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South Platte Decision Support System Groundwater Component Task 33 Field Study Work Plan <br />for Alluvial and Bedrock Well Installation, Testing, and Water Level Monitoring <br />3.1.2 Alluvial Well Drilling and Lithologic Logging <br />CDM's onsite representative will be present for at least 75 percent of all drilling <br />activities and will maintain a log of all significant well construction and testing <br />activities according to the Procedure for Field Documentation and Quality <br />Assurance/Quality Control presented in Appendix B. Drilling and well installation <br />will be performed by a qualified drilling subcontractor under the supervision of a <br />registered Colorado professional engineer or a professional geologist according to <br />CRS 34-1-201 (3). <br />As was done in Phase 1 of the SPDSS, CDM recommends the use of direct push <br />technology (DPT) for the installation of 1-inch diameter observation wells as <br />described in Appendix C1. Lithologic characterization can be performed using a <br />combination of discrete-depth core sampling and electroconductivity (EC) logging as <br />described in Appendix C2. The DPT method can be applied in unconsolidated <br />(alluvial) sediments where large cobbles or boulders are not encountered in the <br />subsurface. Consequently, in the event that existing driller's logs indicate presence of <br />cobbles in the well completion zone, or DPT is unsuccessful at a location, hollow-stem <br />auger (HSA) equipment will be used. Observation wells will be completed with an <br />estimated maximum depth of 110 feet, and an anticipated average depth of 60 feet. <br />The choice of drilling method will be determined by CDM after review of geologic <br />logs from nearby wells, driller's recommendations, and best professional judgment. It <br />is anticipated that of the 27 wells being installed by CDM, 25 will be installed using <br />DPT and 2 will be installed by HSA. If site-specific conditions allow, all wells will be <br />installed by DPT. <br />Each borehole will be logged and sampled and soil samples will be visually classified <br />and described by a qualified geologist or engineer. Lithologic logging of materials <br />encountered during drilling will be performed using EC. Cores will be photographed <br />for additional documentation. In the event that HSA is used, split-spoon samples will <br />be collected on 10-foot intervals to allow determination of general lithology. <br />An EC log will be performed and examined in the field to guide lithologic sampling <br />and well screen placement. After determination of the depth representative of the <br />aquifer materials in the well's sensing zone, a second boring will be advanced <br />adjacent to the EC boring. Depending on site conditions and depth, either a 2- or <br />4-foot long discrete core sampler will then be advanced to the depth of the well's <br />sensing zone, opened, and advanced to obtain a sample for detailed lithologic <br />description and confirmation of the EC log interpretation. The procedure for discrete <br />sample collection is presented in Appendix C3. Up to two core samples per site will <br />be obtained in this manner. <br />At locations where DPT is used to install the wells, electroconductivity (EC) logging <br />will be attempted to a depth of approximately 20 feet below the proposed observation <br />well's total depth, up to a depth of 80 feet below ground surface (bgs). At wells with <br />intended depths greater than 80 feet bgs, EC logging will only be attempted to the <br />~.~.-~ <br />10 <br />Z:\SPDSS\10-CDM DELIVERABLES\PHASE 2 FIELD WORKPLAN\FINAL DRAFT\FINAL SPDSS TASK 33 2 WORK PLAN.DOC <br />