[en] Hydrologic and geochemical characterization of the WIPP site has progressed significantly since the 1980 WIPP Final Environmental Impact Statement. In 1980, the entire Rustler Formation was modeled as a single hydrologic unit, assumed to be isotropic, single-porosity, and completely confined. Variability within the Rustler was evaluated only on the basis of testing at individual wells. In the 1983 WIPP Site and Preliminary Design Validation effort, the Salado Formation, in which the WIPP facility is being constructed, was assumed to be anhydrous, except for fluid inclusions and mineralogically bound water. Recent hydrologic and tracer testing at the WIPP indicates: (1) the local importance of dual-porosity behavior in hydraulic response and transport in parts of the Culebra Dolomite Member of the Rustler Formation; (2) the presence of distinct high- and low-transmissivity regions within the Culebra; and (3) the possible importance of vertical fluid flow within the Rustler. Recent analyses indicate that fluids encountered in the WIPP facility and in experimental brine-migration studies are grain-boundary fluids, chemically distinct from fluid inclusions. Fluid-inclusion compositions appear to have been determined shortly after halite deposition. Because of the times required for diagenetic reactions controlling their compositions, the grain-boundary fluids within the Salado probably have a residence time of several million years. 12 refs., 5 figs., 1 tab