Summary of Major Issues and Findings in the Central Nebraska Basins Study Unit
What factors are important in determining water-quality conditions in the Central Nebraska Basins?
Land use in central Nebraska appears to affect water quality significantly; streams in rangelands generally had fewer occurrences and smaller concentrations of pesticides than did streams in croplands where corn and soybeans were planted extensively. Subbasins with greater proportions of rangeland, such as the Dismal River, had negligible herbicide concentrations. The largest pesticide concentrations were in storm runoff following pesticide applications. Because some pesticide concentrations may exceed the U.S. Environmental Protection Agency's (USEPA) drinking-water Maximum Contaminant Levels (MCLs) in storm runoff, the timing and intensity of rainfall has implications for drinking-water supplies. Pesticides in streams from storm runoff may enter alluvial aquifers as a consequence of ground-water withdrawals. Sites with degraded water chemistry commonly had degraded physical habitats as well. Streamflow regulation of the Platte River has affected water quality through habitat alterations that are deleterious to native species. The combination of degraded physical and chemical environments commonly resulted in structurally simple fish communities.
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Agricultural chemicals are applied to enhance crop production.
Nitrate content in water is related to agricultural land management (p. 6-7)
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Nitrate concentrations in ground water were greatest where more than 60 percent of the area was in corn production, water tables were within 50 feet of the land surface, and soils were most permeable. Nitrate concentrations in ground water often exceeded the MCLs in the Platte Valley, where fertilizer is applied to large areas that produce corn every year.
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Nitrate concentrations varied substantially and tended to be smaller in surface water than in ground water for similar environmental settings. The smallest concentrations were measured in streams during the summer when nitrate uptake by aquatic plants was greatest.
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Land-management practices in the central part of the Platte Valley include constraints on the application of fertilizer. In areas where these constraints are most restrictive, median nitrate concentrations in domestic wells have decreased markedly.
Agricultural activities potentially affect the management of public-water supplies withdrawn from the Platte River and the alluvial aquifer (p. 8-9)
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Alachlor, atrazine, cyanazine, and metolachlor, all organonitrogen herbicides, were the four most commonly applied and detected pesticides for corn, sorghum, and soybean production in the Study Unit.
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Concentrations of these herbicides in surface water varied seasonally and were generally larger in the spring and summer following application than in the fall and winter.
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The Elkhorn River drainage basin contributed the majority of the herbicides flushed from the Study Unit.
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Of 46 pesticides analyzed in water samples from the Platte River at Louisville, it appears that alachlor, atrazine, and cyanazine are the most likely to influence the quality of public-water supplies withdrawn from the Platte River alluvial aquifer downstream from the confluence of the Platte and Elkhorn Rivers. Atrazine is most likely to exceed the MCL in public-water supplies withdrawn from the alluvial aquifer downstream from this part of the aquifer.
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Nutrient concentrations in the Platte River increase downstream as it flows through the Study Unit; however, all measured concentrations were well below the MCLs.
Water quality in the Platte River alluvial aquifer may be affected by surface-water quality in areas of ground-water withdrawals (p. 10-11)
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Ground water is the principal source of drinking water for public- and self-supplied domestic uses in Nebraska.
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The Platte River alluvial aquifer that underlies the Platte Valley subunit is the single most important source of water for public supply and is connected hydraulically to the Platte River.
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The increase in ground-water withdrawals in the Platte Valley has induced additional recharge from the river, and has the potential to change ground-water quality near the river where many of the public supply wells are located.
Aquatic environments potentially are altered by human activities (p. 12-13)
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Streams where row crops dominate the drainage basin had large concentrations of nutrients and pesticides in the water, the largest residues of pesticides in fish tissues, and degraded fish communities. Conversely, streams draining rangelands had small concentrations of pesticides in water and fish tissues and supported diverse fish communities. Chemicals normally associated with urban and industrial sources were detected at small concentrations in water, streambed sediments, and fish tissues throughout the Study Unit.
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Organochlorine concentrations in fish tissues generally were near minimum reporting limits; however, more frequent detection and somewhat larger concentrations of p,p'-DDE, polychlorinated biphenyls (PCBs),
trans-nonachlor, and dieldrin were detected from areas dominated by row-crop agriculture. One of three fish-tissue samples from the Platte River at Louisville exceeded a water-quality criterion for protection of fish-eating wildlife because of large concentrations of p,p'-DDE.
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Streams that drain basins with large percentages of cropland had fish communities tolerant of physical or chemical water-quality degradation. Fish communities from the Dismal River, which drains primarily rangelands, contained a mix of tolerant species, species with intermediate tolerance, and intolerant species. In contrast, the fish communities at Prairie and Shell Creeks, where nearly all the land is used for corn production, were characterized by a few tolerant species such as green sunfish, common carp, and fathead minnow; no pollution-intolerant species were present. Habitat quality also was degraded at sites draining areas where cropland was the predominant land use.
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Wetlands in drainage basins dominated by row crops had herbicide (atrazine and total organonitrogen herbicides) concentrations that were significantly greater than those from wetlands in either rangelands or mixed land uses. These findings imply that a mixture of land use may reduce the potential for water-quality degradation from agricultural sources.
Aquatic and migratory species are affected directly by changes in the physical characteristics of the Platte River (p. 14-15)
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Channel width has been reduced by as much as 90 percent near North Platte. The narrowing process is continuing at sites downstream. Near Grand Island, the channel has narrowed from 40 to 60 percent of its historical width, and trees have become established on sandbars, which develop into permanent islands. Flow regulation upstream from the Study Unit, water withdrawals, and possibly climate changes have been the primary causes of channel-width reductions.
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Migratory birds, including sandhill cranes and the endangered whooping cranes, rely on critical habitats in the Platte Valley. Habitat loss has created overcrowded conditions, which may lead to outbreaks of diseases such as avian cholera.
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Native fish species also have been adversely affected by habitat loss. Some fish species may be absent or in decline as a result of reductions in habitat. The current fish community contains non-native species that are tolerant of degraded habitats. Some sensitive native species are no longer found or are rarely collected.
