Hydrogeochemistry of a Small Tropical River Basin with Special Reference to Ground Water Recharge Estimation Using Conservative Chloride Mass Balance Method: a Case Study from Southwest Coast of India

River Nethravati is a small west flowing river originating in the Western Ghatsand falling into the Arabian Sea after traversing for 147km. The river flows through a densely vegetated forest with steep gradient in the youthful stages of the river which rapidly decreases in the plains. The catchment of the river is spread out in an area of 3657km2 with a discharge of 388m3/sec. The watershed receives frequent and intense southwest monsoonal rainfall with an annual avearage of 4300mm. During the monitoring period, the surface waters from the river Nethravati were collected on a monthly basis for a period of three years, groundwater were collected once in three months for a year and rain waters were collected during the monsoon for a year from the study area and subsequently analysed for major ions. Multivariate statistical techniques like ANOVA, PCA/FA and CA are applied to understand the temporal variability, sources of major ions and to explain the hydrogeochemical process which could explain the water chemistry.The statistical results shows very less heterogeniety in water chemistry temporally and spatially. The anthropogenic source of major ions are negligible during the study period and silicate mineral weathering are the dominant source of major ions in the catchment. In this study, an attempt is also made to explain the interacton between surface water and ground water. The results suggests homogeneity of chemical composition between these compartments which could be explained by shallow ground water table, steep gradients and local recharge. Recent studies on small watersheds highlighted the significance of deep ground water in estimation of chemical outputs (Marechal et al., 2011). In this study, the chloride mass balance approach is used to estimate the annual groundwater recharge to this shallow unconfined aquifer. Natural concentrations of chloride, dissolved in precipitation and groundwater are used to quantify the rate of ground water recharge which is accomplished by using a mass balance of the conservative chloride ion. It is found that the mean chloride concentration in ground water is 4.8mg/L and mean chloride concentration in precipitation is 1.3mg/L. The calculated ground water recharge is >15% of the total precipitation. The ground water discharge is estimated by balancing the mass of chloride in surface runoff, precipitation and groundwater.