[en] The major and minor element data and isotopic data from the ERDA-6 and WIPP-12 testing indicate that the brine reservoirs encountered in the Upper Castile Formation are largely in equilibrium with their surrounding host rock environment. This contention is supported by thermodynamic and stable isotope data. It is not possible to assign an absolute age to the brine based on uranium disequilibrium considerations, but the data do indicate that the brine reequilibrated with a new rock environment at least two million years ago. Information and data evaluated herein indicate the likelihood that the brines encountered are predominantly, if not entirely, derived from a trapped seawater source subsequently modified by diagenesis. Major ion/bromide ratios indicate that halite dissolution has occurred to some extent subsequent to deposition of the Castile anhydrites and entrapment of the seawater brine. Mechanisms for additional halite dissolution are discussed. Based on the degree of present halite saturation, it is concluded that the potential for future dissolution of halite is minimal