Arsenic-contaminated groundwater in South and Southeast Asia accounts for the most widespread poisoning of a human population in history. The high primary productivity and monsoonal flooding in topical deltas promote anaerobic microbial processes leading to As release to groundwater. What remains unresolved, however, is the variation in reactivity of organic C and As-bearing Fe oxides across the landscape and within sedimentary profiles that control pore-water concentrations of As – information critical for projecting groundwater As concentrations and their changes within land and river systems.  Here we decipher variations in the reactivity of organic C and As-bearing solids with sediment depth across distinct hydrologic positions within the landscape of the minimally disturbed upper Mekong Delta. Reactivity of organic C coupled to As(V)/Fe(III) reduction is greatest in near-surface clays of continuously saturated wetlands, and represent the only locations (depth and landscape) where active As release occurs on the time-scale of our measurements.  Short-term As release is not observed in deeper clays, the underlying aquifer sands, or in seasonally saturated near-surface clays. Native dissolved organic C is sufficiently reactive to drive reductive dissolution of ferrihydrite, but not of the sedimentary Fe oxides (i.e., goethite and hematite) present in the deeper clays and aquifer sands, suggesting a thermodynamic limitation to microbially driven As release. Taken together, our results suggest that low-As zones downgradient of continuously saturated wetlands are vulnerable to future As contamination.<br/>

Wetland hydrology dictates organic carbon reactivity controlling microbial arsenic release within the Mekong Delta

249th National Meeting (2015)

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