WORK PLANS - Laserfiche WebLink

Home Browse Search

WORK PLAN FOR ADDITIONAL SOIL VAPOR SURVEY Proposed Assessment Activities May 30, 2014 approximately 200 ml).This purge rate is recommended to limit stripping, prevent ambient air from diluting the samples, and to reduce the variability of purging rates. At each soil gas sample location target depth, the appropriate volume of "stagnant air", based on the results of the purge volume test, will be purged. Potential intrusion of atmospheric air into the sampling train can dilute the samples and compromise integrity. Furthermore, contaminants in ambient air can also enter the sampling system and may be interpreted as a false detect. Accordingly, leak tests of the sampling train will be conducted using a shroud apparatus with helium as a tracer gas.The laboratory will be notified of the selected tracer gas prior to sampling. The purged vapor and the concentration of helium in the shroud is measured using a portable helium detector (Mark Products model 9821 or equivalent). If helium is detected in the purge sample at a concentration equal to or greater than 5% of the shroud concentration, the soil vapor monitoring point and all connections will be checked, tightened, and resampled. Since helium is routinely used by laboratories as a carrier gas, helium analysis will be specifically requested on the chain-of-custody form. Shallow soils at the site have been classified as clay and silt with lesser amounts of silty sand, clayey sand, sand, and gravel. As such there is a potential for low permeability conditions to be present. If a probe cannot sustain a flow rate of approximately less than 200 mL/min with an applied vacuum of less than 100 inches water column (in. wc.) for 2 to 3 minutes, low flow conditions are considered to exist. An alternate procedure for sample collection under low-flow conditions may include collection of a smaller aliquot of soil gas followed by a period of time for the vacuum to dissipate (dependent of gas permeability of soils around sand pack). The process is repeated until approximately 800 mL of soil gas is collected in the Summa canister. In accordance with the DTSC and CETC guidance documents, at least one duplicate per day will be collected and analyzed for Quality Assurance/Quality Control (QA/QC) purposes. Duplicate samples will be collected in separate containers simultaneously using a duplicate sampling T. Soil gas samples will be collected using 1 liter stainless steel, passivated, batch-certified (10% certified) Summa canisters obtained from the project laboratory with the proper vacuum of approximately 30 inches of mercury (in. Hg.) or if an onsite mobile lab is used for analysis using a 60 cubic centimeter, gas-tight syringe. With the leak test enclosure still in place, collection of soil vapor samples from a particular probe will begin within 10 minutes of purging. After the SummaTM canister valve is opened and the canister begins to fill, the pressure gauge on the flow controller will be observed to verify that the vacuum in the canister is decreasing over time. If the flow controller is working correctly, it will take approximately 5 minutes for the vacuum to decrease to 5 in Hg; however, the actual sampling duration may be slightly more or less than 5 minutes. The SummaTM canister valve will be closed and sampling will cease when a vacuum of 5 in Hg is obtained. 3.8