SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Pacific Gas and Electric • - 2 - • 30 April 2008 McMullin Dehydration Station Rationale for No Further Action Source: Leakage from above ground storage tanks which contained natural gas condensate caused soil and groundwater impacts. These impacts were limited to the property boundary, which is approximately 80 feet wide and 115 feet long. All equipment and tanks were removed in 2003. Actions: • Eight groundwater monitoring wells and four piezometers were installed between 1997 and 1998 and sampled quarterly, • In October 2002, a total 4,500 cubic yards of contaminated soil was excavated. The first excavation event removed 3,000 cubic yards of impacted soil. Confirmation soil samples indicated some impacted soil remained, so an additional 1,500 cubic yards was excavated. As seen in Attachments 3 and 4, residual soil impacts of TPH-g and TPH-d remain in the soil at maximum concentrations of 20 mg/kg and 7.7 mg/kg, respectively. • Groundwater monitoring wells along the perimeter of the Site and an additional monitoring well within the former source area delineated the groundwater plume and monitored the effects of remediation. • PG&E continued groundwater monitoring in the five remaining monitoring wells for an additional four continuous quarters. Extent defined: Impacts to soil and groundwater were delineated laterally and vertically. Estimated Residual Petroleum Mass: The estimated mass remaining in soil of TPH-g is 0.40 kg. The estimated mass remaining in soil of TPH-d is 1.53 kg. The mass of contaminants remaining in groundwater is estimated to be 0.0003 kg of Benzene,Toluene, Ethyl-benzene, and Xylenes (BTEX), 0.0075kg of TPHg, 0.0075kg of TPHd, and 0.0752kg of Total Petroleum Hydrocarbon in motor oil range (TPHmo). Threat to Groundwater: Site remedial activities have eliminated the threat to groundwater quality. BTEX and TPH-range hydrocarbons have been at or below water quality objectives in all five wells since 2003. See Attachment 5 for groundwater analytical data. Threat to Human Health: Regional Water Board staff concurs with PG&E and the analysis provided by their consultant, Parson, that there is no threat to human health from vapor intrusion or dermal contact because contaminated soil has been removed and groundwater impacts are below water quality objectives..