Deterioration of water quality is a major problem in both developed and developing countries. The quality of water is currently worsening globally due to widespread economic development, changes in demographics, and climate change. About 2 billion people, who constitutes 26% of the world’s population, lack access to safe drinking water (UNEP, 2021). Although water quality assessments have been carried out globally, the data representing the actual scenario of freshwater quality are still scarce, thus putting 3 billion people globally at risk (UNEP, 2021). Hence, the protection of freshwater resources has gained importance, and the UN has also emphasized this in its sustainable development goals (SDGs), where SDG 6 aims to provide clean water and sanitation to the world’s population. After the advent of COVID-19, water quality issues have been addressed at the forefront globally. Most of countries are falling behind the target of achieving sustainable water management by 2030 (UN water 2021). The challenges faced by developing countries are more severe, since they face greater risks to ecological and human health, mostly due to lack of access to clean water. Recent studies state the increase of salinity in rivers, especially in Asia has worsened since 2010, indicating the risk to humans who consume polluted water (Gupta and Deshpande, 2004; United Nations Environment Programme, 2016). In addition, South Asian countries are the predominant users of groundwater in the world, where groundwater contamination is a major threat, especially in developing countries such as India. In India, the piped water supply is not provided in most of the rural regions, where it is assumed that the groundwater is safe for consumption. The Dependence on groundwater is high in the country, especially in southern India, due to limited surface water resources, non-perennial rivers, and frequent failure of monsoonal rains. About 70% of the freshwater systems are contaminated in India (Aayog, 2019), where 600 million people face high to extreme water stress. The gross domestic product (GDP) of the country has been reduced by one-third due to environmental degradation, leading to an annual loss of $80 billion (World bank 2019). In addition, the data systems related to the water of India are limited in their coverage, robustness, and efficiency.

Hence, the constant monitoring of surface water and groundwater quality is of prime importance. However, studies worldwide have assessed groundwater quality and surface water quality separately, although surface water and groundwater are interconnected. In the case of shallow aquifers, the groundwater is contaminated due to the percolation of contaminated surface water, which can be vice versa as well. Hence, surface water and groundwater need to be considered a single entity in most cases, emphasizing the need for conducting an integrated assessment of the quality of both river water and groundwater. This is critical for the sustainable and efficient management of water resources of river basins. Very few studies have assessed the interaction of surface water and groundwater and its effect on water quality (Brindha et al., 2014; Zhang et al., 2016; Zhu et al., 2019; Johnstone et al., 2022; Khan et al., 2019; Khan and Wen, 2021). Such surface water and groundwater interactions holds much significance in non-perennial rivers since they possess peculiar hydrology. The changing times between flow and non-flow periods will have a greater effect on the surface water and groundwater interactions and, subsequently, on the water quality. This aspect of the impact of water quality during the flow/non-flow periods in non-perennial rivers has not yet been studied in detail.

The Cauvery river, is a major non-perennial river of Southern India covering three states of the Country. The river branches into several distributaries, forming a delta at the lower part, and empties into the Bay of Bengal. The distributaries along with the main river Cauvery are have been the major source of water for both irrigation and domestic uses for the population residing in the Cauvery deltaic zone. The lower part of the Cauvery river is one of the significant hubs for rice production in India, which has been recently categorized as a protected special agricultural zone by the State Government of Tamil Nadu. Despite large-scale utilization of the river basin for irrigation and drinking purposes, poor water management practices along with unplanned urbanization and industrialization have deteriorated the water quality predominantly, especially in the lower part of the basin. Most of the water quality studies in the lower Cauvery Basin are mostly confined to groundwater. The quality of groundwater in few regions of the Cauvery delta regions were assessed for domestic and irrigation uses by various researchers (Rengaraj et al., 1998; Vetrimurugan and Elango, 2007; Nagarajan et al., 2010; Vetrimurugan et al., 2013; Gnanachandrasamy et al., 2015; Sivakumar et al., 2017). In addition to this, the assessment of groundwater pollution due to heavy metals was also studied (Vetrimurugan et al., 2017; TamilMani and Venkatesan, 2021). Multivariate statistical analysis was also used by a few researchers to decipher the contamination of groundwater in the lower Cauvery basin (Solaraj et al., 2010; Lalitha et al., 2021). However, studies pertaining to river water quality in the lower part of the basin are limited. RamyaPriya and Elango (2018) and RamyaPriya and Elango (2021) studied both groundwater and river water quality and its dissolved load; where they have studied only on the main stretch of the Cauvery river, indicating poor quality of water in the delta part of the river. Sharma and Ravichandran et al. (2021) studied river water quality in a smaller areal extent of the basin. However, most of the existing studies have neglected the river water quality of the distributaries present in the lower Cauvery basin. All the distributaries of Cauvery river are non-perennial, where the flow in these distributaries is mainly due to regulated flows from the upper catchment. Also, the effect of seasonal variation on the quality of river water and its adjoining groundwater has also not yet been studied. Hence, the present study was carried out to understand the factors controlling hydrogeochemistry and also to evaluate the quality of surface water and its adjoining groundwater in the lower part of the Cauvery Basin.

