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A.Teichert&Son Inc.Mobile Equipment Shop 103 N.'E'St.Stockton.CA. Remedial Action Plan Mach, 1984 1_3 Previous Work and Historical Summary 1.3.1 Previous Work Previous work at the site Y completed b other parties since 1986 has removed gasoline, p diesel, and waste oil underground storage tanks. Samples from the UST excavations indicated elevated concentrations of gasoline constituents. Prior to soil excavation from the former UST and piping areas, numerous soil borings were advanced and sampled to evaluate the horizontal and vertical extent of product release. Monitoring wells MW-1 through 5, and MW-9 through MW-13 were constructed and sampled. A drop in the water e table beneath the site has resulted in five remaining wells intersecting the water table. One of these wells, MW-9, is obstructed and conventional water sample collection is infeasible. Historical results of groundwater sampling from the four wells indicated that detectable concentrations of petroleum hydrocarbons have occurred in wells MW-10, MW- 11 and MW-13. Historical water level measurements from the five wells indicates that groundwater flow is to the northeast, east or southeast. The locations of existing wells are illustrated in the Site Map, Figure 2. 1.3.2 Subsurface Summary Soil samples collected from the soil excavation, soil borings and monitoring wells have delineated the lateral and vertical extent of elevated TPH in soil. Liquid sample collection and analysis, and potentiometric surface mapping has characterized groundwater gradient and a zero line of TPH gasoline in groundwater. A substantial portion of the hydrocarbon impacted vadose zone evaluated and defined by numerous soil borings was removed during soil excavation in September 1987. Sidewall and excavation bottom sample analysis indicated elevated concentrations of of TPH gasoline only in sample North Wall 32'. During the period January 1987 -August 1992, the water table dropped over 54 feet, resulting in peripheral monitoring wells remaining above the water table (dry). Residual TPH in soil beneath the current excavation backfill has been evaluated by three recent borings, MW-13, B92-1 and MW-14. These borings were advanced and sampled subsequent to the water table falling to it's current temporarily stabilized position at approximately 70 feet bgs. Well MW-12 was constructed and sampled to the southeast of this area. Analysis of soil samples from MW-12 and B92-1 indicated concentrations of TPH gasoline and BTEX below minimum detection limits. These data points, therefore, illustrate a zero line in the vadose zone to the north and southeast, respectively. Soil samples for laboratory analysis were not collected from MW-4 and 5, located much farther from the center of the hydrocarbon plume. Soil samples were collected from well MW-9 during drilling, and all samples indicated concentrations of TPH as gasoline below minimum detection limits. Several samples near the water table indicated concentrations of toluene and xylenes at or just above the detection limit of 0.005 mg/kg, or ppm. Well MW-13 appears to have been advanced in the center of the soil r