[A-HW23] Tracer Hydrology: Advances in Measurement and Modelling

[AHW23-01] Online Flow Cytometry as a new online tracer tool to detect groundwater origins and mixing and monitor drinking water quality

Friederike Currle¹, Théo Blanc^4,3, Yama Tomonaga¹, Johannes Schorr³, Juliane Hollender^3,2, Rolf Kipfer^3,2, Daniel Hunkeler⁴, Philip Brunner⁴, *Oliver S. Schilling^1,3 (1.Hydrogeology, Department of Environmental Sciences, University of Basel, Switzerland, 2.ETH Zurich, Switzerland, 3.Eawag, Swiss Federal Institute of Aquatic Science and Technology, Switzerland, 4.Centre for Hydrogeology and Geothermics, University of Neuchatel, Switzerland)

Keywords:hydrological tracers, groundwater, flow cytometry, aquifer, surface water-groundwater interactions, drinking water quality

Hydrological tracers enable us to evaluate hydraulic properties as well as groundwater origins, mixing and residence times over many different temporal and spatial scales. Yet, most established hydrological tracer methods rely on the measurement of solutes, of the isotopic signature of the water molecule itself, or on temperature. While these methods are unrivaled to track surface water and groundwater dynamics, they are less suited for the detection of the flow paths and transport processes that are relevant for waterborne pathogens. The reason for this lies in the fact that waterborne pathogens are not solutes but suspended particles of colloid size and thus do not move through the subsurface exactly like solutes or water molecules. For the detection of waterborne pathogen transport dynamics, non-classical hydrological tracer methods thus must be considered.
Luckily, over the course of the last two decades, a new method for the measurement of microbial cells directly in the field and continuously in near-real time has been developed: Online flow cytometry (FCM). Online FCM counts microbes by targeted staining of microbial cells and subsequent optical detection of induced fluorescence. Owing to its reliability, online FCM has recently been set as a new legal standard method for the assessment of microbiological drinking water quality in Switzerland. However, online FCM is not only relevant for microbial cell counting. In addition to being able to detect the total cell count, online FCM also allows detection of so-called cytometric fingerprints of the microbial communities present in water. Here, we present the broad capabilities of online FCM to detect not only microbial transport processes but also groundwater flow paths, origins and mixing. Specifically, we demonstrate how online FCM can be used to: (1) differentiate between groundwater types by cytometric fingerprinting (e.g., to detect upwelling deep groundwater), (2) uncover dynamic groundwater mixing processes through changing microbial community patterns, (3) detect impacts to riverbank filtration wellfields from changing river water discharge or river restoration activities, and (4) identify time-varying contributions from multiple conduits in karstic systems.