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Mr Dale Clemons <br /> July 30, 1999 <br /> Page 2 <br /> Recommendations from our geological unit indicate that the geologic logs and Cone <br /> Penetrometer results indicate several sites have extensive clay and silty clay lithology, which will <br /> tend to limit the migration of Volatile Organic Compounds contamination. Air permeability <br /> testing shows that steady state air permeabilities range from 0.950 to 6.134 darcies (SWMU 1, <br /> 1.014 to 2.861 darcies; Area I - Building 237, 0.950 to 3.492 darcies; SWMU 20, 1.756 to 6. 134 <br /> darcies). Although there is documentation to support use of Soil Vapor Extraction (SVE) at <br /> permeabilities less than I darcy, DTSC has concerns on the effectiveness of SVE systems in the <br /> low permeability clay lithology. <br /> The Basewide Record of Decision specifies a groundwater clean-up standard of 2.3 ppb <br /> for trichlorethylene as a Water Quality Objective, but the soil clean-up standard, for soil gas, is <br /> established at the detection limit of 0.5 ppb in leachate. At a minimum, the soil gas numbers could <br /> be 4 to 5 times higher and still meet the groundwater clean-up requirements. This should be kept <br /> in mind when determining if clean-up of soil gas is sufficient. <br /> Overall, the design will meet cleanup objectives, however, it may be excessive at some <br /> sites (SWMU 20/SB204). <br /> The enclosed comments further detail the specific sections in the document which require <br /> additional information and/or modifications to the proposed text. Should you have any questions, <br /> please feel free to call me at (916) 255-3713. <br /> Sincerely, <br /> Hortensia Muniz, P.E <br /> Project Manager <br /> Federal Facilities Unit <br /> Enclosure <br /> cc: See next page. <br /> HM.dm <br /> IIM51 W OA <br />