Page 7: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Radiological Parameters
Part II: Science and Technical Considerations
The approach used to assess radionuclides in drinking water is based on an annual dose and annual risk of cancer, and is consistent with the approach adopted internationally. This is an appropriate approach, since exceedances are expected to be investigated and addressed immediately, thereby eliminating lifetime exposure to exceedances. Drinking water guidelines for radionuclides are established based on a reference dose level of 0.1 mSv/year, which is one-tenth of the public (i.e., an individual of the general public not exposed to radiation in the workplace) dose limit recommended by the ICRP and embodied in Canadian regulations by the CNSC. This is consistent with the reference level of 0.1 mSv/year set by WHO (2008) in deriving its guidelines for drinking water quality. In setting its reference level, WHO conservatively assumed that only 10% of the total ingestion dose arises from drinking water, with the remaining 90% originating from food. Furthermore, the drinking water limit is meant to include all radionuclides in drinking water, whether naturally present or introduced by human activities. A level of 0.1 mSv represents less than 5% of the annual dose from natural background radiation.
It can be estimated that the lifetime risk of fatal cancer or other health detriment from 0.1 mSv is less than 1 in 100 000 (< 10-5). At this low level of risk, it can be concluded that no further actions are necessary to reduce the amount of radioactivity in the drinking water source.
Figure 2 - The MAC for a given radionuclide in drinking water is derived using the following formula:
- 0.1 mSv/year is the reference dose level from 1 year's consumption of drinking water.
- 730 L/year is the yearly drinking water consumption for an adult, which corresponds to a daily consumption of 2 L/day. This value is consistent with the approach used by WHO and the U.S. EPA, but is somewhat higher than the average intake of 1.5 L/day used by Health Canada to derive guidelines for the chemical parameters.
- DC is the dose coefficient, based on the 50-year committed dose from ICRP (1996). This provides an estimate of the 50-year committed effective dose for adults resulting from a single intake of 1 Bq of a given radionuclide. Specific information is provided in Appendix D for radionuclides for which a MAC has been established.
The adult dose coefficient is assumed to be adequately protective of both children and adults for the following reasons:
- the MACs are based on a dose of only 0.1 mSv/year, which is a small fraction of the annual dose received from the natural background; and
- the higher age-dependent dose coefficients calculated for children do not lead to significantly higher doses because of the lower mean volume of drinking water consumed by infants and children and their higher metabolic rates.
Although MACs have been established for the radionuclides most commonly detected in Canadian drinking water sources, guidance is also provided in Appendix A for an additional 78 radiological parameters for which ICRP dose coefficients have been established but which are not expected to be found in Canadian drinking water sources. It is not recommended that monitoring be carried out for these additional radionuclides. Concentrations have been calculated, for information purposes only, using their dose coefficients and the same formula and assumptions as the MACs. They represent the theoretical level at which potential health effects could occur following long term exposure to an individual radionuclide from drinking water.
The radiological effects of two or more radionuclides in the same drinking water source are assumed to be additive. Thus, the following summation formula should be satisfied in order to demonstrate compliance with the guidelines:
Figure 3 - Summation Formula
where Ci and MACi are the observed and maximum acceptable concentrations, respectively, for each contributing radionuclide. Only those radionuclides that are detected with at least 95% confidence should be included in the summation. Detection limits of undetected radionuclides should not be substituted for the concentrations Ci. Otherwise, a situation could arise where a sample fails the summation criterion even though no radionuclides are present.
A corollary of the above discussion is that the lower limit of detection (LLD) for a given radionuclide should always be less than its MAC. The specification of an LLD for a given procedure (Currie, 1968) means that if one cannot detect the radionuclide, one can be 95% confident that it is not present at a concentration greater than its LLD. If there were only one radionuclide of concern, then in theory it would be acceptable to have its LLD = MAC. A problem arises if more than one radionuclide is present, and this often occurs for natural radionuclides in drinking water. Here it is assumed that no more than five radionuclides are likely to be present at concentrations approaching their respective MACs. Therefore, any testing procedure should aim to achieve an LLD not greater than 20% of the MAC of any radionuclide likely to be present. In the experience of most laboratories involved in the radiological analysis of drinking water samples, these detection limits are readily achievable.
Chemical limits should not be included in the radiological summation formula. This applies particularly to isotopes of uranium,Footnote 2 where both chemical and radiological limits may apply. MACs for chemical carcinogens are derived using different assumptions, and it is not appropriate to combine them with radiological MACs in the summation formula.
MACs in drinking water have been established for three natural (210Pb, 226Ra, and total uranium in chemical form) and four artificial (tritium, 137Cs, 90Sr, and 131I) 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 for 1 year's consumption of drinking water, assuming a consumption of 2 L/day at the MAC.
MACs for radionuclides do not take into consideration treatment or analytical limitations. Treatment of water supplies for radionuclides should be governed by the principle of keeping exposures as low as reasonably achievable, social and economic considerations being taken into account. MACs apply to routine operational conditions of existing or new water supplies, but they do not apply in the event of contamination during an emergency involving a large release of radionuclides into the environment.
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, then 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 indoor air.
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