Many rivers contaminated by towns and
industry are healthier today than 20 years
ago.
Image: iStockphoto

Two decades of monitoring river water quality by National Institute for Water and Atmospheric Research (NIWA) has provided important information that is helping to care for New Zealand’s iconic rivers.

More than 18 000 river water samples have been collected by New Zealand’s National Rivers Water Quality Network (NRWQN) over the past 20 years to monitor river conditions.

The monitoring shows that overall New Zealand’s river water quality is in good condition by international standards, especially rivers in native forest and high country areas.

However, rivers running through pastoral areas are degraded by nutrient enrichment, fine sediment reducing visual clarity, and contamination by faecal microbes. The microbial pollution is of particular concern for reducing suitability of our rivers for swimming.

Trend analysis also shows that pollution of New Zealand rivers from point sources (such as discharges of wastewater from towns and industries) has reduced appreciably since the late 1980s.

However, nitrogen and phosphorus levels have increased at many sites due largely to ‘diffuse pollution’ from pastoral farming – with increased stocking rates and use of fertilisers, and conversion of land used for sheep/beef farming to dairy or deer farms.

New Zealand has about 425 000 kilometres of rivers and streams, which are highly valued for recreational activities such as swimming and boating, and provide essential services such as drinking water supply, irrigation, and hydro-electricity.

Every month since 1989, NIWA field staff have visited 77 sites, (mostly upstream and downstream), on 35 of New Zealand’s larger rivers to take measurements and collect samples for assessing water quality. This complements monitoring on smaller waterways carried out by regional authorities.

Our staff measure the dissolved oxygen concentration in the water, its temperature, and visual clarity while on site. They also assess the algae growing on the river bed (known as ‘periphyton’).

A sample of water is collected for laboratory analysis of pH, salt content, water cloudiness, coloured dissolved organic matter, nitrogen, and phosphorus. A separate sample is tested for E. coli (a bacterium that is usually harmless, but indicates faecal contamination).

The NRWQN is mainly funded by the New Zealand Foundation for Research, Science and Technology (FRST), and operated by the NIWA.