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1Z/I/~3 :.,:. WFC SUBMITTAL 2 VOLUME OF WATER FLOWING FROM ALLUVIUM TO MINE (APACHE, CIRUELA AND SANTISTEVAN CANYONS) Based on six years of data collected at the Madrid gaging station, the average runoff is 29,980 acre-feet. The area of the drainage basin above the Madrid station is 550 square miles. The average runoff is 0.085 feet (1.02 inches) per acre. The normal precipitation for the area is about 16.9 inches of which approximately 13.2 inches is available for infiltration and runoff. Since the runoff averages about one inch, the amount of water that infiltrates averages about 12 inches. Based on water balance calculations all of the infiltrated water is removed from the soil profile through evaporation and/or evapotranspiration. Considering the alluvium and stream as a single system, the volume of surface runoff essentially represents the total available water within the system. The runoff from the system manifests itself as both surface and subsurface flow but it is not possible to determine the amounts of each. However, seepage will occur only under the portion of the drainage basin which contains alluvium. An estimate of impact to the hydrologic regime can be made by comparing the seepage volume to the total runoff volume for each drainage basin. The estimated vertical saturated hydraulic conductivity of the overburden materials has been estimated to be 2.1x10-4 ft/day. Under the conditons that exist during mining, the rate at which water will flow from the alluvium to the mine is equal to this conductivity value. Therefore, the unit area amount of seepage is about 0.92 inches per year. Areas of each of the drainage basins were obtained by planimeter. In addition, the areas of alluvium in Ciruela and Apache Canyons were estimated from the map delineating stream-laid deposits. These areas were approximated by assuming rectangular shapes for the alluvium. Therefore, these values are probably conservatively high. Unfortunately the alluvium in Santistevan Canyon was not mapped making it necessary to estimate this area. This was accomplished by multiplying the ratio of alluvium area to drainage basin area for Ciruela and Apache Canyons by the area of the drainage basin for Santistevan Canyon. Using the areas of alluvium and the estimated seepage rate, the yearly volumes of seepage were calculated. In a similar manner, the total volumes of available water were calculated by multiplying the drainage basin area by the average runoff depth. Percentages of total available water lost to seepage were then calculated. The results of all calculations are presented in Table 1. As the results in Table 1 indicate the volume of water lost to seepage is quite small (less than 1%) when compared to the volume of water available to the alluvium-stream system. The primary reason for the small amounts of water that seeps from the alluvium is the low vertical saturated hydraulic conductivity of the overburden materials. Since the seepage volumes are small, the hydrologic balance of these alluvial stream systems should not be materially affected.