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<br /> <br /> <br />In order to quantify this hydrologic variable a[ the Trapper Mine, a groundwater <br />recharge study was done. Based on the following data, mass balance calculations <br />were used to estimate monthly groundwater recharge rates: <br />1) average monthly precipitation data Eor Craig, Colorado (30 years, 1974- <br />1970); <br />2) an infiltration rate measured on the compacted spoil benches using a dou- <br />ble-ring infiltrometer; <br />3) surface runoff estimates using conservative runoff coefficients (ranging <br />from 0.0 to 0.25) which underestimated runoff; <br />4) calculated evapotranspiration rates ranging from 0-12 cm/month (total <br />annual evapotranspiration was about 64 cm); and <br />5) a calculation of soil moisture storage in 4 feet of soil (thickness of eva- <br />po[ranspira[ion zone). <br />The effects of water storage as frozen precipitation, frozen soil profiles, and <br />spring snowmelt/runoff were simulated by varying runoff coefficients. Because of <br />the mass balance relationships present at [he mine, frozen precipitation can be <br />stored as soil moisture in the model. While this creates monthly water balances <br />within the model which appear unrealistic, annual percolation (groundwater <br />recharge) is properly simulated. It is the annual results which are significant <br />to [his study. <br />The results of these initial calculations as presented in Table 4.3-9 showed [hat <br />no moisture Erom precipitation will exit [he bottom of [he evapotranspiration <br />zone during a normal year, using monthly precipitation averages. Average monthly <br />precipitation is no[ sufficiently greater Chan the evapotranspiration Co exceed <br />the soil moisture storage capacity and produce percolation. The calculated <br />annual percolation rate was zero. <br />4-46 <br />