SOURCES, TRANSPORT, AND FATE OF TRIHALOMETHANES IN GROUNDWATER FROM DRINKING-WATER WELLS IN THE UNITED STATES

Samples of groundwater from drinking-water wells across the United States were analyzed for trihalomethanes (THMs) by the U.S. Geological Survey in order to understand THM occurrence in drinking water supplies at a national scale. Samples of untreated groundwater from 2,492 public and domestic wells were collected during 1996-2007 and analyzed for THMs. Chloroform was the most frequently detected THM and was detected in 37% of public wells and in 18% of domestic wells. The brominated THMs were detected much less frequently than chloroform but were more commonly found in public wells compared to domestic. Most concentrations of THMs were less than 0.02 micrograms per liter.

The sources of THMs to groundwater from drinking-water wells are believed to be primarily anthropogenic. The most likely source of THMs in groundwater from urban public wells is the recycling of chlorine-treated drinking water, which may contain THMs from the treatment process. This is supported by the detection frequency and concentration patterns of THMs as well as their frequent co-occurrence with other anthropogenic contaminants. In rural areas, household products disposed in septic systems may be a source of THMs to public and domestic wells.

Relational analyses indicated that the dissolved oxygen content of groundwater was the variable most strongly associated with THM occurrence. Biodegradation is believed to be the primary fate factor controlling the occurrence of THMs and dissolved oxygen strongly influences the microbiological activities that transform THMs. The nature of the relation between dissolved oxygen and THM occurrence is consistent with published literature on the hypoxic biodegradation of THMs.

The finding that dissolved oxygen is strongly related to THM occurrence has relatively low value for most groundwater resource managers in terms of controlling or limiting THMs because little can be done to affect the redox or transport conditions of an aquifer. However, understanding the redox conditions of an aquifer can alert managers to the increased possibility of contamination of groundwater by THMs where both relatively high dissolved oxygen conditions and sources of THMs exist.