Assessment of Groundwater Geochemistry of Vel River basin, Western Maharashtra, India

1. Introduction

In freshwater, groundwater is very important source on the earth (Brindha et al., 2011) and it is major source for drinking in the rural parts of the developing nations (Brindha et al., 2010; Wagh et al., 2016). The chemistry of groundwater modifies by natural and anthropogenic activities (Wagh et al., 2018). Rock weathering and its dissolution is the most important mechanism that modifies the chemistry of groundwater (Weijden and Pacheco, 2006; Pawar and Nikumbh, 2007). Along with it, interaction of water with aquifer minerals along the flow path in the sub-surface and evaporation and precipitation of surface waters also modifies its chemistry (Domenico, 1972; Wallick and Toth, 1976; Adimalla et al., 2018; Kadam et al., 2020; Mukate et al., 2020; Kumari and Rai, 2020). Geochemical scatter plots give important clues about the sources of ions and pollution in groundwater (Pawar et al., 2008; Wagh et al., 2018; Wagh et al., 2019; Gaikwad et al., 2020).

Due to rapid rise in and growth of agricultural and industrial sectors and demand of groundwater has increased in many folds (Brindha and Elango, 2014; Adimalla et al., 2018; Wagh et al., 2020). Due to this development, pollution of water bodies has been increasing (Brindha and Elango, 2014; Adimalla et al., 2018). If groundwater contains dissolved ions more than standard limit, then it is harmful and unsuitable for use (Pawar et al., 2008; Brindha et al., 2011; Wagh et al., 2019a; Wagh et al., 2020; Kadam et al., 2020; Mukate et al., 2020). So the quality of groundwater for drinking as well as for irrigation purpose also can be studied comparing it with WHO standards and using different parameters like Sodium Absorption Ratio (SAR), Soluble Sodium Percentage (SSP) and Kelly Ratio (KR), respectively.

In absence of any scientific work in the area under investigation except some government organizations like CGWB (2013) and GSDA (2003) and Gaikwad et al., (2018), it is necessary to find out sources and pathways of migration of these ions in the groundwater.

The objectives of this study are to understand the spatial-temporal distribution of major ion in groundwater as well as to identify the geogenic as well as anthropogenic pollution sources in it and to investigate the groundwater quality for drinking and irrigation purposes.

2. Study Area

Study area is Vel (Velu) River basin; a small tributary of Bhima River flows through Pune district (Figure 1). It originates near Matewadi and meets to the Bhima River near the Vitthalwadi. Vel River is on the Northern side of Pune city. Major localities in the basin are Shikrapur, Talegaon Dhamdhare, Peth, Wafgaon, Varude, Vitthalwadi and Pargaon. The basin is bound by 73º 50ʹ to 74º 10.5ʹ E longitudes and 18º 37ʹ to 19º 00ʹ N latitudes and it is a part of Survey of India’s (SOI) toposheets No. 47 F/13, 47 J/1 and 47 J/2 of scale 1: 50,000. The study area is about 352.13 sq.km. The major water harvesting structures includes Pargaon and Thitewadi Bhandhra. The average annual precipitation of the basin is about 650 mm (Supekar, 2011). It is also observed that the rainfall is moderately heavy in western part of the study area and decreases rapidly towards lower part of the basin. The temperature ranges from 10 to 32 °C in the basin.

3. Materials And Methods

3.1 Geology

The study area is a part of the Deccan Volcanic Provinces (DVP). The DVP covers an area of more than 500,000 km², in central and western India (Powar, 1987). The many workers have studied DVP from chemostratigraphy, mineralogy and geochemistry point of view including Beane et al. (1986), Mitchell and Widowson, (1991), Subbarao et al. (1988), Subbarao et al. (1994), Sethna et al. (1996) and Melluso et al. (1999), etc.

The major lithologies exposed in the Vel basin of Lonavala and Diveghat Subgroup of Deccan Trap Supergroup (Figure 2).

