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05-N-20OU Cveo.Sok LLC- d eofogic Envesfgafion for Nunan Cage 6 QV U9 Methyl isobutyl ketone (MIBK) was identified in soil sample MW-3 at 16 to 16'/s feet 1 bgs @ a concentration of 0.006 ppm. Methyl ethyl ketone (MEK) was identified in soil sample MW-2 at 451/2 to 46 feet bgs @ a concentration of 0.017 ppm. Since no trends in concentrations were identified for these chemicals onsite, the source could 1 be from offsite. (See Appendix B fou- Laboratory bato ghcWs) (Seg ?able Q f®? Hydrocarbons En Sail). UL-H—M03 O: The soil borings for wells MW-1, MW-2, and MW-3 were excavated with an eight (8) inch hollow-stem auger drill,with 5-foot long sections to depths of 120 feet, 100 feet and 98 feet below ground surface (bgs), respectively. After the soil borings were completed, bentonite pellets were used to seal the open soil boring from the 1 bottom to 98 feet bgs. One foot of No. 3 sand was placed from 98 to 97 feet bgs. A twenty (20) foot long 0.01 inch PVC screen with a seventy-seven (77) foot long blank casing was lowered into each open soil boring which had an eight (8) inch inner diameter. The blank PVC casing was then extended from 77 feet into the well box where it was cut below the AC surface. No. 3 Silica sand was then placed in the annular space from the bottom of the borehole to within two (2) feet above the top of the screen (i.e. from 77 to 75 feet bgs). A 3 foot thick bentonite seal was then placed on top of the sand from 75 to 72 feet bgs. A Type II Cement bentonite grout was then placed to within approximately 1 1/2 feet of the top of the surface cover. A continuous concrete pour was then made on top of the grout to the surface where it was finished with a 3 inch high concrete apron around a Boart Longyear well box and locking well cap (Sac rviguvc a ftv Mail QDnatmetion Details). Well MW4 was sealed with bentonite from 120 to 100 feet bgs and MW-2 was sealed from 100 to 98 feet bgs to seal the opera borehole from the wells which were constructed from 98 feet bgs to the surface. A 0.01 inch slotted PVC screen was used to accommodate the coarse sands encountered in the perched aquifer zone. Although groundwater was initially encountered at a depth of 82 feet bgs in MW-1 during excavation, it was not known if a perched zone had been encountered. Further exploration to 120 feet bgs smeared the zone shut so that the static water level erroneously appeared to be at a depth of 102 as measured after stabilization of the water table had occurred. When the auger string was pulled, after the bridging of the bentonite seal during well construction,the static water level was confirmed to be at approximately 82 feet bgs. Since the thickness of the perched zone was estimated to be between 80 and 90 feet bgs after the excavation of MW- 1, a screen length of 20 feet,was placed between 97 and 77 feet bgs to account for potential rises in the water table and greater perched aquifer thicknesses which could potentially be encountered at other locations to be drilled onsite. A 20 foot screen length was chosen to minimize the potential for dilution of the water samples to be collected yet was considered to be long enough to intercept potentially thicker hydrostatigraphic regimes. Continuous coring within the perched i zone encountered in MW-2 and MW-3 later revealed that the zone was no greater ■ than six(6) feet thick between 80 and 86 feet bgs. In order to maintain the consistency of future well sampling and measurement data between the three (3) '0 groundwater monitoring wells, MW-2 and MW-3 were similarly screened between 97 and 77 feet bgs. I 05-03-20O V Oeo.Soly, LLC-Hydregeologic kwastlgaflon for Nunan Page 4 of TO