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r � AEMA City of Stockton Fire Station #b Soil Remediation Plan Site Code 1013 June 24, 1992 Page 2 Excavation proceeded until site attributes including public and private thoroughfares prevented further enlargement of the pit. Subsequent to excavation, RESNA personnel collected 5 soil samples from the pit walls and one sample from the floor of the pit (Figure 2). Laboratory analysis of the samples indicated that petroleum hydrocarbon constituents are present in soils still in place on the south and east walls of the pit as well as the pit floor. Laboratory results of soil samples are summarized in Table 1. Due to safety considerations, the pit was subsequently backfilled with clean engineered material and compacted to grade. Soils removed during excavation were stockpiled on-site and aerated for a 5-month period under authorization of the San Joaquin County Air Pollution Control District (APCD). The soils were kept moist and were tilled several times during aeration to increase natural bacterial decay of diesel contaminants. The aerated soils were subsequently relocated to the City of Stockton Corporation yard with the authorization of the San Joaquin County Public Health Services Environmental Health Division (PHSIEHD) for use as roadbase material at various city paving projects. Laboratory results of the soil composite sample collected from the soil pile after aeration (02/14/91) is summarized in Table 1. Based on the laboratory results of confirmation samples collected during the re-excavation, further investigation of the extent of soil and possible groundwater contamination was required by the PHSIEHD in order to evaluate remedial action alternatives. On April 29, 1991, RESNA personnel advanced two soil borings (SBI and MWI) to respective depths of 36.5 and 38 feet below grade to determine the full extent of hydrocarbon impacted soils. Soil Boring MW1 was subsequently converted into a groundwater monitoring well. The soil boring and monitoring well were located as close as possible to the area of suspected soil contamination. The location of the two borings are exhibited on Figure 2. The laboratory analysis of soil samples collected at 10.5 and 25.5 feet below grade in Boring MW1 and also at 20.5 and 25.5 below grade in Boring SB1 did not detect the presence of diesel constituents above the method detection limit. The laboratory test results are summarized in Table 1. The depth to first groundwater beneath the site was recorded at approximately 28 feet below grade. Analysis of groundwater collected subsequent to the development of MW 1 indicated SFSb92.TJL