[A-HW20] Advances and Emerging Methods in Tracer Hydrology

[AHW20-05] Tracing and quantifying microbes in riverbank filtration sites combining online flow cytometry and noble gas analysis

*Friederike Currle¹, Theo Blanc², Yama Tomonaga¹, Rolf Kipfer^3,4,5, Daniel Hunkeler², Philip Brunner², Oliver S. Schilling^1,3 (1.Hydrogeology, Department of Environmental Sciences, University of Basel, Switzerland, 2.Centre for Hydrogeology and Geothermics, University of Neuchâtel, Neuchâtel, Switzerland, 3.Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dubendorf, Switzerland, 4.Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland, 5.Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland)

Keywords:microbial transport, online flow cytometry, surface water – groundwater interactions, filtration efficiency, hydrological tracers

Understanding microbial transport in surface water – groundwater systems is crucial for drinking water management. Particularly in the context of climate change the quality of groundwater pumped near streams might be affected by high microbial loads after heavy rain and peak flow events. Dissolved noble gases have been shown to be conservative tracers and provide information on pathways and travel times of groundwater. Although it is known that due to size exclusion, microbes appear to travel faster than solutes, most hydrological tracer methods target groundwater movement and solute transport, while specific tracers for microbial transport are not yet considered for protection zone delineation of drinking water supply wells. Recently, online flow cytometry (FCM) has been shown to be a promising tool to track on site, continuously and in near-real time the movement of microbes in riverbank filtration settings [1]. Beyond direct cell counting, high (HNA) and low (LNA) nucleic acid content microbes, often referred to larger and smaller prokaryotes, can also be distinguished.

Aiming to identify the preferential transport pathways of microbes and develop tracer methods to track their movement, we combined online FCM with online (noble) gas analysis at a riverbank filtration site in the Emme valley, Switzerland [2]. Dissolved gas concentrations (measured using the gas equilibrium-membrane inlet portable mass spectrometer miniRUEDI [3], Gasometrix GmbH) and microbial community patterns (measured using the online flow cytometer BactoSense, bNovate Technologies SA) were monitored continuously over a period of several months in the river, a piezometer next to the river and a nearby pumping well. A 10-year discharge event in December 2022 caused a massive microbial breakthrough in the groundwater. Remarkably, while in the near river piezometer both the total microbial load as well as the fraction of HNA increased significantly, at the pumping well only an increase in the total load was observed, while the fraction of HNA remained stable.

In summary, this combination of state-of-the-art analytical techniques allows to track and quantify microbial pathways from surface water into and through an alluvial aquifer. Filtration efficiencies of the different compartments (riverbed, riverbank vs. fluvial sediments) for distinct microbial clusters (e.g., HNA vs. LNA) can be assessed in a straightforward manner. Furthermore, the setup increases understanding of reactive microbial transport compared to the transport of conservative dissolved gases and, thus, of the suitability of microbes as natural tracers.

[1] Besmer, M. D., Epting, J., Page, R. M., Sigrist, J. A., Huggenberger, P., & Hammes, F. (2016). Online flow cytometry reveals microbial dynamics influenced by concurrent natural and operational events in groundwater used for drinking water treatment. Sci. Rep., 6, Article 38462. https://doi.org/10.1038/srep38462
[2] Schilling, O. S., Partington, D. J., Doherty, J., Kipfer, R., Hunkeler, D., & Brunner, P. (2022). Buried paleo-channel detection with a groundwater model, tracer-based observations, and spatially varying, preferred anisotropy pilot point calibration. Geophys. Res. Lett., 49(14), e2022GL098944. https://doi.org/10.1029/2022GL098944
[3] Brennwald, M. S., Schmidt, M., Oser, J., & Kipfer, R. (2016). A Portable and autonomous mass spectrometric system for on-site environmental gas analysis. Environ. Sci. Technol., 50, 13455-12463. https://doi.org/10.1021/acs.est.6b03669