Biomass, benefits not always linked in stormwater treatment swales

Dense stands of wetland plants excel at taking up nutrients and trapping sediment, so their presence signals improved water quality downstream, right?

Mat of algae

Not necessarily, says an interdisciplinary research team from the University of Wisconsin-Madison. In three experimental wetlands designed to treat urban stormwater, two with sparse vegetation—but with flourishing mats of algae and mosses—unexpectedly bested a site thick with cattails.

In fact, five of six ecosystem services that people normally associate with wetlands, including erosion control, nutrient removal, and support of biodiversity, were lower in the more productive, cattail-dominated wetland than in the other two.

In other words, “the most obvious visual metric—wetland plant biomass—was a very poor indicator of the hydrologic, water quality, and diversity services that society cares most about,” says UW-Madison hydrologist and ecologist, Steve Loheide. “This means that stormwater managers can’t visit a site and assume it’s providing high levels of ecosystem services simply because it’s densely vegetated.”

The work—led by UW-Madison restoration ecologist Joy Zedler, biological systems engineering professor Anita Thompson, and Loheide—appears in the journals Ecosystems and Ecological Engineering.

The scientists conducted their study in the UW Arboretum, a 1,200-acre natural area, and research and teaching center just south of campus. During the last two decades, the site has been beset by nutrient- and sediment-laden runoff from the surrounding urban watershed, prompting projects to enlarge and remodel its existing stormwater treatment facilities.

Because of the research mission of the Arboretum, its scientists wanted the facilities to include an experimental component. So they decided in this project to examine the capacity of wetland swales to treat stormwater from a 140-acre watershed, and “provide all the benefits that people assume they do,” Loheide says.

Aerial view of swales

The three swales were excavated beginning in 2008, covered in a half-foot of local topsoil, and then planted in 27 native wetland species in November 2009.

Many surprises

More and more stormwater managers and engineers are interested in adding a biological component to traditional stormwater treatment designs, Thompson says. And one key element they try for is a thick cover of vegetation, because of its ability to slow flows, remove nutrients, and reduce erosion.

But working with biological systems can also be “difficult and somewhat unpredictable,” Thompson adds. In short, the experiment produced some surprises. The biggest: While the three swales were designed as replicates, water drained from each of them at varying rates. This led in turn to different hydroperiods (frequency and degree of inundation) as measured by Loheide’s student Jeff Miller, as well as plant communities, documented by Zedler’s student James Doherty.

Specifically, one swale was underlain by a thick clay layer that caused water to pond continuously at the surface. A second swale contained only patches of thin clay, allowing water to seep quickly into the soil, and the third was intermediate between the other two. Despite being seeded with native plants, the swale with standing water was soon overrun by cattails. The less waterlogged conditions in the others, however, promoted greater plant diversity, a more open canopy, and profuse growth of algae and mosses.

Young cattails growing in wetland

Plants and other organisms—especially “lower” life forms like algae and mosses—aren’t usually considered in water quality models, but what the study indicates is that the biota shouldn’t be ignored, Zedler says. In particular, Thompson and her student, Stephanie Prellwitz, discovered that the algae and mosses were excellent soil stabilizers. They often withstood the maximum erosive pressure the scientists’ tests were capable of producing: 60 pounds per square-inch.

Meanwhile, the loose muck that developed beneath the cattails eroded at forces 3- to 4-times lower than this. It eroded just as easily, in fact, as bare soil. The cattail-dominated wetland also released suspended solids, nitrogen, and phosphorus downstream, rather than retaining them as expected.

But the other two swales also exported some phosphorus, and none of the three wetlands were “top-performers” at improving water quality, Thompson says. This may have stemmed in part from the facility’s design. The researchers suspect that phosphorus leached from the layer of topsoil that was added to the newly constructed swales.

Stephanie Prellwitz using field equipment

To meet current regulations, the overall system also included an upstream detention pond that removed suspended solids and nutrients from stormwater before it reached the vegetated swales. The detention pond made it difficult to assess the swales’ potential capacity.

“So it would have been preferable to compare this system to one with no pre-treatment to see what the swales by themselves would be able to do,” Thompson says.

Given the uniqueness of the system—with its three wetlands of differing hydroperiod—lingering questions are inevitable. And for the researchers, asking questions is precisely the point. Managers can’t just assume a constructed wetland will treat stormwater or provide ecosystem services. They need to make the measurements.

“Assessments of wetland services, and especially of stormwater treatment facilities, need to be more science-based,” Zedler says.

The research was funded by the U.S. EPA Great Lakes Restoration Initiative.

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