Pesticides entering streams from run-off or spray drift can harm native wildlife. Photo: Stella McQueen

A new study highlights we don’t know what pesticides are in our streams, we haven’t set acceptable environmental levels for many of them – or tested their effect on native species. 

Freshwater streams studied in areas of agricultural New Zealand contained a cocktail of six different pesticides. 

Two streams in Waikato and one in Otago contained six pesticides and 78 percent of the 36 streams tested contained two or more, some of which are banned overseas. 

In some streams, concentration levels of pesticides exceeded levels considered safe for fish.

University of Otago professor Christoph Matthaei, who co-authored a study into pesticides presence in streams, said groundwater monitoring was regularly completed but streams and rivers were not tested for pesticides.

“This lack of knowledge on the distribution of pesticides and their concentrations in our waterways needs to be addressed. Not only are our freshwater fish species at risk, but so too are the animals they eat – aquatic insects such as mayflies and other invertebrates.”

The study tested 36 streams in agricultural areas over one spring and summer. Matthaei worries that due to a record-breaking drought, the levels detected were an under-estimation of normal levels. He would like to repeat the tests on at least 200 streams for a full year. 

The results 

Seven pesticides were targeted in the study. These were atrazine, chlorpyrifos, clothianidin, diazinon, 2,4-dichlorophenoxyacetic acid (2,4-D), imidacloprid, and thiamethoxam.

Around 86 percent of streams contained chlorpyrifos, a substance banned in the United States, several European countries and in residential New Zealand.  

It’s used to protect crops from insects and mites, as well as manage grass grubs in pasture. 

In humans it’s been linked to intellectual disabilities. For wildlife it can be lethal or damaging. A 2016 study found honey bees exposed to levels considered safe became ‘dumbed down’. For fish, prolonged exposure to even levels considered sublethal can lead to anaemia and reduced growth. 

In two streams the level of chlorpyrifos was higher than recommended levels for fish.

The study notes: “It is clear from these results, combined with the high detection frequency for this insecticide discussed earlier, that more monitoring of chlorpyrifos in New Zealand streams is needed and mitigation strategies may be needed.”

They found two or more pesticides at 75 percent of the sites, and four or more pesticides at 39 percent of sites.

Stab in the dark

The Environmental Protection Authority (EPA) manages hazardous substances in New Zealand, but does not gather data on what volumes of chemicals are used, or where they are used.

Last year, talking to Newsroom about the plan to create a chemical map of New Zealand, CEO Dr Allan Freeth put it bluntly: “I have to be honest and say we really don’t know what’s out there.”

One study completed by AgResearch estimates in 2004, 1278 tonnes of pesticide was spread over pastoral land. Around 36 percent of which ended up on dairy pasture to combat bugs such as grass grubs, weevils and beetles. Without pesticides, the intensification of agriculture could not have happened. 

Matthaei said the selection of streams was a “stab in the dark”. Without data showing where pesticides are used heavily he worries the small number of streams combined with the drought may have painted a rosier picture than reality.

“To get a better picture, this study needs to be repeated at a much, much larger scale.”

He would like to do studies in areas where pesticide use is known and test nearby streams before and after pesticide application. 

“Then you zoom in on the streams with the highest pesticide concentrations and you try to target sampling during rainfall events when you know the surface run-off to really get a handle of peak concentration.

“It’s a massive effort to get a comprehensive picture.”

Unlike New Zealand, Matthaei said in North America and Europe thousands of sample sites had been tested over 10 to 15 years.

The nymph of Deleatidium, NZ’s most common mayfly, has single, leaf-like gills. Photo: Landcare Research CC BY 4.0

Another stab in the dark is understanding the exact effect on native species. Around 76 percent of New Zealand’s indigenous fish species are threatened with extinction, along with 33 percent of plants which rely on fresh water and 25 percent of native freshwater invertebrates.

Tests are often based on species other than those likely to be affected in New Zealand streams.

“They’re mainly based on theoretical values and certainly not ecotoxicological tests with the sensitive organisms. The standard lab rat in ecotoxicological tests is daphnia, the water flea.”

The water flea is six times less sensitive to neonicotinoids than New Zealand’s mayfly are. 

Crayfish are another example. It’s thought an insecticide sprayed by helicopter was the probable cause of the death of hundreds of freshwater crayfish in Waikato. It was being sprayed to kill beetles on eucalyptus forestry. A local who caught the scene on camera said he could have filled a car trailer with dead crayfish.

Overseas the same chemical is used to kill pest crayfish. University of Waikato’s Nicholas Ling, who helped with the crayfish investigation, noted the lack of New Zealand-specific evidence.

“Some of the testing that’s been done overseas shows that there’s up to 20-fold differences in sensitivity between different crayfish species. We don’t know where our species fall on that continuum. Presumably, from the evidence of this incidence, New Zealand’s crayfish are at the sensitive end but we just don’t know because we’ve never tested them.”

Matthaei thinks pesticide levels in water is a topic that’s gone under the radar while nutrient run-off, algal blooms and sedimentation has been gaining attention. 

“I’m guessing with pesticides it will probably take 10 years of research effort as well, talking to policy-makers, talking to politicians. It’s not going to be something that is going to happen overnight.”

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