For Public Comment -- Deadline April 2, 2004
Drinking water standards are regulations set USEPA to control the level of contaminants in the nation's drinking water. These standards are part of the Safe Drinking Water Act's "multiple barrier" approach to drinking water protection, which includes assessing and protecting drinking water sources; making sure water is treated by qualified operators; ensuring the integrity of distribution systems; and making information available to the public on the quality of their drinking water. With the involvement of USEPA, states, tribes, drinking water utilities, communities and citizens, these multiple barriers ensure that tap water in the United States and territories is safe to drink. In most cases, USEPA delegates responsibility for implementing drinking water standards to states and tribes.
There are two categories of federal drinking water standards:
A National Primary Drinking Water Regulation is a legally-enforceable standard that applies to public water systems. Primary standards protect drinking water quality by limiting the levels of specific, individual contaminants that can adversely affect public health and are known or anticipated to occur in water.
A National Secondary Drinking Water Regulation is a non-enforceable guideline regarding contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. USEPA recommends secondary standards to water systems but does not require systems to comply. However, states may choose to adopt them as enforceable standards.
In developing drinking water standards, EPA must first make determinations about which contaminants to regulate. These determinations are based on health risks and the likelihood that the contaminant occurs in public water systems at levels that may cause harmful human health effects. The National Drinking Water Contaminant Candidate List, published on March 2, 1998, lists contaminants that are not regulated under the Safe Drinking Water Act, may have adverse health effects, and are known or anticipated to occur in pubic water systems. The regulation describing the list is discussed in the section on how the federal government addresses unregulated contaminants. Section VI.1..
Once it is determined that contaminant should be regulated, USEPA sets a Maximum Contaminant Level Goal (MCLG), the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, and which allows an adequate margin of safety. MCLGs are non-enforceable public health goals. Adequate studies of the health effects of the specific chemical by exposure pathways relevant to drinking water are necessary in order to develop an MCLG. When determining an MCLG, USEPA considers the risk to sensitive subpopulations (infants, children, the elderly, and those with compromised immune systems) of experiencing a variety of adverse health effects. As MCLGs are based only on health effects and do not consider the limits of detection or treatment technology, they may be set at levels that are not measurable or quantifiable by currently available analytical methods.
- Non-Carcinogens (not including microbial contaminants): For chemicals that are of concern because they can cause adverse non-cancer health effects, the MCLG is based on the reference dose. A reference dose (RfD) is an estimate of the amount of a chemical that a person can he exposed to on a daily basis that is not anticipated to cause adverse health effects over a person's lifetime. In RfD calculations, sensitive subgroups are considered, and the uncertainty in the RfD may span an order of magnitude.
- The RfD is combined with exposure assumptions, including typical adult body weight (70 kg) and daily water consumption (2 liters) to provide a Drinking Water Equivalent Level (DWEL).
- The DWEL is multiplied by the percentage of the total daily exposure
assumed to be contributed by drinking water (often 20 percent), known as the Relative Source Contribution factor, to determine the MCLG.
- Carcinogens: If there is evidence that a chemical may cause cancer in humans, and that this may occur through a non-threshold mechanism, the MCLG is set at zero. If carcinogenicity is judged to occur through a mechanism which has a threshold, the MCLG is set at a level above zero at which there is considered to be no risk.
- Microbial Contaminants: For microbial contaminants that may present public health risk, the MCLG is set at zero because ingesting one protozoa, virus, or bacterium may cause adverse health effects. USEPA is conducting studies to determine whether there is a safe level above zero for some microbial contaminants. So far, however, this has not been established.
Once the MCLG is determined, USEPA sets an enforceable standard. In most cases, the standard is a maximum contaminant level, the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. The maximum contaminant level is set as close to the MCLG as feasible, taking into consideration analytical detection limits, best available treatment technology, and cost. When there is no reliable analytical method that is economically and technically feasible to measure a contaminant, a treatment technique is set rather than a maximum contaminant level. A treatment technique is an enforceable procedure or level of technological performance that public water systems must follow to ensure indirect control of a contaminant. Examples of treatment technique rules are the Surface Water Treatment Rule (disinfection and filtration) and the Lead and Copper Rule (optimized corrosion control).
After determining a maximum contaminant level or treatment technique. USEPA must complete an economic analysis to determine whether the benefits of that standard justify the costs. If not, USEPA may adjust the standard to a level that "maximizes health risk reduction benefits at a cost that is justified by the benefits." USEPA may not adjust the standard if the benefits justify the costs to large systems and to small systems that are unlikely to receive variances. (Stales are authorized to grant variances from standards for systems serving up to 3,300 people under certain conditions. However, exemptions cannot be granted for microbial contaminants.)
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