2011-08-04_REVISION - M1999006 - Laserfiche WebLink

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Dewatering Evaluation Report Varra Gravel Operation Kurtz Pit Platteville, Colorado Page 3 direction of due north was assigned to the model. A bedrock elevation was assigned a relative elevation of 10 feet. Eight line sinks were used to simulate dewatering operations. The line sinks were superimposed over the perimeter of the area identified as Tract B and assigned a head of 15 feet, or five feet above the bedrock surface for a total drawdown of 15 feet over the entire mine area. Four line sinks were used to simulate the settling pond (identified as 0.95 acre pond). The Cogburn Tributary is a perennial stream and was not accounted for in the model. Possible inflows from Last Chance Ditch were not accounted for and the model should provide a conservative prediction of affects on local hydrology. The settling pond line sinks were assigned a head of 40 feet or ten feet above the water table. Steady state simulations were then ran and the results are depicted in Attachment A. The head distribution map generated by the model shows that the gravel quarry is a groundwater sink for the majority of the model area south of the St. Vrain Creek, which is a groundwater divide. The model results indicate that the water table will be depressed by eight to ten feet below the wetlands located some 400 feet directly west of the settling pond. The model was then modified to simulate an unlined drainage ditch north of Tract B from the western edge of the wetlands (just east of Cogburn tributary) to the settling pond. The ditch was simulated by adding an additional line sink with an assigned head of 38 feet (eight feet above the water table). The model predicts the settling pond in combination with a drainage ditch completely abates any drawdown beneath the wetlands. The head distribution map of the second model run is also presented in Attachment A, along with MODFLOW model runs.