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What is a Silt Trap?
A silt trap is a designated area where water that is contaminated with suspended sediment as a result of construction activity or water runoff is contained. While the water is in the trap, the sediment can settle to the bottom of the trap until it can be removed. These devices can be made using silt curtains, silt fences or a series of shallow ponds to naturally filter the sediment from the water before it reaches a stream or clean body of water.
Environmental protection efforts, such as the silt trap, are often seen in conjunction with mining or construction. Activity from these industries can result in the production of grain-size particles, stone dust and other components that create suspended sediment when caught up in water runoff during rainfall. As the rainwater carries these particles and other pollutants to streams, rivers and lakes, the suspended sediment can cause a serious issue for the fish and other wildlife that inhabit these waters.
The silt trap is designed to prevent the ecological disaster that could follow if these waterways were suddenly to become filled with suspended sediment as a result of mining or construction activity. Using the laws of gravity and the physics associated with liquid suspensions, the trap allows the denser, heavy particles of the suspended sediment to drop out of the suspension by creating an artificial break in the current. These particles are easily picked up by water runoff moving with a certain amount of speed, but when the water's current is slowed or completely eliminated by the silt trap, the heavier particles of sediment naturally drop to the bottom of the trap.
If left unchecked, silty soil and suspended sediment could travel long distances in fast-moving water. The potential for damage to aquatic wildlife is very great. By incorporating the silt trap into a mining or construction site, these industries can prevent unnecessary pollution or damage to the local watershed, thus helping the environment.
In some cases, the silt trap might also serve an additional purpose. Particularly in the mining industry, it becomes the last opportunity to reclaim precious metal ores that otherwise might have been lost. Settling tables, sluices and gold pans are time-tested methods of reclaiming gold, and these methods all work on the same principles as a silt trap. Many mining companies that produce ore in areas where precious metals are found might also regularly remove the accumulated sediment in their traps and process it to reclaim the precious metals that can be found.
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![Silt curtains used to create a silt trap.]()
By: Pshaw-PhotoSilt curtains used to create a silt trap.
Discussion Comments
@jcraig - The sedimentation ponds you described are very good at sediment control. They are great places for wildlife, too. The problem is that they can be expensive. They can only be put in certain areas and cost some money to set up. Given a choice, the ordinary farmer doesn't care where the water goes if it costs him money. To this point, most of the sedimentation ponds have been installed as pilot projects by universities.
I believe that strip mines do have regulations saying that they must have ponds, though. The nutrient runoff from farms is bad, but runoff from mines can be toxic to everything around it. Most of the pollution is a combination of the chemicals used to mine the ore as well as things used to separate the ore from rock.
Sulfuric acid is often used to mine copper, and this has turned into a huge problem in Chile where copper is common. The sulfuric acid is extremely dangerous and leaks into the water supplies, because they don't have very good regulations in place to filter to mine runoff.
I used to live in central Illinois where farming is the major industry. One of the big problems there is that sediment and fertilizer from the fields ends up running off into streams and ponds. A lot of the nutrients eventually end up in the Mississippi River and the Gulf of Mexico, which has caused some major problems.
Something that they have started to do around the larger farms is putting in sedimentation ponds. Basically, once the water flows off of the field, it goes into the pond instead of the drainage pipes. The ponds aren't anything big. They just cover a relatively small area, but they work great from what I have been told. They have done a few studies and found that almost all of the nutrients and sediment gets filtered out in the pond so that only clean water ends up in the pipes. i think it is a great method of erosion and sediment control, and they should be more common.
For any construction project, they should have permits in place to do whatever they are doing. You could check with the city and see that they have the right permits. If the stream is a small one, the city probably makes its own regulations about what can and can't go into it. If it is a larger one, though, it make be regulated by the EPA. I wouldn't go snooping around the construction site, though, looking for these things. I'm sure the workers wouldn't appreciate that much.
How exactly would you identify a silt trap if it were in place? I guess I could see the silt pond, but what would a silt fence look like? How effective are these things at actually stopping the silt from entering the water supply?
I ask because there is a construction project going on in our town, and I get the feeling that the workers are not using the proper methods to stop material from getting into the stream that is nearby. I would like to get an idea of whether or not they are doing things the way they are supposed to.
If they were violating the regulations, who would you call to report something like that? Would it be an issue that the city was supposed to take care of, or would that be something for a larger organization like the state or national EPA?
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