J. Earth Syst. Sci. (2019) 128:49 c© Indian Academy of Scienceshttps://doi.org/10.1007/s12040-019-1078-9

Hydrogeochemical modelling to understand the surfacewater–groundwater interaction arounda proposed uranium mining site

S Manoj, M Thirumurugan and L Elango*

Department of Geology, Anna University, Chennai, Tamil Nadu 600 025, India.*Corresponding author. e-mail: elango34@hotmail.com

MS received 7 February 2018; revised 11 May 2018; accepted 12 June 2018; published online 23 February 2019

The interaction between surface water and groundwater is a complex process and is considered as animportant component for controlling the mining activities. The objective of this study is to understandthe interaction between surface water and groundwater around a proposed uranium mining site bygeochemical modelling. Surface water and groundwater samples along the groundwater flow path werecollected from September 2013 to June 2016 across the uranium mineralised region located near Gogi,Karnataka, India. Collected water samples were analysed for major ion and uranium concentrations.This hydrochemical data was used as input in the geochemical modelling code PHREEQC to calculatethe uranium speciation and saturation indices. Inverse geochemical modelling was performed along theflow direction by considering the mineralogical composition of host rock. Measurement of surface waterand groundwater level indicates that the recharge and discharge of this region were primarily controlledby rainfall. Relation between the temporal variation of rainfall and saturation index of mineral revealsthe various scenarios of interaction between surface water and groundwater around the mineralisedregion. Silicate/carbonate weathering, irrigation return flow and dissolution of evaporites are the majorprocesses indicated by inverse geochemical modelling, which controls the hydrogeochemical evolution ofwater in this region. Geochemical modelling was effectively used to understand the temporal changes inthe interaction between surface water and the groundwater in a uranium mineralised region.

Keywords. Uranium; Bhima basin; PHREEQC; geochemical speciation; saturation index; inversemodelling.

1. Introduction

Mineral exploitation plays a major role in world’seconomic and industrial development. It guaran-tees the continuous supply of raw materials tothe construction and manufacturing sectors for theeconomic development of the country (Hein et al.2004). Uranium is one such raw material, extractedthrough conventional mining and is used to gen-erate nuclear energy. Due to rise in demand anddepletion of existing uranium reserves, it becomesessential to find new deposits and also increase the

production of the existing deposits (Mason 2014).In general, mining of economic minerals inducessignificant changes in surface hydrology, ground-water systems and water quality (Booth 2006).Diversion of surface water, creation of additionalponds, changes in stream alignment, interactionbetween surface water and groundwater as wellas changes in water quality are some of the chal-lenges need to be addressed during mining (Kayet al. 2006). Vance et al. (2014) compared thecurrently leading approaches of managing envi-ronmental and health impacts of uranium mining

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with the outdated practices which suggest thatinnovative, modern mining practices combined withstrictly enforced regulatory standards will reducethe environmental and health risks caused due tomining.

Understanding the interaction between surfacewater and groundwater is one such essential stepfor the optimal management of regional waterresources during mining operations. In miningareas, extra care was taken to maintain the qual-ity as well as regional fluctuation in the watertable to preserve the surface water and ground-water ecosystem (ANZECC 2000). The interactionof groundwater with adjoining lakes, reservoirs,streams and canals is the major aspect that gov-erns the inflow, outflow and flow direction of theregion. Numerous studies based on stable isotopes(Katz et al. 1997; Paces and Wurster 2014; Ala-aho et al. 2015), hydrochemistry (Soulsby et al.2005; Ayenew et al. 2008; Martinez et al. 2015)and modelling (Pahar and Dhar 2014; Voeckleret al. 2014; Hu et al. 2016; Yi et al. 2016) werewidely applied to understand the surface waterand groundwater interactions. Apart from these,hydrogeochemical evolution from surface water togroundwater through the unsaturated zone usinginverse modelling have been carried out by var-ious researchers (Lecomte et al. 2005; Federicoet al. 2008; Sharif et al. 2008; Belkhiri et al. 2010)to understand the geochemical evolution of waterand the process responsible for the evolution. Earyet al. (2003) also assessed the water quality changesin connection with the mining operations using theinverse modelling and Brindha and Elango (2014)have used the geochemical modelling to understandthe geochemical changes in groundwater due toleaching from uranium tailing ponds.

