Hydrochemistry

• Outside the tectonic lens, for example at drill site 4 and in the area around Lake Eckarfjärden, the groundwater level in the bedrock may be well above the groundwater levels in Quaternary deposits in nearby low-lying areas, implying that flow systems involving the bedrock may have local discharge areas.

• The lake water level-groundwater level relationship indicates that the lake sediments and the underlying till have low vertical hydraulic conductivities. If the hydraulic contact had been good, the situation with groundwater level drawdown from evapotranspiration extending below the lakes, and the quick and extensive drawdowns from the pumping would not have appeared.

Figure 4-12 shows the groundwater levels in wells in Quaternary deposits and bedrock at drill site 1 located within the tectonic lens. The groundwater levels in the Quaternary deposits are above those in the rock except during the summers of 2003 and 2006. Figure 4-13 shows the groundwater levels in a monitoring well in till below the middle of Lake Bolundsfjärden (SFM0023) and in a nearby percussion-drilled borehole (HFM32) located on a small island in the lake. Water levels in the sea and in the lake are also shown in the figure. The lake level and the groundwater level in till are considerably higher than the levels in the four sections of HFM32. The heads are lowest in the two deepest sections. The results indicate a downward flow gradient from the lake and the Quaternary deposits to the bedrock.

In /Tröjbom et al. 2007/, the method of principal component analysis (PCA) was applied to establish an ion-source model for the Forsmark data. In a first step, only data from percussion-drilled and core-drilled boreholes were used to optimise a model for separating the groundwater types found in the bedrock. This model was then applied to observations of surface water and groundwater in Quaternary deposits, revealing similarities in hydrochemical composition between these observa-tions and the main patterns found in the groundwater in the bedrock.

In Figure 4-14, the ion-source model developed in /Tröjbom et al. 2007/ is shown for labelled moni-toring wells in Quaternary deposits with different possible ion sources and possible groundwater types. Five major water types are identified:

• Modern sea water.

• Water with influence of relict marine water (Littorina).

• Deep saline water significantly influenced by shield brine (shield brine is a highly saline groundwater present at great depths in the granitic environment of the Scandinavian Shield).

• Altered meteoric water of meteoric origin, but significantly altered by processes in the Quaternary deposits.

• Freshwaters include both surface water and shallow groundwater, showing “immature” ion signatures from biogenetic CO2 and calcite dissolution.

Most of the samples are classified as belonging to the fresh water and altered meteoric groundwater groups. The wells placed in till below lakes and the sea show quite different chemical compositions.

In general, the waters from these wells have a high salinity, including high chloride content. The well below Lake Gällsboträsket (SFM0012) and the wells located in the Gällsboträsket depression (SFM0011 and SFM0013) are classified as belonging to the influence from relict marine water group Figure 4‑13. Water level in the Baltic Sea and Lake Bolundsfjärden plotted together with groundwater levels in till below the lake (SFM0023) and in sections in the bedrock borehole HFM32 (Depth in m RHB 70: HFM32:1: –198.75 to –96.27; HFM32:2: –95.27 to –30.95; HFM32:3: –29.95 to –24.97;

HFM32:4: –23.97 to +0.97).

-1,00 -0,80 -0,60 -0,40 -0,20 0,00 0,20 0,40 0,60 0,80 1,00

06-02-01 06-04-02 06-06-01 06-07-31 06-09-29 06-11-28 07-01-27 07-03-28

Date

m RHB70

Sea level Lake level HFM32:1 PWH HFM32:2 PWH HFM32:3 PWH HFM32:4 PWH SFM0023

The well in the middle of Lake Bolundsfjärden (SFM0023) also belongs to the influence from relict marine water group and has a chloride concentration of c. 3,775 mg/L. The chloride concentration is approximately the same as in the nearby percussion-drilled borehole HFM32 down to a depth of c. 100 m. Also, the well SFM0022 below Lake Fiskarfjärden shows a chemical signature clearly influenced by relict marine water. The water from the well below Lake Eckarfjärden (SFM0015), however, shows a quite different chemical composition and is in the ion-source model closest to the group altered meteoric water. The chloride content in the SFM0022 well is c. 300 mg/L.

The occurrence of water belonging to the group influenced by relict marine water (Littorina) below Lake Bolundsfjärden, Lake Fiskarfjärden and Lake Gällsboträsket is a strong indication of very low flow rates in the flow systems involving these parts of the regional site investigation area. In the perspective of the total annual water balance of the area, the water can be considered as stagnant.

At Lake Bolundsfjärden, no flow from below reaches the till at present according to the groundwater levels measured in the Quaternary deposits and the bedrock (see Figure 4-14). Furthermore, the water composition indicates that the leakage from the lake through the sediments must be very small.

The hydrogeological and hydrochemical interpretations indicate that shallow groundwater flow systems involving only Quaternary deposits have discharge areas around the lake and in the near-shore parts of the lake, while deeper systems are drained by the highly transmissive shallow bedrock.

The chemistry of the water flowing out of Lake Eckarfjärden and Lake Gällsboträsket gives an opportunity to investigate to what extent relict marine water and deep saline groundwater are present in the flow systems generating surface discharge. Chloride is considered as a good tracer since it follows the water, and due to the big difference in concentrations between water containing chloride Figure 4‑14. The ion-source model showing labelled monitoring wells in Quaternary deposits with different possible ion sources and possible groundwater types (modified from /Tröjbom et al. 2007/, where a detailed description of the model is given). Note that FM and two zeros have been deleted from the well numbers, and that the last part of the well number is the mean elevation (m RHB 70) of the sampling interval (therefore, it is preceded by either “+” or “–“).

S01-4

S02-3 S03-9

S05+4 S06+2

S08-2 S09+2

S11-2 S12-3 S13-1

S14+4 S15-2

S16-2 S17+2

S18+1

S19-1 S20-2

S22-5

S23-4 S24-3

S25-6

S26-15 S27-6

S28-7 S29-7 S30-2

S31-2

S32-2 S34-1

S36-1 S37-2

S49-1S51-3 S53-6 S56-3

S57+0

S59-1 S60-3

S62-3

S63-3

S65-4 S74-3

S82-4

-4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5

-3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 5.5

PC2

PC1

KFM HFM SFM P well Lake Stream Sea

%Na %K

%Ca

%Mg %Li

%Sr

%HCO3

%Cl

%SO4

%Br

%F

Shield brine Influence from

relict marine water (Littorina) Sea water

Altered meteoric groundwater

Fresh waters Weathering products,

Biogenic CO2

Calcite

Influence from deep saline groundwater Modern

sea water

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.