From the Nuclear War Survival Skills by Cresson Kearney, available for free download at http://www.oism.org:
REMOVING FALLOUT PARTICLES AND DISSOLVED RADIOACTIVE MATERIAL FROM WATER
° Filtering
Filtering through earth removes essentially all of the fallout particles and more of the dissolved radioactive material than does boiling-water distillation, a generally impractical purification method that does not eliminate dangerous radioactive iodines. Earth filters are also more effective in removing radioactive iodines than are ordinary ion-exchange water softeners or charcoal filters. In areas of heavy fallout, about 99% of the radioactivity in water could be removed by filtering it through ordinary earth. 73
Fig. 8.11. Expedient filter to remove radioactivity from water. ORNL
DWG 77-18431 (Illustration) 
(see online at http://www.oism.org/nwss/s73p919.htm)
To make the simple, effective filter shown in Fig. 8.11, the only materials needed are those found in and around the home. This expedient filter can be built easily by proceeding as follows:
1. Perforate the bottom of a 5-gallon can, a large bucket, a watertight wastebasket, or a similar container with about a dozen nail holes. Punch the holes from the bottom upward, staying within about 2 inches of the center.
2. Place a layer about 1 inches thick of washed pebbles or small stones on the bottom of the can. If pebbles are not available, twisted coat-hanger wires or small sticks can be used.
3. Cover the pebbles with one thickness of terrycloth towel, burlap sackcloth, or other quite porous cloth. Cut the cloth in a roughly circular shape about 3 inches larger than the diameter of the can.
4. Take soil containing some clay almost any soil will do from at least 4 inches below the surface of the ground. (Nearly all fallout particles remain near the surface except after deposition on sand or gravel.)
5. Pulverize the soil, then gently press it in layers over the cloth that covers the pebbles, so that the cloth is held snugly against the sides of the can. Do not use pure clay (not porous enough) or sand (too porous). The soil in the can should be 6 to 7 inches thick.
6. Completely cover the surface of the soil layer with one thickness of fabric as porous as a bath towel. This is to keep the soil from being eroded as water is poured into the filtering can. The cloth also will remove some of the particles from the water. A dozen small stones placed on the cloth near its edges will secure it adequately.
7. Support the filter can on rods or sticks placed across the top of a container that is larger in diameter than the filter can. (A dishpan will do.)
The contaminated water should be poured into the filter can, preferably after allowing it to settle as described below. The filtered water should be disinfected by one of the previously described methods.
If the 6 or 7 inches of filtering soil is a sandy clay loam, the filter initially will deliver about 6 quarts of clear water per hour. (If the filtration rate is faster than about 1 quart in 10 minutes, remove the upper fabric and recompress the soil.) After several hours, the rate will be reduced to about 2 quarts per hour.
When the filtering rate becomes too slow, it can be increased by removing and rinsing the surface fabric, removing about 1 inch of soil, and then replacing the fabric. The life of a filter is extended and its efficiency increased if muddy water is first allowed to settle for several hours in a separate container, as described below. After about 50 quarts have been filtered, rebuild the filter by replacing the used soil with fresh soil.
Settling is one of the easiest methods to remove most fallout particles from water. Furthermore, if the water to be used is muddy or murky, settling it
before filtering will extend the life of the filter. The procedure is as follows:
° Settling
1. Fill a bucket or other deep container three quarters full of the contaminated water.
2. Dig pulverized clay or clayey soil from a depth of four or more inches below ground surface, and stir it into the water. Use about a 1-inch depth of dry clay or dry clayey soil for every 4-inch depth of water. Stir until practically all the clay particles are suspended in the water.
Book Page: 74
3. Let the clay settle for at least 6 hours. The settling clay particles will carry most of the suspended fallout particles to the bottom and cover them.
4. Carefully dip out or siphon the clear water, and disinfect it.
° Settling and Filtering
Although dissolved radioactive material usually is only a minor danger in fallout-contaminated water, it is safest to filter even the clear water produced by settling, if an earth filter is available. Finally as always the water should be disinfected.
