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2.2.2 Depth to Groundwater The plot of water level elevation versus time for UMW-1 (Figure 4) shows a response to cvcics of wct and dry years. Water levels rose approximately 30 feet between the early 1990s and 1999, to approximately 35 feet bgs in June 1999, After 1999, the trend reversed and water levels declined approximately 15 feet through September of 2004 to about 5.3 (bet bg . "S Rising groundwater levels have since reached approximately 22 feet bgs in March 2007. Comparing water levels nneasured in the two deeper wells, DMW-7 and DMW-8, which are screened to depths of 80 to 90 feet bgs and 60 to 70 feet bgs, respectively, with water levels measured in the shallower wells shows no notable difference. This suggests that there appears to be no vertical gradient between the two depth intervals, and indicates in turn that all of the wells are screened within the same shallow hydrogeolo-ic zone. An examination of the litholog within the saturated portion of the shallow aquifer in which the monitoring wells are completed supports this conclusion, as no continuous aquitard has been identified within the depth range of the monitoring wells. 2.3 Site Stratigraphy Groundwater beneath the site is contained in shallow, unconsolidated deposits of.Late Pleistocene to Recent alluvium comprised of intcrfan or flood basin deposits, The lithology of these deposits consists of.clays, silts, and fine sands. Boring logs show approximately equal amounts of line- grained and coarse-grained materials. The fine-grained sediments consist of clays, silty clays, clayey silts, and silts, and the course-grained materials are described as silty fine sands and silty fine to medium sands. As is shown in the lithologic cross sections (Figures 5 and 6), these materials occur predominantly as lenses without strong lateral continuity. 2.4 Source Areas and 9,2-DCA Distribution 2.4.1 Source Areas As discussed above in Section 1,t, two USTs, a 10,000 gallon diesel tank at the west end of the €ornner maintenance shop, and a 1,000 gallon waste oil UST at the cast end of the shop, were removed in 1988, Following their removal a total of approximately 2,000 cubic yards of impacted soils were removed from the two excavations, based on a remedial objective of 10,000 milligrams per kilogram(mg/kg) of TPH as diesel('I-PFI/'D), Based on analytical data from soil borings it kvas concluded (Dames & Moore, 1994) that the removed soil represented approximately 47_ percent of the TPII/D in soil at the site. It was also concluded that 88 percent of the remaining Soil had a concentration of less than 5,000 mg/kg TPH/D. The effect of contamination remaining in soil on groundwater quality is considered to be negligible. Model simulations were used to estimate the impact of the remaining soil contamination on groundwater quality. The modeling was based on naphthalene, the most mobile constituent of TPM/D, and conservatively neglected the effects of biodegradation. A period of 25 years using three groundwater level scenarios was modeled. Depths of 35 feet blas and 70 feet bgs were considered the extreme ranges, and an average depth of 54 feet bgs was considered the most li cl_y long-term average water level depth. The model predicted that after 25 years of water levels at an average depth of 35 feet bgs, the naphthalene concentration in groundwater would reach 36.5 micrograms per liter (µg/L), while at the more likely average water level elevation of 54 feet bgs a naphthalene concentration of 2,98 µg/L would be reached after 25 years. This concentration is below the proposed water quality goal for the site of 20 µg L, which is the EPA 4