Page 11: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Radiological Parameters
The radionuclides most commonly found in Canadian drinking water and with potential long-term health consequences are described below. The incorporation of new information has caused the MACs for some radionuclides to increase, whereas others have decreased or remained the same.
- Lead-210
- Lead-210 is a bone-seeking radionuclide that often occurs together with its alpha-emitting decay product, 210Po. The MAC of 210Pb is 0.2 Bq/L, up from 0.1 Bq/L, its previous guideline. This change is due mainly to a reduction from 20% to 10% in the reported uptake factor for lead from the gastrointestinal tract.
The radiological MAC for 210Pb should not be confused with the chemical MAC for stable lead of 0.01 mg/L. A 210Pb concentration at the radiological MAC of 0.2 Bq/L would correspond to a total lead concentration of only 7 × 10-8 µg/L. The presence or absence of stable lead in drinking water has no bearing on the presence or absence of 210Pb.
- Radium isotopes
- Radium, like strontium, is a bone-seeking element of the alkaline earth family. About 20% of ingested radium is absorbed by the gastrointestinal tract. Radium-226 (half-life = 1600 years) and 224Ra (half-life = 3.66 days) undergo decay by alpha particle emission. Radium-228 (half-life = 5.75 years) decays through beta emission to 228Th, which is an alpha emitter. In the current guidelines, the MAC for 224Ra remains unchanged at 2 Bq/L; that for 226Ra is reduced slightly from 0.6 to 0.5 Bq/L, and that for 228Ra is reduced by a larger amount, from 0.5 to 0.2 Bq/L. These changes are due to more refined models for the internal metabolism of radium that have become available.
- Uranium isotopes
- Natural uranium consists of three isotopes - 234, 235, and 238 - with half-lives ranging from hundreds of millions to billions of years. As a result of these long half-lives, the specific activities of the uranium isotopes are very low. The MAC for uranium in drinking water is based on its chemical toxicity rather than its radiological properties. However, radiological MACs of about 3 Bq/L for each uranium isotope have been included in Appendix A so that they can be included in the radiological summation formula. At equilibrium, a total uranium concentration at its chemical MAC of 20 µg/L would correspond to 0.25 Bq/L of 238U, 0.01 Bq/L of 235U, and 0.25 Bq/L of 234U. Note, however, that for uranium dissolved in groundwater, the 238 and 234 isotopes may be in disequilibrium by up to a factor of 2.
Experimental studies in adult humans consistently show that absorption of uranium by the oral route is less than 5% (Health Canada, 1999), with the average being 1-2% (ATSDR, 1999). Once in the blood, approximately 67% of uranium is filtered in the kidneys and leaves the body in urine within 24 hours; the remainder distributes to tissues. This may explain why no radiation-related cancers from uranium exposure have been identified in humans following exposure to naturally occurring levels of uranium, even following exposure to highly enriched uranium (ATSDR, 1999).
For information on the chemical aspects of uranium toxicity, the reader should refer to the Guideline Technical Document on uranium (Health Canada, 1999).
- Tritium
- Tritium, with a half-life of 12.3 years, exists in the environment mainly as tritiated water (HTO), but it may also occur in vegetation as organically bound tritium (OBT). Ingested HTO is quickly absorbed into the bloodstream and remains in the body for 2-18 days. Tritium gives off low-energy (18.6 keV) beta particles, with no associated gamma rays or X-rays. Consequently, the dose coefficient for tritium is quite low, and the resulting MAC is 7000 Bq/L, unchanged from its previous value.
- Cesium-137
- Cesium-137, an alkali metal with properties similar to those of potassium, is one of the more important fission products because of its relatively high yield and its ability to bioconcentrate in some food chains. With a half-life of 30.17 years, it has been released into the environment from nuclear weapons tests and from reactor emissions (e.g., the Chernobyl accident in 1986). Fixation by sediments in aquatic environments reduces its concentration in water bodies. Ingested 137Cs is readily absorbed into soft tissues, but is eliminated relatively quickly. Consequently, the MAC is set at 10 Bq/L, which is somewhat less restrictive than for 90Sr and is unchanged from its previous value.
- Strontium-90
Strontium-90 has been monitored extensively in the environment since the beginning of nuclear weapons testing. It decays by pure beta emission with a half-life of 29 years. As an alkaline earth element, strontium is similar to calcium and follows calcium through the food chain to the human body, where it is retained largely in teeth and bone, with half-times of 3-7 years. Because of this long residence time in bone and because of the relatively high energy (846 keV) of its beta particles, the MAC of 90Sr has been set at 5 Bq/L, unchanged from its previous value.
Radioactive strontium (90Sr) should not be confused with stable strontium. The two species of strontium have quite different origins, and their concentrations in drinking water are not correlated.
- Radioiodine
- Radioactive isotopes of iodine are produced by nuclear fission and activation processes. They have received extensive study in view of their mobility and their selective irradiation of the thyroid gland when taken into the body. A number of iodine isotopes are routinely used in nuclear medicine procedures and thus have the potential to be released into water bodies through sewage effluent. Fortunately, all of the iodine isotopes except 129I are short-lived. The different iodine isotopes have MACs ranging from 1 to 10 Bq/L, which remain unchanged from the previous values.
- Cesium-137
- Cesium-137, an alkali metal with properties similar to those of potassium, is one of the more important fission products because of its relatively high yield and its ability to bioconcentrate in some food chains. With a half-life of 30.17 years, it has been released into the environment from nuclear weapons tests and from reactor emissions (e.g., the Chernobyl accident in 1986). Fixation by sediments in aquatic environments reduces its concentration in water bodies. Ingested 137Cs is readily absorbed into soft tissues, but is eliminated relatively quickly. Consequently, the MAC is set at 10 Bq/L, which is somewhat less restrictive than for 90Sr and is unchanged from its previous value.
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