Reactive transport processes involve complex interactions between solid materials, fluid flow, and solute concentrations. In many cases, they are also influenced by the presence and activity of microbes. These processes occur in the subsurface, where conditions vary across time and space, making them difficult and expensive to study using traditional methods.

Geophysics offers a non-invasive way to explore the subsurface without needing to take destructive samples. This is especially useful for understanding solid phase properties, such as the concentrations of substances attached to surfaces and the presence and activity of microbes.

In our research group, we use the geophysical method spectral induced polarization (SIP) to monitor various types of reactions in both natural and engineered porous materials. We often conduct dynamic experiments in flow through cells (columns) and combine geophysical data with geochemical information and reactive transport models to enhance our understanding of these processes.

Current research questions include:

• What causes the polarization of microbes in different metabolic stages, and how can this help us monitor microbially-driven cleanup of contaminated aquifers?
• How does sorption of organic and inorganic compounds influence polarization signals?
• Can SIP be used to detect when the sorption capacity of a reactive barrier is exhausted?
• How can geophysical data be integrated into reactive transport models to improve predictions of reaction rates?

Our work is highly interdisciplinary and connects to other areas of research within our group, including reactive transport modeling, microbial processes, and contaminant hydrogeology.

Microbes in the subsurface catalyze reactions that govern the fate of nutrients and contaminants in the environment. They provide key services in the context of monitored natural attenuation effectively breaking down harmful substances. The rate and extent of reactions depends on microbial activity, their abundance and their dynamics. Microbial communities are highly dynamic and change rapidly in time.

In our research group we focus on better understanding microbial dynamics in porous media. We aim to improve our ability to measure microbes in situ using the geophysical method spectral induced polarization (SIP), taking advantage of their surface charging properties.

Moreover, we aim to understand their dynamics by explicitly modelling their behavior and biodegradation efficiency in reaction models. In that context, we investigate the quantitative potential of molecular biological datasets (e.g., genes) as reliable markers of microbial activity.

The fate of contaminants in the environment not only depends on biodegradation reactions, but also on adsorption and desorption reactions onto solid matrices as (ground)water parcels flow through porous media or filtration cells.

We investigate the efficiency of remediation schemes for the retention of harmful substances such as heavy metals and herbicides by both engineered and natural media, as well as the removal of contaminants from solid matrices such as municipal solid waste.

Our studies in these areas are typically paired with geophysical monitoring of target processes, and we use reactive transport models to quantify the reactive processes of interest.

Reactive transport models simulate the movement of solutes in the environment and the reactions and processes that control their fate. In our research group we focus on models relevant in porous media with a focus on the microbial growth, contaminant biodegradation and adsorption and desorption.

We develop models for specific experimental applications. Our models implement process-based mathematical formulation of reaction rates, and reaction terms are coupled with flow at different spatial scales ranging from the experimental scale in one and two dimensions (1D & 2D) as well as the aquifer scale in three-dimensional (3D) systems.

Karst aquifers exhibit dual-flow behavior, with slow, diffuse flow through the rock matrix and rapid, preferential flow through fractures and conduits. Understanding water movement and storage in these systems is key to predicting groundwater availability, especially in drought-prone or seasonally variable regions. In this research focus area led by Dr. Lysander Bresinsky, we use process-based numerical models to simulate variably saturated flow in karst catchments, focusing on the infiltration dynamics through the vadose zone and recharge to the phreatic zone. A dual-permeability approach is applied to capture both diffuse and preferential flow.

Our case study is the Western Mountain Aquifer, a large carbonate aquifer in the Mediterranean region. This system provides an opportunity to analyze the interactions between climate variability and groundwater recharge in semi-arid environments, where extreme rainfall events and extended dry periods significantly affect water resources. To address these conditions, we integrate high-resolution climate projections into our models to predict the effects of future weather patterns on groundwater availability in the region.

Key research questions include:

-  How does dual-permeability modeling enhance understanding of water storage and movement?

- What role does the vadose zone play in groundwater recharge under variable climate conditions?

- How can we better predict groundwater availability in semi-arid regions impacted by extreme weather?

By improving the understanding of subsurface flow processes, our work contributes to the development of more effective groundwater management strategies, particularly in the context of climate change.

This DFG-funded project is a collaboration between research groups in hydrogeology, sedimentology & organic chemistry and microbial ecology at the universities of Tübingen, Vienna and Kassel. The aim of the work is to advance the process-based understanding of the sedimentological, hydrogeological and microbial-ecological controls that modulate the ability of aquifers to reduce the agricultural contaminant, nitrate, via denitrification. Specifically, we seek to quantitatively link aquifer hydraulic and biogeochemical properties with the “reactivity” of aquifer sediments. Via coupled hydrogeological, sedimentological and microbiological field and lab investigations and reactive transport modelling, the project aims to yield reliable estimates of aquifer-specific reactivity to feed larger scale predictive models.

At the University of Kassel, we will develop complex reaction models that simulate denitrification activity in aquifer sediments as a basis for the development of virtual aquifer models. The upscaling will rely on travel-time based simulations and the parameterization of relative reactivity to predict the effect of sedimentology on an aquifers ability to reduce nitrate.

Principal Investigators

1. Prof. Dr. Adrian Mellage, University of Kassel, Hydrogeology
2. Prof. Dr. Jan-Peter Duda, University of Göttingen, Geobiology
3. Prof. Dr.-Ing. Olaf A. Cirpka, University of Tübingen, Hydrogeology
4. Prof. Dr. Christian Griebler, University of Vienna, Limnology

The project, funded by DFG, aims to isolate the geophysical signal of bacterial cells in the absence of parallel contributing polarization mechanisms. It seeks to link the combined effects of cell abundance and activity, provide a refined conceptual understanding of cell polarization in the field of biogeophysics, and progress toward developing a framework for using spectral induced polarization (SIP) as a monitoring tool for microbial dynamics in the subsurface.

Principal Investigator

1. Prof. Dr. Adrian Mellage, University of Kassel, Hydrogeology