Geochemical overview of the Waste Isolation Pilot Plant near Carlsbad, New Mexico

The Waste Isolation Pilot Plant is a US Department-of-Energy-sponsored repository for the long-term storage of transuranic defense-related nuclear waste, located in the Delaware Basin near Carlsbad, New Mexico. During excavation, numerous large fluid inclusions were collected from highly recrystallized halite at the repository horizon. The inclusion fluid compositions define two distinct populations, based on analyses for Ca, Mg, K, Na, Cl, Br, and SO/sub 4/. Chemical and mineralogical evidence supports our conclusions that the fluid inclusion compositions arise from early diagenetic changes involving alteration of calcium sulfate (gypsum or anhydrite) to polyhalite, and calcium carbonate (calcite or aragonite) to magnesite. Other brines occurring in the repository (as ''weeps'' oozing from the walls or collecting in drill holes in the floor) were also analyzed. These fluids were found to differ significantly from the fluid inclusions mainly by depletion of magnesium relative to potassium. The stratigraphic association of these fluids with argillaceous and anhydritic halite suggests an origin in evaporating seawater that was subsequently modified by dewatering of clays and/or gypsum, and selective uptake of magnesium relative to potassium during clay diagenesis. This interpretation is compatible with observations of a suite of authigenic minerals that includes quartz, magnesite, and magnesium silicates