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4.0 CONCLUSIONS AND RECOMMENDATIONS This environmental site investigation included the drilling of twelve sod borings for soil and 9 9 9 groundwater grab sampling using a Geoprobe rig Field observation and analytical laboratory results indicate that diesel and gasoline hydrocarbons have impacted soil and groundwater beneath the site, the city of Tracy right-of-way (Hamilton Alley) and other private property parcels to the north and east of the site Soils beneath the site were found to consist of a fining-upward sequence of siliciclastic sediments The uppermost soil is a silty clay to a depth of approximately 7 feet bgs Below 7 feet bgs and to a total depth of 20 feet bgs, the sod consists of a sility fine sand with a generally decreasing content of clay Groundwater was encountered at approximately 7 feet bgs Soil samples collected from sod borings located within and in close proximity to the former UST g p Y excavation contained the highest concentrations of diesel and gasoline hydrocarbons Petroleum hydrocarbons were detected in soil samples collected at 10 feet bgs. at concentrations as high as 1,800 ppm of TPH-as-diesel, 400 ppm of TPH-as-gasoline and 5 4 ppm of benzene MTBE was not detected in any of the sod samples analyzed Vertically, the extent of impacted soil is defined to less than 15 feet bgs The most heavily impacted sod is present from dust above the current groundwater table at approximately 7 feet bgs to a depth of approximately 12 feet bgs Laterally, impacted soil extends to the north of the site beneath Hamilton Alley and the private property parcels east and north of the site The lateral extent of impacted soil to the north and east of the site is not fully defined Groundwater grab samples collected from all of the sample locations contained detectable concentrations of diesel and gasoline hydrocarbons Petroleum hydrocarbons were detected in close proximity to the former UST excavation at concentrations of 1,300,000 ppb of TPH-as- diesel, 140,000 ppb of TPH-as-gasoline, 11,000 ppb of benzene and 400 ppb of MTBE Some free product was observed during the soil and groundwater sampling in and around the former UST excavation Laterally, impacted groundwater extends to the north and east of the site under Hamilton Alley and private property The lateral extent of impacted groundwater is not defined Based on the results of the soil and groundwater grab sampling, Ramage Environmental recommends the following course of corrective action 1 Based on the results of the soil and groundwater grab sampling, position and install a network Of groundwater monitoring wells to monitor the concentration and migration of the plume of impacted groundwater Given the relatively large size of the plume, at least six wells may be . necessary to monitor the plume Groundwater should be monitored and sampled from each of 11