2021-01-22_REVISION - M1977526 - Laserfiche WebLink

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Ms. Ioana Comaniciu, P.E. June 22, 2015 Page 2 EXISTING SITE CONDITIONS The Stagecoach Pit is an active surface gravel mine currently mined by Albert Frei and Sons. Figure 2 presents a site plan of the as-designed alignment of Stagecoach Slurry Wall with surveyed locations of monitoring wells, the staff gage and locations of the dewatering trench and sumps. The bottom of the reservoir is relatively dry with a series of dewatering trenches running through the site draining to a sump on the northeast end of the Stagecoach Pit. A second sump is located at the northwest end of the pit. The dewatering trench system appears to be collecting the small amount of leakage through the reclamation slope. No noticeable seeps have been observed in the bottom of the pit. LEAK TEST PLAN The proposed testing of the slurry wall at the Stagecoach Slurry Wall will follow the procedures outlined in the publication"State Engineer Guidelines for Lining Criteria for Gravel Pits," August 1999, for lining gravel pits. Inflow and outflow of water from the site will be measured and recorded as required for the 30-day test period. At the end of the test period, the data will be analyzed to determine the amount of unregulated groundwater entering the reservoir. A summary of the findings, as well as monthly data collected during the test will be prepared in a report and submitted to the SEO for review. The following items will be measured during the test period: ■ Observation of Surface Water Any surface water will be monitored for increase or decrease in size during the leak test. The surface area of any significant pooled water will be approximated using surveying techniques. These areas will be used as inputs in calculating evaporation volumes during the test period. ■ Evaporation At the commencement of the leak test,the bottom of the pit is expected to be generally dry. Based upon the results of the observations during the 30-day leak test, a determination will be made as to whether or not the evaporation losses from any of the known seeps and exposed water in the dewatering sump shall be incorporated in the leak test water balance equation. If the dewatering trench and/or the sump contain open surface water during the test, evaporative losses from the length of the trench with open water will incorporated into the water balance equation. Evaporation from any exposed water surfaces in the pit will be calculated based on NOAA Technical Report NWS33.