Water Resources of North Dakota

Simulation of Effects of Discharging Treated Wastewater to the Red River of the North at Fargo, North Dakota

By Edwin A. Wesolowski

Abstract: North Dakota Water and Pollution Control Conference, Quarterly Issue, April, May, June 1993, v.LXVIII, nos. 10, 11, and 12

The single or combined effect of point- and nonpoint-source discharges on the water quality of the Red River of the North from Fargo, North Dakota, to its confluence with the Buffalo River near Georgetown, Minnesota, is not known. A water-quality model was used to characterize and evaluate the effects of discharging treated wastewater to the 31-mile reach. This paper presents selected data and information from and selected results from the study, including identifying and summarizing selected data; calibration and verification of the model; simulation of dissolved-oxygen concentration; sensitivity of dissolved-oxygen concentration to selected properties, constituents, and coefficients; and simulation of two hypothetical streamflow and water-quality conditions. Measurements of cross-section depth and width, discharge, traveltime, and reaeration were made in 1989 and 1990 to define the river's hydraulic characteristics and instream reaeration-rate coefficients. Two 24-hour water-quality samplings were made to define the river's water-quality characteristics. Streamflow in the study reach was 60 to 523 cubic feet per second during data collection. The Enhanced Stream Water-Quality Model was calibrated and verified to simulate biochemical processes in the study reach, and its companion program was used for uncertainty analysis. Calibration of the transport component of the model consisted of comparing simulated widths, depths, and areas with measured values. Calibration of the water-quality component of the model consisted of comparing simulated and measured values at nine sites. The model simulates values or concentrations of specific conductance, water temperature, dissolved oxygen, ultimate carbonaceous biochemical oxygen demand, total nitrite plus nitrate as nitrogen, total ammonia as nitrogen, total organic nitrogen as nitrogen, total phosphorus as phosphorus, and algae as chlorophyll a. For the purpose of this paper, only the simulation of specific conductance and dissolved oxygen is discussed. Model sensitivity analyses generally indicate that increases in dissolved-oxygen concentration are most sensitive to maximum specific algal growth and that decreases in dissolved-oxygen concentration are most sensitive to point-source total ammonia as nitrogen downstream of treated wastewater outflow. The model, using verified conditions, was used to simulate two hypothetical streamflow and water-quality boundary conditions. When the model simulated streamflow in the upstream reach as 75 cubic feet per second, outflow from the Fargo wastewater-treatment plant as 37.8 cubic feet per second, and total ammonia as nitrogen as 15 milligrams per liter, the water-quality standards were exceeded by the simulated total ammonia as nitrogen, total nitrite plus nitrate as nitrogen, and dissolved oxygen.

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