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Dirt Ranks #1 Water Polluter

July 11, 2003

It’s a Dirty Little Secret

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 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 accelerate 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 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 sediments that are deposited in the stream cover up these areas, sometimes even entombing young fish and eggs. The sediment may also destroy a stream’s natural ‘riffle and pool’ pattern and can make a stream shallower.

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 on construction sites, driveways and parking areas to a minimum. 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 water 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 ribbon of trees and shrubs, referred to as a vegetated buffer, on the down slope between places like your home and the open 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 for clean Maine waters!

This column was submitted by Kathy Hoppe, an Environmental Specialist with the Maine DEP's 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.