Land-use practices in the Denver metropolitan area can affect the quality of streams and shallow ground water in various ways. For example, fertilizers and pesticides applied to lawns, parks, and roadways can wash into streams. Sediment from construction sites also can wash into streams. Fuels and solvents used throughout the urban area can migrate into shallow aquifers.
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Color indicates type of pesticide: Herbicide and Insecticide.
All 36 of the surface water samples collected at two urban sites in the Denver metropolitan area contained at least one pesticide (Kimbrough and Litke, in press), and water from 90 percent of the wells (27 of 30 wells) sampled in the urban alluvial aquifer contained at least one pesticide (Bruce and McMahon, 1996, 1998). Twenty-eight different pesticides were detected in surface water, but only 9 pesticides were detected in ground water. Atrazine, prometon, and simazine were frequently detected in both surface water and ground water.
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Surface water and shallow ground water in Denver, Colo., are susceptible to contamination by urban activities.
Pesticide concentrations were generally low, with only seven pesticides having median values above the method detection limit. However, the health advisory limit was exceeded for diazinon for one surface-water sample, and aquatic-life criteria were commonly exceeded for carbaryl, diazinon, and chlorpyrifos. No ground-water sample had a pesticide concentration in excess of a regulatory criteria. The frequent detection of pesticides, however, indicates that pesticide use in Denver does affect water quality.
Volatile organic compounds derived from substances commonly used in urban areas, such as gasoline and cleaning solvents, were detected in 86 percent of the wells sampled from the alluvial aquifer in Denver (Bruce and McMahon, 1996; McMahon and Bruce, 1997). Sixty-two percent of the samples had more than one VOC present. The most frequently detected VOC was methyl tert-butyl ether (MTBE), a gasoline additive used to reduce carbon monoxide emissions from vehicles during winter. The second most frequently detected VOC was tetrachloroethene (PCE), a common cleaning solvent. Benzene and PCE were the only VOCs whose concentrations exceeded drinking-water standards (see Summary of Compound Detections and Concentrations section, p. 28-34 for range of concentrations). These findings indicate that gasoline and solvents used in the urban area affect water quality.
Thirty-one (ten most frequently detected VOCs shown) of 59 VOCs were detected in shallow urban ground water.
Nutrients such as ammonia, nitrate, and phosphorus have many potential sources in the Denver metropolitan area, including fertilizers applied to lawn areas and discharges from wastewater treatment plants. However, nutrient concentrations in urban streams did not exceed any existing USEPA drinking-water standard (Litke, 1996). Three of 30 samples of shallow alluvial ground water had nitrate concentrations that exceeded the 10-mg/L drinking-water standard (Bruce and McMahon, 1996). Nutrient concentrations in urban streams generally were highest immediately downstream from wastewater treatment plants. The effect of wastewater discharges on water quality are discussed on page 18 of this report.
Urban sources of surface water and alluvial ground water are not currently (1998) used for drinking water in the Denver metropolitan area. However, it is possible that these supplies could become drinking-water sources as demand for water increases.
Fish and stream habitat in the urban environment have been affected by land-use activities. For example, tissue from common carp and white sucker from urban streams contained the banned compounds, PCBs and chlordane, both of which were widely used historically in the urban environment (Tate and Heiny, 1996). Fish communities also were dominated by suckers, a family of fish that is tolerant of degraded water-quality conditions. Channel modifications for flood control and bank stabilization in Cherry Creek and the South Platte River have resulted in high suspended-sediment concentrations and degraded habitat for biotic communities. Despite these negative effects of urban land use on fish communities and habitat, an IBI developed for fish communities in the South Platte River Basin as a tool to assess the ecological health of streams across the basin indicated water-quality conditions were only moderately degraded.
Channel modification has a direct effect on stream habitat and biological communities in the South Platte River.
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Development of areas along the South Platte River is ongoing.
The NAWQA study determined that streams and shallow ground water in the urban area are susceptible to contamination from pesticides and VOCs, indicating that better management of potentially toxic substances may be needed to protect water resources for possible future as drinking-water supplies. |
