ARCHIVED REPORTS_SITE INVESTIGATION REPORT

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' Site Investigation Report for the Former UST Locations at Stockton AASF ' California Army National Guard EXECUTIVE SUMMARY ESA This site investigation report was prepared for the California Army National Guard (CA ARNG)under contract to the United States Army Corps of Engineers, Sacramento District. This report was prepared to assess whether the soils and groundwater in the area surrounding the former underground storage tank(UST) locations at the CA ARNG Stockton Army Aviation Support Facility(AASF) have been impacted with petroleum constituents. The evaluation was accomplished ' as follows: • Twelve soil borings (B3 through B14)were drilled surrounding the two former 20,000-gallon ' UST locations and the three former 5,000-gallon UST locations; soil and groundwater samples were collected. Samples were analyzed to determine whether total petroleum hydrocarbons (TPH)as gasoline (TPH-G); TPH as diesel(TPH-D); benzene, toluene, ethylbenzene, and xylenes(BTEX); oxygenates; and lead are present downgradient of the UST locations. Soil ' samples from the borings showed indication of impacts(discoloration,odors, and/or photoionization detector [PID] readings above background).In addition to the above contaminants of concern, a number of volatile organic compounds (VOCs)were detected in both soil and groundwater; however,the only VOCs detected exceeding action levels were trichloroethene (TCE), carbon tetrachloride(CC14),and cis-1,2-dichloroethene(DCE), all in groundwater. • Based on the investigation results,the three existing groundwater monitoring wells are recommended for quarterly sampling(with approval from the Regional Water Quality Control ' Board [RWQCB] and CA ARNG). These wells have only been sampled twice in the past seven years.Because the wells have not been purged and sampled since March 2004, it may be necessary to redevelop them. If,during the initial purging of the wells,high turbidity is present ' and does not decrease during purging,the wells will be redeveloped.Development will consist of purging and bailing approximately 10 casing volumes of water from the well. A minimum of 48 hours following development, the wells will be purged and sampled. ES.2 The Stockton AASF is located at 2000 Stimson Road, Stockton, California, 95206 (Figure ES-1).It includes a former refueling area with dispenser island,three 5,000-gallon USTs ' (removed); a wash pad(rack); two 20,000-gallon USTs(removed); and an existing 15,000-gallon and 20,000-gallon UST. ES.3 The investigation described in this report includes the area of the two former 20,000-gallon USTs and the three former 5,000-gallon USTs. Information pertaining to and describing a previous investigation in the area of the three former 5,000-gallon USTs and fuel spill area was documented in the AASF work plan (URS Group, Inc. [URS],2007);this work plan describes subsurface conditions and provides the previous investigation sampling results. The AASF previously contained a fuel island for fueling aircraft,with associated dispensers and fuel lines originating from all of the USTs. The source of petroleum impacts investigated at this site are the locations of five USTs (two 20,000- gallon tanks and three 5,000-gallon tanks)and associated piping and fuel dispensers. The two 20,000-gallon USTs were removed and replaced in the mid-1990s. During removal and excavation of the three adjacent 5,000-gallon USTs and associated piping, soil samples were collected. Based on soil sample analyses,releases of diesel fuel and jet fuel had occurred within the vicinity of the three adjacent 5,000-gallon USTs (Versar,Inc.,2004). It is unknown whether soil removed from the excavation was placed back into the UST excavation pit or if the remaining space was filled with ' clean fill material. K:\Wpnxess\00251\CA ARNG\Stockton AASF SI\FINABSI Rpt.doc 1 June 2008