Spatial, vertical and temporal variation of arsenic in shallow aquifers of the Bengal Basin: Controlling geochemical processes

- Hydrogeochemistry at two piezometer sets (2 × 5) is monitored over 20 months.
- Temporal variation of As is observed only in the shallowest part of aquifer (< 30 m).
- Competitive ion exchange couples to the dissolution of Fe oxides to mobilize As.
- Redox zonation in the aquifer determines vertical distribution of As in groundwater.
- Extent of aquifer flushing regulates spatial distribution of As in groundwater.

A detailed understanding of the geochemical processes that regulate the spatial, temporal and vertical variation of dissolved arsenic (As) in shallow aquifers (< 50 m) is a prerequisite for sustainable drinking water management in the Bengal Basin. The present study conducted at Chakdaha Block of the Nadia District, West Bengal, India, combined a high resolution hydrogeochemical monitoring study over 20 months from two sets of piezometers (2 × 5) to the sediment geochemistry at areas with high (average: 146 μg/L, n = 5) and relatively low (average: 53.3 μg/L, n = 10) dissolved As concentrations in groundwater. The determination of the isotopic composition of δ2H and δ18O in groundwater of the two sites indicated the recharge of evaporative surface water to the aquifer. The concentrations of major aqueous solutes (Ca2 +, Mg2 +, Na+, K+, HCO3− and Cl−) and electrical conductivity were considerably higher in wells at the high As site compared to the low As site. Additionally, at the high As site, the major ions, Fe, SO42−, electrical conductivity, δ2H and δ18O showed markedly greater enrichment in the shallowest part (< 24 m) of the aquifer compared to the deeper part, reflecting vertical layering of groundwater composition within the aquifer. The oxidation of pyrites has been attributed to the high rate of mineral dissolution resulting in such greater enrichments in this part of the aquifer. In addition, the anthropogenic input with recharge water possibly increased the concentrations of Cl − in this part of the aquifer. The vertical layering of groundwater was absent in the aquifer at the low As site. The absence of such layering and relatively low major ion concentrations and electrical conductivity could be linked to the enhanced aquifer flushing and decreased water-sediment interactions influenced by local-scale groundwater abstraction. The seasonal variations of As concentrations in groundwater were observed only in the shallowest part of the aquifers (< 30 m). Furthermore, the As concentrations in groundwater at the uppermost part of the shallow aquifers (< 21 m) increased continuously over the monitoring period at both sites. This study supports the view that the reductive dissolution of Fe oxyhydroxides coupled with competitive PO43 − sorption reactions in the aquifer sediment enriches As in groundwater of the Bengal Basin. However, the additional Fe released by the weathering of silicate minerals, especially biotite, or the precipitation of Fe as secondary mineral phases such as siderite, vivianite and acid volatile sulfides may result in the decoupling of As and Fe enrichment in groundwater. The redox zonation within the aquifer possibly regulates the vertical distribution of As in the groundwater.