Effects of temperature variations on aquifer biogeochemistry during high-temperature thermal energy storage operation: flow-through laboratory experiment insights

Reducing the carbon footprint of building heating and cooling is essential for reaching climate change mitigation goals. Seasonal High-Temperature Aquifer Thermal Energy Storage (HT-ATES) is a promising method to achieve these goals. However, the injection of high-temperature water may significantly alter aquifer geochemistry and microbiology by influencing redox equilibria and mineral solubility, and by strongly impacting the structure and activity of microbial communities, favouring shifts toward thermophilic microorganisms and changes in metabolisms. Although microbial life is ubiquitous in aquifers, biogeochemical processes occurring under HT-ATES conditions remain poorly understood, raising concerns regarding environmental impacts, system performances and long-term sustainability.

We investigated the effects of heating and cooling cycles on the structure and functions of aquifer microbial communities. HT-ATES conditions were simulated in the laboratory using a pressurised (13 bar) flow-through column with the BRGM’s BioREP platform. Groundwater from a monitoring HT-ATES well in TU Delft campus (Netherlands) was injected through the aquifer sediments while the temperature of the experimental device varied cyclically within a range typical of HT-ATES warm wells (30°C to 50°C), before a last phase returning to the natural aquifer temperature (12°C). Geochemical parameters, such as pH, redox potential, conductivity, redox-sensitive elements were monitored in circulating water at the inlet and outlet points of the column. Changes in microbial community composition of sediments and circulating water were assessed through 16S rRNA genes Illumina sequencing.

Preliminary results indicate that the aquifer sediments are quartz-rich, with presence of carbonates and clay minerals, and that the groundwater is of reduced brackish Na-Cl type. While intermediate and final sediment mineralogy analysis are still ongoing, groundwater analysis show that the chemistry remained stable throughout the experiment. A gradual clogging of the column (increase of the inlet pressure) was observed. Upon opening the experimental setup, precipitates were observed at the outlet of the column. Their origin, whether chemical or biological, are under investigation.

Initial microbial analysis of the groundwater revealed a dominance of bacteria (89%), with major phyla including Firmicutes, Bacteroidota, Proteobacteria, Desulfobacterota, and Spirochaetota along with Halobacterota. Many of these taxa are associated with anaerobic and slightly saline environments and include fermentative microorganisms. Several dominant groups are mesophilic and/or non-spore forming taxa. Higher and fluctuating temperatures may promote alternative, more thermotolerant microbial assemblage. Ongoing metagenomic analyses aim to determine how temperature perturbations under HT-ATES conditions influence microbial communities’ composition and possible functions and assess the implications for associated biogeochemical processes.

Acknowledgements: Funded by the European Union under grant agreement 1011096566 (PUSH-IT project). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor CINEA can be held responsible for them.