Got Contaminants?

By Larissa Larsen, PhD, and Andrea McFarland

The Most Holy Redeemer Church in Southwest Detroit has a stormwater problem — an expensive one. The Detroit Water and Sewage District (DWSD) recently increased its drainage charges, levying a fee based on a property's impervious surface area. For the church and a school sharing the site, the impervious area totals 3.9 acres of the 4.1-acre site, and adds up to $2,500 in fees annually.

DWSD discounts its fee when property owners add on-site green infrastructure systems to reduce stormwater volumes. Our research team developed a plan to do that, while also selecting interventions that reduce the contaminants from the parking lot and buildings. Synthetic organics, heavy metals, sediment from car byproducts, lawn fertilizer, deicers, salt, and road debris pose the greatest concerns.

The solutions? Pervious pavement in overflow parking areas and a rain garden near an entry to the school. As you'll see below, rain gardens are highly effective in addressing the contaminants of concern, while also decreasing peak stormwater flows. Using the U.S. Environmental Protection Agency's SWMM computer simulation software, our intervention would reduce the stormwater volume from 0.49 ft3 per second to 0.14 ft3 during a six-hour heavy precipitation event — and save $1,250 in annual drainage fees.

Many people recognize that green infrastructure can help reduce stormwater runoff, but fewer people understand how to select an appropriate green infrastructure system to improve water quality. Here is a brief look at the relative effectiveness of various systems in addressing different types of common water contaminants.

Green infrastructure here refers to nature-based solutions that use vegetation, soil, and/or infiltration to retain stormwater and naturally filter out contamination. "Green" costs less than "gray" infrastructure approaches — and it offers a number of additional benefits: reducing heat island effects, increasing urban biodiversity and habitat, enhancing climate resilience, and providing attractive green spaces.

We included eight common green infrastructure systems:

RETENTION BASINS retain runoff, thus reducing peak flows. While the water level varies, retention basins always contain some water.

RAINWATER HARVESTING collects rainwater for later on-site use.

CONSTRUCTED WETLANDS are engineered systems that channel runoff through a vegetated path and remove pollutants through the soil, vegetation, or the inherent bacterial community.

DETENTION BASINS hold runoff during rain events and slowly release the water until empty.

BIOSWALES, while similar to rain gardens, are generally elongated in shape, deeper, and typically use engineered soils. Bioswales slow and filter peak runoff and frequently are placed adjacent to roads and parking lots.

RAIN GARDENS are shallow vegetated basins often next to areas of impervious surfaces.

GREEN ROOFS have vegetation and/or stormwater collection basins for infiltration and evapotranspiration.

PERMEABLE PAVEMENT, such as pervious concrete, porous asphalt, and pavers, can permit the infiltration of stormwater into the ground and thus reduce sheet flows.

The table below also looks at seven common types of stormwater contaminants and the land use typically associated with each. Putting it all together, we assess the relative effectiveness of each green infrastructure system in removing the contaminants common in each land-use type.

Relative Effectiveness of Green Infrastructure Systems in Removing Contaminants

Looking at the pollutants in each common land-use type, we assess how well GI systems work to eliminate them.

Source: "Guide for Using Green Infrastructure In Urban Environments for Stormwater Management" (2019), Environmental Science: Water Research and Technology. Photos: Hockney, iStock/Getty Images Plus; Glenn-Specht, iStock/Getty Images Plus; Heike Hoffman, Flickr; Courtesy Smc (Mdswm.Com); Center for Neighborhood Technology; D.A. Horchner/Design Workshop; Fotografixx; Banksphotos, iStock/Getty Images Plus.

Common Stormwater Contaminants by Land Use

We identify the major sources of each individual contaminant class from each land use and rank the comparing severity of each contribution.

Source: "Guide for Using Green Infrastructure In Urban Environments for Stormwater Management" (2019), Environmental Science: Water Research and Technology. Photos: IP Galanternik D.U.; iStock/Getty Images Plus: Willopix; Bgpix; Ozgurdonmaz; Michel Seelen; Arim Tatarinov; Jazzirt.

Larissa Larsen is an associate professor of Urban and Regional Planning at the Taubman College of Architecture and Urban Planning at the University of Michigan. Her research focuses on environmental planning and urban sustainability. Andrea McFarland is an engineer designing water treatment and conveyance systems at Sickels and Associates, Inc.