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• shallow aquifer. Background ground water quality tests performed on <br /> ground water from upgradient and downgradient wells in December 1992, <br /> indicates that EPA Secondary MCLs were exceeded by the following <br /> constituents: chloride, sulfate, aluminum, iron, and manganese. <br /> 3.0 BACKGROUND SITE CONDITIONS <br /> ' 64 onsite and 12 offsite soil borings have been drilled at the site between 1988 and 1993, <br /> and 12 onsite test holes were excavated by backhoe in 1989. The maximum concentrations <br /> of total petroleum hydrocarbons as gasoline (TPH-G) and total petroleum hydrocarbons <br /> as diesel (TPH-D) detected at the site prior to remedial efforts were 1,900 and 8,200 parts <br /> per million (ppm), respectively. <br /> • Eight onsite and 4 offsite ground water monitoring wells are present at the site. <br /> • Onsite, adsorbed-phase hydrocarbons were excavated to below the standing water level in <br /> July 1990. Greater than 6,000 cubic yards of soil were hauled to appropriate disposal <br /> facilities. Approximately 7,300 cubic yards of clean, imported fill was then compacted <br /> and brought to grade. Figure 2 indicates the approximate limits of the area of excavation. <br /> 0 Dissolved-phase hydrocarbons have historically been detected in Monitoring Wells AW-3 <br /> • and AW-9, along the northern margin of the site. Maximum concentrations of TPH-G, <br /> TPH-D, and benzene for AW-3 were 4,200, 10,000, and 0.5 parts per billion (ppb) in <br /> October 1990, and 72, 65, and 15 ppb in ground water samples collected from AW-9 in <br /> April 1992. <br /> • Free product has not been observed in wells at this site. <br /> 4.0 FIELD ACTIVITIES <br /> 4.1 DRILLING AND SOIL SAMPLING <br /> On March 25, 1993, three ground water monitoring wells (AW-10 through AW-12)were drilled <br /> to a depth of 25 fbg. Ground water was encountered at approximately 11 fbg. The wells were <br /> installed to a depth of 25 fbg (Figure 2). The ground water monitoring wells were developed <br /> using a surge block and bailer immediately following installation of the sand filter pack, and <br /> prior to the placement of the bentonite seal. During surging the sand filter packs were observed <br /> to settle an average of 1.2 feet. <br /> Soil samples were collected continuously from AW-10, AW-11, and AW-12 with a 5-foot core <br /> barrel. Refer to Appendix A for details regarding general field procedures, boring loss. and <br /> ground water monitoring well construction details. <br /> 3 <br />