various high pressure isolation structures are operative to prevent brine or other unwanted liquid from seeping through incompetent or soluble rocks . in one embodiment , a pair of barrier dams are constructed in spaced relation within an underground passageway , and the interface between each dam and the host rock is sealed . a hydrophobic liquid is introduced under pressure into the passageway between the dams so as to impregnate the microfracture system within the host rock . a monitoring installation may be employed to sample the area on the flood side of the upstream barrier dam . alternate embodiments of isolation structures in the form of expandable bulkheads may be employed to effect accelerated rock repressurization so as to close the microfractures either individually or in combination with another barrier dam with hydrophobic liquid .