The Indrayani and Karla Formation are part of Lonavala Subgroup while Diveghat Formation is from Diveghat Subgroup (GSI, 2001). Diveghat Formation overlies the Karla Formation. It is exposed in the western and north-eastern regions of the study area. Outcrops of this formation are witnessed on the hills and along the hill slopes above 700 m AMSL (Supekar, 2011). It comprises mainly aphyric simple flows of AA type (GSI, 2001). Lava flows of this formation are characterized by presence of vesicular, plagioclase basalt with medium grained groundmass. It is parted from the upper formation by a geomorphic break (Godbole et al., 1996). The Karla Formation occurs mostly in the central part of basin and overlying the Indrayani Formation. It essentially comprises of compound lava flows revealing pahoehoe characters (GSI, 2001). The Indrayani Formation is exposed in throughout the study area comprising of simple flows covering more than 80% of the total area.

3.2 Water Samples and Chemical Analysis

The sampling sites were selected to cover accessible part of the Vel River basin. Sixty (60) groundwater samples of wells and bore wells in different formations were collected using standard procedure for major ions analysis (APHA, 2005; CGWB, 2012; Wagh et al., 2016; Gaikwad et al., 2018; Gaikwad et al., 2020a). The location of water sampling stations in Vel River basin is shown in Figure 3.

Volumetric method was employed for analysis of total alkalinity, total hardness and chloride of water samples from study area are analyzed using the standard methods given by APHA (2005). For the determination of total alkalinity of groundwater samples, acidic-base titration method was used. While for determination of chloride, precipitation argent metric method of titration was used. The complex metric method of titration with EDTA (disodium salt) was used for the analysis of total hardness of groundwater. Sodium (Na⁺), Potassium (K⁺) and Calcium (Ca²⁺) were analyzed on the Photometer (Systronics, Make in India) in the Geochemistry Lab, in the Geology Department of Savitribai Phule Pune University (India). While Magnesium (Mg²⁺) analyzed by gravimetric analysis with the standard methods (APHA, 2005). Anions (Cl^-, HCO₃^- and SO₄^2- were analyzed on High Performance Ion Chromatography (HPIC) using standard Method (Gaikwad et al., 2020b). The physico-chemical analysis of water sample is shown in the Table 1.

4. Results And Discussions

4.1 Spatial Variation in pH and EC

The high pH reported in the eastern and northern part of the basin while low is in the central parts (Figure 4). The pH is slightly alkaline (range: pH 7.0-8.1) which is due to loss of CO₂ and mineral salts precipitation. High EC (9160 µS/cm) is observed towards lower part in the basin, while low EC value (616 µS/cm) is reported in BW-33 in middle part of the basin (Figure 5). So, an average values of EC (2641 µS/cm) is indicating high mineralization of groundwater (Pawar et al., 2008; Gaikwad et al., 2020a).

4.2 Spatial Variation in Cations

Calcium (Ca²⁺), Magnesium (Mg²⁺), Sodium (Na⁺) and Potassium (K⁺) are the main four cationic constituents in all types of waters (Pawar et al., 2008; Wagh et al., 2016; Gaikwad et al., 2020b).

Increase in Na⁺ concentration from northwest to southeast due to hydro-geomorphological conditions (e.g., slope) (Figure 6). Low K⁺ (in Figure 7) concentration in groundwater observed due to non-appearance of K-bearing minerals (Subbarao et al. 1994; Pawar et al., 2008; Gaikwad et al., 2020a); excluding quiet increase in central part (BW-52) is excessive use of fertilizers (Pawar et al., 2008). Concentration of Mg²⁺ in groundwater ranges from 30.4 to 110.0 mg/L (Figure 9). In lower part of basin, high Mg²⁺ observed due to high salinity zone (Gaikwad et al., 2018).

Elevated Ca²⁺ values are found in lower part of the basin (Figure 8). The average, Na⁺ + Ca²⁺ representing 75.4 % of total cations signify weathering of felsic minerals such as plagioclase feldspar is major supply from lithologies (Pawar et al., 2008; Wagh et al., 2018;  Gaikwad et al., 2020a). While, Ca²⁺ + Mg²⁺ values accounts for 51.5 % (range: 28-69%) of the total cations indicating its supply is from olivine and pyroxene (Gaikwad et al., 2020b).

4.3 Spatial Variation in Anions

In anions, Bicarbonate (HCO₃^-) is the principal anion followed by, Chloride (Cl^-) and Sulphate (SO₄^2-).

