Comparison of hydraulic and chemical methods for determining hydraulic conductivity and leakage rates in argillaceous aquitards

The capacity of argillaceous aquitards to effectively separate fresh water aquifers from lower quality waters has been widely studied for several decades using both hydraulic and chemical methods. Hydraulic methods, namely laboratory, field and regional groundwater flow modelling, are used to estimate hydraulic conductivity (K), whereas chemical methods, namely aquitard porewater and aquifer groundwater chemistry, are used to estimate either leakage rates or porewater velocity (V). We reviewed a total of 40 formations where either K or V (or both) have been estimated. Typical vertical hydraulic conductivity (K_Z) values in argillaceous materials estimated using hydraulic methods are 10^-12-10^-9ms^-1. Usual V values, estimated using chemical methods, ranges between 0.01 and 1mmy^-1, although the range is as wide as 10^-4-10³mmy^-1, when inferred from hydraulic K_Z measurements. Based on a Péclet number of 1, we calculated the lower limits of porewater velocity that can be reliably identified for different tracers, in most cases ranging from <1mmy^-1 for ²H, ¹⁸O and Cl^- to >100mmy^-1 for temperature. Despite the limited number of sites where both methods were applied, comparison between hydraulic and chemical-derived values showed a reasonable correlation, although the range of K_Z and V estimated using hydraulic methods is larger than that obtained using chemical methods. Methods applied to the whole aquitard thickness (some field hydraulic methods, regional groundwater flow modelling, aquitard porewater and aquifer chemistry) consistently result in lower K_Z values, most probably indicating the presence of very low K_Z layers within the aquitard, likely to be missed while using field hydraulic methods that test only a section of the entire thickness. K_Z was observed to decrease with depth, presumably due to an increase in consolidation resulting in loss of porosity and smaller aperture of fractures and joints. Multi-disciplinary studies involving the use of hydraulic and chemical methods at different scales, combined with geophysical techniques to locate fractures at local and regional scales, are highlighted as promising avenues for the study of inter-aquifer leakage.