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Agricultural Chemicals: Sources, Transport, and Fate

Approximately 40 percent of the land in the United States is used for agriculture. When land is converted to agricultural use, the hydrologic processes of the natural landscape are modified toward optimizing agricultural production. Oftentimes these modifications have unintended environmental impacts, including changes in water quantity and water quality. 

The U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program is assessing the sources, transport, and fate of chemicals applied to crops in agricultural basins across the Nation (referred to as “study units,” see map and table). Chemicals selected for study include nutrients (nitrogen and phosphorus) and about 50 commonly used pesticides and their transformation products, including triazine and acetanilide herbicides such as atrazine and metolachlor, and organophosphorus insecticides such as chlorpyrifos and diazinon.

The basins in the studies represent a range of agricultural settings—with varying crop types and agricultural practices related to tillage, irrigation, artificial drainage, and chemical use—as well as a range of landscapes with different geology, soils, topography, climate, and hydrology. Consistent methodology and analysis allow comparisons among the different basins. This study design leads to an improved understanding of the many factors that can affect the movement of water and chemicals in different agricultural settings.

Information from these studies will help with decision-making related to chemical use, conservation, and other farming practices that are used to reduce runoff of agricultural chemicals and sediment from fields. This information also will benefit the U.S. Environmental Protection Agency, the Department of Agriculture, local and regional water managers, and agricultural chemical manufacturers who are involved in managing chemical use and pesticide registration.

Objectives of the agricultural chemicals study

1. Quantify the amount of water and selected agricultural chemicals entering, leaving, and accumulating within the watershed (referred to as an annual “mass budget”).
2. Determine rates of transport and residence times of water and chemicals in ground water, the soil zone, and in streams.3. Assess transport and transformation of selected chemicals in different parts of the hydrologic system, as affected by natural processes, chemical properties, and agricultural management practices.4. Develop tools and quantitative methods, such as models, to characterize the transport and fate of chemicals within the watershed, and extrapolate the findings to similar, unmonitored agricultural and environmental settings.5. Interpret study results as to the implications for managing the water and water-quality impacts of agricultural systems.

Complex factors control the transport and fate of agricultural chemicals

These USGS studies address the complex processes controlling the transport and fate of agricultural chemicals in streams and ground water, as well as how these processes are affected by natural factors, agricultural practices, and chemical properties.

Not all of the agricultural chemicals remain in the soil or are taken up by plants; a small amount can move upward into the atmosphere, downward through the soil into shallow ground water and underlying aquifers, or across the land into streams. This movement is controlled, in part, by natural factors such as hydrogeology, climate, topography, and soils, making water resources in some basins more vulnerable to contamination than others.

For example, throughout much of the upper Midwest, large amounts of chemicals are used on corn and soybean fields, but underlying ground water generally has relatively low concentrations of agricultural chemicals because the low permeability soils and glacial till minimize downward movement. On that part of the Delmarva Peninsula in Maryland, ground water is more vulnerable because of permeable soils underlain by sand and gravel, which enable rapid infiltration and downward movement of water and chemicals. Streams are most vulnerable to contamination from agricultural chemicals where poorly drained soils, agricultural practices, and topography encourage the rapid movement of water off of fields, or where tile drains and ditches quickly transport agricultural runoff from fields to streams, such as in the White River Basin in Indiana. Natural transformation processes, such as denitrification, and chemical properties play an important role in transport.

For example, poorly drained and poorly oxygenated soils with organic-rich sediments can promote denitrification, which can thereby reduce nitrate concentrations entering streams. Some chemicals, such as atrazine and nitrate, readily dissolve and move with water in both streams and aquifers, whereas many forms of phosphorus and some pesticides, such as chlorpyrifos and permethrin, attach to soil particles and are transported to streams with eroded soil, particularly during times of runoff from precipitation or irrigation. Ground water typically is less vulnerable to contamination by chemicals that strongly attach to soils.

Some chemicals, such as atrazine, are slowly transformed by natural processes and can persist in soil and water for years or even decades. Other pesticides, such as diazinon, are relatively unstable in water and break down to other compounds within days or weeks. Chemicals that persist for a long time are more likely to be transported farther than compounds that are short lived.

Chemical transport also is influenced by agricultural practices, such as methods of tillage and drainage, and chemical use. For example, farmers may leave the soil surface undisturbed from harvest to planting (referred to as “no-till”), and may plant and maintain buffer strips around fields and streams. By decreasing runoff and soil erosion, these practices also decrease chemical transport. Use of drip irrigation in lieu of furrow irrigation decreases the amount of water lost to ditches or evaporation, and allows better control of the amounts of pesticides and nutrients added to irrigation water. A unique feature of these studies is the simultaneous assessment of agricultural chemicals throughout the hydrologic system at many scales.

How study results can be used

The findings of these studies will provide a better understanding of the transport and fate of selected agricultural chemicals in various agricultural and environmental settings. This understanding will enable the development of models to extrapolate information from direct water-quality measurements to similar, yet unmonitored, agricultural areas. Farmers will gain a better understanding of how their operations affect water quality. The findings also will guide future scientific research on processes affecting transport and fate of agricultural chemicals and help water policy makers:

• Optimize selection and timing of monitoring on the basis of understanding geographic areas and water resources most likely to be affected;

• Evaluate conservation strategies such as the protection of riparian areas and buffer strips, crop management, and chemical-use management. Stakeholders associated with the U.S. Department of Agriculture and local and regional water managers will better anticipate the effectiveness and timing of these strategies for controlling transport of chemicals and sediment from fields, in streams, and within the ground-water system; and

• Improve registration and regulation of pesticides on the basis of a better understanding of their transport (including transport of breakdown products), as affected by chemical properties and natural processes.

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