Sources and transport of phosphorus and nitrogen during low-flow conditions in the Tualatin River, Oregon, 1991-93

Abstract

In the 1980s, significant nutrient-related water-quality problems that impacted beneficial uses were identified in the Tualatin River during the low-flow summer months, defined as May 1 to October 31. Nuisance algal blooms resulted in fluctuations in oxygen concentrations and pH conditions; reduction of phosphorus concentrations was determined to be the most effective control mechanism for these conditions. Elevated ammonia concentrations also contributed to low dissolved oxygen concentrations. Because standards for beneficial uses were not being met, the Oregon Department of Environmental Quality established Total Maximum Daily Loads (TMDLs) for phosphorus and ammonia in the Tualatin Basin, as required by the Clean Water Act. To provide necessary context for the TMDL process, data were collected during the period 1991-93 to characterize the sources and transport of water, phosphorus, and major forms of nitrogen in the main-stem Tualatin River during the summer. A significant source of water to the river was not accounted for by surface-water inputs, and was consistent with direct discharge of ground water to the main-stem river channel. Ground water is also the primary source of water for the tributaries during the summer low-flow season. Because large natural supplies of highly mobile phosphorus exist in the upper 500 feet of valley-fill sediments throughout the Tualatin Basin, ground water in the basin is naturally enriched with phosphorus. While improvement in wastewater treatment efficiencies and land management practices have resulted in significant reductions in nutrient concentrations in the Tualatin River, phosphorus concentrations continue to exceed TMDL criterion concentrations. The presence of significant geologic sources of phosphorus in the basin will confound the achievement of current TMDL criteria for phosphorus in the Tualatin River and its tributaries. In contrast, natural sources of all forms of nitrogen to the Tualatin River are insignificant relative to the effluent from the wastewater treatment plants in the basin. Efficient wastewater treatment is, therefore, an effective means for controlling ammonia concentrations in the main-stem river.