Using Ce-Qual-W2 to assess the ammonia assimilative capacity of the tualatin river; oregon

摘要

A moidfied version of the U.S. Army Corps of Engineers model CE-QUAL-W2 was used to simuate flow, temperature, and water quality in the lower reaches of the Tudalatin River, a low-gradient stream that meanders through a mix-ture of urban and rural landscapes on the western side of the Portland, Oregon metropolitan area. Simulated water-quality constituents included nutrients (nitrogen and phosphorus), phytoplankton, and dissolved oxygen. Once calibrated, the model was used to quantify the river's ability to assimilate ammonia wasteloads fromthe basin's two largest wastewater treatment plants without causing violations of the State of Orggon minimum dissolved oxgyen standard. This assimilative capacity increases with increasing river discharge and solar insolation, and decreases with increasing water temperature. Model simualtions were used to determine that the 30-day mean dissolved oxygen concentration would decrease by only about 0.2 milligrams per liter under the most sensitive observed conditions when each of the wastewaer treatment plants releases a constant load of 100 pounds per day of ammonia nitrogen. This determination of the ammonia assimilative capacity will aid in the ongoing reassessment of a Total Maximum Daily Load for ammonia in the Tualatin River.