4 Results
4.12 Iodide diffusion
The results obtained by the radiography method are listed in Tab. 4-40. Diffusion coefficients all refer to the direction normal to bedding.
Tab. 4-39: Isotope composition of rocks and vein minerals.
Calcite [wt%]
Sheet silicate + accessories [wt%]Sr (ppm)Sr (ppm) normalized to calcite content
87Sr/86SrSr (ppm)87Sr/86SrSr (ppm)87Sr/86Srδ18O (‰VPDB)δ18O (‰VSMOW)δ13C (‰VPDB)δ18O (‰VPDB)δ18O (‰VSMOW)δ13C (‰VPDB) SLA 60OMM1490.708048-4.4626.260.51 SLA 90OMM1910.709050-5.6825.00-0.22 SLA 120OMM2750.708898-7.922.72-2.68 SLA 150USM3050.709855-6.4524.21-4.4 SLA 300USM2180.709479-7.7722.85-2.17 SLA 450USM2180.708981-8.1522.46-2.8 SLA 520Plattenkalk2490.707481-4.9425.771.61 SLA 610Massenkalk /Quaderkalk830.707315-6.6224.041.5 SLA 700Wohlgesch. Kalke2090.707362-3.9726.772.7 SLA 750Effinger Schichten94.02.38160.707588-3.7926.952.46-11.519.011.55 SLA 750.84Effinger Schichten67.026.53254850.707285-2.2128.582.81 SLA 755.83Birmenstorf-Sch. and Glaukonit- Sandmergel3990.707774-9.8520.712.25 SLA 757.24Birmenstorf-Sch. and Glaukonit- Sandmergel6290.707802-9.4621.111.4 SLA 760.65Wutach-Fm.4480.707884-14.0716.36-0.23 SLA 761.65Variansmergel-Fm.2040.707461-2.827.970.96 SLA 765.88Variansmergel-Fm.5260.707917-13.4616.98-0.44 SLA 769.11Parkinsoni-Württembergica-Sch.37.033.23480.708110-6.0724.601.48-9.8820.680.58 SLA 778.82Parkinsoni-Württembergica-Sch.26.042.72238580.707796-4.9125.800.87 SLA 783.87Parkinsoni-Württembergica-Sch.68.026.1544210.707900-3.4627.29-2.27-12.1618.32-0.45 SLA 800.01Humphriesioolith-Fm.33.035.52487510.707537-5.1225.582.06 SLA 839.85Opalinuston10.066.1888800.708837-3.6927.06-1.33 SLA 872.12Opalinuston22.030.41557020.708008-9.3621.21-1.62 SLA 915.67Opalinuston14.054.81349580.708267-5.5825.110.82 SLA 939.48Opalinuston11.063.911210210.708315-5.9624.72-1.88 SLA 951.00Jurensismergel46.038.73968610.707534-4.1426.590.66 SLA 952.33Jurensismergel93.04.619080.708511-2.528.281.04-9.4521.120.2 SLA 957.7Posidonienschiefer35.052.02080.708471-6.6623.991.34-9.6920.870.71 SLA 961.78Posidonienschiefer21.048.522510700.708090-6.124.57-1.44 SLA 1020Stubensandstein-Fm.3190.708870-3.5827.17-5.59 SLA 1080Gipskeuper4790.708574-5.0825.62-3.76 Vein calciteSr in vein calcite + celestite (HCl leach)Sr in vein calcite (HCl leach) Sample IDLithostratigraphic unit
Sr in whole rock carbonate (acetic acid leach)XRD + CS-MatWhole rock carbonate (acetic acid leach)
NAGRA NAB 12-54 160 Tab. 4-40:Summary of experimental data of iodide diffusion tests with the radiography method. Depth [m]UnitArtificial porewater recipe [mol/l]Tracer NaI [mol/l]Dp [m2 /s]σa [m2 /s]Ιb [-]wb [-]Ι/w [-] 735.22 Effingen MemberNaCl: 0.0724 Na2SO4: 0.1214 CaSO4: 0.0045
0.46
1.8E-10 4.7E-110.035 0.078 0.44 741.60 Effingen Member1.6E-105.8E-120.045 0.111 0.41 762.56 Variansmergel Fm. 3.7E-101.2E-110.070.143 0.49 780.53 Parkinsoni-Württemb. Beds NaCl: 0.0992 Na2SO4: 0.0382 CaSO4: 0.0016
0.22
3.2E-10 4.8E-110.058 0.150 0.39 795.95 Humphriesioolith Fm. 2.0E-102.8E-110.042 0.069 0.61 808.89 Wedelsandstein Formation 3.5E-101.4E-110.066 0.124 0.53 825.53 Wedelsandstein Formation 3.6E-101.5E-110.070 0.204 0.342 857.80 Opalinuston NaCl: 0.212 KCl: 0.002 MgCl2: 0.017 CaCl2: 0.026 SrCl2: 0.001 Na2SO4: 0.014 NaHCO3: 0.005
0.39
9.3E-11 5.8E-120.066 0.139 0.48 857.80 repl. Opalinuston6.3E-114.2E-120.057 0.139 0.41 896.43 Opalinuston5.3E-115.2E-120.057 0.122 0.47 916.82 Opalinuston5.1E-111.5E-120.063 0.132 0.47 929.22 Opalinuston6.0E-114.6E-120.046 0.133 0.34 954.35 Posidonienschiefer4.0E-11n/a 0.020 0.045 0.45 954.35 repl. Posidonienschiefer3.0E-117.1E-120.018 0.045 0.40 963.70 Numismalis-Amaltheen Beds1.0E-108.2E-120.049 0.121 0.41 977.64 Obtusus Beds6.2E-112.9E-120.038 0.133 0.29 a The standard deviations for Dp values are determined by fitting the analytical solution for 1-D diffusion to multiple time-series profiles of relative tracer concentration. As such, they represent internal variations in the diffusion properties of a single sample. b I = iodide-accessible porosity, (w) = water-loss porosity.
