REACTION-TRANSPORT MODELS FOR RADIOCARBON IN GROUNDWATER - THE EFFECTS OF LONGITUDINAL DISPERSION AND THE USE OF SR ISOTOPE RATIOS TO CORRECT FOR WATER-ROCK INTERACTION

We expand a model for isotope ratios in groundwater-rock systems to co nsider the ratio of C-14 to dissolved inorganic carbon, or ''percent m odern carbon.'' The model couples one-dimensional advection and disper sion with water-rock interaction and radioactive decay. We use a Ficki an formulation to model longitudinal macrodispersion and present a for mula for the groundwater velocity error introduced when dispersion is ignored, as a function of dispersivity and decrease in percent modern carbon. For example, if the longitudinal dispersivity were one tenth t he length of observation and percent modern carbon decreased tenfold a long a flow path, the error would be 23%. We also present a method for inversion of Sr-87/Sr-86 and C-14 data to obtain carbonate dissolutio n rates and dissolution-corrected water velocities and apply this tech nique using published data from the Lincolnshire aquifer, England. The results are similar to those obtained using the delta(13)C method.