Introduction

Development of agricultural activities depends upon the availability of sufficient quantity and quality of water resources. Since the last two decades, groundwater plays an important role in the agricultural sector due to non-availability of surface water in time. In India, irrigation practices utilize about 80 % of the groundwater resources for proper outcome of crop yields, which contributes to increase of national economy.

However, the sources of geogenic (toxic minerals, clay horizons, seawater transgression and incursion, etc.) and non-geogenic (dumping of domestic wastes, leakage of drainage waters and septic tanks, heavy usage of agrochemicals, etc.) origin can deteriorate the quality of groundwater. Back (1966), Drever (1988), Stallard and Edmond (1983), Hem (1991), Karanth (1977) and Subba Rao (2002) explain the causes of changes in the groundwater quality with respect to soil–rock–water interaction, anthropogenic activities and marine sources. Freshwater and salt water interactions are evaluated in Jeju island of South Korea by Kim et al. (2008). Pandey (1983) and Saha (2007) give details on the geochemical characteristics and evolution of groundwater in the Pleistocene deeper and shallow aquifers of South Ganga Plain, Bihar, India. Hydrogeochemical characteristics of groundwater in phreatic aquifers of Alleppey district, Kerala are reported by Shaji et al. (2009). Afroza et al. (2009) evaluated the impact of rock weathering on the groundwater quality with respect to dolomite dissolution and calcium precipitation was discussed in parts of Lower Tista Floodplain, Northwest Bangladesh. Manjusree et al. (2009) explain the groundwater chemistry of the coastal sandy clayey aquifers of Alappuzha district, Kerala, India. Hydrogeochemistry of the coastal aquifers in Tuticorin, Tamil Nadu, India is studied by Mondal et al. (2011) to assess the influence of water–rock interaction, anthropogenic contamination and seawater. Subba Rao et al. (2012) elucidate the chemical characteristics of groundwater from the river basin of Varaha, Visakhapatnam district, Andhra Pradesh, India. Subba Rao et al. (2014a, b) give details on the causes of variation of groundwater chemistry in the coastal region of Chirala–Ongole, Andhra Pradesh, India.

The Western Delta region of the River Godavari, Andhra Pradesh is known as one of the major rice-producing centers in the country (India). But, there are some constraints for developmental activities, including agricultural output. Because, the groundwater occurring in the alluvial deposits in the region is suitable for domestic and irrigation purpose, while that in the floodplain and coastal plain is not fit for any purpose. However, no research has so far been done for assessing the controlling processes of the chemistry of groundwater for taking up suitable remedial measures in the region. Therefore, this region is selected for the present study to evaluate the geochemistry of groundwater for sustainable development of the area.

Study area

The study region, lying between north latitudes 16°18′05″–17°2′ and east longitudes 81°53′ and 81°52′ (Fig. 1), falls in Survey of India toposheet Nos. 65H/5, 6 and 9–11. It is flanked by upland crystalline terrain on north, Bay of Bengal on south, the river Vasista Godavari on east and the river Upputeru and Lake Kolleru on west. The region experiences a tropical humid type. The minimum temperature is 18 °C in January (winter) and the maximum temperature is 30 °C in May (summer). Mean annual rainfall is 1078 mm. Most rainfall (75 %) received is from southwest monsoon. Mean monthly humidity is 80 % in forenoon and 62 % in afternoon. Mean monthly wind speed ranges from 5.4 km/h in March to 12.7 km/h in July. The annual potential evapotranspiration is 1467 mm.

Fig. 1
figure 1

Location map of the Western Delta region of River Godavari, Andhra Pradesh, India

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The study region has a plain land, sloping gently (<3°) toward southeast. Important landforms are valley fills, channels, levee, back swamp, channel, point bar of fluvial and active beach, paleo-beach ridges, back water and tidal flats, spits, mangrove swamps of marine origin (Rengamannar and Pradhan 1961; Nageswara Rao et al. 2005). Kankar (concretion of CaCO3) occurs in the soil zone.

