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In 1999 and 2000, 30 water-quality monitoring wells were installed in the central High Plains aquifer to evaluate the quality of recently recharged ground water in areas of irrigated agriculture and to identify the factors affecting ground-water quality. Wells were installed adjacent to irrigated agricultural fields with 10or 20-foot screened intervals placed near the water table. Each well was sampled once for about 100 waterquality constituents associated with agricultural practices. Water samples from 70 percent of the wells (21 of 30 sites) contained nitrate concentrations larger than expected background concentrations (about 3 mg/L as N) and detectable pesticides. Atrazine or its metabolite, deethylatrazine, were detected with greater frequency than other pesticides and were present in all 21 samples where pesticides were detected. The 21 samples with detectable pesticides also contained tritium concentrations large enough to indicate that at least some part of the water sample had been recharged within about the last 50 years. These 21 ground-water samples are considered to show water-quality effects related to irrigated agriculture. The remaining 9 groundwater samples contained no pesticides, small tritium concentrations, and nitrate concentrations less than 3.45 milligrams per liter as nitrogen. These samples are considered unaffected by the irrigated agricultural land-use setting. Nitrogen isotope ratios indicate that commercial fertilizer was the dominant source of nitrate in 13 of the 21 samples affected by irrigated agriculture. Nitrogen isotope ratios for 4 of these 21 samples were indicative of an animal waste source. Dissolved-solids concentrations were larger in samples affected by irrigated agriculture, with large sulfate concentrations having strong correlation with large dissolved solids concentrations in these samples. A strong statistical correlation is shown between samples affected by irrigated agriculture and sites with large rates of pesticide and nitrogen applications and shallow depths to ground water. Introduction Knowledge of the quality of the Nation’s water resources is important because of the implications to human and aquatic health and because of the significant costs associated with decisions involving land and water management, conservation, and regulation. In 1991, the U.S. Geological Survey (USGS) began full implementation of the National Water-Quality Assessment (NAWQA) Program. The long-term goals of the NAWQA Program are to describe the status and trends in the quality of the Nation’s surfaceand ground-water resources and determine the natural and anthropogenic factors affecting water quality (Gilliom and others, 1995). More than 50 major river basins and aquifer systems are targeted for investigation under the NAWQA Program. Together, these include water resources available to more than 60 percent of the population and cover about one-half of the land area in the conterminous United States. The High Plains Regional Ground Water study began in October 1998 and represents a modification of the traditional NAWQA design in that the ground-water resource is the primary focus. The High Plains aquifer is a nationally important water resource that underlies about 174,000 mi2 in parts of eight Western States (fig. 1). About 27 percent of agricultural land in the United States is in the High Plains, and about 30 percent of all the ground water used for irrigation in the United States is pumped from this aquifer (Dennehy, 2000). Water quality in the High Plains aquifer might be vulnerable to effects from land-surface activities; however, large depths to ground water in some areas could provide a buffer from these influences. The lack of a regionally extensive geologic barrier to impede downward migration of contaminants contributes to the potential for water-quality degradation from land-surface activities. Similarly, applications of large amounts of water at the land surface can be a driving force for downward migration of chemicals. Residential and urban settings, agricultural activities, and oil and gas exploration/ development are potential sources of contaminants to this drinking-water aquifer. 2 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000 Northern High Plains |
