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the City of Manteca This temporary solution will remain in place until a
<br /> more permanent solution can be implemented
<br /> 2- 16 Seytf mher 2000 CPT Drilling and Monitoring Well installation
<br /> In September 2000, ASE drilled two additional soil borings using a C r
<br /> and installed five additional groundwater monitoring wells Groundwater
<br /> in the shallow zone (less than 19-feet bgs) flowed primarily to the north
<br /> beneath the site at a gradient of approximately 0 001-feet/foot
<br /> Groundwater in the 55 to 70-feet bgs zone flowed primarily to the
<br /> south/southeast at a gradient of 0 003-feet/foot Groundwater in the
<br /> deeper zone (below 100-feet bgs) flowed to the southeast at a gradient of
<br /> 0 0018-feet/foot
<br /> In general, groundwater samples collected from monitoring wells on the
<br /> northern portion of the site, north of the USTs and pump dispensers,
<br /> contained the highest hydrocarbon concentrations Monitoring well
<br /> MW-5 contained 0 01-feet of free-floating hydrocarbons Groundwater
<br /> samples collected from recovery wells RW-1 through RW-5 contained up
<br /> to 140,000 ppb TPH-G, 840 ppb benzene, 18,000 ppb toluene, 4,500 p p b
<br /> ethyl benzene, 25,000 ppb total xylenes, 8,100 ppb MTBE, 21 ppb TAME,
<br /> 1,100 ppb TBA, and 380 ppb ethanol
<br /> Groundwater samples collected from all off-site, downgradient
<br /> monitoring wells screened in the water-bearing zone less than 19-feet bgs
<br /> contained MTBE concentrations exceeding the DHS MCL for drinking
<br /> water The highest hydrocarbon concentrations were in the groundwater
<br /> sample collected from monitoring well MW-13 which contained 2,000
<br /> ppb MTBE, 1,000 ppb benzene, 80 ppb toluene, 1,100 ppb ethyl benzene,
<br /> 1,300 ppb total xylenes, 86 ppb TAME, and 600 ppb TBA Groundwater
<br /> samples collected from monitoring well MW-16, located near
<br /> downgradient monitoring well MW-13 but screened in a deeper water-
<br /> bearing zone 49 to 64-feet bgs, contained 26 ppb MTBE, 8 8 ppb TBA, 1 1
<br /> ppb ethanol and 58 ppb methanol Although this MTBE concentration
<br /> exceeds the DHS MCL for drinking water, the concentration still
<br /> represents a significant decrease in MTBE concentration with depth
<br /> Groundwater samples collected from monitoring well MW-6, dust south of
<br /> the gasoline USTs, contained 1,100 ppb toluene, 1,200 ppb ethyl benzene
<br /> and 7,200 ppb total xylenes
<br /> Groundwater samples collected from all other portions of the property,
<br /> iincluding monitoring wells MW-1, MW-2, MW-3, MW-4, MW-7, MW-8,
<br /> Franks One Stop Report — March 2001
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