Geologically, the area is underlain by coarse sand with black clay (buried channels zone), black silty clay of recent origin (floodplain zone) and gray/white fine sand of modern beach sediment of paleo-beach ridges and active beach ridge of marine origin (coastal zone; Fig. 2). Lithologs reveal that the topsoil is followed by sticky clay zone, fine sand zone, clayey zone, coarse to medium sand zone and clay–silt zone.

Fig. 2
figure 2

Geological map of the study region (BC buried channel, FP floodplain, C coastal zones are marked in the map)

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Groundwater occurs under unconfined to semi-confined conditions. Extraction of groundwater is through open dug wells (<10 m depth), filter point wells (10–25 m depth) and tube wells (30–60 m depth). Depth to water level varies from less than 1–22 m. The direction of groundwater flow is NW–SE, following the drainage.

About 95 % of the study area is occupied by agricultural land and the remaining 5 % of the area by tree crops, mangroves, aquaculture practices and small water bodies. The principal crops are paddy, sugarcane and banana. The other crops are cashew nut, mango, coconut, tobacco, turmeric, etc.

Materials and methods

Seventy-two water samples were collected from open dug wells and filter points from the study region during May, 2010 (Fig. 1). Out of which, 29 samples (1–29) belong to buried channel (BC), 30 samples (30–59) to floodplain (FP) and the rest 13 (60–72) to coastal (C) zone.

The pH and electrical conductance (EC) were measured in the field, using their portable meter. The water samples were analyzed for calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), bicarbonate (HCO3 −), chloride (Cl−), sulfate (SO4 2−), nitrate (NO3 −) and fluoride (F−), following the standard water quality methods of APHA (1992). The EC was used to compute the TDS (Hem 1991). The concentrations of TDS and chemical species are expressed in milligrams per liter (mg/L) except pH.

Cation–anion balance was computed, taking their concentrations in milliequivalents per liter (meq/L), which is observed within the limit of ±5 % (Domenico and Schwartz 1990).

Results and discussion

The pH varies from 7.17 to 8.82, being a mean of 7.83 in BC, 7.93 in FP and 7.83 in C zones, indicating alkaline condition (Table 1). Naturally, the pH is controlled by CO2–CO3–HCO3 − equilibrium (Drever 1997). The CO2 with H2O forms H2CO3, resulting in a formation of HCO3 −, which causes a pH (Berner and Berner 1987). It dissolves the salts and/or minerals present in the soils and/or geological formations. Finally, they reach the groundwater body, depending upon the source material. In the present study area, the geological formations of the each zone are intermixed with each other (Fig. 2). Therefore, the pH shows a close variation in the three zones (BC, FP and C). As a result, the hydrogeochemical facies are dominant by HCO3 − ion.

Table 1 Chemical composition (mg/L) of groundwater in the Western Delta region of River Godavari, Andhra Pradesh, India
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The EC ranges from 266 to 12,900 µS/cm, which is high in C zone (mean 3887 µS/cm), following FP zone (mean 2859 µS/cm) and BC zone (mean 1639 µS/cm). This indicates the variation in the enrichment of salts in the groundwater. The TDS is from 170 to 8256 mg/L. The low TDS is observed from BC (mean 1046 mg/L), medium TDS from FP (mean 1770 mg/L) and high TDS from C (mean 2487 mg/L) zones. On the basis of TDS classification (Fetter 1990), the quality of groundwater shows fresh (TDS <1000 mg/L) to brackish (TDS 1000–10,000 mg/L), the latter being dominant in C zone. The variation in the TDS indicates the influence of canal and river water in BC zone, anthropogenic source in FP and marine source in C zone. The TH is from 120 to 1260 mg/L, being high in C zone (mean 493 mg/L), medium in FP zone (mean 320 mg/L) and low in BC zone (mean 220 mg/L). As per the TH classification (Sawyer and McCarty 1967), the groundwater comes under moderately hard (75–150 mg/L) to very hard (>300 mg/L) categories.

