The wire you run to the well will be more effective than the well itself.
For RF grounding, depth is no advantage. You want the return current to have as low a resistance path as possible near the surface of the earth, where most of the RF current will be.
If you have room for ground radials, they can be quite effective, especially if you place a metal ground screen in the immediate vicinity of the base of the vertical radiator, where the current density is highest. Many American LowFERs have had good experience with chicken wire fencing for this purpose. (Are you aware we have a US-Canadian reflector, by the way? I've noticed you posting reports to other North American stations here, but not on that one. Fortunately, there is some overlap, but to ensure you're not missing anything, you might want to check: http://lwca.org/groups.htm
Alternately, if a radial system is not feasible, you might find it useful to get more toploading into the air. If you can better distribute the return current over a 360 radius around the vertical element, rather than concentrating it right under the horizontal run of an L or a T topload, the losses appear to be reduced by quite a useful amount.