Groundwater dynamics and arsenic contamination in Bangladesh

Although arsenic contaminated groundwater in Bangladesh is a serious health issue, little is known about the complex transient patterns of groundwater flow that flush solutes from aquifers and carry solutes into the subsurface. Hydrologic modeling results for our field site in the Munshiganj district indicate that groundwater flow is vigorous, flushing the aquifer over time-scales of decades to a century, and also transporting solute loads into the aquifer with recharge from ponds, rivers and rice fields. The combined hydrologic and biogeochemical results from our field site imply that the biogeochemistry of the aquifer system may not be in steady-state, and that the net effect of competing processes could either increase or decrease arsenic concentrations over the next decades. Modeling results suggest that irrigation has greatly changed the location, timing and chemical content of recharge to the aquifer, flushing water through the system more quickly, and also cycling large fluxes of water through rice fields during the dry season that could mobilize arsenic from oxides in near-surface sediments. Furthermore, the hydrologic model reveals that ponds, many of which have been excavated over the last 50 years, now provide much of the groundwater recharge. These ponds receive most of the waste from the villages and thus provide another potential source of organic carbon to the groundwater system.