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SECTION 4 FIELD ACTIVITIES AND RESULTS • Deep monitoring wells MW-DIS-14D and MW-DIS-11D were installed on November 6, 2001 and December 11,2001. MW-DIS-14D was installed to monitor the deeper water bearing zone between 154 and 164 feet bgs,downgradient from CPT boring CPT-23. The groundwater sample from CPT-23 at 144 feet bgs contained 1,2-DCA at 45 µg/L. MW-DIS-11D was installed to monitor groundwater beneath the plume (between 134 and 144 feet bgs),directly downgradient of the site, • Well MW-DIS-11D was developed via swabbing,bailing,airlifting and pumping on December 13,2001. All other wells were developed via swabbing,surging,bailing and pumping between November 13 and 16,2001. 4.2 Results 4.2.1 Hydrogeology This section presents a supplemental description of the hydrogeology of the Del Monte site. The description is based largely on compilation of information available from CPT borings CPT-1 through CPT-24,monitoring well installation,and recent groundwater monitoring events. The CPT borings indicate alluvium as much as 150 feet thick in the vicinity of the study area. The alluvium consists of unconsolidated deposits of clay,silt,and sand. A 5-to-10 foot thick sand unit is located approximately 90 to 110 feet bgs directly beneath the site. The unit consists of sand and silty sand with some clay lenses. The overlying alluvium is comprised primarily of clay and silts. The sand unit appears to be as much as 50 feet thick northwest of the site in the vicinity of MW-DIS-8S/8D,and thins to 5 to 10 feet in the vicinity of CPT-11, -12, and CPT-19 and-20. A stiff,fine-grained/clay unit underlies this permeable zone. A discontinuous deeper water bearing zone was encountered in some CPT borings at approximately 145 to 165 feet bgs.Monitoring wells MW-DIS-14D and MW-DIS-11D are screened across this zone. 4.2.2 Analytical Results Groundwater beneath the Disco site contains detectable levels of total petroleum hydrocarbons (TPI)as gasoline,BTEX compounds (benzene,toluene,ethyl benzene,and total xylenes),1,2-dibromoethane,also known as ethylene dibromide (EDB),and 1,2-dichloroethane(1,2-DCA). These compounds are known as lead scavengers an w ( d were a P g common gasoline additive prior to 1991. Tert butyl alcohol,another gasoline additive used as an octane booster,was also detected in several groundwater samples. This field investigation focused on areas further downgradient and cross-gradient of the previous study. The results of the sampling and analysis are summarized below. 4 Groundwater samples were collected at discrete depth intervals from all CPT borings.Two groundwater samples,one shallow and one deep,were collected from each boring,except CPT-16 and CPT-24.Samples were collected from permeable zones,as determined from the CPT logs.Groundwater samples were also collected from monitoring wells screened in the permeable zones. Groundwater samples were analyzed for petroleum hydrocarbons, oxygenates,EDB,and 1,2-DCA. Table 1 summarizes recent groundwater analytical data for the CPT and monitoring well investigation. A 1,2-DCA groundwater plume map,based on results of groundwater samples collected from this investigation as well as previous studies, SFOICPTMW_REPORT KS_.DOCIM670036 4.2 I� I