Laserfiche WebLink
Dewatering Evaluation Report <br />Vana Gravel Operatlon <br />Greeley, Colorado <br />Page 4 <br />thickness of the aquifer above the shale bedrock at the seepage face is roughly five feet <br />(as estimated by visual observations). The radius of influence created by dewatering <br />operations was estimated at 600 feet (estimated from analytical model simulations). <br />Based on these parameters the calculated drawdown at the Carbaugh well resulting from <br />mining operations is 9.27 feet. It is our understanding that the well owner provided a <br />measured static water level of 17 feet below grade (15 feet of saturated thickness), which <br />equates to a drawdown of 11 feet from static conditions (1991 data). The close <br />approximation of calculated water levels to measured values indicates an effective radius <br />of influence of 600 feet is reasonable. <br />The effects of dewatering on groundwater flow within the study area were evaluated by <br />use of atwo-dimensional analytical groundwater flow model (TWODAN). TWODAN is a 32 <br />bit Windows program for the modeling of two-dimensional groundwater flow. The program <br />is accepted by the scientific community for evaluating two dimensional groundwater flow <br />problems and was evaluated in Groundwater, v. 36, No. 3, May-June 1998. <br />A uniform flow field was defined in the model with an unconfined aquifer. The model was <br />set at approximately 624 acres (6800 x 4000 feet). The hydraulic conductivity of the <br />aquifer at the Durham site was estimated at 250 feet per day. The ground surface was <br />modeled as 32 feet above the bedrock surface and the water table surface was set at 26 <br />feet (the bedrock surface was assigned an arbitrary elevation of 100 feet). A static <br />groundwater flow direction of north 70 degrees east was assigned to the model. <br />Ten line sinks were used to simulate dewatering operations. The line sinks were <br />superimposed over the drainage ditches and were assigned a head of 102 feet or two feet <br />above the bedrock surface. Eleven line sinks were used to simulate the effects of the <br />Cache la Poudre River and adjacent settling pond. The line sinks were assigned a head <br />based on published hydraulic gradients relative to an arbitrary reference point assigned a <br />head of 126 feet -the estimated elevation of the water table prior to mining. The reference <br />point for the model was 600 feet south of the southern most seepage face and is depicted <br />