Association of State Wetland Managers - Protecting the Nation's Wetlands.

The Wetland Wanderer: Reducing Stormwater Pollution and It’s Impacts by Increasing Imperviousness: How Wetland-related Planning and Management Can Play a Role

by Brenda Zollitsch

Here in Maine and across the Northeast we are in a “wait and see” pattern about how all recentsnow3615-1the current snow will melt.  In Portland, Maine we have more than 90 inches of snow so far this year, much of it still sitting on the sides of roads, frozen in place.  Although it seems that this has been the winter that will not end, indeed at some point all this snow is going to start to melt.  The snowmelt water will be chock full of chlorides, as well as sand, petroleum products, pet waste that was never properly disposed of, and other pollutants.  This water will drip from roofs, run across roads and parking lots and other impervious surfaces to find its way into the nearest stormdrain.  Many people don’t know that storm drains carry these polluted waters directly to local water bodies, without treatment.

Impervious cover/surface refers to “anything that water cannot penetrate. Ranging from residential rooftops, patios and driveways to town roads, public build¬ings, commercial structures and parking lots, impervious cover prevents rain and snow from soaking into the ground” (Rhode Island Department of Environmental Management).  If the level of stream030615-2impervious cover rises too high in these areas, the pollution that results can cause irreversible damage to drinking water quality, to groundwater supply-ing private wells, and to aquatic wildlife habitat.  According to studies conducted and reviewed by the Center for Watershed Protection, as impervious cover rises above 10% if the landscape there is almost always a measurable loss in water quality. Between 10% and 25% these impacts increase, and pollution and flooding are both evident. Above 25% impervious cover, water quality impacts can be so severe that it may not be possible to restore water quality to pre-existing conditions.

Most efforts to combat stormwater pollution are based on trying to reduce pollutants, prevent spills and remove illicit discharges.  Some part of this work also focuses on what is known in the stormwater world as “post-construction” measures (EPA NPDES Stormwater Management Post-Construction Minimum Control Measure.  These practices look at an alternative method of controlling stormwater, which focuses on increasing infiltration of water onsite.  This approach mimics pre-development hydrology.  Constructed or natural infrastructure are used to absorb, store (at least for a limited time) and filter water onsite.

streetside0306015-3Here is where the conversation opens a door for greater collaboration between stormwater and wetland managers.  Stormwater managers frequently think about addressing the problem of impervious through the use of tree box planters, pervious pavement, rain gardens, and green roofs.  In some areas of the country, large projects expand integration to include the use of constructed wetlands.  However, there is limited consideration nationwide about preserving, restoring and creating natural wetlands as a way to improve onsite infiltration and storage in critical areas where stormwater runoff habitually creates management challenges.

Across the nation, wetland loss continues.  Although some of this loss has little connection to the areas where stormwater management concerns are the most pressing, wetlands and riparian corridors do have an impact on other areas and play a critical role on predevelopment landscapes that are now urban.  Especially in areas where imperviousness has not yet exceeded 10%, nonstructural techniques such as riparian buffers, wetland restoration and upland forest retention have been found to be more effective at ameliorating the impacts of impervious surfaces than structural management practices.

protwet3615-4Various strategies to reduce impervious surfaces and their impacts on water resources can be applied to community planning, site-level planning and design, land use regulation and both stormwater and wetland management.  Efforts that wetland and stormwater managers can collaboratively promote include, but are not limited to:

  • Wetland preservation, restoration and creation to provide natural infrastructure that retains clean stormwater onsite;
  • Conservation of critical wetland habitats and the water sources that restore and retain them;
  • Protection and stewardship of wetland buffers;
  • Consideration of impervious cover when reviewing Section 404/401 wetland permits and certifications; and
  • The use of tools such as impervious cover TMDLs to encourage restoration of wetlands and other aquatic resources that can positively impact impaired streams by reducing pollution and slowing runoff velocity. (Example: Maine’s Impervious Cover Total Maximum Daily Load Assessment (TMDL) for Impaired Streams)

As we better understand the critical role that impervious surface plays in aquatic resource impairments, it is beneficial to consider the use of natural infrastructure or combinations of natural and constructed infrastructure to treat waters onsite before they pick up pollutants and enter our waterways.  There are many resources available to learn more about the impact of imperviousness on water quality.

For more information, please check out the following resources:

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