2010-03-18_REVISION - M1977285 (6)

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Response to DRMS COmmBnfS Enuironmenfa/Profecfion Pfan, Denison SundayMineS Group v=-K(w) a? az In the unsaturated zone, the hydraulic conductivity is a function of the pore-water pressure head (y/), which is related to the water content by the soil water retention curve. The variable 0 is the total head and z is vertically up. The variable v is the specific discharge. Pore-water pressures are always negative in the unsaturated zone, although the negative sign is not used in the UNSAT-H model, as shown by initial pressure heads of 1 x 107 cm. The soil water retention curve was defined for the West Sunday and Topaz rock piles with site specific samples analyzed at Advanced Terra Testing in Lakewood, Colorado. In dry soils, the pore water pressure head may be several thousand centimeters at a short distance beneath the surface. At the onset of precipitation, the pore water pressures at the surface are near zero as the soil approaches saturation, which results in a large pressure gradient causing water to infiltrate into the soil. In dry soils, the hydraulic conductivity function is low, but the pressure gradient dominates (Guymon 1993). As the soil profile wets, the pressure gradient decreases, resulting in slower infiltration of water into the soil with time. The UNSAT-H model is a one dimensional model that evaluates vertical infiltration, evaporation, and percolation. If the precipitation rate exceeds the rate at which the precipitation can infiltrate into the soil, the excess precipitation that cannot infiltrate into the soil is classified as runoff. The HPR sensitivity analysis shows that the soil surface saturates and runoff occurs at precipitation rates of greater than approximately 1 cm/hr. The sensitivity analysis shows that higher hourly rates of precipitation result in more runoff and less percolation in the UNSAT-H simulations. Therefore, the low value of HPR used in the initial UNSAT-H modeling provides a conservative estimate of total percolation with respect to the hourly rate of precipitation. Comment 10 Addendum fo theffvdroloeic?valuafion Pg. 2: "it is hypothesized that the mine pool will return to the naturally more reducing conditions present within the aquifer before significant migration of the mine pool occurs " and Pg. 3: "Once mining ceases, mine portals and ventilations shafts will be sealed to prevent ingress of oxygenated air into the underground mine tunnels. " Response: DRMS agrees with the hypothesis regarding ground water f ow and re-equilibration of the mine pool water, but this will depend upon the successful exclusion of oxygenated air from the underground workings. Please provide detailed specifications of the proposed mine seals, and a post-mining monitoring program to ensure success. • Final Sunday Mines RTC_v2.doc