ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec�9 approximately 50 feet north of the former Decaffeination Building, were 88 and 3.4 pg/L, respectively (Figure 8). As shown in Figures 4, 5, and 6, TCE concentrations were substantially lower in groundwater samples from deeper samples from the Upper A-zone, where the TCE concentration was reported at 18 pg/L at CPT-2 from a depth of approximately 67— 70 feet bgs; the shallower sample at this location was reported to contain TCE at a concentration of 7,800 pg/L from a depth of approximately 51 — 54 feet bgs. The vertical distribution of TCE in the Upper A-zone beneath the parking lot area west of the Site is different from the onsite distribution. In the parking lot area west of the Site the greatest TCE concentration for the Upper A-zone samples was 970 pg/L at CPT-4 (approximately 64 — 67 feet bgs). This concentration is an order of magnitude higher than TCE concentrations reported in samples from the stratigraphically equivalent zone east and beneath the Site (Figure 4). Middle A-zone Three Middle A-zone groundwater samples were reported to contain TCE at concentrations greater than 100 pg/L: CPT-4 (82 — 85 feet bgs; 190 pg/L), CPT-5 (84 — 87 feet bgs; 160 pg/L) and CPT-6 (87 — 90 feet bgs; 110 pg/L) (Figures 4 and 5). These groundwater samples were collected from locations in the parking lot area west of the Site (Figure 3); the highest TCE concentration reported for Middle A-zone groundwater samples collected from beneath the Site was 33 pg/L. Lower A-zone (A-zone) Lower A-zone groundwater samples were reported to contain TCE at concentrations less than 100 pg/L in depth-discrete groundwater samples collected by Geomatrix in 2005 (Figures 4 and 5). Groundwater samples were collected from onsite A-zone wells M-1A, M-3A, M-12A, and offsite A-zone well M-6A (in the parking lot area west of the Site) in April 2005 (the same year of the Geomatrix investigation); TCE concentrations were 48 pg/L, 92 pg/L, 20 pg/L and 58 pg/L, respectively. Although early groundwater samples from lower A-zone well M-1A were reported to contain TCE above 1,000 pg/L, leading to the conclusion that there was a TCE source in the Lower A-zone (RUST, 1994a), recent groundwater samples collected by Geomatrix in 2005 show no evidence for a TCE source in Lower A-zone groundwater. The higher concentrations of TCE reported in groundwater samples from well M-1A in the early 1990's and late 1980's appear to have rapidly attenuated, likely due to ongoing groundwater extraction, consistent with the decreasing trend for TCE concentrations in samples from well M-1A (Appendix C). AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 20