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Memo to Wallace Erickson 8 Mav 3. 2001 availability of drill logs, DMG cannot make an independent assessment of the potential impact of drawdowns of different magnitudes on the yield of the Robinson Well. For this reason, and for the other reasons listed here, the DMG discussed the need for water table monitoring with the Applicant, and the Applicant has agreed to install a piezometer near the south property line. The piezometer, which will be installed to bedrock if feasible, is needed to better assure that the prevailing hydrologic balance is protected for the following reasons: • The yield of the Robinson Well is not known and the length of the yielding formation penetrated by the Robinson Well is subject to question. • The completion report generated by installation of the piezometer will provide additional information on the subsurface stratigraphy and water table elevation proximal to the Robinson Well. • Baseline data will be collected on natural fluctuations in water table elevation during the period of time prior to initiation of dewatering in the proposed pit. • Water table elevation readings in the piezometer following initiation of dewatering will allow the DMG and the Applicant to calibrate their drawdown models. • Monitoring of water table elevation is needed due to the uncertainties in assumptions inherent in ground water modeling, and the possibility that the drawdown predicted by the models will be inaccurate due to unanticipated hydrogeologic complexity. • The water table data generated by the piezometer will allow DMG, the pit operator, and potentially affected neighbors to evaluate the magnitude and significance of off site impacts, and to design, implement, and track the effectiveness of mitigation plans if unacceptable impacts are detected. • Water table data from the piezometer will provide a defense for the pit operator against unsubstantiated claims of off site damage. As alluded to previously, DMG believes there may be processes influencing the hydrogeology in the pit area that would limit drawdown in the Robinson Well to less that those predicted by the Neuman solution. The primary process at work would be aquifer recharge. The Neuman solution assumes a flat aquifer with no recharge sources. In reality, a drawdown cone will extend only as far as it takes to intercept recharge that equals the pumping rate. At that point, the drawdown cone will stabilize. The primary recharge source in the vicinity of the Robinson Well is the Dolores River. The attached figure 2(a) illustrates the impact of a recharge source on the shape of a cone of depression, which is obviously to limit the magnitude of drawdown. The fact that the alluvial aquifer pinches out against bedrock west of the highway (permit application, revised Exhibit G, page 27-B) creates the opposite effect on the shape of the cone of depression. This is illustrated on the attached figure 2(b). However, fire amount of water flowing into the cone of depression from the river more than counterbalances any enhanced depression caused by the bedrock boundary to the west. Recharge of the aquifer will also come from the discharge of water pumped from the pit back into the river at a location upstream of the Robinson Well (the drawdown models assume that the pumped water is completely removed from the system). The drawdown model does not consider other sources of recharge such as precipitation and irrigation. By neglecting all these sources of recharge, the drawdown model tends to be conservative by significantly overestimating the magnitude of drawdown caused by pit dewatering.