A WATERSHED PERSPECTIVE ON THE CONTROLS OF ARSENIC MOBILITY

Oxidation-reduction reactions, formation of surface and soluble complexes, and production of organic species each may strongly influence the speciation and mobility of arsenic on a watershed. The overall pattern of arsenic distribution on a watershed involves a spatial and temporal mosaic of these and other processes, interacting with physical transport to create a complex system whose behavior seems difficult to predict or model. Watershed-wide analysis of arsenic species and fluxes on the Aberjona Watershed, however, suggests that some subset of these reactions often dominates at any given location. Investigation of a comprehensive suite of ecosystems on this watershed, within which geochemical controls vary but source functions are linked by watershed structure, yields useful insights into geochemical controls of this element. Extremes of redox potential are found to be a powerful influence, as expected, but the agents of transformation and the identity of elements coupled to arsenic redox cycles hold some surprises. For example, nitrogen redox reactions appear to control arsenic oxidation state in some cases. Likewise, while surface complexation by iron is, as expected, a strong control on arsenic, the speciation of iron does not always conform to expected models. Under sulfidic conditions, unexpectedly high mobility of arsenic may occur due to the formation of thioarsenites and perhaps related species. In surface waters, biomethylation can dominate over other transformations and result in a majority of arsenic occurring in organic forms. Notable priorities for future research include improved knowledge of both soluble and surface complexes, and a more comprehensive picture of biotic processing of arsenic.