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Mr Kyle Christie Project 20805-144 004 July 26, 1996 Page 2 In November 1991, RESNA observed the removal of five USTs and associated piping and collected compliance soil samples from the UST cavity and piping trenches The former UST cavity was not overexcavated and soil and fill material removed concurrent with the UST removal was used to backfill the cavity Soil containing residual petroleum hydrocarbons was also left in-place in the piping trenches New USTs and piping were installed in November and December 1991 RESNA directed additional site investigation activities in October 1992 Eleven soil borings were drilled to depths of 36 to 63 feet bgs Four borings were converted to vapor extraction wells and four borings were converted to groundwater monitoring wells Groundwater was encountered in the four borings converted to monitoring wells at a depth of approximately 54 feet bgs Boring and well locations are shown on Figure 2 Quarterly groundwater monitoring events began in October 1992 Over the duration of the monitoring program, groundwater samples collected from all monitoring wells (MW-1 through MW-4) were below laboratory detection lirruts for total petroleum hydrocarbons as gasoline (TPHG), benzene, toluene, ethylbenzene, or total xylenes (BTEX) with the exception of one sampling event conducted in October 1993, in which ethylbenzene was detected at a concentration of 0 7 micrograms per liter (MW-3) Historical groundwater analytical results are presented in Appendix A The depth to groundwater recorded over the duration of the quarterly monitoring program ranged from approximately 50 to approximately 54 feet bgs (see Appendix A) Perched groundwater conditions may explain the difference between the depth to groundwater encountered during the October 1990 investigation and the depth groundwater was encountered during all future investigations and monitoring Only three of the five soil borings encountered groundwater during the October 1990 investigation, and in those three bonngs groundwater was encountered on top of a silt unit TPH MASS CALCULATIONS The TPH mass calculations are based on estimated lateral and vertical extent of soil impacted by TPHG The lateral and vertical extent of impacts were evaluated using the soil sample analytical results obtained from the former UST cavity, former piping trenches, and exploratory borings TPHG isoconcentration maps were prepared for four depth intervals based on the distribution of petroleum hydrocarbon impacts and soil types • Two to 6 feet bgs for evaluation of near surface impacts, generally related to the piping trenches Soil type generally consists of sand to silty sand is 5AOX\08051440 DOC-961dms 1