The Great Plains contains the High Plains (or Ogallala) Aquifer, the largest aquifer system in the United States that stretches from South Dakota to Texas, and has a myriad of rivers, lakes, and prairie wetlands. Vast amounts of groundwater are contained within the sands and gravels with silt lenses of the Miocene age Ogallala Formation, the Brule and Arikaree Formations in western Nebraska, and overlying Quaternary age sands and gravels.
Water quality indexes as measurements of the suitability of water for irrigation, human and animal consumption, and fish and wildlife have been developed by the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Agriculture (USDA). Different limits of exposure are based on human and plant health and environmental considerations. No single compilation of water quality over the vast Plains region exists, though various local assessments have been made. Investigations of groundwater quality in the High Plains Aquifer in the early 1980s were an outgrowth of studies of water depletion from irrigation. The investigations confirmed that mining of groundwater by irrigators since the 1940s had lowered the groundwater table by more than 100 feet in parts of Texas, eastern New Mexico, the Oklahoma Panhandle, and Kansas. Follow-up studies indicated that water table declines greater than forty feet occurred from 1980 to 1995. Total dissolved solids (TDS), a measure of water purity, increased from north to south in the aquifer, suggesting an association between mining and deteriorating quality. Very high TDS levels occurred in the Ogallala Formation groundwater beneath southwestern Kansas and the Texas Panhandle. According to the usda salinity scale, irrigation with this groundwater will promote development of saline soil conditions that render the soil useless. High TDS levels are generally encountered in deeper, older waters that have become more mineralized with time. Upward movement of naturally occurring chemicals from underlying bedrock is a major source of increased TDS. In Kansas and Texas, underlying Permian age bedrock contains salt beds and saline water, providing high amounts of tds to the Ogallala Aquifer. Wells in these areas also generally contain elevated sodium concentrations that are known to adversely affect plant growth and soil properties.
Although the Ogallala Aquifer has received intense scrutiny, the focus of Great Plains water quality concern is now on the impact of agrichemicals and the generation of animal wastes by agriculture. Areas that are most sensitive to contamination are alluvial valleys with less than fifty feet to groundwater. These river and stream valleys generally have rich, well-drained bottomland and terrace soils that are heavily irrigated by shallow wells and have been row-cropped for decades. Agriculture, the economic mainstay of the Great Plains, is heavily reliant on agrichemicals. In areas vulnerable to leaching, underlying groundwater commonly contains nitrate and herbicides in levels that may adversely aãect human health. Nebraska and Kansas have very high incidences of nitrate in well water. The nitrate is derived from commercial fertilizer and animal waste and exceeds public health limits. The herbicides atrazine and metolachlor are used throughout the region on corn land and are frequently detected in surface water and in many shallow aquifers. In soils and water, pesticide transformation occurs, and the resulting degradates are often detected in concentrations that considerably exceed those of the parent pesticides. These degradate compounds may present a future health hazard and environmental concern. Pesticides are also a problem in surface water. Each spring, pesticides are flushed from farms in the Great Plains in runoã. These pesticides accumulate in lakes and reservoirs and can be introduced into municipal wells situated next to creeks and rivers. In addition to nitrate, ammonia from animal and human waste lagoon overflows has been responsible for several fish kills in the Plains. Other chemicals found in groundwater include carbon tetrachloride and ethylene dibromide, which were used as fumigants and commonly occur beneath and near grain elevators.
Contaminants associated with military bases and industries are also a problem in the Great Plains region. In these locations the groundwater may be contaminated with munitions such as RDX and TNT as well as the commonly used degreaser trichloroethylene. Large trichloroethylene plumes in Hastings, Nebraska, and Wichita, Kansas, are the result of industrial solvent disposal.
Roy F. Spaulding University of Nebraska-Lincoln
Exner, Mary, and Roy F. Spalding. Occurrence of Pesticides and Nitrate in Nebraska's Ground Water. Lincoln: University of Nebraska Water Center, 1990.
Kolpin D. W., E. M. Thurman, and D. A. Goolsby. "Occurrence of Selected Pesticides and Their Metabolites in Near-Surface Aquifers of the Midwestern United States." Environmental Science and Technology 30 (1997): 335–40.