Urban expansion and the correlated anthropogenic activities are having a far-reaching negative impact on the biota in aquatic ecosystems, such that there is no river or coastline unaffected by human influence. Most aquatic ecosystems are simultaneously impacted by multiple stressors, such as pollution from industrial effluents, sewage, mining, urbanization and agriculture. These stressors not only impact the ecological health of the water ecosystems but also diminish their benefits to society.

Globally, untreated sewage (also known as raw sewage) is major concern for human health risk, due to the potentially high loadings of human pathogens associated with human faecal material.

Raw sewage is also a complex and heterogenous matrix, which includes high concentrations of nutrients, chemicals from industry and home waste, as well as pharmaceuticals and illicit drugs. In most developed cities, sewage undergoes varying levels of treatment prior to its release. However, even in the most modern and well-maintained sanitary systems, raw sewage can enter waterways during large rainfall events, via illegal discharge, or as result of damaged or leaking infrastructure. Detecting the presence of raw sewage is challenging, and for several decades has been founded on coliform counts and enumeration from culture-based approaches, primarily E. coli and enterococci. These approaches have limitations as they do not discriminate between the source of the faecal material. For example, high counts could be from wild bird populations, agricultural run-off or domestic pets; and many cases may not pose a true risk to human health.

An alternate approach, and one which is still relatively novel, is to use molecular tools which specifically target human-gut associated microbiota (e.g. Bacteroides and Lachnospiraceae) as surrogates of human-derived faecal material in waters. This approach is analogous to the current routine catchment-level monitoring of sewage for fragments of COVID-19.

In this paper, we used a weight-of-evidence approach to identify the potential presence of raw sewage in Sydney’s Hawkesbury-Nepean River (HNR) catchment. The HNR is the largest catchment in the Sydney region and receives a mix of anthropogenic inputs such as treated sewage, stormwater and agricultural runoff. The lines of evidence included: total nitrogen and phosphorus concentrations; culture-based traditional methods for E. coli and enterococci; and qPCR analysis of the human-gut bacteria Bacteroides and Lachnospiraceae.

Despite a background of diffuse inputs from recent high flow events and the influence of treated wastewater, we found no gradient of faecal (raw sewage) contamination within the HNR system or its tributaries. There was evidence of untreated sewage contamination in two sites, however, these likely originated from industrial runoff and a possible dry weather sewage leak.

Collectively, the findings of this research demonstrated the potential of a multiple-line of evidence approach for identifying the contamination of human faecal material within complex catchments. Furthermore, our findings add credence for the need to include human-specific markers, rather than the sole reliance of traditional and non-taxa specific coliform counts and culture-based approaches. This also has broader applications, e.g. the routine monitoring of beaches, especially after large rainfall events.

This project was a collaboration between SIMS, Macquarie University, the NSW Department of Planning, Industry and Environment, Sydney Water and the School of Freshwater Sciences from the University of Wisconsin – Milwaukee.  The complete publication can be found at https://www.sciencedirect.com/science/article/abs/pii/S0269749121001536

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