Page 2: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Radiological Parameters
Maximum acceptable concentrations (MACs) have been established for the most commonly detected natural and artificial radionuclides in Canadian drinking water sources and are listed in the table below. A guideline for radon in drinking water is not deemed necessary and has not been established.
The MACs are based on exposure solely to a specific radionuclide. The radiological effects of two or more radionuclides in the same drinking water source are considered to be additive. Thus, the sum of the ratios of the observed concentration to the MAC for each contributing radionuclide should not exceed 1.
|Natural radionuclides||MAC||Artificial radionuclides||MAC|
|Total uraniumTable a Footnote 1||0.02 mg/L||Tritium (3H)||7000 Bq/L|
|Lead-210 (210Pb)||0.2 Bq/LTable a Footnote 2||Strontium-90 (90Sr)||5 Bq/L|
|Radium-226 (226Ra)||0.5 Bq/L||Iodine-131 (131I)||6 Bq/L|
|Cesium-137 (137Cs)||10 Bq/L|
Table 1 Footnotes
Radionuclides are naturally present in the environment; they may also enter the environment as a result of human activities. Natural sources of radiation are responsible for the large majority of radiation exposure (greater than 98%), excluding medical exposure. Additional exposure can result from human activities associated with radioactive materials. This document focuses on routine operational conditions of existing and new water supplies and does not apply in the event of contamination during an emergency involving a large release of radionuclides into the environment.
This Guideline Technical Document assesses the human health risks of radionuclides in drinking water, taking into account new studies and approaches, including dosimetric information released by the International Commission on Radiological Protection (ICRP) in 1996 (ICRP, 1996). Maximum acceptable concentrations in drinking water have been established for three natural (210Pb, 226Ra, and total uranium in chemical form) and four artificial (tritium, 90Sr, 131I, and 137Cs) radionuclides. These represent the natural and artificial radionuclides that are most commonly detected in Canadian water supplies. The MACs are derived using internationally accepted equations and principles and are based solely on health considerations. They are calculated using a reference dose level of 0.1 mSv for 1 year's consumption of drinking water, assuming a consumption of 2 L/day at the MAC.
A guideline for radon is not considered necessary. The health risk from ingesting radon-contaminated drinking water is considered negligible, because most of the radon escapes at the faucet or water outlet, leaving only minimal amounts in the water itself. However, it should be noted that radon levels in drinking water, if sufficiently elevated, can significantly affect airborne radon concentrations.
Various mechanisms are responsible for radiation damage. Exposure to radiation from all sources can result in changes to sensitive biological structures, either directly through the transfer of energy to the atoms within the tissue or indirectly by the formation of free radicals. Since the most sensitive structure in the cell is the deoxyribonucleic acid (DNA) molecule, exposure to radiation may damage the DNA, causing the cells to die or to fail to reproduce. This can result in the loss of tissue or organ function or the development of cancer. The likelihood of these events occurring increases with the amount of radiation received. Types of cancer most frequently associated with radiation exposure include leukaemia and tumours of the lung, breast, thyroid, bone, digestive organs, and skin. These cancers can develop between five years and several decades after exposure. This latency period depends on several factors, including individual sensitivities to radiation exposure, the type of radionuclides to which an individual has been exposed, as well as the level of the dose and the dose rate. The MACs are based on a reference dose of 0.1 mSv/year, which represents a lifetime excess risk (i.e., above background levels) of both fatal and non-fatal cancers of 7.3 × 10-6 if an individual is exposed to the MAC for one year.
The occurrence of natural radionuclides in drinking water is associated most commonly with groundwater. Natural radionuclides are present at low concentrations in all rocks and soils. In the cases where groundwater has been in contact with rock over hundreds or thousands of years, significant concentrations may build up in the water. These concentrations are highly variable and are determined by the composition of the underlying bedrock as well as the physical and chemical conditions prevailing in the aquifer. Although rare, natural radionuclides have also been known to occur in shallow wells.
Increased levels of radionuclides in surface waters may be linked to industrial processes, particularly uranium mining and milling operations, fallout from nuclear weapons testing (mostly before 1963), emissions from nuclear reactors, as well as cosmogenic and other artificial radionuclides. Surface waters in close proximity to point sources may contain higher levels of radionuclides; however, levels in groundwaters are less likely to be influenced by point sources.
Although the establishment of drinking water guidelines for a contaminant usually takes into consideration the ability to measure the contaminant and remove it from drinking water, the MACs for radionuclides are based solely on health effects. However, most radionuclides can be reliably measured to levels below the established MACs.
Most radionuclides, with the exception of tritium, can be effectively treated in municipal-scale treatment facilities, with removal efficiencies ranging from 70 to 99%, depending on the type of treatment. However, for artificial radionuclides such as tritium, the strategy should be to prevent contamination of the source water.
At the residential scale, treatment devices are available for the removal of radionuclides with the exception of tritium, with efficiencies generally similar to those of municipal-scale treatment. However, they cannot necessarily be certified to recognized standards, as standards are not available for all radionuclides. In addition, appropriate authorities should be consulted for the disposal of liquid and solid waste from the treatment of drinking water containing radionuclides.
Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.
MACs have been established for three natural (210Pb, 226Ra, and total uranium in its chemical form) and four artificial (tritium, 90Sr, 131I, and 137Cs) radionuclides. These represent the most commonly detected radionuclides in Canadian drinking water supplies. Every effort should be made to maintain radionuclide levels in drinking water as low as reasonably achievable. The levels of radionuclides normally encountered in drinking water are far below the threshold for acute effects of radiation. In virtually every case, the MACs are based on chronic or cumulative exposure over a period of one year.
