Arsenic Mobilization from Source Sediments in an Aquifer System of West Bengal, India

Abstract:

Geogenic As contamination of groundwater is an enormous public health problem in West Bengal and neighboring Bangladesh. The source of As and its release mechanisms are investigated at a field site in rural West Bengal where As concentrations up to 531 μg/L are measured in domestic wells. The shallow groundwater system is hosted by sediments deposited in a meandering fluvial environment, a typical setting in the Bengal Delta Plain. A numerical groundwater flow model is developed in order to establish the hydrogeological context for As contamination patterns. Reverse particle tracking is then used to map the flow path from each well screen back to its point of recharge. High-As groundwaters are found to be recharged in ponds marking an abandoned river channel; however, the source of As is located within the underlying channel-fill sediments. Analyses of environmental tracers reveal that As release within these sediments is accompanied by a concomitant release of Br and DOC indicating that these species may be decay products of natural organobromines co-deposited along with As. Mass transfer of As to the dissolved phase and its flushing from source sediments are described using a simplified reactive solute transport model. By fitting this model to observations, a characteristic reaction time for mass transfer is estimated at 6.7 years. With the advent of intensive irrigation pumping, bulk groundwater residence times in the source are estimated to have declined from 16.6 to 6.6 years. The ratio of residence and reaction times, a Damköhler number, has declined correspondingly from 2.49 to 0.99, indicating a shift from transport to reaction rate limited As mobilization. Greater insight into the As problem in Asia may be achieved by switching the focus of field investigations from plumes in aquifers to potential contamination sources in aquitards.