Texans and Hibernation

I need some good advice on hibernating adult texans. They’re wild caughts from Texas, and I have spent alot of money on a breeding tank/area. I live in Northern Arizona, and this causes many complications, one the temperatures drastically change from periods of night and day (at times 40 degrees). Durring December it snows up to 6 inches, and is well below freezing. Their tank is held outside, due to the size, weight and space needed, so its a must to work around these conditions. I need to keep their water above freezing, any heaters come to mind that would let me keep the water around 44 degrees? Should I put potting soil, and sand mixture on the bottom of the tank for them to cover themselfs. And if ice formed on the water, would it kill the terrapins. Any ideas? Thanks. David