When I was in training to become a SERE Specialist I was sent on a second solo evasion living exercise because I did terrible at my first one. The weather wasn’t terrible and I mostly had sun. The temperature dropped well below freezing at night but I was allowed to have a fire. Life was quite peaceful and I settled into the beautiful mountainside that I had made my home. There was a large meadow perhaps 50 meters from my camp that had an ice cold meandering stream running through it. Disregarding my training because I’m an idiot I looked at the mountains surrounding me, jutting up into the clouds and thought, “This here is pure mountain water, purified by the soil and cleansed by sunshine.” And I happily drank without purifying.

Day 3 of my exercise I collected enough water to make a small journey to see if I could find some materials. I followed the “pure mountain stream ordained by the sun” upstream for around 500 meters before I came to another meadow equally as beautiful as the one I lived next to. I heard a noise moments later and watched a herd of perhaps 80 or so cows cross the stream and shit into it. I was lucky. Purify your water.

In a survival situation you should purify all of your water. Even rainwater as it’s descending to you can hit the tree canopy and collect fecal mater from various animals that live in trees. There are various methods to purify water that include the use of purchased products and ones that need nothing but your ingenuity.

Pre-filtering you water: This is important, if your water is full of particulate matter and other disgusting crap you might want to consider doing a pre filter. I typically do this by putting a shirt over the mouth of my canteen and pouring the water through my shirt. This limits the amount of debris getting into your water and lessens the chance of you getting a water-borne pathogen.

Iodine tablets: Light and portable these are my go to method for purifying water when navigating. The bottle will tell you precisely how much is needed to purify your water. If your water is super cloudy / dirty you can double the dosage of iodine and the time for purification (usually from 30 minutes to an hour). Don’t forget to “burp” your water. Which entails holding the canteen upside down 5-10 minutes after dropping the iodine tablets in your water and unscrewing the cap slightly to allow water with iodine in it to flow through the threads of the cap and purify any remnants of contaminated water in there. A couple of notes, don’t add any powders drink that contain Vitamin C prior to the purification process completing as it reduces the effectiveness of the iodine. Iodine also reduces the uptake of radioactive iodine.

Bleach: Make sure that you purchase bleach without any other fancy chemicals in them. You want bleach that is 4-6% Sodium Hypochlorite. With this percentage, you can use the “You must be 21 to drink” rule. Which is that you need 2 drops of bleach per 1 liter of water. Once the bleach is in, your wait time is 30 minutes. Or if the water is particularly nasty double the dosage and the time. 4 drops per liter and 1 hour.

SODIS (Solar Water Disinfection) method: Use a clear PET water bottle or clear soda bottle. Glass bottles will not work as they block UV-A rays. Remove any labels from the bottles. Fill the bottles with water (pre-filtered) and place them in an area where they will be exposed to direct sunlight. Placing foil behind the bottles increases the amount of light that reaches the water. In direct sunlight, the water will purify in approximately 6 hours. In a cloudy environment, it could take 2 days. This method is most effective in areas with lots of sunlight.

Boiling: An extremely effective way to purify water. Bring the water to a roiling boil for 1 minute to be extra safe. At elevations above 6500 feet you should boil the water for 3 minutes to be safe. In reality, the time it takes to bring the water to a boil is typically sufficient to kill any harmful organisms.