Chemical composition of squeezed waters

Water samples were analysed by ion chromatography at RWI, Uni Bern. Major-ion composi-tions of squeezed waters are listed in for squeezing pressures of 200, 300, 400 and 500 MPa.

The water type evolves from Na-Cl at 200 MPa to Na-(Ca)-Cl-(SO4) at 500 MPa. While the concentrations of Na⁺, K⁺, Cl^-, Br^-, F^- and NO3- decrease with pressure, Ca²⁺ Mg²⁺, Sr²⁺ and SO42- remain constant or even slightly increase (Fig. 4-49). As shown in Tab. 4-21, average ion concentrations at 500 MPa are 0.7 – 0.8 times those at 200 MPa for Na⁺, Cl^- and Br^-, 0.34 times for K⁺, and 1.05 – 1.3 times for Ca²⁺, Mg²⁺, Sr²⁺ and SO42-. These differences are probably related to the size- and charge-dependent ion mobility in the nano-scale pore network and probably due to chemical reactions (e.g. pressure-dependent mineral dissolution).

Fluoride and nitrate exhibit unexpected high concentrations for the 200 MPa solutions. With increasing pressure, these compounds display a strong initial increase followed by fairly constant values (F^-) or decreasing close to the detection limit (NO3-). This behaviour is not shown by the other measured compounds. This suggests contamination of fluoride and nitrate by the filter equipment. Note also that squeezed waters are also strongly oversaturated with regard fluorite (see below).

Tab. 4-20: Chemical composition of waters squeezed at 200, 300, 400 and 500 MPa.

Sample IDSub- samplePressureStratigraphyNaKCaMgSrFClBrNO3SO4AlkalinitypH MPamg/lmg/lmg/lmg/lmg/lmg/lmg/lmg/lmg/lmg/lmeq/l SLA 780.66780.66/2200Parkinsoni-Württemb.-Fm.3321.475.0423.3106.315.512.35133.66.14.31283.4 SLA 780.66780.66/33003256.750.0451.5116.918.95.45080.66.02.51435.818.527.97 SLA 780.66780.66/44002576.928.7463.6121.517.94.14171.75.0<1.61334.83.87.89 SLA 780.66780.66/55002125.721.1448.2124.718.03.63538.54.3<1.61209.2 SLA 796.53796.53/2200Humphriesioolith-Fm.3350.679.7487.2111.719.09.35319.66.14.71390.3 SLA 796.53796.53/33002909.551.2494.0111.019.04.34645.25.8<1.61357.8 SLA 796.53796.53/44002662.833.2491.2113.620.83.34310.25.0<1.61404.7 SLA 796.53796.53/55002360.825.4489.4112.917.83.33876.94.6<1.61323.8 SLA 807.51807.51/2200Wedelsandstein Fm.4605.9108.8482.199.417.424.96966.58.011.71273.5 SLA 807.51807.51/33003824.465.5564.6115.716.98.66037.27.33.01347.0 SLA 807.51807.51/44003323.842.3628.9128.519.46.25525.06.8<1.61347.4 SLA 807.51807.51/55004325.198.2576.5113.518.07.76251.47.74.52205.57.44 SLA 816.93816.93/2200Wedelsandstein Fm.3525.076.6497.9111.417.410.85685.86.25.51210.1 SLA 816.93816.93/33003000.243.2533.1123.821.05.54999.75.71.91188.9 SLA 816.93816.93/44002533.028.8559.5134.622.24.04415.75.0<1.61152.83.317.91 SLA 816.93816.93/55002254.822.1575.4136.022.63.94118.44.71.41108.6 SLA 825.65825.65/2200Wedelsandstein Fm.3389.777.1535.1125.821.710.45482.66.04.11298.8 SLA 825.65825.65/33003238.351.3554.4124.222.94.75315.26.11.71362.2 SLA 825.65825.65/44002705.630.2559.4113.823.33.34519.95.2<1.61289.0 SLA 825.65825.65/55002316.922.4574.5116.223.93.24052.94.9<1.61217.9 SLA 878.45878.45/2200Opalinuston4061.880.5514.3105.417.112.06351.36.512.11323.8 SLA 878.45878.45/33003937.562.3592.8134.921.57.86372.86.85.11426.4 SLA 878.45878.45/44003398.433.8681.6177.822.54.15917.76.41.91338.26.547.88 SLA 878.45878.45/55003326.126.2675.0183.524.03.35810.16.3<1.61369.8

Tab. 4-20: (continued)

