A Garden of Opportunities for Cleansing Urban Storm Runoff
The beauty of a thriving rain garden like the one above located in Centreville, Maryland, is a bonus. Its main purpose is to limit how much stormwater floods into local waterways during a big rain. Credit: Erica Goldman.
AMANDA TRITINGER HAS GOTTEN AN INTRODUCTION TO DIRT this summer. Right now, the college student stands at the edge of a wide pit at the base of a parking lot in Columbia, Maryland. It's 80 feet long, 3 feet deep, and filled with loose soil and mud. In the pit, a few workers around Tritinger's age toil away in the heat, shoveling piles of dirt and breaking up the turf with pickaxes. Soon, they'll fill in this pit and turn it into a vibrant garden full of purple irises and other flowers. That final product will be what's called a rain garden — a tool for storing and filtering urban stormwater.
While this unusual construction team has built well over a dozen rain gardens so far this summer, few have been so big. Most could fit in a front lawn or next to a sidewalk. "This one's not a rain garden," says Tritinger, a crew leader on the project who attends the University of Central Florida in Orlando. "This one's a rain forest."
Last school year, Tritinger, who studies environmental engineering, took a class that dealt with hydrology, or how water flows over and under the land. Today, she's actually changing the hydrology of this suburban region with her own hands as part of a project funded by Howard County, Maryland, called Restoring the Environment and Developing Youth (READY). She's gotten good at it, too. At the start of the season, she hated swinging pickaxes. Now she loves it. "I don't know what changed. ...I mean, I guess I do know what changed — the guns," she says, referring to her buff biceps, which she flexes to make her point.
Howard County is also flexing its muscles, showing that it's beginning to take its water and dirt seriously. And for good reason. As the multistate and federal effort to clean up the Chesapeake Bay charges forward (See A Model Plan), counties like this one will have to look for new ways to cut the excess nutrients streaming off their lands. Rain gardens trap the stormwater and, in the process, the nitrogen, phosphorus, and sediments that wash off parking lots, sidewalks, and other paved landscapes. And since rain gardens are usually small, individual homeowners can even dig their own, contributing a bit to the larger cleanup plan. But even as urban areas like Columbia get started building them, scientists are scrambling to find new ways of reducing nutrient and sediment pollution — and to make existing tools like rain gardens work better.
For towns and cities around the Bay, many of which face big bills for installing nutrient control measures by 2025 as required by the Chesapeake Bay cleanup plan, these findings couldn't come soon enough. "It's going to be a Herculean effort to meet the 2025 goals, and I'm not sure we're going to be able to just because of the sheer number of management practices we have to put on the ground," says Bill Stack, the deputy director of programs for the Center for Watershed Protection in Ellicott City, Maryland. But "while it is a Herculean task, it's something that we have to address."
Working through the heat, Amanda Tritinger (above left) and Raymoan Clay, (below, seated) drop river rocks into a channel that makes up the border of their newest rain garden in this parking lot in Columbia, Maryland. Those rocks should act like the garden's first line of defense, filtering out some of the sediments caught in streams of stormwater. Watershed specialist Amanda Rockler (above right) helped to teach these two READY crew leaders about installing rain gardens. The scientist is proud of her Maryland roots. Earlier today, the two Amandas shared a high five after discovering that they both live in Montgomery County, at least for the summer. Rockler works with communities there and in Howard and Frederick counties. Credit: Daniel Strain.
The Watershed Guy
Tom Schueler has been a big part of that effort for nearly 30 years. He uses the e-mail handle "watershed guy," which sums up his life's work. His career began in the 1980s when he was employed by the Metropolitan Washington Council of Governments to help clean up the then heavily polluted Anacostia River. Today, he directs the Chesapeake Stormwater Network, a professional organization for those who specialize in managing urban stormwater.
Decades ago, experts focused much of their attention on the excess nutrients coming from the region's farms, usually through fertilizers or manure. Agricultural areas, after all, cover a lot more space than the watershed's cities and towns. But, as urban zones expanded across the region, scientists realized that the stormwater from developed lands also contributed a large share to the problem. Engineers like Schueler have begun to think more about how to design greener cities, a trend called the low-impact development movement. But the effort is hampered by its costs. Estimates suggest that, today, developed areas like Howard County may have to pay tens of thousands of dollars or more to trap the nutrients from each acre of land that sheds stormwater. And that could saddle the Bay cleanup plan with a price tag in the hundreds of millions of dollars — per county.
