—by Naomi Jackson, HPC Staff
I expect the question of "permeability" is an odd one for most of you. You might wonder if your soil is fertile, or if you have the proper pH balance, or how to get rid of dandelions without spraying. But permeability? That’s a new one.
Permeability refers to how quickly water moves through layers of soil, sand, and rock. For example, consider the hard-packed soil of a playground. 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.
In recent months, you have probably seen at least one article about rain gardens. For a while, every paper I read seemed to have something more to say about this new-fangled style of gardening. Rain gardens are designed to catch and filter rain water. The impetus behind them is the growing impermeability of metropolitan land surfaces, as fields and wetlands are paved over, and the concomitant degradation of nearby lakes, streams, and rivers.
A couple of years ago, I took a class from Dr. Marvin Bauer at the University of Minnesota. I learned that he and his colleagues are using satellite imagery to identify impervious surfaces in the seven-county Metro Area, and to examine water quality in the same area. They are trying to educate people about how necessary permeable surfaces are in maintaining water quality. 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. Bauer and his associates have noted that between 1986 and 2000, impervious surfaces in the seven-county Metro Area increased 60%.
As people are becoming aware that water should seep into rather than run off of the land, gardening and construction techniques are beginning to change. Wetlands are being re-created. A car dealership in Maplewood has a parking lot made of pervious pavers. (Is anyone old enough to remember when city streets were made of paving stones instead of tar and cement? It was much better for our environment!) Rooftop gardens are becoming popular; the gardens absorb rainwater that otherwise would run down drain pipes and into the storm sewer. Some communities are building rain gardens alongside roadways, rather than funneling rainwater into drains that run to the river. The rain gardens can hold and absorb large amounts of run-off, and the water gets cleaned before it gets to the river (that’s the purpose of wetlands, too).
The Minnesota Landscape Arboretum has a model parking lot where you can see for yourself how different drainage methods affect water quality. Water from a traditionally paved parking area runs into one holding pond. A second section uses permeable pavers, and a third adds a rain garden to the permeable pavers. You can walk by each holding pond and look at the difference in the water. In my own neighborhood, lawn signs are sprouting as people learn the importance of permeable soil. We have two “watershed friendly” yards on our block, which means that the yards are designed to retain and absorb rain water rather than allowing it to run off. Most city lots are too small for rain gardens, which need to be located a certain distance away from one’s basement. But at our house we’ve been looking at other ways to ensure that rainwater doesn’t pour down our front sidewalk and onto the street. 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. One thing I’d particularly like to do is buy permeable 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, wander around your yard and observe the patterns of absorption and run-off. Then start thinking of ways that you could encourage rain water to stay in your yard and be absorbed by the soil. It might be as simple as relocating downspouts. Perhaps 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? Or that ugly sidewalk in the back yard. How about stepping stones instead?
If you'd like more information about permeability or making your yard watershed-friendly, check out the resources listed in the sidebar
Resources on permeability:
- Batalden, Rebecca. “Rain gardens provide bonus for street renovation work in Como Park area,” Park Bugle, May 2006.
- Dickson, Tom. “Gardens for a Rainy Day,” Minnesota Conservation Volunteer, Minnesota DNR, May-June 2000.
- Friends of the Mississippi River provides information and workshops about rain gardens another means of improving water quality.
- “Impervious Surface Mapping using Remote Sensing,” (PDF). Remote Sensing and Geospatial Laboratory Fact Sheet 1, University of Minnesota College of Natural Resources.
- Metro Blooms, a private non-profit organization, offers rain garden workshops: http://www.minneapolisblooms.org/. [Ed. This organization still offers workshops as of 2010.]
- A Quick Reference Guide to Earth-Friendly Home Landscaping is available from Hennepin County Environmental Services, www.hennepin.us.
- Rice Creek Watershed District Education program "Planting for Clean Water".
- Sartor, Nancy. “Go green roofs, go rain gardens,” Southside Pride: Riverside Edition, April 2006. One of Sartor’s resources is "Green Roofs for Healthy Cities".
- Underwood, Lynn. “Rainy Day Plants,” Minneapolis StarTribune, May 17, 2006.
- The University of Minnesota’s Remote Sensing and Geospatial Laboratory provides links to some of Dr. Bauer’s work.
- The University of Wisconsin Extension offers "Rain Gardens: A How-to Manual for Homeowners" for $4.25 [Ed. 2010 price). It can be purchased on line.