Dissolution of Evaporites and its Possible Impact Cn the Integrity of the Waste Isolaticn Pilot Plant (WIPP) New Mexico,
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DISSOLUTION OF EVAPORITES AND ITS POSSIBLE IMPACT CN THE INTEGRITY OF THE WASTE ISOLATICN PILOT PLANT (WIPP) NEW MEXICO,
USA
SIAVCSH M. ZAND Environmental Evaluation Group, New Mexico Health and Environment Department (At present, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 USA).
INTRODUCTION The purposes of this report are: (1) to discuss the manifestation of dissolution processes (such as land subsidence and method of formation of breccia pipes and brine reservoirs) in the bedded salt of southeastern New Mexico in the vicinity of the WIPP site, (2) to postulate conditions under which breaches of the repository might occur, ces of such breaches, and (4)
(3)
to discuss radiological consequen-
to identify areas that need further
investigation. The extent and activity of the dissolution processes both fKow and in the future are of importance in evaluating the geologic adequacy of the WIPP site. The dissolution of evaporites produces dissolution residues and may lead to collapse features such as breccia pipes. Dissolutions may also be responsible for brine reservoirs. The dissolution of evaporites begins by unsaturated water dissolving the soluble materials and continues as long as unsaturated water and evaporites are in contact.
The residual solid matrix is weaker structurally and more porous than the original evaporite bed. This porous residual matrix gradually crumbles and consolidates under the overburden weight forming a new layer. All these processes may occur simultaneously at a given time at different locations within the Basin. The resulting residual formations may provide a significant pathway of the radionuclides to the biosphere if a nuclear repository that is located within the bedded salt is subjected to dissolution. Mathematical models of radionuclide transport are utilized to evaluate the radiological consequences of the nuclear waste repository breach. Hydrogeologic and geochemical properties of the residual formations are of crucial importance in these nodels. The cumulative effect on overburden materials of the compaction of the dissolved evaporites is represented by the land subsidence on the ground surface. The dissolution of evaporites may be divided into two broad classes: (a) shallow dissolution, and (b) deep dissolution. Most scientists agree on the
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mechanisms that control shallow dissolution and on its possible impacts on the integrity of the repository. However, deep dissolution and its possible impact on the integrity of the repository is a matter of controversy. SHALLCW DISSOLUTION Shallow dissolution is the dissolving of surficially exposed evaporites by climatic precipitation and surface water.
The evaporite formations sub-
jected to shallow dissolution in the Basin are the Rustler Formation and upper part of the Salado Formation.
The most pronounced remnant of this process is
Nash Draw two miles to the west of the outer boundary of the WIPP site.
The
leading edge of the dissolution is referred to as the "dissolution front." Bachman' suggests th
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