ARSENIC, CHROMIUM, URANIUM, AND VANADIUM IN ROCK, ALLUVIUM, AND GROUNDWATER, WESTERN MOJAVE DESERT, SOUTHERN CALIFORNIA

Trace elements in groundwater that originate from aquifer materials and pose public-health hazards if consumed are known as geogenic contaminants. The geogenic contaminants arsenic, chromium, and vanadium can form negatively charged ions with oxygen known as oxyanions; uranium complexes with bicarbonate and carbonate to form negatively charged ions having aqueous chemistry similar to oxyanions. Naturally occurring concentrations of arsenic, hexavalent chromium, uranium, and vanadium in water from 1,780 samples from 500 wells in a 3,500 square mile area of the western Mojave Desert were 360, 140, 1,470, and 690 micrograms per liter, respectively. Concentrations within rocks and minerals in geologic source terranes are not the sole factor controlling concentrations of geogenic contaminants in groundwater and high concentrations also result from (1) the fraction of these elements weathered from and sorbed to the surfaces of mineral grains; and (2) redox conditions and pH of groundwater. At the mineral-grain scale, sequential chemical extraction data from 94 samples of aquifer material show arsenic and uranium weathered from mineral grains and sorbed to the surfaces of those grains are more available to groundwater under the alkaline, oxic conditions prevalent in the study area than chromium or vanadium, which largely remain within unweathered mineral grains. At the aquifer scale, hydrogeology (including isolation of water in aquifers from surface sources of recharge and older groundwater age) combines with geochemical processes (such as silicate weathering) to produce alkaline groundwater which may mobilize arsenic, chromium, and vanadium from surface sorption sites into groundwater. Aqueous geochemistry and concentrations of geogenic contaminants in the study area also are affected by anthropogenic activities including (1) discharge of treated municipal wastewater, (2) return from irrigated agriculture, (3) groundwater pumping and subsequent water-level declines, and (4) managed aquifer recharge.