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Stormwater in Parks: Is There Reason to Worry?

The following is an excerpt from City Parks, Clean Water, a report by the Trust for Public Land’s Center for City Park Excellence that examines the role of urban parks in managing stormwater. This is the fourteenth installment in a series of 19 posts.

When a park is doing double duty as both a recreation space and water management area, certain citizen concerns may come to light. Worries regarding standing water, salt or chemical residues in runoff, and previous site contamination are all valid, and certain precautions must be taken to prevent any health impacts.

There’s no question that standing water has its drawbacks, and hydraulic engineers walk a fine line between holding water back for too little time and for too much. Shorter retention periods and the sewage plants get overwhelmed; longer retention, and the water may allow mosquitoes to breed and algae to grow. Mosquito larvae need 72 hours to hatch, so holding ponds should be designed to drain sooner than that. To be on the safe side, New York City requires emptying in 48 hours; Cincinnati is even stricter at 24 hours. If water must be held longer, continuous mechanical circulation and aeration through fountains can help, as can the selection of vegetation that absorbs excess nutrients or supports insect eaters such as bats or fish.

Standing water may also cause concern about the safety of children and other visitors. However, this issue is standard for virtually all park departments and it should not require any special rules: every park system already has rivers, streams, ponds, lakes, or ocean beaches to deal with. When a stormwater-treating wetland in Shoreline, Washington, seemed hazardous for children because of its steep sides and long drop from an adjacent walking path, the city eventually installed safety cables. New York avoids this problem by keeping its water depressions as shallow as possible in neighborhood parks, generally under six inches.

Cromwell Park

Cromwell Park installed safety cables around their retention basin to address concerns about small children playing in the vicinity. (City of Shoreline)

As for residues, they don’t pose a significant threat. “Although the public often has concerns that soils may become contaminated from street and right-of-way runoff,” says Robert Goo, an environmental protection specialist in the Office of Water at the U.S. Environmental Protection Agency, “municipalities have not reported significant problems in terms of contaminant accumulation. In general, heavy metals and hydrocarbons that wash off impervious surfaces onto vegetated areas are filtered and trapped in the upper layers of the soil and by plants, and can be removed with normal maintenance and disposal practices.”

While road salt in high concentrations can kill plantings, most other pollutants and excess nutrients are not harmful, and heavy metal concentrations grow so slowly that significant levels have not yet accumulated in even the older stormwater installations. Most state and EPA manuals recommend a minimum of two feet between the bottom of an infiltration basin and the top of the water table to allow enough space for all pollutants to be filtered. “However,” Goo added, “care should be taken to avoid discharging runoff that has high concentrations of soluble pollutants such as nutrients, pesticides, or chlorides from de-icers, due to the potential for groundwater contamination.”

What about brownfields and past contamination? Although it might seem counter-intuitive to channel stormwater through former industrial sites, these places not only offer great opportunities for new parks but can even alleviate stormwater problems. For one thing, while many sites are classified as brownfields because of the suspicion of pollution, testing may reveal that no toxins are in fact present. In other cases, where a brownfield has only a relatively small, confined area of pollutants, the property can be cleaned to residential standards by removing and replacing all toxic soil.

In the most severe cases, where hazardous wastes cannot be removed and are instead capped in place with a waterproof liner, stormwater can still be managed in a limited fashion. Tree boxes and other shallow containers can be installed above the liner near the surface, where plants can absorb water and then slowly release it through evapotranspiration. Alternatively, runoff can be transported laterally, on top of the liner, to a clean part of the sit e or to somewhere off-site. Because of this, the U.S. Environmental Protection Agency allows the installation of green infrastructure on brownfields as long as the property is well regulated and the owner does due diligence.

“There is often a misconception that brownfields aren’t appropriate for green infrastructure,” explained the EPA’s Robert Goo. “Actually, they can be ideal places for these approaches, and the EPA Brownfields Program has provided grants to do just that as part of community revitalization efforts. For one thing, brownfields don’t always have contaminated soils. Even where they do, green infrastructure can still work if proper practices are selected to protect groundwater and prevent the mobilization of pollutants.”

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