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1 <br /> GEQMATRIX <br /> shallow coarse-grained unit, it is thought the PHCs slowly infiltrated the fine-grained sediment <br />' from a leak in the USTs or ancillary piping In 1988, the source of PHCs was removed when <br /> the tanks and soil beneath and surrounding the tanks (to a depth of 11 feet bgs) were removed <br />' The PHC release apparently migrated through the fine-grained sediment and into lenses of <br />' coarse-grained sediment within the riddle fine-grained stratigraphic unit The PHCs may <br /> have migrated either as free product or dissolved in groundwater or both Some portion of the <br /> gasoline became attached to soil particles and immobilized During periods of rising water <br /> levels, groundwater would fill the PHC-impacted coarse-grained lenses within the middle <br /> fine-grained strata and force lateral migration of the PHCs Lateral migration of PHCs <br />' between high and low groundwater levels (smear zone) continued until residual saturation of <br /> gasoline in the fine-grained soil was reached or the PHCs had been immobilized Because <br /> groundwater velocity is low in the middle fine-grained unit, the groundwater flow direction <br /> has little impact on the distribution of PHCs in that unit <br /> In addition to lateral migration of PHCs with groundwater flow, vertical and Iateral diffusion <br /> of PHCs dissolved in groundwater could occur Diffusion occurs as higher concentrations of <br /> constituents in groundwater move toward lower concentrations The PHCs, in groundwater <br /> below the fine-grained statigraphic layer likely migrated from the fine-grained layer by <br /> diffusion to a maximum depth of about 35 feet bgs A portion of the PHCs in groundwater has <br /> become immobilized as the PHCs attached to soil particles The concentration of PHCs in <br /> groundwater decreases both laterally and vertically away from the former excavation <br /> PHCs dissolved in groundwater in the lower coarse-grained unit could migrate due to <br /> groundwater flow However, because the source was removed 15 years ago and because of <br /> the slow vertical migration processes, PHC concentrations dissolved in groundwater are stable <br />' and PHCs have not migrated more than about 200 feet from the former release Findings in <br /> Recommendations to Improve the Cleanup Process for California's LUFTs (RICP-LUFTs, <br /> 1 1995) indicate that in most cases PHCs in shallow groundwater did not extend more than <br /> 200 feet from the former release, and once the source is removed, the plume will stabilize and <br /> naturally degrade <br /> Based on the soil and groundwater data, the affected area has a small lateral and vertical <br /> extent The PHCs have had at least 15 years to stabilize (tanks removed in 1988) Migration <br /> has occurred through diffusion through groundwater and mass transport of PHCs dissolved in <br /> 1 <br /> I TROiECf16000A6437 00016437-08 doe 13 <br />