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• <br />• <br />• <br />flood irrigated except for a very small portion <br />which can be considered part of the Purgatoire <br />AVF; it is economically unfeasible to divert <br />Purgatoire River water to this drainage; the areal <br />extent of water dependent vegetation species and <br />results of backhoe pits and limited slug tests <br />exhibit no subirrigation characteristics in the <br />canyon; and, only small localized and isolated <br />pockets of alluvial groundwater have been <br />identified and, as such, would not be classified as <br />an alluvial aquifer. <br />Probable Aydroloeic Consequence <br />Based on data collected at the USGS Madrid <br />gaging station, the average runoff is 29,980 acre- <br />feet. The area of the drainage basin above the <br />station is 550 square miles. The average runoff is <br />0.035 Ceet (1.02 inches) per acre. The normal <br />precipitation for the area is about 16.9 inches of <br />which approximately 13.2 inches is available for <br />infiltration and runoff. Since the runoff averages <br />about one inch, the amount of water that <br />infiltrates averages about 12 inches. Based on <br />water balance calculations, all of the infiltrated <br />water is removed from the soil profile through <br />evaporation and/or evapotranspiration. <br />Considering the alluvium and canyon stream <br />as a single system, the volume of surface runoff <br />essentially represents the total available water <br />within the system. The runoff from the system <br />manifests itself as both surface and subsur[ace <br />Row but it is no[ possible to determine the <br />amounts of each. However, seepage will occur <br />only under the portion of the drainage basin <br />which contains alluvium. An estimate of impacts <br />to the hydrologic regime can be made by <br />comparing the seepage volume to the total runoff <br />volume for the drainage basin. <br />The estimated vertical saturated hydraulic <br />conductivity of the overburden materials has been <br />estimated to be 2.1x10' feet per day. Under the <br />conditions that exist during mining, the rate at <br />which water will flow from the alluvium to the <br />mine is equal to this conductivity value. <br />Therefore, the unit area amount of seepage is <br />about 0.92 inches per year. <br />Areas of the drainage basin containing <br />potential alluvium and/or stream-laid deposits <br />were estimated. Using these areas and the <br />estimated seepage rate, the yearly volumes of <br />available water were calculated by multiplying the <br />drainage basin area by the average runoff depth. <br />Percentages of total available water lost to <br />seepage were then calculated. The results of the <br />calculations indicate that Santistevan Canyon has <br />a drainage basin of 2181 acres with runoff being <br />186 acre feet per year. Alluvial areas were <br />determined to be 15 acres. Utilizing a 60 percent <br />seepage loss, this area seepage would than relate <br />to 1.1 acre-feet per year. The volume of water <br />lost to seepage is quite small (less than 1 percent) <br />when compared to the volume of water available <br />to the alluvium-stream system. The primary <br />reason for the small amounts of water that seeps <br />from the alluvium is the low vertical saturated <br />hydraulic conductivity of the overburden materials. <br />Since the seepage volumes are small, the <br />hydrologic balance of this system should not be <br />materially affected nor will the quantify of water <br />supplied to the adjacent Purgatoire AVF be <br />effected. <br />Undermining Santistevan Canyon should not <br />result in material damage to surface and/or <br />alluvial aquifer systems present. This conclusion <br />is supported by calculations which demonstrate <br />that even at conservatively assumed high surface <br />flow conditions, the low transmissivities of the 500 <br />to 600 feet of overburden will not allow <br />significant amounts of surface or alluvial water to <br />reach the mine workings. These assumptions are <br />exhibited by the approximately three square miles <br />of the western part of the New Elk Mine which <br />has been undermined with total extraction by <br />longwall and removal of pillar mining methods <br />over the past 30 years. There is no surface <br />indication of subsidence, as evidenced by downed <br />power poles or timber, surface cracks or slope <br />slides or slumps. Additionally, based on <br />subsidence data collected over longwall and room <br />and pillar mining areas at the New Elk Mine <br />(Woodward-Clyde, 1983), subsidence should not <br />materially increase the transmissivity, as the <br />overburden subsides as a unit with very little <br />horizontal strain or surface expression. Partial <br />subsidence data results indicate a maximum <br />horizontal displacement of less than one foot. <br />8 <br />