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

We present here analyses of macro- and 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 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). 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 and cannot be explained by mixing of the fluid inclusion populations. Finally, holes in the facility floor that filled with brine were sampled but with no stratographic 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 and Na/Cl ratios. However, the Ca and SO4 values for the floor holes are relatively close to the gypsum saturation curve, suggesting that brines filling floor holes have been modified by the presence of gypsum or anhydrite, possibly even originating in one or more of the laterally continuous anhydrite units referred to in the WIPP literature as marker beds. In conclusion, the wide compositional variety of fluids found in the WIPP workings suggest that (1) an interconnected hydrologic system which could effectively transport radonuclides away from the repository does not exist; (2) brine migration studies and experiments must consider the mobility of intergranular fluids as well as those in inclusions; and (3) near- and far-field radionuclide migration testing programs need to consider a wide range of brine compositions rather than a few reference brines