Aquitards are gaining importance as barriers that are integral to environmental and safety assessments and mitigation strategies for unconventional oil and gas development, for carbon sequestration, and for nuclear waste disposal. To meet the challenge of protecting shallow groundwater and surface water from such activities, it is becoming imperative to demonstrate the integrity of aquitard barriers. This involves detailed studies of solute and gas transport within low-permeability regimes and on time scales appropriate to the geological setting and contaminants of concern. Considerable progress has been made in recent years towards understanding the solute transport characteristics of aquitards and finding reliable methods to characterize aquitard porewater and gases.

The focus of this special issue is on the state-of-the-science and ongoing research to characterize aquitard properties and solute profiles as a basis for models to simulate contaminant transport in these critical barriers.

Potential topics include but are not limited to the following:

• Issues on fluids and gases in sedimentary and crystalline aquitards
• Methods for determination of porewater isotopes and conservative tracers in aquitards
• Gases in aquitards: origin, production, and transport
• Derivation of transport parameters
• Sorption properties of clays and cations in saturated and unsaturated materials
• Modelling conservative and reactive solute transport at the pore scale to field scale
• Direct and indirect approaches for dating pore fluids