Numerical Examinations of Transient Chemical Osmosis Effects on Groundwater Flow System

The current key issues in radioactive waste disposal are the uncertainties of fluid pressure anomalies which could be caused by chemical osmosis, and the influences of chemical osmosis on the groundwater system in a geological time scale. The possibility of chemical osmosis and the magnitude of potential osmotic pressures have recently been studied for argillaceous rocks and it has been suggested that some rocks can act as effective semi-permeable membranes and can generate osmotic pressures in the orders of 10 MPa under appropriate conditions. This study focuses on the sustainability of chemical osmosis in geological formations. The duration time of osmotic pressures and the net fluid flux, including chemically-induced and pressure-driven fluxes, were examined by a numerical approach. Assuming laterally extending sedimentary formations containing a semi-permeable layer, 1-D hydrogeological models were used in the numerical simulation. The governing equations for the chemical-hydraulic coupled-transport process were normalized using a couple of dimensionless quantities, consisting of hydraulic and diffusive parameters, and the reflection coefficient representing the ability as semi-permeable membrane. A series of numerical examinations showed that the osmotic pressure in the semi-permeable layer could last for the geologic time scale and the duration time depends mainly on the diffusive parameters. The increase of the Peclet number shortens the time to reach the quasi-steady state, where the chemically-induced and pressure-driven fluxes are almost equilibrating. In the cases with common boundary conditions, once the quasi-steady states are made, the dimensionless osmotic pressure distributions are almost same despite the difference in the values of dimensionless quantities. In addition, the apparent fluid flux calculated from the fluid pressure distributions tend to be larger than the net fluid flux approximately by a factor of the ratio of hydraulic diffusivity to apparent diffusion coefficient. The knowledges obtained from this study will be useful for preliminary estimations of the sustainability of chemical osmosis based on the field observations of fluid pressure and salinity distributions. This research project has been conducted under the research contract with the Nuclear and Industrial Safety Agency (NISA).