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purge volumes. If a new purge volume is selected in areas of different lithology,then the soil gas investigation may be continued with the revised purge volume in the remaining The purge volume(also referred to as the"dead space volume")can be areas with the same lithology. estimated by summation of the following components:(a)the internal volume of tubing used;(b)the void space of the sand pack around the probe Up;and(c)the 2.6 Purge/Sample Flow Rate and Applied Vacuum dry bentonite in the annular space. Sample containers are not included in the purge volume calculation,except when non-evacuated glass bulbs are used. In Purge/sample flow rates between 100 to 200 mUmin and vacuums less than 100 those instances,the volume occupied by the non-evacuated glass bulbs should inches of water for standard small diameter(1/8 to 1/4 inch)tubing should be be added to the purge volume to account for mixing and dilution of gasses inside maintained to minimize partitioning of vapors from pore water to soil gas(i.e., the glass bulb. The step purge tests of one(1),three(3),and ten(10)purge stripping),prevent ambient air from diluting the soil gas samples,and to reduce volumes should be conducted to determine the purge volume to be applied at all the variability of purging rates from contractor to contractor. Low flow sampling locations. Select the appropriate purge volume based on the highest purge/sample rates and vacuums of less than 100 inches of water increases the concentration of the compound(s)of concern detected during the step purge likelihood that representative samples will be collected.The purge/sample rate volume test. Optimize the purge volume for the compound(s)of greatest may be modified based on conditions encountered in individual vapor wells. concern. Avoid extensive purging for soil gas samples collected at less than five These modified rates should be documented in the soil gas report. A flow rate (5)feet bgs.Conduct the step purge test and purging at the same flow rate and greater than 200 mUmin may be used in certain cases,such.as when larger applied vacuum as soil gas samples. If the pump is battery operated,check the diameter tubing are used with deeper vapor wells that are greater than 40 feet batteries before and during operation to insure a proper charge is maintained. bgs. However,a vacuum of 100 inches of water or less must be maintained during sampling whenever a higher Flow rate is used.Large volume sample A default of three(3)purge volumes should be extracted prior to sampling in the containers,such as the 6L-SUMMA®canisters should be avoided for shallow following cases: samples collected at less than five feet bgs. • If VOCs are not detected in any of the step purge tests Evaluate lithologic conditions adjacent to the vapor well to determine if the • If a SUMMA®canister is used for sampling soil gas purge/sample Flow rate is attainable. A vacuum gauge or similar device should • For shallow soil gas samples(collected at less than five feet bgs) be used between the soil gas sample tubing and the soil gas extraction devices, such as a vacuum pump,or SUMMA®canister. Gas tight syringes may also be Include the purge test data in the report to support the purge volume selection. used to qualitatively determine if a high vacuum soil condition is present.If a high The data set should include the purge volume test as well as the Flow rate, vacuum condition due to tight or low permeability soil is present,the sampling vacuum exerted on the formation,and duration of each purge step. technician will have a feeling of suction while the plunger on the syringe is being withdrawn. Conduct soil gas permeability testing at an optimal vacuum to Additional Purge Volume Tests determine the gas permeability of the soil surrounding the probe tip(McAlary,et al.,2009)(See Appendix D). Additional purge volume tests may be necessary when warranted by site-specific situations. For example if widely variable or different site soil types are encountered during continuous coring conducted prior to the soil gas survey,or if 2.7 Sample Containers the default purge volume of(3)is used and another VOC of concern not detected previously is subsequently detected. Collect samples in gas-tight containers and handle in a manner that will prevent photo degradation of the target analytes. Sample containers should not If a new purge volume is selected in areas of similar lithology after additional step compromise the integrity of the samples. purge volume tests are conducted,then ten percent of the previously-completed vapor wells should be re-sampled using the new purge volume. Re-sampling 1. If a syringe is used,it should be leak-checked before each use by closing the may be necessary for all previously sampled vapor wells depending on site exit valve and attempting to force a®bient air through the needle. The use of conditions. The soil gas investigation may then be continued with the revised gas-tight glass syringes with Teflon seals is preferred. The use of plastic purge volume in the remaining areas. syringes is discouraged because of the interaction between the plastic(or rubber)of syringes and some target analytes. 3/3/2010 -25- 3/3/2010 -26-