Abstract Diffusion-driven changes in pore water salinity are prevalent in natural environments. We show that the electrical conductivity of saturated porous rocks at a given salinity depends on the history of salinity change, that is, whether salinity was reached by increasing or decreasing concentration. Understanding this salinity-induced conductivity hysteresis is crucial for accurate geophysical interpretation of subsurface processes. We measure complex conductivity spectra across sixteen diatomite and chalk samples subjected to both increasing and decreasing pore water salinity. For a given salinity, in-phase conductivity is up to ~120% higher on the decreasing salinity path. The hysteresis is minor above 3 S/m bulk water conductivity but becomes increasingly pronounced below that threshold, where bound water increasingly contributes to conduction. Our data indicate that the hysteresis arises from distinct equilibrium states of the electrical double layer for each salinity path, not from kinetics. The magnitude of the consequent surface conductivity hysteresis scales with Archie's m-exponent and the fraction of pore space occupied by bound water. In contrast, the induced polarization (IP) response, characterized by the Cole-Cole parameters of relaxation time, distribution exponent, and normalized chargeability, shows minimal dependence on salinity history. This stability indicates a decoupling between conduction and polarization, where IP timescale and distribution are governed by pore geometry, while its magnitude reflects the equilibrium state of the interface. Using the equilibrium surface conductivity at high- and low-frequency limits, we introduce a new method to estimate the bound water fraction involved in polarization. The analysis shows that ~9–26% of the bound water contributes, with greater spatial extent along the increasing salinity path. This trend reflects that adsorbed ions become more hydrated during the increasing salinity path, and more dehydrated during the decreasing salinity path. These findings provide a new framework for interpreting geophysical data in variable-salinity environments.
Proestakis et al. (Thu,) studied this question.