T H M S in drinking water The director of EPA’s criteria and standards division of the Ofjce of Drinking Water traces ‘thecase for regulating trihalomethanes
Joseph A. Cotruvo Office of Drinking Water LIS. Environmental Protection Agency Washington, D.C. 20460
On Nov. 29, 1979, the U.S. Environmental Protection Agency (EPA) promulgated regulations limiting the permissible levels of trihalomethanes (THMs) in drinking water (I). The culmination of a long and complex regulatory process that began in late 1974 with the discovery of THMs in drinking water ( 2 ) . these regulations were based on a systematic assessment of the occurrence and sources of THMs in drinking water, epidemiological and toxicological investigations to evaluate the potential human health risks, and technological and economic assessments to evaluate and develop control options and determine their feasibility and cost. The authority to regulate THMs derives from the U S . Safe Drinking Water Act ( 3 ) .passed, at least in part, in response to the discovery of THMs in drinking water. The act provides for the development of National Interim Primary Drinking Water Regulations to protect health to the extent feasible by means that the EPA administrator determines are generally available (taking costs into consideration) on the date of enactment of the law, Decemher 1974. The law requires a finding that a substance “may have an adverse 268
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effect on the health of persons”; once that finding has been made, EPA has two regulatory options: developing a maximum contaminant level (MCL) or designating specific treatment requirements. The treatment-requirement option can be chosen only if monitoring for the substance is not technically and economically feasible. Why regulate? EPA’s decision to regulate T H M levels in drinking water and tospecify a maximum contaminant level was based on a number of factors: Drinking water is the major source of human exposure to THMs; THMs are the most ubiquitous synthetic organic chemicals found in drinking water in
the U S . and are generally found at the highest concentrations of any such chemicals; THMs are introduced in the course of water treatment as byproducts of the chlorination process and thus are readily controllable; feasible and low-cost means to reduce their concentrations in drinking water are generally available; monitoring is feasible; and THMs are also indicative of the presence of a host of other halogenated, oxidized, and potentially harmful by-products of the chlorination process that are concurrently formed in even larger quantities but which cannot be readily characterized chemically. In concluding that exposure to THMs in drinking water poses a human health risk, EPA followed the
This article not subject to US. Copyright Published 1981 American Chemical Society
four principles on human risk assessment set forth in the 1911 report of the U S . National Academy of Sciences (NAS), “Drinking Water and Health” ( 4 ) . which EPA feels are representative of the consensus of scientific opinion. As stated in the proposal, they are as follows: Effects in animals, properly qualified, are applicable to man. Methods do not now exist to establish a threshold for long-term effects of toxic agents. Exposure of experimental animals to toxic agents in high doses is a necessary and valid method of discovering possible carcinogenic hazards in man. Material should be assessed in terms of human risk, rather than as “safe” or “unsafe.” In the case of THMs, EPA relied primarily on studies conducted by the National Cancer Institute (NCI) that demonstrated the carcinogenicity of chloroform in both rats and mice. Metabolic data demonstrated covalent binding of chloroform metabolites to DNA and the probable intermediate formation of phosgene as a metabolite. The available epidemiological evidence has not been conclusive, but is a t least suggestive Of a risk’ NAS3 in its review of 13 preliminary epidemiological studies, affirmed EPA’s interpretation and concluded that the risks were probably small but that important confounding factors could not be distinguished in indirect ecological studies to allow a precise evaluation of the contributions from THMs. They pointed out the lack of sensitivity of epidemiological procedures due to lack of exposure data for individuals; population diversity and mobility; inability to control for all known contributing variables such as smoking, occupational exposures, diet, alcohol consumption, socioeconomic and urbanization factors; and the usual 20-40-year latency period required for most cancers. NAS also pointed out that sufficient evidence was available from animal toxicology studies to conclude that exposure to chloroform did pose a risk to human health. In setting a maximum contaminant level, EPA concluded that it would be inapproprii~te3t this time to distinguish between chloroform and other THMs. As a family of compounds, chlorinated and brominated THMs are similar in chemical composition and are formed concurrently during the chlorination of drinking water. Their relative distribution in finished water is a function of organic and halide precursor concentrations, which
can be highly variable and unpredictable. The analytical method concurrently analyzes all four chlorinated and brominated THMs, and available treatment methods would simultaneously reduce them all. EPA’s selection of an interim MCL of 0.10 mg/L was based on a balancing of public health benefits against the feasibility of achieving a reduction in T H M levels. This balance reflected the generally available technology for water treatment, which relies heavily on the proven use of chlorine to produce biologically safe water. It included the existence of monitoring methods and trained personnel, economic considerations, the limited amount of technical assistance available from EPA and the states, and, most important, the risks of compromising protection against pathogenic microorganisms which could be introduced from fundamental changes in disinfection practice. Thus, the interim MCL should not be construed as an absolutely “safe” level, but rather a feasible level
achievable with water treatment technology available since 1974. The preponderance of current scientific thought on human exposure to substances that are proven carcinogens in animals is that they be considered potential carcinonenic risks to humans. The presumpcons are that human health risk is related to the extent of exposure and that no threshold level without risk can be experimentally demonstrated for a genetically diverse population. As a statement of regulatory policy, exposure should be minimized to minimize unnecessary risks.
How to get there To achieve that end, there are three general alternatives: use of an oxidant disinfectant that does not generate THMs in water (or that generates less) treatment to reduce precursor concentrations prior to chlorination treatment to remove THMs after formation. Many possible choices exist within
Summary of THM regulations Maximum contaminant level (MCL): o,lo mglL total trihalomethanes App/icabi//ty: Community water svstems that add disinfectant to the _. . . ~. treatment process (ground and surface) Effective dates: Systems > 75 000 two years after promulgation (Nw. 29, 1981) Systems 10-75 000: four years after promulgation (Nov. 29, 1983) Systems < 75 0 0 0 state discretion Monllorlng requlremenls: Runnlng
annual average of a minimum of four samples per quarler per plant taken on same day. Systems using multiple wells drawing raw water from a single aquifer may, with state approval, be considered one treatment piant for determining the required number of samples. Effective dates: Systems > 75000 one year after promulgation (Nov. 29, 1980) Systems 10-75 000 Wee y e a r s after promulgation (Nov 29. 1982) Systems 75 0 0 0 state discretion sample locations: 25% at extreme of distribution system, 75% at locations representative of population distribution. frequency; For groundwater systems, reduced monitoring may be a p
