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Quarterly Monitoring Report,Apnl 2000 . (—anepa's Car Wash(Park Street) October 20,2000 Page 5 groundwater sample collected from monitor well MW-1 TPH-G and all BTEX constituents were detected in the groundwater samples collected from MW-2 and MW-5 A low concentration of ethyl benzene was detected in the groundwater sample collected from MW-3 A low concentration of toluene was detected in the groundwater Isample collected from MW-7 Low concentrations of MTBE were detected in the groundwater samples collected from MW-3, MW-4, and MW-6 The second quarter, 2000, laboratory analytical results indicate that the area of groundwater contamination most impacted by petroleum hydrocarbons is in the northern portion of the site in the vicinity of monitor wells MW-2 and MW-5 5 2.1 DISCUSSION OF BENZENE CONCENTRATIONS The concentration of benzene has decreased over tirne in monitor well MW-2, but there has been an increase in MW-5 (Figure 7, Appendix A) It appears that as the groundwater elevation has risen, the concentration of I benzene in MW-2 has decreased, but in MW-5 has increased It is important to note that both wells are drowned, with 18 to 22 feet of water above the well screen 5.2 2 DISCUSSION OF MTBE CONCENTRATIONS Concentrations of MTBE in monitor well MW-6 have decreased slightly over time(Figure 8, Appendix A) There has also been a reappearance of low concentrations of MTBE in monitoring wells MW-3 and MW-4 10 5 2.3 DISCUSSION OF OTHER ANALYTE CONCENTRATIONS Concentrations of toluene, ethyl benzene, total xylenes, and TPH-G have decreased over time in monitor well MW-2, but all have increased in monitor well MW-5 (Table A, Appendix B) In general, concentrations of analytes have decreased as the groundwater elevation has increased, except in MW-5, which has seen an increase in contamination It is important to note that since all of the monitoring wells are drowned, we are probably not sampling the shallowest water with the highest concentration of contamination Since we are not sampling these high concentrations, the actual concentrations of the plumes have not been measured and are not known According to Figures 4 and 5, the plumes probably extend beyond the site, therefore the current monitoring wells cannot measure the extent of the contaminant plumes b RECOMMENDATIONS Condor recommends that additional investigation be conducted to further characterize the extent of ' contamination at the site, including the installation and sampling of additional monitor wells 1 he existing monitor wells at the site are screened well below the current groundwater table and do not provide sufficient information pertaining to the vadose zone/groundwater iinterface These wells should be replaced with well screens that intercept the groundwater table A work plan describing the recommended installation of four additional monitor wells (MW-8 through MW-1 1) was prepared and submitted to the PHS/EHD in November 1998 The work plan is currently pending approval by the PHS/EHD • L� CONDOR