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April 29, 198$ <br /> Harding Lawson Associates <br /> PW 88-4924.03 <br /> Mr. Hal Miller <br /> The Southland Corporation <br /> Page 5 <br /> soil or ground water can be detected by measuring their con- <br /> centrations in soil-gas. It is generally not possible to <br /> estimate quantities of organic chemicals in soil or ground <br /> water from soil-gas data alone, because of complex interac- <br /> tions between organic compounds and underground soil or <br /> water. These interactions are a function of soil particle <br /> 4 size and mineralogy, natural soil organic content, soil <br /> moisture, temperature, lithology, and heterogeneity. How- <br /> ever, measurement of soil gas can be an excellent qualita- <br /> tive indicator or screening tool for the presence of <br /> volatile petroleum hydrocarbons in the subsurface. <br /> We have found that a soil-gas survey can .help minimize the <br /> number of borings, wells, and chemical analyses required to <br /> establish the "zero line.' Nevertheless, a soil-gas survey <br /> requires confirmation by chemical analyses of soil and <br /> ground-water samples, as <br /> presented in Task 3. <br /> We propose to initially Perform 20 to 25 probes according to <br /> the following four transects: <br /> Transect 1: North-south, parallel to Buthmann Street <br /> and inside the Southland property line <br /> ' Transect 2: East-west, behind the convenience store <br /> t <br /> • Transect 3: North-south, on the east side of Buthmann <br /> A Street within the City of Tracy right-of-way <br /> • Transect 4: East-west, from the tank area toward <br /> Buthmann Street. ' <br /> We` have assumed that Southland controls a piece of property <br /> Y F <br /> ora° the -:Zcan side of the convenience store so that we an <br /> complete Transect 2 and remain on Southland property. <br /> The soil-gas-probes will be spaced"on approximately- - <br /> - centers. Beforecommencinq the field ely 25-foot <br /> attempt to ,locate all utilities in thepareaaof the of <br /> transects, and obtain permission from the Cit y of .TProposed <br /> ll <br /> Tracy to <br /> f perform all off-site probes . All probe holes will:be back- <br /> filled with sand, with An asphalt plug placed. at the ground <br /> e.1 ourlaae. <br /> The soil-gas survey is performed by a mobile van haat pushes <br /> f galvanized steel probes into;the soil, extracts air from the <br /> probe, samples the soil gas, and analyses it by gas chro- <br /> matography its the mobile van. All soil-gas probes will be <br /> r . <br /> � f - <br />