Vertical distribution of HTO and 125I− transport parameters in Boom Clay in the Essen-1 borehole (Mol, Belgium)

• We examine the transport properties over the entire depth of the Boom Clay layer in Essen.
• We measure the porosity and apparent diffusion coefficient for tracers HTO and iodide.
• The values of the transport parameters in Boom Clay are compared at three sites: Essen, Mol and Doel.

In the frame of the R&D activities performed on the Boom Clay for assessing the suitability of deep clayey formations for radioactive waste disposal, the transferability of the scientific results obtained on the Boom Clay in Mol to the whole Campine Basin is investigated. Boreholes were drilled at different locations (e.g. Mol, Doel, Essen) and cores were sampled over the entire thickness of the Boom Clay formation on which the migration parameters for iodide and tritiated water (HTO) are determined.At Essen, the transport parameters in the Boom Clay can be considered as homogeneous in the range from 159 m to 241 m Below Drilling Table. The average hydraulic conductivity is (5.4 ± 1.7) × 10−12 m/s. The average ηR value for iodide is 0.25 ± 0.03 and 0.42 ± 0.05 for HTO. For HTO, this high value is mainly due to a higher value in the Putte Member (0.46 ± 0.03) compared to the other members (0.39 ± 0.02). The apparent diffusion coefficient is (1.3 ± 0.1) × 10−10 m2/s for HTO and (1.1 ± 0.2) × 10−10 m2/s for iodide. The expected effect of ionic strength (increasing with depth) on the ηR value of iodide is of the same size as the measurement error, what might explain why it was not observed.On a lateral (horizontal) level, the hydraulic conductivity at the Essen-1 borehole (5.4 × 10−12 m/s) lies between that of Boom Clay in Mol-1 (2.5 × 10−12 m/s) and that of Boom Clay in Doel-2b (1.4 × 10−11 m/s). For iodide, the higher ηR value in Essen-1 and Doel-2b (ηR ≈ 0.25) than in Mol-1 (ηR ≈ 0.16) can partly be explained by a higher ionic strength of the pore water. Apart from the Putte Member at Essen-1, the HTO porosities of the Terhagen Member and the Transition zone in Essen are in the range of the average porosities observed in Mol and Doel (ηR ≈ 0.37–0.39). For both iodide and HTO, the value of the apparent diffusion coefficient Dapp is similar in Mol-1 and in Doel-2b, with a clearly higher value for HTO than for iodide. In Essen-1, the apparent diffusion coefficients for iodide and HTO are nearly equal, and slightly smaller than the iodide value in Mol-1/Doel-2b. Accordingly, the HTO apparent diffusion coefficient is considerably smaller in Essen-1 than in Mol-1/Doel-2b.