Schueler has dug five rain gardens on his own property in Catonsville, Maryland, so he's a believer in these measures. But Schueler's also mindful of the costs. He's part of a team that works with the Chesapeake Bay Program, which oversees the Bay cleanup effort, to help evaluate various tools, or "best management practices," for reducing excess nutrients. "It's a grave responsibility to sit there and try to assess which techniques or practices are the most effective because we're talking, ultimately, about billions of dollars of social investment over the next 15 years," he says.
Rain gardens may look scraggly when they’re first installed, but within a season, the flowers, shrubs, and grasses planted inside may grow to fill every nook and cranny. Planted in suburban lawns and elsewhere, they could play an important role in improving water quality miles away in the Chesapeake Bay. Credit: Amanda Rockler.
Back at the READY pit in Columbia, Amanda Rockler inspects some of that investment first-hand. If Schueler's the watershed guy, then she's a soil junkie. In fact, the young scientist sometimes corrects her colleagues when they call it dirt. "It's just so much more respectful to call it soil," she jokes.
As a watershed specialist for Maryland Sea Grant Extension, Rockler educates the public about the importance of rain gardens and other ways to control stormwater locally in order to improve water quality on the Baywide level. She also helped to train the READY team. Now she's observing roughly 15 students as they work, some piling river rocks at the edge of the parking lot, located in Columbia's Oakland Mills Village Center. Rockler is impressed by what she sees. Without a rain garden, the terrain here "would be slick," she says, shouting over the sound of falling rocks. Stormwater "would just run off into the street — into the storm drain system where the water is never treated." And from there, into the Bay. With a rain garden here, however, things should be different.
Rockler explains how this type of landscaping works. The gardens tend to be shallow depressions, like the pit behind us, although usually a lot smaller. They're filled in with a mixture of soil, compost, and sand. Such a concoction should act like a sponge, sopping up stormwater as it slides off parking lots or even common turf lawns, which, despite appearances, can't soak in much water. Once trapped in a rain garden, that stormwater will either evaporate, trickle out into the surrounding ground, or get sucked up by plants. That, in turn, limits how much water will flood into the Bay with each storm. "Hold water, slow it down, and soak it in. That's the motto we use," Rockler says.
But rain gardens also do something else — they treat the water. It often works like this: as rainwater rushes down sidewalks, it picks up particles of silt and sand. This sediment, in turn, carries a host of potential pollutants, including phosphorus molecules. But when those same sediments trickle into a rain garden, they're trapped by the rocks, soil, and mulch inside. And so are the pollutants they carry. Other, free-floating pollutants are also removed through a variety of different means. In a study conducted on the campus of the University of Maryland, College Park, scientists showed that two working rain gardens removed, on average, about three-quarters of the phosphorus they took in.
To remove nitrogen, however, you may need a bit of greenery. Rain gardens tend to be dotted with an array of native flowers and grasses, Rockler explains. In Maryland, it's plants like black-eyed Susan, blue flag iris, or tickseed. They're not just for show. Many of these plants also have long roots, capable of sucking in lots of water — and also nitrogen, which the plants then use to grow their stems and flowers. While estimates vary, studies suggest that rain gardens can remove more than half of the nitrogen they take in, some of it going to vegetation and some going to nitrogen-digesting microbes. No fancy pollution equipment is required, beyond what nature provides in the soil and the plants' green stems.
Or so say scientists working alongside the Bay Program. The partnership now recognizes rain gardens as effective tools to restore the watershed, estimating that these features can remove, on average, about 25 to 80 percent of the nitrogen they collect during big rains — depending on what kind of soil they've been dug into, among other factors. That means that counties like Howard can get credit from the Chesapeake Bay Program for installing these features. And that should help the county draw closer to meeting its targets for reducing nutrients and helping to restore the Bay. Other acceptable practices for urban and suburban areas include planting trees, restoring wetlands, and placing permeable pavement, porous surfaces that allow water to flow through and soak into the ground below.
Each of these practices has its pluses and minuses. Rain gardens, for instance, are expensive. It costs an average of $50,000 to $187,000 to build enough rain gardens to drain and treat stormwater from one acre of pavement, according to estimates released in 2011 by the Maryland Department of the Environment. And that's just the price of installation and planting — new mulch needs to be added every few years, and vegetation needs to be pruned back, too. There are cheaper options. It costs a Marylandwide average of $33,000 per acre treated to plant trees next to urban streams, but estimates suggest that these features may not remove as much nitrogen, on average, as certain rain gardens do.
