Assessing Water Quality through Three Dimensional Hydrodynamic Models

The Challenge

Accurately assess water quality

The requirements set by the Clean Water Act of 1972 give the U.S. Environmental Protection Agency (EPA) the responsibility to assess the water quality of waterways in the U.S.. After obtaining water samples from the Neuse River Estuary in North Carolina, the EPA Region 4 found that the waterway contained high levels of phytoplankton algae (measured by chlorophyll-a). The waterway was immediately declared "impaired."

Algal blooms and high levels of chlorophyll-a are typically due to high levels of nutrients, specifically nitrogen and phosphorus. Nitrogen and phosphorus are naturally occurring constituents in soils and water. However, when levels are excessive due to pollutant loadings, algal blooms can occur. These algal blooms are measured by taking water quality samples and measuring chlorophyll-a in a laboratory. In order to determine why the waterway was impaired, the EPA commissioned Tetra Tech Inc., an environmental engineering and consulting firm, to conduct a study and model of the waterway.

The Solution

Correctly model how water moves in the system

The Neuse River Estuary is located in North Carolina at the convergence of the Neuse River and Pamlico Sound. The Neuse River is 179 miles long and its watershed drains approximately 5,700 square miles from the coastal plain and piedmont provinces of the state. In order to effectively study the waterway, the engineers at Tetra Tech had to first get a thorough understanding of how the water moves within the estuary.

Steven Davie, the director of Water Resources for Tetra Tech, Inc., has worked for Tetra Tech for over six years. In his department, the engineers specialize in complex water modeling for diverse applications to look at water transport and pollutant loadings of waterways such as lakes, river, and estuaries.

For this project, a water quality model was coupled with the EFDC model called the Water Quality Analysis Simulation Program (WASP), developed by EPA. EFDC and WASP were used to predict a response in chlorophyll-a and dissolved oxygen concentrations as a function of nutrient (nitrogen and phosphorus) loadings and transport throughout the estuary. The model was used to evaluate various loading scenarios and their impact on water quality.

However, before the engineers could use the EFDC and WASP model to assess the water quality of the estuary, they had to make sure that the model would correctly predict water movement in the system. In order to validate the code, the engineers first used the model to simulate the salinity of the waterway.

"If someone shows us a salinity plot of measurements, then their next question is show me that your model can do the same thing," says Davie, "so we have to prove to them that the model is actually reproducing what is happening in the real world."

To create the salinity animation, Davie created output from the EFDC model, used a macro from Tecplot visualization software that allowed for multiple frames, and created five frames for the animation. By simulating the salinity of the estuary, Davie and his team were able to get a better understanding of how water moves throughout the waterway.

"Salinity is a good indicator of transport in the system," says Davie. "Is also allowed us to see how wind can incorporate enough energy into the surface waters that a bulging effect occurs as the surface waters are pushed to one side. The bulging of the surface waters will push the deeper, denser waters to the opposite side of the estuary. This phenomenon is recorded by salinity measurements on the side of estuary," says Davie.

Understanding the Complexities of Water Transport

Even the company's engineers were surprised by the results of the simulation. "The way the water moves in that estuary is not typical," says Davie. "Most estuaries have strong tides and the water flushes in and out. This one was really different because there is a small tidal range at the mouth near the ocean and wind is the main factor of why the water moves around. That was the first eye-opening experience."

The model also showed that when the wind subsides, the water is released and a sloshing of the deeper dense waters occurs. This lateral mixing and sloshing generates complex transport patterns that have a clear impact on the fate and transport of phytoplankton, salinity, and the upwelling of low dissolved oxygenated bottom waters. Under certain conditions in the system, strong lateral gradients exist. In addition, salinity and temperature are not only stratified top to bottom, but also side to side.

Davie, who has used Tecplot software since 1995, says that Tetra Tech uses the software to generate both time series plots and animations to present the results of the complex 3D models to their clients and the public. Among the software's greatest strengths, Davie attributes its ability to manipulate large files in a timely manner, good flexibility in layouts and frames, as well as its excellent graphical output.