Water-Quality Conditions in a National Context

Central Nebraska Basins Study Unit

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White areas indicate other NAWQA Study Units sampled during 1992-95.

EXPLANATION

Ranking of stream quality relative to all NAWQA stream sites-- Darker colored circles generally indicate poorer quality. Bold outline of circle indicates one or more aquatic-life criteria were exceeded.

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Five of seven sites in the Central Nebraska Basin Study Unit scored higher than the 75th percentile for nutrients, when compared to the other NAWQA sites. Three of the seven--Prairie Creek near Ovina, Shell Creek near Columbus, and Maple Creek near Nickerson--ranked above the 90th percentile when compared nationally. Water samples collected from drainage basins dominated by the production of corn, soybeans, and sorghum had nutrient concentrations among the largest in the Nation. At Prairie Creek, monthly median ammonia concentrations exceeded the USEPA's criterion for protection of aquatic life more than 30 percent of the time. Prairie Creek's basin, like other tributary basins in the Platte Valley, is largely dedicated to the production of corn, but livestock and dairy production near the streams may contribute to elevated nutrient concentrations. Drainage basins with a large percentage of rangeland, such as the Dismal and Loup Rivers drainage basins, had nutrient concentrations in streams more typical of sites in other NAWQA Study Units.

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Semivolatile organic compounds were present at concentrations comparable to national medians. The most commonly detected compound was p-cresol.

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Pesticide concentrations in the Central Nebraska Basins Study Unit were among the highest in the Nation. At all of the four sites--Prairie Creek near Ovina, Shell Creek near Columbus, Maple Creek near Nickerson, and the Platte River at Louisville--included in the national comparison, pesticide scores exceeded the median scores for sites in the 20 NAWQA Study Units and also exceeded the aquatic-life criteria for atrazine. At both Prairie Creek and Shell Creek, monthly median pesticide concentrations exceeded the aquatic-life criteria 25 percent of the time and median pesticide concentrations were about 10 times greater than the median for other NAWQA Study Unit sites. Prairie Creek had the second highest monthly median atrazine concentration of sites nationally. Sites in the eastern part of the Study Unit--Shell Creek, Maple Creek, and the Platte River at Louisville--were more likely to exceed the aquatic-life criteria for cyanazine. Maple Creek had the highest monthly median cyanazine concentration (24.1 µg/L in June 1993) of any site in the NAWQA Program.

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Concentrations of PCBs and organochlorine insecticides such as DDT and chlordane in the Central Nebraska Basins Study Unit were compar able to median concentrations nationally. Because PCBs and organochlorine insecticides have an affinity for fatty tissue, they tend to concentrate in fish rather than in water or sediments. Sampling efforts at Prairie and Shell Creeks did not yield sufficient tissue for analysis; therefore, results from those two sites include only sediment data and may not be comparable to other NAWQA Study Unit sites. One fish sample from the Platte River at Louisville that was collected following widespread flooding in the Midwest had DDT concentrations that exceeded the criterion for the protection of fish-eating wildlife.

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Trace elements in streambed sediments ranked similarly to those from other NAWQA Study Units, although upstream sites on the Platte River had larger-than-average concentrations of selenium. Platte River sediments at Grand Island contained selenium concentrations three times greater than the median for the 20 NAWQA Study Units.

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Fish community degradation in central Nebraska as indicated by large percentages of pollution-tolerant species was correlated with the amount of cropland in a basin. Prairie and Shell Creeks, both basins dominated by corn production, had fish communities that were among the top 25 percent of the most degraded of the 20 NAWQA Study Unit stream sites across the nation. In contrast, the Platte River at Brady drainage basin has little cropland and supports a relatively diverse fish community. However, even the Platte River at Brady displayed signs of historical trends towards a degraded fish community. The large-river sites at the Loup River at Palmer, the Elkhorn River at Waterloo, and the Platte River at Grand Island all ranked in the lower half of the NAWQA sites in the nation.

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Eroding banks with little vegetative cover contribute not only to degraded quality habitats for fish but also yield sediments and associated contaminants to the stream. When evaluated using these criteria, habitat condition was best in rangelands and along the main stem of the Platte River. Both the Dismal River near Thedford and the Platte River near Grand Island were among the 25 percent of the NAWQA sites with the best habitat scores. Prairie, Shell, and Maple Creeks, which drain croplands, had stream-habitat quality among the lowest 25 percent of the NAWQA sites nationally.

CONCLUSIONS

In the Central Nebraska Basins Study Unit, compared to other NAWQA Study Units:

Streams in basins with greater proportions of rangeland, such as the Dismal River, have negligible herbicide concentrations. Streams with degraded water chemistry commonly have degraded physical habitats as well. The combination of degraded physical and chemical environments often results in fish communities with fewer total species and a higher percentage of tolerant species.

Central Nebraska Basins Study Unit

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White areas indicate other NAWQA Study Units sampled during 1992-95.

Five major water-quality characteristics were evaluated for ground water in each NAWQA Study Unit. Ground-water resources were divided into two categories: (1) drinking-water aquifers, and (2) shallow ground water underlying agricultural or urban areas. The Central Nebraska Basins Study Unit team conducted one limited survey of 11 wells completed in the Platte River alluvial aquifer, a shallow aquifer underlying an agricultural area. Summary scores were computed for each characteristic for the Platte River alluvial aquifer and compared with scores for all aquifers and shallow ground-water areas sampled in the 20 NAWQA Study Units during 1992-95. This comparison provides a ranking of aquifers and shallow ground-water areas relative to national water-quality conditions. Results are summarized by national percentiles; higher percentile values generally indicate poorer quality compared with other NAWQA ground-water studies. Water-quality conditions in the alluvial aquifer also are compared to established standards and criteria for protection of drinking water. Methods used to compute rankings and evaluate standards and criteria are described by Gilliom and others [37].

EXPLANATION

Ranking of stream quality relative to all NAWQA stream sites-- Darker colored circles generally indicate poorer quality. Bold outline of circle indicates one or more aquatic-life criteria were exceeded.

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The median nitrate concentration in the Platte River alluvial aquifer was among the highest when compared with the other NAWQA Study Units. Fertilizer is the principal source of the nutrients. Nitrate in ground water is a concern in some parts of the Study Unit, especially in areas having a shallow water table, sandy soils, and intense row-crop production. Nitrate concentrations commonly exceed USEPA's MCL of 10 mg/L for drinking water in these areas.

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The median dissolved-solids concentration in the Platte River alluvial aquifer was in the highest 25 percent of all sites when compared to the other Study Units.

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Not enough samples for analysis.

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The median volatile organic compound (VOC) concentration in the ground water of the Platte River alluvial aquifer was among the lowest when compared with the other Study Units; however, the findings are represented by a limited number of samples. The VOC concentrations in water in the alluvial aquifer were all below the detection limit. Urban and industrial land use is minor compared to agricultural land use, limiting both the sources and quantities of potential VOCs that could enter the ground water.

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The Platte River alluvial aquifer scored among the highest 25 percent nationally for percent detections of pesticides. Pesticides were detected in 100 percent of the ground-water samples; however, no concentrations exceeded the USEPA's MCLs or HALs for drinking water. Pesticides are used extensively for weed control in row-crop production, and some of the compounds percolate to the ground water. This occurs primarily in areas with permeable sandy soils and shallow ground water.

CONCLUSIONS

Nitrate concentrations in ground water exceeded the USEPA's MCL in areas with shallow water tables, sandy soils, and extensive row-crop production. Although pesticides were detected frequently in ground water, the concentrations measured were well below the USEPA's MCLs or HALs.