Simulation of water quality and the effects of wastewater effluent on the South Platte River from Chatfield Reservoir through Denver, Colorado

Abstract

Projected increases in population and discharges of wastewater effluent in the Denver metropolitan area makes compliance with water-quality standards increasingly difficult and necessitates controlling discharges of wastewater effluent to the South Platte River. In 1989, the State of Colorado adopted the U.S. Environmental Protection Agency's QUAL2E water-quality model as the preferred method for estimating effects of effluent on Colorado rivers. The Denver Regional Council of Governments entered into a cooperative agreement with the U.S. Geological Survey to use the QUAL2E model to evaluate the effects of wastewater effluent on the South Platte River from Chatfield Reservoir through Denver during low-streamflow conditions. Streamflow, dissolved-solids concentration, 5-day carbonaceous biochemical oxygen demand (CBODs), and concentrations of dissolved oxygen and nitrogen species were simulated for the river reach and for the Bear Creek and Cherry Creek tributaries. The QUAL2E simulation model was compared to the previously verified U.S. Geological Survey water quality (USGS-QW) simulation model for a 14.5-river-mile reach of the South Platte River from the Littleton streamflowgaging station to the streamflow-gaging station at 50th Avenue in Denver. Verification and simulation results for both models were similar for streamflow, dissolved-solids concentrations, and CBOD 5 ; however, concentrations of dissolved oxygen and nitrogen species were different. The QUAL2E program and reaction coefficients were modified to obtain a satisfactory fit to the measured values. The QUAL2E model data set then was revised to include a 4.5-river-mile reach that extended upstream from Littleton to the regulation dam at Chatfield Reservoir, the Bear Creek and Cherry Creek tributaries, and the Centennial and Glendale Wastewater Treatment Plants. The revised model then was recalibrated for the 19-river-mile reach. Critically low-streamflow conditions were simulated for 1989 and 2010. The simulations for 2010 included wastewater-treatment-plant effluent volumes and constituent concentrations. Simulations indicated that predicted CBOD 5 was about 1.5 to about 15 milligrams per liter greater in the South Platte River for 2010 conditions than for 1989 conditions; predicted dissolved-oxygen concentrations were similar; predicted total ammonia (as nitrogen) concentrations were about 0.2 to about 1.1 milligrams per liter greater upstream from the Bi-City Wastewater Treatment Plant and were about 0.7 to about 10 milligrams per liter less downstream from the plant; and predicted total nitrate (as nitrogen) concentrations were about 0.7 to about 6 milligrams per liter greater for 2010 conditions. Onsite measurements of specific conductance, pH, water temperature, and dissolved oxygen were made when each water-quality sample was collected. Reaeration coefficients were measured in selected reaches to evaluate methods of estimating reaeration coefficients. Reaeration-coefficient estimates used in the model were calculated by an equation developed b...