The impact of major earthquakes and subsequent sewage discharges on the microbial quality of water and sediments in an urban river

- E. coli was a better predictor of pathogens than C. perfringens.
- F-RNA phage, alongside E. coli, is a potential indicator of untreated human sewage.
- Protozoa persisted in river sediments after cessation of sewage discharges.
- F-RNA phage and Campylobacter did not accumulate in sediments.
- Sediment re-suspension increases health risk from re-mobilisation of pathogens.

A series of large earthquakes struck the city of Christchurch, New Zealand in 2010-2011. Major damage sustained by the sewerage infrastructure required direct discharge of up to 38,000 m3/day of raw sewage into the Avon River of Christchurch for approximately six months. This allowed evaluation of the relationship between concentrations of indicator microorganisms (Escherichia coli, Clostridium perfringens and F-RNA phage) and pathogens (Campylobacter, Giardia and Cryptosporidium) in recreational water and sediment both during and post-cessation of sewage discharges.Giardia was the pathogen found most frequently in river water and sediment, although Campylobacter was found at higher levels in water samples. E. coli levels in water above 550 CFU/100 mL were associated with increased likelihood of detection of Campylobacter, Giardia and Cryptosporidium, supporting the use of E. coli as a reliable indicator for public health risk. The strength of the correlation of microbial indicators with pathogen detection in water decreased in the following order: E. coli > F-RNA phage > C. perfringens.All the microorganisms assayed in this study could be recovered from sediments. C. perfringens was observed to accumulate in sediments, which may have confounded its usefulness as an indicator of fresh sewage discharge. F-RNA phage, however, did not appear to accumulate in sediment and in conjunction with E. coli, may have potential as an indicator of recent human sewage discharge in freshwater. There is evidence to support the low-level persistence of Cryptosporidium and Giardia, but not Campylobacter, in river sediments after cessation of sewage discharges. In the event of disturbances of the sediment, it is highly probable that there could be re-mobilisation of microorganisms beyond the sediment-water exchange processes occurring under base flow conditions. Re-suspension events do, therefore, increase the potential risk to human health for those who participate in recreational and work-related activities in the river environment.