GEOCHEMICAL PROCESSES AND MECHANISMS OF ARSENIC CONTAMINATION IN SOUTHWESTERN IDAHO GROUND WATER

We have initiated geochemical investigations aimed at understanding the sources of elevated dissolved arsenic concentrations in ground water (up to 100 micrograms per liter) and the processes by which arsenic is stored and mobilized in Miocene fluvial and lacustrine unconsolidated sedimentary aquifers and confining units underlying the Boise Metropolitan Area. Understanding these processes is critical for future management and mitigation of arsenic contamination of drinking water in this rapidly-growing region. Previous work demonstrated that arsenic contents of unconsolidated sediments of the Treasure Valley aquifer system rarely exceed average crustal abundances (about 6 ppm), but that redox reactions probably play a dominant role in concentrating naturally-occurring arsenic in ground water. A persistent, laterally-extensive, reduced clay unit within the aquifer (known locally as the “blue clay”) contrasts visually and geochemically with stratigraphically higher oxidized sands and clays. Our current efforts focus on understanding the role of this redox boundary in controlling the fate and transport of arsenic in local groundwater. We are mapping the distribution of the blue clay using drillers' lithologic logs and correlating it to water-quality data to understand the relations among redox conditions in the aquifer materials, the redox state of the ground water, and the concentration of arsenic and other metals in the ground water.