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4.0 GROUNDWATER CHARACTERIZATION INFORMATION 4.1 SUMMARY OF FINAL QUARTERLY MONITORING EVENT In response to concerns expressed by PHS/EHD, CRC directed SECOR to perform a twelfth and final quarterly groundwater monitoring event The sampling work was performed on October 24, 1994 and observed by PHS/EHD staff In an attempt to lower the water level in the onsite monitoring wells below the screened intervals during purging, an alternative technique was employed This technique involved the following activity performed by Lawrence Tank Testing, ' of Downieville, California • After SEVOR staff checked for floating product and depth to water, Lawrence personnel assembled, without solvent glues, a Iength of clean, Schedule 125 PVC plastic pipe for use downhole as "vacuum-lift" casing • The vacuum ng blower is started and lifts water at an average rate of approximately 1 to 2 gallons per minute • When the graduated purge receiving tank has collected 10 gallons, a grab sample is collected, the piping is removed, and the water level is measured with a sounder ® While the purge water parameters are being determined, the system is restarted and repeated until 30 gallons (approximately six or more casing volumes) are recovered Measurements performed in MW-2 immediately after purging indicated that the water level had ' been lowered at least 0 1 ft below the top of the well screen All other wells monitored immediately after purging indicated that water levels were lowered only a few hundredths of a foot below the static water level as a result of this alternative technique Groundwater level measurements for this event are summarized in Table 5 The alternative purging technique was useful for the purpose of removing substantially more wellbore water prior to sampling than previous monitoring events The collected samples were promptly transported under chain of custody to QUANTERRA, a certified analytical laboratory ' No BTEX compounds were detected in any of the six monitoring wells or the MW-2 duplicate sample, MW-7 The VOC 1,2 DCA was detected in MW-2 and MW-5 PCE was detected only in I MW-4, -5 and -6 The laboratory analytical results including chain of custody documentation for this event are provided in Appendix E and the results are also summarized on Table 6 ISECOR International Inc (SECORO) CRC Env¢onmental Risk Management,Inc N0026-OOI 01 add 13 November 30 1994 I