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Dirt Ranks #1 Water Polluter, In Our Back Yard

September 28, 2007

It’s a dirty secret; soil erosion is the number 1 source of water pollution in Maine. Each year rainstorms and snowmelt wash tons of dirt off the land and into waterbodies around the state. How could something so ‘natural’ be so bad? Soil erosion is natural after all. However, when we change the landscape from forest to yards, streets, farm fields, shopping centers and roads, we speed up soil erosion. In fact, across the nation, soil is eroding at about 17 times the rate at which it forms. According to DEP scientist Jeff Dennis, soil erosion and the resulting silt in Maine’s lakes, rivers and streams pose the single greatest threat to surface water quality.

We have all seen a stream in our neighborhood turn cloudy or brown after a good rainstorm. This cloudy water makes it difficult for fish to see and feed properly. But the dirt causes more problems than just murkiness.

The dirt particles act as sandpaper against a fish’s gills, causing damage and making breathing difficult. Also, many fish and aquatic insects lay their eggs in gravel beds. The silt covers up these areas, sometimes even burying young fish and eggs.

To add to the problem, eroded soil particles are more than just “plain dirt”. They carry hitchhiking pollutants like oil, fertilizers, pesticides and bacteria. These contaminants further harm water quality.

In lakes, one particular hitchhiker can be devastating; it’s phosphorous. Phosphorus, like dirt, is natural, but as we change our landscape, we increase the amount of phosphorus running off the land. Phosphorus does help plants to grow, but we don’t want plants like algae growing in our waters. We don’t want our lakes turning green - we want them blue! So, if we want Maine’s lakes to be blue, and our streams to be clear and healthy, then we all need to help. We all need to work to stop soil erosion.

The first step is to stop soil from moving in the first place. Cover bare soil with grass, shrubs and trees so the rain doesn’t have a chance to move it around. Keep exposed areas to a minimum on construction sites, driveways and parking areas. When areas must be exposed, mulch the areas every night, so that the soil is never left uncovered. And be sure to finish projects and seed the areas within two weeks.

On dirt roads and driveways where the soil can’t be covered, it’s important to catch the soil before it reaches the stream or lake. If the volume of stormwater isn’t great, divert it off driveways, roads and gardens into a stable vegetated area so the dirt can be trapped. The best idea is to plant a wide ribbon of trees, shrubs, and ground cover (called a vegetated buffer) on the down slope between places like your home and driveway and the water to capture the soil and pollutants.

It may be a well-kept secret or a little-known fact, but the word is out - soil is water quality enemy number 1. Stop soil erosion to protect our clean Maine waters!

This column was submitted by Kathy Hoppe, an Environmental Specialist with the Maine Department of Environmental Protection's (DEP) Bureau of Land and Water Quality. In Our Back Yard is an informational column of the Maine Department of Environmental Protection. E-mail your environmental questions to or send them to In Our Back Yard, Maine DEP, 17 State House Station, Augusta, ME 04333.