Answer to Question #3399 Submitted to "Ask the Experts"

Question 484 dealt with radium in drinking water and I suspect that the calculations you would like discussed are those that were used to convert the concentration of the radium into an annual dose equivalent and an associated risk. Ultimately, such a calculation involves assumptions about the amount of water consumed, the fraction of the radium absorbed into the blood, the ultimate distribution of that absorbed radium throughout the body and its residence time in the various tissues of the body, the energy deposited in those tissues by the radiation emitted during the decay of the radium, and the relative sensitivity of those tissues.

The simple approach, and, given the numerous uncertainties, a perfectly acceptable one, is to use dose factors of the sort published in Federal Guidance Report 11. This report can be found on the Environmental Protection Agency Web site. For radium-226 the dose factor is 3.58 X 10^-7 sieverts (Sv) per becquerel (Bq) ingested. This more or less equates to 1.3 X 10^-3 millirem per picocurie.

The answer to Question 484 assumed that 1.86 liters of water per day were consumed (perhaps something of an overestimate) and that the concentration of radium-226 in the drinking water was 6.4 picocuries per liter. In such a case, approximately 4,000 picocuries per year would be ingested. Using the aforementioned dose factor, I come up with an annual dose equivalent of a little over 5 millirem. This represents one to two percent of the 300 mrem per year (or thereabouts) that the average American receives due to natural radiation.

With regard to the estimated risk, the highly uncertain working assumption is the risk of dying due to radiation-induced cancer is approximately 5 X 10^-7 (five in ten million) per millirem. As such, the risk due to drinking this water for one year would be roughly 2 or 3 in a million. Judging from the report you referenced, the situation in Lake Zurich would not be very different. If you are concerned about the safety of your drinking water, it would be easy to install a purifier in the water supply. You also asked about the impact of the radium in the water when it came to cultivating fruit trees (and consuming the fruit) and taking a shower. In these situations, the radium concentration in the water is really a nonissue. For further information regarding all of this, do not hesitate to contact your state authorities.

Paul Frame, CHP, PhD