Zürich Airport (Flughafen Zürich AG) is pioneering a large-scale sustainable energy initiative by developing an Aquifer Thermal Energy Storage (ATES) system within a deep, buried glacial channel approximately 300 m below the surface. The open-loop system is designed to cyclically store excess thermal energy during summer for subsequent retrieval in winter. The technical success and economic efficiency of such a system, which involves the injection and abstraction of groundwater at varying temperatures, rely on careful consideration of interactions between the geothermal fluid, the subsurface environment, and the technical infrastructure.

An aspect in the planning and optimization of an ATES system is understanding the microbial community of the aquifer. Microorganisms are an integral part of the natural system and, when properly managed, can contribute to sustaining ecosystem functions and long-term environmental stability. Functional groups may affect the hydraulics and accelerating pitting corrosion which can be mitigated through measures such as the use of high-quality stainless-steel piping, which provides strong resistance to corrosion and ensures operational stability.

Our study focused on characterizing the native microbial community of the 300 m deep, water-bearing gravel aquifer and assessing its suitability for ATES. Online flow cytometry during a pumping circulation test established a microbial baseline with a Total Cell Count of approximately 50,000 cells/mL, comparable to values observed in shallower aquifer systems with drinking water quality (Besmer et al., 2016; Kötzsch and Sinreich, 2014). The analysis also revealed a high fraction of High Nucleic Acid (HNA) cells (~80%), indicating relatively large pore spaces within the aquifer. However, uncertainty remains regarding bacteria or nutrients that may have been introduced and stimulated growth prior to the measurements.

These data provide a first basis for estimating operational parameters according to the thermal regime of the project and for designing management strategies that maintain long-term system efficiency and longevity.

Furthermore, future measurements during operation will help track microbial dynamics, revealing ecological changes and supporting adaptive optimization strategies.

Literature:

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, 38462. https://doi.org/10.1038/srep38462

Köztsch, S. & Sinreich, M. (2014): Zellzahlen zum Grundwasser. Bestimmung mittels Durchflusszytometrie. Aqua et Gas 94(3), 14-21.