A ubiquitous contaminant of soils and sediments, arsenic arguably poses the greatest threat to human health of any naturally occurring compound. Nowhere is the threat more apparent than in the groundwater systems of South and Southeast Asia. Derived from sulfide minerals within rock outcrops and coal seams, arsenic is transferred to iron(III) oxides (inclusive of oxyhydroxides and hydroxides) upon weathering of the Himalayan rock strata. Transported down the large river systems during the annual monsoonal storms, the arsenic hosting iron oxide particles are deposited across the vast floodplains of the low-lying regions. With continued burial, the arsenic-bearing particles progressively reside deeper within the sediments; once the height of the water table exceeds the particle position, anaerobic metabolic processes lead to the reductive dissolution of the arsenic (and iron). Arsenic, upon conversion to arsenite, is then free to move with the advecting water, resulting in wide-spread contamination of groundwater. The rate of arsenic release, however, varies depending largely upon the organic carbon content and pathway of decomposition. In combination with heterogeneous groundwater flow paths, arsenic concentrations can be highly variable across even short distances of the aquifer. Herein, the specific combination of hydrologic states and resulting biogeochemical processes responsible for arsenic liberation and distribution in groundwater are described.

The groundwater threat of arsenic is clear, and mitigation or adaptation strategies have been underway for many years. The threat posed to food supplies, and specific rice, are, by contrast, still being revealed. We illustrate that the combined effects of increasing temperature (resulting from global warming) couple with arsenic have devastating impacts on both rice yield and grain quality. Our findings show a nearly 40% reduction in yield with a 5 °C increase in temperature for the major rice growing regions of the world. Further, the concentration of inorganic arsenic within the rice grain doubles with the temperature change. As a result of changes in plant physiology and soil biogeochemical processes, varying soil arsenic concentrations and elevated temperature, along with carbon dioxide, lead to a complex shift in arsenic cycling within the rhizosphere. In combination, the impacts of soil arsenic and climate change will lead to a devastation in food production.