Data evaluation: experience from Mont Terri samples
The radiography method is constantly under development and until recently it had only been applied to indurated samples from the Michigan Basin in North America. In 2010 the team began working with Opalinus Clay samples from Mont Terri (BDR experiment) and shortly afterward the Schlattingen samples were received. The Opalinus clay samples from Mont Terri were difficult to prepare and displayed minor crumbling near the influx boundary due to swelling. This caused concern that the Dp and I (iodine accessible porosity) results obtained from the unconfined measurement cell (Fig. 3-5) could be biased. Although there are not a lot of comparable data, it appears now that the radiography data for Dp and I from Mont Terri are biased high. The Dp values (mean normal to bedding is 2.0 × 10-10 m2/s and mean parallel to bedding is 4.6 × 10-10 m2/s) are approximately two to three times higher than expected, and iodide-accessible porosity values (0.11) are also slightly higher than expected.
Data evaluation: Schlattingen samples
The Schlattingen samples were received just when the potential for a swelling-related artefact in the Mont Terri sample data was beginning to be realised. Swelling and sample deterioration was not observed with the radiography samples from Schlattingen, but minor swelling was noted during the re-saturation stage of the water-loss porosity measurements. In cases where the entire length of the sample fits within the field of view for the radiographs it is possible to measure changes in sample dimensions over time. This was done with four of the Schlattingen samples (SLA 857.80, SLA 929.22, SLA 963.70, SLA 977.64). Measurements indicate that swelling occurred over 2 to 3 days following installation in the cell, and the maximum swelling of 3.3 % was observed in the Opalinus Clay. The measured data are presented in Tab. 4-41. No swelling was detected in the radial dimension.
Tab. 4-41: Measurements of sample swelling parallel to the core axis (= normal to bedding).
Sample SLA 857.80 SLA 929.22 SLA 963.70 SLA 977.64
Increase in length 3.3 % 2.8 % 2.2 % 3.0 %
Comparison of water-loss porosities in Opalinus Clay samples from Schlattingen
At the University of New Brunswick, the radiography and water-content analyses were conducted on immediately adjacent but not identical rock materials. Spatial heterogeneity of the samples on the cm scale is subordinate, thus the obtained water-loss porosities are thought also to be representative for the adjacent materials used for radiography. Prior to the water-content measurement, the rock material was immersed in artificial porewater. The rationale for this was to reproduce as closely as possible the porosity of the rock material that was used for radio-graphy. In the setup that was applied, the latter material was contacted with water reservoirs on both ends without mechanical confinement, and this essentially corresponds to immersion. Five samples of Opalinus Clay were studied and yielded an average water-loss porosity of 13.3 ± 0.7 vol.-%. The corresponding value for 14 water-content measurements by RWI Bern (Tab. 4-5) is 10.6 ± 0.8 vol.-%. The difference is due to the fact that the RWI samples were not in contact with an external water reservoir prior to the first weight measurement, and they are thought to closely represent the in-situ wet water content. The higher values obtained by the University of New Brunswick are most probably caused by swelling, consistent with the observed change in sample length.Comparison of diffusion coefficients for Opalinus Clay samples from Schlattingen
Once the issue of swelling became evident, two samples of Opalinus Clay were also subjected to classical through-diffusion experiments. The experimental setup was similar to that applied at PSI, with the exception that the cell was made of plastic materials instead of steel. This cell clearly lessened swelling but still provided less mechanical confinement than a steel apparatus.The experimental results are listed in Tab. 4-42. A comparison of the resulting De values with those obtained from radiography yields that the former are only marginally smaller, and that the cell pressures are low when compared to the tests conducted at PSI (4 – 15 MPa). It is con-cluded that the plastic cells have a limited stiffness that still allows some swelling to occur.
Data from PSI using samples from Schlattingen are not available at this stage for direct com-parison. Data from Benken (summarised in Nagra 2002) yield De(36Cl) = 8E-13 m2/s and De(125I) = 5E-13 m2/s. In comparison with these values, the radiography data from Schlattingen are 4 – 6.5 times higher, and the through-diffusion data are 3.5 – 5.5 times higher. However, these comparisons need to be interpreted cum grano salis because the underlying data base is limited and does not refer to the same borehole. Better constraints are expected once PSI data for Schlattingen samples become available.
It is also worth noting that the PSI data represent tracer diffusion coefficients for 36Cl and 125I, whereas the data from the University of New Brunswick are counter-diffusion coefficients that reflect diffusion of bulk I in one and Cl in the other direction. However, given the fact that there is no net salinity gradient across the samples and because the diffusion coefficients for I and Cl and similar (at least in free water), it is concluded that tracer- and counter-diffusion coefficients should be comparable and so cannot explain the observed differences.
Tab. 4-42: Results of radiography and through-diffusion tests on samples of Opalinus Clay conducted at the University of New Brunswick.
Sample ID Dp (I) from