If I were in your shoes I would take her out of the pond and hibernate her in another container or bring her inside to over winter. One of the ways that snappers deal with the cold is to "bask" in bacterial laden detritus using the heat from the metabolism of the bacteria to warm themselves. If you put a heater into the pond then you run a huge risk of a turtle with some form of respiratory infection. My snapper lives indoors over the winter and has gotten to the point he begs for food when I walk by his tank. Good luck with the turtle.