High values of Bicarbonate (HCO₃^-) are observed in central and upper parts of the study area while low in the southern side. High values of HCO₃^- in groundwater (Figure 11) entail abundant supply of CO₂ by rainwater recharge and availability of larger surface area for rock-water interaction and vice-versa (Matthess and Harvey, 1982; Drever, 1982; Pawar et al., 2008). Chloride ranges from 29.6 to 888 mg/l with average of 235 mg/L. High concentrations of chloride is observed in the lower part of the basin (Figure 10). Thus, lower and high concentrations of Cl^- corresponding with recharge and discharge zones, respectively (Gaikwad et al., 2020b).

High concentration of chloride (888 mg/L in BW-55), is due to the combination of high evaporation, deeper source and use of fertilizers (Figure 10). High concentrations of SO₄^2- are reported in lower part i.e. south central part, while low concentration is in upper part of the basin (Figure 12). But high concentration in central part may be due to the use of excess fertilizers and agricultural runoff (Pawar et al., 2008; Wagh et al., 2018).

4.4 Geochemical Plots

In study of hydrochemical evolution it is imperative to know geological inputs, agricultural activities, precipitation and climatic condition, that are influencing the chemistry of groundwater (Wagh et al., 2018).

The Na⁺ vs. Cl^- scatter diagram (Figure 13) shows positive correlation (r = 0.95) in post monsoon season indicating combined influence of anthropogenic activities and ion exchange (Gaikwad and Pawar, 2008; Tiwari and Singh, 2014; Wagh et al., 2018; Gaikwad et al., 2020).

Similarly the cross plot of Ca²⁺+Mg²⁺ vs. HCO₃^- (Figure 14) depicts a positive correlation (r = 0.71), indicating influence of mafic minerals on groundwater geochemistry (Sami, 1992; Pawar et al., 2008; Wagh et al., 2018; Gaikwad et al., 2020a). The cross plot Na⁺+K⁺ vs. HCO₃^- plots also shows positive correlation (r = 0.64), suggesting that there is a contribution of ions from felsic lithologies in the area (Figure 17) (Pawar et al., 2008; Gaikwad et al., 2020a).

The plots of Ca²⁺ + Mg²⁺ vs. Cl^- + SO₄^2- (Figure 16) and Ca²⁺ + Mg²⁺ vs. SO₄^2-  + HCO₃^- (Figure 15) also show positive correlation (r=0.79 and 0.71, respectively), indicating that these are preferred ion pairs from silicate and carbonate lithologies (Pawar et al., 2008; Wagh et al., 2018; Gaikwad et al., 2020). The correlation between Ca²⁺ + Mg²⁺ vs. Na⁺ + K⁺ is also positive (r = 0.76), suggesting additions of ion from weathering of silicate rocks in the area (Figure 18) (Pawar et al., 2008; Gaikwad et al., 2020a). The positive correlation coefficients between Ca²⁺ and Mg²⁺ (r = 0.51), further confirm this reflecting role of Fe-Mg bearing silicates as their source (Figure 19).

In the (Ca²⁺ + Mg²⁺) vs. (HCO₃- + SO₄^2-) (Figure 15) scatter diagram, the ionic concentrations in meq/L are falling both the side of equiline indicating both carbonate and silicate weathering. Such a plot also shows that Ca²⁺ +Mg²⁺ are in excess of HCO₃^- possibly pointing to the process of ion exchange reaction (Rajmohan and Elango, 2004; Wagh et al., 2018).

4.5 Hydrochemical Facies

Physiochemical data of groundwater plotted on Piper Trilinear diagram to know its facies (Piper, 1944). The details are shown on figure 20 and given in table 2. The dominant hydrochemical facies Ca²⁺ + Mg²⁺, Na⁺ + K⁺; Cl^- + SO₄^2- HCO₃^- facies, found in 83.3 % samples indicating the alkaline earth is exceeding the alkalis and the strong acids exceeds the weak acids (Table  2).

4.6 Quality of Groundwater

The quality of water is very important to the mankind, because it has a direct link with human health and welfare. As the study area is falls in the zone of low rainfall, high temperature, and drastic evaporation, which modifies the quality naturally with anthropogenic activities and its pollution (Wagh et al., 2020; Gaikwad et al., 2018).

4.7 Groundwater Quality for Drinking Purposes

The water to be used for drinking purposes should be pure from physical, chemical and biological point of view (Nikumbh, 1997). The data compared with World Health Organization standards found that pH, Total Hardness (TH) and Magnesium (Mg²⁺) of the samples are more % of samples falling above Desirable limit. Otherwise the quality of groundwater is good for drinking (Table 3). 

