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Bolundsfjärden

4.4.3 Solute transport

from atmospheric deposition only and relict marine and deep saline water. In /Johansson 2008/, an evaluation of the transport of chloride from Lake Eckarfjärden and Lake Gällsboträsket is reported.

The quantification of chloride transport is based on the hydrochemical analysis in /Tröjbom et al.

2007/ and the hydrological data presented in /Johansson and Öhman 2008/.

The average Cl concentration measured at the outlet of Lake Eckarfjärden is 5.1 mg/L. This chloride concentration corresponds to what can be assumed to originate from atmospheric deposition.

The chloride concentration in the discharge from Lake Gällsboträsket, measured upstream of the conjunction with the brook from Lake Eckarfjärden, is considerably higher (the average is 29 mg/L) and it was concluded in /Johansson 2008/ that an additional source besides atmospheric deposition has to exist.

By using the continuous discharge and electrical conductivity (EC) measurements and the correla-tion between EC and the chloride concentration, daily values of the transport of chloride from Lake Gällsboträsket have been calculated. The average annual chloride transport for the period Dec. 8, 2004-March 31, 2007 was approximately 9,900 kg. The annual atmospheric deposition in the catch-ment area was 1,800 kg, leaving approximately 8,000 kg originating from another source. From the regolith depth and stratigraphy model, the volume of the Quaternary deposits in the Gällsboträsket depression below 2.5 m was calculated, and the total water volume in the Quaternary deposits was estimated from values of the total porosity.

Based on this volume and the mean chloride concentration of c. 2,000 mg/L in the three wells in Quaternary deposits in the depression, the storage of chloride in the Quaternary deposits was estimated at c. 500 tonnes. With the current transport rate, this storage will be depleted in approximately 60 years. Further analysis of the relationship between discharge and hydrochemical composition indicated influence of deep saline water. This, together with the current outflow rate compared with the estimated storage in the Quaternary deposits, raises the question of whether there is an additional source of chloride, i.e. upward flow of deep saline groundwater, in the Gällsboträsket area (see /Johansson 2008/ for a more detailed description of the analysis).

Concentrations of 3H, δ18O and 2H, as well as the concentration of chloride, may contain information on the origin of the sampled groundwater that is not used in the ion-source model. These parameters are plotted on the ion-source model in Figure 4-15. Specifically, the three wells below the lakes Bolundsfjärden (SFM0023), Eckarfjärden (SFM15) and Gällsboträsket (SFM0012) are labelled.

In general, the SFM0012 and SFM0023 wells show similarities for the presented parameters.

SFM0015, below Lake Eckarfjärden, deviates significantly from SFM0012 and SFM0023, with slightly higher 18O and lower 2H excess values as well as a considerably lower chloride concentra-tion. The combined picture is difficult to interpret, but may be a result of a mixed water with component(s) exposed to evaporation.

Figure 4‑15. The parameters 3H, δ18O and 2H as well as the concentration of chloride in the wells in Quaternary deposits, plotted on the ion-source model, with the wells SFM0012, SFM0015 and SFM0023 labeled (modified from /Tröjbom et al. 2008/).

3H (TU)

PC2

-2 0 2 4

PC1

-4 -2 0 2 4 6

18 16 14 12 10 8 6 4 2 0 Littorina

influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

δδ18O (o/oo) SMOW

PC2

-2 0 2 4

PC1

-4 -2 0 2 4 6

-5 -6 -7 -8 -9 -10 -11 -12 -13 -14 Littorina

influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

2H excess (o/oo SMOW)

PC2

-2 0 2 4

PC1

-4 -2 0 2 4

6 8

6 4 2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 Littorina

influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

Cl (mg/l)

PC2

-2 0 2 4

PC1

-4 -2 0 2 4 6

5000 4000 3000 2000 1000 0 Littorina

influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

-2 0 2 4

-4 -2 0 2 4 6

18 16 14 12 10 8 6 4 2 0 Littorina

influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

-2 0 2 4

-4 -2 0 2 4 6

18 16 14 12 10 8 6 4 2 0 Littorina

influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

/

-2 0 2 4

-4 -2 0 2 4 6

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

-2 0 2 4

-4 -2 0 2 4 6

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

-2 0 2 4

-4 -2 0 2 4 6

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

-2 0 2 4

-4 -2 0 2 4 6

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

-2 0 2 4

PC1

-4 -2 0 2 4 6

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

-2 0 2 4

PC1

-4 -2 0 2 4 6

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

Littorina influence

Deep saline influence

Calcite influence Sea water

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

SFM0015

SFM0023 SFM0012

Figure 4‑16. Example illustration of possible solute transport paths to a recipient from a source in the subsurface.

In general, the particle-tracking simulations performed as part of the site descriptive modelling confirm that most flow paths discharge in the sea. However, some differences regarding the extent of deep groundwater discharge on land and horizontal transport distances below the sea can be observed when comparing results from different models /Follin et al. 2008a, Bosson et al. 2008/.

This indicates that the modelling of the horizontal fractures/sheet joints and their interactions with the regolith are associated with uncertainties, which implies that different scenarios need to be considered in the forthcoming safety assessment.

Figure 4-16 illustrates another uncertainty related to the near-surface part of the flow paths, namely the role of the Quaternary deposits-bedrock interface. If upward, vertical transport through the upper bedrock takes place, the flow paths may continue along the Quaternary deposits-bedrock interface and/or vertically through the Quaternary deposits. As indicated in the figure, this implies that different flow paths in the Quaternary deposits (i.e. spreading associated with spatial and temporal variability in flow) and discharge points in different parts of the surface water system are possible.

Comparison of results from slug tests in till in groundwater monitoring wells installed across the Quaternary deposits-bedrock interface and in Quaternary deposits only shows that K-values are higher in the interface region. Furthermore, the hydrochemical evaluation summarised above

indicates that the groundwater is more or less stagnant below some of the lakes in the Forsmark area, i.e. in areas that in absence of the horizontal fractures/sheet joints would be considered as typical discharge areas. These observations indicate that the Quaternary deposits-bedrock interface could Figure 4‑17. Conceptual illustration of transport domains at the Forsmark site.

Till layer

Bedrock surface

Horizontal sheet joints (ground water flow paths) Middle layer

Biomass

Upper layer

Major deformation zones

Engineered barrier Minor deformation

zones

Immediate far field Till layer

Bedrock surface Middle layer

Biomass Upper layer

Till layer

Bedrock surface Middle layer

Biomass

Upper layer

Till layer Bedrock surface

Middle layer Biomass Upper layer

Terrestrial Mires Lake/brook Marine

Surface water flow paths

Bedrock

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.