HOW EARTH PROCESSES CAN POISON MILLIONS BUT ALSO PROVIDE A SOLUTION: THE CASE OF WELL-WATER ARSENIC IN SOUTH ASIA

There is nothing unusual about the arsenic content of Himalayan sediment deposited in the Bengal basin. Just about any river-borne sediment rapidly buried with organic matter will probably release arsenic to ambient pore water in response to microbial reduction of iron oxides. What is unusual is that tens of millions of villagers across the Bengal basin are chronically exposed to toxic levels of arsenic by drinking water from their shallow well. The best estimates available indicate that a staggering 5% of total mortality in Bangladesh, the most affected country, is attributable to cardio-vascular disease triggered by chronic arsenic exposure. One of the key features of the distribution of arsenic in groundwater across the region is extreme spatial variability. Concentrations of arsenic in neighboring wells can differ by more than an order of magnitude and not necessarily because they extend to different depths. The origin of this spatial variability has bewildered earth scientists for almost two decades and reflects some combination of hydrogeological and biogeochemical factors that can be difficult to tease apart. The same heterogeneity has important public health implications because most households exposed to arsenic live within walking distance of a low-arsenic well or drilling distance of a low-arsenic aquifer. In fact, accessing such low-arsenic groundwater has become the dominant form of arsenic mitigation throughout the region as early hopes that surface water treatment, groundwater treatment, or rainwater collection could reduce arsenic exposure proved to be unrealistic. For reasons that are still not fully understood, arsenic concentrations have generally proved to be stable over time. Time series, where available, indicate that there are limits to the assumption of temporal stability of arsenic concentrations that are attributable to massive pumping for rice irrigation and the municipal supply of large cities. These concerns are worthy of further study but do not justify questioning a new US$250M arsenic mitigation plan recently approved by the Bangladesh government that relies principally on targeting low-arsenic aquifers for the installation of public wells.