Pore water colloid properties in argillaceous sedimentary rocks

Area of Science:

• Geochemistry
• Environmental Science
• Hydrogeology

Background:

• Colloids in pore water are critical for understanding radionuclide transport in geological repositories.
• Opalinus Clay and other sedimentary rocks are potential host formations for radioactive waste in Switzerland.
• Direct measurement of colloids in undisturbed host rock pore water is challenging.

Purpose of the Study:

• To evaluate the nature, concentration, and size distribution of colloids in the pore water of potential Swiss radioactive waste repository host rocks.
• To develop predictive models for colloid behavior based on field and laboratory data.
• To assess the relevance of colloids for radionuclide transport in these geological environments.

Main Methods:

• Utilized predictive modeling based on extensive field and laboratory studies.
• Employed single particle counting techniques for experimental quantification of colloid size distribution.
• Analyzed colloid concentrations in relation to hydrogeochemical parameters and analogue systems.
• Conducted laboratory batch experiments on clay colloid generation and suspension evolution.
• Developed and applied a novel colloid distribution model incorporating generation, aggregation, and sedimentation rates.

Main Results:

• Predictive modeling successfully estimated colloid nature, concentration, and size distributions.
• Groundwater colloid concentrations were quantified experimentally and analyzed against geochemical factors.
• Laboratory experiments showed that colloidal suspension size distribution evolves towards a common state, dependent on attachment factors.
• The study identified factors influencing colloid concentrations and size distributions under in-situ conditions (batch vs. fracture).

Conclusions:

• Clay colloid concentrations in the pore water of considered sedimentary host rocks are predicted to be very low (<1 ppb for 10-100 nm particles).
• The low concentration of clay colloids suggests a restricted role in radionuclide transport within these specific geological formations.
• The findings contribute to the safety assessment of radioactive waste repositories by characterizing colloid behavior in relevant host rocks.