Guidelines for Canadian Drinking Water Quality - Summary Table

Guidelines for radiological parameters

In setting dose guidelines for radionuclides in drinking water, it is recognized that water consumption contributes only a portion of the total radiation dose and that some radionuclides present are natural in origin and therefore cannot be excluded. Consequently, maximum acceptable concentrations for radionuclides in drinking water have been derived based on a committed effective dose of 0.1 mSv** from one year's consumption of drinking water. This dose represents less than 5% of the average annual dose attributable to natural background radiation.

To facilitate the monitoring of radionuclides in drinking water, the reference level of dose is expressed as an activity concentration, which can be derived for each radionuclide from published radiological data. The National Radiological Protection Board has calculated dose conversion factors (DCFs) for radionuclides based on metabolic and dosimetric models for adults and children. Each DCF provides an estimate of the 50-year committed effective dose resulting from a single intake of 1 Bq*** of a given radionuclide.

The MACs of radionuclides in public water supplies are derived from adult DCFs, assuming a daily water intake of 2 L, or 730 L/year, and a maximum committed effective dose of 0.1 mSv, or 10% of the International Commission on Radiological Protection limit on public exposure:

When two or more radionuclides are found in drinking water, the following relationship should be satisfied:

where C_i and MAC_i are the observed and maximum acceptable concentrations, respectively, for each contributing radionuclide.

MACs for radionuclides that should be monitored in water samples are listed in Table 7. If a sample is analysed by gamma-spectroscopy, additional screening for radionuclides that may be present under certain conditions can be performed. MACs for a number of additional radionuclides, both natural and artificial, can be found in the sixth edition of the guidelines booklet.

Water samples may be initially screened for radioactivity using techniques for gross alpha and gross beta activity determinations. Compliance with the guidelines may be inferred if the measurements for gross alpha and gross beta activity are less than 0.1 Bq/L and 1 Bq/L, respectively, as these are lower than the strictest MACs. Sampling and analyses should be carried out often enough to accurately characterize the annual exposure. If the source of the activity is known, or expected, to be changing rapidly with time, then the sampling frequency should reflect this factor. If there is no reason to suppose that the source varies with time, then the sampling may be done annually. If measured concentrations are consistent and well below the reference levels, this would be an argument for reducing the sampling frequency. On the other hand, the sampling frequency should be maintained, or even increased, if concentrations are approaching the reference levels. In such a case, the specific radionuclides should be identified and individual activity concentrations measured.

Table 7. Primary list of radionuclides

Radionuclide		Half-life t½	DCF (Sv/Bq)	MAC (Bq/L)
Natural radionuclides
Lead-210	210Pb	22.3 years	1.3 × 10-6	0.1
Radium-224	224Ra	3.66 days	8.0 × 10-8	2
Radium-226	226Ra	1600 years	2.2 × 10-7	0.6
Radium-228	228Ra	5.76 years	2.7 × 10-7	0.5
Thorium-228	228Th	1.91 years	6.7 × 10-8	2
Thorium-230	230Th	7.54 × 104 years	3.5 × 10-7	0.4
Thorium-232	232Th	1.40 × 1010 years	1.8 × 10-6	0.1
Thorium-234	234Th	24.1 days	5.7 × 10-9	20
Uranium-234a	234U	2.45 × 105 years	3.9 × 10-8	4
Uranium-235a	235U	7.04 × 108 years	3.8 × 10-8	4
Uranium-238a	238U	4.47 × 109 years	3.6 × 10-8	4
Artificial radionuclides
Cesium-134	134Cs	2.07 years	1.9 × 10-8	7
Cesium-137	137Cs	30.2 years	1.3 × 10-8	10
Iodine-125	125I	59.9 days	1.5 × 10-8	10
Iodine-131	131I	8.04 days	2.2 × 10-8	6
Molybdenum-99	99Mo	65.9 hours	1.9 × 10-9	70
Strontium-90	90Sr	29 years	2.8 × 10-8	5
Tritiumb	3H	12.3 years	1.8 × 10-11	7000

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