Weathering profile in a near-surface aquitard : example of the teguline Clay, East Paris Basin, France

Abstract : Near-surface aquitards are targeted for the containment of low-level radioactive waste (LLW) in near-surface environments because they could act as natural low-permeability protective barriers for underlying groundwater resources. Few detailed mineralogical and geochemical studies have been dedicated to surficial clay formations (Hendry and Wassenaar, 2000), compared to the abundance of data made available for reduced clay formations foreseen for deep nuclear repository systems. Their study is however of primary interest for the near-surface waste storage. The objectives of this study were (1) to describe the distribution of major ions in pore waters of a 80 m-thick clay-rich aquitard near the surface, and (2) to identify geochemical processes controlling the distribution of these ions. These objectives were completed by measurements of several parameters which are currently used to model pore water chemistry in deep reduced aquitards: (1) mineralogy and diagenetic sequence, (2) water content and porosity, (3) cation exchange capacity and cation distribution on the clay exchanger as well as the determination of anion concentrations (Cl and SO4) through milliQ leaching, (4) partial pressure of CO2, O2, and N2.