Sulphate

A depth profile with SO42- data from different methods is shown in Fig. 5-7. The squeezing data cover the range 1000 – 2400 mg/l. In contrast to the profiles of Cl^- and Br^-, the SO42- profile is more scattered, and no unique depth trend with squeezing pressure can be identified. SO4

2-concentrations may evolve in both directions with increasing squeezing pressure, so there is also no unique trend as that of decreasing concentrations identified for Cl^- and Br^-. The sample from the organic-rich Posidonienschiefer (SLA 958.21) has a distinctly higher SO42- concentration.

There is good correspondence with the one value from advective displacement. The SO42- con-centration in the groundwater sample from the 'Brauner Dogger' is only sligthly below the gene-ral trend obtained from squeezing. Based on these comparisons, it is concluded that sulphide oxidation was limited, if present at all in the squeezing experiments.

It is further noted that the SO4/Cl ratio exhibits a systematic increase with increasing squeezing pressure. The increase of this ratio from the lowest at the highest squeezing pressure varies from 10 to 90 %. The processes behind the observed variability of SO42- and increasing SO4/Cl ratio with squeezing pressure are not understood as yet. They may be related to the pressure-induced dissolution of sulphate-bearing phases. The possibility of dissolution of a sulphate phase is discussed below.

Fig. 5-7: Sulphate concentrations for squeezed waters, adevective displacement and ground-water sample.

Comparison of squeezing and leaching data

In the following we compare data at the lowest squeezing pressure and corresponding data from aqueous leachates. As illustrated in Fig. 5-8, there is an apparent mismatch between sulphate data from squeezing compared to leaching data re-calculated to the anion-accessible porosity.

The re-calculated leachate data show about 3 – 8 times higher values than those obtained from squeezed samples. The sulphate data obtained from squeezing are also supported by the advec-tive displacement measurements and the data from the groundwater sample. The range of sulphate concentrations at gypsum saturation is also shown in Fig. 5-8. These were calculated by taking the squeezing data and fixing measured Ca concentration and allowing for gypsum equilibrium. This reveals that extrapolated sulphate concentrations from leaching are signifi-cantly higher than would be expected from gyspum equilbrium. The data also illustrate that squeezed waters are slightly undersaturated with regard to gypsum.

There are different possible reasons for this apparent mismatch or "excess sulphate" derived from leachate data, including:

 Dissolution of a naturally occurring sulphate phase (e.g. CaSO₄ or SrSO₄)

 Oxidation of pyrite (or FeS) before and/or during water extraction tests

 Oxidation of sulphur-bound in organic matter during extraction tests

 Decrease in sulphate concentration during squeezing and advective displacement by micro-bial sulphate reduction

Although precautions were taken to minimise possibilities 2) and 3) (e.g. extraction with degassed water under controlled O2-free atmosphere) these experimental artefacts cannot be completely excluded. Possibility 4) is unlikely, notably in the squeezing tests, and will not be discussed further. Possibilities 1) to 3) are discussed below.

Fig. 5-8: Sulphate profile for squeezed water (200 MPa) in comparison with leachate data re-calculated to porosity = 0.5  WL-porosity and advective displacement data.

Calculated sulphate concentrations in equilibrium with gypsum from 200 MPa squeezing data at measured Ca concentration also shown.

Mass balance considerations

As shown in section 5.1.1, the chloride data obtained from the lowest squeezing pressures represent close to in-situ porewater concentrations. Compared to the sulphate concentrations in water obtained from 200 MPa, squeezing tests sulphate concentrations from aqueous leaching data recalculated to the anion-accessible porosity are higher. The apparent "excess sulphate"

concentrations observed from leachates and when compared to squeezed water can be derived from the difference between sulphate concentrations in the leachate and the squeezing data according to:

 

w sq L

S leach

w C r

SO  C 





:

4 (5-3)

where [SO4] is the "excess sulphate" concentration in mol/kg rock, Cleach, Csq are the concentrations in the leachate and squeezed waters, respectively (mol/l), rS:L is the solid/liquid ratio of the leachate,  is anion-accessible fraction and w_w is the gravimetric wet water content.

