Spatial variability of transport parameters in the Boom Clay

Abstract

Several transport parameters (as hydraulic conductivity K, apparent diffusion coefficient D p and diffusion accessible porosity η of HTO and iodide) have been intensively measured in the laboratory on high-quality cores taken at the Mol-1 borehole of the Mol site, in Belgium. The borehole was cored in 1997 from about 145 to 325 m depth, including the whole thickness of the Boom Formation, a Tertiary clay situated between 186 and 288 m depth (ground level), and part of the surrounding layers. The hydraulic conductivity measurements confirm the low permeability of the Boom Clay. An upper 90-m-thick clay layer within this formation can be considered as homogeneous with respect to the hydraulic conductivity. The vertical hydraulic conductivity K v (i.e. K perpendicular to the bedding) is in the order of magnitude of 10 −12 m s −1 and the average is 2.3×10 −12 m s −1. This layer comprises from top to bottom the “Transition Zone”, the Putte Member, the Terhagen Member and the top of the Belsele-Waas Member of the Boom Formation. The 12 m at the base of the Formation, which corresponds to the lower part of the Belsele-Waas Member is characterised by larger K v values (ranging between 10 −11 and 9×10 −11 m s −1). The same thick clay layer can also be considered as homogeneous, regarding the values of the apparent diffusion coefficient and the diffusion accessible porosity η of tritiated water (HTO) and iodide. The average value of the diffusion accessible porosity is 0.37±0.03 for HTO and 0.16±0.02 for iodide. The apparent diffusion coefficient varies from 1.1×10 −10 to 5.5×10 −10 m 2 s −1 for HTO and from 9.1×10 −11 to 5.2×10 −10 m 2 s −1 for iodide.