The significance of flow in the matrix of the Chalk unsaturated zone

Abstract

The significance of flow in the matrix of the Chalk unsaturated zone, in comparison with flow in fractures, has been the subject of much debate. In this article, important elements of the literature are discussed in detail and several simple modelling analyses based on steady-state flow are presented. A study of the sensitivity of solute spreading to fracture spacing in models that ignore matrix flow shows that this latter assumption is generally incompatible with observed solute profiles, unless unrealistically small fracture spacings are assumed. The effect of air phase continuities (e.g. bedding planes) on matrix flow has also been examined. These discontinuities are frequently interrupted by points of connectivity between matrix blocks. An issue therefore is the relationship between connectivity and its effect on inter-block conductance. A simple analysis of the Laplace equation shows that just 1% connectivity represents an effective pathway equivalent to 18% of the local rock hydraulic conductivity. Obviously, when there is no fracture flow, solute spreading is significantly reduced. However, dual permeability model simulations show that matrix flow reduces solute spreading in the presence of persistent fracture flow. All of the above studies suggest that flow in the matrix of the Chalk unsaturated zone is significant and that ignoring it may result in a serious misunderstanding of the system.