Building a Better Toolbox
Towns could save money, however, if rain gardens did their job better, says Allen Davis. He's an environmental engineer from the University of Maryland, College Park, who specializes in managing urban stormwater. He experiments with different rain garden designs outdoors and in his lab. He takes long tubes, about 6 inches wide and 3 feet tall, piles them full with soil and wood chips, and studies what happens next.
When it comes to removing the nitrogen from stormwater, he says, plants are only a short-term solution. "Plants will take up nitrogen," he says. "But if you don't remove the plants, then the nitrogen doesn't really go anywhere." In fact, as plants decompose, the same nitrogen could reenter the Chesapeake Bay's waters. The better solution may be bacteria. But not just any bacteria — specifically, those that thrive in oxygenless, or anoxic, environments. These microbes can convert nitrate, a common type of nitrogen molecule, into nitrogen gas, the harmless gas that makes up most of our atmosphere. These organisms live in rain gardens already, Davis says. It's just a matter of growing more. "If you want to denitrify, you need anoxic conditions and time," he says.
There's more to a rain garden than can be seen from the surface. This drawing shows some of the components of a rain garden — and a bit about how to build one on your own lawn. Illustration by Doug Adamson, courtesy of USDA-NRCS, Iowa.
To get both, you may need a sump. That can be a small hole that extends down like a nipple from the main rain garden. When a rain garden gets wet, water should trickle down into the sump and then pool there, creating a rich bacterial soup — something like a mini-cesspool. That soup should, in turn, digest large quantities of nitrogen. Davis plans to compare different designs for sumps to find out which produce the maximum ecological benefits. No matter what the research finds, he says, counties will likely have to rely on more than just rain gardens to meet their cleanup plan goals. That may include installing more swales, which are ditches, often next to roads, that collect and store stormwater. Rain gardens are "a tool in a toolbox," he says.
The engineer isn't alone, either, in investigating how to improve existing methods of removing nutrients from urban areas. Sujay Kaushal, a biogeochemist also at the University of Maryland, College Park, explores how restoring buried streams could help reduce the nutrients oozing from developed areas. "If you actually looked at a map of streams for New York City or Washington, D.C.," he says, "basically you see there are larger rivers that flow through those urban areas, but...there are no small streams." They've all been buried under dirt and cement over decades of development. He says that by digging these small waterways back up, it's possible to restore at least some of their ability to gobble up nitrogen.
Kaushal has been studying restored streams, such as Minebank Run just north of Baltimore, to learn how best to do that. Not every restoration is created equal, he notes. Streams with wide floodplains, for instance, give the water a chance to spread out and soak into the surrounding soil, where it's more easily treated by microbes and plant roots. Kaushal is currently working with the Chesapeake Bay Program to encourage them to consider stream restoration as an acceptable practice for reducing nutrient pollution.
The heavy costs of the Bay cleanup plan may wind up promoting the development of other new, affordable solutions for reducing nutrients on land before they reach the estuary. A new industry could expand to meet the need. "We have to let the market drive the innovation," says Bill Stack of the Center for Watershed Protection. "I think we're going to see costs come down because of it."
Researchers and companies are already starting to build a better toolbox, albeit slowly. Several new methods for reducing nutrients in urban areas are currently under review by the Bay Program or might be soon. These new projects include floating wetlands, rafts of plants that bob around in ponds, sucking up nutrients in the water through their roots.
Some of the most effective ways to clean the Bay also appear to be among the cheapest. Rain gardens, restored streams, and permeable pavements help — but so can simply changing your behavior, says Andrew Lazur of the University of Maryland Extension. You can avoid fertilizing your lawn because the nutrient-laden residue often winds up in the Bay. In fact, lawn fertilizers make up 10 to 25 percent of the nutrient contents in urban and suburban stormwater runoff, according to a 2011 report by the National Research Council. And you can pick up after your dog during walks, too, because that waste could also make its way into the Chesapeake's waters. "You take 18 million people on the watershed, and they take up one of these practices," Lazur says, "the impact is huge."
Call it a new way to flex those guns.
vol. 11, no. 3
Modeling and the TMDL
Cleaning Up the Watershed