ARSENIC IN SHALLOW AND DEEP AQUIFERS OF BENGAL DELTA PLAIN: A FIELD SCALE STUDY IN WEST-BENGAL, INDIA

High exposure of arsenic (As) from drinking water and chronic toxicity among the population is well established in SE Asia, especially in the Bengal Delta Plain (BDP). The extent of arsenic contamination in groundwater is heterogenous, with considerable spatial variability. The present study deals with groundwater quality and hydrogeochemistry in two As affected areas of BDP namely Chakdaha in Nadia district and Baruipurin South 24 Pargana district of West Bengal India. Groundwater pH are circum-neutral to alkaline (pH 6.4-8.1). The EC values are low to moderate in Chakdaha wells (av. 692 µS/cm in shallow wells and 512 µS/cm in the deep wells) and generally high in Baruipur (av. 1028 µS/cm in shallow wells and 2083 µS/cm in deep wells). The major anions are dominated by HCO₃^- followed by Cl^- and PO₄^3-, and relatively low concentrations of NO₃^- and SO₄^2-. Groundwaters of shallow aquifers (5-70 m) are often enriched with DOC (1.2-7.0 mg/L), whereas deep aquifers (100-304 m) contain low amount of DOC (0.4-1.9 mg/L). Concentration of dissolved As spans over four order of magnitude (<3-1059 µg/L) with high concentration of Fe and Mn. The presence of DOC in shallow aquifers also indicates that organic matter is relatively young and more reactive. On the other hand, alkaline earth elements Ca²⁺ (23-277 mg/L), Mg²⁺ (14-69 mg/L), Sr²⁺ (78-1015 mg/L) and Ba²⁺ (31-529 mg/L) also vary considerably in the shallow and the deep groundwater and controlled by carbonate mineralogy of the aquifers. High p_CO2 values, relatively high redox sensitive elements, low Eh and the absence of DO are the principal hydrogeochemical fingerprints of the BDP shallow aquifers. On the contrary, deeper aquifers of Chakdaha are As bearing with low concentration of chloride where as deeper aquifers of Baruipur are saline with or without dissolved As. The deep aquifers at the both regions also contain high bicarbonate, that indicates that carbonate mineral dissolution play important role in As mobilization at least in deeper aquifers. The study also deals with the role of secondary minerals (mica/clay) in As mobilization. The XPS studies on mica further strengthen that the surface chemistry and of Fe(II) exerts a significant role in the differential mobility in the shallow and deep aquifers in BDP.