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waste mass, dissolves soluble constituents of the waste and either <br /> percolates into the ground or runs off to surface drainage. The <br /> volume of leachate generated depends on the characteristics of the <br /> precipitation event, antecedent moisture conditions, the ability of <br /> the waste to hold moisture and the evaporation of moisture from the <br /> waste following the precipitation event. The amount of water which <br /> percolates into a soil with vegetative cover can be calculated from <br /> a knowledge of the monthly precipitation, evapotranspiration and <br /> soil moisture storage by using the water balance method proposed by <br /> Thornwaite(1) . In general , due to the low rainfall (12 inches/year) <br /> and high potential evapotranspiration (greater than 30 inches/year) <br /> in the Paradox Valley, no net infiltration occurs. <br /> In considering the production of leachate from waste rock, prior to <br /> reclamation, no transpiration due to plant respiration will occur. <br /> Rather, any return of water to the atmosphere will occur as evaporation <br /> from the moisture-saturated waste rock. Analyses of bare soils have shown <br /> that the evaporation rate is highest after initial saturation and drops <br /> off rapidly as the water content of the soil decreases (Philip 2) . <br /> Infiltration occurs when the soil receives water in excess of its <br /> moisture-bearing capacity. This moisture-bearing capacity is a <br /> function of the percent of fines in the soil , and of its composition <br /> (clay, sand, loam, etc. ) . At the present time the exact character- <br /> istics of the waste rock are not known, but a conservative estimate is <br /> that 50% of the annual precipitation falling on the waste rock would <br /> percolate through the pile and become leachate. After reclamation, <br /> i <br />