Sample IDSub- samplePressureStratigraphyNaKCaMgSrFClBrNO3SO4AlkalinitypH MPamg/lmg/lmg/lmg/lmg/lmg/lmg/lmg/lmg/lmg/lmeq/l SLA 896.31896.31/2200Opalinuston4568.9100.6528.4108.017.216.07356.37.18.9981.6 SLA 896.31896.31/33003901.761.9685.8157.622.48.86549.06.73.91349.4 SLA 896.31896.31/44003460.438.1719.3167.623.84.65950.75.91.61408.26.37.99 SLA 896.31896.31/55003085.626.9755.1175.821.04.05484.45.5<1.61360.2 SLA 915.87915.87/2200Opalinuston3563.777.1553.5127.718.513.65820.55.65.41162.9 SLA 915.87915.87/33003882.260.7590.9141.920.56.46323.46.22.41393.37.91 SLA 915.87915.87/44003309.833.5605.3145.320.94.15509.25.4<1.61400.0 SLA 915.87915.87/55002842.624.7626.2148.620.43.64852.54.8<1.61337.5 SLA 937.89937.89/2200Opalinuston3620.461.6579.8132.318.68.95725.95.93.01572.9 SLA 937.89937.89/33003029.834.3609.5137.818.95.44855.95.1<1.61592.34.957.94 SLA 937.89937.89/44002401.319.3637.9143.119.84.14051.44.2<1.61503.3 SLA 937.89937.89/55001991.914.1669.1148.719.33.93559.43.5<1.61412.97.92 SLA 958.21958.21/3300Posidonienschiefer4221.483.1519.8113.015.913.86058.46.05.32178.47.91 SLA 958.21958.21/44003593.049.5628.8121.920.25.25223.95.4<1.62291.4 SLA 958.21958.21/55002983.533.6639.8119.216.14.54399.64.5<1.62148.87.9 SLA 987.40987.40/2200Psiloceras-Schichten2340.547.2164.534.4<1013.02408.02.64.51929.6 SLA 987.40987.40/33001791.623.6163.430.7<106.81747.52.0<1.61734.8 SLA 987.40987.40/44001390.516.1210.541.4<105.91426.2<1.6<1.61539.6 SLA 987.40987.40/5500979.010.1233.446.3<105.41077.6<1.6<1.61236.48.09

Fig. 4-49: Evolution of ion concentrations in squeezed waters as a function of pressure for sample SLA 896.31 (Opalinus Clay).

Tab. 4-21: Ratios of ion concentrations in waters squeezed at 500 MPa over those squeezed at 200 MPa.

Sample ID Unit Na⁺

[-] K⁺ [-] Ca²⁺

[-] Mg²⁺

[-] Sr²⁺

[-] F -[-] Cl

-[-] Br -[-] SO4

2-[-] Cl/Br [-]

SLA 780.66 Parkins.-Württ. Beds 0.64 0.28 1.06 1.17 1.16 0.29 0.69 0.71 0.94 0.97 SLA 796.53 Humphriesiool. Fm. 0.70 0.32 1.00 1.01 0.94 0.36 0.73 0.76 0.95 0.96 SLA 807.51 Wedelsandstein Fm. 0.94 0.90 1.20 1.14 1.04 0.31 0.90 0.97 1.73 0.93 SLA 816.93 Wedelsandstein Fm. 0.64 0.29 1.16 1.22 1.30 0.36 0.72 0.76 0.92 0.96 SLA 825.65 Wedelsandstein Fm. 0.68 0.29 1.07 0.92 1.10 0.31 0.74 0.82 0.94 0.90 SLA 878.45 Opalinuston 0.82 0.33 1.31 1.74 1.40 0.27 0.91 0.97 1.03 0.94 SLA 878.45 Opalinuston 0.82 0.33 1.31 1.74 1.40 0.27 0.91 0.97 1.03 0.94 SLA 896.31 Opalinuston 0.68 0.27 1.43 1.63 1.22 0.25 0.75 0.78 1.39 0.96 SLA 915.87 Opalinuston 0.80 0.32 1.13 1.16 1.10 0.27 0.83 0.87 1.15 0.96 SLA 937.89 Opalinuston 0.55 0.23 1.15 1.12 1.04 0.44 0.62 0.60 0.90 1.03 SLA 958.21 Posidonienschiefer n.d.* n.d.* n.d.* n.d.* n.d.* n.d.* n.d.* n.d.* n.d.* n.d.*

SLA 987.40 Psiloceras Beds 0.42 0.21 1.42 1.35 n.d. 0.42 0.45 n.d. 0.64 0.00

Average 0.70 0.34 1.20 1.29 1.17 0.32 0.75 0.82 1.06 0.87

* not water obtained at 200 MPa

Compositional trends and mineral saturation states

The chemical data for all samples are given in Tab. 4-20. The composition for all waters is dominated by sodium and chloride. Sulphate and Ca are the second most important anion and cation, respectively. Because of small sample volumes, only a few measurements of alkalinity could be performed. These analyses are inflicted by rather large analytical uncertainties because of small sample volumes and moreover are affected by the presence of organic acids (see below). Therefore, alkalinity data should be viewed with care. To some extent, this holds also for pH measurements which may have been affected by CO₂ degassing.

