrix gives information about the interrelationship among the set of variables.Pearson’s Correlation coefficient between ion pairs is the best method of measuring the correlation because it based on the method of covariance(Swan and Sandilands 1995).The Pearson’s correlation coefficient value lies between+1 to-1 and the degree of correlation is said to be perfect,high,moderate and low if the coefficient value will be±1,±0.75 to±1,±0.25 to±0.75 and＜±0.25 respectively.The correlation matrix of groundwater samplescollected from study area is shown in Table 5.

Study of this correlation matrix depicts the following:

Ions of the same type of charge and different valence number:A relatively strong but not significant correlation exist between Mg2+and Na+(r=0.59),Ca2+and Na+(r=0.52),and Cl-andWhereas statistically insignificant correlation exists between Ca2+and Mg2+with K+(r=0.24,r=0.25 respectively).

Ions of opposite charge and equal valence number:A positive and statistically strong correlation is exhibited between Ca2+and SO42-(r=0.56),Mg2+and(r=0.60),Na+and Cl-(r=0.80),Na+and(0.69)with weaker correlations of K+with Cl-,and NO3-(r=0.34,0.47,and 0.11 respectively).

Ions of the same type of charge and equal valence number:Correlation between Ca2+and Mg2+(r=0.68)is significant.Moreover,correlation between Cl-and HCO3-(r=0.37),Cl-and NO3-(r=0.06),and HCO3-and NO3-(r=0.17)is relatively weak.

Ionsof the opposite type of charge and different valence number:Strong correlation observed between Ca2+and Cl-(r=0.81),Mg2+and Cl-(r=0.79),Na+and SO42-(r=0.65).A negative and statistically insignificant correlation of Mg2+,Na+,and Cl-with PO43-(r=-0.15,-0.07,-0.07 respectively)is noticed.

EC shows good correlation with Ca2+,Mg2+,Na+,Cl-,and SO42-which implies that they are the significant ions for theincrease of ECin groundwater(Georgeet al.2014).Cl-shows good correlation with Ca2+,Mg2+,and Na+indicating the influence of leaching and ion exchange process in the study area(Subramani et al.2009).Significant correlation is observed between ions like SO42+,Ca2+,Mg2+,Na+,and Cl-and between Na+,Cl-,and HCO3,which relates to anthropogenic activities like Na+and Cl-might be derived from septic tanks(Panno et al.1999a)and that of weathering when correlated with Na+and HCO3-(Chidambaram et al.2013).

4.3.1 Factor analysis

The purpose of factor analysis is to interpret the structure within the variance,covariance matrix of multivariate data collection.The technique uses the extraction of the Eigen values and Eigen vectors from the matrix of correlations and covariances.The interpretation is based on rotated factors,rotated loadings,and rotated Eigen values.Factor loadings are sorted according to the criteria of(Liu et al.2003),i.e.strong,moderate,and weak;corresponding to absolute loading values of＞0.75,0.75–0.50 and 0.50–0.30respectively.Factors with Eigen values greater than 1 are taken into account(Table 5).Three independent factors were extracted from the data set,and it explained 67.69%(Tables 6,7)of the total variance of the original data set,sufficient enough to give a good idea of the data structure.

Table 5 The correlation matrix of major ions(bold indicates strong and statistically significant correlation among the set of variables)

Table 6 Total data variance

Factor 1 accounted for 40.34%of thetotal variancewith positive loadings on EC,Ca,Mg,Na,Cl,and SO4(Tables 6,7).The association of these ions indicates the anthropogenic processes like Na,Cl from cesspool salts and other ions from municipal and domestic sewage(Panno et al.1999a).Thus infiltration of municipal and domestic sewage into some aquifers and other problems like sewer leakage,faulty septic tank operation,and land fill leachates contribute high EC and Cl to the aquifer(Panno et al.1999a).The elevated level of Ca and SO4can be attributed to the use of chemicals in agricultural fields and domestic sewage from the highly urbanized area(Fianko et al.2009)or it may bea mixing of two processes like ion exchange and anthropogenic factor.

