Radioactive Water Filters

It turns out there's a problem with one system used to remove radon from water.

Because the filters have been removing low-level radioactive radium from the city's water for more than 16 years, they have become slightly radioactive.

"When we went to remove them, we learned they were considered low-level radioactive waste and it would have cost $100,000 for disposal," he said.

Radon has a half-life of 3.823 days. That means that after a month, only 0.4% of the original radon would be left. Each succeeding month would reduce whatever radon is in the system by another factor of 230. So why have these filters managed to accumulate radioactivity for 16 years?

Radon is part of a decay series. Uranium decays to thorium, which decays to protactinium, and so on. Radon is seven steps down from uranium, and after radon decays to polonium, there are another eight steps before the series ends with lead-206, a stable isotope.

One of the steps is lead-210, which has a half-life of 21 years. While it's decaying away slowly, more radioactivity is being added as more radon is captured and starts the decay process in the filter. To be sure, eventually the levels would reach a break-even point, but that would be after several decades. (After 63 years, you're up to 95% of the final level; after 84 years, you're up to 98%.)

Lead-210 is a mixed beta and gamma emitter, so you need to spend at least a little effort shielding the radiation. And the level of the radiation is not the level that's found in the water, but rather the level found in however many millions of gallons pass through the filter over a decade. If you have a picocurie of radon per liter of water, and you process one million gallons per day, you wind up with some 15 or 20 millicuries of lead-210 trapped in that filter. That's quite a lot of radioactive material.

Apparently, it's going to cost $100,000 to dispose of the filters in a low-level waste dump.

Update: The article said "radium" and I was thinking "radon", because it's a consideration in local water sources. The half-life of radium is 1620 years, so it will pile up in filters for quite a while -- up to whatever quantity the filter will hold.