Determination of the Cation Exchange Capacities and Exchangeable Cations of Deep Drilling Core Samples from the Siting Regions Jura Ost, Nördlich Lägern, and Zürich Nordost
Baeyens, B. & Fernandes M. M. (2022): Determination of the Cation Exchange Capacities and Exchangeable Cations of deep Drilling Core Samples from the Siting Regions Jura Ost, Nördlich Lägern and Zürich Nordost. Nagra Arbeitsbericht NAB 21-01.
Opalinus Clay, an argillaceous sedimentary rock formation, is selected as the potential host rock for the deep geological disposal of radioactive Waste in Switzerland. In Stage 3 of the Sectoral Plan (Sachplan Geologisches Tiefenlager, Etappe 3) (Swiss Federal Office of Energy 2008), Nagra is thoroughly investigating the three remaining siting regions, namely Jura Ost (JO), Nördlich Lägern (NL) and Zürich Nordost (ZNO), where Opalinus Clay is present and meets the requirements related to its thickness and depth. In addition to 3D seismic measurements and quaternary boreholes, deep borehole investigations have the aim to complete the already existing overall picture of the underground geological environment in these regions. The outcome of these investigations and the safety-based comparison of the siting regions will contribute to guide Nagra's choice of the region best suited for a deep geological repository.
Site-specific physico-chemical rock data such as cation exchange capacities (CEC) and exchangeable cation occupancies are, amongst other geochemical and mineralogical parameters, part of the deep borehole investigations and are required for each potential region for the site selection procedure. This report focuses mainly on the CEC of the rock samples and on the cation occupancies of exchangeable alkaline and alkaline earth elements. The CEC is a measure of the total negative charge of the rock samples, resulting predominantly from the presence of 2:1 phyllosilicates on which cations adsorb on the planar sites via electrostatic bonding. CEC can also be defined as the sum of exchangeable cations at the clay mineral surfaces of the rock samples. The CEC is directly correlated to the content and type of clay minerals in the rock samples, and consequently to their adsorption properties towards radionuclides. Its variability is an excellent indicator for the homogeneity of the clay mineral content along the Opalinus Clay formation as well as to the upper and lower confining units for each borehole. The in-situ fractional occupancies of exchangeable cations, defined by the quantity of adsorbed cation (in meq/kg) divided by the CEC (in meq/kg) of the rock samples, can act as a “fingerprint” for the in-situ porewater compositions of the formation. In general, the composition of the porewater of rock formations consists essentially of solutes such as Cl resulting from the hydrologic evolution of the rock, and of solutes resulting from equilibrium with the minerals constituting the rock. In argillaceous rocks, the concentrations of dissolved cations are largely determined by cation exchange equilibria with the clay mineral fraction and by the equilibrium with carbonate and sulphide/sulphate minerals.
The objective of this work, mandated by Nagra, is to quantify the CEC and the pool of exchangeable cations of rock samples from 7 boreholes across the geological siting regions Jura Ost (JO), Nördlich Lägern (NL), and Zürich Nordost (ZNO). The samples were selected to represent the different lithologies of the natural barrier system, i.e., Opalinus Clay and the upper and lower confining units. 141 rock samples have been analysed with the focus on CEC and cation exchange occupancies. This report gives a detailed overview of the results obtained for the representative rock samples from Bözberg-1, Bözberg-2, Bülach-1, Stadel-2, Stadel-3, Trüllikon-1 and Marthalen-1.