Agricultural Pesticides and the Atmosphere

Humans have waged war against plant and animal pests that threaten their food supplies ever since the first crops were planted thousands of years ago. The arsenal of toxic chemicals used to control weeds and insects nowadays, however, may be causing casualties beyond the borders of our farms.

Studies carried out by scientists at Environment Canada (EC) and Agriculture and Agri-Food Canada (AAFC) show that some pesticides escape into the atmosphere through a process known as volatilization. After application, these chemicals are released as gases or adhere to particles, such as soil dust. Some of these pesticides can travel long distances in the atmosphere before they wash back down to earth in rainfall or settle out through dry deposition. This not only poses a serious threat to non-target areas, such as wetlands and other sensitive ecosystems, but also represents a significant economic loss to farmers.

Pesticides are somewhat unique among industrial chemicals in that they are designed to be highly toxic, yet are distributed and applied widely in the environment. In the Canadian prairies, where agriculture is a primary industry, most cultivated land is treated with pesticides. Herbicides are the most commonly used, with more than 20 000 tonnes applied to farms in Manitoba, Saskatchewan and Alberta every year. Although herbicides are mainly toxic to plant systems, they can be toxic to mammals as well, and little is known about the long-term effects of low levels of such chemicals or their combined effects on other organisms. The use of insecticides, which are generally more toxic to mammals, is less common but has increased significantly in recent years due to a rise in insect-susceptible crops such as canola and lentils.

Saskatchewan uses almost half of all the pesticides applied to crops in the Canadian prairies. To track the movement of these chemicals, EC and AAFC took soil and water samples on treated cropland and nearby sites, and air and bulk-deposition samples at these sites and others located far from agricultural activity. To ensure that measurements reflected the significant quantity of dry deposition that occurs under prairie conditions, scientists developed a system to measure both wet and dry deposition—a self-rinsing steel tray that washes material deposited on its surface into a reservoir for separation and later analysis.

Concentrations of a number of herbicides were detected in the atmosphere as well as in the water and surface film of farm ponds or dugouts. Among the most prevalent were 2,4-D and triallate—pesticides that are applied to crops mainly by tractor-drawn equipment in order to control weed infestations in cereal crops. Both are used extensively in the prairies, where it is estimated that more than 3.8 million kilograms of 2,4-D and 2.7 million kilograms of triallate are applied annually. Results showed that concentrations of pesticides in farm ponds were higher than normal after the growing season—evidence that atmospheric transport is a major source of these contaminants. Studies at agricultural research stations in the province estimated post-application volatilization rates for 2,4-D and triallate at about 18 per cent.

Other evidence of the atmospheric transport of pesticides came from air samples taken near Regina, Saskatchewan, in Yellowknife, Northwest Territories, and in the Arctic—all of which contained pesticides that were not from local sources. Of particular concern were concentrations of the insecticide lindane, a suspected carcinogen that has the potential to accumulate in the fatty tissues of animals. An estimated 20 400 tonnes of lindane are applied annually around the world—500 tonnes of it in Canada alone, where it has been used primarily as a treatment on canola seed.

While the Regina air samples contained fairly high concentrations of the form of lindane used on canola crops in Canada, Arctic air samples also contained another form used in India and other countries, proof that the pesticide has the capability to travel thousands of kilometres in the atmosphere. To determine just how much lindane was entering the atmosphere from the five million hectares of canola grown in the prairies, where about 95 per cent of canola seed is pre-treated with lindane, Environment Canada scientists undertook the first-ever tests of volatilization from pesticide-treated seed.

Air concentration and wet-dry deposition samplers were stationed in a treated canola field northwest of Saskatoon, Saskatchewan, an abandoned farmyard two kilometres away, and just outside of Waskesiu, in Prince Albert National Park. Data were gathered over two growing seasons, and additional tests were carried out using flux chambers placed directly on the soil to trap escaping gases and particles. Based on their findings, which included atmospheric concentrations of lindane as high as 16.1 nanograms per cubic metre at the treated field, scientists estimate that up to 30 per cent of lindane applied to canola seed enters the atmosphere through volatilization. This represents an atmospheric loading of up to 188.8 tonnes in the prairies alone during the six-week period following planting.

A high-volume air sampler measures concentrations of lindane and other airborne pollutants above a canola field in Saskatchewan.

One reason for this high rate of volatilization is that lindane has a high vapour pressure, and the treated seed is planted at a depth of only about three to five centimetres. This makes it readily exposed to the air, particularly in the coarse soil typical of the prairies. It was also noted that soil moisture increases the rate of volatilization, perhaps by displacing the pesticide in the soil or by drawing it out through evaporation. A similar effect has been documented with triallate, which is tilled into the soil as a granule before or after seeding.

The results of the study confirm that quantities of lindane released from treated seed during the growing season contribute significantly to regional atmospheric concentrations. Although the significance of Canada's prairies as a source of lindane to other North American and global ecosystems is currently under study, forward trajectories have shown that both the southern Ontario/Great Lakes region and the Arctic receive lindane from the West through atmospheric deposition. The paths being taken by other pesticides that enter the atmosphere are more difficult to pinpoint because those chemicals are in broader use across the country and, therefore, their sources are less easily defined.

Lindane manufacturers in Canada voluntarily withdrew the use of lindane as a seed treatment on canola at the end of 1999, although producers were allowed to use existing stock until July 2001. The use of lindane as seed treatment on canola is now discontinued in Canada, although it is still permitted for use on some cereal and vegetable crops. Although they represent a small amount of product use in comparison to lindane use on canola, these remaining uses are undergoing review by Health Canada's Pest Management Regulatory Agency in cooperation with the United States Environmental Protection Agency. Both agencies have expressed interest in the results of the atmospheric transport study.

Scientists are hoping that the results of these studies, all of which have recently been or soon will be published in scientific journals, will encourage more funding for research on the toxicity, behaviour, transport and non-target effects of pesticides currently in use. For example, more than two dozen new herbicides have come into use in Canada within the past two decades that are registered for application in very low concentrations, but are very toxic to plants. As the use of aerially sprayed insecticides increases, so too does the need for studies on the movement and effects of these chemicals.

Exposure to annual releases of some pesticides through breathing, consuming contaminated water, or eating contaminated plants could have significant effects on the health of humans and other animals. Low levels of exposure to a mixture of toxic herbicides may also threaten vegetation in non-target habitats. The more we know about the behaviour and effects of these pesticides, the better equipped we will be to safeguard environmental and human health over the long term.

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