Evaluating the suitability of cultivation-based faecal indicator bacteria as a water quality parameter for alternative water resources at the example of urban stream daylighting

A stream emerges from a pipe outlet and flows through an open landscape. Arrows point from the stream, at both the piped and open sections and at the outflow, to a sampling bottle, representing water collection at each location. A magnifying glass leads from the bottle to a box showing the analytical framework applied to the samples, divided into a conventional and a molecular approach. An arrow labelled 'Usability?' connects the framework box to a section on service water applications, illustrated with images for irrigation and recreation. Tackling the challenges of climate change and the urban heat island effect is driving urban water demand to new heights. Diversifying water supply through the inclusion of alternative water sources is thus gaining increasing relevance and attention. One measure that could provide both urban cooling and unlock additional water sources is stream daylighting, the practice of reopening urban watercourses from buried conditions. However, to date, it remains unclear whether the microbial water quality of piped or reopened streams is of sufficiently high quality to enable their use for urban applications. The presented study thus investigated microbial water quality in two stream daylighting projects through conventional cultivation-based methods, supported by modern molecular biology techniques. Cultivation-based techniques indicate high levels of faecal indicator bacteria in all investigated streams. Furthermore, faecal indicator levels increased in culverted stream sections despite the absence of detectable faecal contamination. Molecular analyses revealed that the detected enterococci are at least partially of plant origin, overall suggesting the presence of naturalised colonies of faecal indicator bacteria. Regarding the use of alternative water sources, the findings indicate that current assessment methods for microbial water quality should be scrutinised toward a more nuanced, integrated microbial risk assessment.