PhD defence: Aquitard hydraulic conductivity Investigating heterogeneity with lab measurements and pumping test data

Groundwater is one of our most important sources of clean drinking water. It mainly flows through layers of sand and gravel, which are often covered or seperated by poorly permeable layers of clay, silt, or peat: the so called aquitards. These layers act like a protective blanket. They block pollution and ensure that interventions in the subsurface, such as water extraction, have only limited effects at the surface. 

Yet we still know little about how water actually flows through aquitards. Their permeability can vary greatly, even within the same material. This makes it difficult to build reliable models that predict groundwater flow. This study therefore examines how aquitards can be better described and understood. 

Using laboratory measurements, pumping tests, and groundwater flow models, the research investigated how heterogeneous aquitards are and which properties most strongly determine their permeability. Machine learning showed that factors such as clay content, depth, and geological unit have a major influence. Pumping tests at different locations further demonstrated that the structure and spatial variation within aquitards play a significant role in how groundwater moves. 

An important insight is that the scale of observation matters. Because water mainly flows through relatively permeable parts, and the likelihood of such pathways increases in larger areas, permeability increases with scale. The study shows that by combining data and models we can better predict how aquitards function. This not only helps protect our drinking water resources, but also supports, for example, the planning of sustainable energy projects in the subsurface.