It’s well known that rain gardens are great for infiltrating stormwater but people may not realize that they also help destroy common stormwater pollutants. Several studies have found that rather than accumulating pollutants in their soils, rain gardens tend to biodegrade them instead. One study (LeFevre et al., 2011) investigated petroleum hydrocarbon levels in 58 rain gardens in Minneapolis, MN representing a wide range of sizes, vegetation types, and contributing area land uses. The researchers found that petroleum hydrocarbon levels were well below regulatory limits in all the rain gardens sampled. And a tip for future rain garden installers, rain gardens planted with more robust vegetation with deeper roots did a better job at breaking down pollutants than those planted with only turf grass.
A rain garden’s ability to biodegrade pollutants is in contrast to what happens in more conventional stormwater management structures like retention ponds. Retention ponds are often installed with larger developments to receive a large volume of stormwater from impervious areas (ex. houses and roads in a subdivision, roof and parking lot of a Home Depot). Other studies (Van Metre et al., 2009; Van Metre et al., 2000; Kamalakkannan et al., 2004), found that pollutants like PAH’s (polycyclic aromatic hydrocarbons), a type of petroleum hydrocarbon, accumulate in the sediments of stormwater retention ponds. This creates a very expensive maintenance issue for retention pond owners when the time comes to remove and dispose of built up contaminated sediments.
Side note – stormwater can pick up PAHs from dust on pavements treated with coal tar sealants which are commonly used on parking lots, driveways, and playgrounds (but they have recently been banned from use on State and local highways in CT).