Shallow ground-water quality in the Platte River Valley alluvium, Nebraska, October-November 1997

Contamination of surface and ground water from nonpoint sources is a national issue. Examples of nonpoint- source contaminants from agricultural activities are pesticides (fungicides, herbicides, and insecticides), sediments, and nutrients (nitrate and phosphorus). The Platte River Valley in Nebraska is a flat lowland area underlain primarily by unconsolidated alluvial deposits of Quaternary age that have a maximum thickness of less than 100 ft (feet) and consist mostly of gravel, sand, silt, and clay (Gutentag and others, 1984, p. 9). These deposits are the single most important source of water for public supply for Nebraska.s largest cities including Omaha, Lincoln, Grand Island, and Kearney. These deposits pro- vide about 117 million gallons per day, nearly 50 percent of the total daily ground-water production for Nebraska (Nebraska Natural Resources Commission, 1994). The Platte River Valley also includes levels, and extensive areas of irrigated land used to grow corn. These conditions increase the vulnerability of the ground water to agricultural contaminants. The U.S. Geological Survey.s National Water Quality Assessment Program is designed to describe the status and trends in the quality of the Nation.s surface- and ground-water resources and to provide a sound understanding of the natural and human factors affecting the quality of these resources. Important components of the program are assessments of more than 50 major river basins and aquifers, called study units. The Central Nebraska Basins study unit was among the set of 20 study-unit investigations begun in 1991. The Central Nebraska Basins comprise sandhills, loess hills, and glaciated areas adjacent to the valley of the Platte River, the main river draining the study unit. This study provides data regarding water quality in Platte River Valley alluvium that was not included in the previous study (Frenzel and others, 1998). In 1997, 27 wells completed in the Platte River Valley alluvium were randomly selected and sampled as part of a CNBR Study (Sub)-Unit Survey. Quality of shallow ground-water (water in alluvial deposits) of the Platte River Valley is described in this report. A total of 27 wells were sampled. Well depths ranged from 12 to 124 ft and the water table ranged from about 2.9 to 31 ft below land surface. Physical properties of specific conductance, pH, and dissolved oxygen were measured on-site at the time of sample collection. Water was collected from the wells using nationally consistent sampling methods developed as part of the NAWQA program, and was analyzed by the U.S. Geological Survey.s National Water Quality Laboratory in Arvada, Colorado for the presence of major ions, nutrients, trace elements, radionuclides, volatile organic compounds, and pesticides. The following is a description of the quality of the water in the Platte River Valley alluvium based on the results of this SUS.