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 chlorination.

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 filters.