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Harding Lawson Associates <br /> animal life. A long-term monitoring program of some type may be <br /> required. <br /> This alternative has been demonstrated to be a most <br /> fr <br /> } � practical and desirable alternative in situations where, because <br /> of physical and/or chemical characteristics of the site, contami- <br /> nation will have minimal impact on humans or the environment. <br /> Such characteristics have been encountered at this site and are <br /> wrot <br /> summarized below. <br /> The affected ground water is in a very shallow, low- <br /> yielding formation that is apparently not used for <br /> CCom; drinking, industrial purposes, or irrigation. <br /> - The results of our investigation indicate that <br /> contaminated soils are at a depth of approximately five <br /> f feet and are isolated from the around surface. The <br /> danger they pose to human health is therefore limited. <br /> y We have identified only one main source of hydrocarbons <br /> on the site, the abandoned 10,000-gallon unleaded fuel <br /> tank. . We believe that the mechanism by which hydro- <br /> PA. carbons could have been transported in soils is through <br /> :. ground-water movement. The area experiences seasonal <br /> 7 fluctuations of the potentiometric surface. Thus, as <br /> ground water slowly transported gasoline hydrocarbons <br /> I }. downgradient from the source and the ground-water level i <br /> fluctuated, residual amounts of hydrocarbons have been ' <br /> periodically deposited in soils. The only mechanism <br /> � i for continued movement of hydrocarbons would be the downgradient .flow of the ground water and fluctuations <br /> ;r 3" in the potentiometric surface. Because a ground-water. f remedial system is planned at this site, the hydro- <br /> carbons will eventually be carried to the system and <br /> !f removed. <br /> ... _ _.-..-......... _....._ -..._. - <br /> -- The area where hydrocarbon-bearing soils have been <br /> identified is covered predominantly with asphalt or <br /> _ ^5� concrete. Although these materials are not imper- <br /> meable, they greatly. reduce vertical infiltration of <br /> any surface flow at the site. The chances of vertical . <br /> migration of hydrocarbons from surface soils to ground <br /> water are very small. <br /> The stiff, moist clays in which the hydrocarbons are <br /> located help restrict their migration. <br /> Y ; <br /> k �t <br /> 20 . <br /> F <br />