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Revised Work Plan for Additional Site Investigation Raymond Investment Corporation Former Helena Chemical Facility February 27,2018 Page 8 5.5 MONITORING WELL DRILLING AND INSTALLATION The proposed monitoring wells will be installed using two-inch outside diameter (0.D.) PVC casing. The monitoring well borings will be drilled using a drilling rig equipped with eight-inch diameter, continuous- flight, hollow-stem augers. Drilling activities will be performed under the supervision of a Condor geologist in accordance with the provisions of the EHD permits. The soils will be logged according to the USCS. Geologic logs will be prepared containing field observations including soil type and the presence of staining or odor. Condor anticipates that soil samples will be collected from a depth of approximately 10 feet bgs for laboratory analyses, subject to field observations. Consistent with existing monitoring wells, the proposed borings for MW-4 and MW-5 will be drilled to a depth of approximately twenty-five feet bgs. The proposed wells will include a screened interval between five and twenty-five feet, and will be constructed with 20 feet of 2-inch 0.D., Schedule 40, flush-threaded, machine-slotted PVC screen (0.010-inch slot size). A flush-threaded PVC bottom cap will be placed at the bottom of the screen section in the well. Blank PVC casing will be installed above the screens to ground surface. A filter pack of #2/12 Monterey sand will be placed in the annular space to approximately one and one-half feet above the screens while the augers are in place. The depth of the sand in the annulus of the well will be monitored with a weighted tape measure while the sand is being placed and the augers will be gradually retracted to avoid bridging. To form a protective seal, approximately two feet of bentonite chips will be installed above the sand in the well as the augers are retracted. The bentonite will be hydrated using potable water and allowed to set at least 30 minutes before installing the surface seal. A surface seal of neat cement will be poured from the surface into the annulus of the well and will extend from the bentonite to the ground surface. The monitoring wells will be completed with locking caps and traffic boxes installed flush with the ground surface. Details of the well construction will be noted on the geologic log, including the depth intervals of the screen, sand pack, and seals. A generalized well construction diagram is included as Figure 7 (Appendix A). 5.6 FIELD QUALITY ASSURANCE QUALITY CONTROL In addition to the primary soil and groundwater samples collected and analyzed as described above, field QA/QC samples will also be collected. Duplicate soil and groundwater samples will be collected at a ratio of one duplicate sample for every twenty primary samples. Thus, Condor anticipates that two field duplicate soil samples and one duplicate "grab" groundwater sample will be collected during the investigation. In addition to the field duplicate samples, two equipment rinsate samples will be collected and transported to the laboratory with the other samples under the same chain of custody procedures. One equipment rinsate sample will be collected from the northern portion of the Site and one equipment rinsate sample will be collected during drilling of the step-out borings near well MW-3. All field duplicate and equipment rinsate samples will be labeled in a manner similar to the primary soil and groundwater samples and submitted blind to the laboratory for analysis of constituents of concern. A travel blank will accompany each shipping container that contains groundwater samples. The field duplicate soil samples will be collected within one foot of each corresponding counterpart, and the duplicate groundwater sample will be collected immediately after its counterpart using the same tubing/check valve apparatus. If the field duplicate laboratory analytical result is within 60 percent of the result of its primary counterpart, then other primary analytical results should be considered representative of subsurface conditions. CONDOR