The Cauvery river, one of the most important non-perennial rivers of southern India, has a basin area of 81,555 km² and flows along a length of 800 km. It originates from the Brahmagiri hill ranges of the Western Ghats at Thalakaveri at an elevation of about 1,341 m and drains into the Bay of Bengal, forming a delta. The Cauvery delta forms the lower part of the Cauvery river system, which falls under the state of Tamil Nadu and has complex water issues. The mean temperature of the lower part of the basin is between 26°C and 30°C. April is generally the hottest month of the year with a mean monthly maximum temperature of 34.31°C (CWC, 2014). The main climatic feature in the lower region of the Cauvery river basin is the monsoon rain. The northeast monsoon provides the greater portion of annual precipitation; with the average annual rainfall of about 1,000 mm. The delta region receives greater rainfall during October, November, and December in the coastal districts, while further inland, the peak monthly rainfall is during early October (Supplementary Figure S3). Since the upper part of the basin receives rainfall during the southeast monsoon, the lower part of the Cauvery basin has river flow only from June to January and remains dry during the remaining periods (CWC, 2014). The Cauvery delta is river-dominated and is occupied by several of its distributaries. At Tiruchirappalli, three medium-sized reservoirs are in operation, of which the Upper Anicut reservoir is in the upstream of the urban region, while the Grand Anicut and Anaikarai are located downstream. The northern branch of the river is called the Kollidam, while the southern branch retains the name Cauvery and then flows directly eastward into Thanjavur District. From Thanjavur, the river splits and flows to a few places in the Cauvery delta Central groundwater board (CGWB, 2020). Some of the important distributaries of Cauvery forming the delta are Nandalar, Vanjiyar, Vettar, Kudamurutti, Vennai, Vennar, Pamani, Nanilam, and Thirumalarajanar (Figure 1). The present delta has developed a triangular pattern with three distinct morphological areas consisting of a marginal denudational unit, a central fluvial–marine depositional unit, and a coastal marine depositional unit (Babu, 1991). Geologically, the lower basin of the Cauvery river chiefly comprises of Quaternary alluvium; along with exposures of Cretaceous sediments (Uttatur, Ariyallur, and Tiruchirapalli formations) and sandstone (Cuddalore Formation) of the Mio-Pliocene age (Supplementary Figure S2). The Quaternary sediments are fluvial sediments, middle Holocene beach ridges, and late Holocene dune sands (Kunz et al., 2010). Also, the rock formations of Precambrian crystalline to Quaternary sediments exist in Cauvery delta. In the Quaternary alluvial sediments, there are both shallow (unconfined) and deep aquifers (semi-confined). The Cauvery delta region is underlain by different aquifers, namely, Archean, Cretaceous, Eocene, Miocene, and Quaternary alluvial aquifers Ministry of Water Resources (2012). The study area is an intensively irrigated region, where most of the area is covered by agricultural lands and lakes. The delta area is the most fertile tract in the basin. The main crops grown in the study region are paddy, pulses, groundnut, cotton, sugarcane, coconut, and vegetables. Principal soil types found in the basin are black soils, red soils, laterites, alluvial soils, forest soils, and mixed soils. Alluvial soils are commonly found in the delta areas. The lower Cauvery basin is also known for its hydrocarbon prospects, and the deltaic region has potential aquifer systems, particularly along the paleochannels and levee deposits (Das, 1991).

The analytical results have been evaluated to assess the suitability of river water and groundwater for drinking and agricultural uses. The summary of the analytical results for all the parameters of both river water and groundwater samples is shown in Table 1.

