Arsenic toxicity in Groundwater of Brahmaputra Floodplains of Assam, India: Concerns for drinking water quality

Abstract

Arsenic (As) contamination in groundwater is a serious environmental and public health issue, particularly in regions where groundwater is a primary source of drinking water. Many Asian countries, particularly Bangladesh, India and parts of Southeast Asia are adversely affected with As contaminated aquifers. The present study thus aims to explore the source, distribution and release mechanism of As into the groundwater in the parts of southern bank of the Upper Brahmaputra floodplains in Assam, India.  Groundwater samples (n=100) were collected from the shallow aquifers (< 30 m) from two the districts Jorhat and Golaghat, and were analysed for major ions (Ca²⁺, Na⁺, K⁺, Mg^2+, Cl^-,HCO₃^-, NO₃^-,SO₄^2-) and trace elements (As, Fe, Mn, Pb, Co, Cu, Zn). Concentration of As, Fe and Mn in the aquifers has exceeded the permissible limits set by WHO (World Health Organisation) posing serious threat to human health. 54% of groundwater samples have exceeded WHO permissible limit of 10 µg/L for As (range: bdl (below detection limit) to 480 µg/L, mean: 31µg/L). While 94% (range: 0.076 mg/L to 41.37 mg/L, mean: 10.92) and 77% (range: 0.002 mg/L to 9.06 mg/L, mean: 0.6 mg/L) of groundwater samples have exceeded the WHO permissible limit of 0.3 mg/L and 0.05 mg/L for Fe and Mn respectively. Aquifers enriched with As was found adjacent to Naga foothills while low As was found near the Brahmaputra river. Aquifer lithology reveals the presence of thick clay layer near the Naga hills (also indicated by higher Al₂O₃ in the sediments) and subsequently minerals like illite and kaolinite was found in these clay layers (confirmed by the XRD peaks). The clay minerals might have acted as the active site for adsorption of As, thus acting as the host for As in the studied region. Moreover, the average value (mean: 80) of Chemical Index of alteration (CIA) indicates intense chemical weathering at the source area in warm and humid condition leading to formation of copious amount of clay minerals like kaolinite. No strong co relation was seen between As and the redox sensitive elements viz; Fe and Mn, nor with HCO₃, NO₃ or SO₄ indicating multitudinous processes or reactions viz competitive exchange with anion, reductive dissolution and pH dependent sorption might have control the release of As in the studied region. Higher LREE compared to HREE indicates the source of these clastic sediments could be from felsic or intermediate composition of rocks. Principle Component Analysis (PCA) and cluster analysis indicated the dominance of geogenic factors as the main contribution of these contaminants in the groundwater of the study area. Regular monitoring and intervention of groundwater in the region is crucial for its prolong use. The present study will assist stakeholders and policy maker in taking evidence-based decision and providing As safe drinking water to the affected communities

Key words: Arsenic; Groundwater; Brahmaputra floodplains; Sediment Geochemistry; Hydrochemistry