STABLE ISOTOPE VARIATIONS OF HEXAVALENT CHROMIUM IN GROUNDWATERS OF THE MOJAVE DESERT, CALIFORNIA, USA

Chromium in potable water supplies recently has become of increased health concern. Likewise recently, the use of transition metal isotopes has found broader application to the interpretation of sources and processes controlling metal concentrations in waters. Chromium isotopes may be particularly useful for confirming the reduction of chromate to chromic ion, the predominant immobilization mechanism. Previous work during development of groundwater supply and injection wells in the Surprise Springs and Deadman Basins in the western Mojave Desert showed elevated concentrations of Cr (up to 38 µg/L) and As. In May 2001, about thirty groundwater samples were collected from production and monitoring wells in this area for complete isotope and chemical characterization, including dissolved Cr redox speciation. Using initially added ⁵⁴Cr/⁵⁰Cr double-spike solutions, hexavalent chromium was separated from 16 groundwater samples and the ⁵³Cr/⁵²Cr ratios were determined by thermal ionization mass spectrometry. The d⁵³Cr values ranged from –0.3 to +3.2 per mil relative to the NIST 979 Cr isotopic reference standard, with an overall variability among duplicates averaging ±0.15 per mil. The NIST standard yielded a ⁵³Cr/⁵²Cr ratio of 0.11339±0.00003, comparing favorably with the certified value. Preliminary interpretations indicate that isotope ratios are shifted to heavier values in the lower aquifer, which is recharged by locally derived runoff and is very old. Lighter ⁵³Cr⁵²Cr values are observed in the younger waters of the upper aquifer, which is recharged by infiltration and runoff from the San Bernardino Mountains. This could mean that Cr(III)/Cr(VI) redox reactions in the lower aquifer are enriching the ⁵³Cr in the dissolved phase by an equilibrium fractionation mechanism. The Cr source for the lower aquifer is unlikely to be anthropogenic, as anthropogenic sources of Cr typically have d⁵³Cr values very near zero. These results represent some of the first known Cr isotope ratios for groundwater that have been determined with this kind of precision.