Answer to Question #729 Submitted to "Ask the Experts"

Q

What are the implications of naturally occurring uranium-contaminated well water of upwards of 1,600ug/L? Also, what correlation if any can be drawn between those levels and radon and or radium in the water?

A

In 1982, eight household wells were sampled around the Lac du Bonnet batholith as part of baseline studies carried out by Atomic Energy of Canada (AECL) prior to construction of the Underground Research Laboratory in Eastern Manitoba. High levels of radioactivity (alpha and beta) were found. These results prompted the Government of the Province of Manitoba to conduct a survey of 300 domestic wells throughout Southeastern Manitoba. More than half of these wells contained ²³⁸U/²³⁴U and ²²⁶Ra well in excess of the Canadian Drinking Water Guidelines set at that time (100 microgram/L U and 1 Bq/L ²²⁶Ra). In the Manitoba Government survey, uranium values up to 2,020 microgram/L U were found. The wells sampled in the AECL and the Manitoba Government surveys are all in or adjacent to the granitic rock along the southern edge of the Canadian Shield. A summary of the study results is given by Gascoyne in Applied Geochemistry, Vol. 4, pp 577-591 (1989). The Manitoba experience is not dissimilar to other jurisdictions, particularly for granitic rock. In the listed publication, samplings from other jurisdictions are also summarized, including a result from Helsinki Finland where the uranium (including the uranium in the undissolved solids) topped 14,000 micrograms/L. What is interesting about the AECL and Manitoba studies is that ²²⁶Ra is also quoted. You can expect that ²²²Rn and progeny in bottled and sealed water will be in equilibrium with ²²⁶Ra. In fact, radium concentrations in water were measured by AECL by extraction and quantitative analysis of radon gas. The groundwater leach rates and adsorption coefficients for uranium and radium are different since they are chemically different. Since the two chemicals are leached and transported at different rates, there is no reason to expect radium to be in equilibrium with its ultimate parent ²³⁸U. In fact the AECL data indicates (once concentrations by mass are converted to activity concentrations) that radium ranges from equilibrium concentrations to 1/10 the equilibrium values. The highest radium values measured in the Lac du Bonnet domestic wells was 3.52 Bq/L almost in perfect equilibrium with the ²³⁸U/²³⁴U. The health implications for water consumption from wells with these concentrations of radionuclides may be judged from estimates of cancer risk based on the Annual Limits on Intake (ALIs) from ICRP-61. 1600 ug/L of uranium corresponds to approximately 20 Bq/L of uranium. The ALI for oral ingestion of uranium is 8×10⁵ Bq. The ALI for ²²⁶Ra is 9×10⁴ Bq. The drinking water guidelines were based on a 70 kg adult consuming 1.5 L/day or 547 L/y. If all were consumed from the same household well with a concentration of 1,600 micrograms/L and radium in equilibrium, the intake would amount to about 0.15 ALI weighted for both uranium and radium and also taking into account the ²²⁴U present. This amounts to a dose commitment of approximately 3 mSv/y over each successive 50 years. Such an exposed individual would receive the full 3 mSv in the 50th year. For exposures of members of the public, it is normally assumed that the exposure occurs over 70 years, which would further increase the dose and the risk of probabilistic effects such as cancer. For argument's sake, if the exposure from this well were to continue for 50 years, the total dose commitment would be 150 mSv. A lifetime whole-body exposure of 150 mSv would correspond to an excess cancer risk of about 1% of which 20% are nonfatal. A person's lifetime risk of cancer now is about 25-30%, depending on region. Approximately 5% of the 25-30% are nonfatal. So health effects from water consumption from such wells would be difficult to observe, and indeed no health effects attributable to this source were identified in the Manitoba studies. The reality is that the exposures are likely much less than this because a person's water supply rarely comes from the same source for a whole lifetime or even 50 years. Also, much of the dose is received later in life, often long after water consumption from the source is discontinued. I should mention that since the Manitoba and AECL studies, the Canadian Drinking Water Guideline for Radium has been reduced to 0.6 Bq/L. The Manitoba standard is still 1 Bq/L (Public Health is a Provincial Jurisdiction) but will be reduced soon to be in line with the National Guidelines. John Barnard, Ph.D. Director of Facilities Acsion Industries Incorporated