WORK PLANS - Laserfiche WebLink

Home Browse Search

0 • Work Plan for Groundwater Investigation and Groundwater Monitoring Well Installation Field Maintenance Shop #24, Stockton, California exceeded the primary maximum contaminant levels (MCLs) of 1 pg/L for benzene and 300 pg/L for ethylbenzene. The highest concentrations of toluene (up to 6,000 pg/L) and xylenes (up to 4,900 tag/L) were reported in boring B30, and exceeded the primary MCLs of 150 pg/L for toluene and 1,750 tag/L for xylenes (Versar, 2004). The detected concentrations of contaminants described above indicated that groundwater beneath the former diesel and gasoline UST locations was impacted by leaks related to the former diesel and gasoline USTs, and occurred at concentrations greater than potentially applicable standards for groundwater, including primary MCLs. Because contaminant concentrations exceeded primary MCLs in each of the borehole locations, the extent of contamination had not been fully defined by the Versar investigations. 2.3. MAY 2007 SITE INVESTIGATION (URS) The site investigations conducted by URS in May and October 2007 were documented in the December 2007 Site Investigation Report, and included the collection of soil, soil gas, and groundwater samples from five hollow stem auger (HSA) borings at the site (FMS24SB01 through FMS24SB05; Figure 3). Borings were placed in and around the location of the two former USTs and associated piping. Soil, soil gas, and groundwater samples were collected and analyzed for TPH-g and TPH-d by USEPA Method 8015; VOCs, including methyl tert-butyl ether (MTBE)/BTEX/fuel oxygenates, by USEPA Method 826013; and metals by USEPA Method SW6010B (URS, 2007). In addition, soil samples were collected from four hand auger borings to 5 feet bgs, and were submitted for metals analysis. The May 2007 URS investigation showed that soil, soil gas, and groundwater contamination extends in all directions from the former USTs, and that BTEX and TPH concentrations exceeded potentially applicable regulatory clean-up levels. In addition, the lateral and vertical extent of contamination was not defined. Based on these results, a second phase of sampling was conducted by URS in October 2007. 2.4. OCTOBER 2007 SITE INVESTIGATION (URS) The site investigation conducted by URS in October 2007 included the collection of soil and groundwater grab samples from six HSA borings (FMS24SB06 through FMS24SB11) and four soil gas samples from shallow direct -push probes (FMS24DP01 through FMS24DP04) for analysis by an onsite laboratory. Sampling locations for the October 2007 investigation are indicated in Figure 3. The borings were placed further from the former UST locations relative to previous borings to help define the lateral extent of contamination. Soil, soil gas, and groundwater grab samples were analyzed by the on-site lab for BTEX and TPH-g/TPH-d compounds. Direct -push shallow soil gas samples were collected at 8 feet bgs adjacent to four buildings in the vicinity of the USTs, to determine whether a potential health risk exists to workers within the buildings (URS, 2007). Analytical results for all soil samples collected during the October 2007 investigation were reported as non-detectable for all analytes. BTEX constituents, 1,2-dichloroethane (1,2 -DCA), TPH-d, and TPH-g were detected at concentrations exceeding potentially applicable regulatory standards in groundwater grab samples collected from borings SB06 and S1307 (Figure 3) (URS, 2007). Based on results from the two phases of investigation in 2007, URS concluded that the UST source contamination had reached groundwater and the contamination plume had migrated OTIE 0