Monitoring of the effects of a temporally limited heat stress on microbial communities in a shallow aquifer

Heterotrophic nitrate reduction potential of an aquifer microbial community from psychrophilic to thermophilic conditions

High temperature-aquifer thermal energy storage (HT-ATES) aims at the seasonal storage and extraction of large quantities of heat in the subsurface. However, the impacts of temperature fluctuations caused by HT-ATES toward biodiversity and ecosystem services in the subsurface environment with respect to the nitrogen cycle remain unclear. Hence, understanding possible temperature adaptation mechanisms of aquifer microbial communities is crucial to assess potential environmental risks associated with HT-ATES. In this study, we investigated the effects of temperatures between 12 °C and 80 °C on a pristine aquifer microbial community and its capacity to reduce nitrate, a common global groundwater contaminant. 13C-labeled acetate was used as easily consumable carbon and energy source for nitrate reduction, allowing precise activity measurement by analysis of released 13CO2. We observed nitrate reduction coupled to acetate mineralization at 12 °C, 25 °C, 38 °C, 45 °C and 60 °C but not at 80 °C. The rates of acetate mineralization at 12 °C to 38 °C were significantly higher than rates at 45 °C and 60 °C. Temperature significantly affected the composition of the acetate-mineralizing, nitrate-reducing microbial communities. Members of the families Pseudomonadaceae and Comamonadaceae mainly developed in enrichments incubated at 12 °C and 25 °C, whereas phylotypes affiliated to Rhizobiaceae dominated at 38 °C. At 45 °C and 60 °C, phylotypes belonging to Symbiobacteriaceae, Paenibacillaceae and Planococcaceae mainly developed. These findings indicate that the indigenous aquifer microbiome can maintain the ability to reduce nitrate over a wide temperature range, providing support that HT-ATES may allow thermal energy storage while simultaneously attenuating nitrate pollution.

Rapid Consumption of Dihydrogen Injected into a Shallow Aquifer by Ecophysiologically Different Microbes

The envisaged future dihydrogen (H2) economy requires a H2 gas grid as well as large deep underground stores. However, the consequences of an unintended spread of H2 through leaky pipes, wells, or subterranean gas migrations on groundwater resources and their ecosystems are poorly understood. Therefore, we emulated a short-term leakage incident by injecting gaseous H2 into a shallow aquifer at the TestUM test site and monitored the subsequent biogeochemical processes in the groundwater system. At elevated H2 concentrations, an increase in acetate concentrations and a decrease in microbial α-diversity with a concomitant change in microbial β-diversity were observed. Additionally, microbial H2 oxidation was indicated by temporally higher abundances of taxa known for aerobic or anaerobic H2 oxidation. After H2 concentrations diminished below the detection limit, α- and β-diversity approached baseline values. In summary, the emulated H2 leakage resulted in a temporally limited change of the groundwater microbiome and associated geochemical conditions due to the intermediate growth of H2 consumers. The results confirm the general assumption that H2, being an excellent energy and electron source for many microorganisms, is quickly microbiologically consumed in the environment after a leakage.

Predictability of initial hydrogeochemical effects induced by short-term infiltration of ∼75 °C hot water into a shallow glaciogenic aquifer

Despite their potential in heating supply systems, thus far high-temperature aquifer thermal energy storages (HT-ATES) currently lack widespread application. Reducing the potential risks by improving the predictability of hydrogeochemical processes accelerated or initiated at elevated temperatures might promote the development of this technology. Therefore, we report the results of a short-term hot water infiltration field test with subsurface temperatures above 70 °C, along with associated laboratory batch tests at 10, 40 and 70 °C for 28 sediment samples to determine their usability for geochemical prediction. Most groundwater components had lower maximal concentrations and smaller concentration ranges in field samples compared to the batch tests. This indicates that the strongest geochemical effects observed in laboratory tests with sufficient site-specific sediment samples will likely be attenuated at the field scale. A comparison of field measurements with predicted concentration ranges, based on temperature induced relative concentration changes from the batch tests, revealed that the predictive power was greatest, where the hot infiltrated water had cooled least and the strongest geochemical effects occurred. The batch test-based predictions showed the best accordance with field data for components, with significant temperature-induced concentration changes related to ion exchange and (de)sorption processes. However, accurate prediction of concentration changes based on other processes, e.g. mineral dissolution, and downstream reversals in concentrations, requires further investigation. The here presented procedure enables the prediction of maximal expectable temperature-dependant concentration changes for most environmentally relevant ancillary groundwater components, e.g. As, with limited effort.