OCCURRENCE OF HEXAVALENT CHROMIUM IN GROUND WATER IN THE WESTERN PART OF THE MOJAVE DESERT, CALIFORNIA

As part of a study of the sources and distribution of Cr in groundwater of the western Mojave Desert, California, about 200 samples from selected public supply, domestic, and observation wells completed in alluvial aquifers underlying the western part of the Mojave Desert were analyzed for total dissolved Cr and Cr(VI). Because Cr(VI) is difficult to preserve, samples were analyzed in the field using both a direct colorimetric technique and EPA method 218.6. Comparisons between the methods were excellent (r²=0.9993). Total dissolved Cr concentrations in uncontaminated areas ranged from less than the 0.8 mg/L detection limit to 60 mg/L, and almost all the Cr present was Cr(VI). Near recharge areas along the mountain front Cr(VI) concentrations were below the 0.1 mg/L detection limit, and Cr(VI) concentrations and pH values increased downgradient. In groundwater underlying the desert floor, decreasing Cr(VI) concentrations were positively correlated with decreasing dissolved O₂ concentrations along flow paths - especially as groundwater approached discharge areas near dry lakes. Chromium(VI) concentrations up to 60 mg/L occurred in the Sheep Creek fan alluvial deposits of mafic rock derived from the San Gabriel Mountains. Near Twentynine Palms, CA, Cr(VI) concentrations up to 38 mg/L were detected in groundwater from alluvial deposits of weathered volcanic and metamorphic rocks that compose the lower aquifer system underlying the Surprise Springs aquifer. Chromium(III) was the predominant form of Cr only in areas where dissolved O₂ concentrations were below 1 ppm and was detected at a maximum concentration of 2.6 mg/L, owing to its low solubility in water of near-neutral pH. To evaluate differences in Cr concentrations and speciation at varying depths within alluvial deposits, samples were collected under pumping conditions from different depths in selected production wells. In one well, Cr(III) was the predominant form of Cr from the water table down to 610 feet below land surface, where dissolved O₂ concentrations were near zero. In the same well, from 610 to 820 feet below land surface, dissolved O₂ was present, Cr(VI) concentrations were up to 35 mg/L, and Cr(III) was absent. Combined with Cr stable isotope ratio results, these results will enhance geochemical interpretations of Cr in groundwater in this region.