Hydrochemical Appraisal and Geochemical Evolution of Groundwater with Special Reference to Nitrate Contamination in Aquifers of a Semi-Arid Terrain of NW India

Hydrochemical characteristics of 85 groundwater samples collected from the semi-arid terrain of Ajmer district of Rajasthan state, India, during pre-monsoon season, indicate generally hard-to-very hard, fresh-to-brackish, alkaline and medium-to-high saline waters with the dominance of Na-Cl, Na-HCO and mixed water types. Hydrofacies and statistical analysis reveals that the dominant processes controlling the chemistry of groundwater are evaporation, salinisation, mineral precipitation and dissolution, ion exchange and anthropogenic activity. The groundwater quality is poor compared to the drinking water specifications of Bureau of Indian Standards and significant spatial variability exists in nitrate concentration. Nitrate content, high in groundwater samples of hard rock areas, shows negative correlation with well depth due to denitrification processes at higher depth. The nitrate content is also reduced with an increase in distance from ridges and pediments, particularly along rivers due to dilution effect. Domestic sewerage and industrial wastes aggravated by fertilizer inputs are responsible for nitrate contamination in the study area. Bivariate plots of ions, correlation, and factor analysis support the role of anthropogenic activities influencing the nitrate contamination of groundwater.