USGS Publications Warehouse

Abstract

This report describes the occurrence of trihalomethanes (THMs) in the Nation's ground water and drinking-water supply wells based on analysis of 5,642 samples of untreated ground water and source water collected or compiled during 1985?2002 by the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program. THMs are a group of volatile organic compounds (VOCs) with natural and anthropogenic sources that are of interest because they are associated with acute and chronic health problems in humans. THMs occur in water primarily from chlorination and are classified as disinfection by-products. In this report, the four THMs are discussed in the order of chloroform, bromodichloromethane, dibromochloromethane, and then bromoform; this sequence corresponds to largest to smallest chlorine content and smallest to largest bromine content. Four trihalomethanes were detected in less than 20 percent of samples from studies of (1) aquifers, (2) shallow ground water in agricultural areas, (3) shallow ground water in urban areas, (4) domestic wells, and (5) public wells. Detection frequencies for individual THMs in the five studies ranged from zero for shallow ground water in agricultural areas to 19.5 percent for shallow ground water in urban areas. None of the samples from aquifer studies, domestic wells, or public wells had total THM concentrations (the sum of the concentrations of chloroform, bromodichloromethane, dibromochloromethane, and bromoform) greater than or equal to the U.S. Environmental Protection Agency Maximum Contaminant Level of 80 micrograms per liter (?g/L). Comparisons of results among studies of aquifers, shallow ground water in agricultural areas, and shallow ground water in urban areas were used to describe the occurrence of the four THMs in ground water for three different land-use settings?mixed, agricultural, and urban, respectively. At the 0.2-?g/L assessment level, one or more of the four THMs were detected in 7.9 percent of the samples from aquifer studies, 2.2 percent of the samples from shallow ground water in agricultural areas, and 19.5 percent of the samples from shallow ground water in urban areas. In general, detection frequencies and concentrations of the four THMs were greater in shallow ground water in urban areas compared to aquifer studies and to shallow ground water in agricultural areas. For all three of these studies, the most common two-THM mixture at the 0.2-?g/L assessment level was chloroform?bromodichloromethane, and this was the only two-THM mixture found in samples of shallow ground water in agricultural areas. Comparisons of results between studies of domestic wells and public wells were used to describe the occurrence of the four THMs in two different supplies of ground water used for drinking water. At the 0.2-?g/L assessment level, one or more of the four THMs were detected in 5.2 percent of the domestic well samples and in 14.7 percent of the public well samples. In general, detection frequencies and THM concentrations were greater in samples from public wells than from domestic wells. At the 0.2-?g/L assessment level, the six possible two-THM mixtures occurred about six times more frequently in samples from public wells than from domestic wells. One of the most common two-THM mixtures in samples from domestic and public wells was bromodichloromethane?dibromochloromethane. Detection frequency is associated with the chlorine content of the THM compound. In general, for each of the five studies, as the chlorine content of the THM compound decreased, the detection frequency at the 0.2-?g/L assessment level also decreased. The exception was the study of public wells in which the detection frequency of the THMs decreased in the following order: chloroform, bromoform, dibromochloromethane, and bromodichloromethane. At the 0.2-?g/L assessment level, the median concentration for one or more of the four THMs ranged from 0.3 ?g/L (shallow ground water in agricultural areas) to 0.6 ?g/L (shallow ground water in urban areas). For the other three studies (aquifers, domestic wells, and public wells), the median concentration was 0.5 ?g/L. Generally, as the chlorine content of the THM compound decreased, the maximum concentration of the THM also decreased. At both the 0.2-?g/L and the 0.02-?g/L assessment levels, the most common finding was that most samples did not have any detected THMs. Furthermore, the number of samples with no THMs was much greater than the number of samples with 2, 3, or 4 THMs combined. At the 0.2-?g/L assessment level, one THM was detected in 6.7 percent of the samples from aquifer studies (mixed land use), 2.1 percent of the samples from shallow ground water in agricultural areas, and 17.7 percent of the samples from shallow ground water in urban areas. For the same studies at the same assessment level, two or more THMs were detected in 1.3, 0.1, and 1.8 percent of the samples, respectively. At the 0.2-?g/L assessment level, one THM was detected in 4.6 percent of the domestic well samples and 9.4 percent of the public well samples. For the same studies at the same assessment level, two or more THMs were detected in 0.6 and 5.3 percent of the samples, respectively. For all five of the studies, chloroform was the most frequently detected individual THM, and if only one THM was detected in a sample, the THM was most likely to be chloroform. Analyses of mixtures were performed using the 0.2-?g/L assessment level for shallow ground water in agricultural areas, shallow ground water in urban areas, and public wells and using the 0.02-?g/L assessment level for aquifer studies and domestic wells. No mixtures occurred in 1 percent or more of the samples collected from shallow ground water in agricultural areas. Comparing the results for the other four studies was difficult because of the different assessment levels, but some mixtures were more prevalent in all four studies. Generally, the most common mixtures included chloroform. Chloroform?bromodichloromethane was the most frequently detected or one of the most frequently detected two-THM mixtures in the four studies, and the mixtures of chloroform?perchloroethene and chloroform?methyl tert-butyl ether were the most frequently detected or one of the most frequently detected THM and non-THM mixtures in the four studies. For bromodichloromethane, dibromochloromethane, and bromoform, the most common mixtures were with other THMs. In samples collected from shallow ground water in urban areas and in samples collected from domestic wells, these three THMs were only detected when one or more of the other THMs were detected.