TOWARDS AN UNDERSTANDING OF THE HAZARDS PRESENTED BY ARSENIC-BEARING SEDIMENT: X-RAY ABSORPTION SPECTROSCOPY CASE STUDIES

The chronic arsenic poisoning of millions of Southeast Asians (particularly Bangladeshis) from ingestion of naturally-occurring, arsenic-enriched ground water is an extreme example of one type of hazard presented by arsenic-bearing sediment. Even in the U.S., where documented cases of chronic arsenic poisoning are rare, the recent lowering of the arsenic drinking water standard from 50 micrograms per liter to 10 micrograms per liter has generated concern, both about the health effects of arsenic and about the economic costs of implementing the new standard (particularly in the western states). The common inorganic forms (species) of arsenic are known carcinogens, and there is no risk-free level of exposure to these species when dissolved in water. Understanding the mechanisms by which arsenic is transferred from solid phases in sediment to the dissolved phase (i.e., pore water or ground water in sediment interstices) is important for minimizing that transfer. Case studies characterizing arsenic species in sediment from the Lava Cap Mine Superfund Site (Nevada City, CA) and from sites in east-central and southern Bangladesh will be presented. These studies will be used to illustrate that: (1) of the many possible chemical reactions regulating arsenic mobility in the environment, only a few appear to dominate; (2) most sediment arsenic is bound in one or more recalcitrant forms, but it is the labile fraction that may control the arsenic accumulation in sediment interstitial water; (3) common microbiota (as opposed to arsenic-respiring microorganisms) are important to the cycling of arsenic in small ponds and streams, due to their extreme uptake of this element, and its association with their biomass upon death and burial. Although ingestion of arsenic in drinking water is the primary route of exposure to this element around the world, there are notable exceptions. In a final case study, the application of XAFS in identifying sources of particulate arsenic to the threatened desert tortoise (Gopherus agassizii), will be presented to highlight the potential health hazards presented by inhalation of arsenic-bearing particles and to illustrate future research directions.