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Ile 27 October 2004 AGE-NC Project No 96-0254 Page 9 of 10 ' lateral migration of dissolved MTBE from the UST at the southern portion of the site,across the site and towards the location of well W-2 and less to well MW-4 Vertically MTBE has been locally documented to have migrated tofeet bsg in soil bonng B8, located at well MW-1 MTBE has not ' migrated vertically at wells MW-2 or MW-5 JYi t,TJr9 ►t �'C p� ir7 Cr3� in ,boi?7Ld �� 1ftl� `. DISTRIBUTION OF DISSOLVED MTBE VrA Since Initiation of ground water monitoring in January 2000, the depth to ground water has ranged from approximately 14 t,0 feet (recently) below casing grade Ground water flow has been consistently north-northeast J"tA- aQ 0 v'� sj'Ss _ Along with general lateral migration, the MTBE-impacted ground water plume migrated approximately 70 feet to well MW-2 and twice was detected in well MW-4, located approximately 100 feet from well MW-1 Laterally the MTBE-Impacted groundwater plume migrated not greater," � yJ than 40 feet northeast and 50 feet northwest to well MW-2 The dissolved concentrations of MTBE ow in the stratigraphic cross sections are presented on Figure 4 The distribution of dissolved MTBE ' during July 2004 is depicted on Figure 5 The general trend of dissolved MTBE on the site has steadily declined since ground water monitoring began in January 2000 and the MTBE plume has remained stable within the monitoring well network Therefore, consideration for site closure is warranted ' 8.0. SUMMARY AND CONCLUSIONS The key components of the MBP-Manteca site conceptual model include ' • The subsurface soil stratigraphy consisted of sand with interbedded silt and clay to a depth of 50 feet bsg ' • Ground water elevations at the site has fluctuated between 14 and 20 feet below MSL since ground water monitoring began in January 2000 Generally, ground water flow has been to the north, A/WA 6 0 +- S Jfq • Distribution of MTBE contamination in ground water indicates an initial release In the area of the gasoline USTs, which then migrated laterally to well MW-2 below the MCL and ' veitic,ally tliruu li the 9011 et well MW-1 Vertically, soil contamination does appear to J extend below feet big, howci er rhe vertical extent of ground water contamination is 4; <7undefined below e µg/l ' The concentration of 220 µg/1 of MTBE in one ground monitoring water is above the California Department of Health Service's Maximum Contaminant Levels (MCL) for Advanced GeoEnvironmental,Inc