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S E C O !R ARCO 595 May 5, 2004 • Page 5 Graphs of TPHg, benzene, MtBE, and 1,2-DCA concentrations vs time are Included In Attachment 5 r, Y CONCLUSIONS This Site Conceptual Model was meant to summarize site activities and provide a guide for future f assessment and remediation Figure 11 presents a graphical interpretation of the general conceptual site model The following conclusions result from the above collection of data • Petroleum hydrocarbons detected in soil and groundwater during the removal of product lines W and dispensers in 1999 and subsequent site investigation resulted from operations of gasoline USTs and associated piping and dispenser at the site The majority of hydrocarbons in soil are in the area of the dispensers and product lines at depth of 3 to 7 feet bgs • Well survey's for the site indicate the presents of 8 wells within Y2- mile of the site including onsite well (WSW) and the domestic well at Wlenerschnitzel (70 downgradient of the site) Wienerschnitzel well DW-1 is currently being treated for 1,2-DCA impacts • Monitoring and sampling performed at the site since 2000 Indicates the presence of TPHg, benzene, MtBE, 1,2-DCA dissolved In groundwater beneath the site The extent of Al petroleum hydrocarbons in groundwater cross and downgradient of the site are defined in the— h ort o shallow zone The lateral extent of MtBE and 1,2-DCA in the deep zone Is not currently �xs delineated ,Ir Exposure to contaminants associated with petroleum hydrocarbons adsorbed to soil and dissolved In 4 groundwater at the site is depicted in Figure 11 Potential exposure medium include the following • groundwater, soil, and air Groundwater is encountered at the site through use of domestic wells at the ARCO station and Weinerschnitzel, during monitoring well gauging and sampling, and well installation Soil is typically only encountered during well installation or onsite excavation Despite the presence of asphalt at the site, air (ambient) may be encountered via cracks in the asphalt surfaces should the site be redeveloped Figure 12 presents an exposure pathway model, depicting sources responsible for potential exposure and potential pathways of exposure Based on the above information the site should, 1) continue to be sampled on a quarterly basis to confirm groundwater concentrations and to verify plume stability, 2) additional deep monitoring wells (total depth 100 feet) should be installed to complete the delineation of MtBE and 1,2-DCA to MCLs, 3) two additional shallow groundwater wells should be installed to replace source area wells MW-2 r and MW-4, and 4) one monitoring well should be Installed adjacent to well MW-4 to an approximate depth of 150 feet bgs to complete vertical delineation at the site Additional delineation to the'westof E,,,7 well VIW-1 does not appear warranted because no receptors within 2000 of the site were noted to the ' n� fes ? west and no potential upgradeint source has been noted Additionally, historical review of the property north of ARCO 595 will be completed to determine If minor concentrations detected in wells MW-5 and MW-8 are related to the site being closed with residual contamination " The information contained in this report represents SECOR's professional opinions, and Is based in part on information supplied by the client These opinions are based on currently available information, and are arrived at in accordance with currently accepted hydrogeologic and engineering practices at this time and location S H4t 11ARCOX05951Reports\Site Conceptual Model doe h aM