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' height vs. time. Two tests were performed on wells W-4 W-6 and W-7 d tw g p M M M an o curves were generated (Appendix D). Curve A illustrates the rate at which injected water infiltrated the filter pack and aquifer. Curve B shows the rate at which water levels recovered after removal of the slug. Only a slug withdrawal test was performed on well MW- 1. ' 3.4.3u` e Ay if r Model The aquifer model used in this test is Model 18 of Dawson and Istock (1991), which is based on a modified version of the Thiem equation. Model 18 applies to slug tests performed on unconfined aquifers when the well does not penetrate the full thickness of the aquifer. Appendix E contains the mathematical calculations used to determine the hydraulic conductivity and transmissivity at the site. ' Using the model to interpret the recovery curves, the calculated hydraulic conductivity of the aquifer ranges from .68 ft/day (MW-1, slug withdrawal test) to 143.2 ft/day (MW-6, slug ' addition test). This range appears to be reasonable, because MW-6 was completed within the medium-grained sand that forms the main aquifer at the site, whereas MW-1 was completed in the silty interval above this sand. The data for wells MW-4, MW-6, and MW-7, ' all of which were completed within the aquifer, indicate that the aquifer is significantly more permeable in the vicinity of MW-6 than in the vicinity of MW-4 or MW-7. None of the wells at the site has penetrated the full thickness of the aquifer, so that estimates for aquifer thickness have been based on prior experience with the stratigraphy of the Pleistocene deposits in the eastern San Joaquin Valley. Assuming a thickness of 50 ' feet, the aquifer transmissivity ranges from 375 ft /day to 7,150 ft'/day. ' 4.0 SOIL CONTAMINATION 41 Depth or Contamination ' Gasoline and volatile aromatics were detected at moderate to high concentrations ncentrations at the bottom of vadose zone well RW-1 at a vertical depth of 48 feet (Figure 5). Similar ' concentrations were detected at 50 feet in MW-1 (near the groundwater interface in 1987), and organic vapor readings taken during drilling of MW-1 suggest that contamination may ' extend to the bottom of the well at 70 feet (see Weiss Associates report dated October 16, 1987). Contaminated soil below 50 feet is within the capillary fringe ("smear zone"), and contaminated soil below 60 feet has been within the saturated zone since before ' investigation began at this site. 4.2 Lateral Extent of Contamination Nine soil borings have been drilled since assessment of this site began. A total of 27 soil nnn MEW-28-94/1 a7 19 1