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i ~ Memo to Wallace Erickson 2 Ntay 3 2001 In addition to parametric sensitivity analysis and assessment of the range of potential water table impacts on a theoretical basis prior to dewatering, ground water modeling results can be calibrated using empirical water table and pumping rate information generated once dewatering has commenced. This calibration provides for adjustments in the predicted ground water system response and implementation of well interference mitigation if needed. The DMG's analysis of the proposed pit dewatering and recommendation for monitoring of the water table during dewatering are described below. The overall conclusion of the DMG is that pit dewatering as proposed will effectively minimize impacts to the hydrologic balance, will not create deleterious well interference, and that this issue need not deter DMG from recommending approval of the Line Camp Pit application. An effort is made to conduct the following discussion in laymen's terms as much as possible to facilitate the review of the DMG's decision by parties objecting to the application. Well yield can be defined as the maximum pumping rate that can be supplied by a well without lowering the water level in the well below the pump intake. For example, if the Robinson Well were to be pumped at a high rate offlow for a sustained period, a point may be reached were the well would dry up and pumping would have to cease until the well bore was recharged with water by a recovering water table. Well interference occurs when the cones of depression in the water table created by proximal pumping wells overlap. This overlap exaggerates the depression of the water table, and the water table depression reduces the well yield. The importance of the well interference effect in the field of ground water resource evaluation has resulted in standard practices to predict and measure the impacts, and has also led to legislative and regulatory protection of potentially affected well owners. The Office of the Colorado State Engineer requires that if anotfter well is located within 600 feet of the perimeter of a dewatering pit, consent of the well owner must be obtained or a hearing before the State Engineer must be conducted to determine if a gravel pit dewatering well permit can be issued. This regulation is an indication of the unlikelihood of damaging well interference for wells that are more than 600 feet apart. In the case of the Line Camp Pit, the Robinson Well is more than 1000 feet from the proposed pit perimeter. However, geologic systems are highly variable, and the potential for well interference outside of the regulatory 600 foot radius cannot be ruled out without further analysis. Theoretical analyses of drawdown are based on an understanding of the physics offlow toward a well during pumping. The fundamental hydrologic parameters are hydraulic conductivity, as discussed above, as well as porosity, and compressibility of aquifer media. From these basic parameters, the derived parameters of irnnsmissivity and sroraiivity have been defined to simplify the analysis of well hydraulics. Pumping induces horizontal hydraulic gradients toward a well resulting in decreased hydraulic head in the aquifer around the well. To evaluate the extent of the reduction in hydraulic head, the fundamental hydrogeologic concepts are input to a boundary-value problem that represents flow to a well in an aquifer, and the theoretical repponse is examined. The mathematics of the drawdown solution for a confined aquifer were developed by Theis (1935) using an analogy to heat-flow theory. The analytical solution for an unconfined aquifer, such as the aquifer at the Line Camp Pit location, was advanced by Neuman (1972, 1973, 1975). It can be observed that water-level drawdowns in piezometers adjacent to