Sources. Sediment may come from eroding soil on bare areas or unstabilized construction sites, or from impervious surfaces in developed areas where soil particles are deposited by traffic and wind. All land areas will contribute some sediment, but unprotected or disturbed areas contribute huge amounts of sediment from relatively small areas. The Soil Conservation Service (SCS) estimates that erosion on construction sites is about 10 times greater than erosion on cropland, about 200 times greater than land in pasture, and about 2000 times greater than land in forest (USDA 1970). Also, construction vehicles often carry mud onto roads and parking lots, which can be washed into streams with the next storm.
Intensively developed areas also will contribute large amounts of sediment. The very high flows from the large amount of impervious surface severely erode unprotected stream banks, causing direct deposition of sediment in the stream.
Any particles of dirt and dust on the ground, or lying on a road, roof or parking lot, will wash off during a storm and become sediment in stormwater. Industrial air emissions of dust, particulates and even smoke become sediment in stormwater.
Impacts. Sediment poses a double threat to water. It is a pollutant, and it also carries other pollutants with it. Many pollutants form weak chemical bonds to sediment particles and are said to be sediment-bound or associated with sediment. In many instances, the sediment-bound pollutants are much more harmful to the stream.
Sediment that settles to the bottom can smother plants and animals, create mudbars, and fill in lakes and impoundments. Also the sediment carries sediment-bound pollutants such as toxic metals and organic pollutants, effectively concentrating them in the bottom deposits of the receiving waters. These toxic pollutants can be released to the aquatic environment when high storm flows resuspend the sediment, thus posing a future risk to the stream and to aquatic life.
Very fine sediment that stays in the water without settling out is known as suspended sediment or suspended solids. Suspended sediment causes the water to become cloudy or muddy. This increased cloudiness damages the aquatic environment by reducing the distance light penetrates into the water, thereby inhibiting or killing plants and sight-feeding predators. The fine particles can clog the gills of fish and shellfish (particularly juveniles), causing them to suffocate. In addition, fishing and other recreational uses are impaired.
Indicator parameters. Sediment can be recognized directly as the muddy deposits, mud bars, sand bars and mud flows on the bottoms of ponds and channels and on other land that has had stormwater flowing over it. Sediment stains appear on trees and other objects, marking the high water level.
Source reduction. Most sediment in water can be prevented by using these steps:
Manage stormwater runoff to minimize the velocity and peak flows of runoff on disturbed or bare land.
Spread large, concentrated flows of stormwater runoff in thin sheets over grassed areas or stable areas such as forested areas to reduce the volume and velocity of the runoff.
Reduce sediment from industrial sources by containing dust generated in crushing, grinding or other similar dusty operations. Commercial and industrial areas can clean areas such as parking lots and roads to remove dust, dirt and other particles that settle on those surfaces.
Removal. A high level of sediment can be reduced inexpensively by allowing it to settle in dry detention basins or wet retention ponds with extended holding times. Longer holding times are especially important because a longer holding time means more sediment and finer sediment particles will be removed. The ponds used for settling must be cleaned out periodically, which can be expensive.
Sediment may also be removed by vegetative filters such as grassed filter strips, grassed swales, and forested buffer strips. These measures are used as pretreatment for stormwater entering another stormwater control device to protect the device from excess sediment.
Return to: Surface Water Pollutants and Stormwater Runoff