Impact Investigation into Faecal Contamination of the Arahura Mussel Beds
This investigation aimed to determine the relative importance of the likely sources of faecal contamination of the Arahura Mussel Beds after Crown Public Health determined in June 2001 that the flesh of the mussels were contaminated with faecal bacteria at levels rendering them unsuitable for consumption. Several complaints and enquiries were fielded by West Coast Regional Council concerning this contamination and the safety of consuming the shellfish. This report provides information about the effects of dairy farm effluent discharges that will be useful for resource management purposes.
These mussels (kutai) are a valued source of kai moana for Maori at Arahura Pa and Te Runaka o Kati Wae Wae and recent illness within the local mussel-eating community prompted further investigation. The Arahura Mussel Beds are located approximately 6 kilometres to the north of Hokitika and extend south from the Arahura River mouth for over a kilometre.
Feacal coliform and E.coli samples were taken on five events at 10 locations from Arahura to Hokitika Rivers. Total faecal contaminant loading was estimated from flow estimations.
The results of this investigation suggest the most important source of contaminants is from the Hokitika River over a range of different flow conditions. As the dominant sea current direction is from south to north, the Hokitika River is considered the single biggest contributor to contamination of the mussel beds. Samples comparing faecal bacteria from the north and south ends of the mussel beds also confirm the major source is from the south.
Short-term moderate levels of contamination were discharged to the Arahura River from Paddy's Creek upstream of the confluence of Arahura River Mouth after high rainfall events. The high level of contamination in Paddy's Creek is not significantly mitigated by dilution from Arahura River or the sea at the northern end of the mussel beds. Northerly near-shore surface sea currents were confirmed during times when creeks were in high flow. This current is likely to deposit faecal contaminants from Paddy's Creek over the Arahura mussel beds. The discharges from two dairy farms in the area appear to be the main cause of poor water quality of Paddy's Creek north branch and the Unnamed Creek and Arahura Valley Road. There were also high concentrations of the faecal bacteria in "Mair Creek". It is unknown whether the source of this faecal contamination is of animal or human origin (from the deer abattoir or sewage/septic tanks).
Click here to download the Contamination of the Arahura Mussel Beds Report (700kb PDF)