We have an old cisterna in the middle of the Casa which now has no water in it! and its about 12 ft from our external walls, all floors which were dirt floors have been tanked with Visqueen 2 inches of polystyrene block insulation and 2 inches of concrete floor on top then tiled there is a 10ft of concrete walkway all round the building and still the walls above the old cisterna get damp, you are on a loser I'm afraid short of injecting every piece of rock with silicon at floor level!!