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NNEW Quik Stop Market No. 132 Workingo lteto � lature Project No. F7132.93 une , To determine the extent of excavation, removed soil was monitored for the presence of gasoline related compounds with a portable organic vapor meter (OVM) and then followed with soil sampling for laboratory analysis. Vertical excavation in the tank area was continued to a depth of approximately 15 feet where groundwater was encountered. As water began to collect in the excavation, sections of the sidewalls began to collapse and present a safety hazard, so continued downward excavation was halted. Lateral excavation was continued until the presence of gasoline related compounds was not detected with the OVM. Excavation sidewall and bottom samples were collected with the approval of Ms. ° Kasey Foley and Mr. Steven Schneider of the SJCPHS. Laboratory analysis of these samples for TPHG and BTEX indicated that in soil samples collected from 10-11 feet TPHG concentrations varied from non-detectable to a high of 3,304 ppm (sample QS-2) and benzene concentrations varied from non-detectable to a high of 10 ppm (sample QS-2) (see Plate 4 and Table 2). Due to these concentrations excavation ' was continued until groundwater and subsequent side wall caving was encountered at approximately 15 feet. Laboratory analysis of soil samples from 14 to 15 feet (bottom of the excavation) consisted of excavation sidewall samples and excavation bottom samples(see Plate 5 and Table 2). Analytical results of these samples for TPHG varied from non-detectable to a high of 1,600 ppm (sample T3-E) and for benzene from non-detectable -_to a high of 6.5.PPm (sample Tl-E). r.w .__ �It is clear from this data that gasoline related compounds were detected in the bottom of the excavation. As mentioned before the bottom of the excavation was at or below the level i of groundwater and at this depth sidewall caving was observed. From field observations it 01 was noted that a layer of soil 1 to 2 feet thick immediately above the groundwater, at or within the capillary fringe, contained gasoline contaminated soil which was overlain by 13 to 14 feet of clean soil. In view of these observations and the safety hazard presented by y' sidewall caving, it was the opinion of Exceitech (now RESNA) that this contamination could be more effectively and safely dealt with as a groundwater problem than a soil problem and__ 1. i excavation was halted. - —�-- The location with the highest remaining detected concentration of TPHG in the bottom of the excavation was sample location T3-E at 1,600 ppm (see Table 2 and Plate 5). The histone apparent groundwater flow direction has been,an is presently, easterly with a slight southern component (see Plates 2 and 3). Located downgradient from this location (T3-E) are monitoring wells MW-1R and MW-5. MW-1R is designed to monitor the local lower aquifer and MW-5 the local upper aquifer (Exceltech, July 1991). As can be seen in Table 1, TPHG or BTEX have never been detected in groundwater samples collected'from monitoring wells MW-1R (lower aquifer). TPHG and BTEX have been detected in groundwater samples collected from monitoring well MW-5 (upper aquifer). However, as can be seen in Table 1, concentration levels have dropped dramatically from a high of 4,200 ppb on October 15, 1991 to 280 ppb on March 17, 1993. Located further downgradient are 4