Table 7 Rotated principal component matrix(bold indicates the representation of the parameters in the respective factor loadings)

Fig.16 Spatial distribution of factor 1

The Factor 2 accountsfor 15.02%of total variance with strong positive loadingson pH,K,and HCO3(Tables 6,7),which indicates the weathering process(Drever 1988).The strong positive loadings of K and HCO3reveals predominant K-feldspar weathering.

During weathering,a structured aluminosilicate(feldspar)is converted into a cation-poor,degraded aluminosilicate(clay),so cations and silicic acid increases in solution.Thus CO2is consumed and HCO3is produced(Chidambaram 2000).Both the carbonate and silicate reactions consume CO2and produce HCO3and cations in solution.Higher concentration of HCO3-ionsindicates the predominant weathering process which is an indirect indication of recharge along the foothills(Ophori and Toth 1989;Subba et al.2007;Srinivasamoorthy et al.2008).

Factor 3 showsstrong positiveloadingsof PO4and NO3with 12.32%of total variance(Table 6,7)which explains theleaching of nitrateand phosphatefrom fertilizer applied in agricultural lands to ground water(Chidambaram et al.2013).Intensive agricultural activities tend to use of chemical fertilizers which lead to an elevated level of NO3in the region.Excessive cultivation,spreading of animal droppings,sewage mud,and waste matter can all leads to leaching of NO3(Chidambaram et al.2013).The prime characteristic of NO3leaching is that it occurs due to the presence of excessnitrate in the soil in one season and later leached into groundwater by thenext season(Calvet 1990).Heavy precipitation and denitrification rate also influence NO3content in leached water(Fianko et al.2009).

4.3.2 Factor score

Spatial variation of a factor and its zone of representation are estimated by factor score which can be determined by two methods like weighted least square and regression method(Chidambaram et al.2012).Factor scores are computed by the regression method.Extremely positive factor scores indicate the areas that are most affected and extremely negative scores indicate the areas unaffected by the chemical process represented by the factor.The factor score for each sample was calculated and spatial distributions of three factors were explained.

The spatial distribution of factor scores for the First factor shows that the higher scores are located in the northwestern,central,and southern part of the study area(Fig.16).The spatial distribution of contamination observed indicate that main contamination sources result from the rural and urban area indicating the influence of human activity.Infiltration of aquifers by cess-pool salts,municipal,and domestic sewage and agrochemicals are the major outcomes of anthropogenic activities.It is obvious that shallow groundwater tends to have the highest contamination as compared to deeper groundwater because leaching will be more prominent in shallow groundwater level(Morriset al.2003).Thehigher valuesarenoted along theraised groundwater gradientand near theresidential area which clearly indicates the infiltration of contaminants leached out from the rural and urban areas into the aquifers.

The spatial distribution of factor scores for the second factor is well represented in the northern,northwestern,and southern part of the study area(Fig.17).Positive scores indicate these parts are mostly affected by weathering i.e.K-feldspar weathering.Thisisbecauseof the fact that these areas are mainly underlain by charnockite and gneiss which contain K-feldspar predominantly.The weathering of K-feldspar is the major process which recharging the aquifer as well as it controls the chemical composition of groundwater.

The significant representation of the third factor with higher scoresisnoted in thenorthern and southwestern part of the study area(Fig.18)which is mostly affected by agricultural activities.Intensive agricultural activities lead to use of chemical fertilizers and sewage in the agricultural lands.Because of these dominant agricultural practices,the elements like NO3and PO4gets leached into the groundwater.Thus a sharp increase of these two ions concentration is observed in this area.

Figure 19 represents higher scores of both factor 2 and factor 3 noted in the northern and central part of the study area,whereas all three factors are predominant along the southern part.It can be inferred from the Fig.19 that the factor 1 falls away from the mountain front,representation of urban sprawl along the downgradient of the water table and factor 2 and 3 are represented near the mountain region.But the third factor as inferred is an anthropogenic factor and is represented by the agriculture predominant regions.The second factor as inferred is represented by weathering and noted along the foothills and also represented in the regions predominantly with water bodies,which influences weathering.

