SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Caltrans 1603 B Street Stockton CA 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 only one vapor extraction well,VW1, would be sufficient to remediate the approximately 5,000 cubic yards of contaminated soil, based on a minimum radius of influence (ROI) of 0.25 to 0.5 inches of water vacuum. Radius of influence 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. As PHSIEHD indicated in correspondence dated August 29, 1995, the effectiveness of the vapor extraction well, VW1, is questionable due to its extremely long screened interval, from 12 to 98 feet bgs and the site's stratigraphy. The problems inherent with using a vapor extraction well with such a long screened interval are due primarily to the preferential vapor pathways through more permeable soil layers. Another problem with this particular well is the possible vertical leakage related to the sandy gravel used to backfill the former excavation. 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. Sol borings, for interim effectiveness evaluation would be required to demonstrate that the design is effectively remediating the contamination as proposed. Please prepare an addendum to address PHSIEHD's comments regarding the above referenced reports and to fulfill the requirements of the California Underground Storage Tank Regulations. If you have any questions, please do not hesitate to contact me at(209)468-0337. Donna Heran, REHS, Director �Environmental Health Division Mary Meays, Senior REHS Margaret Lagorio, REHS Site Mitigation Unit Supervisor MM/1603BSTREET cc: Elizabeth Thayer, CVRWQCB ee: Richard Walls, Geocon