Analyses of Regulatory Water-Quality Data

Professional Paper 1885-D
Prepared in cooperation with the Lahontan Regional Water Quality Control Board
By:  and 



Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the Pacific Gas and Electric Company (PG&E) Hinkley compressor station in the Mojave Desert 80 miles northeast of Los Angeles, California. The Pacific Gas and Electric Company has monitored groundwater near Hinkley, California, for Cr(VI) and other constituents since the late 1980s. By June 2017, more than 20,000 samples had been collected and analyzed for Cr(VI) for regulatory purposes. Most Cr(VI) samples were analyzed using the U.S. Environmental Protection Agency (EPA) Method 218.6 with a laboratory reporting level (LRL) of 0.2 micrograms per liter (μg/L). Between July 2012 and June 2017, selected samples were analyzed for low-level Cr(VI) concentrations using a modified version of EPA Method 218.6 with an LRL of 0.06 μg/L. Field-blank data and duplicate samples collected during this period indicate a study reporting level (SRL) of 0.2 μg/L for most analyses and a SRL of 0.12 μg/L for low-level Cr(VI) analyses. The overall precision for Cr(VI) data analyzed by both methods at the interim regulatory Cr(VI) background concentration of 3.1 μg/L was 0.09 μg/L, or about 3 percent.

Hexavalent chromium concentration trends were calculated for 564 monitoring wells for the period from July 2012 through June 2017. Upward Cr(VI) concentration trends were present in water from 102 monitoring wells throughout Hinkley and Water Valleys. Upward Cr(VI) concentration trends in water from wells near the margins of the October–December 2015 (Q4 2015) regulatory Cr(VI) plume (1) within strands of the Lockhart fault east and southeast of the Hinkley compressor station and (2) in water from shallow wells within the northern subarea were consistent with expansion of the Cr(VI) plume in these areas between 2012 and 2017. Upward Cr(VI) concentration trends were widely distributed elsewhere in Hinkley and Water Valleys outside the Q4 2015 regulatory Cr(VI) plume and were commonly associated with declining water levels. These upward trends may result from natural Cr(VI) sources, including movement of Cr(VI) containing groundwater from (1) weathered bedrock, (2) fine-textured deposits, or (3) secondarily oxidized material distributed throughout aquifer deposits. Downward Cr(VI) concentration trends were observed in 146 monitoring wells. Downward trends were largely within the Q4 2015 regulatory Cr(VI) plume and can be attributed to remediation activities downgradient from the Hinkley compressor station. Hexavalent chromium concentration trends also were calculated for 219 domestic wells from July 2012 through June 2017. Upward Cr(VI) concentration trends in 8 domestic wells and downward trends in 23 domestic wells were clustered largely within former residential areas west of the Q4 2015 regulatory Cr(VI) plume. Results of Cr(VI) trend analyses (including upward, downward, and no trend) were used with other data as part of a summative-scale analysis (chapter G) to define the extent of anthropogenic Cr(VI) and natural Cr(VI) within Hinkley and Water Valleys.

Suggested Citation

Izbicki, J.A., and Seymour, W.A., 2023, Analyses of regulatory water-quality data, Chapter D of Natural and anthropogenic (human-made) hexavalent chromium, Cr(VI), in groundwater near a mapped plume, Hinkley, California: U.S. Geological Survey Professional Paper 1885-D, 28 p.,

ISSN: 2330-7102 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • D.1. Introduction
  • D.2. Data Availability
  • D.3. Sample Collection, Laboratory Analyses, Data Quality, and Statistical Methods
  • D.4. Hexavalent Chromium Concentration Trends in Water from Wells
  • D.5. Comparison of Hexavalent Chromium Concentration Trends with Water-Level and Other Data
  • D.6. Conclusions
  • D.7. References Cited
  • Appendix D.1. Quality Assurance and Environmental Hexavalent Chromium Data from Selected Monitoring and Domestic Wells Sampled for Regulatory Purposes by Pacific Gas and Electric Company, Hinkley and Water Valleys, Western Mojave Desert, California, July 2008 through June 2017
Publication type Report
Publication Subtype USGS Numbered Series
Title Analyses of regulatory water-quality data
Series title Professional Paper
Series number 1885
Chapter D
DOI 10.3133/pp1885D
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) California Water Science Center
Description Report: viii, 28 p.; 6 Appendixes
Country United States
State California
City Hinkley
Additional Online Files (Y/N) Y
Google Analytic Metrics Metrics page
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