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1 77 jl Field Sampling Plan for the Former UST Location at Stockton FMS #24 California Armv National Guard above indicate that groundwater beneath the previous diesel and gasoline UST locations has been impacted by leaks from the former diesel and gasoline USTs and/or from the concrete wash pad/holding tank north of and adjacent to the former UST locations, and TPHG, TPHD, and BTEX contaminant concentrations are present within groundwater greater than Taste and Odor Thresholds and primary MCLS, respectively. Concentrations of fuel oxygenates and lead were not detected in any of the groundwater samples collected at borings B30 through B32 at the former diesel and gasoline UST locations. Figure 3-5 shows the detected concentrations for each analyte in groundwater beneath the former diesel and gasoline UST locations. Concentrations of TPHG, benzene, toluene, and ethylbenzene have not been defined laterally from borings B30 to B32 in all directions; in addition, all concentrations for these contaminants exceed their water quality goals for all boring locations. Concentrations of TPHD have been defined laterally to the south and southwest by borings B31 and B32 where TPHD was not detected; however, TPHD remains undefined laterally in all other directions from the former UST location because concentrations of TPHD within boring B30 were detected greater than Taste and Odor Thresholds (100 µg/L). Concentrations of xylenes have been defined only laterally to the south by boring B31 where xylenes were detected less then primary MCLS; however, xylenes remain undefined laterally in all other directions from the former UST location because concentrations of xylenes within borings B30 and B32 were detected greater than primary MCLS (1,750 µg/L). No groundwater samples have been collected beneath first encountered groundwater; therefore, the vertical extent of groundwater contaminant concentrations for TPHG, TPHD, and BTEX has not been determined. 3.2.4 North and adjacent to the former diesel and gasoline UST locations is a wash pad and holding tank for the wash pad. No samples have been collected within the vicinity of the wash pad/holding tank. Soil beneath the former diesel and gasoline UST locations contains low levels to no detections of TPHD, TPHG, and BTEX and only one sample (B31 at 35 feet bgs) contained a concentration of TPHD and ethylbenzene greater than Tri -Regional Board Guidelines and residential PRGs, respecticely. The sample at the next depth below the 35 feet bgs sample in boring B31 at 40 feet bgs did not detect TPHD (non -detect), and ethylbenzene was 0.041 mg/kg (less than its residential PRG of 8.9 mg/kg). As a result, data indicate that groundwater contaminant concentrations, which are several orders of magnitude greater then their respective water quality goals, may not entirely originated from the former diesel and gasoline UST locations; but, may have also originated from the adjacent wash pad and holding tank. 3.3 Contaminant Migration Potential 3.3.1 Quantitative information has not been obtained regarding the physical properties of surface and vadose zone soils in the former UST area; therefore, the rate of surface water infiltration and the rate of migration of possible contaminants cannot be evaluated. Surface water runoff at the site flows toward the west. The potential for surface water infiltration exists because approximately 90% of the site is unpaved. Therefore, any contaminants present in the subsurface soils could continue to migrate to groundwater. 3.3.2 Potential migration pathways at FMS #24 include indoor/outdoor air, surface water, soil, soil gas, and groundwater. The primary release mechanism is the former UST location; thus, leaching of contaminants downward within the subsurface to groundwater is the potential pathway. 3.3.3 Based on the limited information available and site conditions at FMS #24, the potential for the exposure of receptors (e.g., workers at the FMS) to any contaminants from the former UST I K:\Wprocess\00251\CA ARNG\Stockton FSP\FINAUStockton FSP.doc 17 August 2006