Karst aquifers are particularly susceptible to contamination due to their high permeability and limited natural attenuation capacity. Conventional delineation of karst catchments and flow pathways has largely relied on solute tracer experiments. While effective for identifying groundwater connections, such tracers do not adequately reflect the transport dynamics of particulate contaminants, including colloids and microorganisms, which are increasingly relevant in assessing groundwater vulnerability.

Over the past years, a comprehensive series of tracer experiments has been conducted at a karst spring in southwestern Germany. These tests employed a diverse set of tracer substances, including fluorescent dyes, organic micropollutants, silica microspheres, and microplastic particles. Most recently, the tracer suite was extended to include non-pathogenic microorganisms to more closely mimic pathogen transport behavior. The comparative analysis revealed high overall transport velocities with predominantly conservative transport behavior. Only slight variations in transport velocities among the different tracer types regardless of transport length were detected. However, notable discrepancies in tracer recovery rates point at distinct retention and removal mechanisms acting on the particulate tracers. These findings underscore the complexity of particle transport in karst systems and highlight the limitations of traditional tracer approaches.

The study further emphasizes the need for methodological advancements—particularly in tracer selection and analytical techniques—to improve of tracer tests involving particulate and biological agents. Future research should focus on optimizing the design of such experiments, including the development of standardized microbial tracers and the integration of advanced detection methods.

The incorporation of the World Health Organisation's water safety plans into German law through the groundwater catchment regulations (Trinkwassereinzugsgebietsverordnung), which will come into effect in 2025, will allow for, but also demand, new investigative groundwater monitoring tools. As part of the BMBF project iMolch, new tools and strategies are being developed to enable sustainable groundwater management. These tools will provide new insights into the ecological and chemical status of drinking water catchments. Two catchments within the Düsseldorf water supply network will be presented as examples. Both are affected by multiple land uses and different proportions of bank filtration from the Rhine. To evaluate the chemical status, non-target analytical methods are compared with hot target analytical methods based on the principle of indicator compounds (over 136 organic micropollutants) and main water chemistry. The ecological status will be investigated using an improved DAC index and beta diversity. Community composition was analysed using 16S rRNA gene sequencing; cell density was measured using flow cytometry; and internal ATP was determined using the BacTiter-Glo microbial cell viability assay. Principal component analysis (PCA) is used to analyse the ecological data and non-target chemical analytics, and the results are presented and discussed. The impact of floods in the Rhine on the groundwater composition was evaluated using both chemical and ecological data. Furthermore, it was possible to calculate the amount of bank filtrate in the individual supply wells based not only on the indicator compounds, but also on the ecological data.

Heavy metal contamination of abandoned mines affects natural ecosystems long after mining has ceased. Recent analytical developments have shown that Sb isotopes can be used as a powerful geochemical tool to trace sources and processes of contamination in regions affected by Sb mining [1]. However, despite its toxicity, the Sb cycle in aqueous systems apart from Sb-mine contaminated waters remains poorly understood.

To address this issue and better understand Sb isotope behavior in abandoned mines without a (known) occurrence of Sb mineralization, we investigated Sb isotopes in drainage waters of the Ernst-August Stollen in Bad Grund, Harz Mountains (Germany), which was famous for its Zn-Fe-Pb mineralization. The water samples distributed throughout the mine and (anthropogenic) floating iron at the separator basin were collected during winter and summer seasons in 2024. The variable Sb concentrations (0.1 to 0.9 μg/L) and Sb isotope compositions of 0.2 μm filtered water (0.13 ≤ δ¹²³Sb ≤ 0.50 ‰) are most likely related to natural Sb weathering and adsorption processes at variable conductivity (635 to 5300 μS/cm). In contrast, the floating iron has high Sb concentrations (~30 μg/g) and a heavier Sb isotope composition (~1.3 ‰). This Sb isotope fractionation between floating iron and circumneutral drainage water may reflect a combination of adsorption, weathering and subsequent leaching processes.

