WATER-QUALITY CONDITIONS IN A NATIONAL CONTEXT: Comparison of Stream Quality in the South Platte River Basin Study Unit with Nationwide NAWQA Findings.

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Seven major water-quality characteristics were evaluated for stream sites in each NAWQA Study Unit. Summary scores for each characteristic were computed for all sites that had adequate data. Scores for each site in the South Platte River Basin Study Unit were compared with scores for all sites in the 20 NAWQA Study Units sampled during 1992-95. Results are summarized by percentiles; higher percentile values generally indicate poorer quality compared with other NAWQA sites. Water-quality conditions at each site also are comparto established criteria for protection of aquatic life. Applicable criteria are limited to nutrients and pesticides in water and semivolatile organic compounds, organochlorine pesticides and PCBs in sediment. (Methods used to compute rankings and evaluate aquatic-life criteria are described by Gilliom and others, in press).

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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Nutrient concentrations in urban, mixed (urban/agriculture), and agricultural land-use areas were some of the highest compared to other NAWQA Study-Unit sites. Ammonia concentrations exceeded aquatic-life criteria for fish (salmonids) toxicity at most sites in these land uses. In contrast, nutrient levels in mountain and rangeland sites were among the lowest nationally.

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Pesticide concentrations in the Denver urban area and in an agricultural area near Greeley, Colo., are among the highest of all NAWQA Study-Unit sites. Both urban sites exceeded the aquatic-life criteria for three compounds during the summer.

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Concentrations of organochlorine pesticides and PCBs including DDT, chlordane, and dieldrin in the South Platte River Basin in urban, agricultural, and mixed (urban/agriculture) land uses rank among the highest of all sites sampled in NAWQA Study Units.

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Though some trace elements occur naturally in soils, South Platte River Basin sites in urban areas and sites affected by mining were ranked among the highest compared to sites in other NAWQA Study Units.

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A majority of the sites in the South Platte River Basin sampled for SVOCs were ranked above the national median for all NAWQA sites sampled. Only two urban sites had compounds that exceeded threshold concentrations known to have adverse effects on aquatic life.

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Fish communities are directly related to the habitat available and the chemistry of the environment in which they live. Sites in urban, agricultural, and mixed (urban/agriculture) land-use types have fish communities that are among the most severely degraded of all NAWQA Study-Unit sites. These degraded fish communities are characterized by a large percentage of contaminant-tolerant species and correlated to stream habitat degradation and high concentrations of chemicals in water, sediment, and fish. In contrast, fish communities in forested mountain and rangeland sites were ranked lower than the national median; the exception occurs at a mountain site having high concentrations of trace elements in sediment as a result of upstream mining.

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Physical characteristics of stream channels (natural compared to modified) and streambanks (vegetation cover, bank erosion) strongly affect water quality and the ability of streams to support biological communities. In the South Platte River Basin, the physical condition of most stream habitat assessed was lower than the national median; the exceptions occurred at sites located in agricultural and urban areas, which also had degraded fish communities.

CONCLUSIONS

Physical, chemical, and biological measures indicate that urban, mixed(urban/agriculture), and agricultural land-use areas generally have poorer water quality not only within the South Platte River Basin but also compared to all sites sampled for the NAWQA program. In contrast, sites in forested mountain and rangeland areas have good water quality both within the basin and in comparison to all study units.