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Emergency Remediation - 2480 Tracy Blvd., Tracy, CA. • Page 9 below the water table. However, there is a trend in the data that suggests that the concentrations of gasoline decrease rapidly with depth below the water table. The observed distribution of contaminants in the soil is typical of sites down gradient from a leaking, underground, fuel storage tank, where the leaked fuel floats on the water table beneath the tank and is carried down gradient under the action of groundwater flow. The floating layer of fuel, as it advances with its associated plume of dissolved hydrocarbon compounds, is smeared over, and adsorbed onto, the soil closely above and below the water table. The thickness of the zone thus affected by high concentrations of hydrocarbons increases as the water table rises and falls seasonally. MTBE was first used as an additive to gasoline in California in the winter of 1992. Thus, its presence in the soils and groundwater beneath the 2480 Tracy Boulevard property indicates that the source of discharge to the groundwater that has contaminated the site was on going at least up until that time. ' Figure 3 is a cross section through the basement structure of the new car-wash and lubrication-service facility that illustrates the relationship between: the soil sampling locations, the basement excavation, the basement slab, the water table, the property boundaries and the zone of the subsurface found to be affected by gasoline. (Note: As shown on Figure 2, Section A-A! on Figure 3 passes through push-probe sampling location PP-6. Other nearby sampling locations that have been projected into the section are indicated by dashed lines.) Based on in-situ permeability testing of similar soils in other locations in the region, it is estimated that the silty clay has a hydraulic conductivity around 2.0 x 10-3 cm/sec. If it is also assumed that the gradient of the water table is around 0.002 ft/ft to 0.003 ft/, which is typically found in and around Tracy at sites with similar hydrogeologic settings, and that the porosity of the soil is 0.5, then a reasonable estimate of the rate of down-gradient groundwater flow is 0.04 ft/day. The above estimate for the down-gradient flow rate, combined with the known date when MTBE was first added to gasoline in California, places the source of the gasoline affecting the subsurface beneath the 2480 Tracy Boulevard property no more than 48 ft. up the groundwater gradient from the basement excavation. If a significantly more conservative estimate of 1.0 x 10-2 cm/sec for the hydraulic conductivity of the silty clays is used, the isource of the gasoline found under the 2480 Tracy Boulevard site is projected to be no more than 103 ft. up gradient of the basement excavation. Both of those estimated distances place the source of contamination well within the boundaries of the 575 West Grant Line Road property. REMEDIATION OF SOIL AND GROUNDWATER Mr. Sanchez contracted with Dietz Irrigation of Tracy, California, an experienced remediation contractor holding a Class A General Engineering License with a Hazardous-