2019-01-23_REVISION - M2014043 - Laserfiche WebLink

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Albert Frei & Sons, Inc. Page 1 Hatchery Pit Permit # M-2014-043 Revised 1/16/19 Groundwater Monitoring Plan. The plans are to install slurry wall liner or compacted liner around the 2 excavated areas and then backfill them to present ground levels . To do this there are two different scenarios that have to be addressed in a different manner. They also have common issues that can be addresses in general . We use the term line as a general term to describe either the Slurry wall liner of compacted liner depending on the state being discussed Phase 1 involved installing a Slurry Wall around the area to be mined and then mining the alluvial deposit of gravel . The slurry will be place at grade around the area to be mined and backfilling will start when all of the gravel is removed. Stage 2 is mostly a terrace deposit with the water table located approximately 30 feet below the surrounding surface at approximately 5079 . 8 . The plan is to install a compacted liner around the bottom of the excavated area that is keyed into the bedrock. The high point of the liner is 5083 . 3, at a point hat is approximately 300 feet south of the northeast corner and slopes to 5077 . 3 on the northwest corner of Stage 2 and 5077 . 6 on the Southwest corner of Stage 2 . This places the top of the slurry wall approximately 3 . 5 feet above the average ground water elevation. The bottom of the French drain pipe is installed at a maximum elevation of 5079 .4 and slopes to 5073 . 2 on the northwest corner of Stage 2 and 5073 . 6 on the Southwest corner of Stage 2 . The French drain will maintain the height of the groundwater table at 2013 elevations . The higher liner top will provide adequate height to prevent ground water from entering the filled area when reclamation is complete. When the Certificate of Designation was approved by Adams County they did not include a groundwater monitoring requirement since all fill was to be isolated from the groundwater table. Seven (7) monitoring wells were installed around the mine site on June 14, 2016 (prior to mining commencing) to be used to monitor the height of the groundwater table and to determine the height of the liners . Map Exhibit Gm-1 (revised) shows the locations of the piezometers as installed. From June 2016 to March 2018 samples were collected prior to dewatering starting on Stage 2 to establish background elevations for the water table. The Groundwater Monitoring Data Analysis discussion explains how the data collected will be used to address the proposed mitigation measures if problems occur to the surrounding groundwater table. This site and surrounding properties have existing groundwater and surface water features that impact groundwater elevations due to their proximity to the mine. On the north is an old gravel mine that was backfilled with inert materials similar to what is proposed at this site. Along the west side, the Bull Seep parallels the property line from the SW corner, where the property line turns back to the east, to the northwest corner where it leaves the site. The Bull Seep is a warm water slue that carries water year around. West of the Bull Seep is a slurry wall liner installed by the City of Thornton around a water storage reservoir that runs the entire length of the west side of the permit area. An old gravel lake is located in the northwest part of the mine area and previous owners installed lateral ditches and ponds along the interface of the terrace deposit and the alluvial deposit to divert water that comes to the surface, around the site and back to the Bull Seep. All of these water features have an impact on the current groundwater conditions and the flow of it across, around and thru the permit area.