USGS Releases Long Awaited & Anticipated Chromium-VI (Cr-6) Background Study Report for the Hinkley Valley, California

Debate over the lateral extent of PG&E’s Cr-6 groundwater impacts in Hinkley have occurred for over 20 years. Project Navigator, Ltd. was consensually retained by the Hinkley Community Advisory Committee, PG&E and the California Regional Water Quality Control Board (RWQCB) over 10 years ago to perform in the role of an Independent Review Program (IRP) Manager with two main objectives: 1. Understand the technical aspects of PG&E’s remediation project and 2. Communicate and report our observations, especially on behalf of the Hinkley Community. That is, build “trust.”

USGS was retained in 2015 in an arrangement between PG&E and RWQCB to perform an exhaustive study comparing the lateral extents of background (natural) Cr-6 and anthropogenic Cr-6; i.e. that released to groundwater, via cooling water disposal, by PG&E during the timeframe 1952 to 1964.

A technical working group (TWG) was formed, which PNL as the IRP, actively participated in. The TWG consisted of the IRP Manager, PG&E, Water Board, USGS and Hinkley community members.

USGS released the Background Study’s final report on April 25, 2023, which includes the following takeaways:

  • The purpose of the study was to establish scientifically defensible estimates of naturally occurring Cr-6 in Hinkley.
  • The study took over ten years to complete and included developing the scope of work, contracting, stakeholder engagement, field work and report preparation.
  • The study was divided into four subareas to incorporate the different geological features of the Hinkley Valley.
  • A Summative Scale Analysis (SAS) approach was utilized to delineate the extent of PG&E plume. See map below showing the SAS plume boundary and background numbers.
  • The majority of PG&E plume was identified in the eastern sub-area and the estimated background Cr-6 concentration is 2.6 parts per billion (ppb).

The results of the USGS Background Study will be used by the Water Board to determine the final remedial target as part of the revised Cleanup and Abatement Order.

Link to the study is: https://pubs.usgs.gov/publication/pp1885