[nine_k]

Beta decay is emission of electrons. They quickly slow down in the air. A beta emitter can only burn you if you literally pick it with you fingers, or maybe swallow it. Throwing it in a campfire is not a great idea, but nickel is not going to burn or melt in a campfire, so it won't become a dangerous pollutant. Nickel is also chemically pretty stable, so underground waters are not going to carry around any significant amounts of it after you have extinguished the campfire and put some soil on its site.

172m2 Hafnium decays into regular 178 Hafnium, but it readily burns in the air, so if you throw a battery with in into a campfire, chances are you'll add some beta radioactivity to the environment nearby, or inhale part of it yourself.