Generally, water quality is influenced by geogenic factors such as lithology and soil type; anthropogenic activities such as urban runoff, discharge of sewage, effluents, and agricultural runoff; and climate factors, such as precipitation and temperature, in addition to other factors. The Pearson correlation analysis was used to study the relationship of major, minor, and trace elements and to understand the factors affecting the quality of river water and groundwater. The correlation matrix with a positive correlation of HCO₃ with Ca (r ² = 0.832), Mg (r ² = 0.553), and Na (r ² = 0.777) was observed during the wet and dry seasons (Supplementary Tables S1, S2). The HCO₃ ions correlated with Ca (r ² = 0.613), Mg (r ² = 0.563), Na (r ² = 0.656), and Cl (r ² = 0.596) can be observed both in river water and ground water (Supplementary Tables S3, S4). This indicates that weathering of minerals is one of the major contributors of ions to surface water and groundwater. The weathering of silicate minerals from hard rocks such as quartz, plagioclase feldspar, pyroxenite, and mica could have contributed these ions to water. In river water, the correlation was observed between NO₃ and PO₄ (r ² = 0.533) in both seasons, whereas in groundwater, the correlation of PO₄ (r ² = 0.480) indicates the contamination from agricultural runoff, where NPK fertilizers are extensively used. A moderate correlation was observed between Zn with Cu (r ² = 0.457) and Cd (r ² = 0.481) during the wet season, and Zn with Cu (r ² = 0.478), Cd (r ² = 0.434), and Cd with Pb (r ² = 0.480) was observed during the dry season in river water. In groundwater, the relationship between Fe with Cu (r ² = 0.664) and Zn with Cu (r ² = 0.446) during both seasons indicates that they could have evolved from municipal sewage, industrial effluents, runoff containing fertilizer inputs from agricultural fields, and leaching from galvanized pipes. This indicates that anthropogenic activities influences the water chemistry of river water and groundwater. The mobility of Zn increases in water with high carbonate concentration, influencing the formation of hydrogen carbonates in water (Namieśnik and Rabajczyk, 2010).

The selected geochemical parameters are considered variables for principal component analysis (Figure 8) during both dry and wet seasons. Three principal components (PCs) have been extracted based on an eigenvalue greater than one. The factors account for a total variance of PC1 of 37.68% during the dry season and 31% during wet seasons in river water, whereas in groundwater, PC1 of 35.51% during the dry season and 32.16% during the wet season can be observed. PC1 has a strong positive load of Ca, Mg, Na, K, and HCO₃. This factor possesses the influence of geogenic inputs and also the effect of mineral dissolution in the study area. PC2 consists of 18.05% in river water during the dry season and 16.27% during the wet season. In groundwater, 12.08% during the dry season and 10.38% in the wet season can be observed with a positive load of Na, Cl, Cu, Fe, Zn, NO₃, PO₄, and Pb. This factor suggests the influence of agricultural inputs, fertilizer runoff, and corrosion of metals. PC3 constitutes 12.68% during the dry season and 9.07% during the wet season in river water; while in groundwater, 9.37% in the dry season and 7.78% in the wet season of the total variance, consisting of positive loading of F, U, Cd, NO₃, and PO₄, can be observed. This factor indicates that the concentration was attributed due to anthropogenic inputs, agricultural and urban land pollution, and corrosion of underground pipes.

The hydrogeochemical process plays an important role in determining the chemical constituents of the river and groundwater. The Durov (1948) plot in both river water and groundwater shows similar water types of Ca–HCO₃ type, followed by SO₄–Na (Figure 9). This shows that the river water and groundwater have the same type of chemical composition in this study area. Gibbs (1970) proposed a diagram representing the plots in which TDS vs. Na/(Na + Ca) for cations and TDS vs. Cl/(Cl + HCO₃) for anions were used to assess the functional sources of dissolved chemical constituents, which could be precipitation dominance, rock dominance, or evaporation dominance. The rock–water interaction was the dominant process, followed by the evaporation process for river water in the study region. For groundwater, it was found that the rock–water interaction was the most dominant process.

The effect of evaporation on the water quality was also assessed, since evaporation is a natural process that increases the concentrations of all dissolved ions in water, especially during the dry season, leading to an increase in the salinity of the water. The relationship between the sodium chloride ratio vs. EC of water is an indication of evaporation or evapotranspiration. If evaporation is dominant, assuming that no mineral species are precipitated, then the Na/Cl ratio would be unchanged (Jankowski and Acworth, 1997). Hence, the plot of the Na/Cl ratio is an indicator of an increase in concentration by evaporation or evapotranspiration. The EC vs. Na/Cl scatter diagram (Figures 10A, B) shows that the trend line is horizontal, which means that evaporation plays a significant role in controlling the river water chemistry during both seasons. When evaporation is dominant, the concentration of ions in the soil zone increases, thus leading to the precipitation of salts in the soil zone. In groundwater (Figures 10C, D), the inclined trend line is found, indicating that evaporation is not the dominant process in controlling the geochemistry.

The rock–water interaction is the major contributor of ions for both river water and groundwater in the lower Cauvery Basin. The rock–water interaction could be due to the weathering of rock and the ion exchange process. It is identified that the weathering of rocks contributes to most of the major ions in both river water and groundwater (Figure 11). Therefore, the concentration of major ions in river water and groundwater has been used to determine the weathering process. The role of silicate weathering is dominant if sodium-normalized Mg values are between 0.1 and 1 and the value of Ca is between 0.5 and 5 (Gaillardet et al., 1999). Figure 11 indicates that the samples fall in the zone of silicates, which indicates the dominance of silicate weathering in the lower Cauvery Basin.