Speciation, pCO₂ and saturation state calculations performed in an analogous manner as for the leachates (see section 4.3) are presented in Tab. 4-22. Because of the scarce alkalinity and pH measurements, calculations for pCO₂ and saturation states of carbonate minerals can only be performed for rather few samples. Note in particular that no such data are available for the water squeezed at the lowest squeezing pressure of 200 MPa. As outlined in the methods section, total alkalinity data is affected by low molecular-weight organic acids (LMWOA), particularly by acetate and formate, for which semi-quantitiave data from IC analysis are available. Using these data, carbonate alkalinity was roughly estimated by the difference between total alkalinity and the sum of LMWOA, whose fraction is 15 – 78 % of total alkalinity (Tab. 4-22). Subsample 780.66/3, for which no clear equivalence point could be detected, exhibits a very high alkalinity value. The more reasonable value of subsample 780.66/4 was selected instead for speciation calculations.

Considerable oversaturation for most waters squeezed at higher pressures for both calcite and dolomite is noted (SIcalcite 0.3 to 1.0, SIdolomite = -0.3 to 1.2). Possible reasons for this feature are (i) CO2 degassing in the sample vials during storage, (ii) pressure-dependent increase of solubility of carbonate minerals and (iii) pressure solution effects leading carbonate mineral deformations. The last two points are discussed below. The waters exhibit conditions close to equilibrium with strontianite (SI = 0 to 0.6).

The waters are slightly undersaturated with gypsum (SI -0.7 to -0.2) and are closer to saturation with celestite (SI -0.4 to 0.0). Note that the waters squeezed at the lowest pressure, hence at conditions affected the least by filtration and pressure effects, systematically display the lowest SI values for these sulphate phases. The near-zero saturation indices and the behaviour of sulphate with increasing squeezing pressure suggest control by some sulphate phase, possibly present as solid solution (see section 5.1.2).

The waters are strongly oversaturated with fluorite (by up to 2 SI units). This feature is possibly the consequence of the release of fluoride from filter materials (see above).

Tab. 4-22: Calculated pCO2 and saturation indices for all squeezed samples.

Labels /2, /3, /4 and /5 refer to squeezing pressures of 200, 300, 400 and 500 MPa.

* Alkalinity contribution from low molecular weight organic acids (see text).

Sample IDSubsampleStratigraphypHAlkAlklogSISISISISISI noLMWOA*CarbonatepCO2calcitedolomite(dis)gypsumcelestitestrontianitefluorite meq/lmeq/lbar SLA 780.66 780.66/2Parkinsoni-Württemb.-Sch.-0.53-0.211.28 SLA 780.66 780.66/37.97-0.45-0.100.60 SLA 780.66 780.66/47.892.161.64-3.030.590.41-0.43-0.11-0.360.40 SLA 780.66 780.66/5-0.45-0.120.39 SLA 796.53 796.53/2Humphriesioolith-Fm.-0.45-0.121.08 SLA 796.53 796.53/3-0.42-0.110.40 SLA 796.53 796.53/4-0.40-0.040.15 SLA 796.53 796.53/5-0.40-0.110.17 SLA 807.51 807.51/2Wedelsandstein-Fm.-0.54-0.261.93 SLA 807.51 807.51/3-0.43-0.221.12 SLA 807.51 807.51/4-0.37-0.160.82 SLA 807.51 807.51/57.44-0.26-0.030.99 SLA 816.93 816.93/2Wedelsandstein-Fm.-0.50-0.231.26 SLA 816.93 816.93/3-0.46-0.130.77 SLA 816.93 816.93/47.912.580.73-3.420.34-0.13-0.42-0.09-0.600.46 SLA 816.93 816.93/5-0.42-0.080.48 SLA 825.65 825.65/2Wedelsandstein-Fm.-0.45-0.101.20 SLA 825.65 825.65/3-0.41-0.050.61 SLA 825.65 825.65/4-0.39-0.040.20 SLA 825.65 825.65/5-0.38-0.030.23 SLA 878.45A 878.45/2Opalinuston-0.49-0.231.33 SLA 878.45A 878.45/3-0.41-0.121.01 SLA 878.45A 878.45/47.883.862.68-2.840.911.06-0.36-0.11-0.100.49 SLA 878.45A 878.45/5-0.36-0.080.20

