The effect of dispersion on the establishment of a paleoclimatic record from groundwater

A mathematical model which can account for the dispersion of solute in underground water over time scales exceeding 10 4 yr. is described. In its simplest form, the model postulates a diffusion limited exchange of material between flowing groundwater and the stationary water in the confining aquiclude, leading to the spreading of the solute in the mobile water and a consequent reduction in its concentration variability. The model is used to study the increasing attenuation with “age” of the perturbations in groundwater composition induced by paleoclimatic oscillations in the late Quaternary. Specific reference is made to noble-gas concentrations, stable-isotope ratios and chloride levels. The extreme attenuation of stable-isotope ratios in the Australian Great Artesian Basin cannot yet be adequately explained and may provide the basis of a critical evaluation of different dispersion models in large hydraulic basins.