Stormwater management can protect water quality: study

North storm drain showing people recreating too close to a storm water spill in Banks Channel. Photo: Mallin, Grogan study

In American bays and estuaries, stormwater runoff is the second leading cause of water degradation. North Carolina’s coastal watersheds are no exception, making stormwater management techniques necessary to maintain the state’s water quality.

A study published in the Journal of Environmental Management in January by Amy Grogan and Michael Mallin of the Center for Marine Sciences, University of North Carolina Wilmington, examined the effectiveness of a set of stormwater management best practices, or PGBs, implemented on a study site. at Wrightsville Beach.

The study, “Successful mitigation of nutrient, fecal bacteria and suspended solids load from stormwater in a recreational beach communityFound that the BMPs implemented at Wrightsville Beach reduced the overall amount of contaminants in the study area.

Wrightsville Beach is an island community on the North Carolina coast near Wilmington. Between the town’s two islands is Banks Channel, an estuarine sound dotted with docks, a beach, and places to hire kayaks and paddleboards.

The study examined the drainage area of ​​two stormwater outlets that flow into Banks Channel. Like many urbanized areas of the shoreline, the Wrightsville Beach study area was covered with impermeable pavements. Impervious pavements do not allow water to seep into the ground, but they are washed in waterways. It takes with it all the contaminants it has come into contact with. Stormwater pollution has been identified as a problem deserving special attention in the community of Wrightsville Beach.

Funding for the project was provided by the Coastal Federation of North Carolina, thanks to a grant from North Carolina Land and Water Fund, formerly known as the North Carolina Clean Water Management Trust Fund. The Coastal Federation publishes Coastal Review.

A BMP system was installed to replace the impermeable pavement with a permeable pavement, designed to let stormwater seep into the ground, at the Hanover Seaside Club in Wrightsville Beach. An infiltration chamber was also built in the parking lot. This BMP system was installed in spring 2018.

The impermeable pavement is replaced by a permeable pavement around the south storm drain, the Hanover Seaside Club is in the left background.  Photo: Grogan, Mallin study
The impermeable pavement is replaced by a permeable pavement around the south storm drain, the Hanover Seaside Club is in the left background. Photo: Grogan, Mallin study

The Mallin and Grogan study set out to test the effectiveness of BMPs in the study area. They tested the stormwater discharge in the two discharge pipes in the Banks Channel before and after the installation of the BMPs. They tested things like fecal bacteria, suspended solids, and nitrogen. In coastal communities, high nitrogen levels can be the canary in the coal mine for bigger problems.

“(When) you load nitrogen into the water, you increase your chances of causing algae blooms,” Mallin said. “And some of these flowers could be poisonous.”

Harmful algal blooms have become increasingly common in the United States. As they can be destructive to the ecosystems in which they are found and produce toxins toxic to humans, there is significant interest in monitoring nitrogen levels in rivers.

Mallin and Grogan found that the BMP system reduced overall stormwater runoff by 62%. Total nitrogen was reduced by 87% and bacteria indicative of enterococcal fecal contamination by 76%. Mallin considers these overall results to indicate that BMPs have been successful in a way that positively impacts the community of Wrightsville Beach.

Water quality is important not only for the habitats and ecosystems in contact with Banks Channel, but also for the local economy and residents. During warmer weather, Banks Channel is a popular spot for sunbathers, swimmers, and kayakers. These are all tourist activities that depend on the quality of the water.

Shellfish farmers in the region are also directly dependent on the quality of the water for their work. When faecal microbial pollution reaches high levels in coastal waters, shellfish farmers have to shut down their operations. For an island community like Wrightsville Beach, the usability of the canal is important to the local economy.

“It is not only becoming a health issue, but it is an economic pressure on the local municipality,” Mallin said.

According to Mallin, these BMPs have been proven to work in Wrightsville Beach, but could also prove useful in comparable coastal communities, as well as some inland riparian habitats. That being said, the effectiveness of BMPs must be individualized according to the different fields of study.

“BMPs that work in one location may not work in another, depending on the geology of the area,” Mallin said. Much depends on the type of soil. Wrightsville Beach is very sandy and comparable areas can see similar results.

Although the overall numbers indicate success, the split between the two pipes studied shows that although enterococcus decreased in the north pipe after installation of the BMPs, it actually increased in the south pipe.

The northern stormwater outlet in Banks Channel at high tide, Photo: study E. Grogan and MA Mallin

According to Dr. Rachel Noble of the Institute of Marine Sciences at the University of North Carolina at Morehead City, this shows that the effectiveness of BMPs like these is an ongoing conversation.

Much of Noble’s work has focused on water quality and microbial contaminants, and she was part of a research team that identified stormwater in the Wrightsville Beach area as contaminated and deserving of attention. further study. Noble says there’s a big picture of the systemic issues that are causing this pollution.

“We cannot continue to develop the coast and increase impervious surfaces without making sure to promote infiltration for rain and flood water,” Noble said.

According to Noble, there are multiple pressures on North Carolina’s coastal systems that challenge effective stormwater management. These pressures include things like aging sewage systems, rising sea levels, height of groundwater, and wastewater management. In addition, the coastal topography does not have much variability in gradient. This makes it difficult to always move the water in the desired direction. The variability of the study’s results, Noble said, illustrates the complexity of the issue.

“There is just no quick fix,” Noble said.

So while Noble believes that BMPs like permeable pavements hold promise and offer hope, systemic issues need to be addressed in tandem.

Noble said the more information you can collect, the better coastal communities can do to keep moving in the right direction. And stormwater BMPs like this one show promise for reducing some contaminants.

“It shows some possibilities,” Noble said. “But there’s still a lot to do.”


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