A novel device for in situ point measurements of fluorescent tracers in sediment pore water

Understanding transport and biogeochemical turnover processes of solutes in riverine systems is vital in order to preserve aquatic and terrestrial ecosystem health. The hyporheic zone is the well-studied interface between stream water and underlying aquifer. It acts as a hydrodynamically driven bioreactor, i.e. its reactivity is strongly controlled by its dynamic subsurface flow patterns. Fibre optic sensing techniques and, in particular, fluorescence spectroscopy using a suite of fluorescent substances can provide unique information about physical, chemical, and biological processes occurring in the hyporheic zone. However, current devices for in situ fluorescence measurements in sediment pore water are insufficient in measurement reproducibility due to significant shortcomings of their mechanical design. Therefore, a novel design concept for a robust fluorescence sensor system performing reproducible real-time in situ fluorescence measurements in sediment pore water was developed. The holistic design concept includes the mechanical protection of the fibre optic tip, constant optical properties of the measurement environment, significantly reduced turbidity effects, and de-risk the formation of interfering gas bubbles. The study demonstrates the robustness and suitability of the prototype for in situ fluorescence measurements in laboratory and field using the fluorescence dye uranine. The presented proof of concept for the fluorescence sensor system may provide a fundamental basis for a sensor system performing any fluorescence spectroscopy analysis in sediment pore water in real time.

Automated summary

Understanding transport and biogeochemical turnover processes of solutes in riverine systems is essential for ecosystem health. The study presents a novel design concept for a robust fluorescence sensor system that can perform reproducible real-time in situ fluorescence measurements in sediment pore water. The prototype demonstrated robustness and suitability in laboratory and field settings, providing a basis for future sensor systems performing fluorescence spectroscopy analysis in sediment pore water in real time.