Chemistry of brines in salt from the Waste Isolation Pilot Plant (WIPP), southeastern New Mexico: a preliminary investigation

The Waste Isolation Pilot Plant (WIPP) is a US Department of Energy-sponsored facility for the eventual disposal of defense-related transuranic nuclear waste, located in the Delaware Basin near Carlsbad, New Mexico. We present here analyses of macroand microscopic (intracrystalline) brines observed within the WIPP facility and in the surrounding halite, with interpretations regarding the origin and history of these fluids and their potential effect(s) on long-term waste storage. During excavation, several large (~1O Jim to several millimeters) fluid inclusions were recovered from an area of highly recrystallized halite in a thick salt bed at the repository horizon (2150 ft below ground level). Two populations of inclusions were distinguished on the basis of major element content, using analyses for Mg, Ca, K, Na, CI, Br, and S04. We propose that the inclusion compositions arise from the alteration of calcium sulfate to polyhalite and calcium carbonate to magnesite, respectively. Overall, the inclusion compositions suggest a significant departure from a simple seawater evaporation model. In addition, 52 samples of brine "weeps" were collected from walls of recently excavated drifts at the same stratigraphic horizon from which the fluid inclusion samples are assumed to have been taken. Analyses of these fluids show that they differ substantially in composition from the inclusion fluids (mainly by depletion of magnesium relative to potassium) and cannot be explained by mixing of the fluid inclusion populations. Since weeps are associated with argillaceous or anhydritic halite, these fluids may have originated by dewatering of clays and/or gypsum. Their compositions may have also been modified by selective uptake of Mg during clay diagenesis and by the formation of authigenic magnesite and magnesium silicates. Finally, holes in the facility floor that filled with brine were sampled but with no stratigraphic control; therefore it is not possible to interpret the compositions of these brines with any accuracy, except insofar as they resemble the weep compositions but with greater variation in both K/Mg