Advancing Tracer Techniques: Comparative Analysis of Solute, Particulate, and Microbial Transport in Field Experiments

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.