2012-09-06_REVISION - M2008070 (25)

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Design and Operations Plan October 24, 2011 Western Gravel E & P Waste Disposal Facility Page 38 of 51 3.10 GROUNDWATER/SURFACE WATER MONITORING The proposed landfill facility will be located atop low permeable Wasatch Formation. However, because the White River is located proximal to the facility, proposed cell designs (e.g. double liner, LDS, LCRS) exceed those required under Regulation for a solid waste facility in order to provide a higher level of environmental protection. Because of designed engineering controls and existing site conditions (e.g. Wasatch Formation properties, bedrock bedding dipping away from the White River to the south) adverse environmental impacts to the White River are considered remote. Based upon published information, regional bedrock groundwater ultimately discharges to the White River Alluvium. Because of the dilution factor in the White River aquifer, it is unlikely that groundwater monitoring along the river floodplain would identify a release from the landfill. Therefore, in accordance with Rio Blanco County's request, a surface water monitoring program will be implemented in order to help determine if potential leakage from the landfill located above is migrating through terrace bedrock and impacting the White River. Any surface water monitoring conducted will be based upon Rio Blanco County requirements and will not be conducted in association with CDPHE solid waste regulations. Based upon the facility design (e.g. double liner system, LDS, LCRS) and geotechnical evaluation results, it appears that facility impacts to bedrock are remote. However, in order to be conservative, wet /dry wells will be constructed at locations more likely to intercept cell leakage to bedrock. In the event that liquid enters a wet/dry well, samples shall be collected in order to help determine the liquid source. A waiver from CDPHE solid waste Regulations Section 2.2 - Ground Water Monitoring is requested in accordance with Section 1.5 of the Regulations. Waiver request information is provided in Appendix A. 3.10.1 Wet/Dry Wells In order to monitor for potential landfill cell leakage and/or impacts to groundwater, five wet/dry wells (WD- 1 through and WD -5) will be installed along the south side of the landfill and inside the COD. Because bedrock bedding dips to the south, wet/dry wells located along the south side of landfill cells are likelier to intercept potential leakage. Wet/dry wells will be phased concurrent with cell development and filling. A wet/dry well will be located within 50 feet of each LDS manhole at the assumed bedrock down dip side of each cell. Five conceptual wet/dry well locations are shown on Figure 3. Based upon the geotechnical evaluation discussed in Section 2.4.2, wet/dry wells will be screened in unsaturated bedrock. The intent is for liquids potentially leaking into bedrock from a cell to be intercepted by a wet/dry well that is used as a cell leak indicator. The wet/dry wells will provide an additional level of environmental protection. The well screens and well seals shall be placed in a manner where surface water will not migrate into the well. In general, a well screen shall be placed from approximately five feet above the lowest point of cell base to approximately five feet below the cell base. General well construction details include a 10 foot long, 2 -inch or 1 1/2 inch diameter, 0.10 slot polyvinyl chloride (PVC) well screen and solid PVC riser pipe including flush - thread joints. Filter sand will be placed around the well screen and extend approximately 1/2 foot below the well screen and approximately 1 1/2 feet above the screen. A bentonite seal will extend from the top of filter sand to approximately 2 feet bgs. A cement or concrete surface seal will be placed from the top of bentonite seal to ground surface. An approximately five foot long protective steel well casing shall be installed over the wells and extend into the surface seal. Protective steel casing will include a locking lid to inhibit unauthorized access. A cement or concrete pad approximately three feet square and 3 1/2 inches thick will be placed around the protective steel well casing. Wet/dry well monitoring will be conducted on a quarterly or semi - annual basis and may be conducted concurrent with landfill gas and/or retention pond monitoring activities. Each wet/dry well will be monitored NWCC, Inc.