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Sites Assessment for Hunan - 09/06/97 Rage a c g There appears to be no preferential pathways for migration of hydrocarbons through the subsurface and the soil itself provides a reasonably good barrier to contaminant migration. 5.0 31_iIM SMVE M-CEPUO sS The stormdrain manhole appears to be located in the back of the building. San Joaquin County Public Works stated, during a telephone inquiry, that they do not have a record of the storm drain at the subject site's location. They did state that the sewer runs out to the alley located in the back of the property. Typical stormdrain and sewer lines, however, are located within 1 the top ten (10) feet of the subsurface and are therefore not likely to be exposed to benzene in soil located between 15 and 25 feet bgs. 1 The current intended use for the site is auto repair which is generally performed by workers within the small, one story structure onsite. Benzene in soil will not migrate vertically upwards into the building, thus exposing 1 people, because the high water-air partition coefficient for benzene will dissolve into vadose zone soil moisture which will draw the benzene vertically downwards (Fetter, Contaminant Hydrogeology, 1993, pp. 221 ' 224). Since the source of the benzene plume in soil was removed six (6) years ago, rthe spill volume has reached its residual saturation in the vadose zone, thus ceasing the vertical migration of the contaminants (Dominico and Schwartz, Physical and Chemical Hydrogeology, �990, pp. 601). Field evidence of this is shown by the cross-sections in Hgurrss 3 & 4 which show the vertical exent of contamination dissipating with depth. ' 6.0 COMCILUEOMS ' The low levels of gasoline ranged organics identified in the vicinity of the former waste oil UST were found to be not associated with benzene. The vertical and lateral extent of gasoline contamination in soil has been ' defined in the vicinity of the former gasoline UST pit area and is associated with low levels of benzene which are not a threat to human health, drinking water, and/or the environment. There are no supply wells close enough to the subject contamination to be ' threatened. Groundwater is too deep, and is protected by very thick, region- wide, clay layers, to be at risk from vertical migration of benzene through the vadose zone.