155 Million Pounds of Pesticide Leach Into Aquifers Every Year

During World War I, modern pesticides were developed as the by-products of research on nerve gas and explosives. After World War II, synthetic organic compounds ushered in a new era of pesticides—one that conveniently found a use for chemicals previously used in war. The chemical industry transitioned to peacetime by, for example, pushing DDT everywhere it could, even with the knowledge that "non-target" species were dying as a sort of collateral damage.

Thankfully, we recognized the horrid toxicological effects of DDT, and it is now banned in much of the world. However, we are still enthusiastic about spraying poison on living things. Currently, around 3 million tonnes of agricultural pesticides are used worldwide every year. In Imperial measurements, that's around 3.3 million tons or 6.6 billion pounds.

It is an enormous number, and here's the thing. We apply these vast amounts of pesticides to crops, but they don't just magically disappear once they have done their job of killing their intended victims.

Researchers now reveal that pesticides, in fact, travel far and wide after initial agricultural application, leaching into aquifers, rivers, oceans, and soils.

The global study analyzed the geographic distribution of 92 of the most commonly used agricultural pesticides and found that approximately 70,000 tonnes (77,000 U.S. tons or 155 million pounds) of potentially harmful chemicals leach into aquifers each year, impacting ecosystems and freshwater resources. 

“Our study has revealed that pesticides wander far from their original source," says lead author Federico Maggi from the University of Sydney’s School of Civil Engineering. "In many cases these chemicals end up a long way downstream and often, though in much smaller amounts, all the way to the ocean.” 

The researchers show that some 80% of applied pesticides degrade into byproducts ("daughter molecules") in the soil that surrounds crops.

“This degradation of pesticides often occurs as a ‘cascade’ of molecules into the surrounding environment, which can persist in the environment for a long time and can be just as harmful as the parent molecule or applied pesticide. One such example is glyphosate. Although it is highly degradable, it breaks down into a molecule known as AMPA that is both highly persistent and toxic,” says Maggi.

The study reveals that only a fraction of pesticides enter river systems. But once they are in the water, most of the active ingredients end up in the ocean. This puts at risk the very basis of marine and freshwater food chains," the authors explain, with potential negative impacts on marine wildlife and coral reefs.

“On paper, 0.1 percent leaching into fresh waterways might not sound like much,” says Maggi. “But it only takes a tiny amount of pesticides to have a negative impact on the environment.” 

And indeed, it amounts to 730 tonnes (1.6 million pounds) of pesticides entering rivers alone each year. As the study explains, about 13,000 kilometers (8,100 miles) of rivers display chemical concentrations above safety limits for a number of aquatic plants and invertebrates—and nobody is sure about what kind of impact this has on river ecosystems. 

“Just because we don’t see pesticide residues in soil and water doesn’t mean they’re not there, impacting critical systems on land, rivers and oceans," says paper co-author Dr. Fiona Tang from Monash University.

Remarkably, the authors say that the numbers are conservative because not all pesticides were included in the analysis. Legacy pesticides, those used in aquaculture, private dwellings, and public spaces, were not included—meaning the risk of these chemicals to ecosystems and people is likely higher.

"We must urgently adopt sustainable management strategies to promote reductions in field applications of harmful pesticides and set in place systems to effectively monitor their use under the 2030 Sustainable Development Agenda," says another one of the paper's co-authors, Dr Francesco Tubiello, Senior Environmental Statistician at the United Nations’ Food and Agriculture Organisation (FAO).

With a rapidly expanding human population and the daunting logistics of growing enough food to feed everyone, the topic of pesticides can become contentious. Advocates argue that they are needed in order to grow adequate food. But Maggi points out that reducing pesticide use worldwide is possible without threatening food security, as long as such initiatives are designed and implemented in consultation with food producers.

“Globally, there is a lot of room to increase efficiencies and yield while still supporting an abundant food supply through new technology and modern crop management practices,” he says. In a separate paper, Maggi laid out recommendations to reduce pesticide use, including calling for a reliable set of indicators and improved monitoring.

"It is important that national authorities disclose statistics on the use of agricultural inputs, be they fertilizers or pesticides, given the effect they have on the environment and ecosystem service."

The study, "Agricultural pesticide land budget and river discharge to oceans," was published in Nature.

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