Gadd, J., Dudley, B., Montgomery, J., Whitehead, A., Measures, R., Plew, D. (2020, June). Water quality of estuary of the Heathcote and Avon Rivers/Ihutai. National Institute of Water and Atmospheric Research.
Commissioned by Environment Canterbury in 2020, this report was designed to provide up-to-date information on water quality in Te Ihutai Estuary and its rivers, to guide environmental management decisions by ECan, the Christchurch-West Melton Zone committee, and partner agencies including local rūnanga, the Christchurch City Council and Avon-Heathcote Estuary Ihutai Trust. To achieve this, physio-chemical and microbiological water quality data collected from 2007-2019 was used alongside statistical analysis, modelling, and ecological and human health assessments to answer eight key questions.
Results from questions 1 and 2 demonstrated that water quality in the estuary had improved since 2007 due to the diversion of nutrient discharge from Christchurch’s wastewater treatment plant in 2010. Results from Question 3 showed that high faecal indicator bacteria indicated human health risks, while high nutrients and turbidity at many sites left the estuary very highly susceptible to macroalgal eutrophication. Question 4 results indicated that many parameters varied seasonally, while Question 5 results indicated that while birds potentially contribute high concentrations of faecal indicator bacteria to the estuary, data was lacking to understand their impact over time. Question 6 results demonstrated that following the diversion of wastewater discharge in 2010, the Ōtākaro and Ōpāwaho rivers remained primary drivers of poor estuarine water quality.
Following Question 7 and 8, the report determined that while the current estuary water quality monitoring programme was adequate to monitor ecological changes across the estuary over time, regular macroalgal monitoring was recommended as a bioindicator of estuarine nutrient and climatic responses. It also recommended that future investigations in the estuary should focus on acquiring water flow and quality information on drains entering the estuary, alongside modelling estuary residence times.