The Ca2+ varies from 8 to 569 mg/L, with a mean of 15, 20 and 65 mg/L in BC, FP and C zones, respectively (Table 1), while the Mg2+ is from 17.39 to 204.20, being a high in C zone (88 mg/L), medium in FP zone (73 mg/L) and low in BC zone (41 mg/L). Relativity, the higher Mg2+ than Ca2+ is a result of ferro-magnesium minerals in soil salts and aquifer sediments, ion exchange between Na+ and Ca2+, precipitation of CaCO3 and seawater intrusion (Todd 1980; Hem 1991; Drever 1997). The Na+ varies from 15.30 to 955.80 mg/L, which is observed to be low from BC (mean 193 mg/L), medium from C (mean 349 mg/L) and high from FP (mean 362 mg/L) zones due to influences of soil salts, deltaic clayey sands, coastal clayey sands, marine salts, anthropogenic activities, etc. on groundwater system. A low K+ (1.42–310.60 mg/L, with a mean of 23, 67 and 92 mg/L in BC, FP and C zones, respectively) indicates its tendency to be fixed by clay minerals (Hem 1991).

The HCO3 − ranges from 84 to 1752 and its mean is low in BC zone (344 mg/L), which is followed by FP (628 mg/L) and FP (752 mg/L) zones. It results from an intense mineral weathering and favoring mineral dissolution (Stumm and Morgan 1996). The Cl− is from 26 to 960 mg/L with its higher concentration (mean 405 mg/L) in C zone, medium concentration (mean 360 mg/L) in BC zone and low concentration (mean 156 mg/L) in FP zone. It derives mainly from the non-lithological sources (domestic waters, septic tanks, irrigation return flows, agrochemicals and marine salts; Todd 1980; Hem 1991; Appelo and Postma 1993; Subba Rao 2002; Nag and Ghosh 2013). The SO4 2− is in between 21.62 and 432.27 mg/L, with a mean of 147 mg/L in BC and FP zones, and 208 mg/L in C zone. The main source of SO4 2− is gypsum, which is used to alter physical and/or chemical properties of soils (Todd 1980). Another source is seawater (Hem 1991). The NO3 − varies from 0.27 to 48 mg/L, which is relatively high (mean 8 mg/L) in C zone compared to BC (mean 4 mg/L) and FP (mean 6 mg/L) zones. The higher concentration of NO3 −, i.e., >10 mg/L, is an indication of man-made pollution (poor sanitary conditions, unlimited application of fertilizers, etc.; (Cushing et al. 1973). The F− is observed to be very low (0.12–0.15 mg/L), which is less than the lowest desirable limit of 0.60 mg/L in the water (BIS 2012).

Further, certain parameters like EC, TDS, Na+, K+, Cl−, HCO3 − and NO3 − show a wide higher range of standard deviations, while others (Ca2+, Mg2+ and SO4 2−) have a lower range of standard deviations in BC, FP and C zones. These suggest the inter-mixing of chemical processes caused by geogenic, anthropogenic and marine origin along with water flow path.

The quality of groundwater shows three types of hydrogeochemical facies based on procedures of Seaber (1962) and Back (1966). They are (i) Na+ > Mg2+ > Ca2+:HCO3 −> Cl−> SO4 2−, (ii) Na+>Mg2+ > Ca2+:Cl−> HCO3 −> SO4 2− and (iii) Mg2+ > Na+ > Ca2+ > or < K+:HCO3 − > Cl− > or < SO4 2−. A dominance of HCO3 − in first and third facies classifies the area as a recharge zone, while that of Cl− in second facies delineates the area as a discharge zone.

Geochemical evolution of groundwater

According to Todd (1980), Hem (1991) and Drever (1997), the chemical species of cations derive mainly from the lithological (geogenic) sources, while those of anions from the non-lithological (anthropogenic) sources. Depending upon the occurrence of source material under climatic conditions, the quality of groundwater varies from place to place. The expected controlling chemical processes in the groundwater are mineral dissolution, leaching of ions, ion exchange, precipitation of ions, evaporation, etc.

Evaporation

In tropical climate, evaporation plays a vital role in the change of groundwater quality. In the present study region, the annual potential evapotranspiration is 1467 mm. The occurrence of kankar in the soil zone supports the evaporation activity in the study region and thereby changes in the groundwater quality. To examine the influence of evaporation on the groundwater, the chemical composition of groundwater of the study region is plotted in Na+ vs Cl− diagram (Fig. 3). Groundwater samples are observed to be close to equiline (1:1) of Na+:Cl− compared to the freshwater evaporation line. It indicates the dissolution of Cl−—salts accumulated by evaporation, as the salts are transferred from groundwater to soil (Drever 1997; Subba Rao 2002). During post-monsoon, the groundwater levels touch the soil zone, which transfers the ions into the soil–water interface.

