One way to purify water is with a "solar still". You can use this with anything containing water; damp vegetation, wet dirt or sand, brackish or dirty water, or some combination of these.
You will need:
1. Something wet.
2. A shovel or digging implement.
3. A sheet of transparent plastic. I recommend stockpiling plastic painter's tarps; they are cheap and available from any home supply or hardware store. (Lowe's, Home Depot, Ace Hardware, etc.)
4. Flexible tubing such as aquarium tubing, medical IV tubing.
5. A cup, pan, or can to catch the water.
6. A small rock.
Dig a pit or depression about 3 feet across and about 18 inches deep. This needs to be in an area exposed to the Sun.
Place wet material in the hole. Place the pan or cup in the center, and run the flexible tubing from the bottom of the pan to a location outside of the pit. Use string, tape or twist-ties to prevent the tubing from pulling out of the pan.
Stretch the plastic sheet over the hold, and anchor it with the dirt from the hole, all the way around. The sheet needs to be relatively level. Place the rock in the center of the sheet directly above the pan, so that the rock forms a small shallow cone depression in the sheet.
When the sunlight heats the pit, water in the pit will be evaporated. The steam will condense on the plastic sheet, and the condensed water will run down the sheet to the center, where the rock is. The water will drip from the depression into the pan, and you can suck the water through the tubing. The amount of clean water produced will depend on the amount of sunlight striking the pit, the moisture content of the damp material, and the size of the pit.