Modeling the hydrogeochemistry of aquitards using minimally disturbed samples in radial diffusion cells

The radial diffusion method is a new technique that, among other applications, allows laboratory study of the chemistry of groundwater in intact aquitard materials. The computer model PHREEQC was used to simulate the changes in hydrochemistry observed in radial diffusion cells. A relatively small number of assumptions were required. In addition to using the mass measurements and the initial concentrations of aqueous species in the cells (after first addition of solution), equilibration of the aqueous solutions with the mineral calcite, and with a pool of exchangeable cations, was assumed for each step. For the majority of dilution steps, one or more of the following redox reactions were employed: oxidation of pyrite, oxidation of organic C, reduction of SO4. The simulated trends in dissolved concentrations were close to the observed data. However, the simulated levels of exchangeable cations based on tests with radial diffusion cells were generally lower than those determined by a conventional method, especially in the case of a Cretaceous “shale” core .