Fig. 3
figure 3

Relation between Na+ and Cl− in the groundwater

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Saturation index of CaCO3 is also computed for examination of the influence of evaporation on the groundwater. Groundwater in BC zone shows unsaturation (−0.33) with respect to the solid phase of CaCO3 (Table 2), indicating a dissolved state, while it is oversaturation (0.48–0.89) in FP and C zones, resulting a precipitated state, which is further supported by the occurrence of CaCO3 concretions in the soil zone due to tropical humid climate of the study region.

Table 2 Results of source rock minerals’ deduction of groundwater in the Western Delta region of River of Godavari, Andhra Pradesh (after Hounslow 1995)
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Rock weathering

Rock weathering is a process by which the chemical species can be released into the groundwater through dissolution and leaching. To find out the sources of Ca2+ and Mg2+ in the groundwater, a combination of Ca2+, Mg2+, HCO3 − and SO4 2− is used (Ritchler et al. 1993). The chemical composition of the present study region is plotted in Ca2+ + Mg2+ vs HCO3 − + SO4 2− (Fig. 4). About 96 % of the groundwater samples are observed below the equiline of Ca2+ + Mg2+:HCO3 − + SO4 2−. It indicates an excess of SO4 2− and HCO3 − over Ca2+ and Mg2+ in the groundwater due to input from the weathering of Ca–Mg minerals (calcite, gypsum and plagioclase feldspar present in the sediments (Cerling et al. 1989; Fisher and Mullican 1997; Srinivasamoorthy et al. 2011). The remaining water samples (4 %) fall above the uniline (1:1) of Ca2+ + Mg2+:HCO3 − + SO4 2−, in which the concentrations of Ca2+ and Mg2+ are more than those of SO4 2− and HCO3 −. The extra source of Ca2+ and Mg2+ ions is caused by reverse ion exchange process. From this observation, it is clear that these ions are derived mainly from the weathering of plagioclase feldspar, pyroxenes, amphiboles, biotite and garnet minerals present in the sediments of the study region. Because of the mineral weathering, most groundwater samples (92 %) fall above the fresh water equiline (1:1) of Na+:Cl− (Fig. 3), reflecting the release of Na+ from the mineral weathering (Meyback 1987). If the water derives primarily by mineral weathering, it would have HCO3 − as the most abundant ion, as in the study region (Table 1).

Fig. 4
figure 4

Relation between SO4 2− + HCO3 − and Ca2+ + Mg2+ in the groundwater

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In plot of Na+ + K+ vs total cations (TC), about 92 % of the groundwater samples fall in between the 1:0.5 and 1:1 line of Na+ + K+:TC and the remaining (8 %) water samples are found below the line of 1:0.50 (Fig. 5). This appears to be caused by rock weathering and/or leaching of soil salts (Stallard and Edmond 1983; Subba Rao 2002). Further, it is also observed from the Na+ + K+ vs Cl− + SO4 2− diagram (6), the concentrations of Na+ and K+ increase simultaneously with the increase of Cl− and SO4 2−. This indicates a common source of these ions from the dissolution of soil salts, as also observed in Ganga–Brahmaputra River Basin (Sarin et al. 1989), Delhi Region (Datta and Tyagi 1996), Guntur District (Subba Rao 2002) and Cuddalore District (Srinivasamoorthy et al. 2011) in India (Fig. 6).

Fig. 5
figure 5

Relation between TC and Na+ + K+ in the groundwater

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Fig. 6
figure 6

Relation between Na+ + K+ and Cl− + SO4 − in the groundwater

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The HCO3 − is plotted against TC (Fig. 7) for understanding the influence of dissolution of carbonate and silicate minerals on the groundwater system (Stumm and Morgan 1996; Kim 2003). From the present study region, all groundwater samples fall above the equiline of HCO3 −:TC, but away from its uniline. Broadly, it infers the dissolution of carbonate and silicate minerals in the groundwater. However, the deviation of the groundwater sampling points from the equiline of HCO3 −:TC suggests that the influence of other activities like anthropogenic and marine origin is also involved on the groundwater system. Otherwise, the deviation of water sampling points may not be possible. Thus, there is also a deviation of most of the water sampling points in HCO3 − vs Ca2+ + Mg2+ diagram (Fig. 8), as also reported in Alappuzha District, Kerala, India (Manjusree et al. 2009).

