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d - I CVATC 1117 Lone Palm Avenue, Suite Modesto, CA 953511 www.atcassociates.com 209-579-2221 ASS U C I ATE S 1 N c . Fax 209-579-2225 Environmental, Geotechnical and Materials Engineers selected soil samples were submitted for laboratory analyses. Each well was constructed with 2-inch diameter PVC casing, with MW8, MW9, MW10, and MWl l extending to a depth of 65 feet bgs in { the shallow groundwater bearing zone, MW5, MW6, and MW12 extending to a depth of 90 feet bgs in the intermediate groundwater bearing zone, and MW7 extending to a depth of 121 feet bgs in the deep groundwater bearing zone. Monitoring wells MW5 , MW6, MW8, MW9, MW10, and MW12 were developed on December 22 and 23 , 2004 . An obstruction encountered in MW7 at approximately 64 to 65 feet bgs prevented development of this well. MW l was purged dry after removing 0 . 5 gallons of water. Efforts to develop this well using a bailer revealed the presence of grout inside the well and the total depth of the well was measured to be approximately 58 feet. i On December 22 and 23, 2004 monitoring wells MWl through MW12 were surveyed relative to NAD 83 and NGVD29 for position and elevation and groundwater samples were collected from MWl through MW6, MW8, MW9, MW10, and MW12 . Petroleum hydrocarbon constituents were detected in soil samples collected from the boreholes of MW5 through MW9 and MW12. Groundwater samples were not collected from MW7 and MWl l due to an obstruction in MW7 and grout infiltration in MW11 . Petroleum hydrocarbons were detected in the groundwater samples collected from MWl through MW6, MW8 through MW10, and MW12. The results of the subsurface investigation activities are presented in ATC ' s Subsurface Investigation Report, dated July 25 , 2005. On April 7, 2005, an ATC geologist supervised the destruction of MWll and the installation of replacement monitoring well MW I IR to a depth of approximately 65 feet bgs using 24rich diameter schedule 40 PVC casing. Refer to ATC' s First and Second Quarter 2005 Monitoring Report, dated October 10, 2005 , for additional information. From September 25 to 30, 2006, an ATC geologist supervised the installation of MW13 and MW14 to approximately 60 feet bgs, and MW15 to approximately 151 feet bgs. To evaluate the lateral extent of impacted groundwater in the shallow zone, MW13 was installed off-site to the west and MW 14 was installed in the northern portion of the site. Monitoring well MW15 was installed on-site downgradient from the source area to evaluate the vertical extent of impacted groundwater. The results of the subsurface investigation activities are presented in ATC ' s Subsurface Investigation Report, dated December 20, 2006. 2.0 DISCUSSION The following paragraphs summarize the results of the subsurface investigations performed at the project site. 1 . The vadose zone, from 0 to approximately 42 feet bgs, generally consists of clays to approximately 15 feet bgs and is underlain primarily by sands or silty/clayey sands to approximately 35 feet bgs. Silts, clays, and clayey sand are generally present from approximately 35 to 60 feet bgs. Soils consisting of sands with increasing silt and clay fractions with depth were encountered from approximately 60 feet to approximately 80 feet s:\environmental\28349\reports\femibility.doc 2