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


Task 1

Evaluation of existing data

Water-level and water-chemistry data collected by PG&E and their consultants are available from existing domestic wells, and from monitoring wells installed to monitor the plume. These data have been assembled and reviewed by PG&E and their consultants, and were discussed during TWG meetings beginning in January 2013. Although this review familiarized TWG members with available data, the opportunity to work directly with the data will give U.S. Geological Survey project staff greater familiarity with the spatial and temporal distribution and the quality of the data prior to the start of new data collection. As a framework for the initial analysis, Principal Component Analyses (Kshirsagar, 1972; Gnanadesikan, 1974) will be used to evaluate water-level and water-quality data relative to the occurrence of Cr VI with respect to pH, specific conductance, major-ion chemistry, and temporal and spatial differences within the study area.

The Hinkley CAC has expressed concern over possible trends in Cr VI concentrations and the destruction of domestic wells having long-term Cr VI data as PG&E acquires property within and near the mapped plume. Although in many cases, domestic wells were replaced with monitoring wells (having short screened intervals, completed at different depths within the aquifer), there is concern from the CAC that the Cr VI concentration data from domestic and monitoring wells, and the spatial density of the data may not always be comparable.

Historic data will be examined to determine if there are trends in water quality, including total Cr and Cr VI concentrations, in monitoring wells and domestic wells. Data will be analyzed using the nonparametric Mann–Kendall trend test (Mann, 1945; Helsel and Hirsch, 2002) to determine the significance of Kendall's t correlation between total Cr and Cr VI concentrations with time. The Sen slope estimator will be used to estimate trend magnitude (Sen, 1968; Hirsch and others, 1991). A minimum of four analyses for each well are necessary to attain a statistically significant result (p-value less than 0.1) for the Mann–Kendall test, although at least 8 points are recommended for statistical analysis of trends (Grath and others, 2001). To the extent possible data will be evaluated to determine if seasonality, water level, well-construction data (where available), or other factors may influence the presence or absence of trends in wells (Kent and Landon, 2013). Decreasing, stable, or increasing chromium concentrations identified as part of this analysis will be used to guide data collection for later Tasks in this proposal. Water-quality trends in parts of the study area do not necessarily represent increases or decreases in the plume extent.

Data collected by PG&E and their consultants since the 2007 background study will likely be the easiest to analyze. Older data collected prior to the 2007 study and data collected in the area by other agencies may need to be interpreted in light of changes in field collection, changing collection intervals, analytical techniques, and reporting levels; but will be considered for analyses if available.

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

California Water Conditions

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