Hi in Virginia, We are in southern NC, and built a large pond in the backyard last year. It is about 1000-1200 gallons, one side is about 3 foot deep and the other half is about 24 inches. Now the winters are mild here for the most part, you are not too far away, so you shouldnt have too many problems with a pond for yoru turts. Ours is filled up pretty high, and we kept the filtration and spitter going all winter, we filled the bottom of the deep end with leaves and mud and an old plastic bucket. The two of our turts hibernated and just emerged last week unscathed from the elements. It did get down to 20's and teens at night here for quite a few weeks, no snow but lots of freezing nights. We compensated by starting the winter off for them with a pond de-icer unit which we let sit at the bottom of the shallow end and a tank heater on max setting to keep the pond from freezing, and both did perfextly, maintained about 38-40 all winter long. We also adapted an old screen tent frame to surround the pond and covered it with the screen part(roof nylon cut out and sewed together) to keep out unwanted limbs and anything that may fall in. We were nervous at first letting them hibernate for the first time, one of them being 7 years old and the other 2, but it was fine. As long as you make it deep enough by 3 foot or more, and keep it from freezing, and make it mucky enough for them to hide and protect themselves to keep warm, they will be fine. Here is our pond as of before winter.
Hope this is helpful to you.
