Saturday, December 21, 2019

Penn State Extension: What Is Sediment, Why Is Stormwater A Pollutant?

By Danielle Rhea, Extension Educator

Sediment is made up of soil particles that have been detached from the land by a process called erosion. In Pennsylvania, water is the primary cause for erosion, and sediment is often dislodged by rainwater and transported by stormwater runoff
Sediment can range in size from small, pea-sized gravel to tiny soil particles, less than 2 millimeters in diameter, and is present in both native soil and some materials used for building unpaved roads, driveways, and farm lanes. 
Consequently, the source of sediment can be from bare soil from construction sites or farm fields, poorly maintained dirt and gravel roads, or degrading stream banks. 
Any soil that is not protected from rainfall or runoff may be vulnerable to erosion and become a source of sediment pollution.
Raindrops that fall from the sky have enough force to dislodge soil particles from uncovered soil. 
Rain that is not absorbed into the ground becomes stormwater runoff and flows downhill, building momentum and picking up unprotected sediment until it reaches a waterway such as a stream, river, lake, or pond. Initially the shallow flow of water over the land is spread out in a process called sheet flow. 
But as stormwater continues to flow downhill, it can concentrate and form small channels called rills, or larger channels called gullies, that intensifies the force of stormwater runoff that detaches and transports additional sediment. 
Eventually, this sediment laden stormwater will reach our waterways, turning the affected surface water a muddy brown color.
Sediment pollution can also originate within a stream channel itself. During rainstorms and snowmelt, more water fills the banks of streams and rivers than the typical base flow. 
Historically, when stream levels rose, water escaped into the floodplain where energy was dissipated, and water was absorbed into the ground. 
However, because of development, there are more hard, impervious, man-made structures like roads and rooftops in both urban and rural areas, and streams in both landscapes now handle more storm water compared to those in natural settings. 
Additionally, many stream channels have been physically altered-- such as being lined with concrete, covered in culverts, or straightened through ditching--  to keep them contained. 
Combining increased runoff with concentrating flows in modified channels causes stream flows to be deeper and to travel faster than they did historically. 
This deep, fast flowing water has great erosive force and removes stream bed and bank sediment in a process called streambank or channel erosion. 
As bed and bank materials are removed, sediment is washed downstream and the elevation of the stream bed is lowered, forming vertical banks that further constrict water flow and continue to intensify erosion and sedimentation over time.
Once sediment reaches our waterways, it can degrade water quality in many ways. 
Small sediment particles may remain suspended in the water column or deposited onto the streambed. 
Suspended sediments increase the turbidity of the water, which causes the water to be cloudy, obstructs sunlight and limits photosynthesis of aquatic plants, reduces biologically available oxygen, and increases water temperature. 
Increased turbidity can also make it more difficult for fish gills to absorb oxygen and makes it harder for visual predators, such as brook trout or largemouth bass, to forage. 
Additionally, the cost of treating a source of drinking water with high levels of sediment is greater than the costs to treat clearer, cleaner water.
Sediment can also blanket the stream bed in a process called sedimentation. 
Over time, this process of mud building up on the stream bottom can reduce viable habitat for aquatic insects, fish, amphibians, and other wildlife by clogging the spaces between larger gravel, cobble, and boulders. 
Overall, the population of more sensitive species will be reduced, leading to a less diverse aquatic community.
Sediment poses a greater water quality risk than just soil particles alone, because it often carries other pollutants, such as nutrients, heavy metals, organic chemicals, bacteria and other pathogens along with it. 
These pollutants originate from sources such as agriculture, industrial waste, mine spoils, and urban contaminants and can have short-term and long-term effects. Some will be dissolved into the water and washed downstream quickly, while others may remain stuck to sediment on the bottom of the stream bed for years
If you have additional questions about stormwater, or you are just interested in learning more, you can find a full series of videos and articles in the Penn State Extension Stormwater Basics series.
[For more information on controlling stormwater pollution, visit DEP’s Stormwater Management webpage.]

(Reprinted from the latest Penn State Extension Watershed Winds newsletter. Click Here to sign up for your own copy.)
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[Posted: December 21, 2019]

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