Laserfiche WebLink
3.2.6 Soil Gas Sampling Strategy <br /> A total of 22 soil gas sample probes were installed at the Site during this RI/SSI. Soil gas sample <br /> locations were selected to generate the data necessary to meet the needs of a baseline HRA for <br /> the Site and also to provide the appropriate amount of information necessary for use in <br /> determination of potential impacts in the subsurface from VOCs. Additionally, this information <br /> will be used in the HRA for the Site. <br /> Soil gas borings were logged for lithology and were visually inspected for the potential presences <br /> of impacted material. Of the 22 soil gas sampling probes installed at the Site, 15 soil gas <br /> locations were placed in a 100 foot by 100 foot grid to identify potential VOC impacts in soil <br /> gas. The remaining seven locations were biased locations based on results of samples collected <br /> during the initial phase of investigation for the RI. <br /> At one RI location (A3-SG1), the soil gas location corresponds with the location of the former <br /> gasoline UST. Based on results from A3-SGI, an additional five soil gas locations (A3-SG2 <br /> through A3-SG6) were placed and sampled as part of the SSI to delineate identified VOC <br /> impacts and to evaluate the potential for vapor intrusion. The data from these additional <br /> locations serves as the delineator of potential impacts remaining after the removal of the gasoline <br /> UST (i.e.,the source). <br /> Based on samples collected during the RI from soil gas point Al-SG1, tetrachloroethene (PCE) <br /> concentrations exceeded the commercial California Human Health Screening Level (CHHSLs) <br /> in shallow soil gas of 603 micrograms per cubic meter (ug/m). As such, two additional soil gas <br /> points (A1-SG3 and Al-SG2) were installed approximately 10 feet upgradient and downgradient <br /> (relative to groundwater flow direction), respectively, to laterally delineate potential PCE <br /> impacts. The source of the PCE is not known. Soil gas purge and sample logs for these samples <br /> are provided in Appendix I2. <br /> 3.2.7 Groundwater Sampling Strategy <br /> Groundwater samples were collected from four existing monitoring wells at the Site that were <br /> installed during the previous investigations and the additional well (TW-5) installed during this <br /> RI/SSI. Groundwater samples were analyzed for one or more of the following constituent <br /> groups: PAHs, TPH, Title 22 Metals and VOCs. The information generated from the sampling <br /> of the wells (TW-1 through TW-5) and a grab groundwater sample (E3-2GW) collected during <br /> the SSI provides preliminary evidence of the potential impacts, or lack thereof, in groundwater <br /> associated with former MGP operations and/or residual impacts from the gasoline UST removed <br /> from the Site. The locations of the wells in relation to groundwater gradient are provided in <br /> Figure 4. <br /> 3.3 DEVIATIONS FROM THE WORKPLANS <br /> During the implementation of the RI/SSI, there were minor deviations from the DTSC approved <br /> workplan (Parsons, 2008) and/or the Final Technical Memorandum (Parsons, 2009). These <br /> deviations are as follows: <br /> • Due to insufficient recovery, Borings C1-1 and D1-1 could not be sampled at five feet <br /> bgs as outlined in the April 2008 Workplan; however, samples were collected from each <br /> of these borings at a depth of 6 feet bgs; <br /> PARSONS 29 FINAL RI REPORT-FORMER TRACY MGP <br /> APRIL 2010 <br />