Answer to Question #5568 Submitted to "Ask the Experts"

The Baltic Sea received a great deal of Chernobyl-associated fallout and is considered highly contaminated with elevated ¹³⁷Cs (among others) levels. I have found average levels in Baltic seawater quoted at 60 Bq m^-3. Toronto city water compares at 0.25 Bq L. What is the comparison? If one were to travel on a cruise ship on the Baltic Sea and the ship uses seawater in a desalination process, what level of activity would a passenger be consuming? Would that be a safe level of consumption?

It is not clear from your question whether the 0.25 Bq L^-1 that you quote for Toronto represents ¹³⁷Cs. If so, it would translate to 250 Bq m^-3, which seems unlikely. The 0.25 Bq L^-1 may well refer to tritium in the water supply, the number being more reasonable for tritium, which is ubiquitous in all surface waters with concentrations commonly in the 0.2 to 1 Bq L^-1 range. 

The Baltic Sea has the highest levels of ¹³⁷Cs of any marine body in the world, although the level of 60 Bq m^-3 that you quote is about 100 times less than the highest value that was measured in the Baltic Sea following the Chernobyl accident. To my knowledge, most ship desalinization systems remove salt through a process of reverse osmosis. I would expect this process to be very effective, probably greater than 99 percent, in removing cesium and other possible radioactive cations that might be present in the water. Having said this, I should point out that the value of 60 Bq m^-3 for ¹³⁷Cs, while high compared to many other oceanic bodies, is not sufficiently high to pose any significant threat to people, even if they received all of their drinking water from the Baltic. Drinking such water 365 days per year would result in an effective whole-body dose to the affected individual of less than 0.1 mrem, a totally inconsequential number compared to the typical background radiation dose of around 300 mrem from natural sources.

I hope this answer is helpful to you.

George Chabot, PhD, CHP