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In the April 1998 Soil Assessment Report, Upgradient illustrated the extent of TPH-g and benzene contamination in the most contaminated depth interval of the soil (Figures 3 and 4) 1 his depth interval corresponds to the upper part of a fine-to-coarse-grained sand bed that appeaj s to be a linear stream-channel deposit trending roughly north-south A hydraulic conductivity of 6 7 x 10 4 was measured on a sample from this bed at a depth of 27 feet This value indicates that the bed is relatively transmissive of groundwater in comparison to the surrounding silt beds, w'vch have hydraulic conductivities in the range of 10"5 to 10-7 As Figures 3 and 4 show, the contaminant plume appears to be slightly elongated north-south, parallel to the axis of the sand bed 3.0 PROPOSED ASSESSMENT In order to confirm the trends shown in Figures 3 and 4 and to determine whether the groundwater plume is similarly elongated north-south;four additional monitorin wg ells are proposed (Figure 5) All of these are located in the street or sidewalk, on City of Stockton property Wells VM 5 through VM-7 will be located upgradient of the site, but both VM-5 and VM-7 are expected to be near the upgradient edge of the contaminant plume Well VM-8 will be located downgradient of the site, along the projected trend of the contaminated channel deposit This well must be located in the sidewalk north of Main Street, heavy traffic prevents drilling in the parking lanes on Main Street The wells will be drilled to a depth of 20 feet below the soil/groundwater interface and will be completed with 25 feet of 2-inch-diameter perforated casing The depth to groundwater is estimated to be approximately 35 feet Details of the well design are illustrated in Figure 6 Soil samples will be collected at 5-foot intervals, beginning at 15 feet Because all of the wells are located beyond the mapped limit of soil contamination, no more than 2 samples pei well are planned for laboratory analysis (one at the groundwater interface) Drill cuttings will be placed in drums and stored at the south end of the site Samples will be analyzed for total petroleum hydrocarbons in the gasoline range, BTEX, and MTBE (methyl tertiary butyl ether) If MTBE is �i reported by method 8020, the result will be confirmed by method 8240 All groundwatf r samples will be analyzed by method 8240 4.0 PROPOSED FEASIBILITY TESTING Appendix A of the Tri-Regional Guidelines specifies that feasibility tests of potential remediation methods should be conducted during the assessment phase in order to evaluate the cost- effectiveness of various technologies At the Valley Motors site, where gasoline and benzene are the contaminants of primary concern and the contamination is largely confined to a relatively shallow, sandy channel deposit, both ex-situ and in-situ methods should be considered 3efore in- situ methods can be evaluated, however, short-term pilot tests must be conducted Tests of three possible methods are proposed enhanced bioremediation; soil vapor extraction, and air sparging These tests will require specially-designed wells, which will be dnlled a the same tume_.ts_the oft- site monitor wells