More About Green Infrastructure
Trees, shrubs, grasses, and soils naturally absorb and purify water. When they are replaced with hard surfaces like pavement, buildings, or compacted soil, rainwater and snowmelt that is usually absorbed and infiltrated is turned into "runoff"—excess water that runs overland until it ends up in a water body or storm sewer. The picture below shows how, in a naturally-occurring ecosystem, about 10% of water from a typical rainstorm will turn into stormwater runoff. In a highly urbanized environment, close to 55% of water will turn into runoff.
This extra runoff picks up pollutants on the ground—like heavy metals and petrochemicals from vehicles, bacteria and viruses from animal waste, nutrients and organic compounds from fertilizers—and deposits them directly into lakes, rivers and streams. All these pollutants can make our water unsafe for drinking or swimming, can harm or kill fish, birds and other animals, and cause other problems.
What's more, all that extra water flowing as runoff can overwhelm our communities, causing flooding. Urban floods like the one we saw in Toronto in 2013 were a result of very intense rainfalls and a lot of impervious surfaces that made it so all that water had nowhere to go but overland.