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Work Plan for Additional Subsurface Investigation <br /> Former Helena Chemical Facility <br /> July 13,2020 <br /> Page 6 <br /> process, if warranted. The tenants, the property owner, and the adjacent property owner will approve the <br /> boring locations and confirm the location of underground utilities. <br /> Prior to initiating fieldwork at the Site,all field personnel will familiarize themselves with the Site Specific <br /> Health and Safety Plan(HASP). In addition, a tailgate safety meeting will be held prior to commencement <br /> of fieldwork. The HASP pertains to field activities and includes information on potential hazards expected <br /> to be encountered at the Site,personal protective equipment to be used during field activities,and directions <br /> to and telephone numbers for the nearest hospital.A copy of the HASP is included in Appendix D. <br /> 5.2 MONITORING WELL DRILLING AND INSTALLATION <br /> Condor proposes the installation and sampling of three groundwater monitoring wells. One groundwater <br /> monitoring well will be installed in the area of former soil borings B39, B42, and B44, where highest <br /> concentrations of 1,2,3-TCP in groundwater were documented(MW-6); one groundwater monitoring well <br /> will be installed on-Site east of former soil boring B48 (MW-7); and one groundwater monitoring well will <br /> be installed northeast of proposed wells MW-6 and MW-7 on the adjacent property to the north at 1604 Tillie <br /> Lewis Drive (MW-8). The locations of proposed groundwater monitoring wells are presented on Figure 6 <br /> (Appendix A). <br /> The three proposed monitoring wells will be installed to approximately 25 feet bgs using two-inch outside <br /> diameter(O.D.) PVC casing. The bore holes will be drilled by a state-licensed drilling contractor using a <br /> drilling rig equipped with eight-inch diameter, continuous-flight, hollow-stem augers. Drilling activities <br /> will be performed under the supervision of a Condor geologist in accordance with the provisions of the <br /> EHD permits. Cored soil samples will be collected at five-foot intervals for soil identification. The soils <br /> will be logged in general accordance with the Unified Soils Classification System(USCS). Geologic logs <br /> will be prepared containing field observations including soil type and the presence of staining and/or odor. <br /> Condor anticipates that soil samples collected from depths of approximately 5 feet, 10 feet,and 25 feet bgs <br /> from each well boring, subject to field observations, will be submitted to a state-certified analytical <br /> laboratory for analyses of chemical constituents of concern. <br /> Consistent with existing monitoring wells,the proposed borings for the three groundwater monitoring wells <br /> will be drilled to a depth of approximately 25 feet bgs. The proposed wells will include a screened interval <br /> from 5 feet to 25 feet bgs and will be constructed with 20 feet of 2-inch O.D., Schedule 40, flush-threaded, <br /> machine-slotted PVC screen (0.010-inch slot size). The extended screen length is necessary based upon <br /> field observations during drilling and subsequent depth to groundwater measurements; groundwater is <br /> generally encountered at a depth of approximately 10 to 12 feet bgs during drilling,but groundwater levels <br /> rise inside the bore hole/well casing indicating that groundwater exists in a semiconfined condition.A flush- <br /> threaded PVC bottom cap will be placed at the bottom of the screen section in the well. Blank PVC casing <br /> will be installed above the screens to ground surface. A filter pack of#2/12 Monterey sand will be placed <br /> in the annular space to approximately 1 to 1.5 feet above the screened interval while the augers are in place. <br /> The depth of the sand in the annulus of the well will be monitored with a weighted tape measure while the <br /> sand is being placed and the augers will be gradually retracted to avoid bridging. The well will be partially <br /> developed using swab and bail technique during installation of the filter pack to promote settling of the <br /> filter pack. To form a protective seal,approximately 1.5 feet of bentonite pellets or granular bentonite will <br /> be installed above the sand in the well as the augers are retracted. The bentonite will be hydrated using <br /> potable water and allowed to set approximately 15 to 30 minutes before installing the surface seal.A surface <br /> seal of neat cement will be poured from the surface into the annulus of the well and will extend from the <br /> bentonite to the ground surface.The monitoring wells will be completed with locking caps and traffic boxes <br /> installed flush with the ground surface. Details of the well construction will be noted on the geologic logs, <br /> ��r CONDOR <br />