Summary description of the surface system at Forsmark

of retention properties, including mineralogy and geochemistry, are presented in /Hedenström and Sohlenius 2008/. Based on this information and on hydrochemical data available in the Forsmark data freeze 1.2, /Grandia et al. 2007/ evaluated potential retention processes for a set of selected radionuclides. The result of the process evaluation is shown in Table 4-4.

In /Grandia et al. 2007/, the conceptual retention process models for different radionuclides were also implemented in a numerical model. Results of test simulations of transport of uranium, strontium and caesium in till and clay systems are reported in /Grandia et al. 2007/. In short, these results demonstrate that the regolith may provide significant radionuclide retention, but also that predictions of its retention capacity can be highly sensitive to assumptions made in the development of the process models.

The candidate area is characterised by a low altitude and is almost entirely located below 20 m.a.s.l.

Till is the dominant Quaternary deposit and granite is the dominant rock type. The annual precipita-tion and runoff are 560 and 150 mm, respectively. The main lakes are Fiskarfjärden, Bolundsfjärden, Eckarfjärden and Gällsboträsket. The lakes are shallow with a maximum depth ranging from 0.4 to 2 m. Seawater flows into the most low-lying lakes during events of very high seawater levels.

Wetlands are frequent and cover between 25 and 35% of some of the delineated sub-catchments.

There are no major water courses in the candidate area. The brooks downstream of Lake Gunnarsbo-träsket, Lake Eckarfjärden and Lake Gällsboträsket carry water most of the year, but can be dry for long periods during dry years such as 2003 and 2006. Substantial parts of the brooks in the area have been artificially deepened for drainage purposes.

Measurements show that the horizontal hydraulic conductivity and specific yield of the till are typical of, or slightly higher than, those of the surrounding region. Groundwater levels in Quaternary deposits are very shallow; on average less than 0.7 m below ground surface for 50% of the time.

Shallow groundwater levels imply a strong interaction between evapotranspiration, soil moisture and groundwater. Diurnal fluctuations of the groundwater levels, driven by evapotranspiration cycles, are evident in many groundwater wells. Furthermore, groundwater level measurements in the vicinity of the lakes show that the lakes may act as recharge sources to till aquifers in the riparian zone during summer.

There is a close correlation between the topography and the groundwater levels in the Quaternary deposits. For groundwater levels in the upper bedrock there is no such strong coupling to the topography. This is most evident in the central part of the candidate area, where the groundwater level gradients in the bedrock are very small, indicating a high transmissivity. Here, the groundwater levels in the till are often considerably higher than in the bedrock. This implies that local, small-scale recharge and discharge areas, involving groundwater flow systems restricted to Quaternary deposits, overlie the more large-scale flow systems associated with groundwater flow in the bedrock. In the middle of Lake Bolundsfjärden, located in the central part of the study area, the lake water level and the groundwater level in till are considerably higher than the levels in the bedrock down to 200 m depth. This indicates a downward flow gradient from the lake and Quaternary deposits to the bedrock.

The surface water and shallow groundwater at Forsmark is characterised by high pH-values and high contents of major constituents, especially calcium and bicarbonate /Sonesten 2005, Tröjbom and Söderbäck 2006/. The main reason for this is glacial remnants, mostly in the form of a till layer, which were deposited during the Weichselian glaciation and deglaciation /Fredén (ed) 2002/. This till layer, originating from the sedimentary bedrock of Gävlebukten about 100 km north of Forsmark, has a rich content of calcite.

The marine ecosystem in the Forsmark area is a relatively productive coastal area in a region of otherwise fairly low primary production. This is due to up-welling along the mainland /Eriksson et al. 1977/. The surface water has a nutrient concentration ranging from 330 to790 µg/L total nitrogen and 12 to 25 µg/L total phosphorous. The seabed consists mostly of erosion and transport bottoms with heterogeneous and mobile sediment, mainly sand and gravel with varying fractions of glacial clay. Some areas of the seabed close to the mainland have rocky bottoms, partly covered with coarse till. The results from modelling show that although most parts of the area are heterotrophic, the mean of the whole model area is autotrophic, i.e. more carbon is fixed in biomass by primary producers than is released by all organisms /Wijnbladh et al. 2008/.

