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� 0 i //VSEa GEOMATRIX Based on our review of the chromatograms, "detections"of TPH in the grab groundwater samples GB-1 and GB-4 are likely the result of interference from petroleum-affected soil particulates (<0.7 microns in size for filtered TPHd analysis) entrained within the sample. Because soil particles affected by crude oil may have been present in the sample due to the field techniques used to collect the groundwater samples, two temporary monitoring wells were ' proposed to SJCPHS for the purpose of confirming the detections. The detection of PAHs may ' also be due to petroleum-affected soil particulates present in the grab groundwater samples; however, concentrations are well below screening criteria(i.e., Maximum Contaminant Levels [MCLs] or U.S. EPA Preliminary Remediation Goals [DRGs], 1998). ' 3.0 GROUNDWATER INVESTIGATION On 11 September 1998, two temporary monitoring wells (TW-1 and TW-2)were installed in the same approximate locations of soil borings GB-4 and GB-1 (Figure 2). Prior to field activities, Geomatrix obtained a soil boring permit from the SJCPHS and notified Underground Service Alert. All drilling and soil sampling equipment was steam cleaned prior to use. Well ' bores were advanced to depths of 14 feet below ground surface (bgs) using hollow-stem auger drilling equipment with 8-1/4-inch augers; the borings were continuously cored for lithologic control. Petroleum affected soil was observed in the boring for TW-2 from approximately 4.5 to 12 feet bgs. The borings were completed as temporary groundwater monitoring wells constructed using 2-inch-diameter, Schedule 40 flush-threaded PVC casing and factory slotted well screen with 0.020-inch slots. Each well was screened across the water table between the ' depths of 4 and 14 feet. A sand filter pack(Lonestar#2/16) was placed from the bottom of the well screen to approximately one half foot above the top of the well screen. A one-foot ibentonite seal was placed above the sand filter pack. Cement grout was used to fill the remaining annular space. The well was capped with a locking waterproof expansion cap, and ' completed with a locking "stovepipe" well cover. Boring logs are presented in Appendix B. The temporary wells were developed and purged on 14 September 1998 until water quality parameters stabilized. Prior to purging, depth to groundwater was 7 feet bgs. Although a petroleum sheen was observed in TW-2 prior to sampling, TW-2 was sampled at the request of ' \\sf3\deptdata\Doc Safe\4780.01\cisrrpt.doc 3