Plants Really Do Clean Pollution From Stormwater

You might have seen them around new buildings and roadways: little basins and ditches, planted with various small growing things. They’re designed to stop crud from washing into the gutters and down the storm drains.

Now, a new report from the U.S. Geological Survey shows this idea is working.

The report analyzed a project done in Rapid City, South Dakota, the gateway to Mount Rushmore. The river that runs through it from the Black Hills, Rapid Creek, offers excellent trout fishing and serves as a city water source for part of the year.

Rapid City made special arrangements for its river after 1972, when a massive flood killed hundreds of people and wiped out 1,335 homes. The city withdrew from the river’s floodplain and now allows only compatible uses there, like parks and golf courses.

In 2005, the city started taking steps to keep pollution out of Rapid Creek by controlling stormwater runoff: rainwater that’s carried away through gutter grates and storm drains. Stormwater flushes out a wide range of toxic metals, dissolved chemicals and germs. It’s not as bad as sewage and it doesn’t need to be treated as aggressively, but its effects on rivers and lakes can be serious.

The dirtiest runoff in Rapid City came from its downtown area, where the ground is mostly pavement and concrete. The urban street grit washing off this area went straight into the river, and the runoff exceeded the state of South Dakota’s water-quality limits for suspended sediment and germs. Germ levels are based how much E. coli and fecal coliform bacteria are found in the water; both are associated with human waste.

In 2011, Rapid City installed several vegetated channels along the downtown drainage route, a “best-management practice” recommended to cleanse urban runoff.

Vegetated channels are little artificial wetlands, engineered and planted to slow the flow of stormwater runoff. The goal is to give the dirty water some face time with plants and soil, which absorb toxic metals, filter out water-clouding sediment and neutralize noxious germs.

In its study, the USGS took water samples from three vegetated channels, each about 300 feet long and 30 feet wide. The tests showed the channels cut the average levels of suspended sediment by nearly half. Germ levels were reduced by as much as 36 percent. All it took to do this was slowing down the runoff by at least 8 minutes.

The channels also improved other water-quality measures that were already below the standard limits. Nutrient levels (phosphorus and nitrogen) were lowered by about one-third. So were concentrations of metals like lead, zinc and copper. All these benefits were just what the city’s water department had predicted in its official analysis.

Stormwater systems are a way to restore, at least in part, what was lost when cities moved streams into concrete culverts. Originally, the idea was to contain the natural messiness — floods, wildlife, mosquitoes, soft property lines — and keep it out of the built environment. But, in important ways, culverts are overkill.

And you don’t have to lose the flood control benefits of the culvert when you route water through vegetation instead, as one large Oakland project demonstrates. For the Lion Creek Restoration Project, crews dug and planted a new stream bed next to the existing culvert. They planted more than 9,000 creek and wetland plants to create an acre-and-a-half wetland, with a pedestrian path, near the Oakland Coliseum. Now the stream runs through the reeds and grasses like it used to, while the culvert is still there to handle floods.

The project cost more than $4 million, contributed by a range of sources, including the state, taxpayers and the Alameda County Flood Control and Water Conservation District.

Stormwater systems are essential for the managed landscapes of the Anthropocene world. Where cities have turned the landscape unnatural, these systems serve the same functions as natural stream beds and floodplains. They require human skills in design, construction and maintenance, but we can provide that.