From Burial to Barrier: How burial history controls the hydraulic conductivity in argillaceous formations

Abstract. Deep geological repositories for high-level radioactive waste (HLW) rely to a large extend on the long-term hydraulic integrity of host rocks to limit fluid flow and radionuclide migration. Low hydraulic conductivity (K < 10^-10 m/s) is a key factor for effective long-term barrier performance, and argillaceous formations are promising candidates due to their strong aquitard characteristics. However, predicting their bulk hydraulic behaviour across temporal and spatial scales remains difficult, as it reflects the combined effects of intrinsic material properties and post-depositional evolution. This study compiles 782 hydraulic conductivity measurements from six European argillaceous formations, including laboratory and field scales. By integrating petrophysical, mineralogical, and reconstructed burial history data, we identify systematic links between burial evolution and hydraulic behaviour. Results show that maximum burial depth and associated stress and temperature conditions exert a first-order control on matrix-scale hydraulic conductivity, which is largely retained after uplift. In contrast, bulk hydraulic behaviour at the rock-mass scale reflects interactions between maximum burial depth and present-day depth, defining processes such as decompaction, fracturing, and self-sealing processes. Three evolutionary trends emerge from the compiled data: (1) Shallowly buried (<400 m), poorly indurated formations show limited hydraulic variability and scale independence; (2) Moderately buried (~800 m – 2,000 m), overconsolidated formations retain low matrix hydraulic conductivity after uplift, but exhibit gradually (partly significantly) enhanced hydraulic conductivity at depths <100 m due to the evolution of a pronounced decompaction zone. When devoted to less pronounced uplift and at greater present-day depths (>250 m) matrix and bulk hydraulic conductivities converge and predominantly range within a natural variability between 10^-14 to 10^-12 m/s, indicating effective self-sealing processes; (3) deeply buried formations (>2,000 m) become increasingly indurated and brittle, with reduced self-sealing capacity due to the loss of swellable clay mineral phases and fracture-dominated bulk hydraulic behaviour. Matrix and rock-mass hydraulic conductivities may diverge by several orders of magnitude. These trends provide predictive insights into the long-term barrier performance of argillaceous host rocks in HLW repositories.