SR-Site - sulphide content in the groundwater at Forsmark

Sulphide concentrations in groundwater play a key role in the long-term reliability of the metal canisters containing the radioactive waste within a disposal facility for nuclear waste. This is because sulphide in the groundwaters circulating in the vicinity of the deposition tunnels can react with copper in the canisters causing corrosion and therefore reducing their expected lifetime; in a worst case scenario erosion of the bentonite buffer material will expose the canister more rapidly to the fracture groundwater.Sulphide in the groundwater is predominantly microbially produced and thereby controlled by the content of oxidised sulphur sources, organics (carbon sources), reductants (mainly Fe(II), DOC, H2 and CH4), and also flow and mixing of different groundwater types. In addition, achieved saturation in respect to amorphous Fe-monosulphide will control the possible maximum values and will also limit the Fe2+ and S2- values in the groundwater. The aim of this report is to assess realistic, representative and reliable sulphide groundwater concentrations at present conditions in Forsmark and also to evaluate possible changes during different climatic conditions covering the repository operation period (some tens to hundreds of years), post closure conditions (some thousand of years) and the proceeding temperate period (some tens of thousands of years) which may be extended due to enhanced greenhouse effects etc. It is expected that this period will be followed by the onset of the next glaciation during which periglacial (permafrost), glacial and postglacial conditions may succeed each other. To achieve these aims, an evaluation is performed of all the sulphide-related data reported from the Forsmark site investigations /Laaksoharju et al. 2008/ and later monitoring campaigns, all of which are stored in the Sicada database. This evaluation shows that values from the Complete Chemical Characterisation (CCC) sampling are usually lower than those measured during the monitoring phase. When both CCC and monitoring values are available from one borehole section two values are used, one representing each of the sampling methods. When time series are measured in the monitoring sections, in most cases the sulphide values are higher in the downhole tubing and in the borehole section compared with groundwater from the surrounding fractures. The routinely carried out removal of five borehole section water volumes prior to monitoring are in most cases inadequate to obtain representative samples from the bedrock fractures, i.e. without elevated sulphide due to contamination from the initial stagnant water from the borehole section. It is, however, assumed that the two groundwater samples from the same water conducting structures, i.e. the sample from the monitoring and the sample from the CCC sampling, the latter usually showing lower sulphide, spans the interval of representative sulphide values in the formation water. For most of the CCC and monitoring sections the last sample in the time series is suggested as representing the 'best possible' value. It is worth noting that the plug flow calculations have proven to be very useful in order to judge the water volumes needed to be pumped at each monitored section before obtaining a sample sufficiently representative of the fracture groundwater. Practically all sulphide concentrations at Forsmark are ≤ 0.013 mmol/L (∼0.4 mg/L). There is, however, a probability that for some deposition location in the repository the surrounding groundwaters may have sulphide concentrations as high as 0.12 mmol/L (∼4 mg/L). In order to estimate canister corrosion rates in the SR-Site during a full glacial cycle, it is recommended that the observed distribution of sulphide in groundwaters at Forsmark during the present temperate conditions should be used. Furthermore, it should be noted that the present sulphide concentrations have been sampled after a recent period of possible intrusion of marine sulphate-rich waters.