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Stewart Black, P.G. 2- July 19, 2017 <br />In 2004, four groundwater monitoring wells were installed at the site. Each well was constructed <br />with a screen interval of about 45 to 65 feet. Groundwater is about 50 feet below ground surface. <br />Groundwater was collected from the monitoring wells in 2004 and 2005. None of the groundwater <br />samples collected contained detections of PCE or any fuel constituents. <br />In 2014, four soil vapor extraction (SVE) wells were installed and connected to a 100 cubic feet per <br />minute SVE blower. The SVE system was operated during mid- May through August 2015. The <br />SVE influent was monitored using a photoionization detector (RID). Influent PID concentrations <br />ranged up to 1.2 parts per million by volume (ppmv) of volatile organic compounds near the <br />beginning of the SVE operation and decreased to and remained at 0.0 ppmv for the last 5 weeks of <br />operation. Laboratory samples were collected from the SVE influent a few days after starting SVE <br />operation and a week later from the SVE well located at the area of the maximum historical PCE <br />concentration. Neither sample contained detectable concentrations of PCE or any other volatile <br />organic compounds. <br />To confirm the effectiveness of the SVE remediation, two independent soil vapor sampling points <br />(SV-1 and SV-2) were installed in late 2016 and sampled in early 2017. The soil vapor probe <br />borings were each drilled using a hand auger to a depth of 5.5 feet. Isopropyl alcohol was used for <br />leak checking during sampling. Soil vapor samples were collected using 1-liter Summa canisters <br />for analysis of volatile organic compounds. PCE was found in one of the vapor probes (SV-1) at a <br />concentration of 5.5 micrograms per cubic meter (pg/m3 ). SV-1 also contained 2.8 pg/m3 of <br />toluene and 2.7 pg/m3 of dichlorodifluoromethane (also known as Freon-12). Sample location SV-2 <br />also contained 2.7 pg/m3 of dichlorodifluoromethane as well as 81 pg/m3 of isopropyl alcohol. The <br />81 pg/m3 of isopropyl alcohol in sample SV-2 is not considered significant since the air within the <br />shroud was analyzed and found to contain 840,000 pg/m3 of isopropyl alcohol. If the detection of <br />isopropyl alcohol in SV-2 indicates a leak of shroud air into the sample, then the sample contained <br />less than 0.01 percent of air from the shroud. <br />Extent defined: Yes <br />Estimated Residual Mass: Low PCE soil gas concentrations detected in 1997 appeared to be no <br />longer present following completion of a 3-month SVE system operation in 2015. <br />Threat to Groundwater: No groundwater pollution was detected in site groundwater samples. <br />Low level residual PCE concentrations in soil gas do not threaten groundwater. <br />Threat to Human Health: <br />Vapor Intrusion Hazard: The highest soil gas PCE concentration collected at the site was <br />detected in 1997. Location P-19 contained 10,600 pg/m3 of PCE at a depth of 20 feet. In <br />2017, after SVE remediation had been completed, a 5-foot sample near P-19 contained 5.5 <br />pg/m3 of PCE. The DTSC screening levels for PCE in soil gas for current and future <br />residential settings are 230 pg/m3 and 460 pg/m3. Based on these screening levels, the <br />recent maximum PCE detection of 5.5 pg/m3 represents an incremental lifetime cancer risk <br />significantly less than one in a million for current and future residences at the property. <br />Therefore, residual PCE concentrations do not pose a vapor intrusion hazard.