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Remedial Action Plan Addendum January 14, 1993 Southland Store No. 20304, 455 Grant Line Rd., Tracy, CA 3 ..! hydrocarbons, along the eastern side of the site, appears to be in the capillary fringe at approximately 5 feet below grade. The north-south trend in this area is probably caused by lithology, as Indicated by the generalized stratigraphic cross sections prepared by HLA (f=igure 3). It Is unknown whether soil excavated from the tank pit during excavation and tank removal was returned to the excavation or was disposed of and replaced with dean fill. 1.2.2 Saturated Zone V Lithology. The uppermost aquifer, which is typically encountered approximately 5 feet below grade, .. is composed of clay and slit with lenses of silty, sandy, and gravelly clay. Hydrogeology. Evaluation of site survey data and depth to water measurements indicates that the groundwater gradient across the site is approximately 0.003 foot per foot (ft/ft), or 0.3 percent, in the northeasterly direction. The data suggest that a narrow easterly trending trough through the central �. area of the site appears similar in materials and structure to a former stream bed. The observed hydrocarbon distribution from the suspected source area and the lithologic logs of the soil borings appear to support the presence of a subsurface trough. The data also seem to Indicate that the transmissivity of materials in the trough is significantly greater than in the surrounding fine-grain .. soils. A short-term step drawdown aquifer pumping test was performed at monitoring well MWA on November 25, 1992. The preliminary data,generated indicates that the maximum aquifer yield is approximately 20 to 40 gallons per minute (gpm). The duration of the test was not sufficient to `"' rameters. A longer test was not performed because of the evaluate the aquifer type or capacity pa 9 limited volume of wastewater that could be handled. The long-term sustainable aquifer yield may be significantly different from the estimates based on the preliminary step drawdown test data. The monitoring wells are installed to varying depths ranging from 5.72 to 23.56 feet, but the data Indicate +-• that each of the screened intervals within the saturated zone are communicative. Hydrocarbon Distribution. Sixteen groundwater wells have been installed, 14 of which are currently being sampled quarterly. Results of former and current chemical analyses Indicate that hydrocarbons dissolved in groundwater have been migrating primarily along an apparent easterly trending subsurface trough. There may be a northerly component to the migration, but because of R3263A1.SLL �.. DOW GROUNDWATER TECHNOLOGY, INC.