Manufactured Gas Plant (MGP) Experience —
Site Assessments: Tetra Tech successfully completed 18 site assessments for various MGP sites in California for Sempra Energy/The Gas Company. Our staff performed a thorough records search and document review of the sites, prepared detailed geological and hydrogeological studies, and conducted full-scale human health and ecological risk assessments. Four of the investigations resulted in no further action (NFA), with the remainder requiring further investigation/remediation. For these sites, Tetra Tech developed a cost-effective methodology consisting of comparison of site-specific risk-based screening levels (RBSLs) for future on-site commercial workers, industrial workers, construction workers and residents to exposure point concentrations. The RBSLs were developed according to USEPA risk assessment guidelines and site-specific fate and transport modeling, including soil vapor intrusion to indoor air and chemical leaching to groundwater. RBSLs were used to determine whether a site warranted remediation and also to guide site remediation, notably demonstrating that soil and soil gas removal actions achieved RBSLs.
Risk Assessments Under Preliminary Endangerment Assessments (PEAs): Tetra Tech successfully completed 12 PEAs at various Gas Company Sites. The PEAs were conducted in accordance with California Environmental Protection Agency (Cal/EPA) guidance. More than half of the PEAs resulted in NFA, with the remainder requiring further investigation/remediation. Tetra Tech also completed human health risk assessments as part of the PEAs conducted for PG&E at the Colusa, Marysville, and Grass Valley MGP sites, which resulted in NFA for all three sites.
Remedial Action Designs: Tetra Tech successfully negotiated with DTSC regarding the approach and cleanup goals for remediating several former MGP sites. The projects were efficiently conducted such that DTSC responded by using the projects as examples of successful teaming efforts between business and government on remediation actions.
Remedial Action Work Plans: Tetra Tech successfully streamlined the remedial investigation/feasibility study and removal action work plan approach to expedite the removal work via successful negotiations with the regulatory agency, which resulted in substantial savings of time and money.