HR: 1340h
AN: H33F-0542    [Abstracts]
TI: Investigations of Observed Water Level Rise in the Culebra Aquifer; WIPP Site, Carlsbad, NM
AU: * McKenna, S A
EM: samcken@sandia.gov
AF: Sandia National Laboratories, P.O. Box 5800 MS 0735, Albuquerque, NM 87059 United States
AU: Lowry, T S
EM: tslowry@sandia.gov
AF: Sandia National Laboratories, P.O. Box 5800 MS 0735, Albuquerque, NM 87059 United States
AU: Beauheim, R L
EM: rlbeauh@sandia.gov
AF: Sandia National Laboratories, National Parks Highway, Carlsbad, NM 88221 United States
AB: The Waste Isolation Pilot Plant (WIPP) in southeast New Mexico has been developed for underground disposal of transuranic waste in halite beds of the Permian Salado Formation. Managed by the Department of Energy (DOE), the WIPP has been operational since March 1999. The most important water-bearing unit of the Salado Formation is the Culebra Aquifer, which lies about 200 m below ground surface and 400 m above the repository. In the assessment of compliance monitoring parameters for the year 2000, freshwater heads were compared to trigger value ranges established for 28 monitoring wells in the Culebra. Of these 28 measurements, freshwater heads in 21 wells appeared to be outside the trigger value ranges, with 20 higher and one lower than expected. Head changes in four of the wells could be explained by problems with well casings and/or leaking packers, leaving 17 wells with unexpectedly high freshwater heads. Hydrographs over time show a steady and consistent head level rise in these wells for the last 15 to 20 years. Two possible scenarios for the rise in heads have been formulated: (1) leakage into the Culebra of refining process water discharged onto potash tailings piles, probably through subsidence-induced fractures and/or leaky boreholes; and (2) leakage into the Culebra of water from units above or below the Culebra through poorly plugged and abandoned boreholes. This research examines the plausibility of these scenarios by separately modeling each case using a different mechanism for adding water to the Culebra Aquifer. For leakage from potash tailings piles, the model assumes recharge from a single potash tailings pile known as the Mississippi East site, which is located 10 to 12 km due north and up-gradient of the WIPP site. Storage coefficient and recharge rate from the tailings pile are calibrated to linearized hydrographs from 13 of the 28 monitoring wells using PEST. The second conceptual model identifies 26 boreholes as probable candidates for leakage to the Culebra, and models the boreholes as point sources to the Culebra. Like the first model, flow rates from the boreholes into the Culebra are calibrated to the linearized hydrographs. Calibrations for both models are conducted across 100 realizations of previously calibrated transmissivity fields, allowing for statistical analysis of the results. Results show that calibrations are generally better for the leaky borehole scenario than for the tailings pile scenario. This is most likely due to the distributed nature of the leaky boreholes versus the large point source of the single potash tailings pile. However, each scenario is able to provide very close fits to the hydrographs under certain transmissivity fields. Analyses are not able to exclude one scenario over the other, indicating that the water level rise is most likely due to a combination of sources. Investigations into the relative importance of each source are currently underway.
DE: 1899 General or miscellaneous
DE: 1829 Groundwater hydrology
DE: 1836 Hydrologic budget (1655)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting