HYDROGEOLOGIC STUDIES TO ADDRESS CHANGING WATER LEVELS AT THE WIPP SITE

The Waste Isolation Pilot Plant (WIPP) is the U.S. Department of Energy’s (DOE) underground repository for transuranic and mixed wastes located near Carlsbad, NM. The principal water-bearing unit at the WIPP site is the Culebra Dolomite Member of the Rustler Formation, which lies approximately 440 m above the repository. When the DOE submitted its Compliance Certification Application to the Environmental Protection Agency in 1996, heads in the Culebra were thought to be in a steady-state condition, perturbed only by drilling and testing activities. Subsequent monitoring showed Culebra water levels to be fluctuating in unexpected ways on nongeologic time scales. DOE therefore initiated an investigation to determine the cause(s) of the water-level fluctuations and evaluate potential impacts on WIPP’s regulatory compliance.

Three hypotheses involving human activities were developed to explain the water-level changes: 1) potash processing effluent discharged onto a tailings pile upgradient of the WIPP site may be leaking into the Culebra through poorly plugged boreholes beneath the tailings pile or through subsidence fractures; 2) water from overlying units may be leaking into the Culebra through poorly plugged potash exploration holes; and 3) brine injected into deeper formations for secondary oil recovery or salt-water disposal may be leaking into the Culebra, either through casing failures in the injection well(s) or through nearby poorly plugged boreholes.

Both field and modeling activities were initiated to address these hypotheses. New wells were installed downgradient of the potash tailings pile and in other areas where data were lacking. Bureau of Land Management and New Mexico Oil Conservation Division databases were searched to: 1) identify exploration holes and wells that may have been inadequately plugged, and 2) obtain information on injection wells, their target horizons, and injection rates. Records from the New Mexico Office of the State Engineer show how much water is pumped for potash refining.

Modeling studies show that some combination of leakage from the tailings pile and leakage through boreholes in other locations is required to match the observed spatial distribution of water-level changes. None of the scenarios considered suggest any threat to WIPP’s compliance with regulations.