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Mr. Jim Barton RWQCB March 29, 2013 Page 11 sample. Drive the probe an additional 2 feet, and reattempt sample collection. iii) If the vacuum releases, pack the space between the ground surface and the exterior of the drive casing and the annular space between drive casing and sample tubing with hydrated bentonite to prevent ambient air intrusion. (1) If the needle moves back to zero very quickly, it indicates that: (a). The soil at the sampling depth is very permeable, or (b) There is a leak in the sampling train that was not found in the leak checks. If this is the case, the sample should be impacted by tracer gas. Purging Once the probe is at the proper sample depth,the downhole leak test has been passed,the sample probe has been pulled up to expose the sample screen, the vacuum has been released in a timely manner, and the annular spaces packed with hydrated bentonite (Steps l through 14, above),purging will begin, as follows: 1) Calculate the system,purge volumes, and purge time: a) One system volume is the volume of air within the sample tubing. Three system volumes will be removed. i) 1 system volume (cubic feet [ft3]) = R X radius (feet) squared X length of tubing(feet)_(ar2L) b) The system volume calculated above is in cubic feet (ft), which is then converted to liters (L) using 28.3 liters per ft3. The resulting system volume is multiplied by 3 to calculate the purge volume: i) Purge volume in L=3 X system volume.(ft3)X 28.3 C) The purge time is calculated by dividing the purge volume in liters by 2 (the purge rate of the vacuum pump is 2 liters per minute) and multiplying' by 60 seconds (the number seconds per minute). i) . Purge time (seconds)=purge volume/2 X 60 2) Purge the system for the duration(number of seconds) calculated. a) While purging, take PID measurements of the soil vapor from the vacuum pump exhaust and record the highest value. 3) After purging the appropriate volume, quickly close the valve at the base of the Sample T (nearest the pump), start a stopwatch, and allow the sample line pressure to return to zero vacuum (zero in Hg).