WATER-QUALITY CONDITIONS IN A NATIONAL CONTEXT

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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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In general, nutrient concentrations in streams of the upper Snake River Basin were among the smallest of those reported in all NAWQA Study Units. Concentrations of nutrients in the Snake River increased in a downstream direction; concentrations were largest between Twin Falls and the basin outlet at King Hill. Sites downstream from agricultural areas contained the largest concentrations of nutrients in the basin. However, none of the concentrations exceeded drinking-water standards for nutrients.

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Only two sites in the upper Snake River Basin had sufficient pesticide data to compare with national conditions. Although numerous pesticides were detected at these sites (Teton River near St. Anthony and Rock Creek near Twin Falls), the concentrations were among the smallest of those reported in all NAWQA Study Units. None of the concentrations exceeded drinking-water standards or aquatic-life criteria.

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Concentrations of PCBs and other organochlorines in bed sediment and fish tissue in the upper Snake River Basin were typical of concentrations reported in other NAWQA Study Units; concentrations at about half of the sites were larger and about half were smaller, compared with the national median determined by NAWQA. Concentrations in fish tissue at two sites exceeded NAS/NAE aquatic-life criteria. These sites were Rock Creek at Twin Falls and the Snake River near Twin Falls. Compounds that exceeded aquatic-life criteria were PCBs, the insecticide DDT, and the DDT breakdown products DDE and DDD [10]. Concentrations were largest at sites downstream from agricultural and urban areas.

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Trace-element contamination does not appear to be a concern in most areas of the upper Snake River Basin. Concentrations in bed sediment were largest in pristine areas where the geology is naturally rich in trace elements.

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Stream habitat conditions can strongly influence the water quality and associated biological communities of streams. Instream modification, bank erosion, and riparian conditions represent useful measures of stream habitat conditions. In the upper Snake River Basin, the habitat conditions were generally better than those at other NAWQA sites nationwide. Sites where habitat was highly degraded were influenced primarily by hydrologic modification and agricultural activities such as excessive grazing. Because bank and riparian conditions generally are not reflective of habitat conditions in large rivers, the large-river sites in the upper Snake River Basin were not evaluated for stream habitat conditions.

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Semivolatile organic com pounds typically are present in streams receiving municipal and (or) industrial discharges. In the upper Snake River Basin, 47 different SVOCs were detected in bed-sediment samples. The most commonly detected SVOCs were phthalates (98 percent of the samples), phenol (86 percent of the samples), and 2,6-dimethylnaphthalene (83 percent of the samples). Concentrations of SVOCs in bed sediment were smallest at sites upstream from the influence of municipal and (or) industrial areas. Bed-sediment samples at Snake River sites downstream from Idaho Falls all contained SVOC concentrations that ranked higher than the national median determined by NAWQA. However, no concentrations of SVOCs in bed sediment exceeded NAS/NAE aquatic-life criteria.

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Fish community indices, such as the number of omnivores, pollution-tolerant species, and non-native fish, indicated that most of the sites assessed in the upper Snake River Basin were more degraded than were sites sampled in other NAWQA Study Units. Fish communities were most degraded in the Snake River and its tributaries downstream from Twin Falls, where multiple sources of pollutants degrade water-quality conditions. Even sites where degradation was minimal showed some degradation as a result of introduced fish species.

CONCLUSIONS

In general, nutrient and pesticide concentrations in streams in the upper Snake River Basin were small compared with concentrations in other streams sampled by NAWQA nationwide. Concentrations of nutrients and pesticides did not exceed drinking-water standards in any of the stream samples collected.

PCBs and other organochlorine compounds are more of a concern in fish tissue than in bed sediment. Concentrations of these compounds exceeded aquatic-life criteria for fish tissue at two sites. PCBs and the insecticide DDT and its breakdown products were the most commonly detected organochlorine compounds in bed sediment and fish tissue.

Fish communities in large rivers downstream from anthropogenic effects were degraded compared with communities at sites sampled in other NAWQA Study Units. Degraded sites supported primarily nongame, warmwater species of fish. Headwater streams and large rivers upstream from anthropogenic effects supported healthy communities of predominantly coldwater fish.

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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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Nitrate is one of the most common anthropogenic-induced contaminants in ground water in the upper Snake River Basin. Nitrate concentrations in ground water from the Minidoka study area were among the largest of those reported in all NAWQA Study Units; concentrations in water from 24 percent of the wells sampled exceeded the drinking-water standard of 10 milligrams per liter. Nitrate concentrations also were larger than the national median in the eastern Snake River Plain, A&B, and Eden study areas.

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Radon concentrations in water from wells in the tributary valley, Jackson Valley, and Minidoka study areas were larger than the national median determined by NAWQA. Concentrations in water from nearly all wells sampled in these three study areas exceeded the USEPA proposed drinking-water standard of 300 picocuries per liter. Radon concentrations were smaller than the national median in the eastern Snake River Plain, A&B, Jerome/Gooding, and Eden study areas. Radon is naturally occurring and is produced by the radioactive decay of uranium-containing minerals in local geologic materials.

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Pesticide detections in water from wells in the Jackson Valley study area were among the smallest of those reported in all NAWQA Study Units. Pesticide detections in water from wells in the Minidoka, A&B, and Eden study areas were larger than the national median determined by NAWQA; water from 86 percent of the wells sampled in the Minidoka study area contained at least one detectable pesticide. None of the concentrations in ground water in the upper Snake River Basin exceeded USEPA drinking-water standards.

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Volatile organic compounds include solvents, fuels, and soil fumigants. Eleven different VOCs were detected in ground water in the upper Snake River Basin. Only 1,2-dichloropropane exceeded USEPA drinking-water standards, and in water from only one well in the Minidoka study area. Water from only one well in the Jackson Valley contained VOCs; the source probably was a local landfill.

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Water from wells in the Minidoka, A&B, and Eden study areas contained some of the largest DS concentrations of those reported in all NAWQA Study Units. Large DS concentrations probably result from leaching of evaporative salts and soil minerals during infiltration of irrigation water.

CONCLUSIONS

In general, ground water in the upper Snake River Basin is safe to drink; the quality of ground water in the Jackson Valley area is some of the highest quality ground water reported in the NAWQA Study Units. However, ground-water quality in some areas of the basin has been degraded as a result of anthropogenic activities. Nitrate poses the greatest concern for public health. Nitrate concentrations in ground water in many areas of the upper Snake River Basin were near or exceeded the USEPA drinking-water standard of 10 milligrams per liter. Nitrate and DS concentrations in ground water in the Minidoka, A&B, and Eden study areas were some of the largest of those reported in all NAWQA Study Units.

Pesticide occurrence in ground water also poses some concern. Even though pesticide concentrations did not exceed USEPA drinking-water standards, the numbers of pesticides detected in some areas were large. More research is needed to establish the toxicological effects of drinking water that contains small concentrations of more than one pesticide or other organic compound.