Geochemical modeling and multivariate statistical approach for assessing the groundwater quality, and mechanism of arsenic mobilization in Bahraich region, Indo-Gangetic plains, India

As contamination in groundwater has been reported from various regions of the Indian subcontinent but no data related to heavy metal contamination of groundwater has been reported for the Bahraich area in the Indo-Gangetic plains. We report the first dataset on arsenic contamination and groundwater hydrogeochemistry, in Bahraich. This includes concentrations of heavy metal such as As, Mn, and Fe, along with major cations (Na⁺, K⁺, Ca²⁺and Mg²⁺) and anions (F^-, Cl^-, NO₃^-, SO₄^2- and PO₄^3-), and dissolved organic carbon (DOC), along with various physico-chemical parameters such as EC, pH, and Eh from samples collected during two extensive field campaigns conducted during pre-monsoon, and post-monsoon seasons respectively. The combined use of geochemical modeling and multivariate statistical approaches such as principal component analysis (PCA) and correlation analysis (CA) suggest several processes affecting the geochemistry of groundwater including the lithological characteristics of aquifers and anthropogenic activities.

The groundwater of the study area predominantly belongs to the Ca-Mg-HCO₃ type hydrochemical facies. HCO₃⁻/Na⁺ and Ca²⁺/Na⁺ signatures of groundwater indicate the influence of silicate weathering and carbonate dissolution processes with the insignificant role of evaporate dissolution mechanism. As concentration was found to range from 0.6 μg/L to ~100 μg/L with almost 40% of the collected samples exceeding the WHO defined limit of 10 μg/L for drinking water. 70 % of the groundwater samples were found to have very high Fe concentrations exceeding the WHO guideline of 0.3 mg/l in drinking water. Mn concentrations in the groundwater samples were relatively low with only ~10 % of the samples exceeding the WHO defined limit for Mn (400 μg/L). The majority of the groundwater samples were found to be anoxic in nature showing low NO₃⁻ & SO₄^2- concentrations, high Fe & Mn and DOC concentrations, and negative Eh values.

Results from this study show that the reductive dissolution mechanism of iron oxyhydroxide is the dominant mechanism responsible for arsenic release in groundwater of the region, ruling out any role of sulfide oxidation and alkali desorption.