2018-03-09_ENFORCEMENT - M2004031

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' EXHIBIT 22 ' and sluice gates to control flows to the drain. The drain sluice gates will be closed when the Fulton Ditch is releasing water to the Bull Seep. This will allow the historic flow pattern of the Bull Seep to continue ' and prevent the water from entering the City of Thornton reservoir. When the releases from the Bull Seep end the sluice gates will be opened and the lingering Bull Seep surface water that causes the rise in groundwater levels will be drained back to the Bull Seep via the City of Thornton reservoir. Stop logs are included in the design to allow the "fine-tuning" of the water levels and flow rates to the drain. This is necessary to maintain the jurisdictional wetlands in the area of the Orr property. Tetra Tech evaluated the possibility of preparing a groundwater model to aid in the design of the Plan. However, any practical groundwater model would not have sufficient accuracy and precision to aid the design. Given that the elevations of pre-mining groundwater within a foot of the ground surface adjacent to the Orr property, the accuracy of the model simulations would not have been sufficient to predict the changes in water level from the Plan. Consequently, Al proposes to use an observational approach that ' includes monthly motoring of groundwater levels and staff gage measurements. Existing staff gauges and Monitoring wells HZMW-2, HZMW11A, and HZMW-11B will be monitored. ' BACKGROUND Tetra Tech has reviewed the hydrologic information available for the area of the Orr property to develop a plan that will restore the hydrologic regime, to the extent practical, to the pre-mining condition. The available information includes: water level information for the monitoring wells adjacent to the Orr ' property, the location of the 100-year floodplain, the presence of wetlands, surface water drainage features and drainage easements. The available information indicates that the western 1/2 to 2/3 of the Orr property is within the 100-year floodplain and the property likely contained wetlands prior to construction of the slurry wall (Figure 1). The Bull Seep is a major source of surface water and groundwater in the area of the Orr property. The Bull Seep is sourced from a turnout on Fulton Ditch south of 1041h Avenue and from a turnout at the southwest property line of the Orr property (See Figure 1). ' Water is generally released from the Fulton Ditch to the Bull Seep during winter months, but the water may be released at any time of year depending on the operating of the ditch. The Bull Seep water has historically flowed adjacent to the Orr property and contributed to the formation of jurisdictional wetlands. Likewise, seepage losses from the Fulton Ditch and groundwater from the uplands to the east likely contributed to the formation of the jurisdictional wetlands. ' Groundwater Levels Figures 4 through 6 are hydrographs of water level measurements in monitoring wells HZMW-2, HZMW- 11A and HZMW-11 B, respectively. These wells were chosen because they are the closest wells to the ' property and are most representative of historic, current and future groundwater levels. Water level measurements in well HZMW 3 were reviewed for this study. However, well 3 is east of the Fulton Ditch and much further from the slurry wall and the Orr property. Consequently, the data from this well is less useful than wells 2 and 11A and 11B and not presented in this study. The historic period of record for wells 2, 11 A and 11 B is February 2004 to May 2007, Recent ' measurements were made on June 7, 2016. The hydrographs have been annotated to show: the ground ' 2 Tetra Tech