GEOLOGIC SOURCES AND GEOCHEMICAL MECHANISMS OF ARSENIC CONTAMINATION IN SOUTHWESTERN IDAHO GROUND WATER: PRELIMINARY RESULTS

Arsenic in drinking water is a serious concern in southwestern Idaho, where concentrations exceeded the new EPA maximum contaminant level (10 micrograms per liter) in 123 of 378 wells tested between 1991 and 2001. Ongoing geochemical investigations with the Idaho Department of Environmental Quality are aimed at understanding the sources of arsenic (As) and the mechanisms by which it is stored and mobilized in Miocene fluvial and lacustrine unconsolidated sedimentary aquifers and confining units beneath the Treasure Valley. Minor- and trace-element analyses of cuttings from 12 wells show that most samples are not enriched in As relative to average crustal abundances, but elevated concentrations are observed locally. Arsenic was detected in 78 of 106 samples (minimum detection level, 1 ppm); eight clay-rich samples exceed 12 ppm As. Arsenic is more abundant in bluish-, grayish-, or greenish-colored sediments (maximum, 33 ppm As) than in reddish- or brownish-colored sediments, suggesting that redox conditions play an important role in storing and mobilizing arsenic. Arsenic is strongly positively correlated with Mo, Sb, U, Cu, Ni, Cd, and Co, and more weakly correlated with Cr, Fe, Zn, and Mg. Results of four-step partial chemical extractions show that As is held in the least available fraction and suggest the presence of arsenic-bearing sulfides. Although sulfides were not identified in these samples, sulfides previously have been noted in drillers’ logs from this region; small quantities would be sufficient to produce the observed bulk As concentrations. Future investigations will use aquifer redox mapping based on drillers’ lithologic logs, well-construction data, and water-quality data to elucidate geochemical controls on arsenic dissolution and mobility.