Tab. 4-22: (continued)

Sample IDSubsampleStratigraphypHAlkAlklogSISISISISISI noLMWOA*CarbonatepCO2calcitedolomite(dis)gypsumcelestitestrontianitefluorite meq/lmeq/lbar SLA 896.31 896.31/2Opalinuston-0.62-0.371.58 SLA 896.31 896.31/3-0.38-0.141.16 SLA 896.31 896.31/47.993.762.54-2.981.011.20-0.32-0.070.000.61 SLA 896.31 896.31/5-0.31-0.130.51 SLA 915.87 915.87/2Opalinuston-0.49-0.221.50 SLA 915.87 915.87/37.91-0.41-0.150.76 SLA 915.87 915.87/4-0.38-0.100.44 SLA 915.87 915.87/5-0.36-0.120.47 SLA 937.89 937.89/2Opalinuston-0.36-0.111.12 SLA 937.89 937.89/37.943.541.41-3.170.650.47-0.30-0.08-0.390.71 SLA 937.89 937.89/4-0.28-0.050.49 SLA 937.89 937.89/57.92-0.26-0.080.52 SLA 958.21 958.21/3Posidonienschiefer7.91-0.30-0.081.43 SLA 958.21 958.21/4-0.190.050.62 SLA 958.21 958.21/57.90-0.17-0.040.65 SLA 987.40 987.40/2Psiloceras-Schichten-0.66-0.151.00 SLA 987.41 987.40/3-0.65-0.130.50 SLA 987.42 987.40/4-0.55-0.150.51 SLA 987.43 987.40/58.09-0.54-0.180.44

Consideration of pressure-driven mineral dissolution effects

Contrary to the monovalent cations Na⁺ and K⁺, the divalent Ca²⁺, Mg²⁺ and Sr²⁺ exhibit an increasing trend with pressure. This hints at pressure-dependent dissolution phenomena. This was evaluated by preliminary numerical modelling using the FLOTRAN code (Lichtner 2007), as described in Appendix G. In short: The model was applied to one dataset of the Opalinus Clay (SLA 878.45) and simulated the squeezing experiment with a simple linear pressure-time relationship rather than the actually conducted step-wise pressure increase. Note that in these calculations it is implicitly assumed that the pore pressure is equal to the applied squeezing pressure, which is not necessarily the case. Further, the observed decrease in chloride with increasing pressure was approximated by a simple dilution relationship by which the total water content was kept constant. No electrostatic effects were included. In a first step, a simple cal-cite-only system was considered. The second step involved more complex chemistry including cation exchange reactions. The outcome of the modelling exercise indicated the following:

 Assuming equilibrium conditions, the simulated concentrations cannot explain the observed ones. Notably, the predicted Ca, Mg and SO₄ concentrations are too high.

 Considering mineral kinetics, the simulations follow the same trends as the experimental data. They predict dissolution of calcite, dolomite and – in case of its presence – of a sul-phate phase (possibly gypsum). At pressures of about 320 MPa conversion from calcite to aragonite is predicted. However, applying constant mineral reaction rates, the Ca and Mg concentrations are still overpredicted.

 The observed data may be explained by a decrease of mineral reactivity and/or of transport efficiency over time.

In conclusion, the preliminary modelling exercise suggests that pressure-driven dissolution of carbonates (calcite, dolomite) occurred in the squeezing experiments. However, the observed effects are smaller than predicted with a simple thermodynamic or kinetic model. It should be also emphasised that the actual pore pressures generated by squeezing are not known.

Chloride and sulphate data at 200 MPa

The samples squeezed with the lowest pressure (i.e. 200 MPa) are considered to be the most representative of porewater conditions and thought to be least affected by filtration effects (see below) and pressure-dependent dissolution.

Chloride concentrations increase with depth, with values of about 5400 mg/l for the uppermost analysed sample at 780.66 m (Variansmergel Formation) depth to about 7400 mg/l at 896.31 m (Opalinus Clay). Chloride shows a decrease for the lowermost sample, reaching about 2400 mg/l at 987.40 m depth (Psiloceras Beds). The chloride data are illustrated in Fig. 4-50a.

Sulphate data indicate rather constant concentrations (1200 – 1300 mg SO4/l) down to a depth of about 900 m from where, contrary to chloride, an increase with depth is noted, reaching about 2000 mg/l at 987 m depth (Fig. 4-50b). No clear correlation of sulphate data with total sulphur content in the rock can be seen (not shown).

(a) (b)

Fig. 4-50: (a) Chloride and (b) sulphate concentrations vs. depth for squeezed waters at 200 MPa.