Hydrogeochemical evaluation for Simpevarp model version 1.2. Preliminary site description of the Simpevarp area

Siting studies for SKB's programme of deep geological disposal of nuclear fuel waste currently involves the investigation of two locations, Simpevarp and Forsmark, to determine their geological, hydrogeochemical and hydrogeological characteristics. Present work completed has resulted in Model version 1.2 which represents the second evaluation of the available Simpevarp groundwater analytical data collected up to April, 2004. The deepest fracture groundwater samples with sufficient analytical data reflected depths down to 1.7 km. Model version 1.2 focusses on geochemical and mixing processes affecting the groundwater composition in the uppermost part of the bedrock, down to repository levels, and eventually extending to 1000 m depth. The groundwater flow regimes at Laxemar/Simpevarp are considered local and extend down to depths of around 600-1000 m depending on local topography. The marked differences in the groundwater flow regimes between Laxemar and Simpevarp are reflected in the groundwater chemistry where four major hydrochemical groups of groundwaters (types A-D) have been identified: TYPE A: This type comprises dilute groundwaters (< 1000 mg/L Cl; 0.5-2.0 g/L TDS) of Na-HCO3 type present at shallow (<200 m) depths at Simpevarp, but at greater depths (0-900 m) at Laxemar. At both localities the groundwaters are marginally oxidising close to the surface, but otherwise reducing. Main reactions involve weathering, ion exchange (Ca, Mg), surface complexation, and dissolution of calcite. Redox reactions include precipitation of Fe-oxyhydroxides and some microbially mediated reactions (SRB). Meteoric recharge water is mainly present at Laxemar whilst at Simpevarp potential mixing of recharge meteoric water and a modern sea component is observed. Localised mixing of meteoric water with deeper saline groundwaters is indicated at both Laxemar and Simpevarp. TYPE B: This type comprises brackish groundwaters (1000-6000 mg/L Cl; 5-10 g/L TDS) present at shallow to intermediate (150-300 m) depths at Simpevarp, but at greater depths (approx. 900-1100 m) at Laxemar. At Simpevarp the groundwaters are mainly Na-Ca-Cl in type but some Na-Ca(Mg)-Cl(Br) types also occur. At Laxemar there is a transition to more Ca-Na-Cl types with depth. Main reactions involve weathering, ion exchange (Ca, Mg) and dissolution/precipitation of calcite. Redox reactions include precipitation of pyrite and some microbially-mediated reactions (SRB). At Simpevarp there is potentially some residual Littorina Sea (old marine) component, commonly in fracture zones close to or under the Baltic Sea. At both the Simpevarp and Laxemar sites there is a glacial component and also a deep saline (non-marine) component. TYPE C: This type comprises reducing saline groundwaters (6000-20 000 mg/L Cl; 25- 30 g/L TDS) present at intermediate to deep (>300 m) levels at Simpevarp, and at even greater depths (approx. 1200 m) at Laxemar. At Simpevarp the groundwaters are mainly Na-Ca-Cl with increasingly enhanced Br and SO4 with depth. At Laxemar they are mainly Ca-Na-Cl also with increasing enhancements of Br and SO4 with depth. Main reactions involve ion exchange (Ca). At both sites a glacial component and a deep saline component are present. At Simpevarp the saline component may be potentially non marine and/or non-marine/old Littorina marine in origin; at Laxemar it is more likely to be non-marine in origin. TYPE D: This type comprises reducing highly saline groundwaters (> 20 000 mg/L Cl; to a maximum of ∼70 g/L TDS) and only has been identified at Laxemar at depths exceeding 1200 m. It is mainly Ca-Na-Cl with higher Br but lower SO4 compared to Type C groundwaters. Main reactions involve water/rock interaction for long residence non-marine brines driven by diffusion. A modelling approach was used to simulate the composition of the highly saline or brine groundwaters and, in the Simpevarp area, concluded that mixing is the main irreversible process. It controls chloride concentration that, in turn, determines the re-equilibrium path (water-rock interaction) triggered by mixing. Coupled transport modelling was used to model the groundwater age, tritium content and calcite dissolution/precipitation processes at shallow groundwater depths at both Laxemar and Simpevarp. The modelled results provide additional support to hydrogeological models by using independent hydrochemical information and added support to the general hydrogeochemical understanding of the site. In this evaluation the groundwater model has been updated, the salinity distribution, mixing processes and the major reactions altering the groundwaters have been modelled down to a depth of 1000 m, and an updated Hydrogeochemical Site Descriptive Model version 1.2 has been produced. More groundwater and isotopic data, together with microbial information, colloids and gases, provided additional site descriptive information. Finally, the introduction of coupled modelling provided additional possibilities to address independently the various processes in question