Computer models in current use for describing water-rock interaction commonly involve the introduction of initially small amounts of rock into large volumes of water. Although this class of models has been very useful for the study of many types of geologic environments, this proposal suggests a new approach, where small amounts of water are put into a volume of rock. The conceptual difference between these approaches translates into a practical one where it is desirable to do calculations at low water-rock ratios, such as might be encountered in rock- buffered geologic environments including fracture development and vein formation or metamorphism.