Constructing Wetlands to Improve Coastal Water Qualityby Claire Payne
South Carolina draws developers, residents, and visitors looking for a mild climate—development of previously wooded tracts of land is at an all time high along the coast. This rapid influx of people and development has brought waterquality problems to previously healthy coastal waterways—the result of both increased pollution and the loss of ecosystem services from the forests and wetlands that once protected coastal systems. Unfortunately, change is outpacing both the technology and the legislation that could mitigate this source of water pollution. As a result, the health of coastal waters appears to be declining, and there is concern that sustainability of the fisheries is threatened.
The relatively infertile lands that lie along the South Carolina coast have not been considered valuable until recently. After the Civil War, much of the land reverted back to natural vegetation, and became home to people who earned their living from fishing, hunting, and working small farms. In the last few decades, this land has become desirable to the more affluent—its woodlands and wetlands replaced with upscale subdivisions, luxury homes, intensively landscaped yards, golf courses, and resorts. With trees disappearing, these landscapes have lost much of their ability to provide the ecosystem services important to the health of estuaries.
Nature’s Water Filter Under Siege
Coastal wetlands have a high capacity for biotransformation, a process where organisms alter the structure of chemical compounds, converting nutrients from one form to another—allowing the natural environment to act as a filter. The nutrient nitrogen, in its mineral forms of nitrate and ammonium, is a major component of runoff from residential and resort areas. In these forms, nitrogen is notorious for stimulating algal blooms along ocean shores. Biotransformation of nitrogen includes a process called denitrification, where microbes convert mineral nitrogen into an inert gas that is released into the atmosphere. Organisms involved in biotransformation also process nutrients by transforming them into energy-rich organic matter, which makes the estuaries into which these rivers drain important nurseries for marine life.
The rise in coastal development, loss of vegetated buffers, increased use of fertilizers, increased proportion of impermeable surfaces on the landscape, and inferior stormwater management practices have resulted in sometimes large amounts of nutrientrich runoff entering coastal waters. Algal blooms have become more frequent, causing oxygen depletion which results in fish kills.
Detention ponds—more than 2,000 covering 15,000 acres built along the South Carolina coast in the last three decades—have become a standard treatment for stormwater runoff in built-out and resort areas of South Carolina. Unfortunately, the ponds collect pollutants rather than filter them out; as they age, the situation gets worse, with nitrogen concentrating in the ponds and leaking out through ground water into nearby tidal creeks and estuaries.
As a research ecologist based in the SRS Charleston, SC, unit, Marianne Burke got interested in improving the best management practices (BMP) developed to protect coastal watersheds in South Carolina. She started working on this issue in 2003, with a research study that focused on two particularly problematic watersheds on Kiawah Island, a barrier island 34 miles south of Charleston. Both watersheds contain residential areas, and each drains into an interconnected series of ponds that end with a polluted pond close to an estuary.
The research attracted interest from others concerned with water quality; over the course of the project, Burke worked with partners from the University of South Carolina (USC), the South Carolina Sea Grant Consortium, the South Carolina Departments of Natural Resources (SCDNR) and Health and Environmental Control, the College of Charleston, Clemson University, and the Kiawah Island Community Association.
With Alan Lewitus from USC and SCDNR, Burke obtained funding from the South Carolina Sea Grant Consortium to study the source and effect of the pollution and to develop ways to mitigate the problem. Together they graduated five students from the Masters of Science in Environmental Studies Program at the College of Charleston, each of whom studied a part of the issue.
Graduate student Krista McKracken conducted hydrologic assessments of the two watersheds; one drains a golf course that receives routine application of fertilizers, while the other drains a residential area vegetated by forest. McKracken found large amounts of mineral nitrogen from fertilizers and other sources entering both ponds through subterranean stormwater pipes. This occurred even in the forested watershed, where pipes channeled stormwater directly from residential streets into the ponds without the opportunity for the forested watershed to process nutrients in the water. McKracken also showed that nitrogen in the impaired ponds was moving out into ground water flowing in the direction of neighboring estuaries.
Meanwhile, Sadie Drescher measured the potential capacity of the two watersheds to process nitrogen. She found that the denitrification capacity was high, particularly in the forested watershed, and was higher when there was more organic matter in the soil. But when she analyzed the detention ponds, she found almost no denitrification was occurring in pond sediments. There were high levels of ammonium, which must be changed to nitrate before it can be denitrified. Nitrification needs oxygen; because the ponds were highly anaerobic, the process could not occur. As a result, large amounts of mineral nitrogen were entering, but relatively little was leaving. The high ammonium concentrations were feeding intense algal blooms, which further reduced oxygen in the ponds—a vicious cycle of impairment.
Drescher concluded that denitrification capacity in the pond sediments would improve if they were more oxygenated, even if only in the shallow areas, and that adding organic matter such as wood fiber into the soil around the watershed would increase its ability to immobilize and process nitrogen, as well as provide energy to denitrifying organisms.
A third graduate student, Bill Strosnider, designed a shallow wetland to sequester the stormwater flowing out of the watersheds long enough for denitrification to reduce the nitrogen load in the water before it entered the detention ponds. He tested the feasibility of the wetland as a retrofit option for already built-out coastal communities and developed a hydrologic model to predict the effectiveness of the design. When he plugged in the data from McKracken’s hydrologic assessment and Drescher’s estimate of watershed denitrification capacity, his model confirmed that the wetland he designed should remove most of the mineral nitrogen before stormwater entered the pond. Alan Fleming showed that polychlorinated biphenyls (a mixture of toxic chemicals) were not an important component in stormwater entering the ponds, while Lara Brock documented the association between pond water quality and harmful algal blooms.
Better Best Management Practices for the Coast
Burke and her collaborators used the results of this study to develop an improved BMP for stormwater in coastal residential and resort areas. They recommended intercepting stormwater with a constructed wetland, subjecting it to a 20–day residence in the wetland, and incorporating wood fiber into wetland soil to provide a substrate for microbes to increase nitrogen processing capacity. Burke and her partners also developed a hydrologic model that can be adapted and used to estimate performance of other designs for BMPs in residential settings.
The project highlighted the ecosystem services provided by forests and wetlands that are lost when development proceeds without the proper BMPs in place. Town, county, and South Carolina State regulatory groups are currently using the results of this study to revise their BMPs for stormwater management and to improve the permitting process along the South Carolina coast.
Burke recently moved from Charleston to Washington, DC, where she is now strategic management systems leader for Forest Service Research & Development.
For more information:
Marianne Burke at 703–605–4181 or firstname.lastname@example.org.