[en] The key parameter for modeling the effects of reaction in groundwater systems is the equilibration length scale, L = v/RMK_D, where v is groundwater velocity, R is reaction rate, M is the solid/fluid mass ratio, and K_D is the solid/fluid distribution coefficient for the element. K_D varies over several orders of magnitude for the elements of interest. Although rock-groundwater reactions proceed slowly, they have significant effects for elements with large K_D or when ground-water residence times are long. A model was constructed for the case of groundwater flowing through devitrifying tuffs at Yucca Mtn. For Sr, C, and O, estimates of L are 10², 10⁵, and 10⁵ meters, respectively. Thus, Sr-87/Sr-86 in Yucca Mtn. groundwater or vein calcite reflects local rock values more than differences in groundwater source. Moreover, relationships between spatial patterns of Sr-87/Sr-86 in fluids (or vein calcite) and rock matrix can be inverted to determine L and flow direction (or paleo-flow direction), information that can be used to predict Sr-90 transport. For oxygen and carbon isotope ratios, the reaction effect is smaller, but potentially significant because l, the groundwater travel distance, is a significant fraction of L. This suggests that the waters' isotopic ratios are slightly shifted toward equilibrium with the rock, and that interpretation of isotope data for paleoclimate and groundwater age studies in this region requires assessment of the groundwater reaction history