USDA study finds higher rates of herbicide volatilization than expected

According to a U.S. Department of Agriculture (USDA) study published in the Journal of Environmental Quality, the volatilization of atrazine and metolachlor, two herbicides known to contaminate surface and ground water, consistently results in herbicide movement off the target site that exceeds nontarget field runoff, varying widely depending upon weather conditions. Linked to endocrine disruption, cancer, developmental effects, and more, increased levels of these hazardous pesticides in the air is cause for concern. When averaged over the two herbicides, loss by volatilization is about 25 times larger than movement from surface runoff, despite low vapor pressures. Agricultural Research Service (ARS) soil scientist Timothy Gish, PhD and ARS micrometeorologist John Prueger, PhD led the investigation, which looks at the field dynamics of these two herbicides commonly used in corn production.

Prior to this field study, many experts believed that volatilization was not a contributing factor to water contamination because atrazine and metolachlor had a low vapor pressure. However, the monitoring of both herbicide volatilization and surface runoff at the field-scale over multiple years had never been done. So the team set up a 10-year study in an experimental field in Beltsville that is equipped with remote sensing gear and other instrumentation for monitoring local meteorology, air contaminates, soil properties, plant characteristics, and groundwater quality. This allowed the team to carry out its studies on a well-characterized site where only the meteorology—and the soil water content—would vary.

Drs. Prueger and Gish observe that when air temperatures increases, soil moisture levels have a tremendous impact on how readily atrazine and metolachlor volatilize into the air, a key factor that had not been included in previous models of pesticide volatilization. When soils are dry and air temperatures increase, there is no increase in herbicide volatilization, but herbicide volatilization increases significantly when temperatures rise and soils are wet. Most of the volatilization from wet soils occurs within the first 3 days after the herbicide is applied.

The largest annual runoff loss for metolachlor never exceeds 2.5%, whereas atrazine runoff never exceeds 3% of that applied. On the other hand, herbicide cumulative volatilization losses after 5 days range from about 5 to 63% of that applied for metolachlor and about 2 to 12% of that applied for atrazine. Additionally, daytime herbicide volatilization losses are significantly greater than nighttime vapor movement.

Atrazine is used to control broad leaf weeds and annual grasses in crops, golf courses, and residential lawns. It is used extensively for broad leaf weed control in corn. The herbicide does not cling to soil particles, but washes into surface water or leaches into groundwater, and then finds its way into municipal drinking water. It has been linked to a myriad of health problems in humans including disruption of hormone activity, birth defects, and cancer. As the most commonly detected pesticide in rivers, streams and wells, an estimated 76.4 million pounds of atrazine is applied in the U.S. annually. It has a tendency to persist in soils and move with water, making it a common water contaminant. Atrazine is a major threat to wildlife. It harms the immune, hormone, and reproductive systems of aquatic animals. Fish and amphibians exposed to atrazine can exhibit hermaphrodism. Male frogs exposed to atrazine concentrations within federal standards can become so completely female that they can mate and lay viable eggs.

Metolachlor is used for grass and broadleaf weed control in corn, soybean, peanuts, sorghum, and cotton, as well as on lawns, golf courses and ornamental plants. It is classified as a possible human carcinogen. Evidence of the bioaccumulation of metolachlor in edible species of fish as well as its adverse effect on the growth and development raise concerns on its effects on human health. Metolachlor is a suspected endocrine disruptor and linked to organ damage.

Avoid contributing to a food system that relies on toxic pesticides that pose hazards to consumers, workers and the environment by eating organic food. Atrazine and metolachlor are routinely applied to conventional corn production. There are 83 pesticides with established tolerance for corn, 36 are acutely toxic creating a hazardous environment for farmworkers, 79 are linked to chronic health problems (such as cancer), 11 contaminate streams or groundwater, and 71 are poisonous to wildlife. Learn more about the hazards associated with chemical-intensive food production on our Eating with a Conscience webpage.

Source: beyondpesticides.org