Millibequerels per liter?
Are we worrying about this?
You literally can't detect levels this small except when measuring by hundreds of gallons *because natural background overpowers it so drastically*. Even on such a scale, you have to evaluate *chemically* unless you have *extremely sensitive equipment*.
We are talking 15mbq per liter. 0.015bq per liter. For reference, take a banana, average, 7". Take a 0.1778mm slice of that banana. Literally so thin you can see through it.
Then blend it up in a 1 liter bottle of water.
THAT IS THE LEVEL OF RADIOACTIVE EXPOSURE WE ARE TALKING ABOUT.