Since the sampleshave been collected for analysisalong the foothills,it is inferred that the sampleshave both higher and lesser ionic strength(0.008–0.013,0.013–0.018),and there are 3 major water types(Ca–Na–Cl–HCO3,Na–Ca–Cl–HCO3,Na–Ca–HCO3–Cl)represented along these foothills(Fig.20).Those which are with higher ionic strength and Ca–Na–Cl–HCO3type are inferred to be recharged or infiltrated from the MB and have traveled a longer distance(longer residence time).And those which have the lesser ionic strength(0.008–0.013)and following water types(Na–Ca–Cl–HCO3,Na–Ca–HCO3–Cl)are inferred to be recharged along the foothills and with alesser residence time in the host rock.It is inferred that Chittar River promotesthe leaching of ionsand river water recharge.

Fig.17 Spatial distribution of factor 2

Fig.18 Spatial distribution of factor 3

Fig.19 Overlay of spatial distribution of dominant factor,land use and elevation

Fig.20 Conceptual diagram evolved from the Hydrogeochemical interpretation of the present study

5 Conclusions

The study has arrived at the following conclusions:

1. High concentration of both the cations and anions are noted along few locationsof northern and southern part of the study area where high ionic strength is also noted.This indicates that the water has been recharging from higher altitude acquiring a longer flow path and higher residence time.High concentration of ions is also noted along the foothills and in the direction of water flow,which is an indirect indication of recharge process.

2. Four types of hydrogeochemical facies have been identified i.e.Na–Ca–Cl–HCO3,Na–Cl–HCO3,Ca–Na–Cl–HCO3,Na–Ca–HCO3–Cl types.Na–Ca–Cl–HCO3type is predominant along the foothills.This water type normally changes to Na–Cl–HCO3due to ion exchange process.Na–Ca–HCO3water type changes to Ca–Na–Cl–HCO3water type due to either Na–Ca ion exchange process or by the addition of Ca,leached out from Ca-rich rock type.Na–Ca–HCO3water type changes to Na–Ca–HCO3–Cl type because of increase of weathering process.This increase of HCO3ion concentration along the direction of water flow shows higher groundwater recharge.Significant weathering process along the direction of water flow reveals higher ionic strength,which infers that water types may have been acquiring longer flow path and recharged from a higher elevation.

3. The relationship of Ca and Mg indicates that most of the sample values are clustered around the equiline indicating a predominance of both silicate and carbonate weathering,as mineral weathering releases both bicarbonate and silica.Furthermore,there are relatively higher values of Mg along the foothills due to weathering of charnockites in the MB.

4.The chemical composition of groundwater is mainly controlled by weathering and anthropogenic activities.But weathering is more dominant than anthropogenic activities.The higher HCO3and H4SiO4concentration are mainly derived from the carbonate and silicate weathering process.Elevated concentration of HCO3ion indicates predominant weathering as well as higher recharge process in this region.

5. The statistical analysis reveals that anthropogenic activities,weathering,and leaching are the major factors controlling the hydrogeochemistry of the area.The spatial distribution of factor score shows that dominant hydrogeochemical regimes are distributed along the direction of water flow,highly urbanized area and near to the agricultural lands.Thus,the hydrogeochemical evolution indicates infiltration of contaminants leached from the residential area due to anthropogenic activities,weathering process along the direction of water flow and leaching of nitrate from agricultural lands.This higher weathering processisan indicative of recharge.

AcknowledgementsThe authors would like to thank the Science&Engineering Research Board(SERB),New Delhi(No:SB/S4/ES-699/2013)for providing necessary financial support to carry out this study and the author Banajarani Panda wish to express her sincere thanks to Department of Science and Technology for providing the Inspire Fellowship(No:DST/INSPIRE Fellowship/[IF150615],27th October 2015).

Compliance with ethical standards

Conflict of interestOn behalf of all authors,the corresponding author states that there is no conflict of interest.

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