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f 1604 B Street July 1997 Page 2 In fact, the Geocon recommendation describing the proposed CAP indicated that soil vapor extraction was the most feasible and cost effective interim corrective action option. The CAP stated that vapor extraction utilizing the two vapor extraction wells, VW1 and VW2, would be sufficient to remediate the contaminated soil, based on a 1.5 hour test. These two wells are located at the lateral boundaries of the soil contamination plume; therefore, when extracting from these two wells, the zone of contamination effectively becomes the "stagnant" zone where air flow would be small compared to the flow moving towards the wells from outside of the contaminated zone. The CAP stated that the effective radius of influence (ROI) was 35 feet given an extraction flow rate of 50 standard cubic feet per minute. Please note that ROI does not necessarily infer an effective remedial zone of capture, since cleanup is directly dependent on the air flux through contaminated soil which in turn is a function of pressure gradient and soil permeability. PHS/EHD has discussed with Caltrans staff the problems inherent with using vapor extraction wells with such long screened intervals. The problems are due primarily to the preferential vapor pathways through more permeable soil layers. An aspect of the cost effectiveness evaluation of a proposed remedial alternative is to estimate the amount of time it will take to remediate the site. An industry standard which has been used is to calculate the amount of time it would take to remediate the 5,000 pore volumes of the soil requiring remediation at the site. The rational for the 12 to 18 month period of time required to remediate the site was not presented. Using a standard ROI based on achieved pressure measurements, clean-up times typically will be overestimated in high flow zones (high permeability) and underestimated in low flow zones (low permeability). Vapor extraction wells should be screened to maximize flow through the contaminated soil which the wells are installed to remediate. If the extraction rate of 5 pounds per day is used as the cost effective cut off for system operation, an inefficiently designed system may achieve an extraction rate reduction without the concomitant reduction of hydrocarbon concentrations throughout the area requiring remediation. Soil borings, for interim effectiveness evaluation would be required to demonstrate that the design is effectively remediating the contamination as proposed. As PHS/EHD indicated in correspondence dated July 3, 1996, additional monitoring data would be necessary prior to PHS/EHD's approval of the installation of the two additional monitoring wells which were proposed downgradient and crossgradient of MW1 1. The most recent groundwater information available would suggest that their installation may be warranted. If the May sampling event's results continue to evidence similar concentrations in MW6 and MW1 1, please submit a drilling permit application and $89 fee. Please prepare an addendum to address PHS/EHD's comments regarding the above referenced reports and to fulfill the requirements of the California Underground Storage Tank Regulations and Health and Safety Code. If you have any questions, please do not hesitate to contact me at (209) 468-0337. Donna Heran, REHS, Director Environmental Health Division 14104� Mary Meays, Senic REHS Margaretagorio, RENS Site Mitigation Unit Supervisor cc: Elizabeth Thayer, CVRWQCB cc: Richard Walls, Geocon