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B. If a new purge volume is selected after additional step purge tests are conducted, the soil gas investigation should be continued as follows: 1. In areas of the same or similar lithologic conditions: a. Re-sample 20 percent of the previously completed probes. This re-sampling requirement may be reduced or waived in consultation with Agency staff, depending on site conditions. If re-sampling indicates higher detections (e.g., more than 50 percent difference in samples detected at greater than or equal to 10µg/L), all other previous probes should be re-sampled using the new purge volume. b. Continue the soil gas investigation with the newly selected purge volume in the remaining areas. 2. In areas of different lithologic conditions: Continue the soil gas investigation with the newly selected purge volume in the remaining areas. 2.4 Leak Test Leakage during soil gas sampling may dilute samples with ambient air and produce results that underestimate actual site concentrations or contaminate the sample with external contaminants. Leak tests should be conducted to determine whether leakage is present(e.g.,the leak check compound is detected and confirmed in the test sample after its application). 2.4.1 Leak tests should be conducted at every soil gas probe. 2.4.2 Leak Check Compounds: Tracer compounds, such as pentane, isopropanol, isobutene, propane, and butane, may be used as leak check compounds, if a detection limit (DL) of 10 µg/L or less can be achieved. These compounds may be contained in common products such as shaving cream. 2.4.3 A leak check compound should be placed at any location where ambient air could enter the sampling system or where cross contamination may occur, immediately before sampling. Locations of potential ambient air intrusion include: A Sample system connections; B. Surface bentonite seals(e.g., around rods and tubing); or C. Top of the Temporary Soil Gas Probe (see Section 2.2.5.13). 01/28/2003 - 10-