Note: Chlorination isn't effective if the water isn't already fairly clean, so don't bother trying this if the water is cloudy or has a lot of suspended matter in it.
8 cups (1/2 gallon) of chlorine bleach per 1000 gallons of water
Mix proper amount of chlorine and water together thoroughly and let stand for 20 minutes. If the water does not have a slight chlorine smell, repeat the dosage and wait another 15 minutes. If after the second dosing, the water still does not have a chlorine odor, it isn't suitable as a drinking water source. If the chlorine taste is too strong, let it stand exposed to the air or pour it from one container to another several times.
You may be able to use the water source if you filter it first. Pour a gallon of water through a paper filter or clean cloth. To treat one gallon of water, use 10 drops of chlorine bleach, mix, wait 30 minutes, smell for chlorine. If no odor, repeat the treatment. If after the second dose of bleach, there is still no odor, don't consume the water.