Hexavalent Chromium in Groundwater in the Davis/Dixon/Woodland Area, Western Sacramento Valley, California

Elevated groundwater hexavalent chromium concentrations not known to be associated with environmental releases are found throughout the Davis/Dixon/Woodland area of the Western Sacramento Valley. As part of an effort to establish groundwater background (ambient) values for a federal Superfund site in Davis, data have been compiled from available public databases and several other sources. Ambient hexavalent chromium concentrations exceeding 200 micrograms per liter (µg/L) have been identified locally in the first hydrostratigraphic unit (HSU-1), a low-to-moderate permeability zone extending from the water table to approximately 80 feet below ground surface (bgs). Ambient concentrations as high as 180 µg/L have been reported in the next deeper unit (HSU-2), a high permeability zone extending from approximately 80 to 120 feet bgs and used extensively for water supply. Concentrations can be highly variable over short distances, especially in HSU-1. The combination of ultramafic sediments derived from the Coast Ranges to the west and oxic/slightly alkaline groundwater allow for the natural oxidation of trivalent chromium to hexavalent; however, the mechanism(s) that produce the wide range and spatial variability of ambient hexavalent chromium concentrations in the area are not well understood. Scientists from the U.S. Geological Survey, Stanford University, University of California Davis and its contractors have been investigating potential mechanisms, including: (1) geochemical changes associated with organic matter, fertilizer use, widely fluctuating water levels, and irrigation; (2) hydrologic factors such as residence time and mixing; and (3) geologic factors such as mineralogy. This presentation will review the regional hexavalent groundwater data and available results related to potential formation mechanisms.