The Culebra Dolomite Member of the Rustler Formation, a thin (10 m) fractured dolomite aquifer, lies approximately 450 m above the repository horizon of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico, USA. Salinities of water in the Culebra range roughly from 10,000 to 200,000 mg/L within the WIPP site. A proposed model for the post-Pleistocene hydrochemical evolution of the Culebra tentatively identifies the major sources and sinks for many of the groundwater solutes. Reaction-path simulations with the PHRQPITZ code suggest that the Culebra dolomite is a partial chemical equilibrium system whose composition is controlled by an irreversible process (dissolution of evaporites) and equilibrium with gypsum and calcite. Net geochemical reactions along postulated modern flow paths, calculated with the NETPATH code, include dissolution of halite, carbonate and evaporite salts, and ion exchange. R-mode principal component analysis revealed correlations among the concentrations of Si, Mg, pH, Li, and B that are consistent with several clay-water reactions. The results of the geochemical calculations and mineralogical data are consistent with the following hydrochemical model:

• 1. solutes are added to the Culebra by dissolution of evaporite minerals
• 2. the solubilities of gypsum and calcite increase as the salinity increases; these minerals dissolve as chemical equilibrium is maintained between them and the groundwater
• 3. equilibrium is not maintained between the waters and dolomite; sufficient Mg is added to the waters by dissolution of accessory carnallite or polyhalite such that the degree of dolomite supersaturation increases with ionic strength
• 4. clays within the fractures and rock matrix exert some control on the distribution of Li, B, Mg, and Si via sorption, ion exchange, and dissolution.