Quantifying in-situ decay-rates of faecal indicators and pathogenic viruses in a river section in Germany.

For assessing the health risk at recreational waters resulting from wastewater discharges from urban catchments, knowledge about the dynamics of the ratio between faecal indicator bacteria and pathogenic viruses is essential. Differences in wastewater concentrations, decay rates, and relevant exposure concentrations may influence how reliable concentrations of faecal indicator bacteria truly reflect existing health risk. Full-scale experiments on decay rates of pathogenic viruses in natural surface waters, especially fresh waters, are largely missing.

In the present study, we quantified the decay rates of faecal indicator bacteria and pathogenic viruses in a natural surface water. To this end, we performed two Lagrangian sampling campaigns after combined sewer overflows (CSO) along a river section in Berlin, Germany. During the campaigns the same body of contaminated water was resampled while travelling through the city. Organic micro-pollutants (Gabapentine, Acesulfame), and inorganic ions (Cl^-, SO₄^2-) were analysed to function as conservative tracer substances. Wastewater and stormwater amounts were estimated in each sample. Furthermore, all samples were analysed for humane Norovirus GII, Adenovirus 40/41, somatic coliphages, f-specifc coliphages, intestinale enterococci, and E.coli. Decay rates were derived by relating the pathogens to the estimated fraction of wastewater. The observation time was 5 and 2 days for the first and second CSO event, depending on the flow of the river. Decay rates indicate a significant variability between organisms but also between sampling campaigns as a result of different physical-chemical conditions. While the oxygen was completely consumed in the wastewater plume of the first event, this was not the case during the second event, which still allowed pathogen removal by grazing of heterotrophic zooplankton. Our results contribute to the general understanding of pathogens and faecal indicators in surface waters.