INSIGHTS INTO CONTROLS ON CHROMIUM(VI) CONCENTRATIONS IN GROUNDWATER IN THE SACRAMENTO VALLEY, NORTHERN CA, PROVIDED BY ENVIRONMENTAL TRACER DATA

In northern California, Cr(VI) concentrations in groundwater are commonly elevated (>5 μg/L) due to the widespread occurrence of Cr-rich ultramafic rocks, posing a potential public health concern. However, specific causes of local variability in background Cr(VI) concentrations remain unclear because processes controlling the oxidation of Cr(III) and subsequent mobilization and transport of Cr(VI) in groundwater are complex and poorly understood. We interpreted groundwater chemistry data from 167 wells in the Sacramento Valley in northern California to determine recharge sources and water-rock interaction history for waters containing widely ranging Cr(VI) concentrations (<1 to 70 μg/L). Samples were collected by the U.S. Geological Survey under the state’s Groundwater Ambient Monitoring and Assessment (GAMA) Program. Groundwater is generally oxic (76% of wells) with pH ranging from 6.3 to 8.7. The data include a suite of environmental tracers: stable water isotopes (¹⁸O and ²H); tritium; noble gases (He, Ne, Ar, Kr, Xe); ³He/⁴He ratio; and carbon isotopes (¹³C and ¹⁴C). Tracer data were used to constrain the age of groundwater, compute noble gas recharge parameters (recharge temperature and excess air), and identify recharge sources. Comparison of this age and source information with Cr(VI) concentrations reveals several correlations. In the southern part of the valley, higher Cr(VI) concentrations are in waters with a modern (<60 yr old) component and an evaporated (heavy) δ¹⁸O signal, consistent with recently applied irrigation water. However, in the middle and northern parts of the valley, higher Cr(VI) concentrations occur in waters 1-10 kyr old with unexpectedly warm recharge temperatures. We hypothesize that these waters infiltrated near the basin margin or in the adjacent foothills where the water table is deeper (>100 m), and that the thick unsaturated zone may be an important factor in the natural oxidation/mobilization of Cr. Pleistocene-age (>10 kyr old) waters are generally anoxic and are not elevated in Cr(VI). Chromite mines in adjacent foothills appear to have little or no influence on valley groundwater. Efforts are ongoing to relate Cr(VI) with other chemical constituents and to better understand the geochemical evolution of the high-Cr waters.