Cation exchange parameters for Opalinus Clay and its confining units

Abstract

The knowledge of cation exchange parameters is important to understand and model the porewater chemistry in clayrocks. The Opalinus Clay (OPA) is a well-studied formation in view of its importance as host rock for radioactive waste repositories, but uncertainties exist regarding its cation exchange parameters, in particular its selectivity coefficients (Kc). In fact, porewater chemistry models for OPA have so far been based on generic selectivity coefficients.Thanks to a recent extensive siting programme, involving eight vertical deep boreholes in northern Switzerland, a large number of porewater and cation exchange data from the OPA and its clay-rich confining units could be analysed. Thus, from the combination of porewater compositions (obtained from high-pressure squeezing and advective displacement), and exchangeable cation data (obtained from Ni ethylenediamine extraction), reliable selectivity coefficients could be derived. The values for KcNa/K, KcNa/Ca, KcNa/Mg are consistent with those obtained in parallel from the Mont Terri underground rock laboratory although the scatter is somewhat larger in the latter dataset. The derived selectivity coefficients indicate no dependency on ionic strength or in-situ temperature. Minor differences in Kc values between OPA and confining units can be attributed to differences in clay mineralogy. The validity of a two-site cation exchange model including illite and smectite for estimating exchangeable Na, Ca and Mg from measured porewater compositions could be shown, while this simple model underpredicts the share of exchangeable K. The exchangeable cation extraction procedure induces the dissolution of Sr-bearing minerals whose amount is in the same range as exchangeable Sr, thus not enabling the derivation of selectivity coefficients for Sr.Finally, derived selectivity coefficients confirm results of previous studies, thus providing confidence in the applied experimental and modelling protocols on the one hand and in the outcome of those studies on the other hand.