Uranium exploration in India dates back fromearly 1950s and the deposits belongs to the Pro-terozoic age has been considered as the potentialtarget (Chaki et al. 2005). Bhima basin is oneamong the seven Purana basins (Palaeoprotero-zoic–Neoproterozoic age) in Indian peninsula (Kaleand Phansalkar 1991), which is characterised byseven major faults (Kale and Peshwa 1995) inwhich the medium grade of uranium depositsoccurs around one of them (Achar et al. 2001).Investigations in the Bhima basin were carried outby an integrated approach with several explorationtechniques was adopted and the extent of uraniumreserve in this region was estimated (Achar et al.2001). This uranium deposit is structurally con-trolled and of hydrothermal vein type (Chaki et al.

2011). To meet the increasing demand of uranium,it is planned to mine uranium ore from this areain future. Manoj et al. (2017a, b) assessed the con-centration of uranium in groundwater of the entireShahpur taluk which ranged from below determi-nation level (<0.05 ppb) to 302 ppb.

Hydrogeochemical methods are commonly usedto establish the relationship between the sur-face water and groundwater and the mechanisminvolved in the geochemical evolution. Hydrogeo-chemical methods based on the concentration ofions in water and saturation indices of mineralsprovide a cost-effective alternative to understandthe groundwater recharge and flow processes overa large area. As this region with uranium deposithas two large lakes with complex hydrogeologicalsetup, it is necessary to understand the interac-tion between the surface water and groundwater.In addition, this study will serve as the baselinedata which will be helpful to assess the presentand future impacts from mining that can possi-bly influence the hydrogeology and geochemistry ofthe surface water and groundwater of this region.Thus, the objective of this study is to understandthe interaction between surface water and ground-water in the uranium mineralised Gogi region,Karnataka, India, by geochemical modelling.

2. Study area

The study area lies in the southern part of theBhima basin, which is located around 12 km westof Shahpur, Yadgir district of Karnataka, India.The geographical extent of the study area is about14 km2 with 1571 houses and about 10,000 resi-dents as per the 2011 census (Chandramouli andGeneral 2011). The area experiences three seasons:(i) summer from late February to mid-June, (ii)southwest monsoon from mid-June to late Septem-ber and (iii) dry winter until January, where thetemperature ranges from 37◦ to 46◦C, 25◦ to 37◦Cand 12◦ to 32◦C, respectively (Karunakara et al.2014). The average annual rainfall in this areais about 839mm and most of the precipitationoccurs during the southwest monsoon. The sur-face runoff resulted in the development of dentriticdrainage pattern in this area. Two major lakesnamely Melinakere (L1 lake) and Kelaginakere (L2lake) are present in the upstream and downstreamof the mineralised zones, respectively. These lakessupport the water demand for domestic and agri-cultural purposes in the study area (figure 1).

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Figure 1. Study area.

The cropping pattern depends on three seasonsnamely kharif, rabi and summer. The kharif crop-ping season is from July to October during thesouthwest monsoon and the rabi cropping season isfrom October to March. The crops grown betweenMarch and June are summer crops. The crops cov-ered in this area are kharif: paddy, jowar, tur,cotton, and sunflower; rabi: jowar, wheat and ben-gal gram; and summer: paddy and groundnut.

3. Uranium mineralisation and geologyof Gogi region

In India, most of the uranium deposits fall underthe low-grade category, which include the depositsin the states of Jharkhand, Chhattisgarh, Megha-laya, Andhra Pradesh, Rajasthan and Haryana.The Gogi uranium deposit in the state of Kar-nataka is of the higher quality (medium grade)among the ores found in the rest of the coun-try (Chaki et al. 2011). Based on the available

lithologs, the cross-section along the Gogi uraniummineralised zone was arrived (figure 2). Uraniummineralisation in Gogi region occurs within themajor E–W trending Gogi–Kurlagere fault(Karunakara et al. 2014), passing through the min-eralised zone, which takes a NE swerve near thesouth of Gogi lake and attains easterly trend nearthe north of the Gogi village (Achar et al. 2001).Chaki et al. (2005) reported that the mineralisationis steeply dipping and the fault is reverse in nature.The strike of the reverse fault is N50◦E−S50◦Wand is dipping towards S40◦E. Intense breccia-tion in limestone, steeply dipping beds and base-ment granites are the characteristic features of thefault zone indicating the involvement of basementrocks during tectonisation. Uranium occurring inuraninite is hosted mainly in sheared phosphaticlimestone, non-phosphatic limestone and basementgranite present in this region. During the drillingby Atomic Mineral Directorate, the surface sam-ples collected contain 0.017–0.084% and 0.02–0.27% of U3O8 in phosphatic and non-phosphatic

