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' Geolo lTec/uurslxc Page 15 Site Conceptual Model Report Project No 724 2 May 2,2006 ' GTI interprets the data in Figure 15 to suggest that a broad plume of TPH-G above a laboratory detection limit of 50 ug/l exists around the intersection of E Charter Way and Grant Street This plume extends eastward of the GASCO site to reach Aurora Street, ' however it does not appear to reach Aurora Street east of the Morita Brothers parcel This situation is evidenced by the lack of TPH-G in CPT-3 and CPT-4's groundwater samples obtained in January 2006 to define the plume east of the Morita parcel Within this broad ' plume of impacted groundwater are two core hot spots depicted as contoured/stippled areas associated with each UST site It is noted that the GASCO site's wells MW-7 and MW-9 contain elevated TPH-G concentrations which suggests that contamination from the west of that parcel is migrating eastward As stated above, data from the GeoTracker website indicates that the 701 E ' Charter Way site had a detection of 95,000 ug/l TPH-G in September 2005 Further analysis of the co-mingled plume situation will be presented in section 5 0 below 4.0 POTENTIAL EFFECTS OF RESIDUAL CONTANIINATION ' When petroleum hydrocarbons are released to the soil, the material percolates and moves deeper under the primary influences of gravity, groundwater flow patterns, and capillary action As the product reaches the water table it concentrates in a pool on the top of the groundwater surface due to its lesser density Petroleum constituents then dissolve from the pool into the groundwater to form a contaminant plume that migrates under the control of the ' groundwater gradient At the same time the dissolved plume is forming and migrating, non- dissolved petroleum product remains in the pore spaces in the soil due to capillary forces These forces make it difficult to remove the non-aqueous phase liquids (NAPL) trapped in ' the pore spaces Fresh water moving through the soil can eventually flush a portion of the NAPL out, but this process can take a very long time and can contribute to an extensive groundwater plume ' As stated above in Section 2 the majority of the residual contamination is below the current water table at approximately 36 feet bgs in February 2006 It will continue to source a ' groundwater plume that will migrate with the groundwater flow that is primarily to the east The RWCQB-CVR developed the Water Quality Control Plan for the San Joaquin River ' Basin and the plan includes numerical Water Quality Objectives (WQO) The WQO for the site's COC and their current maximum levels in groundwater are included in the table below 1