Using an anion-accessible fraction of 0.5, the values shown in Tab. 5-2 are obtained. These indicate roughly a range of 1 – 3 mmol sulphate per kg rock, corresponding to a very low pro-portion of the total sulphur measured in the rock (< 0.4 – 5 % of Stot). It should be noted that leaching and squeezing data do not correspond to the same depth and that in some cases may differ by some meters (Tab. 5-2). This adds additional uncertainty to the results.

This simple estimate highlights that the fraction of the sulphate dissolved in addition to that from the "porewater" (taken from squeezing) is small compared to the total inventory. Con-gruent dissolution of a minor sulphate phase (possibility 1) in the leachates could explain this additional sulphate. Such a minor phase would be very difficult to detect by standard methods, such as XRD. It would correspond only to 0.14 – 0.41 g CaSO₄ or 0.18 – 0.55 g SrSO₄ per kg of rock.

Alternatively, the increase could also be explained by oxidation of pyrite (possibility 2), which constitutes the largest sulphur pool. However, the aqueous extractions were performed under reducing conditions and the availability of oxygen is limited (see below). Possibility 3), release of sulphur from organic matter could also be envisioned, but this is unlikely, because from mass balance this would require that the organic matter would contain 10 – 20 % of sulphur. More-over, all this "organic" sulphur would have been oxidised during the experimental procedure.

Tab. 5-2: Mass balance for "excess sulphate" (see text).

Geochemical considerations – dissolution of a sulphate phase

In the following the plausibility of possibilities 1) and 2) are evaluated by geochemical con-siderations and also by modelling of the leachates with S/L = 1 and 0.1. First the hypothesis of dissolution of a sulphate phase is tested. Second, the possibility of oxidation of pyrite during the aqueous extraction and during the preparation procedure is evaluated.

Stratigraphy Lab no sample ID SO₄

sample

no SO₄ [SO₄] [SO₄] m mol/kg rock mol/kg rock mol/kg rock % total S Parkinsoni-Württemb. SLA-7 SLA 778.70 1.99E-03 780.66/2 1.98E-04 1.79E-03 0.6 Humphriesioolith-Fm. SLA-9 SLA 800.00 3.14E-03 796.53/2 2.05E-04 2.93E-03 1.3 Wedelsandstein-Fm. SLA-10 SLA 812.11 1.62E-03 807.51/2 1.19E-04 1.50E-03 1.2 Wedelsandstein-Fm. SLA-11 SLA 816.73 2.03E-03 816.93/2 1.19E-04 1.91E-03 0.4 Wedelsandstein-Fm. SLA-12 SLA 823.53 1.64E-03 825.65/2 1.35E-04 1.51E-03 0.8 Opalinuston SLA-18 SLA 880.30 1.99E-03 878.45/2 1.24E-04 1.87E-03 2.0 Opalinuston SLA-20 SLA 898.31 1.81E-03 896.31/2 1.01E-04 1.71E-03 1.1 Opalinuston SLA-22 SLA 915.67 1.80E-03 915.87/2 1.10E-04 1.69E-03 5.4 Opalinuston SLA-25 SLA 939.48 2.68E-03 937.89/2 1.31E-04 2.55E-03 0.4 Posidonienschiefer SLA-27 SLA 960.38 3.80E-03 958.21/3 * 4.81E-04 3.31E-03 0.6 Psiloceras-Schichten SLA-30 SLA 987.61 1.68E-03 987.40/2 1.54E-04 1.52E-03 1.0

* 300 MPa squeezing pressure

leachate 1:1 squeezing (200 MPa) "excess sulphate"

The geochemical model is based on the following premises:

 Chloride and sulphate originating from porewater are extrapolated from squeezing data to S/L = 1 and S/L = 0.1.

 Cation concentrations (Na, Ca, Mg, K) are controlled by cation exchange.

 Calcite equilibrium is assumed.

 For the sulphate dissolution hypothesis: "excess sulphate2 derived from mass balance origi-nates from dissolution of a CaSO₄ phase. Alternatively, the possibility of dissolution of a SrSO₄ phase is evaluated.

 The CO₂ partial pressure is fixed to a constant level corresponding to atmospheric condi-tions, i.e. pCO₂ = 10^-3.5 bar for the leachates with a S/L ratio of 1 and 10^-4.5 bar for a S/L ratio of 0.1. This somewhat arbitrary assumption is based on the leachate results (sec-tion 4.3). Note that pCO₂ conditions in the leachate tests are not well defined because of degassing of CO₂ produced by calcite dissolution into the N₂ atmosphere of the glovebox during titration and pH measurement.

