WORK PLANS - Laserfiche WebLink

Home Browse Search

0 Field Sampling Plan for the Former UST Location at Stockton FMS #24 California Army National Guard location through inhalation, ingestion, or dermal contact is possible; therefore, there is a potential health risk at the site. Potential contaminant releases at the former UST location occurred primarily below grade. Most of the site is not paved; therefore, the transport of near -surface contamination through indoor and outdoor air or surface water runoff is minimal. The transport of contaminants through outdoor air, surface water runoff, and infiltration to the subsurface is possible beyond the former UST location. The presence of groundwater contaminants originating from possible releases from the former USTs may have occurred. 4.0 CONCLUSIONS 4.1 Based on information obtained during the preliminary assessment and previous investigation efforts by Versar, Inc., soil and groundwater beneath the former diesel and gasoline UST locations at FMS #24 have been impacted by releases of contaminants to the environment from the former USTs and/or by the concrete wash pad/holding tank adjacent to and north of the former USTs. Historically, FMS #24 has been in operation since the early 1950s. Historical activities performed at the site have impacted the soils and groundwater beneath the site. 4.2 The lateral extent of soil contamination associated with the former diesel and gasoline UST location has not been determined. One of the known sources of soil contamination at FMS #24 is the area where the former diesel and gasoline USTs were located. The former 5,000 -gallon diesel and 10,000 gallon gasoline USTs were removed in April 1996. Since then, soil and groundwater samples were collected during the excavation and during March 2000. Later, a preliminary assessment and an initial investigation was completed by Versar, Inc., in 2004. These previous soil sampling events determined that the soil beneath the former diesel and gasoline USTs locations contain concen- trations of TPHD, TPHG, and BTEX. The investigation completed by Versar, Inc., determined that the soil beneath the former diesel and gasoline USTs locations contain concentrations of TPHD, TPHG, BTEX, and lead at levels less than Tri -Regional Board Guidelines and residential PRGs; however, one soil sample (boring B31 at 35 feet bgs) contained TPHD (260 mg/kg) and ethylbenzene (18 mg/kg) concentrations greater than Tri -Regional Board Guidelines (100 mg/kg) and residential PRGs (8.9 mg/kg), respectively. As a result, the concentrations of TPHD and ethylbenzene have not been defined to levels less than Tri -Regional Board Guidelines and residential PRGs, respectively, laterally to the east, south, and west of B31. The vertical extent of TPHD (non -detect at 40 feet bgs) and ethylbenzene (0.041 mg/kg at 40 feet bgs) concentrations have been determined by the immediate soil sample at 40 feet bgs (below the 35 foot bgs soil sample at borehole B31) to levels less than Tri -Regional Board Guidelines (100 mg/kg) and residential PRGs (8.9 mg/kg), respectively. 4.3 The lateral and vertical extent of groundwater contamination associated with the former diesel and gasoline UST location has not been determined. One of the known sources of groundwater contamination at FMS 424 is the area where the former diesel and gasoline USTs were located. The previous 2004 groundwater sampling by Versar, Inc., determined that the groundwater beneath the former diesel and gasoline USTs contain concentrations of TPHD, TPHG, and BTEX at levels that exceed Taste and Odor Thresholds and primary MCLS. Concentrations of TPHG, benzene, toluene, and ethylbenzene have not been laterally defined in all directions from boreholes B30 through B32 because all the detected analyte levels in groundwater were greater than each of the specific analyte Taste and Odor Thresholds and primary MCLS. Concentrations of TPHD have not been defined to levels less than Taste and Odor Thresholds laterally to the southeast, east, northeast, north, and northwest of borehole B30; but, have been defined to the west and south by borings B31 and B32 which had no detections of TPHD. Concentrations of xylenes have not been defined to levels less K:\Wprocess\00251\CA ARNG\Stockton FSP\FINAUStockton FSP.doc 18 August 2006