The characteristics of the limnic ecosystem in the Forsmark regional model area are, to a major extent, determined by the small topographic gradients in combination with the ongoing shoreline displacement, the close proximity to the sea, and by the occurrence of calcium-rich deposits. The lakes are classified as oligotrophic hardwater lakes, i.e. they contain high calcium levels, but low levels of nutrients, as phosphorus is precipitated together with the calcium. This kind of lake is

The terrestrial vegetation is, among other things, affected by the bedrock, the Quaternary deposits and human land use. The Quaternary deposits are mainly wave-washed till, on which conifer forests are common. In depressions, a deeper regolith layer is found, with fairly high calcium content. The calcareous influence is typical for the north-eastern part of Uppland and is manifested in the flora.

The Forsmark area has a long history of forestry, which is seen today as a fairly high percentage of younger and older clear-cuts in the landscape. Wetlands occur frequently and cover 10–20% of the area in the three major catchments and up to 25–35% in some sub-catchments /Johansson et al.

2005/. A major part of the wetlands are coniferous forest swamps and open mires. Arable land, pastures and clear cuts dominate the open land. Arable land and pastures are found close to settle-ments. Many pastures were earlier intensively used but are today a part of the abandoned farmland, following the nationwide general regression of agricultural activities /Löfgren (ed) 2008/.

In conclusion, the surface system site description has increased the general understanding of how the site functions in terms of properties in different volumes, main functional units, processes and system descriptions arising from different scientific disciplines. The initial conceptual model, developed at the beginning of the work in 2002, has been adjusted to site-specific features, and site data have been used to describe flows and accumulation of matter in and between different parts of the surface system at Forsmark.

The site description will be used in safety assessments to achieve input on properties and processes, and on overall system conceptualisation and understanding. The main supporting background reports developed to be used as input to the safety assessment are:

• Surface system Forsmark, Site descriptive modelling, SDM-Site Forsmark (R-08-11).

• Geological evolution, palaeoclimate and historical development of the Forsmark and Laxemar-Simpevarp areas, Site descriptive modelling, SDM-Site (R-08-19).

• The terrestrial ecosystems at Forsmark and Laxemar, Site descriptive modelling, SDM-Site (R-08-01).

• The limnic ecosystems at Forsmark and Laxemar, Site descriptive modelling, SDM-Site (R-08-02).

• The marine ecosystems at Forsmark and Laxemar, Site descriptive modelling, SDM-Site (R-08-03).

• Hydrochemistry of surface water and shallow groundwater, Site descriptive modelling, SDM-Site Forsmark (R-07-55).

• Description of surface hydrology and near-surface hydrogeology at Forsmark, Site descriptive modelling, SDM-Site Forsmark (R-08-08).

• Description of the regolith at Forsmark, Site descriptive modelling, SDM-Site Forsmark (R-08-04).

The present site description covers all information available on the surface system at Forsmark. The wealth of information, both in the form of data from the site investigations and of conceptual and numerical models, comprises a comprehensive foundation for the environmental impact assessment (EIA). Main SDM references for the EIA are the ecosystem reports covering the biotic and abiotic descriptions of the Forsmark surface system. The hydrology and the properties of the Quaternary deposits are also available, together with the chemical description of Forsmark waters, soils and biota. No description on nature values is made in the SDM, but all relevant information, e.g. the occurrence of threatened or vulnerable species, collected during site investigations is stored in the SKB databases.

A number of properties and models, beside the general description, are available to use as input to the design of the potential repository. Important information for planning the above-ground facilities is contained in the reports describing Quaternary deposits (regolith), surface hydrology, and the digital elevation model.