Arsenic enrichment in the north Gangetic Plains of Laksar, Uttarakhand, India

This study reports new data on arsenic contamination in groundwater in Laksar, a pristine and semi-rural region of India's Himalayan state of Uttarakhand. Based on a systematic investigation of hydrogeochemical parameters influencing arsenic mobilisation, the most plausible mechanism of arsenic release into groundwater has been proposed. The groundwater in the research area is mostly homogeneous, with the Ca-Mg-HCO3 facies accounting for 87.5 percent of groundwater samples. The combined effect of silicate weathering, and the dissolution of carbonate minerals predominantly influence Laksar groundwater composition, as evident from bivariate plots of Mg/Na vs. Ca/Na and HCO3-/Na vs. Ca/Na. The statistical analysis (PCA and correlation analysis) performed on the geochemical data indicates that natural processes such as carbonate weathering, reductive dissolution of Fe/Mn minerals together with anthropogenic activities such as the utilization of fertilizers in agricultural fields, human and animal waste disposal, exert control on the major ion chemistry, and As enrichment in groundwater. The study area has extremely high arsenic concentrations, ranging from 0.60 to 91.21 mu g/L (mean: 15.63 mu g/L), with majority of groundwater samples (>50%) having arsenic contents exceeding the WHO permitted limit (10 mu g/L). Analysis of the Eh-pH diagram shows that As(III) species (H3AsO3) accounts for 79% of samples, whereas As (V) species (HAsO4-) accounts for the remaining. The observed arsenic enrichment in groundwater can be attributed to As desorption from iron oxide/hydroxide minerals via reductive dissolution mechanism under reducing groundwater conditions, which is supported by negative Eh values and high levels of Fe and comparatively low levels of sulphate and nitrate.