Assessment of groundwater quality with special emphasis on nitrate contamination in parts of Varanasi City, Uttar Pradesh, India

In the current study, an effort was made to assess the geochemistry of groundwater by random collection of the samples from 15 different borewells located in various parts of Varanasi City, Uttar Pradesh, India. Geology of the study region is dominated by Quaternary alluvial sediments of Pleistocene to recent times where the younger alluvium receives fresh deposits of silt, clay and loam from periodic flood events. Ca-Mg-HCO(3)was inferred as major hydrogeochemical facies from Piper trilinear diagram. Ca-Mg type and HCO3 (-) type were the dominating cation and anion facies, respectively. Hydrogeochemistry reveals that the cation abundance follows the order Na+> Mg2+ > Ca2+ > K+ and anion abundance HCO3 (-) > Cl- > NO3 (-) > SO4 (2-) > F-. As per Gibb's plot, chemical weathering of rock minerals is affecting the quality of groundwater. Overall, most of the samples for majority of parameters lie within the allowable limits as set by WHO (guideline for drinking water quality, fourth edn, WHO, Geneva, p 340, 2004) except nitrate, which varied in the range of 40.32-78.97 mg/l. 80% of the groundwater samples in which nitrate exceeded beyond acceptable limit (50 mg/l), as per WHO standard, which may be due to poor sewerage, human excreta leakage from septic tanks, poorly maintained disposal of solid waste locally, agricultural activities, wastewater irrigation and irrigation runoff. The Water quality index (WQI) value of the study region depicts that 93% samples lie in the category of excellent water and 7% in good water category. Good positive correlation of NO3-Cl (r = 0.60) and Na-NO3 (r = 0.55) signifies an anthropogenic input of these ions into the subsurface water of the study region. The various indices such as electrical conductivity (EC), salinity, percent sodium, sodium absorption ratio (SAR), residual sodium carbonate (RSC), permeability index (PI), Kelly's ratio and magnesium ratio are used to check the fitness of ground water for irrigation uses which shows that groundwater samples of the study region is good to permissible for agricultural uses.