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City of Manteca This temporary solution will remain in place until a
<br /> more permanent solution can be implemented
<br /> 2_16 Sentemher 2000 CI?T Drilling and Monitoring_ Well Installatrton
<br /> In September 2000, ASE drilled two additional soil borings using a CPT a n d
<br /> installed five additional groundwater monitoring wells (Figure 3 )
<br /> Groundwater in the shallow zone (less than 19-feet bgs) flowed primarily
<br /> to the north beneath the site at a gradient of approximately 0 001 -
<br /> feet/foot Groundwater in the 55 to 70-feet bgs zone flowed primarily to
<br /> the 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 /> 40 ethyl benzene, 25,000 ppb total xylenes, 8,100 ppb MTBE, 21 ppb tert-
<br /> amyl methyl ether (TAME), 1,100 ppb TBA, and 380 ppb ethanol
<br /> Groundwater samples collected from all off-site, downgradient monitoring
<br /> wells screened in the water-bearing zone less than 19-feet bgs contained
<br /> MTBE concentrations exceeding the DHS MCL for drinking water The
<br /> highest hydrocarbon concentrations were in the groundwater sample
<br /> collected from monitoring well MW-13, which contained 2,000 ppb MTBE,
<br /> 1,000 ppb benzene, 8 0 ppb toluene, 1,100 ppb ethyl benzene, 1,300 ppb
<br /> total xylenes, 8 6 ppb TAME, and 600 ppb TBA Groundwater samples
<br /> collected from monitoring well MW-16, located near downgradient
<br /> monitoring well MW-13 but screened in a deeper water-bearing zone 49
<br /> to 64-feet bgs, contained 26 ppb MTBE, 8 8 ppb TBA, 11 ppb ethanol a n d
<br /> 58 ppb methanol Although this MTBE concentration exceeds the DHS
<br /> MCL for drinking water, the concentration still represents a significant
<br /> decrease in MTBE concentration with depth
<br /> Groundwater samples collected from monitoring well MW-6, .Just 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 /> including monitoring wells MW-1, MW-2, MW-3, MW-4, MW-7, MW-8,
<br /> Frank's One Stop Remedial Action Pian -- August 2001
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