Modeling Pollution on the Cape Fear River
Scientist Danica Schaffer-Smith is creating a model that gives a better picture of water quality in the Cape Fear River basin.
The Cape Fear watershed is big—the size of New Jersey. Until now, getting an accurate understanding of water quality across such a large area, has been difficult. Governments and community-based organizations analyze water samples but rarely pull from across the entire basin. Watershed Scientist Danica Schaffer-Smith is working to fill in those gaps by creating a model that gives TNC and other organizations a better picture of water quality across the basin. Then, TNC, governmental agencies and other conservation organizations can use this new data to determine where conservation work will have the greatest impact on water quality.
“You can’t realistically sample all over the basin,” Schaffer-Smith explains. “But this model allows us to take the limited sampling we do have and estimate what’s going on across the basin.
“There’s been a real inequity in how we understand this watershed,” she adds. “There tends to be a bias towards our urban centers where more data is collected. You have big swathes of the Middle and Lower Cape Fear Rivers, where we don’t have enough data. Currently state agencies are not able to determine if many streams and rivers are impaired or if they are healthy.”
Modeling Water Quantity and Quality
Schaffer-Smith used the Soil and Water Assessment Tool (SWAT), an open-source watershed model developed by the U.S. Department of Agriculture, to conduct the initial assessment. The model accounts for land use, point source discharges such as municipal wastewater treatment, nonpoint source discharges or runoff and weather conditions from 1979 to 2019. It looks at nitrogen, phosphorus and sediment going into the river to determine where that pollution originates and how it varies depending on weather and river flows.
“This is a highly variable system. There’s just a lot happening in the basin,” Schaffer-Smith says. “There are times when we can be having flooding and drought at the same time in different parts of the Cape Fear.”
That complexity is reflected in the model’s findings. During normal flows and flooding, nonpoint source pollution accounts for 90% of the pollution going into the Cape Fear. But during drought years, point sources can drive in-stream pollution. “This is where the dynamics of this watershed are just astounding to me,” Schaffer-Smith says. “Pull out a dry year like 2011 and as much as 70% of nutrient pollution is coming from point sources.”
Identifying Priorities for Conservation and Restoration
The model also found that 16% of the watershed contributed the most pollutants across all flow conditions. “This is much easier to tackle than something the size of New Jersey,” Schaffer-Smith says. “This identifies key places that are worthy of the most attention.”
The model findings can be overlaid with earlier mapping that Schaffer-Smith produced showing the areas that repeatedly flooded in recent hurricanes. Together the flood mapping and watershed modeling can help identify areas where restoration would help reduce pollution into the river as well as absorb or slow flooding.
The modeling can help guide future work in the basin. “Our hope is that we will be able to influence where restoration happens on the ground,” she explains. “This can be really valuable to increase the pace and scale of restoration in the basin and using taxpayer dollars wisely.”
Identifying Priority Areas for Interventions in the Cape Fear River Basin
- Journal Article Pre-Print (.pdf)
Improving Water Sources for Nature and People
TNC continues to build on the models. A grant from the U.S. Geological Survey (USGS) will drill down on specific communities, considering socioeconomic vulnerability, and determining how restoration will benefit that community. “This additional work will make the model more locally relevant,” Schaffer-Smith says.
A student from North Carolina Central University (NCCU) will work on this part of the project. NCCU Central Professor Timothy Mulrooney, who will supervise that work, says this is important work for the basin and his student. “A big hallmark of our program is to provide students with experiential learning. This is an opportunity for them to work with TNC professionals, helping to make certain that student is workplace ready.”
NCCU is one of the many partners working on the project. Researchers from Arizona State University (ASU), USGS and North Carolina State University (NCSU) are also a playing role.
For her part, Schaffer-Smith is looking forward to the next stages of her work, which will get her out from behind a computer. She is working on an outreach plan to spread the word about the model and its implications. “We want this to be as useful as possible to as many people as possible,” she says.
Ultimately, she looks forward to the day when the computer modeling becomes reality on the landscape. “That will be the icing on the cake, to actually see a project happen based on this work.”
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