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Figure 2. Cross-section across the uranium mineralised zone (after Chaki et al. 2005, 2011).

limestones, respectively, whereas some of the coresamples of granitic terrain contain up to 20% ofU3O8 (Chaki et al. 2005). Mineralisation in graniteclose to the unconformity contact shows radiomet-ric assay of 0.02–0.3% U3O8 (Achar et al. 2001).Uranium in this region is apparently derived fromhydrothermal leaching of basement granite rockand deposited in the fault zone near to the contactof carbonate rocks due to the favourable geochem-ical environment of uranium precipitation (Senthiland Srinivasan 2002).

4. Methodology

4.1 Sample collection

Surface water (L1 and L2) from the two lakes andgroundwater (G1 and G2) from the nearby wellswere collected so as to approximately representthe general groundwater flow direction (figure 2).

Water sampling was carried out once in 3 monthsfrom September 2013 to June 2016. Geologically,L1 and G1 fall in granitic terrain, whereas L2 andG2 fall in carbonate terrain. The aim and purposeof choosing this location is to understand the geo-chemical variation during the migration of waterfrom granitic terrain to carbonate terrain. Col-lected water samples were filtered through 0.45µmand transferred to the laboratory for the analysisof major ions and uranium concentration.

4.2 Hydrogeochemical characterisation

The pH, electrical conductivity (EC), tempera-ture and redox potential (Eh) of surface waterand groundwater were measured in the field usingportable multiparameter system (Eureka SubManta-2) and the concentrations of carbonate andbicarbonate were estimated using the Merck alka-linity test kit (111109). Cations (Ca2+, Mg2+, Na+

and K+) and anions (Cl−,SO−4 and NO−

3 ) were

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analysed in the laboratory from the filtered watersamples using ion chromatograph (IC Metrohm861). Ion balance error was calculated and wasfound to be within ±10%. The concentration ofuranium in water samples was analysed using alaser fluorimeter (Quantalase LF-2a). This methodmeasures the fluorescence of uranium complex inthe water sample by excitation under ultravioletlight. The fluorescence of uranium complexes ismeasured by a sensitive photomultiplier tube. Theaccuracy of analysis was achieved through the mea-surement of certified standard reference solutionICP-MS-66N and the precision was established byduplicate analysis of every two samples.

4.3 Measurement of surface water andgroundwater levels

Water level fluctuation in surface water bodies, aswell as subsurface, is a primary tool to understandthe degree of interaction between surface water andgroundwater. Aquifer system readily reacts to thechanges that take place during the recharge anddischarge of water with respect to time and sea-son. In order to understand the mechanism of theinteraction process, the water level in the lakeswas measured using a scale fixed at a place withthe known elevation and the groundwater level wasmeasured during the sampling campaign using thewater level indicator (Solinist 101) in the wells (G1and G2) adjacent to the lakes and in three morewells located around the mineralised zone. Basedon the measured surface water and groundwaterlevel, groundwater table map was prepared usingArcGIS 10.1 software for different time periods.

5. Geochemical modelling

5.1 Species calculation and saturation index

Uranium occurs as U4+, U5+ and U6+ ion innatural water; however, from geochemical point ofview, the oxidation states of U4+ and U6+ ions weredominant (Dongarra 1984). U4+ occurs in the formof hydroxides, hydrated fluorides and phosphates,whereas U6+ is the most stable state occurringin the form of U3O8 (uraninite). The distributionof aqueous uranium species in surface water andgroundwater of this region was calculated basedon geochemical data using PHREEQC (Parkhurst1995). Temperature, pH, Eh, and concentrations ofions are the essential data used as an input for thespecies calculation.

