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Hydrochemical investigations of the groundwater and the seasonal effect on the chemical budget of ions along the course of the polluted river Adyar were carried out. From the geochemical results, it has been found that the seasonal effect does not change the order of abundance of both cations and anions, but it does change the concentration of various ions present in the groundwater. Among the chemical budget of ions, sodium and chloride were found to be the most predominant ions. The nitrate concentration in the groundwater ranges from 4.21 to 45.93 mg/l in pre-monsoon and in post-monsoon it ranges from 1.02 to 75.91 mg/l. The nitrate concentrations in the post-monsoon are high in some places especially in the upper stretch of the river. The intense agricultural activities near the upper stretch of the river may be an important factor for the higher concentration of nitrates in these aquifers. In order to determine the geochemical nature of water, the data was interpreted using the piper diagram wherein the results show the predominance of NaCl and CaMgCl types. Equiline diagrams, 1:1, were applied to evaluate the affinity ion relationship between various ions present in these waters. The quality of the groundwater was assessed with regard to its suitability to drinking and irrigation. A comparison of the groundwater quality in relation to drinking water quality standards shows that most of the water samples are not suitable for drinking, especially in post-monsoon period. US Salinity Laboratory’s, Wilcox’s diagrams, Kellys ratio and magnesium ratio were used for evaluating the water quality for irrigation which suggest that the majority of the groundwater samples are not good for irrigation in post-monsoon compared to that in pre-monsoon. Moreover the source of the ions in the water was examined and classified accordingly using Gibb’s diagram. The analytical results reveals that the TDS values of the pre-monsoon samples were found to be lower than the post-monsoon reflecting that leaching predominates over that of the dilution factor.

Geochemical analytical data of 15 representative rock samples, 34 soil samples and 55 groundwater samples collected from Salem magnesite mines and surrounding area in Salem, southern India, were subjected to R-mode factor analysis. A maximum of three factors account for 93.8 % variance in rock data, six factors for 84 % variance in soil data, five factors for 71.2 % in groundwater data during summer and six factors for 73.7 % during winter. Total dissolved solids are predominantly contributed by Mg, Na, Cl and SO4 ions in both seasons and are derived from the country rock and mining waste by dissolution of minerals like magnesite, gypsum, halite. The results also show that groundwater is enriched in considerable amount of minor and trace elements (Fe, Mn, Ni, Cr and Co). Nickel, chromium and cobalt in groundwater and soil are derived from leaching of huge mine dumps deposited by selective magnesite mining activity. The factor analysis on trivalent, hexavalent and total Cr in groundwater indicates that most of the Cr in summer is trivalent and in winter hexavalent. The gradational decrease in topographical elevation from northern mine area to the southern residential area, combined regional hydrogeological factors and distribution of ultramafic rocks in the northern part of the study area indicate that these toxic trace elements in water were derived from mine dumps.