Water: Radionuclides Rule
Basic Information about the Radionuclides Rule
What are radionuclides?
A nuclide is a general term applicable to all atomic forms of an element. Nuclides are characterized by the number of protons and neutrons in the nucleus, as well as by the amount of energy contained within the atom. A radionuclide is an unstable form of a nuclide. They may occur naturally, but can also be artificially produced. The Office of Air and Radiation has additional information about radioactivity and specific types of radionuclides.
What radionuclides are regulated in drinking water and what are their health effects?
The regulated radioactive drinking water contaminants are:
|Contaminant||MCL (year promulgated)||Source||Health Effect|
|Combined radium-226/-228||5 pCi/L (1976)||Naturally occurs in some drinking water sources.||Some people who drink water containing radium –226 or -228 in excess of the MCL over many years may have an increased risk of getting cancer.|
|(Adjusted) Gross Alpha||15 pCi/L (not including radon or uranium) (1976)||Naturally occurs in some drinking water sources.||Some people who drink water containing alpha emitters in excess of the MCL over many years may have an increased risk of getting cancer.|
|Beta Particle and Photon Radioactivity||4 mrem/year (look-up table) (1976)||May occur due to contamination from facilities using or producing radioactive materials.||Some people who drink water containing beta and photon emitters in excess of the MCL over many years may have an increased risk of getting cancer.|
|Uranium||30 µg/L (2000)||Naturally occurs in some drinking water sources.||Exposure to uranium in drinking water may result in toxic effects to the kidney. Some people who drink water containing alpha emitters in excess of the MCL over many years may have an increased risk of getting cancer.|
How do radionuclides get into drinking water?
Most drinking water sources have very low levels of radioactive contaminants ("radionuclides"), which are not considered to be a public health concern. Of the small percentage of drinking water systems with radioactive contaminant levels high enough to be of concern, most of the radioactivity is naturally occurring. Certain rock types have naturally occurring trace amounts of "mildly radioactive" elements (radioactive elements with very long half-lives) that serve as the "parent" of other radioactive contaminants ("daughter products"). These radioactive contaminants, depending on their chemical properties, may accumulate in drinking water sources at levels of concern. The "parent radionuclide" often behaves very differently from the "daughter radionuclide" in the environment. Because of this, parent and daughter radionuclides may have very different drinking water occurrence patterns. For example, ground water with high radium levels tend to have low uranium levels and vice versa, even though uranium-238 is the parent of radium-226.
Are radionuclides more common in some parts of the country?
Most parts of the United States have very low "average radionuclide occurrence" in drinking water sources. However, some parts of the country have, on average, elevated levels of particular radionuclides compared to the national average. For example, some parts of the mid-West have significantly higher average combined radium-226/-228 levels. On the other hand, some Western states have elevated average uranium levels compared to the national average. However, in general, average uranium levels are very low compared to the MCL throughout the United States. While there are other radionuclides that have been known to occur in a small number of drinking water supplies, their occurrence is thought to be rare compared to radium-226, radium-228, and uranium.
Can drinking water be contaminated by man-made radionuclides?
A very small percentage of drinking water systems are located in areas that have potential sources of man-made radioactive contamination from facilities that use, manufacture, or dispose of radioactive substances. Drinking water contamination may occur through accidental releases of radioactivity or through improper disposal practices. Water systems that are vulnerable to this type of contamination are required to perform extensive monitoring for radioactive contamination to ensure that their drinking water is safe. These radionuclides are regulated under the "beta particle and photon radioactivity" standard.
Does EPA regulate tritium in drinking water?
Tritium is a beta particle emitter which forms in the upper atmosphere through interactions between cosmic rays (nuclear particles coming from outer space) and the gases comprising the atmosphere. Tritium can be deposited from the atmosphere onto surface waters via rain or snow and can accumulate in ground water via seepage. Tritium is also formed from human activities, including production of electricity, nuclear weapons, nuclear medicines used in therapy and diagnosis, various commercial products, as well as in various academic and government research activities. Natural tritium tends not to occur at levels of concern, but contamination from human activities can result in relatively high levels.
What systems must meet the requirements of the 2000 Radionuclides Rule?
The rule applies only to community water systems, which are water systems with at least 15 service connections or that serve 25 or more persons year-round. EPA will further consider a future proposal to regulate radionuclides levels in drinking water served by non-transient non-community water systems. These are water systems that serve at least 25 of the same people more than six months per year, such as schools, churches, nursing homes, and factories that supply their own water.
How did the 2000 Radionuclides Rule address Uranium?
Exposure to uranium in drinking water may cause toxic effects to the kidney. In 1991, EPA proposed an MCL of 20 µg/L, which was determined to be as close as feasible to the maximum contaminant level goal (MCLG) of X µg/L. Based on human kidney toxicity data collected since that time and on its estimate of the costs and benefits of regulating uranium in drinking water, EPA determined that the benefits of a uranium MCL of 20 µg/L did not justify the costs. Instead, EPA determined that 30 µg/L is the appropriate MCL, because it maximizes the net benefits (benefits minus costs), while being protective of kidney toxicity and carcinogenicity with an adequate margin of safety.
How did the 2000 Radionuclides Rule change monitoring requirements?
Under the old rule, community water systems only tested water from a “representative point” in the distribution system. The old monitoring requirements did not protect every customer, since water quality may vary significantly within the distribution system. To ensure that water served to all customers meets the MCLs for radionuclides in drinking water, the revised rule required that future monitoring be performed such that all water entering the distribution system is tested. The monitoring frequency requirements were also changed to be more consistent with the “Standardized Monitoring Framework” that other drinking water standards use.This improves consistency in monitoring requirements and provides monitoring relief for those water systems that have very low contaminant levels.
In addition, the new rule corrected a monitoring deficiency in the 1976 framework for monitoring for combined radium-226 and -228. Under the old rule, it was assumed that radium-226 and gross alpha levels could be used to screen for radium-228. Since then, EPA has collected substantial evidence that this assumption is false. The correction involves separate monitoring requirements for radium-228, further ensuring that drinking water system customers will be protected from harmful radioactive contaminant levels.