Ion exchange is one of the important processes responsible for the concentration of ions in water. Chloro-alkaline indices (CAI) are a better representation of the ion exchange process, where CAI I and CAI II are calculated for the river water and groundwater samples by using the following formula (Schoeller, 1967): CAI   I = Cl − Na + K / Cl (13) CAI II = Cl − ( Na + K ) / ( SO 4 − HCO 3 + CO 3 + NO 3 (14)

When there is an exchange between Ca and Mg in the water with Na and K in the aquifer material, both the aforementioned indices would be negative, and if there is a reverse ion exchange, then both these indices will be positive (Schoeller, 1965; Schoeller, 1967). In river water, the reverse ion exchange process occurs during the dry season in most of the locations, whereas in the wet season, both the indices values were negative, indicating the ion exchange process (Figures 12A, B). In groundwater, CAI I and CAI II indices indicate the dominance of the reverse ion exchange process during both seasons (Figures 12C, D). The evidence for cation exchange is given by bivariate plots of Ca + Mg–HCO₃–SO₄ as a function of Na–Cl. Na–Cl represents the amount of Na + K gained or lost relative to that provided by chloride salt dissolution (mostly halite dissolution), while Ca + Mg–HCO₃–SO₄ represents the amount of Ca and Mg gained or lost relative to that provided by gypsum, calcite, and dolomite dissolution. If reverse cation exchange was an important composition-controlling process, the relation between these two parameters would be linear, with a slope of –1 (Fisher et al., 1997; McLean et al., 2000). The reverse ion exchange process occurs in groundwater during both seasons, whereas in river water, the reverse ion process occurs only during the dry season. The flow in the river controls the ion exchange process in the river, where during the dry periods, the river water is enriched with sodium ions which forces the sodium ions toward Ca–Mg-based minerals, thus causing reverse ion exchange. During the wet periods, the flow of the river inhibits the enrichment of ions, thus causing cation exchange to occur in the river water.

The quality of both river water and groundwater in the lower Cauvery basin was found to be good during wet periods compared with the dry season. The flow of the river controls the quality of river water to a greater extent, where the concentration of ions is much lesser during the wet periods due to dilution. Also, the quality of groundwater is heavily influenced by the recharge of river water, where the quality is good during wet periods. The relation between the levels of river water and groundwater indicates that during the dry seasons groundwater is discharged into the river, while the river recharges the groundwater during the wet seasons (Figure 13). Also, the temporal variation of major ions between river water and groundwater in a few locations indicates that they follow a similar pattern, thus indicating their interrelationships. This evidences the interaction between river water and groundwater in the lower part of the Cauvery basin. Since these rivers are of non-perennial nature, the interconnection between the river water and groundwater is very much prominent. Also, unregulated release of effluents, agricultural runoff, and improper sewage disposal in the water systems affects both river water and groundwater, especially during dry periods. Hence, proper stringent measures should be taken to protect both groundwater and river water from such improper disposal of pollutants. This study also necessitates the development of a finer observational network for water quality at the downstream and distributaries for the surface water and groundwater in the lower part of the Cauvery basin. Integrated water management options should be explored in the lower part of the Cauvery basin, considering the groundwater in river basin management planning. Developing capacity-building programs and community participation in river basin management should be encouraged to sustainably protect and preserve the water resources in the Cauvery lower basin.

The quality of the river water and groundwater for drinking and irrigational uses in the lower Cauvery river basin was assessed. Most of the river water and groundwater samples were found to be fresh and were of the soft category. Based on the CCME water quality index, it was observed that most of the river water and groundwater samples were suitable for drinking purposes. Both river water and groundwater were found to be unsuitable for drinking at locations near the coast and near irrigated lands. The irrigation water quality index based on TDS, SAR, RSC, MH, KR, and trace elements, where the results shows that some groundwater and river water samples near the coast and agricultural areas were not suitable for irrigational purposes. In general, it was found that the quality of both river water and groundwater were poor during dry seasons compared with the wet periods. The geogenic sources were the major contributor of ions such as Ca, Mg, Na, K, and HCO₃. Anthropogenic sources also resulted in elevated concentrations of Na, Cl, Cu, Fe, Zn, NO_3, PO₄, and Pb in water. The rock–water interaction, followed by evaporation, were the dominant process controlling the hydrogeochemistry of river water, while the rock–water interaction was the dominant process in groundwater. Silicate weathering was the dominant process, followed by carbonate weathering. The study indicated that the source of contamination of river water and groundwater is also due to the indiscriminate disposal of untreated sewage and solid waste into the river, runoff from agricultural fields, and seawater intrusion in the coastal regions. Hence, it is necessary to strengthen the water quality monitoring network to find out the sources of anthropogenic polluters and it necessary to prevent the disposal of sewage and waste into the Cauvery river.