2008-04-15_REVISION - M1980244 (325)

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Grassy Valley hydrologic conditions 33 Environmental Protection Agency, 1994) was used to estimate the percentage of precipitation to the ECOSA becoming runoff during the post-closure period based on the properties of the low permeability cover, the grading profile, the vegetation cover, the properties of the overburden material, and site-specific climate data. It is estimated that the runoff rate will be about 13.5 percent of precipitation.. This runoff rate is evaluated further in the sensitivity analysis, Section 4.4.9. Infiltration It is assumed that the majority of infiltration to the alluvium in Grassy Valley becomes shallow interflow and reports to Grassy Creek. An infiltration rate to the shallow alluvium of 0.5 inches per year (2.6 percent of precipitation) was used in the base case modeling simulations. Infiltration that does not route to shallow interflow and to surface flow in Grassy Creek infiltrates the diatreme or granite bedrock. Infiltration rates of 15 percent and 30 percent of precipitation to the granite and diatreme bedrocks, respectively, were used in the current conditions modeling simulations. The effect of changes to these estimated bedrock infiltration rates is evaluated in the sensitivity analyses, Section 4.4.9. Although water is expected to infiltrate into the ECOSA during mining, no throughflow is expected. The ECOSA is not expected to receive enough precipitation to reach field capacity during build out activities. Infiltration is expected only to raise the moisture content of the stockpiled overburden. In the post closure period, precipitation which does not run off the ECOSA will either evapotranspire or infiltrate the ECOSA. The demands of ET are met first in the model, and with the enhanced runoff from this facility, there is a limited amount of water available to meet the potential ET requirement. Therefore, simulated actual ET for the ECOSA is often significantly lower than potential ET. Any water in excess of runoff .and potential ET is assumed to become infiltration to the ECOSA. Post-closure infiltration to ECOSA is approximately 0.25 inches of precipitation of the 18.9 inches falling on the whole of Grassy Valley, or equivalent to about 1.8 inches of water per year falling onto the ECOSA. The HELP model also was used to estimate the percentage of precipitation becoming infiltration to the backfilled WHEX portion of the East Cresson Mine following closure. The HELP model analysis was based on the properties of the growth medium cover, the grading profile, the vegetation cover, and the properties of the backfill material. It is estimated that the runoff rate will be about 1 percent of precipitation and the infiltration rate will be about 17 percent of precipitation. Areas Areas were delineated and calculated in GIS for use in water balance calculations of precipitation, ET, runoff and infiltration. In subsequent sections of this report, these areas are referred to as `segments' of Grassy Valley drainage. Table 4.4 presents the calculated areas used in the water balance for current, mining and post-mining conditions. • 2736 Cripple Creek & Victor Gold Mining Company Water Management Consultants