Organic chemical contamination may have a local source. We’ve all heard of and underground gasoline
or fuel oil storage tank leaking into someone’s well.
Many public water supplies contain low levels of organic
compounds created as byproducts of water chlorination. These are called trihalomethanes or THMs. A good example is chloroform, one
of the most common organic compounds found in drinking water and a possible
carcinogen. It is often traced to
chemical reactions between dissolved organic matter and the chlorine used to
disinfect public water supplies.
Health data on THMs tend to be sketchy or incomplete; but
some evidence suggests that collectively, they may contribute slightly to
cancer risk. Public health officials
view the risk as acceptable however, because of the important
disease-preventing benefits of chlorination.
The EPA does not set limits for THMs in small water supply systems
partly because such systems have experienced disease outbreaks from inadequate
The EPA requires water-supply systems serving more than
10,000 people to keep THM levels below 100 parts per billion. If testing shows that your drinking water
exceeds that level, you can reduce it significantly with activated carbon