Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3-48.5 m) and piezometer wells (depth 12.2-79.2 m) demonstrate that the groundwater is mostly the Ca-HCO3 type and anoxic in nature (mean Eh(SHE) = 34 mV). Arsenic concentrations ranged from <10-538 mu g/L, with high concentrations only present in the shallow to medium depth (30-50 m) of the aquifer along with high Fe (0.07-9.8 mg/L) and relatively low Mn (0.15-3.38 mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO3, SO4 and NO2 along with higher values of DOC and HCO3, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain. (C) 2010 Elsevier Ltd. All rights reserved.