Regional-Scale Feedbacks That Affect Pore Scale Processes and the Spatiotemporal Distribution of Arsenic Contamination.

Groundwater arsenic contamination is widespread but heterogeneous throughout South and Southeast Asia.  Although it is well understood that much of this groundwater contamination results from the reductive dissolution of arsenic-bearing iron oxides, it is difficult to reliably predict aquifer arsenic concentrations, and know little about spatial and temporal evolution of groundwater composition. One potential reason for this uncertainty is that it often is difficult to place the molecular- and pore-scale processes responsible for arsenic release into a larger geological setting or context. In this work, we examine the feedback between regional perturbations in groundwater flow induced by large scale municipal pumping in Hanoi, Vietnam, on the sources and composition of groundwater recharge.  This aquifer is typical in that it is located within the floodplains of a large river network and their natural groundwater flow patterns have been altered as a result of heavy exploitation of groundwater for irrigation, industrial, and drinking water supplies.  Regional-scale pumping has changed the source of recharge by changing the groundwater flow direction. In this heavily-pumped aquifer, groundwater is derived from riverbanks rather than surficial soil environments, in environments that contain much more arsenic, higher reactivity iron oxides, and freshly deposited, young organic matter than under natural conditions. As a result, arsenic concentrations in the hyporheic zone are elevated and this arsenic is pulled into the groundwater. Reactive dissolved organic carbon is also advected into the aquifer system, where it can cause additional redox transformations. As a result, groundwater arsenic concentrations are related to their landscape position, the loci of pumping, and susceptible to change.