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_ ATC Associates Inc. CVATC 1117 Lone Palm Avenue, Suite 201 Modesto, California 95351 209-579-2221 ASS O C I ATE 6 INC , faX: 209-579-222$ that also appears to be laterally extensive underlies the sand layer and appears to be thickest at MW12 extending from approximately 31 to 46 feet bgs . Relatively thick beds of sand interbedded with clay and silt were encountered to the terminal depth of approximately 121 . 5 feet bgs at MW14 . SITE CONTAMINANTS Soil The lateral and vertical extent of petroleum hydrocarbons in soil is defined and extends predominantly along the levee , west of the store/garage building . The aerial extent of impact within the levee vadose and saturated zones averages 90 feet in length and 50 feet in width and is primarily limited to depths between 5 and 25 feet bgs . The aerial extent of contaminant impact within both the vadose and the shallow saturated zone, landward further east of the levee, averages approximately 90 feet in length and 40 feet I n width and is limited primarily to depths between 5 and 15 feet bgs . A very low concentration of MTBE was detected in the soil sample collected from MW11 at a depth of 51 .5 feet bgs, and very low concentrations of DRO were detected in the soil samples collected from MW14 at depths of 87. 5 feet bgs and 98 feet bgs . These results are regarded to be anomalies , as impact to soil is unlikely to occur at these depths and soil samples collected from both these borings at shallower depths were not reported to contain petroleum hydrocarbons. A summary of laboratory analytical results of soil samples is presented in Table 2 . The monitoring well locations are shown on Figure 2. Groundwater MTBE and TBA constitute the contaminant groundwater impact at the site . MTBE represents the greatest aerial extent of contaminant constituents in groundwater. The MTBE groundwater plume in Zone A is generally defined . MTBE was detected at concentrations below the Maximum Contaminant Level (MCL) in perimeter wells MW8 and MW9 . The laboratory analytical results indicate the TBA groundwater plume is adequately defined as this constituent was not detected in any of the perimeter wells . Monitoring well MW7 continues to be the most impacted well . Differences in groundwater elevations with respect to the Zone A and Zone B wells and deeper well MW14 indicate the presence of confining layers or aquitards in the subsurface. MTBE was the only constituent detected in Zone B groundwater monitoring wells. Very low concentrations of MTBE were detected in the Zone B groundwater monitoring wells, indicating the lateral and vertical extent of contaminant impact to groundwater in Zone B is generally defined . Petroleum hydrocarbons were not detected in sentinel well MW14 . KIR 2009 WP for Pilot Testing.doe ATC Associates Inc. SMnviromnental\62514 — KIR ATC Project No. 54.62514.0001 Page 6