Solute contaminant transport in variably saturated dual-porosity/dual permeability chalk: field tracer experiments and modelling

Abstract

A detailed field investigation was performed for studying groundwater recharge processes and solute downward migration mechanisms prevailing in the unsaturated zone overlying a chalk aquifer in Belgium. Various laboratory and field experiments were performed, among others tracer experiments in the unsaturated zone. From the experiments performed in the variably saturated chalk, it appears that the migration and retardation of solutes is strongly influenced by recharge conditions. Under intense injection conditions, solutes migrated at high speed along the partially saturated fissures, reaching the saturated zone in a few hours. At the same time, they were temporarily retarded in the almost immobile water located in the chalk matrix. Under normal recharge conditions, fissures were inactive and solutes migrated slowly through the chalk matrix, taking about one year to reach the water table. The tracer experiments performed in the chalk were modelled using the finite element flow and transport simulator SUFT3D. The mathematical representation of the unsaturated properties of the fissured chalk is based on a new modified van Genuchten — Mualem relationship that takes into account the dual-porosity characteristics of the rock. For modelling the transport of the tracers, an adapted first-order transfer, dual-porosity model is used, based on a dynamic partitioning of water between effective and immobile water porosity according to the water saturation of the chalk. Conclusions are drawn in terms of recharge mechanisms and vulnerability of the chalk to pollutions occurring at the land surface.