Alternatives for Stormwater Management
The Character of Storm Water Pollutants
Storm water run-off from the urban landscape contains many different pollutants, including metals, oil, grease, diesel fuel, gasoline, paint, herbicides, pesticides, fertilizer, dust and dirt, paper, cardboard, glass, plastics, leaves, sticks, grass clippings, and a host of other items in varying quantity. Of this list, the first nine items are the most important to be removed from run-off. These pollutants are harmful to us and to wildlife, and if they are allowed to continue to be discharged into waterways, the long-term effects will be disastrous to the local watershed. These compounds ideally should also be prevented from entering the ground water.
Storm Water Wetlands
The ecology of the wetlands is well suited for the treatment of storm water. Indeed, wetlands have evolved as low places in the landscape that receive run-off water and associated sediment and organic matter. The particular characteristics of constructed wetlands that we are interested in for stormwater treatment are the following:
- Sediment reduction
- Bio-degradation of organic compounds
- Bio-degradation of petroleum hydrocarbons
- Nitrogen reduction
- Sequestration of metals in the form of chelated compounds
Wetlands As a Stormwater Solution
Wetlands are naturally adapted to the seasonal rainfall and the ebb and flow of flood events. The particular structure of the wetland plants allows them to be inundated and to bend, rather than break, during exposure to strong currents during floods.
By incorporating wetlands into detention basins, we can clean up and remove the pollutants of concern, and at the same time create an attractive habitat for waterfowl, amphibians, and mammals. Wetlands are particularly suitable where drainage is poor, and the soils are saturated. In areas with sandy soils we can create wetlands by placing plastic liners that are impermeable and non-degradable over the sandy soils. The liners protect the underlying aquifer from contamination. Because of the requirements for impermeable soils, wetlands are therefore not very good as a means of infiltrating water into the ground. There is some vertical movement of water, albeit very small amounts. If the wetlands are built using liners on sandy soils, a subsequent downstream area can be developed as an infiltration area. This is an optimal combination as this allows for treatment prior to percolation and recharge.
Wetlands are ideal in many respects for the removal of pollutants because unlike mechanical systems, little or no maintenance is required beyond periodically removing trash. The ability to biodegrade pollutants, or sequester metals in a passive manner eliminates the need for electricity or equipment of any kind.
As part of the landscape, they create the open space so essential to the urban environment. Once planted with the many species of natives, including flowering plants, the wetlands can provide a pleasing green corridor within the community and at the same time, the plants and associated microbial community clean the water.
When designing wetlands for stormwater run-off, they are usually combined with sedimentation basins and trash removal racks. Sediment can ultimately fill in a wetlands, changing its character entirely. Because of the likely amounts of trash such as paper, cans, bottles, and plastic, a screening device installed immediately in front of the sedimentation basin simplifies the removal and management of this trash, and ultimately keeps it out of the watershed.
Wetlands can be any size – from a tens of square feet to many square miles. The primary vegetation should be native plants; however, there are numerous non-invasive, non-native flowering species of plants that make these stormwater wetlands very attractive. This landscape quality makes wetlands a beautiful option for stormwater treatment. Because of the shallow slopes surrounding the edge of a stormwater wetland, less water can be detained than in a detention basin of equal area; however, habitat and wildlife will often be greatly increased. Wetlands are often combined with pools or ponds, which can increase the storage capacity and habitat value.
Wetlands are encouraged in state and federal regulations, and as part of a mitigation or restoration program they are eligible for additional funding. No urban stormwater plan should be considered complete without the consideration of constructed wetlands for stormwater treatment.