The calculations were carried out using the PHREEQC version 2 code with the Nagra/PSI database. They involved two steps:

In the first one, the initial in-situ cation-exchange population was estimated from the CEC data presented in section 4.6 and the cation data from the squeezing tests. Cation selectivity coeffi-cients proposed by Bradbury & Baeyens (1997/98) for Opalinus Clay were used. The derivation is detailed in section 5.1.3. The obtained cation-exchange populations for the samples used for squeezing are presented in Tab. 5-6.

In the second step, leaching tests of S/L ratios 1 and 0.1 were simulated using the cation exchange occupancies derived in the first step for the squeezed samples. The following further constraints were imposed:

 The chloride concentration was fixed to the measured one.

 The sulphate concentration was assumed to be made up from two sources: the porewater sulphate determined from squeezing data and a CaSO₄ phase whose amount was determined from "excess sulphate" (see above).

 The initial calcium concentration in mol/l was set to that of the total sulphate, thus the sum of the two sulphate sources.

 The initial sodium concentration was fixed by charge balance.

 Calcite equilibrium was assumed.

 CO2 partial pressure was imposed to logpCO2 = -3.5 for S/L = 1 leachates and -4.5 for S/L = 0.1 leachates.

The results are shown for Na, Ca, Mg, K, alkalinity and pH in Tab. 5-3 (S/L = 1) and 5-4 (S/L = 0.1) and compared with the corresponding measured data. The agreement between calculated and measured data is, generally speaking, remarkably well (within a factor of two) considering the simplicity of the model and the uncertainty related to the model input data, particularly with regard to pH/pCO2 conditions and CEC characteristics. An exception is potassium, where measured values are 2 – 3 times higher than calculated ones. This suggests that either the applied selectivity constant for KX (Bradbury & Baeyens 2000) is too high or there is an addi-tional K source from a dissolving mineral (e.g. Bradbury & Baeyens 1998).

Overall, this modelling exercise indicates that the complete dissolution of a CaSO4 phase is con-ceivable to explain sulphate concentrations noted in the leachates from aqueous extraction. The fact that such a CaSO4 phase has not been detected in the SEM study described in section 4.9 might be explained by its low content, as derived by mass balance ( 0.02 – 0.04 wt.-%).

Diagenetically formed gypsum was observed in a thin section of an Opalinus Clay sample from Mont Terri (Bläsi et al. 1990). Other evidence for gypsum or anhydrite formation is lacking so far. It should be noted that squeezing data are not entirely in line with a gypsum (or anhydrite) source, since they show slight undersaturation with CaSO4 phases in squeezed waters (SIgypsum -0.2 to -0.5) (Fig. 5-8).

Tab. 5-3: Comparison of measured and calculated parameters for S/L = 1 leachates (mol/l except pH).

Measured concentrations refer to average concentrations of the two subsamples.

Tab. 5-4: Comparison of measured and calculated parameters for S/L = 0.1 leachates (mol/l except pH).

Measured concentrations refer to average concentrations of the two subsamples.

The possibility of dissolution of SrSO₄ rather than CaSO₄ has also been tested. The results obtained are basically the same, hence supporting also such as phase as a source for observed sulphate levels in the leachates. If dissolved SrSO₄ is the source in the leachates, then the same amount of Sr should be found therein. The released Sr²⁺ would equilibrate with the exchanger and the Sr inventory would be the sum of dissolved and exchanged Sr. Leachate data indicate such an inventory to be in the range of 0.3 – 0.6 mmol/kg, albeit somewhat lower than the

"excess sulphate2 inventory (1 – 3 mmol/kg). This suggests that the SrSO₄ source is too low to make up for the "excess sulphate" inventory. On the other hand, Ba-Sr sulphate has been detected by SEM anaylsis in some of the studied samples. Squeezing data point to slight under-saturation with regard to celestite, with SI from -0.1 to -0.4. Results from advective displace-ment on the 'Brauner Dogger' sample (Tab. 4-30) on the other hand indicate SI values close to zero for celestite.

