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Abstract

During 2002–2004, the U.S. Geological Survey’s National Water-Quality Assessment Program conducted a study to determine the effects of urbanization on stream water quality and aquatic communities in six environmentally heterogeneous areas of the conterminous United States—Atlanta, Georgia; Raleigh-Durham, North Carolina; Milwaukee-Green Bay, Wisconsin; Dallas-Fort Worth, Texas; Denver, Colorado; and Portland, Oregon. This report compares and contrasts the response of stream chemistry during base flow to urbanization in different environmental settings and examines the relation between the exceedance of water-quality benchmarks and the level of urbanization in these areas. Chemical characteristics studied included concentrations of nutrients, dissolved pesticides, suspended sediment, sulfate, and chloride in base flow.

In three study areas where the background land cover in minimally urbanized basins was predominantly forested (Atlanta, Raleigh-Durham, and Portland), urban development was associated with increased concentrations of nitrogen and total herbicides in streams. In Portland, there was evidence of mixed agricultural and urban influences at sites with 20 to 50 percent urban land cover. In two study areas where agriculture was the predominant background land cover (Milwaukee-Green Bay and Dallas-Fort Worth), concentrations of nitrogen and herbicides were flat or decreasing as urbanization increased. In Denver, which had predominantly shrub/grass as background land cover, nitrogen concentrations were only weakly related to urbanization, and total herbicide concentrations did not show any clear pattern relative to land cover—perhaps because of extensive water management in the study area. In contrast, total insecticide concentrations increased with increasing urbanization in all six study areas, likely due to high use of insecticides in urban applications and, for some study areas, the proximity of urban land cover to the sampling sites. Phosphorus concentrations increased with urbanization only in Portland; in Atlanta and Raleigh-Durham, leachate from septic tanks may have increased phosphorus concentrations in basins with minimal urban development. Concentrations of suspended sediment were only weakly associated with urbanization, probably because this study analyzed only base-flow samples, and the bulk of sediment loads to streams is transported in storm runoff rather than base flow. Sulfate and chloride concentrations increased with increasing urbanization in four study areas (Atlanta, Raleigh-Durham, Milwaukee-Green Bay, and Portland), likely due to increasing contributions from urban sources of these constituents. The weak relation between sulfate and chloride concentrations and urbanization in Dallas-Fort Worth and Denver was likely due in part to high sulfate and chloride concentrations in ground-water inflow, which would have obscured any pattern of increasing concentration with urbanization.

Pesticides often were detected at multiple sites within a study area, so that the pesticide “signature” for a given study area—the mixtures of pesticides detected, and their relative concentrations, at streams within the study area—tended to show some pesticides as dominant. The type and concentrations of the dominant pesticides varied markedly among sites within a study area. There were differences between pesticide signatures during high and low base-flow conditions in five of the six study areas. Normalization of absolute pesticide concentrations by the pesticide toxicity index (a relative index indicating potential toxicity to aquatic organisms) dramatically changed the pesticide signatures, indicating that the pesticides with the greatest potential to adversely affect cladocerans or fish were not necessarily the pesticides detected at the highest concentrations.

In a screening-level assessment, measured contaminant concentrations in individual base-flow water samples were compared with various water-quality benchmarks. One or more recommended Ecoregional nutrient criteria were exceeded at about 70 percent of the 173 total sites—less often for sites with less than about 3 percent urban land cover; these criteria are intended to represent baseline conditions for surface water that is minimally affected by human activities. Secondary drinking-water regulations for pH, sulfate, and chloride were exceeded at 24 sites, indicating some possibility of taste and odor problems at these sites if the stream water were to be used as drinking water without treatment. Otherwise, benchmarks were rarely exceeded: one or more human-health benchmarks was exceeded at 15 sites (for nitrate, atrazine, dieldrin, or simazine), and aquatic-life benchmarks at 12 sites (for pH, chloride, ammonia, chlorpyrifos, diazinon, and malathion). Benchmark exceedances were not related to the degree of urbanization, except that the dieldrin exceedances always occurred at sites with more than 60-percent urban land cover. Comparison of ambient stream water concentrations to human-health benchmarks (which apply to lifetime consumption of drinking water), as was done in this study, is not appropriate for human exposure assessment, but serves only to put the data in a human-health context. Because this study sampled stream water only twice per year during base-flow conditions, it is likely that the contaminant occurrence and benchmark exceedance rates described here may underestimate occurrence and exceedances in individual ambient water samples collected at other times, such as in peak pesticide- or fertilizer-use periods or during storm events or irrigation return flow.

The response of stream-water quality in base flow to urbanization differed by chemical constituent and by environmental setting. In areas where land cover in minimally urbanized basins was predominantly forest or shrub/grass, urbanization generally was associated with increasing chemical concentrations, although other nonurban factors may have been related to chemical concentrations as well. In areas where minimally urbanized basins were already affected by other stressors, such as agriculture, water management, or inflow of relatively saline ground water, the effects of urbanization were less clear. Maintenance or protection of stream quality may be addressed by identifying all important stressors and supplementing the management practices currently used in urbanizing areas with additional steps to mitigate the effects of these other stressors.