Pacific Gas and Yvonne J. Meeks 6588 Ontario Road Electric Topock Project Manager San Luis Obispo, CA 93405 Company Chromium Remediation Project Office Gas Transmission & Distribution Mailing Address 4325 South Higuera Street San Luis Obispo, CA 93401 805.546.5243 Internal: 664.5243 Fax:: 805.546.5232 E-Mail:
[email protected] October 31, 2008 Mr. Aaron Yue Project Manager California Department of Toxic Substances Control 5796 Corporate Avenue Cypress, CA 90630 Pamela Innis U.S. Department of Interior P.O. Box 25007 (D-108) Denver, CO 80225-0002 Subject: Technical Memorandum - Summary of Colorado River Bridge Pier Construction and Hydrogeologic Assessment Dear Mr. Yue and Ms. Innis: This letter transmits the Technical Memorandum - Summary of Colorado River Bridge Pier Construction and Hydrogeologic Assessment describing the results of historical research into the construction methods of the subsurface piers of the BNSF rail bridge, the Highway I-40 bridge, and the historic Red Rock Bridge. This memorandum also presents an evaluation of whether the bridge piers could serve as preferred groundwater flow conduits to surface water. This work was done in response to the California Department of Toxic Substances Control (DTSC) letter to PG&E dated September 17, 2008 and the U.S Department of Interior (DOI) letter to PG&E dated September 17, 2008, which directed this evaluation and the establishment of a new RMP surface water sampling location at the westernmost BNSF bridge pier. The new sampling location at the downstream side of the westernmost BNSF bridge pier will be sampled over winter 2008-2009, beginning no later than December.