Sample ID

calc meas calc meas calc meas calc meas calc meas calc meas calc meas

SLA 778.70 8.79 8.685 1.08E-02 1.07E-02 7.64E-05 4.31E-05 2.71E-05 2.01E-05 1.55E-04 4.30E-04 3.66E-03 3.39E-03 1.99E-03 1.99E-03 SLA 800.01 8.69 8.765 1.27E-02 1.24E-02 1.30E-04 7.85E-05 4.17E-05 2.97E-05 1.93E-04 7.52E-04 2.89E-03 2.91E-03 3.14E-03 3.14E-03 SLA 812.11 8.86 8.705 1.18E-02 1.14E-02 5.69E-05 4.06E-05 1.60E-05 1.88E-05 1.78E-04 3.69E-04 4.35E-03 3.66E-03 1.62E-03 1.62E-03 SLA 816.73 8.75 8.480 1.22E-02 1.19E-02 9.28E-05 6.37E-05 3.05E-05 2.44E-05 1.63E-04 3.99E-04 3.33E-03 2.92E-03 2.03E-03 2.03E-03 SLA 823.53 8.76 8.905 1.10E-02 1.10E-02 8.49E-05 4.25E-05 2.96E-05 1.57E-05 1.55E-04 3.81E-04 3.40E-03 3.34E-03 1.64E-03 1.64E-03 SLA 880.30 8.80 8.635 1.19E-02 1.33E-02 7.52E-05 7.26E-05 2.17E-05 2.71E-05 1.48E-04 3.73E-04 3.74E-03 5.43E-03 1.99E-03 1.99E-03 SLA 898.31 8.84 8.580 1.17E-02 1.32E-02 6.28E-05 6.54E-05 1.78E-05 2.85E-05 1.63E-04 3.37E-04 4.13E-03 5.78E-03 1.81E-03 1.81E-03 SLA 915.67 8.76 8.405 1.09E-02 1.77E-02 8.63E-05 1.62E-04 2.91E-05 7.18E-05 1.50E-04 5.00E-04 3.39E-03 1.10E-02 1.80E-03 1.80E-03 SLA 939.48 8.71 9.030 1.22E-02 1.36E-02 1.15E-04 7.12E-05 3.74E-05 2.58E-05 1.33E-04 3.29E-04 3.02E-03 4.59E-03 2.68E-03 2.68E-03 SLA 960.38 8.73 9.065 1.40E-02 1.40E-02 1.14E-04 1.05E-04 3.24E-05 4.57E-05 1.78E-04 4.03E-04 3.22E-03 3.31E-03 3.79E-03 3.79E-03 SLA 987.61 8.91 8.905 9.48E-03 1.01E-02 4.52E-05 2.53E-05 1.25E-05 1.32E-05 1.25E-04 3.51E-04 4.89E-03 6.43E-03 1.68E-03 1.68E-03

SO4 Alk

pH Na Ca Mg K

sample

calc meas calc meas calc meas calc meas calc meas calc meas calc meas

SLA 778.70 9.53 9.68 3.09E-03 2.64E-03 2.33E-05 1.93E-05 5.26E-06 6.52E-05 6.25E-05 1.60E-04 2.44E-03 2.22E-03 2.23E-04 2.22E-04 SLA 812.11 9.57 9.63 3.46E-03 2.82E-03 1.99E-05 1.02E-05 3.25E-06 2.00E-05 7.26E-05 1.59E-04 2.83E-03 2.66E-03 1.73E-04 1.73E-04 SLA 823.53 9.53 9.57 3.15E-03 2.98E-03 2.29E-05 1.03E-05 5.33E-06 1.34E-05 6.06E-05 1.49E-04 2.47E-03 2.55E-03 1.89E-04 1.89E-04 SLA 915.67 9.51 8.94 3.04E-03 3.70E-03 2.44E-05 2.96E-05 5.38E-06 1.15E-05 5.89E-05 1.75E-04 2.34E-03 3.36E-03 2.04E-04 2.01E-04

SO4 Alk

pH Na Ca Mg K

Geochemical considerations – oxidation of pyrite

The possibility of pyrite oxidation by O₂ has also been explored by geochemical considerations.

