MULTI-SCALE APPROACHES FOR ASSESSING ARSENIC SPECIATION AND MOBILITY IN AQUIFERS OF SOUTH AND SOUTHEAST ASIA

Extensive arsenic contamination of groundwater in South and Southeast Asia has created adverse health consequences of an unprecedented scale. Overexposure to arsenic can lead to skin diseases and a host of cancers, and it is estimated that 100 million people across the region who rely on well water as a drinking water source routinely consume arsenic concentrations above the World Health Organization’s recommended limits. Arsenic is naturally derived from eroded Himalayan sediments and is believed to enter solution following reductive release from solid phases. However, relatively low concentrations of arsenic in aquifer sediments and extensive human perturbations to groundwater systems have limited our ability to adequately assess locations of arsenic release to pore water and predict distributions of dissolved arsenic. Here, I describe spectroscopic, laboratory, and field investigations of arsenic in Bangladesh and Cambodia. Microscale analyses of aquifer materials show immobile phases of arsenic but also indicate a paucity of minerals to which arsenic will readily adsorb. A labile pool of arsenic in aquifer sediments is revealed in macroscale laboratory experiments and highlighted by relatively uniform arsenic concentrations along aquifer flow lines. These results collectively indicate that the fate of arsenic is controlled by coupled hydrological and biogeochemical processes, notably liberation from sediments at recharge zones and transport by groundwater through aquifers. Furthermore, the multi-scale approach described here is one that is well-suited for addressing emerging issues regarding arsenic in Asia, including the vulnerability of arsenic-free aquifers to future contamination and mitigation options for high arsenic concentrations in groundwater and soil.