Interpretation of isotopic data in groundwater‐rock systems: Model development and application to Sr isotope data from Yucca Mountain

A model enabling extraction of hydrologic information from spatial and temporal patterns in measurements of isotope ratios in water‐rock systems is presented. The model describes the evolution of isotope ratios in response to solute transport and water‐rock interaction. In advective systems, a single dimensionless parameter (a Damköhler number, N_D) dominates in determining the distance over which isotopic equilibrium between the water and rock is approached. Some isotope ratios act as conservative tracers (N_D ≪ 1), while others reflect only interaction with the local host rock (N_D ≫ 1). If N_D is close to one (i.e., the distance for equilibration is close to the length scale of observation), isotope ratio measurements can be used to determine N_D, which in turn may yield information concerning reaction rates, or spatial variations in water velocity. Zones of high velocity (e.g., as a result of greater fracture density), or less reactive zones, may be identified through observation of their lower N_D values. The model is applied to paleohydrologic interpretations of Sr isotope data from calcite fracture fillings in drill cores from Yucca Mountain, Nevada (Marshall et al., 1992). The results agree with other studies suggesting “fast path” transport in the unsaturated zone. Also, we find that the data do not give a conclusive indication of paleowater table elevation because of the effects of water‐rock interaction.