Lead becomes mobile after acted on by an acid. If you dig out a bullet that has been in the ground for some time, and that ground is not acidic, you will find a white coating on it. That is a lead phosphate coating and it immobilizes the lead.
Unfortunately that coating is broken down by acid. So what I would do if I was worried about it is test the soil for acidity. If it was much below neutral I might mix in some limestone screenings and #8's. Then I would sprinkle some Ag Phosphorus on it a couple times a year, rake that into the soil. Not a lot though, you don't want that running into the pond and causing an algae bloom.
https://www.ncbi.nlm.nih.gov/pubmed/18380230