SITE INFORMATION AND CORRESPONDENCE

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Mr.Gordon Boggs 2 Calif. Regional Water Quality Control Board, Central Valley Region,3443 Routier Rd., Sacramento, CA November 16, 1994 containment levels in a plume diminished in size to the vicinity of one well. The data also show that the shrinking dissolved hydrocarbon plume is stationary. Soil Contamination From February 1987 through January 1991, soil vapor extraction system was in operation at the site. During operation, a 100-cubic-feet-per-minute (cfm) blower was in use. On January 13, 1994, a vapor extraction test was conducted using a 1.5-horsepower (hp) blower. During the test, the maximum measured flow rate was 83 cfm. The test blower produced Induced vacuum on 0.1 Inch of water column at 100 feet from the test wall. The linear regression graph on Figure 10 of the Addendum of Closure Report indicates that there was pressure (vacuum) gradient to at least 100 feet from the vapor extraction well. The slope of the line on the graph represents pressure gradient which induces vapor flow. The calculated radius of capture of 100 feet given in the report is a conservative estimate based on data collected during a short term test and based on laws of physics. Soil samples collected from confirmation soil borings drilled on the January 12, 1993, did not contain petroleum hydrocarbons at concentrations above detection limits. Analysis of vapors collected during January 1994 soil vapor extraction test did not contain benzene. The maximum concentration of TPH-G detected in the soil vapors was 74 micrograms per liter and the hydrocarbon extraction rate during the brief test was approximately 0.55 lbs/day. Typically, however, hydrocarbon concentrations drop rapidly with time when vapor extraction is initiated after a prolonged time of no vapor extraction, and the concentrations reach low asymptotic levels within a few hours or a few days. The data indicate that low concentrations of petroleum hydrocarbons exist locally in the vadose zone, but the results of soil analyses indicate that detectible levels of adsorbed hydrocarbons are not present in the soil. Summary Soil vapor extraction was used to remediate the site from February 1987 through January 1991. Hydrocarbon concentrations from the soil vapor extraction system had reached low asymptotic levels. It is the opinion of Southland that the concentrations of remaining petroleum hydrocarbons in the soil are not sufficient to impact the groundwater. Furthermore, groundwater sampling indicates that while low concentrations of petroleum hydrocarbons are present in groundwater, the random minor occurrences in one well do not constitute a measurable plume, do not demonstrate migration, and are not a threat to public health now, and will not be a threat to public health in the future. The Southland Corporation does not concur with San Joaquin County's opinion as stated in the July 12, 1994 letter. ADDENDM2.LTR(STHL-6) ❑❑s GROUNDWATER OCA❑ TECHNOLOGY