Shadow,thats a red-eared slider or a painted turtle, I think a painted turtle(no red ear), that shell is was to smooth to be a snapping turtle. Turtles(both snappers and painted) have a nasty sharp bite and usually keep what ever they bite

. If the ducks leg is just broken and doesn't have a big piece missing, I doubt a turtle did it.
Here is a list of wildlife rehabbers by state.Some times you have to call a few before you get any action.
http://www.wildliferehabber.org/st_disp_list.php