Hyperfiltration-osmosis effects in compacting sediments: Isotopic and chemical changes in pore waters

Abstract

The marine Callovo-Oxfordian clay formation is found at a depth around 410m in the eastern part of the Paris Basin (France). It is a very low permeability formation investigated by the French agency for nuclear waste management (ANDRA) to study the feasibility of a radioactive waste disposal. Examining hydrogeological and geochemical characteristics of the clay sequence may test confinement properties of this formation. This study uses chlorine isotopes to investigate long-term transport processes which may carry chemical elements out of the clay layer to the surrounding rocks. Detailed chlorine concentration and δ37Cl depth profiles are examined using pore waters and aquifer waters sampled in the clay formation and its surrounding aquifers (the Dogger at the bottom and the Oxfordian/Kimmeridgian/Tithonian unit at the top). They are discussed in terms of chlorine budget and hydrogeological processes.Clay pore waters and aquifer waters show strong chlorine concentration depletion (<3000 mg/L) relative to the original marine interstitial water (∼19000 mg/L). This probably results from an early dilution by meteoric water in limestones (as also indicated by oxygen and hydrogen isotopes).A steep Cl-concentration gradient from the Dogger at ∼500m in depth (∼2500 mg/L) to the Oxfordian/Kimmeridgian/Tithonian aquifer near the surface (≈ 10 mg/L) is associated to a ‘v-shaped’ profile of the δ37Cl values. Modelling Cl transport shows that a hydrodynamic dispersion process explains Cl concentration and δ37Cl profiles in Oxfordian Limestone. This process implies a mean upward flux of chloride in the 2.6 10−8–8.2 10−8 mole/m2/yr range from the clay formation towards upper limestones where a westward advective flow disperses the chloride. The modelling and knowledge of underground water transfer suggest a maximum effective Cl-hydrodynamic vertical dispersion coefficient (= vertical Cl-transport coefficient) of ∼7.6 10−10 m2/s.Chlorine transfer through the Callovo-Oxfordian clay, since deposition 160My ago, can be mainly described by the interplay of an early dilution and a later hydrodynamic dispersion event which has apparently erased most of the isotopic effects of diagenetic events (such as early diffusion, ion filtration etc.).