I trap a lot of rats out of little ponds that sound just like this one. One thing you want to watch too is those little mounds and/or any mound that is in "shallow" water versus the deeper located mounds. When things freeze up, the shallow or "feeder" mounds will be abandoned quickly and the deeper mounds will get all the visitors.
Every mound I've ever set in these little ponds has one or two well-defined little "runs" (they look like little ditches that a #110 will fit into quite well) and I simply blind set every little entrance run I can locate. We have soft bottom ponds here, so when you go sloshing up to the mound you have to move slowly so as not to kick up too much mud so you can see what the heck is going on underwater and where to make sets. When it is too muddy to see, or the wind is blowing and there is too much chop on the water surface to see below it, just carefully feel around with your hand, toe, or a walking stick for the runs. Be sure to set your #110s fairly solidly in that run...I use sticks, one long one that sticks up well above the surface for the stake, then a couple of smaller-pencil sized ones for stabilizing the trap. These smaller sticks go at angles through the top, outside jaw corners and cross slightly over the trap. This also helps direct the rat down through the trap and not around it. A good, well-defined run will sometimes get 3 or 4 traps in it, all at least 3 feet apart. I catch a lot of multiples by setting up the hot runs with several traps.
Just sold 33 rats that all came from one little pond surrounded by cattails. The pond had mounds, and bank dug dens alike. Had to break ice many mornings to check and empty traps, but it was fun and easy money. I averaged $2.50 on those finished December rats, too.
Jim-NE