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Daily News Blog

04
Aug

Transport of Pesticides through Waterways Raises Serious Contamination Problems

(Beyond Pesticides, August 4, 2023) The results of an Australian study published in Nature strike a contrast between land and water contamination with pesticide active substances (PAS), highlighting contamination as pesticides are transported through waterways. 

The study results on water transport raise serious contamination issues. Only about one percent of the pesticides entering rivers degrade, so that long stretches of waterways and the oceans suffer the direct impact of a pesticide’s active ingredient. The lack of degradation also means that water organisms are being exposed to levels of pesticides exceeding many of the regulatory threshold limits set by governments. Although observation data are highly variable, the authors note that measured concentrations of pesticides in some river reaches of North America, East Asia and Europe exceed one or more regulatory threshold limits “at least once a year.” Further, the pesticides can bioaccumulate at each level of the aquatic food web, which can multiply concentrations by a thousand or more in the highest trophic levels, according to the study authors.

The study finds that more than four-fifths of PAS are degraded in the soil, leaving about 10 percent of the original chemical in the soil as residue. Nearly half of that residues migrate into deeper layers of the soil where there are fewer microorganisms to break the active ingredient down. This means that fraction of the PAS likely accumulates in aquifers, mostly in its original chemical form. Aquifers are the source of most well water.

The authors of the study, through their assessment of hydrology and biogeochemistry, have developed their estimate of how much pesticide remains on land, how much reaches the oceans, and how pesticides behave in both ground and surface waters along the way. Understanding how pesticides behave in the global water cycle, from wells and ponds to rivers and the oceans, has been lacking.

The authors note, “[I]n many observed cases PAS may degrade into a cascade of daughter substances which can be as toxic as the parent and occasionally even more persistent.” For example, in 2021 Beyond Pesticides covered a study of pesticide metabolites, stating that “neonic [neonicotinoid] metabolites, such as desnitro-imidacloprid and descyano-thiacloprid, are more than 300 and ~200 times more toxic to mammals, respectively, than the parent compound imidacloprid.”

Globally, approximately 3.3 million tons of pesticides are applied to crops every year. While most of these pesticides are applied on land, some portion of everything on land gets into water and ends up in the ocean eventually. Pesticides are no exception.

The scientists assess the hydrology and biogeochemistry of PAS for the 92 most-used pesticides based on 2015 data. The PAS are a fraction of the total mass of pesticide compounds applied to crops, amounting to 1.1 million tons—a third of the total global usage. Of this, the researchers calculate that 783 tons of PAS are released to the oceans annually. So, according to this study, a fraction of the total reaches the oceans.

Active ingredients are virtually the only component of pesticide compounds whose toxicity is tested and regulated, so both the “inert” or “inactive ingredients” and chemicals resulting from geochemical or microbial action or disinfection processes are omitted from calculations of pesticide harms. For example, as Beyond Pesticides noted in its coverage of the pesticide metabolites study, “Nearly half of all breakdown products (transformation products) from four common-use environmental pesticides produce stronger endocrine (hormone) disrupting effects than the parent compound.”

The rivers receiving the most pesticide runoff from land include the Mississippi and the Sacramento in the U.S., the Parana in Argentina, the Ganges in India, the Yangtze, Pearl and Yellow, in China, and the Irrawaddy and lower Mekong in South Asia. These are not all of the rivers that discharge the most PAS into the oceans, however. The Danube and the Amazon joined the group in that category. In all, fifteen of the most important rivers in the world discharge at least 5,000 kg of PAS each to the oceans annually. The effects of pesticides on the oceans—unlike those of fertilizers, which deplete oxygen and cause harmful algal blooms—are little understood.

The researchers calculated another measure of the pesticide burden by comparing their estimates to actual observations in specific locations in North America, the European Union and Australia. They found that their model “generally underestimated observations,” which means that the burden on rivers and oceans is likely also heavier than they had predicted.

The most common chemicals in the modeled waters were glyphosate, metam potassium (a soil disinfectant), chlorothalonil and chlorpyrifos. The ratios of pesticide types in rivers were estimated at just over half as herbicides, about 36 percent multi-purpose pesticides, about 11 percent fungicides and 0.6 percent insecticides. The ratio reaching the oceans was even more unbalanced, with about 63 percent herbicides, a little over a quarter multipurpose pesticides, about 10 percent fungicides and 0.7 percent insecticides.

A final result of the study: The group analyzes which variables exert the most control over how much PAS remain unchanged in soils and how much is discharged to the oceans. In both cases the main predictor is the rate of application to fields. For soil residues, annual soil water saturation, temperature and organic carbon content come next. For the oceans, the next most controlling variables are the surface area of the crop treated with pesticides, the surface area of the watershed, and the length of the river.

The study is a first approximation of the scale of the problem posed by pesticides traveling through planetary waters. Its results suggest that pesticides’ deleterious effects on the biosphere extend much farther than manufacturers claim. Over time, the global water cycle ensures that everything on land, including mountains, reaches the sea, and putting anything into water diminishes any control humans might have over it.  To interrupt the cascade of pesticide catastrophes, understanding the source is critical: the fields where the pesticides are applied. Stopping the process there is the most direct and effective way to start recovering from the damage pesticides cause.

The U.S. Supreme Court is not making the task any easier. As Beyond Pesticides observed last June, President Biden said the Court’s recent ruling in Sackett v.  Environmental Protection Agency (EPA) “will take our country backwards” because it “dramatically limits the EPA’s ability to protect critical wetland ecosystems,” which are integral parts of continental watersheds. This means the burden of pesticides on waterways will increase, and, according to the Australian researchers, part of their active ingredients will traverse rivers all the way to the ocean.

Government agencies are not monolithic in their failure to press for improved regulation. Beyond Pesticides has covered the U.S. Geological Survey’s critique of the EPA’s regulation of pesticides in water. The USGS reported in 2020 that of its 110 National Water Quality Monitoring Network sampling sites, only 2.2% of the water samples were free of detectable pesticides. It is firmly established that humans are exposed to pesticides and their various related compounds through drinking water and carry body burdens of these chemicals that threaten their health.

You can take action. Urge Secretary of Interior Deb Haaland to expand USGS monitoring and mapping of U.S. waterways. Tell EPA Administrator Michael Regan that pesticides shown to contaminate our waters must be banned. For sample letters, click here. Visit here for more information on threatened waters and action suggestions.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Maggi, F., Tang, F.H.M. & Tubiello, F.N. Agricultural pesticide land budget and river discharge to oceans. Nature (2023). https://doi.org/10.1038/s41586-023-06296-x;             https://www.nature.com/articles/s41586-023-06296-x

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One Response to “Transport of Pesticides through Waterways Raises Serious Contamination Problems”

  1. 1
    Tracy G. Taylor Says:

    Thank you for sharing this informative and concerning article. It highlights the critical issue of pesticide contamination in our waterways and its far-reaching ecological impact. It’s essential that we continue to raise awareness about the consequences of pesticide use and advocate for more stringent regulations to protect our environment and health.

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