SPATIAL AND GEOLOGIC CONTROLS ON CHROMIUM TRANSPORT FROM WATERSHEDS WITH ULTRAMAFIC ROCKS IN NORTHERN CALIFORNIA

Weathering and transport of chromite, an accessory mineral in ultramafic and serpentinized ultramafic rocks, can have environmental and human health consequences for surface and groundwater quality. Hexavalent chromium (Cr(VI)), a known carcinogen, occurs in some Sacramento Valley groundwater wells and has been linked to chromite-bearing alluvium in the valley, with ultramafic sources in the Coast Range, Sierra Nevada foothills, and the Klamath Mountains. Variations in chromium for different bedrock types and potential for dissolution in surface and groundwater were investigated in two adjacent Coast Range watersheds. Thomes and Elder Creek have similar geology, with ultramafic, metasedimetary and sedimentary rocks in the upper watershed. There were several active chromite mines between 1890 and 1946 in the middle and north forks of Elder Creek watershed and no active chromite mining in Thomes Creek. Three springs flowing through ultramafic rocks sampled in the upper watershed had chromium concentrations from 1 to 16 µg L^-1, mostly as Cr(VI), with low Ca/Mg ratios consistent with groundwater interaction with ultramafic rocks. Dissolved chromium concentrations in Thomes Creek tributaries upstream from serpentinized ultramafic rock were <0.5 µg L^-1, with 1 to 3 µg L^-1 in ultramafic reaches of the catchments, predominantly as Cr(VI). The north, middle and south forks of Elder Creek had <0.5 to 2 µg L^-1, but unlike Thomes Creek the detectable chromium persisted downstream into the main stem of Elder Creek, with 0.06 µg L^-1. Invertebrate communities were sampled to determine the extent of chromium uptake in aquatic invertebrates and in predatory spiders, to test whether historic mining activity had exacerbated uptake above natural background concentrations. Streambed sediments and alluvial soils were also collected to follow the transport of chromite-bearing materials into the Sacramento Valley. Implications for elevated natural background chromium concentrations in surface waters from watersheds with ultramafic rocks, regardless of mining history, need to be considered for water resource management. Reservoirs and managed wetlands in Tehama and Glenn Counties could potentially be affected by chromium, both directly from surface water discharge and from mineral dissolution in sediments and soils.