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Channel Street Soil Vapor Study, Condor Project 3849I _ Page 1 of I R Vicki McCartney [EH] From: Don Kresse [dkresse@condorearth.com] Sent: Wednesday, November 29, 2006 12:11 PM To: Vicki McCartney [EH] Cc: Nuel Henderson [EH]; Alex Dewitt; Casey Kipf Subject: Channel Street Soil Vapor Study, Condor Project 38491 Hello Vicky, I discussed the issue of conducting field vapor sampling versus conducting a RBCA type statistical evaluation with our resident risk analyses expert, Alex Dewitt. This site has a lot of soil and groundwater data, with more coming soon, but no soil gas data. If Nuel was wondering about using the soil and groundwater data to estimate subsurface vapor intrusion risk, there are a couple of issues. First, using the existing J&E models that extrapolate soil vapor intrusion risk from soil and/or groundwater concentrations is not as defensible as using actual data (and is becoming less and less accepted in practice). Second, doing a statistical analysis of the data would require using an excessive number of substitutions for left-censored data "NDs" such that the rigorousness of the calculations is not well known and thus less defensible. To summarize: due to inconsistencies between different regulatory agencies, we typically recommend collection of actual soil gas data. Our clients are better served by acquiring actual soil gas data instead of using the extrapolation models that take soil and/or groundwater data and produce "guestimates" of subsurface vapor intrusion risk. These extrapolation models have been recently rejected by regulatory agencies (on numerous occasions) in favor of actual soil gas analytical data because the models provide unreliable and usually inflated estimates of soil gas risk. Using this approach, while less expensive in the short term, exposes all of us (Client, Condor, and regulators)to liability when it comes to making risk management or remediation decisions. Because this area of our environmental practice is changing rapidly(away from extrapolation and toward using actual data), it is recommended that we use the best available methods for helping our clients appropriately evaluate their properties. With this said, if Nuel or you feel more comfortable using statistical evaluation instead of the recommended approach, please let me know and we can potentially prepare a revised work pian. On a practical level, I know that Nuel is very knowledgeable about what to expect from Jim Barton at the water board. One suggestion may be for us to get the soil and groundwater investigation report in your hands first, so you can evaluate the soil and groundwater data, and that may help all of us to decide on the most reasonable course of action for the soil vapor issue. For now, as described, we recommend the soil vapor sample collection approach. Regards, Don Kresse **CONFIDENTIALITY NOTICE: The information contained in this e-mail message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure, or distribution is prohibited. If you are not the intended recipient:, and have received this communication in error, please contact the sender by reply e- mail and destroy all copies of the original message. Thank you. 11/30/2006