Sediment...It's a Dirty Word

(Conservation Currents, Northern Virginia Soil and Water Conservation District, March 2000)

There has been a lot of talk in the news recently about the damage caused by sediment on our lakes, streams, and rivers. While sediment comes from several sources including eroding stream banks, of particular concern is the amount of sediment escaping from construction sites.

What is sediment and how does sediment cause damage to our properties and natural resources?

When land is stripped of grass, trees, and other plants, the soil becomes exposed and loose. Precipitation picks up the loose soil particles, which then get carried away in the runoff on its way to our lakes, streams, and rivers. When the flow of water slows down, the soil particles settle out as sediment. Sediment is the term used to describe soil that has moved from one place to another.

Sediment can be detrimental to the environment. Nutrients such as nitrogen and phosphorous that are attached to the sediment trigger algae blooms in ponds and lakes. Algae reduce water clarity, deplete oxygen, kill fish, and produce unpleasant odors. Excessive sediment blankets the stream bottom destroying the fish and other living organisms on the stream bed. Turbidity (muddiness) from the sediment impairs in-stream photosynthesis thus reducing the aquatic food supply and habitat.

Sediment also has an economic impact. Dredging ponds and lakes and restoring stream channels are costly and labor intensive. Properties adjacent to damaged water bodies often decline in value.

What regulations do we have to prevent sedimentation from happening?

Each year an estimated 80 million tons of sediment are washed from construction sites into the lakes, rivers, and waterways of the United States. As more and more natural land is cleared for residential and commercial use, the chances increase for sediment to enter our waters.

The Fairfax County Erosion and Sediment Control Law addresses erosion problems on construction sites. This law requires that anyone who causes land disturbance exceeding 2,500 square feet must prepare an erosion and sediment control plan. The plan must satisfy certain minimum standards to reduce soil erosion and include practices to prevent eroded sediments from leaving the site. The appropriate authority within the County must approve the plan before land disturbance can begin. During construction, the project is subject to inspection to ensure that the plan has been properly implemented and maintained. If violations are found, the law provides for enforcement actions and penalties to be imposed.

With these measures in place, how does our present erosion and sediment control plan perform?

Unfortunately, inefficiencies exist and occasional failures occur. Sometimes erosion and sediment control plans are not adequately integrated with other stream protection measures such as stormwater management or land grading plans. The developer might not have adequately implemented the plan. And, most importantly, maintenance and inspection of the site might have been inadequate. In fact, without adequate site inspection, even the best programs may fail. In highly urbanized watersheds like those in Fairfax County, frequent site inspection is very difficult because of inadequate manpower.

One proposed solution to inspection difficulties is to prioritize sites based on their environmental sensitivity, which can be determined when developers or owners apply for rezoning or seek plan approval. For sites that the County deems environmentally sensitive, the owners or developers would need to provide baseline, predevelopment information on downstream resources potentially threatened by the planned development. The County would inspect these sites more often. When runoff and sediment from a site cause downstream damage, the County could use the baseline data to assess the extent of the damage.

At present, lack of predevelopment information on damaged water resources is the main obstacle to reliable damage assessment.

What is the County doing to improve the quality of its erosion and sediment control program?

A steering committee appointed by the County’s Planning Commission is now examining several aspects of residential development in Fairfax County. {See page 8.} The committee will review current policies and regulations regarding stormwater management and erosion and sediment control and their effectiveness in ensuring minimum impact of residential development on downstream properties. The committee is expected to come up with recommendations for improving the quality of the County’s erosion and sediment control program.

A separate county task force is reviewing the site inspection program to determine what it would take in terms of manpower and dollars to adequately inspect all building sites in the County.

For more information, contact Asad Rouhi at 703/324-1427 or send an e-mail.