Microbial and Water Quality Dynamics in a Utilidor-Housed Subsurface Drinking Water Distribution System under Low-Use Conditions

Cold-region utilidor-based drinking water distribution systems (DWDSs) are considered climate-resilient infrastructure but remain poorly characterized. Elevated temperatures and water age in such systems may accelerate disinfectant decay and microbial growth. We investigated water chemistry and microbiomes in a utilidor-housed, subsurface DWDS serving a mixed-use sub-Arctic community during low demand following partial closure. Main and premises plumbing samples were collected over eight events spanning 14 months. Physicochemical measurements accompanied 16S rRNA gene amplicon sequencing. Temperatures increased from 7.2 °C at the inlet to 31.9 °C distally, while free chlorine residual declined significantly. α diversity increased with distance and correlated negatively with disinfectant residual and dissolved oxygen. Microbial community composition differed significantly by location and time; Proteobacteria dominated most sites. Genera containing opportunistic pathogens included Legionella (80.7%), Mycobacterium (24.7%), and Pseudomonas (32.5%), with significant associations with total dissolved solids, nitrate, and dissolved organic carbon. These findings demonstrate that elevated temperatures and disinfectant decay may influence microbial communities and favor pathogen-associated taxa under low use. Utilidor-based DWDSs may, therefore, indicate vulnerabilities in buried systems under climate-driven warming and reduced demand. Results highlight the need for design and operational strategies addressing thermal control, water age, and disinfectant stability to protect drinking water quality in climate-stressed systems.