Water-quality conditions in a national context

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 Ozark Plateaus Study Unit were compared with scores for all sites sampled in the 20 NAWQA Study Units during 1992­95. Results are summarized by percentiles; higher percentile values generally indicate poorer quality compared with sites in the 20 Study Units. Water-quality conditions at each site also are compared to 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.

Nutrient concentrations in two streams draining basins with predominantly agricultural land use and one stream downstream from a wastewater-treatment plant were higher than the national median. Beef cattle, dairy cattle, and poultry production provides a major source of nutrients to streams. Ammonia concentrations did not exceed the EPA criterion for protection of aquatic life at any of the agricultural sites. Nutrient concentrations in streams draining basins with predominantly forested land use were among the lowest in the 20 Study Units.

Pesticide concentrations in two intensively sampled streams in agricultural basins were among the smallest in the 20 Study Units. No exceedances of existing criteria occurred. The herbicides atrazine and simazine were the most commonly detected pesticides.

The bed sediment at sites located in lead-zinc mining areas and in or near the Boston Mountains had concentrations of trace elements at a higher level than most other sites in the 20 Study Units. Zinc concentrations and lead concentrations at Center Creek near Smithfield, Mo., were substantially higher than the national median zinc (about 50 times higher) and lead (about 15 times higher) concentrations.

Summed concentrations of semivolatile organic compounds in bed sediment at most sites were lower than at most sites in the 20 Study Units. None of the sites had concentrations above which there is a high probability of adverse effects on aquatic organisms. Two sites with concentrations exceeding the national Study Unit median are in forested areas. The two sites with the highest concentrations are in basins containing some of the more urban areas of the Study Unit.

Fish communities at most sites were less degraded (fewer diseased, tolerant, and omnivorous fish) than at most other sites in the 20 Study Units. Sites with most degradation in the Study Unit generally are on larger rivers affected by agricultural, mining, or urban activities. The fourth site is in a forested basin and was considered to have a degraded fish community because of the large percentage of omnivorous fish such as gizzard shad and common carp.

A well-vegetated riparian corridor contributes substantially to healthy habitat scores at many sites in the Study Unit. Gravel and lead-zinc mining probably have contributed to modified channels and increased sedimentation at some sites. A site on Center Creek, which flows through an area of agricultural land use and historical lead-zinc mining, is the only site that was more degraded than the median of sites in the 20 Study Units.