Polluted waterways: New Zealand’s huge clean-up challenge revealed in report

Scientists have laid out the massive challenge New Zealand faces in cleaning up its polluted waterways, with many lakes and rivers greatly exceeding water quality limits. Photo / Michael Cunningham

Just as Kiwis get set to head to their favourite summer swimming holes, scientists have laid out the massive challenge New Zealand faces in cleaning up its polluted rivers and lakes.

While the worrying state of our waterways is now well known – the latest snapshot graded two-thirds of monitored river sites poorly for harmful E. coli – a just-published report marks the first national assessment of four key contaminants against set standards.

Those contaminants were E. coli, a notorious bacteria linked with human or animal faeces; nitrogen and phosphorus, which can choke waterways with slime and weed growth; and sediment, such as mud and silt, that can make water murky and smother habitats.

For councils, successive national policy statements for freshwater management have set bottom lines for each contaminant – and the new analysis showed substantial reductions were needed in almost every region in New Zealand.

The biggest problem appeared to be E. coli, known to cause vomiting, cramping, nausea and diarrhoea. Three-quarters of New Zealand’s land has waterways where the bacteria is present at higher rates than bottom lines allowed for.

Bringing E. coli loads in line with those standards would require staggering reductions of 91 per cent in Waikato, 90 per cent in Manawatū, 85 per cent in Gisborne and 80 per cent in Northland.

As this map shows, three-quarters of New Zealand's land area is contributing unacceptable amounts of E. coli to waterways. Image / Our Land and Water

Elsewhere, the report, funded by the Our Land and Water National Science Challenge, showed reductions in total phosphorus and nitrogen would be required across catchments comprising 11 and 20 per cent of New Zealand’s land area respectively.

Sediment loads also needed to come down across catchments accounting for just under half of our land area, with Otago, Manawatū, Canterbury and Waikato requiring the biggest drops.

Some areas required the load of more than one contaminant to be slashed by more than 30 per cent – a threshold at which changes in land use or intensity, like shifting to forestry or destocking, would likely be needed.

The report authors said their data didn’t challenge the set bottom lines but rather, set out the size of the job to meet them - and where action was needed most.

“Moving towards these bottom lines is currently the best option we have for improving ecosystem health and swimmability in rivers, lakes and estuaries with poor water quality,” Our Land and Water director Dr Jenny Webster-Brown said.

The former Labour-led Government’s main policy response to the problem was its 2020 Essential Freshwater package, which introduced new environmental standards, mandated farm plans and more stringent rules around fencing and nitrogen loss.

The new coalition Government of National, Act and New Zealand First now planned to replace the current national policy statement to allow councils “more flexibility” in how they meet environmental limits.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.