Waste Containment In Domal Salt

The most widespread concern about hazardous waste containment is the perceived threat of contamination of groundwater. This concern is warranted by the record of groundwater contamination associated with traditional near-surface disposal technologies. Therefore, the key question is whether a salt dome has a particular set of attributes that will prevent the release of contaminants to the environment in both short-term and very long term time frames. From a regulatory perspective, a "no migration" petition must be approved by the Environmental Protection Agency for the containment facility. By "no migration," it is implied that the waste

Injection Zone

Injection Interval

Confining Zone

Injection Interval

Confining Zone

Confining Zone

Figure 13 Waste containment zone.

Confining Zone

Figure 13 Waste containment zone.

must be contained for 10,000 years. A demonstration that this condition will be met will require model calculations, and such models must be based on the physical and chemical characteristics of the waste and the geologic environment. A unique characteristic here is that no-migration petitions are routinely conducted for liquid waste injection wells, whereas the waste form to be disposed at the Dayton facility is in solid form. As fluid flow in deep formations is well understood, the tasks required in a no-migration petition for liquid wastes are routinely performed by specialists in hydrogeology and by petroleum engineers. Well-defined geologic units in such studies include an injection zone and a confining zone (Figure 13). Injection zones have a reasonably high permeability, and the confining zones should be of low permeability. The tasks required in the no-migration petition for a domal salt facility are, however, not routine, for several reasons. First, solid (not liquid) waste will be disposed of in solution-mined caverns that will be on the order of 1800 ft in height and 125 ft in diameter. For migration to take place, the solid waste would first have to be converted to a liquid form. Second, the question of how this might take place in a salt dome is not a simple one. Salt, particularly in salt domes, is not normally considered a flow medium because its permeability, porosity, and water content are very low. Tests at the North Dayton dome, for example, indicate that the total water content of in situ salt varies from 0.001 to 0.002 wt%. For comparative purposes, the bedded salt at the WIPP site contains 0.1-1 wt% water. Even if we assume that the wastes take on a liquid form some time after placement, the question remains as to how they would be transported beyond the walls of the salt cavern. Two mechanisms may be available to do this: fluid flow and molecular diffusion through the "porous" salt rock. Last, the concepts of injection zones, injection intervals, and confining zones as conceived for liquid injection wells have no geologic identity in an extensive salt environment. Some relief is provided here, however, as definitions have been provided by the Texas Waste Commission.

Based on Figure 13, the injection interval is taken as the cavern measuring 38 m in diameter and approximately 610 m in height plus an additional 15.24 m radially, making a circular cylinder 68.5 m in diameter. The injection zone extends another 15.24 m in all directions from the injection interval, and the confining zone is the remainder of the salt-rock environment. The task of the "no migration" petition is to demonstrate that the site conditions are such that the waste will not migrate out of the injection zone. This constraint permits waste transport over a distance not exceeding 30.5 m radially from the cavern wall, or approximately 49 m from the cavern axis.

The following issues must be addressed in the petition.

1. The potential for brine infiltration, infiltration rate, and how quickly an immobile solid waste could potentially convert to a more mobile liquid state.

2. The potential for migration of dissolved waste by advective transport and how far the waste could be transported over the 10,000-year containment period.

3. The potential for migration of dissolved waste by molecular diffusion and how far the waste could be transported over the 10,000-year containment period.

4. The potential for gas transport out of the cavern and how far gas or gaseous waste could be transported over the 10,000-year containment period.

5. The potential for transport of the waste components in the solid state and how far the waste could be transported by solid-state diffusion over the 10,000-year containment period.

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