Assessment of sewage spills in the Huon and D’Entrecasteaux Channel area

Oysters are a high-risk food group as they are filter feeders, concentrating particles that are present in the water, and are generally eaten raw (i.e. have no bacteria kill-step). Faecal contamination of oysters occurs when animal or human faeces or sewage enters oyster growing areas. Once contaminated the oyster can cause illness if ingested. Faecal contamination includes both bacterial and viral human pathogens. Sewage treatment plant failures are therefore of great concern to shellfish food safety as they release large volumes of faecal contamination into the shellfish growing environment. Whilst bacterial pathogens are short lived in seawater, viral pathogens such as Norovirus can survive for long periods in this environment and can cause illness for weeks after a contamination event. Published decay rates range from between 0.27 to 0.05 (d-1) (Tu et al 2008, Boehm et al. 2019). 

Reported spills from Sewerage Treatment Plants (STP) are monitored closely and a conservative approach is adopted in terms of assessing the risk of interaction with nearby oyster farms. In Tasmania, the FAO approach of modelling the isometric spread of the spill in the growing area is adopted, and a mandatory closure instigated if a spill is suspected to reach a growing area, judged by an estimated Norovirus load of 0.04 particles per litre. However, local hydrodynamics play a fundamental role in the transport of contaminants into the leases from source locations, and an asymmetric rather than isometric spread of a spill will occur in many environments.

In this study we are looking at the impact a spill from an STP at Woodbridge would have on two nearby oyster leases, Birches Bay and Fleurty’s Point (Figure 1). The two leases are in the northern end of D’Entrecasteaux Channel, just south of the STP at Woodbridge. The Huon River typically drives hydrodynamics in the D’Entrecasteaux Channel and Huon Estuary (Herzfeld et al. 2016). River flow is heavily influenced by rainfall events in the region, as are sewage spill events. A hydrodynamic model of southeast Tasmania (TASSE) was developed by CSIRO. The domain of TASSE includes the D’Entrecasteaux Channel and Huon Estuary. An online particle tracking tool that uses water currents modelled by TASSE can be used to examine connectivity1 in the region. Complex behaviour can be added to a particle (e.g. decay rate, swim speed) to simulate different types of particles (e.g. viruses or zooplankton). This modelling platform can be used to develop a risk-based assessment of the interaction between oyster leases and sources of contamination within the region.