This oxidation reaction leads to sulphate, ferric hydroxide and acidity production:

FeS₂ + ¹⁵/₄O₂ + ⁷/₂H₂O  2SO₄^2- + Fe(OH)₃ + 4H⁺ (5-4)

Thus, 3.75 moles of O₂ are required to produce 2 moles of sulphate. The type of geochemical modelling described above has been performed, assuming that the "excess sulphate" is induced by the pyrite oxidation reaction (eq. 5-3). The results obtained (not shown) are very similar to those assuming dissolution of a sulphate phase, except that far more calcite is dissolved to compensate for the acidity production. From a mass balance perspective, roughly 20 – 50 mg O₂ per litre would be required to produce the "excess sulphate" observed. This is much more than is soluble in water. Moreover, the leachate tests were performed in N₂ atmosphere with degassed water free of O₂. Hence, pyrite oxidation during the leaching tests is not a conceivable process to explain sulphate concentrations.

A further possibility is pyrite oxidation during sample preparation. The only preparation step in which rock samples were exposed to air for some time (max. 10 min.) was during milling. The net air volume of the mill filled with the rock sample is 0.26 litres. With an air density of 1.184 g/l at 25 C and an O₂ mass fraction of 0.233, an O₂ content of 2.24 mmol in the mill results. There is about 50 g of rock sample exposed to air during milling. Thus, from mass balance, the amount of produced SO₄ is at maximum 1.2 mmol for 50 g or 24 mmol/kg of rock sample, which would largely be sufficient to explain the "excess sulphate2 in leaching waters.

However, pyrite oxidation is constrained by kinetics. Pyrite oxidation rates and corresponding O₂ consumption rates in air are relatively fast, in the range of about 10^-7 to 10^-8 mol O₂/m²/s (Jerz 2002) (where m² refers to the pyrite surface area). In order to estimate the oxidation rates during milling, the pyrite surface area(s) needs to be estimated. This can be done by assuming spherical particles and the geometrical relationship (Nicholson 1994):

s = 6/dpy (5-5)

where d is the pyrite particle diameter and py is the pyrite density (5.01 g/cm³). The size of the pyrite particles in the sampled sediments is in the µm range according to SEM pictures (e.g. Fig.

4-75). Using a "pessimistic" estimate of 1 µm, a pyrite surface of 1.2 m²/g is obtained. The pyrite content in the rock varies between 0.2 and 2.2 wt.-% (section 4.1.1), for the samples that can be compared to the squeezing samples, hence corresponding to a pyrite surface area of 2 – 41 m² per kg rock. Applying a "pessimistic" rate of 10^-7 mol O₂/m²/s and 10 min. of reaction results in a production of 0.07 – 1.6 mmol sulphate. This is 4.8 – 70% of the "excess sulphate".

This means that even when "pessimistic" assumptions are chosen for both the particle size as well as for the dissolution kinetics, oxidation appears to be too slow to explain observed sul-phate concentrations in leachate waters. On the other hand, from these considerations and the underlying uncertainties it cannot be ruled out that pyrite oxidation during milling has con-tributed to the "excess sulphate" observed to some degree.

The effect of sample preparation on oxidation was checked by preliminary in-house tests, in which Fe(II) and Fe(total) were compared for Schlattingen samples prepared in the glovebox with those prepared in ambient air. The interpretation is rendered difficult because of sample heterogeneity (not the exactly same material was analysed), but nevertheless a trend of increased Fe(II) oxidation with increasing pyrite content for samples exposed to air could be deduced. Further systematic tests are necessary to quantify the effect of sample preparation on pyrite oxidation.

Summary considerations

In conclusion, from all these considerations, dissolution of a sulphate phase is a plausible explanation for the observed sulphate concentrations in aqueous leach solutions compared to squeezed water data. A viable candidate is a CaSO₄ phase, possibly present in some form of solid solution. Squeezing and advective displacement data show slight undersaturation with gypsum, thus such a CaSO₄ phase would have to be slightly more insoluble. SrSO₄ is a further possibility and such a phase (or rather a Sr-Ba sulphate solid solution) has been observed in some of the samples. Ni-extraction data however suggest a too low Sr inventory for quantita-tively explaining "excess sulphate" concentration.

Although considered to be less likely, the contribution of pyrite oxidation during sample prepa-ration (milling) cannot be ruled out.