The saturation index (SI) was applied to predictthe reactive mineralogy of the subsurface from thegroundwater sample data without collecting thesamples of the solid phase (Rajmohan and Elango2004). The SI was calculated using the computergeochemical program PHREEQC for surface waterand groundwater samples. It is defined as

SI = logIAPKeq

,

where IAP is the ion activity product and Keq isthe equilibrium constant. Equilibrium is indicatedwhen SI = 0, the water is supersaturated whenSI > 0. If SI < 0, the water is undersaturated.

5.2 Inverse geochemical modelling

Inverse modelling calculates the transfer of moles(minerals/gases) that tend to dissolve or precip-itate, and it also explains the chemical changesbetween the known initial and final solutionswith respect to the available mineral phases. Thisis essential in understanding the causes for thetransformation of chemical composition withinthe groundwater and surface water. To evaluate thegeochemical evolution and chemical reactions alongthe flow direction, inverse modelling was carriedout using PHREEQC. Mass balance calculationalong a specific flow path was performed in theinverse modelling. During this inverse geochemicalmodelling, it was assumed that (i) the initial andfinal solutions represent the flows along the sameflow direction, (ii) dispersion/diffusion do not affectthe chemistry of water, and (iii) the mineral phasesused in the model are exactly present in the aquifer.The modelling was carried out with the measuredchemical composition of waters and the initialand final waters were L1–G1, G1–G2 and G2–L2for all the sampling periods. That is the chemi-cal composition of the surface water (L1/L2) andgroundwater (G1/G2) represents the initial andfinal solutions for inverse geochemical modelling.The mineral phases that are likely to be presentbetween these locations were also considered. Min-eral phases included in the inverse modelling areobtained from the field observation and the geo-chemical studies were carried out in and aroundthis region by Chaki et al. (2005) and Patnaik et al.(2016). The simulations were constrained withinthe pre-defined uncertainty limit of 0.05 (5%) or 0.1(10%) as default for all the periods. By keeping themineral phases constant, the inverse geochemical

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Table 1. Hydrogeochemistry of surface water and groundwater.

Parameter Unit

Mean Range

Surface water Groundwater Surface water Groundwater

pH pH scale – – 7.2–9.3 6.8–7.9

EC µS/cm 1005 1444 617–1674 1189–1909

Calcium mg/l 52 59 40–82 42–75

Magnesium mg/l 22 34 19–30 10–75

Sodium mg/l 95 141 50–143 66–255

Potassium mg/l 5 7 1–7 1–18

Bicarbonate mg/l 235 351 175–341 263–488

Chloride mg/l 113 175 65–164 112–275

Sulphate mg/l 42 76 25–64 53–106

Nitrate mg/l 48 43 21–80 28–66

Uranium µg/l 12 27 2–33 13–52

Figure 3. Hydrogeochemical facies of surface water and groundwater (Durov 1948).

modelling was carried out for all samplingperiods to understand the temporal changes in thesurface and groundwater interaction.

6. Results and discussion

6.1 Chemical characterisation of surface waterand groundwater

The mean and range of pH, EC, calcium,magnesium, sodium, potassium, bicarbonate, chlo-ride, sulphate and nitrate concentration in thesurface water and groundwater samples collectedaround the uranium mineralised zone are given

in table 1. The dissolved solids are comparativelyhigh in groundwater than the surface water asindicated by EC. The concentration of nitrate aloneis slightly higher in surface water, whereas restof the major ions in groundwater is higher thansurface water. Since this region is intensively cul-tivated and the runoff discharges into the lakes,high concentration of nitrate (Bureau of IndianStandards permissible limit 45 mg/l) was observedin both the surface water (53% of samples) andgroundwater (41% of samples). Hydrochemicalfacies identified by a double triangular plot (Durov1948) (figure 3) indicates that the surface water (L1and L2) and groundwater (G2) fall in the mixedCa–Mg–Cl type, whereas groundwater from the

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Table 2. Percentage range of U speciation in surface water and ground-water.

