Projects & Programs
Evaluation of Dissolved Oxygen and Phytoplankton Biomass in the Willamette River, Oregon
The Willamette River Basin Water Quality Study (WRBWQS) is an ongoing interdisciplinary study that includes investigations of river hydrology, sediment transport, toxic organic compounds and trace elements, point and nonpoint pollution sources and aquatic ecosystems. The development of predictive computer models was one goal of the study. As part of the WRBWQS, PNNL staff, while at another organization, were contracted by the Oregon Department of Environmental Quality to select, develop and calibrate a computer model of dissolved oxygen (DO) concentrations in the main stem of the Willamette River during the low-flow conditions of late summer. The low-flow period coincides with the critical period for DO and is suitable for steady-state DO modeling. The selected model was to incorporate the interactions among nutrients, phytoplankton and DO, a step that had not previously been undertaken.
In order to first identify appropriate predictive models, several DO models of varying complexity were identified and evaluated using several selection criteria: 1) dimensionality—a one-dimensional model was considered adequate, 2) temporal characteristics—a steady-state model was considered appropriate for the summer low-flow period of interest, 3) consideration of relevant processes—including the capability to model phytoplankton growth and nutrient interactions, 4) suitability for a range of applications—e.g., temperature or bacteria modeling, 5) data requirements—the data required for model calibration had to be within the resources of the study and 6) ease of use—the selected model needed to be "user friendly" so that water-quality managers could easily use it as a decision-making tool.
Based on these selection criteria, the model QUAL2E-UNCAS (version 3.14) was selected. This model is supported by the U.S. Environmental Protection Agency-Center for Exposure Assessment Modeling. The QUAL2E model is a one-dimensional, steady-state model that incorporates all of the relevant processes and has a menu-driven input and output system that facilitates use. The model also includes applications for component, sensitivity, first-order error analysis and Monte Carlo simulations.
Project Highlights:
- The model was calibrated and developed into a water quality management tool
- The model demonstrated that over 50% of loading to the river was from non-point sources
- The model identified that sediment oxygen demand was a significant sink of DO in the lower 20 miles of the river
- The model showed that by maintaining flows above 6,000 ft 3/sec, DO and phytoplankton issues in the Willamette River would be minimized.