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Mr. David a. Rosso <br /> California State University Stanislaus <br /> 1252 N. Stanislaus Street, Stockton <br /> Page 2 of 2 <br /> logs). Sparged air bubbles rising through the clay sand will probably <br /> encounter the sand/fine-grained unit boundary with a likely decrease of <br /> ` vertical permeability, which may then force the air to move laterally-and <br /> cause lateral migration of the contaminants. <br /> • AS will be tested utilizing helium as a tracer to determine the radius of <br /> influence (ROI). EHD has some concern over the interpretation of the <br /> results utilizing helium as a tracer as the helium atom is smaller than the <br /> relatively larger and chunkier 4- to 20-carbon molecules common to <br /> gasoline and helium has little attraction to soil. The relative permeability of <br /> the soil can be anticipated to be much higher for helium than for gasoline- <br /> range hydrocarbons. Therefore, helium may not well simulate the <br /> migration pathways and ROI that sparged hydrocarbons and chlorinated. <br /> solvents may follow in the subsurface. These limitations should be kept in <br /> mind while interpreting the results of the AS test. If there are correction <br /> factors that can be utilized to convert the. helium results into an ROI <br /> appropriate for the sparged contaminants, please provide these to EHD. <br /> Please address these concerns in a work plan addendum by 17 January 2005. <br /> Include provisions to detect lateral' migration of contaminants. The final <br /> remediation plan should include an effective monitoring method to detect lateral <br /> migration of contaminants. <br /> EHD desires to assist you in this matter as much as possible. Questions,.or <br /> comments may be directed to Nuel Henderson by mail (address on letter head), <br /> by e-mail at'nhenderson@sjcehd.com, or by telephone at (209) 468-3436. <br /> Donna Heran, RENS, Director <br /> Environmental Health Divisio <br /> ;�Ze6�-za�� <br /> Nuel C. Henderson, Jr., RG <br /> Unit IV - Site Mitigation <br /> cc: James L.L. Barton — CVRWQCB <br /> Donald F. Kresse — Condor <br />