From the The Glenwood Springs Post Independent (John Colson):
The compound in question is known as “total trihalomethanes,” Pace said, adding that its formation in the water system is known as a “disinfection byproduct.” It comes about, Pace said, when chlorine comes in contact with organic matter and other sediment within the water system. The longer the chlorine and the sediments sit together, he said, the greater the production of TTHM, as it is known in water treatment circles. He said that is partly why the town, at the end of every summer, flushes its two-mile water line leading to the Coal Ridge High School, in order to bring fresher water to the taps there.
“In large quantities,” Pace said, the chemicals “could be cancer causing,” and the letter from the town advises residents that they might want to switch to bottled water, or some kind of water-hauling service, until the problem is fixed. That is not likely to be until late 2011 or possibly early 2012, because the town first must study the situation to come up with possible solutions, and then get state permission to go ahead with whichever solution is identified.
More water treatment coverage here.