Bacterial stability and site-specific homogeneity in urban groundwater under Low-Temperature aquifer thermal energy storage operation

The bacterial microbiome in groundwater ecosystems is essential for drinking water quality and biogeochemical cycling. Low-temperature aquifer thermal energy storage (LT-ATES) systems may influence these communities, yet in situ evidence remains limited. We hypothesized that LT-ATES operation alters groundwater bacterial diversity and community composition. To test this, we conducted long-term monitoring of an LT-ATES system and urban and rural reference groundwater wells using high-resolution 16S rRNA gene metabarcoding and hydrochemical analyses. Multivariate analyses showed no significant differences in bacterial alpha or beta diversity between the LT-ATES system and the variability observed among the investigated reference sites. Hydrochemical parameters and bacterial community composition remained within the range of spatial and temporal variability among the investigated groundwater wells, with comparable Bray–Curtis dissimilarities within sites. Instead, community structure was primarily shaped by site-specific environmental gradients, including conductivity, redox potential, and boron. Phylum-level partial Mantel tests identified Planctomycetota, Chloroflexi, Acidobacteriota, and Firmicutes as most responsive to these gradients, indicating environmental filtering as the dominant driver. These findings indicate that, under the operational conditions investigated, the LT-ATES systems did not induce detectable bacterial or hydrochemical changes beyond the variability observed among the investigated groundwater sites. The results support the environmental compatibility of LT-ATES under comparable hydrogeological conditions and operational temperature ranges.