Spatial and temporal distribution of arsenic in groundwater of the Brahmaputra River floodplains in Assam, India

Abstract

The present study focuses on spatial and seasonal distribution of arsenic (As) along with the solute chemistry and hydrochemical evolution of groundwater in the southern bank of Brahmaputra floodplains in Assam, India. A total of 100 groundwater samples were collected from shallow aquifers (<30m) that are distributed spatially covering the entire study area during the pre-monsoon (April) and post monsoon (Nov) season in the year 2022.The samples were than analyzed for different physico chemical parameters viz; pH, EC, TDS, Ca²⁺, Na⁺, K⁺, Mg^2+, Cl⁻, HCO₃⁻, NO₃⁻, SO₄²⁻, Fe, Mn and As to interpret the hydrochemistry and groundwater evolution in the study area. Broadly three zones were delineated based on As distribution in the region viz; zone 1 as high As zone, areas adjacent to the foothills of Naga hills,(ranged from below detection level (bdl) to 531 μg/l, mean:93.91 μg/l). Zone 2 is demarcated as low arsenic zone, near the Brahmaputra River, where As concentration was mostly <10 μg/l. Zone 3, lying between the flanks of Mikir Hills and Naga Hills is demarcated as intermediate zone where As concentration ranged from bdl to 50 μg/l. Piper plot indicates Na-HCO₃ as a primary water type during pre-monsoon, while Ca-Mg-HCO₃ type during post monsoon.Groundwater is undersaturated with respect to As phases such as Arsenolite and As₂O₅ specifying that As is in dissolved form in the groundwater. The groundwater is supersaturated with calcite (CaCO₃) and Dolomite (MgCa(CO₃)₂and Fe(III) (Oxyhyroxide). The stable isotopes (δ¹⁸O and δ²H) of groundwater suggest that precipitation is primarily recharging the groundwater with some influence of evaporation. The results of the study will contribute to a deeper understanding of the arsenic distribution dynamics in the Brahmaputra Floodplains along with facilitating evidence-based decision making aimed at providing arsenic safe drinking water to the affected communities.