Performance of on-site flow cytometry for near-real-time microbiological analysis of alpine karst drinking water resources

This study presents the first rigorous, multi-parameter evaluation of near-real-time flow cytometry (FCM) for automated on-site monitoring at a model alpine karst spring used as a drinking water resource. We tested automated FCM for its capacity to indicate microbial cell inputs from the catchment surface (SRI) and to indirectly indicate diffuse faecal contamination (DFC). A nested 10-year study design combined low-frequency monitoring with high-frequency event sampling. The instruments tested in the study performed well under challenging field conditions and reliably quantified microbial cells, as validated against established methods. FCM parameters robustly detected SRI and, indirectly, DFC, in a catchment with diffuse animal faecal contamination sources. The same performance was observed for the online physico-chemical parameters UV254 and turbidity. The combination of these biotic and abiotic indicators outperformed single parameters in early warning analyses, demonstrating their potential for water safety planning by enabling timely and specific responses. The various aspects of the automated on-site monitoring devices used are discussed in detail. While automated FCM already supports the indirect detection of diffuse faecal pollution, further technical advances are necessary to extend its scope toward the specific and sensitive detection of faecal contamination.