[1] Wen et al. 2023 J. Hazard Mater. 446, 130622.

Bank filtration has been applied worldwide to extract raw river water after a passage through subsurface sediments. Commonly, this water is used as drinking water after proper treatment. However, river water, especially if urban structures are part of their catchment, is prone to anthropogenic impacts possibly leading to unwanted chemical substances and compounds in water. A wide range of impacting boundary conditions, e.g., high or extreme low river water levels affect the spatio-temporal evolution of the resulting concentrations in groundwater. In order to improve the understanding in the key impacts influencing the concentration behavior in the extracted water at bank filtration sites, the following steps are applied in this study using a highly urbanized site at Düsseldorf, North-Rhine Westphalia, Germany: Firstly, conceptual models are set-up based on delineation of widely known principles at comparable sites. On the basis of these conceptual models, in a second step hypotheses are made for the spatio-temporal behavior of hydraulic and hydrochemical parameters in the groundwater for a set of scenarios often occurring in river catchments. The resulting assumptions and signals expected from the conceptual models under certain conditions were compared to field data from the water supply company Stadtwerke Düsseldorf and City of Düsseldorf and later-on analyzed using a 2D-vertical numerical model. Finally, improvements in the understanding of the processes can be achieved. Besides this, understanding will help to improve observation information quality by reducing efforts in the same time.

Electron acceptors, such as nitrate, in aquifers are attenuated via biologically mediated redox reactions in groundwater. Their reduction is coupled to the oxidation of electron donors. The main source of electron donors, particularly over long water-sediment contact times is the sediment/rock matrix. The physical and chemical properties of aquifer matrices, thus, govern the rate and extent of natural attenuation. In addition to catalyzing redox reactions, subsurface microorganisms, actively scavenge electrons from reduced mineral phases or, in the case of organic carbon, hydrolyze solid- to dissolved-organic-carbon (DOC). The release of aqueous electron donors is often the reaction-limiting step. Moreover, at the aquifer scale, both the quantity and heterogeneous distribution of electron donors influence the overall extent of reduction. The extent of reaction hinges on flow paths that ensure long-enough contact time between aqueous electron acceptors and solid-phase electron donors, intrinsically linking hydraulic conductivity and sediment makeup with reactivity. Here, we show both analytically and numerically that hydrolysis of DOC from the matrix limits and conditions the rate of electron acceptor reduction. Our results show that this conditioning drives otherwise complex non-linear reactions to simple zero-order kinetics. In ongoing experiments with biostimulated aquifer sand, via a nitrate injection, preliminary results highlight that this limiting step is also relevant for matrices dominated by reduced iron minerals constitute the dominant electron donors. We expect to validate our theoretical analysis experimentally, and propose a framework to address the challenges in better representing both the reaction and transport properties of aquifers by accounting for sedimentological information.

Measuring gravity over time provides direct information about subsurface mass changes and, consequently, changes in groundwater storage. Gravity residuals can be used to estimate the specific yield of aquifers or to calibrate hydrogeological models.
As part of our research project, a high-precision absolute gravimeter will be developed for field applications. The absolute gravimeter is expected to offer significant advantages over existing high-precision instruments, such as a simple setup and low drift. In parallel, we will build a mesh-based physical hydrogeological model simulating both unsaturated and saturated zone processes in a study area experiencing declining groundwater levels. Mass changes in each mesh cell lead to gravity changes, which can be calculated by accounting for local subsurface properties. Model parameters will be constrained by fitting the modeled gravity response to the observed data. While previous studies have successfully calibrated saturated zone models using gravity data, including the unsaturated zone will improve the accuracy of subsurface mass change estimates and enhance the understanding of processes such as groundwater recharge. A comparison with repeated measurements of surface nuclear magnetic resonance will further assess the method’s validity for capturing unsaturated zone processes.