Here’s what happens to the pollutants swept up in stormwater runoff.
If you’re concerned about water pollution, you’ve likely heard this message: The water that gushes off our roofs, driveways, streets, and landscaped yards is to blame for the bulk of the pollution that dirties Puget Sound and numerous Northwest waterbodies. You probably also know about the most popular stormwater solutions, including rain gardens and other green infrastructure that soak up the filthy water, cleaning it before it reaches sensitive waterways that are home to salmon, frogs, orcas, and other wildlife.
But those two ideas taken together are making some people anxious. If stormwater is the source of such devastating amounts of petroleum and heavy metals, won’t the rain garden in my front yard become a mini toxic waste site that could harm children and pets?
Tallying the toxics
Rain gardens and similar environmentally friendly stormwater infrastructure are being embraced worldwide because they do their job so well. They sponge up polluted runoff, keeping the foul chemicals out of the places that are home to beloved wildlife and where people like to play and fish.
The worry is that these same, very efficient rain gardens that are cropping up in our parking strips and front yards are doing their job so well that they could become residential toxic sites. But in fact are they? Not according to the research that’s available. Here’s the score on pollutants in rain gardens, in summary:
Petroleum pollutants/PAHs: Studies from the field and laboratory find that rain gardens do a great job of capturing petroleum pollution, and that the chemicals are largely eliminated when they’re destroyed by bacteria in the soil.
Heavy metals: Soil and mulch in rain gardens contain particles that will adsorb and hold metals including copper, cadmium, lead, and zinc. A small fraction of the metals are sucked into plant roots and vegetation.
While metals are not degraded in rain gardens, they’re present at very low levels. When Northwest counties test for metals in the sediment that’s scooped from the bottom of stormwater ponds or rain gardens that drain parking lots and other city surfaces — material that would likely have higher levels of metals than your average residential rain garden — they found that the contamination levels were still below soil and compost standards meant to protect human health.
Bacteria and viruses: While some research has found bacteria and viruses that can cause disease in humans in stormwater, sunlight as well as other microorganisms in the runoff and soil of rain gardens can destroy the pathogens. Also, most of the microorganisms present come from animal waste and are less likely to cause illness in people.
The bottom line is that the soil in rain gardens is safe for kids and pets. That said, people are advised to wash their hands after working or playing in any soil, which can contain naturally occurring metals, fecal waste from the neighbor’s dog, or any number of compounds one wouldn’t want to ingest. And remember that while rain gardens are attractive landscape features, the plants and soil are also doing an important job, so they need to be treated with some care.