A river in decline
The water quality of Gisborne's Taruheru River is among the worst of rivers in New Zealand, data from Land, Air Water Aotearoa (LAWA) shows.
The river, which starts at Waihirere Domain and ends where it meets Waimata River in Gisborne city centre, is one of only three rivers in the country rated “state D (poor)” for ammonical nitrogen and has also tested high in other categories such as nitrate, E.coli and phosphorous levels.
LAWA says the river's health over a decade has been in a “declining trend”.
“The reasons for this are a combination of contaminants from both urban and rural land use within the catchment,” Gisborne District Council chief of strategy and science Joanna Noble says.
These may include sediment, faecal bacteria, nutrients from rural and urban land, and stormwater and wastewater overflows.
According to State of the Environment 2020 document, the two key nutrient components that greatly affect stream health are phosphorus and nitrogen — both of which have been found in higher concentrations in Taruheru River.
While testing in July found the phosphorus level in the river was within the LAWA guidelines, the level of nitrates (a form of nitrogen) was a concerning 1.5 milligrams per litre.
“The natural amount of nitrate would be barely measurable and as per the ANZECC (Australian and New Zealand Environment and Conservation Council) guidelines, 0.44 milligrams is the recommended limit, so it's three times more than what is the standard,” freshwater ecologist Mike Joy said.
LAWA data also showed the river had higher E.coli levels in June — 1100 CFU (colony-forming units)/100 millilitres.
Dr Joy said anything above 260 CFU/100ml was not safe to swim in.
“So these grades for E.coli are really bad,” he said.
Ms Noble said the council was aware of the state of the river and had proactively engaged with sectors of the community to work towards improving its quality.
“The council is required to undertake actions to improve the health of the Taruheru as part of the Waipaoa Catchment Plan (WCP), which is part of the Tairāwhiti Resource Management Plan,” she said.
The WCP sets the water quality objectives and limits and includes the Taruheru and Waikanae catchments.
“One requirement of the Waipaoa Catchment Plan is to undertake a Taruheru River restoration project,” she said.
Council staff are to do the project this summer, in collaboration with local iwi and the community, to try to further identify sources of contamination upstream of the city.
“Once sources have been identified the council will develop actions to help improve the health of the Taruheru,” she said.
Council environmental science manager Tom Porter said the council would be looking to follow up the project with regulatory and non-regulatory procedures.
“Regulatory would include enforcing landowners to make sure the nutrients that come out are monitored from their land and also non-regulatory things like planting riparian stream banks,” he said.
Mr Porter said as the plan was still in development it would take time for improved results.
With planting it could take 20 to 30 years for effects to be shown.
“It's a very complex situation. It is something the whole world is grappling with and we are learning from other regions and countries as to what works and what doesn't,” he said.
Ms Noble said in terms of monitoring the water quality of the river, the council did monthly water quality sampling.
However, Dr Joy said such a sampling approach was “silly” as the amount of concentrations in different categories of E.coli, nitrates, phosphorus and ammonia were “incredibly variable”.
“For example, imagine the southern motorway in Auckland and say a person took a snapshot of the traffic at three o'clock in the morning. With no cars on the road the person could infer there were no cars on Auckland highway and hence no need for the highway. But a snapshot at about seven o'clock would tell a completely different story — an opposite effect.
“So that's the kind of problem with monthly sampling . . . you need continuous monitoring.”
According to LAWA while the level of phosphorus in the river for July was measured at 0.15 mg/l, in January it peaked at 1.1 mg/l.
Dr Joy said that peak was high.
“It should be very low, less than half a milligram.
The nitrate level peaked at 7.2 mg/l in May and at 2000 CFU/100ml for E.Coli.
LAWA's figure over 10 years had an even higher peak for each category.
Dr Joy said investing in better technology was needed to ensure frequent sampling and more accurate results.
Ms Noble said the council was working with the community on a combination of urban and rural land initiatives.
Some of the rural community had been working on a future-proofing vegetable production project, which has involved working with rural landowners and farmers on nutrient budget decision-making.
“Such land-based actions take time to show improvements to the river water quality,” Ms Noble said.
Dr Joy says the “degrading trend” is problematic.
“It's bad by New Zealand standards but New Zealand standards are bad in lowland rivers anyway, so it's doubly bad.”