I've repeatedly seen the claim that fallout particles tend to settle in a still water body such as a lake or a bucket and that settled water is safe for drinking and cooking, at least from a radiological standpoint. I would still filter it though.
Once entrained in a municipal water distribution line it's difficult to see how sufficient settling would occur. I would therefore suggest creating your own settling operation. A few buckets would do. Let the water stand for several days, carefully dip out of the top so as not to re-mix any sediments, then stir and throw out the bottom half, rinse with a little of the "clean" water from the top half, and re-fill the bucket from your water source for more settling. To facilitate removal of the "good" water a spigot could be mounted at the halfway mark on the bucket.
I would take these steps rather than run city water directly into a filter. For one thing, you may not always have city water or adequate line pressure and will need to collect rain water, pond water, or even ditch water that you would want to settle anyway before trying to filter it. Settling will vastly improve filter life.
Being entirely radiation free is not an option. There is constant destructive background radiation from our sun, other stars, radionuclide decay in rock, radon gas seeping up through the soil, etc. It constantly rips through our bodies, leaving paths of destruction, and our bodies must continually repair the damage, which they seem able to handle at normal levels.
The fallout from several Japanese reactors in meltdown is unlikely to deposit enough particles on our roof or flower beds to cook us with gamma radiation as we huddle in our homes, which is the classic fallout scenario. It is the uptake of the radioactive elements themselves that matters to our survival in this case.
What can kill us are small clumps of unstable elements that either lodge in one place (E.G. the lungs) or are taken atom by atom through the intestinal lining and stored in dangerous concentrations where they irradiate the surrounding tissues at a level the body is not equipped to handle. Unstable isotopes of iodine concentrating in the thyroid gland is the leading example.
Plutonium, an alpha emitter, is of little danger except in the lungs because alpha radiation cannot penetrate our skin and is moved along through the intestines. Due to their need for rapid gas permeability, however, the cell walls of the lungs are necessarily very thin and cannot block alpha radiation. A minute particle of plutonium that lodges in the lungs will therefore damage the surrounding cells and open the door to lung cancers -- not an immediate death sentence but an almost inevitable, slow, painful death, especially amid a societal collapse.
Particle size is important with lung-lodged radionuclides. A single atom may damage only one cell when it deteriorates but a clump of ten thousand atoms may cause more damage than the body is able to repair. (Even though plutonium is a giant heavy element, ten thousand atoms is still microscopic.) Therefore it should be important to establish the size of particles raining down on us from Japan's nuclear disaster. Particles small enough to become airborne again once deposited, yet large enough to do a lot of tissue cooking are of particular concern.
My guess is that, without a thermonuclear explosion and massive updraft, the fallout stemming from the Fukushima plant will be mostly tiny particles and any larger ones will have fallen into the Pacific before they reach us. But guessing is not very useful; particle measurement data is crucial to assess our risk.
New Hampshire and Ecuador.
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