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L <br /> %I, shallow aquifer extends to 25 feet b s and no <br /> g , petroleum hydrocarbons have been detected in <br /> soil samples collected from the bottom clay layer. The screened interval(s) of the well is not <br /> known. If the well is screened in the shallow aquifer, at the worst case, dissolved petroleum <br /> hydrocarbons would reach the well location in approximately 20 years (see Appendix V). <br /> L. <br /> An on-site search for potential receptors was conducted on December 16, 1996 by Smith <br /> Technology personnel. No open ponds or streams were located within 100 feet of the plume. <br /> L <br /> 7_2 Potential Pathways <br /> The site plan showing the facility and the utility lines under the site, as provided by the tenant, <br /> were examined for preferential pathways for the migration of contaminants. The building on- <br /> site is constructed on a thick concrete pad, according to the property owner, and is located <br /> over a portion of the petroleum hydrocarbon plume (Figures 4 and 6). This pad would prevent <br /> �.. any petroleum hydrocarbon vapors apparently located in soil under the building to migrate into <br /> the building. <br /> As shown on Figure 9, numerous utility lines cross the groundwater plume area, especially in <br /> the vicinity of SB2, MW2, MW3 and the former UST area. Petroleum hydrocarbons were <br /> detected in the vadose zone in the S132, MW2 and MW3 borings. The utility lines (electrical <br /> and water) are not considered to be preferential pathways for petroleum hydrocarbons due to <br /> — their construction. Electrical and water utility lines are usually installed within the top five <br /> feet of soil. The groundwater level in the soil is at least seven feet bgs, and thus, groundwater <br /> L, would not intercept the utility trenches and flow down the backfill as a.preferential pathway. <br /> In addition, the open areas of the site are overlain by asphalt which would be expected to <br /> impede vapor contamination pathways. <br /> With regard to vapors intercepting the electrical and water lines in the vicinity of SB2, MW2, <br /> `' MW3 and the former UST area, the vertical native state effective air conductivity of the soil at <br /> 2 to 4 feet (silty clay) is very low, only 3.27 x 10-5 cm/s. Thus, the low conductivity to air of <br /> '- the native soil would impede vertical migration of petroleum hydrocarbons in the vadose zone. <br /> L <br /> a <br /> L <br /> i <br /> w:1951221Plumerpt 16 <br /> • <br /> • SMTN <br />