hole which extends into but not through the impervious stratum c a protective tube which has a diameter intermediate between those of the two bore holes and which extends from the surface to the stratum d a protective mortar which is added to the annular space defined by the sides of the larger bore hole and the tube and e perforated transfer tubing wound around the tube at the level of the protective mortar and adapted to permit fluids to be injected from the surface into the mortar it is important that the protective tube be seated at its lower end on the impervious stratum hence the requirement that the appropriate bore hole extend into but not through the stratum it must naturally extend a sufficient distance into the stratum to permit the establishment of an annulus of protective mortar of an appropriate size this will depend on factors such as size of bore hole and depth of stratum but the skilled person can readily ascertain this preferably the bore hole for the protective tube extends nearly to the bottom of the stratum thus the protective tube with the protective mortar will prevent water from above the stratum from entering the aquifer when such a bore hole is decommissioned the protective tube is capped and left in place and the imperviousness of the stratum is therefore assured the other bore hole equipment used in conjunction.