CHARACTERIZATION OF NATURAL CHROMIUM IN DEEP GROUNDWATER IN CENTRAL CALIFORNIA

In July 2011, the California Office of Environmental Health Hazard Assessment expects to finalize a draft Public Health Goal for hexavalent chromium (Cr⁶) (currently set at 0.02 μg/L). The California Department of Public Health (DPH) will then be required to adopt a new state Maximum Contaminant Level (MCL) for Cr⁶. DPH may also lower the state MCL for total chromium (Cr) which is now 50 μg/L, whereas the federal MCL is 100 μg/L. Concurrently, the USEPA is also evaluating the development of a new MCL for Cr⁶. Naturally occurring Cr⁶ is prevalent in California, and the new drinking water standard(s) may require many water purveyors to treat future groundwater supplies. Due to this concern, a city in northern Solano County, California embarked on further understanding of the natural occurrence of Cr in groundwater in their local aquifer system. The investigation considered analyses of total Cr and Cr⁶ and the governing oxidizing and reducing conditions at selected monitoring well locations, some extending to depths of about 1,800 feet. Samples collected from multiple-completion monitoring wells allowed evaluation of the occurrence of Cr within the deep aquifer unit used by the city, as well as other overlying units of the aquifer system. The Cr and Cr⁶ concentrations in groundwater evaluated during this study (e.g., up to about 44 μg/L in the water supply and monitoring wells of interest) were found to be dependent on depositional and geochemical conditions in the deep aquifer system and associated with geogenic factors related to the proximity of the Coast Range serpentinites of ophiolitic origin. Given the geochemical conditions associated with the local groundwater, adsorption and co-precipitation reactions involving iron hydroxides and oxidation of Cr³ to Cr⁶ by manganese oxides appear to be major factors attributable to the presence of Cr⁶ in the area’s deep aquifer system. Cr presence in groundwater may be linked to mafic/ultramafic rocks, including serpentinites and ophiolites, across the globe. With additional characterization of the local geochemical environment and an understanding of the processes contributing to the presence and variability of Cr⁶ in the aquifer system, water supply well design, particularly zones of completion and sealing considerations, can be employed to minimize Cr⁶ concentrations.