Geochemical evaluation of nitrate and fluoride contamination in varied hydrogeological environs of Prakasam district, southern India

Hydrogeochemical controlling factors for high rate of groundwater contamination in stressed aquifer of fractured, consolidated rocks belonging to semi-arid watershed are examined. The groundwater in mid-eastern part of Prakasam district confining to Musi-Gundlakamma sub-basins is heavily contaminated with nitrate and fluoride. Distinct water chemistry is noticed among each group of samples segregated based on concentration of these contaminants. The nitrate is as high as 594 mg/l and 57 % of the samples have it in toxic level as per BIS drinking water standards, so also the fluoride which has reached a maximum of 8.96 mq/l and 43 % of samples are not fit for human consumption. Nitrate contamination is high in shallow aquifers and granitic terrains, whereas fluoride is in excess concentration in deeper zones and meta-sediments among the tested wells, and 25 % of samples suffer from both NO3 (-) and F- contamination. Na+ among cations and HCO3 (-) among anions are the dominant species followed by Mg2+ and Cl-. The NO3 (-)-rich groundwater is of Ca2+-Mg2+-HCO3 (-), Ca2+-Mg2+-Cl- and Na+-HCO3 (-) type. The F--rich groundwater is dominantly of Na+-HCO3 (-) type and few are of Na+-SO4 (2-) type, whereas the safe waters (without any contaminants) are of Ca2+-Mg2+-HCO3 (-)- and Na+-HCO3 (-) types. High molecular percentage of Na+, Cl-, SO4 (2-) and K- in NO3 (-) rich groundwater indicates simultaneous contribution of many elements through domestic sewerage and agriculture activity. It is further confirmed by analogous ratios of commonly associated ions viz NO3 (-):Cl-:SO4 (2-) and NO3 (-):K+:Cl- which are 22:56:22 and 42:10:48, respectively. The F- rich groundwater is unique by having higher content of Na+ (183 %) and HCO3 (-) (28 %) than safe waters. The K+:F-:Ca2+ ratio of 10:5:85 and K+:F-: SO4 (2-) of 16:7:77 support lithological origin of F- facilitated by precipitation of CaCO3 which removes Ca2+ from solution. The high concentrations of Na+, CO3 (-) and HCO3 (-) in these waters act as catalyst allowing more fluorite to dissolve into the groundwater. The indices, ratios and scatter plots indicate that the NO3 (-) rich groundwater has evolved through silicate weathering-anthropogenic activity-evapotranspiration processes, whereas F- rich groundwater attained its unique chemistry from mineral dissolution-water-rock interaction-ion exchange. Both the waters are subjected to external infusion of certain elements such as Na+, Cl-, NO3 (-) which are further aggravated by evaporation processes leading to heavy accumulation of contaminants by raising the water density. Presence of NO3 (-) rich samples within F- rich groundwater Group and vice versa authenticates the proposed evolution processes.