Tracking the release of a natural poison [electronic resource] : arsenic contamination of groundwater in the Mekong Delta
Keywords: Environmental Earth System Science.

Arsenic (As), a toxic metalloid common throughout the Earth's crust, accounts for the most widespread poisoning of a human population in history. Within the major deltas of South and Southeast Asia, rivers annually deposit As-bearing sediment derived from the Himalaya. The high primary productivity and monsoonal flooding in the topical deltas promote microbially driven anaerobic metabolic processes leading to arsenic release to groundwater, namely dissimilatory As(V) and Fe(III) reduction. Groundwater is a primary source of drinking and irrigation water in the region, especially within rural areas. Prolonged consumption of As-contaminated groundwater ultimately can lead to a myriad of cancers and chronic respiratory conditions. Switching to arsenic-safe wells is the most common mitigation option exercised in the rural areas of the Ganges-Brahmaputra Delta (Bangladesh), where the largest impacted population resides. In order for well switching to be a long-term viable option, an accurate estimate of arsenic-safe well longevity (i.e., when will As contaminate the extracted groundwater) must be made. Further, installation of new wells can be guided by knowledge of the spatial distribution of arsenic (measured or projected), and the probable longevity of low-As conditions. However, the spatial distribution of the reactants governing arsenic release across the landscape and within the sediment profile remains unresolved. The work of this dissertation is performed in the minimally disturbed upper Mekong Delta of Cambodia to examine the arsenic release process within a well-defined groundwater flow regime and stratigraphic sediment profile. Here I decipher the zones, geographically and in-depth, where microbially driven arsenic release occurs and does not occur, and I deduce the limiting reactants within system; in sum, my results allow a biogeochemical framework by which we can model (or delineate) As concentrations in space and time. I show that microbially driven arsenic release actively occurs in the near-surface clays of continuously saturated wetlands and is rate-limited by organic carbon reactivity. However, (active) arsenic release is not observed in deeper clays, the underlying aquifer sands, or in seasonally saturated near-surface clays -- although due to past release and groundwater flow, most of the aquifer system is contaminated with hazardous levels of As. Native dissolved organic carbon is sufficiently reactive to drive reductive dissolution of ferrihydrite, but not of the sedimentary iron oxides (i.e., goethite and hematite) present in the deeper clays and aquifer sands, suggesting a thermodynamic limitation to microbially driven arsenic release. The lack of arsenic release observed in the near surface of seasonally saturated wetlands likely results from aerobically directed oxidation of organic carbon during the dry season. In contrast to the near-surface organic carbon driven arsenic release I demonstrate in this work, buried peat layers have been implicated elsewhere as the principal source of organic carbon driving As release within sediment profiles of the Bengal Basin and analogous basins worldwide. However, I show that buried peat layers have actually accumulated arsenic in the form of arsenian pyrite -- a stable mineral under reducing conditions. The formation of arsenian pyrite likely occurs under conditions of peat deposition, and does not currently regulate arsenic partitioning within the sediment profile as evidenced by pore-water As concentrations. The buried peat would only release arsenic if oxidizing conditions were to ensue. The demonstration of microbially driven arsenic release in near-surface clays of continuously saturated wetlands, rate-limited by organic carbon reactivity, defines future directions for modeling As release rates including the quantification of the organic carbon fermentation rate providing organic acids/H2(aq) capable of driving dissimilatory As(V)/Fe(III) reduction. Further, land management or land use changes that increase flood duration and organic carbon inputs in near-surface sediments should be avoided. Due to similarities in geology and hydrology, results defined here in the Mekong Delta may be transferrable to other major Asian deltas.