The growth of modern society and technology requires an increasing level of protection of the environment, and waste disposal is a growing problem, particularly for radioactive and toxic industrial wastes.

The injection of wastes mixed with cement grout into thick shale formations is a promising method for effectively immobilizing wastes in a nearly impermeable medium. Hydraulic fracturing serves as a tool to increase the permeability of shale during the grout injection. Ion exchange and adsorption agents can be added to the grout when it is mixed. After hardening of the grout, the injected wastes will become an integral part of the shale and remain there as long as the shale is not subject to erosion.

Problems concerning the safety of the method are phase separation and orientation of the hydraulically induced fractures. During hardening of the grout, phase separation may occur, that is, some liquid may separate. In such an instance, the mobility of wastes in the separated liquid may be greatly retarded by the very low permeability and high ion-exchange capacity of shale. If the separated liquid reached a groundwater reservoir, the concentration of contaminants would be greatly reduced further by dilution in the native groundwater.

In bedded shale there is a great difference in tensile strength between the direction normal to and parallel with bedding planes. This difference in tensile strength may favor the formation of fractures along low-angle bedding planes within a zone of limited vertical extent. However, not all shale formations produce bedding fractures, therefore before construction of a waste-disposal plant it is necessary to test the site by water or nontoxic grout injections tagged with radioactive isotopes to judge whether a "zone" of fractures can be induced parallel with the shale bedding planes. Injection pressure, movement of the ground surface, and gamma-ray logs made in observation wells are used to interpret the orientation of the hydraulically induced fractures during the site-selection test . A case history of hydraulic fracturing at West Valley, New York, illustrates the method of selection of a site.

Waste disposal through an injection well is conducted in multiple-layered injection stages. The first injection starts from the deepest depth, then the injection zone is plugged by cement, and the second injection will be started about 10 ft above the first one. The repeated use of the injection well distributes the high cost of construction of injection and monitoring wells over many injections, thereby making hydraulic fracturing economically feasible as a tool for the disposal of certain types of wastes.

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