RETRIEVAL OF RATES OF MINERAL-WATER REACTIONS FROM GROUNDWATER SYSTEMS

Rates of mineral-water reactions can be estimated from groundwater systems by using (1) geochemical modeling to identify reactants and estimate reaction extent, (2) elapsed time estimated from groundwater dating or flow models, and (3) petrographic observations to estimate reactive surface area. In practice, each aspect of this approach has many limitations. Geochemical modeling rarely leads to unique identification of reactions due to the presence of multiple reactive minerals and organic matter in aquifers, and yields net mass transfer, resulting in potential for underestimation of uni-directional rates. Particular challenges include estimation of reactive surface area, interpretation of groundwater age from environmental tracer data or groundwater flow models, and obtaining representative (unmixed) water samples from aquifers. In-situ placement of single crystals in flowing groundwater offers an alternative and promising approach to obtaining kinetic information from aquifers on relatively short time scales (to a few years).

Aquifers span a range of timescales, reaction possibilities, and reaction rates. Some reactions can be shown to occur near equilibrium with rates driven by one or more irreversible reactions, but only the rates of the irreversible reactions can be estimated. In aquifers of inhomogeneous spatial distribution of reactants (examples: anhydrite nodules encased in low-permeability zones; organic matter in semi-confining layers; dual porosity systems), rates of irreversible reactions can appear to be much slower than those in homogeneous systems. Reaction rates derived from aquifers tend to be orders of magnitude slower than those from laboratory experiments, due, in part, to the inhibiting effect of organics and other trace impurities in aquifers that can greatly retard rates near equilibrium relative to rates observed in laboratory experiments. Examples of field-derived rates are given for anhydrite dissolution and organic matter oxidation accompanying dedolomitization on the radiocarbon timescale. In spite of the difficulties involved in obtaining rates of reactions in aquifers, groundwater systems represent a reality that is not easily obtained in the laboratory.