Abstract |
Medicine Lake is a saline lake lying on the western flank of the Coteau des Prairie in Codington County, SD. Total dissolved solids content ranges from 35,000 parts per million to 65,000 parts per million. The source of the salts entering Medicine Lake is the result of 'normal' weathering of geologic materials. The weathering products go into solution with the ground water, which is the primary source of recharge for Medicine Lake. Vertical control data, supported by remote sensing thermal imagery, established Medicine Lake to be a closed drainage. Because evaporation exceeds precipitation large quantities of water are lost from the closed drainage system. Ground-water movement is toward Medicine Lake from the southeast through McKillicans Lake, Round Lake, and Horseshoe Lake. Ground-water movement toward Medicine Lake from the northwest is through a series of potholes. During the time ground water is moving toward Medicine Lake it is exposed to evaporation whenever it occupies one of the other lake basins. As a consequence of this, concentration of the initial ground water starts in McKillicans Lake and progresses, with more evaporation and concentration occurring, each time it is exposed in a lake basin. Medicine Lake, lying at the lowest elevation in the closed drainage system, receives ground water that has already been concentrated during its migration through the chain of lakes and potholes. (Author) |