Local Parameters Influencing Groundwater Arsenic Concentration in Shallow Aquifers in Bangladesh

Recent work conducted in Bangladesh suggests that groundwater flow may be a key factor controlling the spatial distribution of arsenic. An increase of arsenic concentration with groundwater age suggests a possibility for arsenic to release gradually from sediments and accumulates along the groundwater flow path in a single aquifer system. However, a more likely scenario is that groundwater arsenic concentrations are related to sediment mobilizable arsenic concentrations, but the distribution coefficients (Kd) of sediments can vary. Whereas aquifer systems depleted of the labile sediment As by flushing have low groundwater As concentrations, the hydrogeological and geochemical factors that regulate the partitioning of sediment and groundwater As in an incompletely flushed aquifer system is the focus of this study. A new site, Site K, has been identified for field and laboratory experiments to determine parameters that can be used in reactive-transport modeling for arsenic mobilization along a flow path. Using a three dimensional array of monitoring wells, temporal changes in hydraulic gradient and groundwater chemistry have been investigated since December 2006.

Monitoring wells are installed in a sandy, raised village covered and agricultural fields covered with silty clay near a small abandoned channel (stream). Dissolved arsenic increases from less than 5 to 500 µg/L over ~ 300 m from the village towards the stream. A strong redox gradient also exists over this distance, with low dissolved Fe and high SO₄ in the village. Solid phase P-extractable arsenic concentrations vary less than ten-fold over the same range, from 0.4 to ~ 4 mg/kg, but show no systematic spatial trend. Hydraulic gradients reverse direction seasonally, with the gradient directing flow away from a small stream pre-monsoon and toward the stream post-monsoon. Results from push-pull experiments suggest that Kd is a key parameter that determine groundwater As concentration at local scale.