Chemical Controls on Abiotic and Biotic Release of Geogenic Arsenic from Pleistocene Aquifer Sediments to Groundwater.

Over 150 million people in South and Southeast Asia consume unsafe drinking water from arsenic-rich Holocene aquifers. Although use of As-free water from Pleistocene aquifers is a potential mitigation strategy, such aquifers are vulnerable to geogenic As pollution, placing millions more people at potential risk. The goal of this research was to define the chemical controls on abiotic and biotic release of geogenic As to groundwater. Batch reduction incubations on sediments with natural chemical variability from a Pleistocene aquifer in Cambodia were conducted over a 9-month period to evaluate how interactions among arsenic, manganese and iron oxides, and dissolved and sedimentary organic carbon influenced As mobilization from sediments. The addition of labile dissolved organic carbon produced the highest concentrations of dissolved As after 9 months, as compared to sediment samples incubated with sodium azide or without added carbon, and the extent of As release was influenced by the quantities of Mn and Fe mobilized from sediments as well as the initial solid-phase concentrations of the elements. The mode of As release was impacted by the source of DOC supplied to the sediments, with lactate and acetate additions yielding primarily biologically driven As release over time, and humic and fulvic acid additions yielding more abiotic release. Overall, cycling of key redox-active elements and organic carbon reactivity govern the potential for geogenic As release to groundwater, and results here may be used to formulate better predictions of the arsenic pollution potential of aquifers in South and Southeast Asia.