4.8 Groundwater Quality for Irrigation

Total dissolved solids (TDS), proportion of Na⁺, HCO₃^- with Ca²⁺ and Mg²⁺ and toxic substances present in water are major factors for irrigation quality point of view (Alamary, 2005). Along with these, soil properties, water table depth, slope, climatic condition and crop pattern also affect it. Different parameters like Sodium Absorption Ratio (SAR), Soluble Sodium Percentage (SSP) and Kelly ratio (KR) have been calculated to analyze the quality of water for irrigation. The details of it have been given in Table 1 and 4. It is found that according to the SAR parameters all samples are excellent to good for irrigation. In SSP, 33.3 % samples are permissible, while 66.6% samples are doubtful for irrigation purpose. In KR, almost all samples (excluding 04 samples in lower side of basin) are suitable for irrigation.

The US Salinity Laboratory Staff diagram (USSL, 1954) is used to study water quality for irrigation (Figure 21). About 8.3 % samples classified in C2-S1 category indicating low salinity and sodium hazard, while 85.06 % of samples falling in C3-S1 category indicating low salinity hazard but medium sodium hazards. Remaining 6.6 % samples of the lower reaches of the river are falling in C4-S1 zone indicating very high salinity hazard and low sodium hazard. According to Adimalla and Venkatayogi (2018a) high salinity affects the growth of crops and can create nutritional disorders.

5. Summary And Conclusion

To assess the geochemistry of groundwater and its quality for drinking and irrigation purpose from Vel River basin, Sixty (60) samples of Dug well and Bore well were collected and analyzed using standard techniques. It shows order of dominance for cations: Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and in anionic concentration as HCO₃- > Cl^- > SO₄^2- in groundwater. The groundwater pH is slightly alkaline (range: pH 7.0-8.1) while average values of Electrical conductivity (EC) is about 2641 µS/cm indicating high mineralization of groundwater. The cationic values in the groundwater increase in the downstream side (from Northwest to Southeast), suggesting geological control on the composition of groundwater while highest concentration is in lower part of the basin are generally associated with the high salinity. In the major anions, bicarbonate (HCO₃^-) is higher due to rock-water interaction. Average value of chloride in is about of 235 mg/L due to discharge zones along with anthropogenic activities. High concentrations of SO₄^2- are reported in lower and central part of the basin may be due to the use of excess fertilizers and agricultural runoff. The Na⁺ vs. Cl^- scatter diagram shows positive correlation (r=0.95) indicating combined influence of anthropogenic activities and ion exchange mechanism. Similarly the cross plot of and Ca²⁺ + Mg²⁺ vs. HCO₃^- depicts a positive correlation (r=0.71), indicating influence of mafic minerals on groundwater geochemistry. The cross plot Na⁺ + K⁺ vs. HCO₃^- plots also shows positive correlation (r=0.64), suggesting that there is a contribution of ions from felsic lithologies in the area. The plots of and Ca²⁺ + Mg²⁺ vs. Cl^- + SO₄ and Ca²⁺ + Mg²⁺ vs. SO₄ + HCO₃^- also showing positive correlation (r = 0.79 and 0.71 respectively), indicating that these are preferred ion pairs from silicate and carbonate lithologies. The geochemical data plotted on Piper Trilinear Diagram showing The dominant hydrochemical facies Ca²⁺ + Mg²⁺, Na⁺+ K⁺; Cl- + SO₄^2-  HCO₃^- facies, found in 83.3 % samples indicating the alkaline earth is exceeding the alkalis and the strong acids exceeds the weak acids. In drinking water quality, pH, Total Hardness (TH) and Magnesium (Mg²⁺) of the samples are showing more % of samples falling above desirable limit. Otherwise the quality of groundwater is good for drinking. The irrigation quality indices SAR, KR and SSP were considered to evaluate groundwater suitability for irrigation. Study of SAR parameter showed that all samples are excellent to good for irrigation. In SSP, 33.3 % samples are permissible, while 66.6 % samples are doubtful for irrigation purpose. In KR almost all samples (except 04 samples i.e. 6.6 % in lower side of basin) are suitable for irrigation. In nutshell, groundwater geochemistry of Vel River Basin modified due to variations in climate, geology and anthropogenic activities in the basin.