ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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ameO Conversely, water level data from 2002 indicate a more easterly component to the flow within the B-zone. At a time when well N-1 was operating in 1994 at a pumping rate of approximately 140 gpm, a cone of depression in the water table is evident (RUST, 1995). This is an indication that N-1 was extracting groundwater from the B-zone, as well as deeper zones such as the C2-zone above the Corcoran Clay. Groundwater flow in the A-zone beneath the Site has been northerly towards groundwater extraction wells EI-1 and EW-1 (ECM, 2008b). While N-1 was pumping, groundwater flow in the Intermediate Aquifer likely was also towards N-1 beneath the Site, as discussed in Section 4.5.3. Hydrographs for B-zone and A-zone well pairs M-1 B/A, M-2B/A, M-3A/B, M-6A/13, M-15A/B and M-22A/13, are provided in Appendix B. Downward hydraulic gradient values are evident near M-1 B/A where groundwater extraction from the A-zone is occurring at nearby EI-1 and EW-1; water levels for other B- and A-zone well pairs are similar. Upward gradients between Intermediate Aquifer well M-25C1 and Upper Aquifer well M-1A are evident since 2006; previously, water levels for M-1A and M-25C1 were similar, with downward gradients between the A- and C1-zones observed on occasion. There has been little measurable difference between water levels in M-25C1 and M-25C2. Fluctuations in vertical gradients are generally caused by changes in groundwater extraction at the site. Water levels beneath the Site were approximately six to nine feet lower between 1990 and 1995 compared to maximum water levels in 1997 and 2006 (Appendix B). The lowest water level recorded for well M-1 B was 33.09 feet msl in October 1992; the B-zone would have been largely unsaturated at this time, with approximately 4 feet of water above its base at approximately 37 feet bgs near M-1 B. 3.4 SPATIAL DISTRIBUTION OF COCS IN SITE GROUNDWATER TCE is the primary COC detected in groundwater beneath the Site, reported at concentrations above 1,000 pg/L to depths of approximately 60 feet bgs based on recent characterization (Geomatrix, 2006). Other COCs include chloroform, 1,1-dichloroethane (1,1,-DCA), 1,1- dichloroethene (1,1,-DCE), cDCE, vinyl chloride, and PCE; these COCs are generally reported at concentrations much less than 100 pg/L where detected. The vertical and lateral distribution of these COCs is described below, based primarily on depth-discrete groundwater samples collected in 1999 (during the drilling of the exploratory boring for wells M-25C1 and M-25C2; IT, 1999) and 2005 (Geomatrix CPT and Hydropunch investigation; Geomatrix, 2006). 3.4.1 M-25 Depth Profile of COCs Figure 6 shows the vertical distribution of TCE at location M-25C2 from a series of depth- discrete groundwater samples collected in 1999. Stratification of COCs with depth is apparent from the groundwater chemistry profile shown in Figure 6, where TCE concentrations are AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 18