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Page 2 of 3 r From: Michael Carroll [mailto:mcarroll@sjgov.org] Sent: Tuesday, February 07, 2012 11:05 AM To: Robert McClellon [EH] Cc: Natalia Subbotnikova [EH] Subject: Gas Probe Modifications - answers to questions Good morning Robert, Thanks for forwarding these questions. I have inserted my responses,which I hope will provide assurance that inserting the tubes within the existing pipes does not conflict with the objectives of soil gas monitoring, and may enhance our ability to do so. Best regards, Mike Carroll W.Michael Carroll,PE Senior Civil Engineer,Public Works/Solid Waste 1810 E Hazelton Way Stockton,CA 95205 Phone(Direct):209-468-8504 Phone(Office Main):209-468-3066 Fax:209-468-3078 From: Robert McClellon [EH] [mai Ito:RMcClellon@sjcehd.com] Sent: Tuesday, February 07, 2012 8:09 AM To: Michael Carroll Cc: Natalia Subbotnikova [EH] Subject: Gas Probe Modifications Mike here are comments from Calrecycle staff: Hello Natalia: Thank you for forwarding the proposed revision to County landfills gas compliance wells. We do understand the Operator would like to reduce their purge time by inserting polyethylene tubes inside the existing PVC pipes. Perhaps this is not important, but most of our probes are W High density polyethylene (HDPE),although we do have some PVC. Based on Calrecycle's landfill gas monitoring procedures, we have recommended against purging the gas probes and have recommended readings once steady state has been achieved. Purging might actually defeat the purpose of true gas monitoring since by purging, gas that might have had the potential to migrate and accumulate within the probes will be purged. This method of monitoring is exactly what we planned. Perhaps "purged"was the wrong term; it is our intent to clear the probe of gas pulled during the previous measurement so that we measure the current concentrations. We don't intend to pull more gas from the soil than that, but gas from the previous measurement could be trapped in the probe, and should be removed before making a new measurement. The tubes will greatly reduce the volume needed to clear the probe for the new measurement. In fact, reducing the amount of gas needed to clear the probe (by using much smaller tubes to carry the sampled gas)will reduce the potential for the problem brought up by CalRecycle staff—that we do not want to remove so much gas from the soil that we introduce air from the surface,thus diluting the soil gas. 2/22/2012