Fig. 7
figure 7

Relationship between HCO3 − vs TC

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Fig. 8
figure 8

Relation between HCO3 − and Ca2+ + Mg2+ in the groundwater

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Ion exchange

In Na+ vs Ca2+ diagram (Fig. 9), the groundwater sampling points are away from the uniline of Na+:Ca2+. Especially, they fall mainly toward the concentration of Na+. This explains that the ion exchange of Ca2+ with Na+ (Mercado 1985; Al-Khashman 2007; Saha et al. 2008; Shekhar Gupta et al. 2009) and the influence of soil salts, deltaic clayey sands, coastal clayey sands, anthropogenic activities and marine salts are responsible for the higher concentration of Na+ in the groundwater. Otherwise, we could not expect such a deviation from the equiline of Na+:Ca2+. Further, the higher Mg2+ than Ca2+ is found in the groundwater (Fig. 10). This is a result of dissolution of ferro-magnesium minerals occurring in the soil salts and marine salts (Hem 1991; Subba Rao 2002; Subba Rao et al. 2005).

Fig. 9
figure 9

Relation between Na+ and Ca2+ in the groundwater

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Fig. 10
figure 10

Relation between Mg2+ and Ca2+ in the groundwater

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Hounslow (1995) suggests the possible source of ions from the minerals into groundwater system (Table 2). This further supports the changes in the groundwater chemistry as explained above.

Mechanism controlling groundwater chemistry

In Na+ + K+:Na+ + K+ + Ca2+ and Cl−:Cl− + HCO3 − vs TDS diagrams (Fig. 11), the chemical composition of the groundwater samples of the study region is plotted to assess the influence of precipitation (rainfall), rock and evaporation on groundwater system. The groundwater sampling points spread from the rock domain toward the evaporation one. It indicates that the groundwater chemistry is mainly controlled by rock weathering and subsequently modified by the sources of anthropogenic and marine origin. As the increase of Na+ and Cl− by anthropogenic and marine sources, the groundwater shows a higher TDS. Therefore, the water chemistry moves toward the evaporation domain from the rock domain. These mechanisms obviously explain that the initial groundwater quality is subsequently modified by anthropogenic and marine sources. This further supports the evolution process of the geochemistry of groundwater in the region, as explained in the earlier sections.

Fig. 11
figure 11

Mechanisms controlling the groundwater chemistry (after Gibbs 1970)

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Conclusions

The quality of groundwater is mostly brackish and very hard. It shows three facies viz. (i) Na+>Mg2+>Ca2+:HCO3 − > Cl− > SO4 2− > NO3 −, (ii) Na+ > Mg2+ > Ca2+:Cl−> HCO3 − > SO4 2−, and (iii) Mg2+ > Na+ > Ca2+> or < K+:HCO3 − > Cl− > or < SO4 2− facies. The plotting of the chemical composition of the groundwater samples in Ca2+ + Mg2+:HCO3 −, Ca2+ + Mg2+:SO4 2− + HCO3 −, Na+ + K+:TC, Na+ + K+:Cl− + SO4 2−, HCO3 −:TC, HCO3 −:Ca2+ + Mg2+, Na+:Cl− and Na+:Ca2+ diagrams suggests that the quality of groundwater is controlled by rock weathering, mineral dissolution, evaporation and ion exchange. The possible source of ions altered the original composition of groundwater mostly by dolomite dissolution and calcium precipitation and weathering of plagioclase and other ferro-magnesium minerals present in the sediments. The sources of anthropogenic and marine origin are the supplementary factors for brackish water quality. This is further supported by Gibbs mechanisms that control the aquifer chemistry. Thus, the present study suggests that the original chemical composition has been affected by geogenic and subsequently modified by the impact of anthropogenic and marine sources.