By Valerie J. Kelly
USGS Water-Supply Paper 2465-A, 68 pages, 50 figures, 24 tables
During the winter season, defined as November 1 to April 30, four wastewater treatment plants in the Tualatin River Basin discharge about 10,000 to 15,000 pounds per day of biochemical oxygen demand, both carbonaceous and nitrogenous, to the river. These loads often increase substantially during storms when streamflow is also increased. Another issue concerns the discharge of about 2,000 pounds per day of ammonia from the treatment plants during the early winter season, when streamflow is frequently less than the average winter flow. This study focused on the capacity of the river to assimilate oxygen-demanding loads under winter streamflow conditions during the 1992 water year, with an emphasis on peak-flow conditions in the river, as well as winter-base-flow conditions during November 1992.
Concentrations of dissolved oxygen throughout the main stem during the winter remained high relative to the State standard for Oregon of 6 milligrams per liter, except during periods of streamflow less than 500 cubic feet per second and temperatures greater than 10 degrees Celsius. The most important factors controlling oxygen consumption during winter low-flow conditions (streamflow from 500 to 2,000 cubic feet per second) were carbonaceous biochemical oxygen demand from wastewater treatment plants and tributaries, and input of oxygen-depleted waters from tributaries. In spite of increased ammonia loads, nitrification was not significant because of the cold water temperature and reduction in residence time. During peak-flow conditions (streamflow greater than 2,000 cubic feet per second), oxygen depletion was negligible; the effect of increased oxygen-demanding loads was minimized by the greatly reduced travel time and increased dilution associated with the increased streamflow.
During winter low-flow conditions, the combined mean load of carbonaceous biochemical oxygen demand from the wastewater treatment plants was essentially equivalent to the combined mean load from the tributaries. The total mean load increased by two-fold during peak-flow conditions, relative to winter low-flow conditions. This increase in carbonaceous biochemical oxygen demand was exclusively from the tributaries, because the combined mean load from the wastewater treatment plants actually decreased.
During the base-flow period in November 1992 (streamflow less than 300 cubic feet per second, conditions that would be expected to occur once every 3 to 4 years), concentrations of dissolved oxygen at river mile 3.4 consistently fell below 6 milligrams per liter. A hydrodynamic water-quality model was used to identify the processes depleting dissolved oxygen, including sediment oxygen demand, nitrification, and carbonaceous biochemical oxygen demand. Sediment oxygen demand was the most significant sink for oxygen during this period, accounting for more than 50 percent of the oxygen demand. Nitrification was important, but not as significant, accounting for nearly 30 percent of the oxygen demand. The effect of carbonaceous biochemical oxygen demand, about half of which was from the wastewater treatment plants, was slight due to the low rate of in-river decay. These results suggest that further reductions in loads of carbonaceous biochemical oxygen demand from the treatment plants during the base-flow period would have little effect on oxygen concentrations in the river.
Hypothetical scenarios were simulated to evaluate the effect of different loading strategies employed by the treatment plants in the basin during winter base-flow conditions. Streamflow and temperature were determined to be significant factors governing the capacity of the river to assimilate oxygen-demanding loads. For the range of loads simulated, at water temperatures between 12 and 13 degrees Celsius, streamflows near 350 cubic feet per second are required to maintain dissolved oxygen concentrations in the river above the State standard. When water temperatures increase to 18 degrees Celsius, streamflows must increase to 500 cubic feet per second to maintain concentrations above the standard.
John Williams <johnw@usgs.gov> U.S. Department of the Interior | U.S. Geological Survey