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Faecal Source Tracking in the Avon River, Christchurch March-May 2009


Background: Water quality in the Avon River is monitored for the presence of the faecal indicator Escherichia coli (E. coli). E. coli are found in the faeces of humans, animals and birds. The detection of E. coli in water indicates the presence of faeces and therefore the potential presence of faecal-oral micro-organisms (such as Campylobacter, E. coli O157, and Cryptosporidium) that can cause disease in humans. MfE/MoH guidelines recommend that freshwater recreational areas should have less than 260 E. coli/100ml. At levels above 550 E. coli/100ml the guidelines recommend undertaking a sanitary survey, reporting on the sources of contamination, the erection of warning signs and informing the public through the media that a public health problem exists. The problem: Water measurements in the Avon River regularly exceed 260 E. coli/100ml and in many cases 550 E. coli/100ml. For water managers to implement appropriate responses to elevated E. coli levels, information is required on where the faecal pollution comes from. This report specifically addresses the question of whether these E. coli are from a human, animal or wildfowl source. What we did: Collected water samples from the Antigua Boatsheds and Kerrs Reach during low flow conditions, or during rainfall impacted flows. Twelve samples from each site were analysed using chemical and molecular tools capable of identifying faecal pollution and whether the pollution is from a human or animal/wildfowl source. No sewage overflows were reported during the sampling period. What we found: In the absence of rainfall, E. coli levels of up to 540 E. coli/100ml were measured in the Avon River. The primary sources of these E. coli are wildfowl, with secondary contributions from dog faecal material. There was no indication of a human sewage contribution. During, and immediately following rainfall, E. coli counts in the Avon River increased up to 3,600 E. coli/100ml. The faecal source profile changed to be dominated by what appeared to be dog faeces, with secondary contributions from wildfowl. At the Antigua Boatshed there was no indication of a human sewage contribution. In contrast at Kerrs Reach following heavy rainfall faecal pollution with a human signature was detected. What does it mean? Wildfowl and dogs are the primary contributors to degraded water quality in the Avon River. Measures to encourage dog owners to pick up dog faeces, especially along river banks, and to design and install stormwater systems to treat stormwater at source may result in lower levels of E. coli in the river. During the sampling period, a human sewage contribution was only detected during very heavy rainfall.