L. no. Nature of water Speciation form

Percentage

range

L1 Lake (upstream) UO2(CO3)4−3 21–91

UO2(CO3)2−2 9–77

UO2(CO3) 0–2

L2 Lake (downstream) UO2(CO3)4−3 9–76

UO2(CO3)2−2 24–85

UO2(CO3) 0–6

G1 Groundwater (upstream) UO2(CO3)4−3 31–64

UO2(CO3)2−2 36–68

UO2(CO3) 0–1

G2 Groundwater (downstream) UO2(CO3)4−3 17–54

UO2(CO3)2−2 46–80

UO2(CO3) 0–3

G1 well was of Na–Cl type. It indicates that thehydrogeochemistry of groundwater flow from theupstream region is transformed from Na–Cl typeto the mixed Ca–Mg–Cl type when it enters intothe mineralised zone of the downstream region dueto the presence of carbonate rocks and the contin-uous infiltration of surface water (L1).

Uranium concentration during all the samplingcampaign ranges from 2 to 52µg/l. The meanand range of uranium concentration in surfacewater and groundwater are given in table 1. Theconcentration of uranium is higher in ground-water when compared to surface water in mostof the sampling campaigns (figure 3). The ura-nium concentration increases along the flow path(L1–G1–G2) and decreases in L2 (when comparedwith G2). An inconsistent range of uranium con-centration was observed throughout the samplingcampaign between flow path G2–L2, which is dueto the sudden variation of pH, Eh, temperature andexposure to surface water (L2), resulting in insta-bility of uranium complex in water.

6.2 Geochemical speciation

The mobility of uranium in water is controlled byuranium speciation; hence, it is important to knowthe dominant uranium species in order to predictits migration and distribution. Factors controllingthe uranium speciation are pH, Eh, concentrationof ions/ionic strength and the different mineralphases during the interaction process (Bernhardet al. 1996). Based on PHREEQC output, theuranium tetravalent ion complex exists in variousforms of species such as U(OH)5−, U(OH)4, U4+

Figure 4. Temporal variation of U species in surface waterand groundwater of upstream and downstream regionsaround the mineralised zone.

and uranium hexavalent complex exhibits in theform of UO2(CO3)4−

3 , UO2(CO3)2−2 , UO2(CO3),

UO2OH+, UO2+2 , (UO2)2(OH)2+2 , (UO2)3(OH)5+.

Species calculation displays that the uranium com-plexes of UO2(CO3)4−

3 and UO2(CO3)2−2 are com-

mon in aqueous solution, whereas the complexingof the types such as U(OH)5−, U(OH)4, U4+,UO2(CO3), UO2OH+, UO2+

2 , (UO2)2(OH)2+2 ,(UO2)3(OH)5+ were minimal in surface andgroundwater of this region. Formation of uranylcarbonate complexes UO2(CO3)4−

3 and UO2

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(CO3)2−2, is favourable because of neutral to alkaline

pH (Hsi and Langmuir 1985; Pabalan et al. 1996;Nair and Merkel 2011) of water and the weatheringof carbonate rocks present in this region. Table 2shows the percentage range of uranium species dis-tributions in the lake and groundwater present inthe upstream and downstream regions. Temporalvariation of uranium species is shown in figure 4,and it indicates that the groundwater of down-stream region is abundant with uranium speciesdue to increase in the release of uranium from the

Figure 5. Spatial variation of groundwater level during(a) monsoon, (b) non-monsoon, and (c) temporal variationof surface water and groundwater levels (m amsl).

mineralised zone at deeper depth along the slopedirection.

6.3 Validation of interaction based on water levelfluctuations

The groundwater level in this region rangesbetween 1 and 5m below the ground level. L1lake contains water throughout the year, whereasL2 lake contains water during monsoon andpost-monsoon and is almost dry during summer.Discharge of surface water from upstream to down-stream (L1–L2) was observed in the field duringmonsoon where the movement of water was clearlyvisualised. Based on the measured surface waterand groundwater levels, the spatial and tempo-ral variation was prepared (figure 5). Spatial andtemporal variations of surface water and ground-water levels indicate that the rainfall plays a majorrole, which controls the recharge and discharge ofthis region. It also reveals that the direct runofffrom the L1 lake (upstream) to the L2 lake (down-stream) took place during high rainfall periods.Both the lakes were filled quickly in the begin-ning of monsoon itself due to increase in rainfallrecharge and excess runoff from surrounding areas.During the onset of monsoon, the water level inthe lakes L1 and L2 rises and the water seeps intoG1 and G2, respectively. The water from the G1further flows into G2. The groundwater level grad-ually rises during the monsoon to a depth less than1m from the surface. During the post-monsoon, L1seeps into shallow groundwater (G1) from whichthe water flows into G2. The groundwater (G2)further flows into lake L2 due to the differencein water levels. It shows that recharge from lakesto groundwater and groundwater to lakes variesduring pre-monsoon/monsoon and post-monsoon,respectively.

