Wherever you are in the field, if you're not sure the water is safe for drinking, purify it first otherwise you may get typhoid, cholera, or dysentery. This film shows various sources of water at advanced phases and how it can be made safe and drinkable. Surface water such as that in rivers, lakes, ponds and streams is obviously the most easily contaminated but necessary treatment can be quickly determined and accomplished. Groundwater in wells and natural springs though you might think it pure must nonetheless be treated since it is subject to contamination by seepage. Melted snow contains the same type of impurities as surface water so it too must be purified before drinking. Even rain water gathered in natural or man-made collecting places picks up germs or decaying vegetation that gives it a bad odour or taste makes it dangerous. No matter where it comes from or how good it looks, treat it first before drinking it.