The sampling and analyses for individual radionuclides should be carried out often enough to accurately characterize the annual exposure. If the source of the radioactivity is known or expected to be changing rapidly with time, then the sampling frequency should reflect this factor. If there is no reason to expect concentrations to vary with time, then sampling may be carried out seasonally, semi-annually or annually. If measured concentrations are consistent and well below the MACs, this would be an argument for reducing the sampling frequency. In contrast, the sampling frequency should be maintained, or even increased, if concentrations are approaching individual MACs or if the sum of ratios of the observed concentration to the MAC for each contributing radionuclide approaches 1.
Jurisdictions with facilities where environmental releases of radionuclides are likely to affect drinking water sources may wish to establish monitoring programs to ensure that drinking water treatment plant operators are made aware of these releases so that appropriate action can be taken. If a situation exists where ongoing exposure to radiological parameters is likely, a jurisdiction may choose to apply additional measures based on the toxicity, the expected level in the source water, and the frequency of occurrence, in order to mitigate risk.
Water samples may be initially analysed for the presence of radioactivity using techniques for gross alpha and gross beta determinations rather than measurements of individual radionuclides. Alpha emissions are generally associated with naturally occurring radionuclides, whereas beta emissions are generally associated with artificial radionuclides. Although facilitating routine examination of large numbers of samples, these procedures do not allow for confirmation of the identities of the contributing radionuclides. These measurements are generally suitable either as a preliminary screening procedure to determine if further radioisotope-specific analysis is necessary or, if radionuclide analyses have been carried out previously, for detecting changes in the radiological characteristics of the drinking water source. Gross alpha and gross beta screening is also useful in determining if the activities from specific radioisotopes account for all of the activity found in the screening test.
Water samples may be initially screened for radioactivity using techniques for gross alpha and gross beta activity determinations, subject to the limitations of the method. Compliance with the guidelines may be inferred if the measurements are less than 0.5 Bq/L for gross alpha activity and less than 1 Bq/L for gross beta activity. These screening levels are consistent with those established by the World Health Organization (WHO, 2008). Specifically, the screening level for gross alpha activity is based on the strictest MAC (226Ra) for alpha activity, whereas the screening level for gross beta activity will be protective of all beta-emitting species that can be expected to be found in drinking water, including iodine species and 90Sr.
If either screening level is exceeded, then the specific radionuclides should be identified and individual activity concentrations measured. When the sum of ratios of the observed concentration to the MAC for each contributing radionuclide is below 1, no further action is required, and the water is acceptable for human consumption from a radiological perspective. Where the sum exceeds unity for a single sample, the reference dose level would be exceeded only if exposure to the same measured concentration were continued for a full year. Hence, an exceedance from a single sample does not in itself imply that the water is unsuitable for consumption and should be regarded only as a level at which further investigation, including additional sampling, is needed.
Radionuclides emitting low-energy beta activity, such as tritium, and some gaseous or volatile radionuclides, such as iodine, will not be detected by standard gross activity measurements. If their presence is suspected, radionuclide-specific sampling and measurement techniques should be used.
WHO has established screening levels for drinking water at 0.5 Bq/L for gross alpha activity and 1 Bq/L for gross beta activity (WHO, 2008). The gross alpha screening level reflects values near WHO's radionuclide-specific guidance reference dose level. The gross beta activity screening level, in the worst case, would lead to a dose close to the guidance reference dose level of 0.1 mSv/year. The rationale for these screening levels is currently under review by WHO and the International Atomic Energy Agency.
The U.S. Environmental Protection Agency (EPA) has established a gross alpha maximum contaminant level of 15 pCi/L (0.56 Bq/L), which includes 226Ra but excludes radon and uranium. This screening level accounts for the risk from 226Ra at 5 pCi/L (0.19 Bq/L) (the 226Ra maximum contaminant level) plus the risk from 210Po, the next most radiotoxic alpha emitter in the uranium decay chain (U.S. EPA, 2000a). The U.S. EPA's gross beta screening level is set at a fixed dose of 4 mrem/year (0.04 mSv/year) and involves two limits. For normal water supplies, a beta screening level of 50 pCi/L (1.85 Bq/L) has been set, above which speciation would be required to determine which beta emitters are present; for water supplies known to contain radionuclides, the beta screening level has been set at 15 pCi/L (0.56 Bq/L).
The screening level set by Australia for either gross alpha or gross beta activity is 0.5 Bq/L. The gross beta measurement includes a contribution from 40K, which is a natural beta emitter. Water meeting these screening guidelines is expected to result, at worst, in an annual dose of approximately one-third of the minimum dose at which intervention should be considered. If the screening level for gross alpha or gross beta activity is exceeded, specific radionuclides should be identified and their activity concentrations determined.
The health risk from ingesting radon-contaminated drinking water is considered negligible, because most of the radon escapes at the faucet or water outlet, leaving only minimal amounts in the water itself. However, it should be noted that radon levels in drinking water, if sufficiently elevated, can significantly affect airborne radon concentrations. Where indoor air radon concentrations exceed 200 Bq/m3 as an annual average concentration in the normal living area (Government of Canada, 2007), the source of the radon should be investigated, including through the monitoring of concentrations in drinking water. If radon concentrations in drinking water exceed 2000 Bq/L, it is recommended that actions be taken to reduce the release of radon from the drinking water into ambient air.
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