Wastewater-treatment plant effluent can contribute almost all of the flow in the South Platte River downstream from Denver.
There are more than 100 permitted municipal wastewater treatment plants (WWTPs) in the South Platte River Basin. Twenty-five plants located along the Front Range urban corridor discharge about 275 million gallons of effluent per day or about 95 percent of the total daily effluent discharge in the basin. Most plants discharge into streams, and in some locations, effluent can make up a substantial part of the streamflow. For example, Metro Wastewater Reclamation District annually contributes about 69 percent of the streamflow and at times 100 percent of the streamflow in the South Platte River downstream from Denver, Colo. (Dennehy and others, 1995). About 7,000 tons of nitrogen and 1,200 tons of phosphorus enter the South Platte River Basin every year from WWTPs (Litke, 1996).
Nitrate and ammonia were the primary forms of nitrogen present in WWTP effluent in the basin (Litke, 1996). Elevated levels of these constituents can lead to excessive growth of algae and other aquatic plants. Also, high nitrate levels can be a health concern to infants, and high levels of ammonia in its un-ionized form can be toxic to fish. Data collected from synoptic sites indicated that for most streams along the Front Range urban corridor, total nitrogen concentrations were substantially higher downstream from WWTPs.
Phosphorus is the nutrient that often is the controlling factor in causing eutrophication in reservoirs and aquatic plant growth in streams, and the USEPA has recommended a limit of 0.1 mg/L for total phosphorus in streams (U.S. Environmental Protection Agency, 1986). NAWQA synoptic data showed that total phosphorus concentrations in the main stem South Platte River generally exceeded this limit in the 150-mile reach of the river from Denver to Balzac, Colo. On the main stem, total nitrogen and total phosphorus concentrations were largest just downstream from Denver and generally decreased in a downstream direction.
Elevated nutrient concentrations in streams have the potential to affect oxygen and food supplies for aquatic communities through eutrophication. Among the five main-stem sites monitored for IBI, the South Platte River at Henderson, Colo., had an IBI score of 24, which was the lowest for all sites considered and indicated significantly degraded conditions. Elevated nutrient concentrations in the South Platte River at Henderson, primarily caused by effluent from Denver's WWTPs, contribute to degraded conditions at this site.
Total nitrogen concentrations in streams along the Front Range were substantially larger downstream from WWTPs.
Total nitrogen and total phosphorus concentrations in the South Platte River were largest just downstream from Denver and then generally decreased in a downstream direction.
Nutrient concentrations were elevated downstream from WWTPs. Phosphorus concentrations were consistently in excess of USEPA recommended limits.