ARCHIVED REPORTS XR0011742 - Laserfiche WebLink

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�.' San Joaquin County Public Health Services March 31, 1999 Page Two Outline Of The Proposed Corrective Action Work • Significant definition of lateral and vertical extent of the hydrocarbon affected soil has been accomplished during the course of tank and piping sampling as part of the tank removal process. Using this existing data, a push probe mounted on a drill rig will be used to obtain soil samples to characterize the site to a depth of 0 to 12 feet. The water table is at a depth of 8 to 9 feet. This push probe work will be used to obtain the lateral and vertical extent of hydrocarbon affected soil. • Using the soil data, monitoring wells will be located and installed to obtain the groundwater gradient as well be used to determine the natural attenuation of the groundwater with time since the source has been removed. • The soil that has already been removed from the principally affected area, the dispenser island, will be treated on-site with a combination of aeration and bio remediation. The proposed clean-up standard is 100 ppm Tph gasoline, 1000 ppm Tph as diesel, non detect for benzene, toluene, xylene and ethyl benzene as well as MTBE. This treated soil will then be used on-site as backfill underneath future paving. • A risk based analysis will be performed after the natural attenuation of the groundwater has been demonstrated adequately to enable eventual closure of the site. Options Considered and Rationale for Proposed Corrective Action • The local geology and hydrology of the Tracy area, which contains clay soils with premeabilities on the order of ten to the minus six centimeters per second or less limits movement of hydrocarbons in the soil. With very low permeability the opportunity for effective soil vapor extraction of this material has been shown to be ineffective based on several projects which have used this technique in Tracy. Groundwater treatment has long been shown to not be particularly cost effective since large quantities of groundwater must be treated to remove small quantities of hydrocarbon. Excavation and on-site treatment of the soil is the most cost effective method of removing hydrocarbons from the soil but the cost benefit is not judged to be appropriate since there are several acres of affected soil to a depth which could necessitate treatment of tens of thousands of cubic yards. • Since the most severely hydrocarbon affected soil, which was beneath the pump islands and piping, has been removed natural attenuation is considered to be the best long term approach for this site. While the near surface aquifer has been slightly impacted it is not considered a usable aquifer in the area and natural bio remediation - will reduce the hydrocarbons with time. This will be monitored and a risk based