To pave or not to pave - controlling water runoff at your school
You may be surprised to learn that the amount of pavement at your school can increase pollution in nearby waterways, threaten local stream life, and reduce the health of your trees. Consider future pavement carefully and opt for more earth-friendly options to bring you closer to your green school goals. Along the way, your students will learn how to make eco-friendly choices and more about the water in our oceans and rivers and the water that we drink.
Your school probably has concrete or asphalt parking lots, sidewalks, playground blacktops, running tracks, and sport courts. It’s true that paving an area is relatively inexpensive, requires little further maintenance, and creates a non-slip area that doesn’t hold puddles of water. But there are problems with this solution.
When rain falls on pavement or other impervious materials, it runs off of the surface, usually into a storm gutter in a nearby road. Water coming off of roofs carries chemicals from the roofing materials and falls onto the pavement on its way to the storm drain. On its journey, it picks up all of the trash, oils, and chemicals from sealants on the pavement and carries it into the street gutters. These gutters drain directly to culverts, canals, creeks, rivers, and lakes, with some leading to our oceans. This trash either gets stuck somewhere en route, creating hazards for local wildlife, or it travels all the way to the ocean where plastic bags are mistaken for jellyfish by turtles and tiny plastic pieces are picked up by mother albatrosses to feed their young, who starve with bellies full of plastic. Eventually all of the plastic breaks down into a chemical soup, contaminating the ocean water itself.
Scientists have found an area they call the Great Pacific Garbage Patch or the North Pacific Gyre. In this area, currents swirl the water around in circles, and floating trash is accumulating. There’s a similar area in the Atlantic Ocean. The plastic breaks into tinier pieces, but plastic doesn’t dissolve like natural materials. Instead, it is converted into chemicals that mix with our water sources and are taken in by fish and marine mammals and seabirds.
Pervious ground surfaces allow rainwater to filter into the ground slowly. Trash can then be picked up and recycled, composted, reused, or placed in a landfill. As water filters through the layers of rock and soil in the earth, contaminants are removed, and microorganisms work to decompose trash.
Creeks, streams, bodies of water
Water that soaks into the ground nourishes plants and trickles down through the earth and rock, being filtered in the process, returning water to the aquifers, springs, creeks, and rivers gradually. This means that there’s a constant source of water for small streams. This water supports plants and animals that live in and around the stream. Acting as a filter, the soil prevents pollution and trash from reaching the waterways, also important for animals living there. If this water rushes directly through drainage ditches, the streams are inundated after a rain, but dry at other times. This inconsistency is problematic for riparian plant and animal life, and wildlife around the stream dies. The trash and pollution that is washed directly into the water is can also be toxic to this wildlife.
Most trees’ roots extend out to the edge of their branch canopy. When pavement extends underneath this canopy, as when a parking lot or paved patio is placed underneath an existing tree, these roots are deprived of water. Even when water running off of pavement is directed towards trees and plants, often the volume is too great for the water to soak into the soil. Instead, it runs off into the gutter. In nature, rain falls on the earth, where it soaks gradually and deeply into the roots around the tree.
- Avoid hard, impervious pavement where possible. See below for some alternatives.
- Catch water off of roofs. Where impervious areas like roofs drain, catch this water and save it to be used for irrigation or at least released when it is more likely to soak into the earth.
- Install water retention areas for newly-paved areas. Most building codes require that newly-constructed buildings retain run-off water on-site in specially-designed ponds. This slows the water down, allowing it to filter back into the earth. These retention areas also catch trash and contaminants, and they reduce the volume of water running into the community storm sewer system after a rain.
- Install rain gardens. These are areas where water collects and is used to promote plant and animal life. The plants in these gardens also help clean and filter the water.
- Where you must use pavement, opt for porous paving.
- Choose concrete over asphalt. Asphalt is a petroleum product, and the oils from the asphalt and from petroleum-based sealants can contaminate water sources. And because asphalt is darker in color, it can get much hotter than concrete, which may be a real issue in hot climates. Conversely, concrete is more durable; is made primarily of sand and cement, both relatively inert materials; and it stays cooler than asphalt in the summer.
Alternatives to pavement
Wood mulch – A natural product with some cushioning qualities for safety. Allows water to trickle through the spaces between the shredded wood. Our school uses mulch from local tree companies as an inexpensive way to keep unplanted areas from becoming muddy.
Recycled rubber mulch – Made from shredded tires, this is used often on playgrounds.
Rubber playground surfacing – Rubber granules bound by a polymer liquid, poured in place, which later hardens. Some products are porous, and some are not, so choose carefully. This product is often used for playgrounds accessible to wheelchairs or for running tracks or paths. Also look for products made from recycled materials.
Gravel - Also a natural product. Pea gravel, if deep enough and sized appropriately, can cushion falls.
Crushed granite – Good for tracks and walking paths. This will become compacted and can be less porous than some other options.
Rocks – Larger rocks can be used under eaves and next to walkways. Spaces between rocks allow water to trickle through. Place weed fabric beneath to prevent plant growth.
Grass – Select native varieties of grass that don’t require irrigation. Avoid using herbicides and pesticides which can contaminate waterways.
Planted groundcover – Good for areas not subject to foot traffic. Again, native plants make good choices.
Gardens – Vegetable gardens, habitat gardens, flower gardens, butterfly gardens, herb gardens. Keep in mind that gardens require maintenance.
Rain gardens – These gardens are designed specifically to retain water. Porous soil and gravel prevents the area from becoming boggy. Select plants carefully for maximum success (see link below).
Structural plastic grids under turf or gravel – In cases where an area is used part of the time for vehicular traffic, as in a fire lane or an overflow parking area, these grids are placed beneath the surface of the turf to support the weight of the cars, protecting the grass roots while allowing 100% of the water to flow through. www.invisiblestructures.com Grasspave and Gravelpave.
Pervious concrete pavers – These pavers are formed with holes between or within them for the water to run down into the earth. The pavers support vehicular traffic, but prevent the soil in the holes from being compacted. This results in about a 50% reduction in the porosity of the surface, depending on the size of the holes.
Pervious asphalt and concrete products – The spaces between the larger pieces of aggregate found in these products allow water to percolate through the surface into the soil below.
Recycled glass porous pavement – Made from recycled glass with an elastomeric binder, this pavement is an earth-friendly alternative to less porous options. www.prestogeo.com/Filterpave
Decking – Decking with space between the planks allows about 50% of the water to flow through. Cedar is naturally rot-resistant. Wood-plastic composite decking material is made from recycled plastic and wood fiber.
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