Updating equilibrium calculations to derive reference compositions of Essen and seawater

Ondraf/Niras (O/N) decided to use ThermoChimie as the new reference thermodynamic database to perform geochemical calculations supporting the assessment of the HLW-relevant radionuclide behavior in different scenarios and under different environmental conditions. At SCK CEN geochemical speciation and solubility calculations in the past were however carried out using the in-house developed thermodynamic database ‘MOLDATA’. For a purpose of knowledge transfer some important geochemical calculations done in the past should be updated using the new reference database. This report describes the updated calculations done in 2016 to derive reference porewater compositions of Boom Clay at higher salinity as represented by the Essen porewater and seawater. The ThermoChimie v.10a. database was assigned to be used. Results illustrated that replacing MOLDATA by ThermoChimie does not change the calculated reference water compositions significantly. Nevertheless, new compositions are recommended with (1) a slightly lower pCO2 in seawater due to a lower temperature of 16°C considered and (2) a slightly changed iron concentration due to a changed solubility value of siderite selected by ThermoChimie database. This report also originally aimed at prescribing typical water compositions to be used in experimental activities in support to a defensible knowledge transfer from well-studied systems to less-studied systems that may be relevant during the siting and the preliminary evaluation process.