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• 5.0 CONCLUSIONS The specific objectives of this investigation have been achieved With regard to the objectives listed in section 14,we make the following conclusions 1) Monitor well GT-10 is screened from 30 to 60 feet below surface grade, ensuring tha it will be suitable for groundwater monitoring and sampling until the static water level drops belo x 60 feet, which will probably not occur for many months After that, monitor wells MW-5, MW-6, and MW- 9 will allow valid sampling until the water level drops below 75 feet, at which time w ills GT-1, GT-2,and GT-3 will become suitable 2) Diesel concentrations increase downward through the Modesto Channel, reachn ag values S between 10,000 and 20,000 ppm near the base Gasoline concentrations show a somewl tat similar C%t! though less consistent trend, and concentrations between 700 and 800 ppm have been det,.cted near the base in flus and previous phases of drilling 3) Well GT-10 was completed with 4-inch-diameter PVC slotted casing The upper [5 feet of casing are above the present static water level, and the lower 15 feet are within the saturated zone, making this well suitable for aquifer testing, soil vapor extraction, groundwater extrai.tion, and possibly air sparging L�r `°�1 4) The depth to the upper sand bed within the Modesto Formation is variable, and in sc me places . tivs bed is absent In GT-10, it is not present in the anticipated depth range of 15-19 feet This implies that it is either absent or is present between 19 and 25 feet, an interval that was nos sampled 5) Both GT-10 and GT-11 were located close to the axis of the Modesto Channel, and so it samples were collected across the contact between the channel and the underlying silt bed PID re..dings and laboratory analyses indicate that high concentrations of diesel and moderate concenl rations of gasoline extend below the contact into the silt bed Concentrations then decline through tlne silt bed but extend locally into the underlying Riverbank Formation 6)The hydraulic conductivity of the lower snit bed (Modesto Aquntard)ranges from 1 2x 10`6 to 44 x 10-8 cm/sec It has the lowest conductivity of any bed that has been tested at the site, pi oving that it is the most significant aquntard within the stratngraphic sequence of concern 7) Soil samples were collected at 5-foot intervals from the groundwater interface (approximately 45 feet) to the bottom of GT-11 (120 feet) Moderate hydrocarbon concentrations were deter ted for 10 feet below the groundwater interface, but thereafter concentrations declined abruptly, nm}flying that contaminated groundwater in the borehole did not significantly cross-con tet son I deeper nn the baring However, the detection of very low concentrations of dies 1 8-to-20—~ d/ pier samples may be due to cross-contamination from the groundw t r during sample retnevaf S)The maximum depth of confirmed soil contamination is 71 feet, where a TPH-d concei)f ation of 740 ppm was recorded Projection of the maximum depth of contamination near GT- 0 suggests tQ of 10 I