Hydrogeologic Characterization of the Saline Aquifers, East Texas Basin-Implications to Nuclear Waste Storage in East Texas Salt Domes

Groundwaters in the deep aquifers (Nacatoch to Travis Peak) range in salinity from 20,000 to over 200,000 mg/l. Based on their isotopic compositions, they were originally recharged as continental meteoric waters. Recharge probably occurred predominantly during Cretaceous time; therefore, the waters are very old. Because the basin has not been uplifted and faulting of the northern and western sides, there are no extensive recharge or discharge zones. The flanks of domes and radial faults associated with domes may function as localized discharge points. Both the water chemistry and the hydraulic pressures for the aquifers suggest that the basin can be subdivided into two major aquifer systems: (1) the upper Cretaceous aquifers (Woodbine and shallower) which are hydrostatic to subhydrostatic and (2) the deep lower Cretaceous and deeper formations (Glen Rose, Travis Peak, and older units), which are slightly overpressured.

The source of sodium and chloride in the saline waters is considered to be from salt dome dissolution. Most of the dissolution occurred during the Cretaceous. Chlorine-36 analyses suggest that dome solution is not presently occurring. Salinity cross sections across individual domes do not indicate that ongoing solution is an important process.

The major chemical reactions in the saline aquifers are dome dissolution, albitization, and dedolomitization. Albitization and dedolomitization are important only in the deeper formations. The high Na concentrations in the deeper aquifers system result in the alteration of plagioclase to albite and the release of Ca into solution. The increase in Ca concentrations causes a shift in the calcite/dolomite equilibrium. The increase in Mg results from dissolution of dolomite.

The critical hydrologic factors in the utilization of salt domes for disposal of high-level nuclear waste are whether the wastes could leak from a candidate dome and where they would migrate. The following conclusions are applicable to the problem of waste isolation in salt domes:

(1) Salt domes in the East Texas Basin have extensively dissolved. The NaCl in the saline aquifers is primarily from this process. Major dissolution, however, probably occurred in the Cretaceous time. There is little evidence for ongoing salt dome dissolution in the saline aquifers.

(2) If there was a release to a saline aquifer, waste migration would either be along the dome flanks or laterally away from the dome. If there is a permeability conduit along the dome flanks, then contaminants could migrate to the fresh-water aquifers, provided an upward hydraulic gradient exists. Calculation of performance assessment scenarios must take into account whether there is potential for upward flow between saline aquifers at repository level and the fresh water aquifers. If an upward flow potential exists, upward leakage along the dome flanks should be used as the worst-case scenario.