GEOCHEMICAL RELATIONS AMONG ARSENIC, CHROMIUM, AND OTHER TRACE ELEMENTS IN GROUND WATER UNDERLYING THE SHEEP CREEK FAN, WESTERN MOJAVE DESERT, CALIFORNIA

The Sheep Creek fan in the western Mojave Desert, about 60 miles east of Los Angeles, is composed of alluvium weathered from highly mineralized metamorphic rock in the San Gabriel Mountains. As ground water moves from sources of recharge near the mountain front to discharge areas near a dry lake at the down slope end of the fan, arsenic and chromium concentrations increase from less than their detection limits to 42 and 60 µg/L, respectively. Almost all the arsenic was present as arsenate (As(V)) and almost all the chromium was present in the hexavalent (Cr(VI)) oxidation state. Chromium concentrations in water from some wells exceeded the California Maximum Contaminant Level of 50 µg/L. Arsenic concentrations increased when pH increased as a result of silicate weathering. The pH dependence of chromium concentrations was less pronounced because of decreases in dissolved oxygen concentrations in ground water near the dry lake. No effective mineral controls on arsenic concentrations were identified using thermodynamic calculations; however, chromium hydroxide (Cr(OH)₃) may control chromium concentrations at pH values near neutral. Despite the low concentrations, redox values calculated from chromium(III/VI) redox species appeared reasonable and reflected measured ground water chemistry and dissolved oxygen concentrations. The arsenic(III/V) redox couple is rarely in equilibrium with other solution redox couples. Redox values could not be calculated from the iron(II/III) redox couple because most iron concentrations were below detection.