Abstract
In Norway, drinking water is often transported and stored in tunnels and reservoirs carved into mountain rock, taking advantage of favorable geological conditions and stable temperatures. These installations are typically robust structures constructed through rock blasting or excavation. Incomplete sealing and fractures, however, may allow groundwater intrusion, creating potential pathways for microbial contamination. Such vulnerabilities have contributed to documented contamination events in Norway, particularly following heavy precipitation.
This case study, conducted in collaboration with the Municipality of Trondheim, investigates microbial water quality changes associated with mountain-based drinking water installations and evaluates the need for additional protective measures. The study combines quantitative microbial risk assessment (QMRA) with flow cytometry-based monitoring to support operational decision-making and prioritization of mitigation efforts.
Two online flow cytometers are being deployed to compare inlet and outlet water quality across selected installations, including a drinking water tunnel and several storage basins in the distribution network. The monitoring focuses on total and intact cell concentrations as well as cytometric fingerprinting to detect subtle microbial shifts. Microbial changes downstream of treated water may arise from biofilm shedding, planktonic growth during residence time, or groundwater intrusion. Because the water is cold (<10 °C) and surface-area-to-volume ratios are relatively low, the extent to which growth or biofilm processes alone can explain such changes remains uncertain. To strengthen interpretation, installations believed to be well sealed are used as contrasts to sites with documented groundwater intrusion.
In parallel, QMRA is applied to intrusion scenarios to estimate infection risk from six waterborne pathogens under varying assumptions of leakage volume, pathogen source strength, and disinfection barrier performance. To capture a representative range of microbial hazards, the scenarios include two bacteria (Campylobacter and E. coli O157:H7), two viruses (norovirus and adenovirus), and two protozoan parasites (Giardia and Cryptosporidium). The modelling assesses whether secondary disinfection is necessary and sufficient. In Trondheim, a secondary ultraviolet disinfection unit has already been implemented at the outlet of a tunnel, and this study examines the extent to which such measures mitigate intrusion-related risk.
By integrating scenario-based risk estimates with high-frequency microbial monitoring, this case study aims to improve understanding of intrusion-driven vulnerability in mountain installations and provide a practical evidence base for resource prioritization and resilient drinking water safety management under changing climatic conditions.