[A-HW19] Tracer Hydrology: Advances in Measurement and Modelling

[AHW19-P04] Tracking microbes in surface water-groundwater systems by integrating online flow cytometry and numerical simulations

*Friederike Currle¹, Rene Therrien², Theo Blanc³, Yama Tomonaga¹, Rolf Kipfer^4,5,6, Daniel Hunkeler³, Philip Brunner³, Oliver S. Schilling^1,4 (1.Hydrogeology, Department of Environmental Sciences, University of Basel, Switzerland , 2.Department of Geology and Geological Engineering, Université Laval, Québec, Canada, 3.Centre for Hydrogeology and Geothermics, University of Neuchâtel, Neuchâtel, Switzerland, 4.Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dubendorf, Switzerland, 5.Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland, 6.Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland)

Keywords:microbial transport, online flow cytometry, surface water – groundwater interactions, integrated surface-subsurface hydrological model, hydrological tracers

Riverbank filtration is widely used for drinking water production, taking advantage of the natural filtration of surface water in the fluvial sediments. The quality of groundwater extracted near streams may nevertheless be impacted by elevated microbial concentrations after intense rainfall and flood events. Thus, an effective risk assessment requires an understanding of the transport and fate of microbes in these river-aquifer systems. While dissolved noble gases serve as conservative tracers for understanding river-aquifer interactions and offer insights into pathways and travel times of alluvial groundwater, they may not accurately represent microbial transport. As an effect of size exclusion, the transport of microbes is restricted to the larger and usually more conductive pore space which leads to a faster transport and arrival of microbes compared to solutes. Recent studies have highlighted the potential of online flow cytometry (FCM) as a promising tool for real-time and continuous monitoring of microbial movement in riverbank filtration settings (Besmer et al., 2016). Beyond direct cell counting, FCM allows for the differentiation of microbial community patterns, such as high (HNA) and low (LNA) nucleic acid content microbes, commonly referred to as larger and smaller prokaryotes.
Our objective is to assess the preferential pathways of microbes and develop a quantitative tool for reactive transport modeling of microbes in river-groundwater systems. This involves a combination of online flow cytometry (FCM) and noble gas analyses, with integrated surface-subsurface hydrological modeling (ISSHM). Adopting a dual-permeability approach featuring a two-site kinetic deposition mode, we can co-simulate the rapid preferential microbial transport and slower bulk transport. This approach also considers the attachment and detachment of microbes in high and low permeability regions within the pore space (Bradford et al., 2009). The transport algorithm has been implemented in the ISSHM HydroGeoSphere (HGS; Aquanty, Inc.), allowing for multispecies transport representation, including distinct groups like HNA and LNA.
We present two measurement campaigns at riverbank filtration sites in Switzerland which revealed the sensitivity of cell concentrations and microbial community patterns to river water infiltration and travel distance within the alluvial sediments. Flood events, river restoration activities, snowmelt periods, and fluctuations in pumping rates caused notably variations in the microbial patterns. We also present first simulations and quantifications of the observed reactive transport of microbes at the wellfield scale, using the transport of conservative dissolved noble gases for comparison.

REFERENCES

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

Bradford, S. A., Torkzaban, S., Leij, F., Šimůnek, J., & van Genuchten, M. T. (2009). Modeling the coupled effects of pore space geometry and velocity on colloid transport and retention. Water Resources Research, 45(2). https://doi.org/10.1029/2008WR007096