6.4 Validation of interaction based on SI

Hydrogeochemistry of surface water and ground-water is used as an input to represent the extentto which the water is chemically in equilibrium

Table 3. Minimum and maximum of calculated mineral SI.

Phase Composition

Minimum Maximum

Lake Groundwater Lake Groundwater

Calcite CaCO3 − 0.26 − 0.27 1.31 0.47

Dolomite CaMg(CO3)2 − 0.57 − 0.52 2.61 0.9

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Figure 6. Comparison between the temporal variation of the SI with respect to rainfall: (a) L1 lake–G1 well, (b) L2 lake–G2well, (c) L1 lake–L2 lake, (d) G1 well–G2 well, (e) L1 lake–G2 well, (f) L2 lake–G1 well and (g) temporal variation of rainfall.

with respect to the minerals present in the aquifersystem. Mineral phases such as calcite and dolomitewere considered owing to the presence of carbon-ate minerals in limestone rock. Table 3 shows the

minimum and maximum mineral SIs observed fromthe PHREEQC.

Temporal variation of SI was prepared to under-stand the interaction process and is compared with

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the rainfall data, which is a major controllingfactor for mineral saturation level. The carbon-ate minerals such as calcite and dolomite showan increasing trend during monsoon due to thedissolution of limestone rock. Comparison betweenthe mineral SI of lake water and groundwatercollected during several periods of sampling wasplotted to understand the effects of lake waterover groundwater and vice versa. In this study, sixdifferent comparisons were interpreted to under-stand the interaction process which include(i) L1 lake–G1 well, (ii) L2 lake–G2 well,(iii) L1 lake–L2 lake, (iv) G1 well–G2 well,(v) L1 lake–G2 well and (vi) L2 lake–G1well.

Comparison between upstream region (L1–G1)and downstream region (L2–G2) reveals that a

higher degree of interaction was observed in thedownstream region (figure 6a and b). In down-stream region (L2–G2), the influence of lake waterover groundwater is dominant during the mon-soon due to rainfall recharge. Figure 6(c and d)indicates that lake water (L1–L2) and groundwa-ter (G1–G2) express a similar pattern in the SIvalue indicating a strong correlation that depictsthe common source of origin. Cross-comparisonof lake water and groundwater (L1–G2 and L2–G1) (figure 6e and f) expresses a strong cor-relation during the monsoon and less in post-monsoon. This indicates that recharge from L2,which was dominant in monsoon, will inverseduring summer (i.e., groundwater in the upstreamregion recharge the L2 during the dryseason).

Figure 7. Schematic view of surface water and groundwater interaction near the uranium mineralised zone.

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6.5 An idealised interaction processbased on SI value

Based on the comparison between the SI values oflake water (L1–L2) and its adjacent groundwater(G1–G2), the following interpretations are made:

(i) The SI value of L2 and G2 in the down-stream region follows similar pattern due tosame source and continuous recharge from thelake L1.

(ii) Degree of interaction between the lake (L2)and the groundwater (G2) is higher in thebeginning of monsoon due to the recharge fromL2, which occurs only during the onset ofmonsoon.

(iii) A similar trend of SI value was observedin groundwater of upstream (G1) and down-stream (G2) indicating that the groundwater(G2) has evolved from G1.

(iv) Interaction between G1–G2 and L2 is higherduring summer, which indicates that L2 isrecharged from groundwater (G2) which isrecharged by G1.

With the above results, various schematicdiagrams were prepared to understand the interac-tion between surface water and groundwater nearthe uranium mineralised region (figure 7).

