Is Your Yard Permeable?

If you poured a five-gallon pail of water into the middle of your yard, where would that water go? Would it pool up? Would it run towards the street? Ideally, if your yard has the right amount of permeability, the water would slowly soak into the ground.

Permeability refers to how quickly water moves through layers of soil, sand, and rock. For example, consider the hard-packed soil of a playground (or your back yard in a drought!). It’s nearly impervious; water runs off of it as it would from cement. At the other extreme, there is the limestone karst country of southeastern Minnesota. Water rushes down through the cracked and water-worn rock, joining the underground water supply with a minimum of filtration. Karst is highly permeable; contaminated liquids applied to the surface flow directly into the ground water.

Ideally, water seeps slowly into the ground, where it is filtered and cleaned by layers of humus, soil, sand, and gravel. Water that lands on impervious surfaces such as parking lots, roads, and large, flat roofs tends to flow directly into nearby lakes and rivers (usually via gutters and storm sewers), carrying with it a wide variety of contaminants, from motor oil to pesticides.

Between 1986 and 2000, impervious surface in the seven-county metro area increased 60 percent. The more parking lots and developments there are, the more wetlands that are filled in, and the wider our highways become, the more water runs off the land and directly into lakes, rivers, and streams without having any of its impurities removed.

To see for yourself how permeability affects water quality, visit the Minnesota Landscape Arboretum. Water from five mini-watersheds, each using different types of pavement, flows into holding ponds. You can walk by each holding pond and see the difference in water quality.

As people are becoming aware that water should seep into rather than run off of the land, gardening and construction techniques are changing. Rooftop gardens are becoming popular; they absorb rainwater that otherwise would run down drain pipes and into the storm sewer. Some businesses and homeowners are using pervious pavers instead of tar or cement for parking lots and driveways. (Remember city streets made of paving stones? They were much better for the environment!)

Some communities are building rain gardens alongside roadways instead of constructing curbing that funnels rainwater into drains that run to the river. Like wetlands, rain gardens hold and absorb large amounts of run-off, and clean the water before it gets to the river.

What can you do to prevent rain water from flowing out of your yard and down the storm sewer? We’ve talked about a rain garden, but our yard, likemost city lots, is too small. A rain garden needs to be located a certain distance away from one’s basement. However, we could shore up the soil in front of the house, where the lawn slopes down to the sidewalk. We could use mulch to keep garden soil moist, so that less water runs off during a hard rain shower.

I’d like to buy pervious pavers for our driveway. My partner says, “But we already have a permeable driveway.” It’s sad but true. Now that the city inspectors are no longer preoccupied with the hazardous sunflowers we had draped over the back fence, I’m afraid they will notice the healthy crop of weeds pushing aside disintegrating chunks of tarmac behind our garage. Rainwater hitting our driveway doesn’t run off; it sinks through those cracks and gets cleaned and filtered by layers of earth, just like it’s supposed to. But I have to say, that’s one ugly permeable surface!

Next time it rains, grab a raincoat and observe the patterns of absorption and run-off in your yard. Then start thinking of ways that you could encourage rain water to stay in your yard and be absorbed by the soil. Maybe you don’t need that solid cement patio now that the kids are grown. If your driveway is due for a facelift, how about paving stones rather than tarmac?

There are dozens of ways you can insure that rain water goes through your soil rather than directly to the storm sewer. Here are some links:

Minnesota DNR rain garden resources
Minnesota Water Gardens

Anita Doyle, Green Irene Eco-consultant
earthlygoods.minneapolis@greenirene.com
http://greenirene.com/EarthlyGoodsMinneapolis