Aquitards are rocks that hinder movement of groundwater. They fulfil an important role in protecting groundwater from contamination and are also considered as host rocks for radioactive waste repositories. They also act as natural seals for oil and gas reservoirs. Clay-rich sedimentary or crystalline aquitards have been one of our prominent research topics over the past 25 years.
We have developed a number of novel experimental, analytical and numerical methods to study the geochemistry of these rocks and their porewaters. One of the main goals is to understand the transport of solutes, as well as chemical and radioactive contaminants within the scope of radioactive waste disposal. Further goals are to unravel the history of fluid flow and to identify the long-term sealing capacity of aquitards for CO2 storage. A significant part of our research involves international studies in Underground Research Laboratories, such as the Mont Terri Rock Laboratory, the Grimsel Test Site, the French Bure site, the Swedish Äspö Hard Rock Laboratory, and the Finnish site at Olkiluoto.
Minerals precipitated in the rock matrix during diagenesis or hydrothermal alteration, as well as fracture infilling minerals, are archives of past fluid-flow events.
A cardinal prerequisite of any porewater investigation is a sample, such as a piece of drillcore, that represents the in‑situ conditions as closely as possible. This requires approaches in the field that exclude or at least reduce sampling induced artefacts such as evaporative loss of porewater or degassing of dissolved gases by minimizing the exposure time of the drillcore to air.
Pore waters in aquitards reflect the past geochemical evolution of the adjacent aquifers. Solute transport in aquitards occurs predominantly via diffusion and is therefore slow.
Faults and joints are present in most aquitard units and were formed either during tectonic deformation events or as a consequence of decompaction. In aquitards rich in clay minerals, fractures self-seal efficiently and do not constitute preferential fluid pathways, except in transient stages of deformation.