Fletcher Wildlife Garden. This is a non-native species (native to the southern US) which is popular in the pet trade. When the turtles get too big, well-meaning people drop them off at wetlands and ponds, thinking they will survive. Some do, some don't.

This turtle is sitting on the turtle raft at the Fletcher Wildlife Garden that myself and another volunteer made as a basking place for turtles. The chain you see is not tying this guy to the raft! It is holding the raft in place in the pond. When I took the photo, the slider was sharing the raft with a painted turtle.