CONTROLS ON ARSENIC AND SELENIUM IN THE GROUNDWATER OF AN URBANIZING CATCHMENT

Elevated concentrations of the toxic trace metals arsenic and selenium generally co-occur in groundwater wells, springs and artificial drainage seeps along a short reach of a concrete-channeled catchment in Orange County, California. Urbanization of coastal Southern California has destroyed many historic wetlands, including an area known as the Swamp of the Frogs, centrally located in this catchment. Lowered groundwater elevation and increased erosion are clear, documented consequences of this landscape change. Site-specific soil and groundwater evidence along these reaches indicate that the co-occurrence of arsenic and selenium may be purely historical. Sulfate, silicon and other trace metals are found with Arsenic and selenium in the catchment, but the relationships are not linear with both trace metals and are not temporally correlated. The modern mechanisms of arsenic and selenium release do not appear to be linked. Arsenic is likely mobilized from mineral forms by anoxic, saturated-zone desorptive processes in iron- and carbon-rich soils. In contrast, vadose-zone processes control both the spatial and temporal concentrations of selenium oxyanions. The basic redox properties of these elements support this field evidence. Historic hydrologic and geochemical processes controlled the co-deposition of arsenic and selenium in this catchment. However, their modern fate is governed by separate subsurface hydrologic processes perturbed by development of this coastal region.