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. Potentiometric surface measurements in locations with monitoring wells screened at multiple depths such as MW-5 and MW-11, monitoring wells MW-1 and MW-12, monitoring wells MW-6, MW-7 and MW-8, monitoring wells MW-9 and MW-10, monitoring wells MW-13 and MW-16, and monitoring wells MW-17 and MW-18 suggest a vertical (downward) groundwater gradient 10 . 0 CONCLUSIONS On September 25, 2000, groundwater in the shallow zone (less than 19- feet bgs) flowed primarily to the north beneath the site at a gradient of approximately 0 001-feet/foot Groundwater in the 55 to 70-feet bgs zone flowed primarily to the south/southeast at a gradient of 0 003- feet/foot Groundwater in the deeper zone (below 100-feet bgs) flowed to the southeast at a gradient of 0 0018-feet/foot In general, groundwater samples collected from monitoring wells on the northern portion of the site, north of the USTs and pump dispensers, contained the highest hydrocarbon concentrations Monitoring well MW-5 contained 0 01-feet of free-floating hydrocarbons Groundwater samples collected from recovery wells RW-1 through RW-5 contained up to 140,000 ppb TPH-G, 840 ppb benzene, 18,000 ppb toluene, 4,500 ppb ethyl benzene, 25,000 ppb total xylenes, 8,100 ppb MTBE, 21 ppb TAME, 1,100 ppb TBA, and 380 ppb ethanol Since some of the highest hydrocarbon concentrations are present in the recovery wells to the outside (RW-1 to the west and RW-5 to the east), it is not known at this time whether the full extent of the plume is being captured with t h e groundwater remediation system with these existing wells Groundwater samples collected from all off-site, downgradient monitoring wells contained MTBE concentrations exceeding the DHS MCL for drinking water The highest hydrocarbon concentrations were in the groundwater sample collected from monitoring well MW-13, on the northern shoulder of West Yosemite Avenue, which contained 2,000 ppb MTBE, 1,000 p p b benzene, 8 0 ppb toluene, 1,100 ppb ethyl benzene, 1,300 ppb total xylenes, 8 6 ppb TAME, and 600 ppb TBA There is currently no downgradient definition of the extent of MTBE contamination, except from the definition provided from the sampling of downgradient domestic and irrigation wells (Please see ASE's report dated October 30, 2000 for the results of the sampling of off-site domestic wells) High hydrocarbon concentrations are still present in groundwater samples collected from monitoring well MW-6, dust south of the gasoline USTs Franks One Stop Report — November 2000 -17-