Study of the heterogeneity of hydraulic properties in a chalk aquifer unit, using sequential pumping and tracing experiments with packer systems

Chalk geological formations generally represent strategic aquifers for drinking water supply. In Belgium, more than 20% of abstracted groundwater volumes are from chalk aquifers. From a hydrogeological perspective, chalk is a complex medium. The high porosity value enables the storage of large quantities of groundwater, while fast preferential flow is mainly observed in fractures. The characterization of groundwater flow and solute transport in this dual-porosity medium is crucial for many current issues, such as the set-up of protection zones around pumping areas, or the assessment of the nitrate concentrations’ future evolution. Due to the complexity of the rock medium, these problems remain difficult in chalk aquifers, and the quantification of the related parameter values is still challenging. Consequently, these parameters are often considered as homogeneous over the aquifer units, and used in equivalent porous media models. The objective of this study is to assess the actual hydraulic spatial heterogeneity of the fractured-porous chalk aquifer, with in-situ experiments performed in two adjacent 50 m-deep wells, located in the Mons Basin area (South-West Belgium). Sequential tracer and pumping tests have been performed using inflatable packer systems to isolate and study specific 1-metre sections of the boreholes. The tests have been compared to similar tests carried out using the whole borehole depth. Results have been interpreted in terms of hydraulic conductivity values and fracture apertures, using numerical modelling. They have been correlated with recordings made with a borehole camera system, which has evidenced the presence of different types of fractures with a mean density of about 1 fracture per metre. The different types of fractures have also been correlated with observations, made on similar outcropping rock formations. Results confirm the high heterogeneity of the flow and transport properties at the scale of the studied chalk unit.