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Pre-Design Investigation Workplan Former Manufactured Gas Plant—Lodi, California estimated void space of the dry bentonite in the annular space above the sand pack. Purge volume calculations will be recorded on soil gas sampling forms for each soil gas sample. 4.8.4 Soil Gas Purging and Sample Collection Each soil gas probe will be purged of three purge volumes (per Section 4.7.3) prior to sample collection at a consistent purge rate ranging between 100 and 200 mL/min using a pump. Vacuums less than 100" W.C. will be maintained during purging and sampling to minimize partitioning of vapors from pore water to soil gas, to prevent ambient air from diluting the soil gas samples, and to reduce the variability of purging rates. A flow meter and a vacuum gauge will be located in the sample train between the soil gas probe and the purging pump, but not in-line with the sample container. The sample container will be connected to the soil gas probe and purging line using a "tee" fitting or three-way manifold connection. Soil gas samples will be collected using 1-liter SummaTM canisters. The SummaTM canisters will be appropriately decontaminated and batch-certified by the off-site stationary analytical laboratory prior to use. An evacuated SummaTM canister will be attached to the probe tubing via a laboratory- provided manifold and other fittings assembled prior to the leak check tests. After the soil gas probe has been appropriately purged and the leak check tests completed, the soil gas sample will be collected. A flow regulator calibrated by the analytical laboratory to flow at a rate between 100 to 200 mL/min will be used to collect the soil gas sample in the SummaTM canister. A vacuum gauge will be also used to ensure a vacuum of less than 100"W.C. is maintained during sampling. Low-flow conditions may be encountered where soil gas permeability is too low to allow sustainable purging at appreciable flow rates (minimum flow of 100 mL/min) without applying excessive vacuum (greater than 100" W.C.). If low flow conditions exist, then alternative low flow sampling protocols described in Appendix D of the DTSC ASGI guidance document (DTSC, 2010) will be followed. These alternative sampling protocols allow for a minimum of 1 purge volume equal to the dead volume of the filter pack and probe tubing to be removed from the soil gas probe prior to conducting the leak test and collecting the soil gas sample. Further, purging and sampling where low-flow conditions exist due to fine-grained formations are conducted by removing an aliquot of soil gas over time to allow for dissipation of down-hole vacuum. Other alternative protocols may also include the replacement of the subject probe using different construction specifications and altering the soil gas sampling methodology. Per DTSC ASGI guidance, field quality control (QC) samples including either duplicate or replicate samples will be collected at a rate of one (1) duplicate/replicate sample per 20 samples or per batch shipment to the laboratory, whichever is more often. Duplicate samples are collected simultaneously and replicate samples are collected sequentially. Duplicate samples are collected at the same time as the primary sample using a "tee" fitting and two separate SummaTM canisters, each with pre-set flow control regulators. Replicate samples are collected immediately after the primary sample using a second SummaTM canister. For the proposed sampling plan, a minimum of one soil gas sample duplicate/replicate will be collected. In addition to duplicate/replicate samples, field QC samples will also include equipment blanks (called a soil vapor well material blank in the ASGI guidance) and field blanks (called a method blank in the ASGI guidance). The equipment blank will TERRA PACIFIC GROUP Page 17 April 4, 2012