I am sitting on a limestone plateau. There is little surface water here and no hot springs. Possibly the permeability to water improves the rock's ability to regenerate, but after all, everything was impermeable enough to allow drilling and filling. By the way, for me with the improved filling (so higher thermal conductivity, maybe that makes a difference in the cycles – with you, the area near the pipes might regenerate somewhat more slowly).
As long as your AZ is fine and the brine doesn't continuously drop further until it no longer works, you can still live with it. The heat pumps are designed for this temperature range.