2010-01-04_REVISION - M1977285 (7)

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Calculation Brief Evaluation of the Sensitivity of Percolation Estimates through Mine Rock Stockpiles on the Simulated Precipitation Rate Table 1 and Figure 1 summarize the results of the HPR sensitivity analysis for the Sunday Mines Group. The corresponding UNSAT-H output files are included in Attachment 2. Table 1. Summantj of UNSAT-H modeling results (all results in centimeters). HPR (cm/hr) Annual Percolation (cm) Annual Runoff (cm) 0.1 2.09E-02 4.70E-02 1 1.30E-03 8.41E+00 2 8.76E-04 9.78E+00 4 8.76E-04 9.78E+00 The evaluation shows that higher simulated precipitation rates (HPR) result in decreased percolation and increased runoff. UNSAT-H does not simulate runoff implicitly. Runoff is estimated as the precipitation rate that is in excess of the infiltration rate. The UNSAT-H model simulates infiltration in a two-step process. First, infiltration is set equal to the precipitation rate during each time step. Second, if the surface soil saturates, the solution of that time step is repeated with the surface node saturated. The resulting flux from the surface into the profile is the infiltration rate (Fayer 2000). If the precipitation rate exceeds the infiltration rate, the excess precipitation is assumed to be runoff. The sensitivity analysis shows that at values of HPR greater than approximately 1 cm/hr, the surface soil saturates and the precipitation rate exceeds the infiltration rate. This leads to runoff, and a resulting decrease in estimated annual percolation. 6.0 Conclusions An analysis of the sensitivity of the UNSAT-H model to the rate of precipitation was conducted to address a DBMS (2009) comment regarding whether the UNSAT-H model accounted for potential "high-intensity, short-duration precipitation events such as thunderstorms". The precipitation rate in the UNSAT-H model is controlled by a variable called HPR, which is the rate of precipitation expressed in centimeters per hour. The evaluation shows that precipitation rates of greater than approximately 1 cm per hour result in saturation of the surface soil, an excess of precipitation in relation to infiltration, and generation of runoff. This decreases the amount of water that is available to percolate through the Topaz Mine development rock pile, and decreases the estimated annual percolation as shown in Table 1 and Figure 1. A HPR value of 0.1 cm/hr. results in negligible runoff, and maximizes the amount of water available to infiltrate the surface of the pile and to potentially percolate through the base of the pile. The UNSAT-H estimates included in CDM (2009) used an HPR value of 0.1 cm/hr. Although this precipitation rate does not account for high-intensity, short-term precipitation events, it does provide a relatively higher estimated annual percolation rate as compared to higher HPR values, based on this sensitivity analysis. C:Welson_work\Denison\UNSAT_H\HPR sensitivily\Sunday HPR Sensitivity talc bfietdoc