Water resources and geology of Mount Rushmore National Memorial, South Dakota

Abstract

Ground water suitable for public supply can be obtained from fractured metamorphic and igneous rooks at Mount Rushmore National Memorial, S. Dak. The memorial comprises three main drainage basins: Starling basin, Lafferty Gulch basin, and East Boundary basin. Ground water is most prevalent in Lafferty Gulch basin but Starling basin contributes the most surface water. The total water supply was obtained from springs until 1967 when increasing numbers of visitors required development of additional sources. As a result of this investigation, wells 3 and 4 were drilled in Lafferty Gulch basin and East Boundary basin. Well 3 is 200 feet deep in mica schist and granite. It produced 7.3 million gallons of water in 1968 and 7.7 million gallons of water in 1969, the total supply for the memorial. Well 4 is 500 feet deep, also in mica schist and granite. It is not used at the present time (1970) but will be used in the future when more water is needed. Water from both wells is potable, but the quality of water from well 3 is superior to that from well 4. Mica schist is the most abundant rock in the memorial. The more prominent hills and mountains, however, are in large, northerly striking granite sills, some of which are several hundred feet thick. Pegmatite sills and dikes are also numerous. The western boundary of the memorial is at the east edge of the Harney Peak Granite batholith. The dip of schistosity and bedding in schist adjacent to the batholith is about 30 ? E. but increases across the memorial to about 65 ? E. in the northeast corner. At some locations in the memorial, granite or pegmatite sills act as ground-water dams preventing the movement of ground water down gradient. A pegmatite or granite sill is probably the cause of the accumulation of water in the vicinity of well 3. The well flows when it is not being pumped. The occurrence of ground water is dependent upon the presence of joints and fractures in the schist and granite bedrock. The rocks themselves are relatively impermeable and would yield little or no water in their unaltered state. Mica schist that has been intruded by granite and (or) pegmatite is more fractured and yields more ground water in the memorial than mica schist alone. This condition may be due to jointing and to the greater fracturability of the intruded rocks in the vicinity of granitic intrusions. Ground water is also available from alluvium in major valleys such as Starling basin and the valleys of Grizzly Bear Creek and Battle Creek. Evapotranspiration is the greatest use-item in the water budget. An approximation of evapotranspiration based upon an average annual precipitation of 19 inches is 1,600 acre-feet, or 80 percent of the annual precipitation. Several locations in the memorial have potential as future, sources of ground-water supplies. The most promising areas are near spring 6 in the southeast corner and alluvium in the valleys of Battle and Grizzly Bear Creeks. Developed and potential water resources in the memorial probably are sufficient to meet demands beyond the year 2000.