A viewer asked this question on 4/30/2000:
1. On average, how long does it take for water to be cleansed of pollution that has been in effect for several years?
2. Can this process be completed naturally?
3. What tactics are used most often in cleaning streams? Are there any such tactics?
4. What are the most common pollutants in streams found in residential areas?
5. Which of these usually cause the most damage?
6. How do polluted streams affect the surrounding wild life?
7. What do environmental agencies do to put a stop to/cleanse polluted streams?
JesseGordon gave this response on 5/3/2000:
1. I don't know if anyone can cite an "average", but the amount of time for water to be "cleansed" depends on a number of factors:
a. How much water flow there is (for example, a river would recover faster than a lake, since the lake's water doesn't change)
b. What the source of the problem was (for example, if a river is heavily silted due to deforestation, the forest would have to regrow for the system to fully recover).
c. Whether the living population was wiped out or heavily reduced, or not (for example, if fish with an annual spawning season were affected, it will take until at least the next spawning season or two to recover).
2 & 3. Generally, streams and rivers can recover naturally from pollution. The only "tactic" really needed is to remove the source of the problem and let nature recover. Of course, we often intervene to speed up the process, or to undo specific damage. For example, I've seen the National Park Service use bulldozers in streambeds to undo the damage of previously bulldozed "channelization".
4. The most common pollutants in residential areas are runoff from roads and lawns. From roads you get salt (in the north where salt is used for snow control), and oils, both of which kill freshwater species. From lawns you get fertilizers, which cause algae blooms. And in all built-up areas, you get siltation from building roads and other activity near waterways.
5. What causes the "most damage" depends on what damage you're concerned with. it might mean:
a) "Most dead fish" (the most damage from a recreational perspective); caused by salt, silt, or any other major pollutant.
b) "Most health risk" (the most damage from a park user's perspective); caused by E. coli populations from sewage or animals upstream.
c) "Most wilderness loss" (the most damage from an ecological perspective); caused by channelization or other heavy activity, or a dam.
6. Generally, streams and rivers are the basis for the wildlife in the entire surrounding area. That's the basis for considering pollution from a "watershed" perspective, which is the EPA's latest way of looking at water pollution.
7. What the EPA has focused on for its first 20 years (since the early 1970s) is "point source pollution", which means stuff that comes out of pipes. Over those 20 years, most sewage pipes and other effluents have been cleaned up. Now the EPA is switching its focus to "non-point sources", such as runoff from roads and lawns.