Calibration, verification, and use of a water-quality model to simulate effects of discharging treated wastewater to the Red River of the North at Fargo, North Dakota

A 30.8-mile reach of the Red River of the North receives treated wastewater from plants at Fargo, North Dakota, and Moorhead, Minnesota, and streamflows from the Sheyenne River. A one-dimensional, steady-state, stream water-quality model, the Enhanced Stream Water Quality Model (QUAL2E), was calibrated and verified for summer streamflow conditions to simulate some of the biochemical processes that result from discharging treated wastewater into this reach of the river.

Data obtained to define the river's transport conditions are measurements of channel geometry, streamflow, traveltime, specific conductance, and temperature. Data obtained to define the river's water-quality conditions are measurements of concentrations of selected water-quality constituents and estimates of various reaction coefficients. Most of the water-quality data used to calibrate and verify the model were obtained during two synoptic samplings in August 1989 and August 1990.

The water-quality model simulates 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 algal biomass as chlorophyll a. Of the nine properties and constituents that the calibrated model simulates, all except algae were verified. When increases in dissolved-oxygen concentration are considered, model sensitivity analyses indicate that dissolved-oxygen concentration is most sensitive to maximum specific algal growth rate. When decreases in dissolved-oxygen concentration are considered, model sensitivity analyses indicate that dissolved-oxygen concentration is most sensitive to point-source ammonia. Model simulations indicate nitrification and sediment oxygen demand consume most of the dissolved oxygen in the study reach.

The Red River at Fargo Water-Quality Model and the verification data set, including associated reaction-coefficient values as input, were used to simulate total ammonia as nitrogen, total nitrite plus nitrate as nitrogen, 5-day carbonaceous biochemical oxygen demand, and dissolved oxygen for water-quality conditions that result from three hypothetical boundary conditions. The model was applied to various combinations of three hypothetical waste loads when the headwater streamflow was either 50 or 75 cubic feet per second, when Fargo's wastewater-treatment plant outflow was either 15 or 37.8 cubic feet per second, and when total ammonia as nitrogen concentration of the outflow was either 5,9, or 15 milligrams per liter. For each hypothetical waste load, at least one water-quality standard for either total ammonia as nitrogen, total nitrite plus nitrate as nitrogen, or dissolved oxygen was contravened, and, for one scenario, all three standards were contravened.