California Water Science Center

Cooperating Agency: Lahontan Regional Water Quality Control Board
Project Chief: John A. Izbicki
Phone: 619-225-6131
Email: jaizbick@usgs.gov

Occurrence of natural and anthropogenic hexavalent chromium (Cr VI) in groundwater near a mapped plume, Hinkley, CA

The Problem

Mojave River Groundwater Basin

Figure 1. - Mojave River Groundwater Basin

Problem: The Pacific Gas and Electric Company (PG&E) Hinkley Compressor Station, 3 miles southeast of Hinkley, CA and 80 miles northeast of Los Angeles (fig. 1), is used to compress natural gas as the gas is transported through pipelines from Texas to California. Between 1952 and 1964, water treated with a compound containing chromium was used to prevent corrosion of pipes and machinery within the compressor station. This water was discharged to unlined ponds, resulting in contamination of soil and groundwater within the underlying alluvial aquifer with total and hexavalent chromium (Cr VI) (LRWQCB, 2012a).

The California State Water Resources Control Board requires clean-up of discharges to either background water quality, or to the best water quality reasonably obtainable if background water quality cannot be restored. Background is defined as the water quality that existed before the discharge occurred (LRWQCB, 2012a). In 2007, a study intended to characterize naturally-occurring background concentrations (CH2MHill, 2007) estimated average Cr VI concentrations in the area of 1.2 micrograms per liter (µg/L). The 95 percent upper tolerance limit (UTL) of 3.1 µg/L was determined from the 2007 background study and was adopted by the LRWQCB as the maximum background concentration for the site. On the basis of those data, in 2008 the mapped extent of the plume was about 2 miles north of the compressor station and the plume was about 1 mile wide (LRWQCB, 2008). By 2011, the mapped extent of the plume increased to 5.4 miles long and 2.4 miles wide. The increased extent of the plume may have resulted from a combination of: 1) movement of Cr VI with groundwater (the plume is bigger), 2) more comprehensive sampling of areas surrounding the 2008 mapped plume extent (there are more data), and 3) improved understanding of the distribution of chromium in different layers within the aquifer and how to sample those layers to obtain maximum concentrations (the data are of higher quality) (LRWQCB, 2012b).

The 2007 background study was criticized by independent reviewers for: 1) use of existing wells not specifically designed for groundwater monitoring and often having incomplete construction data, 2) inconsistent spatial and temporal distribution of data from wells used for the background study, 3) statistical handling of the data with respect to less than values, outliers, and representative concentrations from sampled wells, 4) uncertainty as to the historic extent of Cr VI contamination at the site, and 5) lack of a site conceptual model that includes the effects of pumping and ongoing remediation on groundwater flow and contaminant movement (LRWQCB, 2012b). The Lahontan Regional Water Quality Control Board (LRWQCB) subsequently agreed to revisit the 2007 background study in response to criticism of the study’s methodology and the increase in mapped extent of the plume between 2008 and 2011.

In response to criticism of the 2007 background study, PG&E proposed a statically-based sampling approach for a revised background study (Stantec, 2012). That proposal included installation of 32 wells, uniformly distributed near the center of township and range grids throughout the study area, with one-year of data collection from the wells. Although statistically unbiased and designed to estimate the average Cr VI concentrations within the volume of groundwater sampled, the proposed study design provided limited evaluation of the hydrologic history of the area with respect to groundwater movement relative to the compressor station, and limited evaluation of the potential geologic sources of natural chromium within the study area.

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