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Sunland Gasoline March 14, 2000 ' 2085 Fremont Street, Stockton, California Page 8 . Report of Findings, Site Characterization groundwater table (15-45') did not transport the contaminates on-site from an off-site source. At depths of 15-45' high levels of contamination are present in the soil. t The lateral and vertical extent of soil contamination has not been defined. The lateral extent of groundwater contamination remains undefined. ' The soil contamination level sharply decreases at the soil/groundwater interface at fifty- eight feet (58'). Between 55' and 60' there is a decrease of at least 99% for every constituent except Benzene (78% decline) and Ethanol. ' The soil boring, B-2 show relatively low levels of contamination. Contamination in this Y area is most likely due to a former leak in the piping supplying the pumps at the fuel island. This is believed to be the case due to the fact that petroleum hydrocarbon contamination was present at five feet (5') below ground level, the first sampling point. The soil in the boring exhibited decreasing levels of contamination with increasing depth until thirty feet (30'). At the thirty-foot point, the contamination levels seem to be increasing. This may be due to the migration of soil contamination plume associated with ' the leaking UST's approximately forty feet (40') to the south. WHF believes that the levels of contamination found between 5-20' do not pose a threat to groundwater underlying the site at a depth of fifty-eight feet (58'). Natural attenuation of the soil will l occur as the contaminates migrate downward to insignificant levels. Therefore, no action ie is needed regarding the top 5-20' of soil in the vicinity of B-2. ' Groundwater ingestion of contaminates is occurring at this site as a result of the contact of the contaminated soil mass and the groundwater. Therefore, the contaminated soils are the source, or are contributing to the contamination of groundwater underlying the subject ' property. ' 8.0 Recommendations After reviewing the site data obtained from this investigation, WET recommends the following actions to monitor and mitigate the effects of the release: ' • Install three (3) groundwater monitoring wells across the site. This will provide information of groundwater conditions, allow for monitoring of contaminate levels, and possibly provide data for the definition of the groundwater plume. Perform quarterly groundwater monitoring for one year. ' • Complete four (4) additional soil boring to determine the lateral and vertical extent of soil contamination at the subject property.