6.6 Hydrogeochemical evolution by inversemodelling

Inverse modelling was performed with three sim-ulations (L1–G1, G1–G2 and G2–L2) (figure 8)along the same flow path with the primary mineralphases including aragonite, biotite, calcite,chlorite, coffinite, dolomite, gypsum, halite,K-feldspar, plagioclase, quartz and uraninite whichare reported by Chaki et al. (2005) and Patnaiket al. (2016) for this study region. Analytical val-ues (major ions and uranium) of the water samplesfor each period were used to represent the ini-tial and final solutions along the flow direction.Figure 8 shows the suitable model which hasbeen selected based on the criteria of ‘the sum ofresiduals and maximum fractional error’ from allthe models generated by PHREEQC. It represents

Figure 8. Hydrogeochemical evolution/phase mole transfer along the flow line.

49 Page 12 of 14 J. Earth Syst. Sci. (2019) 128:49

Figure 9. Bivariate plots indicating weathering trends: (a) Ca+Mg vs. HCO3+SO4, (b) Ca vs. HCO3, (c) Ca/Na vs. Mg/Naand (d) Ca/Na vs. HCO3/Na.

most possible combinations of reactants andproducts that are responsible for the hydrogeo-chemical evolution. In upstream region (L1–G1),precipitation of aragonite, calcite, quartz,K-feldspar and chlorite indicates the dominanceof silicate weathering and carbonate weathering(figure 8). Since G1 is located in the agriculturalland, the precipitation of gypsum and halite wasobserved along the flow path of G1–G2, whichis derived from the irrigation return flow that isconcentrated with the ions such as sodium and sul-phate which are present in fertilisers and calciumfrom the soils that are derived from the adjacentcarbonate rocks. From G2 to L2, the precipita-tion of carbonates, plagioclase, coffinite, gypsumand halite indicates the combined process of weath-ering and dissolution of evaporites computed bythe inverse modelling which is also confirmed bythe bivariate plot (figure 9). Figure 9(a) indicatesthat the points falling along the equiline may haveoriginated from weathering of sulphate and car-bonate minerals as suggested by Datta and Tyagi(1996). Figure 9(b) suggests that the points fallingalong 1:2 line have resulted from weathering of cal-cite (Mackenzie and Garrels 1971; Holland 1978).Figure 9(c and d) suggests that both silicate andevaporites are responsible for the changes in hydro-geochemistry of both the surface water and ground-water. The present study region is highly complexwith respect to geology; the reactions that are

responsible for the changes in hydrogeochemistryof water do not result from a single source, but frommultiple sources which are depicted in figure 9.Since the precipitation and dissolution kinetics ofsilicate minerals are slow, the moles released fromthese minerals are very less.

7. Conclusions

Hydrogeochemical modelling was used to under-stand the interaction between surface water andgroundwater in a uranium mineralised region.Hydrogeochemically, surface water and ground-water were mainly of mixed Ca–Mg–Cl type exceptin the groundwater of upstream region which isof Na–Cl type. Uranium species in surface waterand groundwater was dominated by UO2(CO3)4−

3

and UO2(CO3)2−2 hexavalent complexes. The lake

on the western side always contributes to ground-water recharge, whereas during most part of theyear groundwater is discharged to the lake onthe eastern side. The rainfall results into changein the geochemical nature of water in the lakeby way of changing the saturation indices fromoversaturated to undersaturated level. The pre-cipitation and dissolution of carbonate mineralswere the major reactions that are responsible forthe changes in the hydrogeochemistry in com-parison with silicate minerals. Hence, carbonate

J. Earth Syst. Sci. (2019) 128:49 Page 13 of 14 49

weathering, silicate weathering and irrigationreturn flow are the dominant processes that gov-ern the hydrogeochemical evolution in this regionduring the interaction process. Thus, the geochem-ical modelling helped to understand the temporalchanges in the interaction between surface waterand the groundwater in a uranium mineralisedregion which cannot be estimated by simple waterlevel measurements.

Acknowledgements

The author would like to thank the Board of Re-search in Nuclear Sciences, Department of AtomicEnergy, Government of India (grant no. 2009/36/71-BRNS/1690) for their financial support.

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Corresponding editor: Abhijit Mukherjee