Pacific Gas and Electric Company (PGE), Former Manufactured Gas Plant Low Risk Groundwater Closure Report

Pacific Gas and Electric Company (PGE), Former Manufactured Gas Plant Low Risk Groundwater Closure Report

DTSC approved a Low Risk Groundwater Closure Report pursuant to the authority granted under the CA Health and Safety Code, Ch. 6.8 and the Federal Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Section 121 for the PGE, Former Manufactured Gas Plant (MGP) site. The approved Low Risk Groundwater Closure Report would modify the existing Remedial Action Plan that addressed soil and groundwater contamination at the Site which was approved by DTSC in a letter dated April 21, 2003. The RAP included soil excavation and disposal of contaminated soils by truck transportation to an off-site disposal facility, on-going groundwater monitoring activities in a network of Site related monitoring wells, and land use controls for the Site. Waiver of groundwater remedial action goals. Under the proposed revision to the remedy, cleanup goals for TPH-d and TPH-g would be waived as requirements for closure based on low risk principles. The cleanup goal concentration for the two constituents in groundwater as determined in the RAP have not yet been achieved in all monitoring wells (TPH-g goal: 50ug/L and TPH-d goal: 100ug/L). However, the Low Risk Report demonstrates that the project groundwater cleanup goals have been met consistently in all monitoring wells for more than five years for all constituents of concern except TPH-g and TPH-d. Only four wells (MW-1D, MW-1S, MW-8S, and MW 14S) have dissolved TPH above the goals. Statistically defensible time versus concentration trends indicate that cleanup will be achieved in two of these wells (MW 8S and MW-14S) in two years or less. Results of many years of quarterly monitoring indicate that remediation is proceeding gradually via natural biodegradation in groundwater. The plume of residual petroleum hydrocarbon groundwater is decreasing in size. Ongoing, naturally occurring biodegradation will achieve groundwater cleanup goals within a reasonable timeframe.

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