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The general stratigraphy consists of silt and clay from the surface to a range from 40- to 50-feet below grade From a range of 40- to 55-feet below grade to approximately 65-feet below grade is a silty sand and gravel unit This unit has been saturated, unsaturated and partially saturated during the historic onsite monitoring period According to the boring logs prepared for the deep borings, there is a clayey and sandy silt unit from approximately 65-feet through 90-feet below grade (limit of borehole depth) The boring log for MW-5 indicates that this silt unit extends from 65- to 75-feet and is underlain by a sand and gravel unit through 90-feet below grade (limit of borehole depth) Soil samples were not collected below 70-feet in MW-6 and MW-7 and were not collected in MW-5 below 75-feet Potential Effects of Residual Hydrocarbon Impacts on Surface Water and Groundwater There is a slight yet unlikely potential for impacts to surface water and groundwater The only possible route of lateral transport is migration of colloidal clay, carrying adsorbed petroleum hydrocarbons This feature would only occur during high water table conditions, and is subject to high retardation factors It is also very likely that the colloidal clay mobility is greatest during the purging process for all wells A portion of groundwater discharge during purging of the deeper screened wells likely comes from the recently saturated shallow screened groundwater zone and carries clay colloids impacted with TPHd into the deeper screened wells Based on the distribution of the TPHd detected in groundwater sediment, it appears that there are likely remaining isolated pockets of low level TPHd impacts at the sand/silt transition zone approximately 65-feet below grade The TPHd levels remaining in soil following over-excavation are below the levels required for dissolution into groundwater Because purging the deeper screened wells likely draws down impacted sediments, destruction of these wells is recommended Based on a detailed review and analysis of the data collected at the subject site, it is the professional . opinion of RTDs California Certified Hydrogeologist that the limited and low level residual diesel (no BTEX) soil impacts delineated at the subject site do not pose a threat to public health or the environment FEASIBILITY STUDIES Feasibility Studies Performed to Date Petroleum Impacted Soil Excavation A field scale remediation feasibility study was initiated in October 1993 in conjunction with the UST replacement process The mass of gasoline and diesel products, and soil removed during the over- excavation activities were calculated in the RSCR (AMV, 1995) Based on the data collected, AMV estimated that approximately 350-gallons of diesel and 50-gallons of gasoline were removed and that 50-gallons of diesel and 5-gallons of gasoline remained in soils following over-excavation activities The remediation performance was evaluated based on one year of semi-annual groundwater monitoring The groundwater elevation and laboratory results are summarized in Table 1 The water table has increased by 20-feet over the last year (during the verification monitoring period) and has re- saturated the interval of the shallow screened wells Since August 1995, all of the deeper screened wells had detections of TPHd at one time or other, and one of the shallow screened wells had detections of TPHd in August 1995 only The December 1995 groundwater sampling event had detections of TPHd in MW-5 and MW-6 in the samples containing sediments, and no detections in the sediment free (decant) samples Low levels of BTEX were detected in the shallow screened wells six years prior to over-excavation of impacted soil, and have not been detected since then Based on the data collected during over-excavation and post removal monitoring, it appears that the majority of diesel and gasoline was removed from subsurface soils The remaining impacts will slowly - 6 -