EVAPORITE DEPOSITION, DISSOLUTION, AND THE WASTE ISOLATION PILOT PLANT (WIPP)

As a disposal site for radioactive waste from US defense programs, WIPP has been intensively investigated for processes, including dissolution, that might affect long-term isolation of these wastes from the accessible environment.

Mudstone equivalents to halite-bearing sections of the Permian Rustler Formation were initially assumed to be post-depositional dissolution residues. Data from large diameter shafts, drillhole cores, and geophysical logs show that Rustler mudstones are landward facies to halite pan deposits east of WIPP. Anhydrite overlying the mid-Rustler halite margin is locally fractured, indicating probable post-depositional dissolution of halite. West of WIPP one drillhole encountered cavernous porosity in sulfate and dolomite interbeds.

Hydraulic testing of the Salado reveals outward hydraulic gradients that limit access of halite undersaturated fluids.

Long-term monitoring of Rustler water-bearing intervals reveals differing flow directions. Pumping tests show these intervals are not connected vertically at the site. Water samples show ages that preclude short-term vertical recharge of these water-bearing intervals.

West of WIPP, Nash Draw is an internally drained, evaporite (mainly gypsum) karst valley displaying active sinkholes, same cave development, alluvial dolines, and some perennial springs with gypsum-rich fluids. The valley developed by a combination of erosion and dissolution, including upper (Permian) Salado halite beds. The Rustler crops out and underlies surficial deposits; karst features are developed in the Rustler. Beneath the Rustler in Nash Draw, a brine-bearing interval of solution residues of the uppermost Permian Salado Formation is present.

At the WIPP site, neither Salado nor the underlying Castile Formation has undergone observable evaporite dissolution.