I have what I believe is a Kirk Walnut in my yard. It has a large hollow that is gathering water. I'm worried that this watery situation will rot the heart of the tree. I'd love to preserve this historic beauty. It doesn't look like it could be cut off so water doesn't collect in there. Someone suggested cement? I am seeking expert advice.
Heart rot has already started and that is why there is a hollow there in the first place. So no matter what you do the rot will continue. These rots are slow and the tree may survive for many years to come. The tree is structurally compromised such that limbs or the entire tree could come down in future storms. So if limbs are over houses, etc. you may want to consider pruning those in the future.
The options are to do nothing (which is ok), to keep water from getting in or to drain the water out. Cement is trouble especially in the future when someone needs to use a chainsaw to cut it down or up (please do not use it). You might figure other ways to keep water out. The easiest solution would be to drill a hole at an angle from below to drain out the water. Yes, it injures the tree but as I said the rot has already started.