Basins in which row crops dominate tended to have larger concentrations of agricultural chemicals in the water, residues of pesticides in fish tissues, and degraded fish communities . Conversely, streams draining rangelands had smaller concentrations of agricultural chemicals in water or fish tissue and supported diverse fish communities. Urban and industrial chemicals generally were detected at small concentrations in water, streambed sediments, and fish-tissue samples throughout the Central Nebraska Basins Study Unit.
In general, chemicals discussed here enter the aquatic environment through nonpoint-source runoff from agricultural lands that dominate the Nebraska landscape. Because some chemicals may be hundreds or thousands of times more concentrated in the tissues of aquatic organisms or in fine sediments these media are well suited for the detection of certain classes of chemicals. Other chemicals are highly soluble and therefore most likely detected in water.
Residues of organochlorine compounds were present in bottom-feeding fish. Generally, concentrations were near method detection limits; however, p,p'-DDE (a degradation product of DDT), PCBs, trans-nonachlor, and dieldrin were detected more frequently and in somewhat larger concentrations in areas dominated by row-crop agriculture. Although some of these compounds, or their parent compounds, have not been used in the United States for a quarter of a century, they persist in the environment. Large-magnitude flooding, such as was experienced in the Midwest during 1993, may resuspend compounds long buried in streambed and flood-plain sediments. The largest concentration of organochlorine residues was detected in common carp sampled near the mouth of the Platte River following the flooding in 1993. The p,p'-DDE concentration of 1,600 micrograms per kilogram (µg/kg) was large enough to exceed the National Academy of Sciences and National Academy of Engineering guideline for the protection of fish-eating wildlife .
Although some organochlorine compounds have not been used for decades, they remain persistent in the environment.
Organochlorine pesticides may also be found associated with organic carbon in streambed sediments. However, the less than 2-mm (millimeter) size fraction analyzed from streams in central Nebraska is mostly sand and has little organic carbon. Consequently only 1 of 28 samples had detectable levels of organochlorine pesticides. There were no detections of these compounds from 8 ground-water samples or 159 stream-water samples.
Volatile organic compounds (VOCs) generally are associated with industrial activities, although some also are used as inert ingredients in pesticide formulations. Because some VOCs are carcinogenic, their presence in water supplies would be of concern. In the Platte Valley, environmental conditions are more likely to allow contamination than in other subunits of the Study Unit, yet no VOCs were detected in any of the 4 surface-water and 11 ground-water samples from streams or the alluvial aquifer.
The same samples were analyzed for alpha and beta radioactivity. Alpha radioactivity has an MCL of 15 picocuries per liter (pCi/L) in drinking water and generally indicates the presence of naturally occurring radium or radon. Alpha radioactivity ranged from 9.9 to 18 pCi/L in streams and from 1.5 to 79 pCi/L in ground water. Beta radioactivity at concentrations greater than 50 pCi/L may indicate the presence of fission products such as potassium-40 or strontium-90. Beta radioactivity ranged from 11 to 35 pCi/L in streams and from 5.2 to 55 pCi/L in ground water. Areas upstream from the Central Nebraska Basins Study Unit contain uranium-rich rock that is a likely source of alpha and beta radioactivity in the Platte Valley.
Generally, trace elements in water, tissue, and streambed sediments of the Central Nebraska Basins Study Unit were not an issue. Water samples from streams and aquifers with trace-element concentrations greater than a detection limit of 1 mg/L were uncommon. No samples exceeded drinking water MCLs for any trace element. However, streambed-sediment samples from the Platte River between North Platte and Grand Island in 1992 had notable selenium concentrations. The North Channel of the Platte River at Brady, with 1.2 micrograms per gram (µg/g), the Platte River at Overton (2.6 µg/g), and the Platte River near Grand Island (2.1 µg/g) all exceeded the median value of 0.7 µg/g from the 20 NAWQA Study Units submitting samples from 1992 to 1995. Samples collected during 1995 at Brady and near Grand Island, following high flows, had selenium concentrations of 2.6 µg/g and 1.8 µg/g.
Because many specific factors should be considered when evaluating water quality, it is often difficult to provide a single result that represents the integration of these factors. However, aquatic biota that are continually subjected to all aspects of physical and chemical factors of their environment provide a means for making an integrated assessment. Species composition reveals information about local environmental conditions. If tolerant species, such as common carp, dominate the community to the exclusion of less-tolerant species, such as smallmouth bass, the implication is that water quality is degraded.
Fish communities were degraded at streams draining mostly cropland.
Fish communities were sampled at nine sites in the Central Nebraska Basins and evaluated using an index of biotic integrity (IBI). In this study, the lowest IBI scores (indicating degraded water quality) resulted from the identification of only a few individual fish, all of highly tolerant species. Shell and Prairie Creeks had the lowest IBI scores. Higher scores are associated with samples collected at the Dismal River and Platte River sites. These sites had fish communities represented by species of all tolerance classes. Sites with larger percentages of cropland in a drainage basin had fish communities indicative of degraded water quality . Although cropland does not directly affect a fish community, factors related to the presence of cropland, as well as the effects of cropland on water chemistry and the physical habitat, likely are responsible for this relation.
Measures of fish communities' tolerance to pollution suggest that degraded water quality is associated with a high percentage of cropland in a basin.
Wetlands provide a variety of functions including flood and erosion moderation, sediment and nutrient retention, ground-water recharge, and wildlife habitat. Decades of agricultural activities in Nebraska have resulted in substantial decreases in wetland acreage. Remaining wetlands in the predominantly agricultural landscape are subjected to a variety of effects from surrounding land use. Some of the herbicides used in Nebraska are among those reported to have reproductive and endocrine disruptive effects on wildlife. With fewer acres of wetland habitat available to wildlife, the quality of remaining habitat becomes increasingly important.
Thirty-one wetlands representing dominant local land uses were sampled for organonitrogen herbicides in May and August 1994. Analysis of land use within 0.6 mile of each wetland revealed three groups of sites. One group of sites had an average of 74 percent rangeland, a second group of sites averaged no more than 30 percent in a single land-use category, and the third group had an average of 78 percent cropland surrounding the sites. The median concentrations of atrazine and the total of all herbicides analyzed in water samples were significantly smaller in rangeland-dominated sites than in cropland-dominated sites in both sampling periods. Sites without a single dominant land use had significantly smaller atrazine and total herbicide concentrations than sites in cropland-dominated landscapes, but were not significantly different than those in rangelands . These results suggest that herbicide concentrations are elevated in water bodies where most of the surrounding land is used for row-crop production.
Wetlands surrounded mostly by cropland had significantly greater herbicide concentrations than wetlands in other land-use areas.
--Steven A. Frenzel