WATER INFILTRATION INTO EXPOSED FRACTURED ROCK SURFACES

Fractured rock media are present at many existing and potential waste disposal sites, yet characterization data and physical relationships a re not well developed for such media. This study focused on water infi ltration characteristics of an exposed fractured rock as an approach f or defining the upper boundary condition for unsaturated-zone water pe rcolation and contaminant transport modeling. Two adjacent watersheds of 0.24 and 1.73 ha with slopes up to 45% were instrumented for measur ing rainfall and runoff. Fracture density was measured from readily ob servable fracture traces on the surface. Three methods were employed t o evaluate the rainfall-runoff relationship. The first method used the annual totals and indicated that only 22.5% of rainfall occurred as r unoff for the 1990-1991 water year, which demonstrates a high water in take rate by the exposed fracture system. The second method employed t otal rainfall and runoff for individual storms in conjunction with the commonly used USDA Soil Conservation Service curve number method deve loped for wide ranges of soils and vegetation. Curve numbers between 7 5 and 85 were observed for summer and winter storms with dry anteceden t runoff conditions, while values exceeded 90 for wet conditions. The third method used a mass-balance approach for four major storms, which indicated that water intake rates ranged from 2.0 to 7.3 mm h-1, yiel ding fracture intake velocities ranging from 122 to 293 m h-1. The thr ee analyses show the complexity of the infiltration process for fractu red rock. However, they contribute to a better understanding of the up per boundary condition for predicting contaminant transport through an unsaturated fractured rock medium.