Why is Silt a Type of Pollution?

Silt is fine-grained soil – if you rub some between your fingers it feels softer than sand but grittier than clay. How can something like dirt be considered pollution? Some soils get washed into streams after most heavy rainstorms – we have all seen Little Sewickley Creek look like chocolate milk, not water. If a stream has a healthy, undisturbed riparian zone (the area around the stream) the stream will generally clear up a few hours after the rainstorm. However when there is a lot of disturbed land in the watershed, due to construction sites or newly plowed farm fields, too much loose soil can get washed into a stream. The stream will look like chocolate milk for days not hours. This can be very harmful to the organisms living in the stream for several reasons:

  • The fine-grained soils can clog the gills of fish and other macro-invertebrates (crayfish, insects, snails, bivalves) living in the stream causing them to suffocate and die.
  • The soils suspended in the water impact how much sunlight can penetrate the water. Aquatic plants need sunlight for photosynthesis, which produces food for the plants (which then are food for the fish, macro-invertebrates, water fowl, etc.) and oxygen (which is also vital to the other organisms living in the stream).
  • The soils also make the water darker. We all know how dark clothing feels on a sunny day – it is hot. Dark water also gets hot and the warmer the water the less dissolved oxygen it can hold. Without adequate amounts of dissolved oxygen fish and other aquatic organisms die.
    The dirt can also be coated with harmful pollutants like herbicides, pesticides, fertilizers and oil. Pesticides and herbicides are designed to kill unwanted plants and insects on the land; if too much of it gets washed into the stream it will also kill plants and insects in the stream. Fertilizers help plants to grow but if too much gets washed into streams it can cause algal blooms which disrupt the natural balance in the stream and deplete the oxygen supply.

That’s why construction sites, like the QVRA site, are required by law to install and maintain Erosion and Sedimentation Controls. For example “catch ponds” are designed to collect all the silt-laden water from the site and store it for a few days until all the silt settles to the bottom and the clear water is released from the site. Other controls include re-vegetating a site, or farm field, as soon as possible so the plant roots hold the soil in place. These straightforward measures are essential in protecting our waterways from a simple pollutant like dirt.

Last updated: March 27, 2022 at 10:04 am