FIELD DOCUMENTS - Laserfiche WebLink

Home Browse Search

ADVISORY—ACTIVE SOIL GAS INVESTIGATIONS volume testing may be warranted to determine the air permeability of the subsurface (see Appendix D for more information). 4.2.4 Additional Purge Volume Tests Additional purge volume tests may be warranted by site-specific situations. Under the following conditions, additional purge volume tests should be conducted: • A previously unknown lithology is encountered; • Variable flow conditions are unexpectedly encountered; or • If the default purge volume of three is used and a VOC of concern not previously detected is subsequently detected. If a new purge volume is selected, then 10 percent of the previously completed soil gas wells should be re-sampled using the new purge volume. Re-sampling may be necessary for all previously sampled soil gas wells depending on results of the re- sample. The soil gas investigation may then be continued with the revised purge volume in the remaining areas. 4.3 PURGE/SAMPLE FLOW RATE AND APPLIED VACUUM Flow rates between 100 to 200 milliliters per minute (mL/min) and vacuums less than 100 inches of water should be maintained during purging and sampling to minimize stripping (partitioning of vapors from pore water to soil gas), to prevent ambient air from diluting the soil gas samples, and to reduce variability between contractors. Maintaining these flow rates and vacuums will increase the likelihood that representative samples will be collected. A flow rate greater than 200 mL/min may be used when purging times are excessive, such as for deep wells with larger-diameter tubing. However, a vacuum of 100 inches of water or less must be maintained during sampling whenever a higher flow rate is used. The pressure gauge used to measure vacuum should be calibrated and in good working order. A vacuum gauge should be used between the soil gas sample tubing and the soil gas purging device to verify that 100 inches of water or less is maintained during sampling. Gas-tight syringes may also be used to qualitatively determine if a high vacuum soil condition is present. If a high vacuum condition is present due to low permeability soil, the sampling technician can feel the suction while the plunger on the syringe is being withdrawn. If low permeability conditions are encountered where 100 inches of water is exceeded, the well can be sampled using the techniques in Appendix D (Soil Gas Sampling in Low Permeability Soil). 4.3.1 Vacuum Pump When a vacuum pump is used, collect samples on the intake side to prevent potential contamination from the internal parts of the pump. To collect the sample in a polymer gas sampling bag, a lung box'is required. Record the vacuum readings and ' A lung box is a small airtight chamber into which the polymer gas sampling bag is placed. The connective tubing to the bag protrudes out a hole in the chamber. The sealed chamber is evacuated by a pump, causing the bag to April 2012 20