California Water Science Center

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

Task 7

Estimation of background hexavalent chromium (Cr VI) concentrations

Results from this study and interpretation of data as part of Task 6 will form the basis for identifying where background Cr VI concentrations are present within the different subareas within Hinkley Valley. After identification of areas containing natural and anthropogenic Cr VI (and mixtures of end-members from sources identified as part of this study), it may be possible to estimate background Cr VI concentration in areas not affected by discharges from the compressor station using existing data and data collected as part of this study. However, given criticism of the 2007 background study, the TWG (and the LRWQCB) may determine that it is necessary to produce a statistically-defensible background Cr VI concentration for one or more of the subareas within the study area. If a statistically-defensible approach is needed, PG&E may need to install additional monitoring wells in areas identified as having naturally-occurring Cr VI. The approach and specific details associated with this task will be outlined in the preliminary report to be prepared in Federal Fiscal Year 2014.

Regardless of the scope of this task, wells selected for inclusion in background Cr VI calculations will need verifiable construction data, and representative spatial distribution within the study area. Scope of work for this task includes quarterly sample collection and analyses of total dissolved chromium, and Cr VI from 30 wells and associated data analyses. The wells will be identified on the basis of data collected as part of this study in consultation with the TWG. Selected wells will be sampled and analyzed quarterly for a period of one year (4 samples) to obtain a data to assess variability in naturally-occurring Cr VI concentrations in different parts of the study area. Cr VI data will be collected by PG&E consultants and U.S. Geological Survey staff. Samples will be split and analyzed by both groups. The average of the two samples will be used for estimation of background. In cases where Cr VI data differ by more than 10 percent, the sample the best fits data from other quarters will be used for estimation of background.

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