River water by nettles = now you're talking! Beautiful stuff. For dirt plants there is no risk of cross contamination provided you do not splash the water on the plants. Some ducks carry botulism...
the key is to collect from healthy locations. look around before you just collect what you see first. you can do harm picking from bad spots, ive never had problems but i don't pick from still ponds or lakes. if collected right, i foliar spray river water on plants all the time and they love it. again its all about the location. good thing its easy to spot a place that looks nice vs. one that looks dead, crappy, and anaerobic.