Coupled hydrologic and biogeochemical processes controlling arsenic in aquifiers of Southeast Asia

Natural arsenic contamination of groundwater in Southeast Asia has caused the largest mass poisoning in history: nearly 100 million people routinely drink well water with hazardous arsenic concentrations. The mechanism for arsenic release to the aqueous phase, the subsequent residence time of arsenic in groundwater, and the human-induced effects on arsenic concentrations remain unresolved. The research presented in this thesis elucidates the coupled hydrological and biogeochemical processes controlling arsenic concentrations within aquifers of Southeast Asia and explores how land use alterations may influence arsenic concentrations.;Reductive dissolution of Fe(III) (hydr)oxides and concomitant arsenic release has become the most widely accepted explanation of high arsenic groundwater concentrations. In order to evaluate the validity of this potential mechanism of arsenic desorption to groundwater, spectroscopic and laboratory batch incubation experiments were conducted with aquifer sediments. Contrary to the prevailing paradigm, Fe(III) (hydr)oxides are not detected in aquifer materials and proxies of active microbial processes are inconsistent with Fe(III) reduction (and arsenic release) at well depth. Instead, a highly labile, and thus mobile, arsenic phase persists in the aquifer that is easily transported with groundwater flow.;A comprehensive analysis of geochemical and hydrological conditions in Bangladesh and Cambodia suggests that arsenic is released in the near-surface environment—rather than within the deeper aquifer—and then transported to depth. Groundwater residence times are sufficiently short to necessitate continued input of arsenic in order to maintain observed concentrations. The only portion of the sediment profile with conditions conducive to arsenic desorption is in the near-surface environment. Following its release to solution, arsenic is easily transported through aquifers by groundwater flow. Under natural conditions, arsenic inputs via sediment deposition are comparable to arsenic outputs via groundwater discharge, and thus, on the bases of coupled hydrologic, biogeochemical, and sedimentary processes, arsenic concentrations in groundwater can be maintained indefinitely. However, while groundwater arsenic concentrations are in steady-state and have persisted for millennia, human-induced changes in land use that disrupt the natural hydrologic regime or arsenic source material will have important consequences for arsenic in the aquifer.