Linking Hydrologic Drivers to Arsenic Contamination in Asia, Results From a Field Site in Cambodia

Attempts to resolve the specific release mechanisms associated with arsenic contamination on the deltas of Asia have been hampered, especially in Bangladesh, by the difficulty of linking geochemical mechanisms with hydrologic drivers. At an arsenic-contaminated (100-1200 ug/L) aquifer in Cambodia, where pumping for irrigation is limited and the hydrologic system is much less complex than in Bangladesh, we have developed a conceptual model of arsenic behavior that integrates the system hydrology and depositional history, providing strong evidence that arsenic release is sensitive to human activities. Specifically, the hydrogeologic regime is dominated by seasonally-variable head gradients between the river and adjacent wetland basins but the net annual flow is from the wetlands downwards into the aquifer (0.04-0.4 m/a) followed by discharge to the river (1-13 m/a). These flow rates produce aquifer residence times on the order of 100-1000 years. When coupled with elevated concentrations throughout the aquifer, this indicates that arsenic contamination predates recent human activities. Within this flow regime, it is possible to track spatial changes in arsenic concentrations along specific flow lines: The greatest increases in dissolved arsenic concentrations (150 ug/L/m) occur in shallow sediments along the first 2-10 m of the flow path, consistent with the depth at which reducing conditions are no longer seasonally disrupted by fluctuating water levels. Estimates of arsenic delivery to the wetlands by flood deposition (1,300,000 kg/a) are of a similar magnitude to the estimated flux, via groundwater, out of the aquifer (1,000,000 kg/a), indicating that arsenic delivery, release, and transport are in approximate steady-state. Given the dependency of arsenic release and transport on hydrologic drivers, human activities that alter the hydrology (inclusive of pumping for irrigation, soil excavation, and upstream river damming) will likely also alter arsenic behavior.