ARCHIVED REPORTS_XR0013116 - Laserfiche WebLink

Home Browse Search

3.2.3.2 Soil Gas Sample Collection The subsurface gases were allowed to re-equilibrate for at least 72 hours after installation prior to sampling. As recommended by the Cal/EPA Advisory, the first soil gas probe sampled during the RI (A3-SG1) was purged for one, three, and seven volumes. A soil gas probe volume was calculated as the internal volume of the tubing and soil gas sampling implant along with the annulus of the auger boring around the soil gas sample point. The initial soil gas samples were collected in a one liter Tedlar bag after each discrete volume was purged and screened using a MiniRae 2000 PID, calibrated daily using 100 parts per million by volume (ppmv) isobutylene gas. The purge volume that correlated with the highest PID value was the volume purged at each sample location thereafter. One, three, and seven purge volumes from the A3-SG1 location yielded 400 ppmv, 467 ppmv, and 510 ppmv, respectively. Based on these concentrations, each soil gas probe sampled during the RI was purged for seven volumes immediately prior to sampling, which equates to approximately 54 milliliters for each probe. Purging was accomplished with a GilAir-58 air sampling pump equipped with a low-flow module calibrated to between 100 and 200 milliliters per minute (mL/min). Soil gas samples were collected into pre-cleaned, six-liter, flow-controlled, evacuated SUMMA® canisters. Canisters were connected to the 3-way valve located at the top of the soil gas probe with Teflon® tubing and threaded Swagelock unions. Dedicated flow-controllers for each sample were calibrated by the laboratory and placed in-line directly before each SUMMA® canister. Initial SUMMA® canister vacuum pressures prior to sampling were less than 28 inches of mercury. SUMMA® canisters were allowed to collect soil gas until the vacuum pressure measured within the canister was at minus three inches of mercury or less. After soil gas collection, the SUMMA® canister valve was shut, a brass cap was placed on the canister port, and the canister was packaged for delivery to Air Toxics, LTD., a State-certified laboratory. Soil gas samples were analyzed for VOCs by USEPA Method TO-15 with naphthalene and isobutane for the RI (Phase 1) sampling activities. For the SSI (Phase 2), soil gas samples were analyzed for VOC by USEPA Method TO-15 with naphthalene and helium. A total of three duplicate samples were collected immediately after the corresponding primary samples were collected. Each soil gas sample was named using an alphanumeric grid along with the sample location and the sample depth as shown in Figures 9 and 12. Soil gas sample locations are signified with "SG". For example, sample number A3-SG1 designates a soil gas sample collected from alphanumeric grid A3 from the first soil gas sample location(SG I). Leak Detection and Quality Assurance/Quality Control(QA/QC)Procedures Leakage during soil gas sampling could dilute samples with ambient air and produce results that underestimate actual site concentrations, or contaminate the sample with external contaminants. As such, each soil gas probe was leak checked in accordance with the test procedures described in Section 2.4 of the Cal/EPA Advisory — Active Soil Gas Investigations (DTSC, 2003). As specified in the guidance, leak tests were conducted at each soil gas point using a tracer compound. It should be noted that the DTSC was in the process of modifying the Advisory guidance between RI (Phase 1) and SSI (Phase 2) sampling at the Site. DTSC requested that PARSONS 22 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010