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The construction of an exploratory shaft 12 feet in diameter into the Salado Formation (repository horizon for transuranic waste material) at the Waste Isolation Pilot Plant site in southeastern New Mexico affected water levels in water-bearing zones above the repository horizon. The responses in the water-bearing zones were recorded at hydrologic test pads HI, H2b, and H3. Interpretation of the construction history of the exploratory shaft produced an approximation of construction-generated hydraulic stresses at the shaft. The magnitude of these stresses was estimated using the hydrographs from hydrologic test pad HI. Whereas flow rates from the Magenta Dolomite and Culebra Dolomite Members of the Rustler Formation into the exploratory shaft were unknown, the ratio of transmissivity to storage (diffusivity) was determined by mathematically simulating the hydrologic stresses with a floodwave-response digital model. In the Magenta Dolomite Member, the time of occurrence of the initial slope break, amplitude, and slope of the hydrograph recorded at test pad HI were matched to a set of type curves generated using a diffusivity of 0.03 foot squared per second. In the Culebra Dolomite Member at hydrologic test pad HI, the timing was matched with a diffusivity of 0.5 foot squared per second. The slope and amplitude of the response were also matched with a diffusivity of 0.5 foot squared per second. In the Culebra at hydrologic test pad H2b, the diffusivity value obtained from matching the time of the response was less than 1.0 foot squared per second and the diffusivity value obtained from matching the amplitude and slope was more than 0.5 foot squared per second. In the Culebra at hydrologic test pad H3, the diffusivity value from the response time was 1.0 foot squared per second and the diffusivity values from the amplitude and slope were 0.5 foot squared per second. These results indicate that the Magenta Dolomite and Culebra Dolomite Members of the Rustler Formation can be modeled as homogeneous, isotropic, and confined water-bearing zones. One simple and consistent explanation, but by no means the only explanation, of the lack of a single diffusivity value in the Culebra water-bearing zone is that local effects in or near the observation wells at the hydrologic test pads dampen the amplitude of waterlevel changes. INTRODUCTION The U.S. Department of Energy is beginning the construction phase for a repository for transuranic waste in the bedded salts of the Salado Formation of Permian age in southeastern New Mexico (fig. 1). The repository, known as the Waste Isolation Pilot Plant (WIPP), is designed to demonstrate disposal technology for transuranic wastes. At the WIPP, water-bearing zones with little permeability or storage (Magenta Dolomite and Culebra Dolomite Members of the Rustler Formation) occur above the repository horizon. Because these zones represent a potential path for the transport of radionuclides, there is a need to evaluate their regional hydrologic properties. This information, when determined, may be used in the design and construction of a monitoring-well network. The construction of an exploratory shaft 12 feet in diameter through the overlying Dewey Lake Red Beds and Rustler Formation into the Salado Formation at the WIPP site affected water levels in the Rustler Formation. The observed water-level changes presented an opportunity to apply flood-wave response techniques to estimate diffusivity values (transmissivity divided by storage coefficient) of the water-bearing zones above the repository horizon. Purpose and Scope The purpose of this report is to refine estimates of hydraulic coefficients of the water-bearing zones in the Rustler Formation. This investigation describes the procedures and interprets the results of modeling the Magenta Dolomite and Culebra Dolomite Members of the Rustler Formation at the WIPP site. The modeling effort simulated water levels measured by the U.S. Geological Survey during and after the construction of the exploratory shaft from June 1981 to December 1981 at hydrologic test pads HI and H3 and observation well H2b at test pad H2 (referred to as hydrologic test pad H2b in this report). I04°I5' 104° 00' I03°45' T. 22 S. ZONE H BOUNDARY

water-resources investigations report

Determination of diffusivities in the Rustler Formation from exploratory-shaft construction at the Waste Isolation Pilot Plant in southeastern New Mexico