23-46. Water from lakes, pools, ponds, and other standing sources is likely to be heavily contaminated; though most of the heavier, long-lived radioactive isotopes will settle to the bottom. Use the settling technique to purify this water. First, fill a bucket or other deep container three-fourths full with contaminated water. Then take dirt from a depth of 10 centimeters (4 inches) or more below the ground surface and stir it into the water. Use about 2.5 centimeters (1 inch) of dirt for every 10 centimeters (4 inches) of water. Stir the water until you see most dirt particles suspended in the water. Let the mixture settle for at least 6 hours. The settling dirt particles will carry most of the suspended fallout particles to the bottom and cover them. You can then dip out the clear water. Purify this water using a filtration device.