2014:48 Technical Note, Independent assessment of groundwater sulphide content in the long-te

Groundwater sulphide content is a key factor in the determination of canister lifetimes because of its role in corrosion of copper.  SKB has based the canister lifetime analysis on a distribution of measured sulphide concentrations which are assumed to apply throughout the long-term analysis period.  The distribution is based on expert judgement in the selection of samples that are regarded as representative.  The objective of this report is to make an assessment of the validity of SKB’s approach. 

Sulphide in groundwater derives from biogeochemically-mediated redox transformation of sulphate.  Sulphide concentrations in groundwaters with normal microbial activities, dissolved iron concentrations and organic carbon contents are generally lower than 10-4 mol.dm-3 because of low sulphate reduction kinetics and/or because of control of dissolved sulphide concentrations by iron sulphide equilibrium.  Equilibrium modelling shows that measured sulphide contents are below saturation levels for iron monosulphide, so either reduction rate is controlling concentrations or another iron sulphide phase is involved.