A popular herbicide is showing up in drinking water supplies, threatening its future use by Indiana farmers.

Producers who use weed control products with the active ingredient atrazine need to be extra careful when applying the herbicide to cropfields near watersheds, says Bill Johnson, a Purdue University Extension weed scientist.

High levels of atrazine have been found in almost a dozen watersheds in Indiana. Farmers can reduce the risk of atrazine runoff by not making applications nearer to water sources than recommended or at times when the herbicide is more likely to wash away, Johnson says.

Atrazine is used extensively in corn and sorghum production. It has not been linked to health problems in humans.

In 2003 the U.S. Environmental Protection Agency began requiring that atrazine manufacturers increase monitoring of selected surface water supplies, says Leighanne Hahn, water quality program specialist in the Office of the Indiana State Chemist (OISC). The OISC has regulatory oversight of pesticide use in Indiana.

"We began recognizing the concern with atrazine in surface water supplies when the federal Safe Drinking Water Act was implemented," Hahn says. "The law applies to all public drinking water supplies."

Indiana community water systems in the atrazine monitoring program include Batesville, Bedford, Fort Wayne, Indianapolis (Eagle Creek), Jasper, Logansport, Santee Utilities, Stucker Fork, Versailles, Westport and Winslow. Together, the 11 water systems serve more than 1.1 million people.

Millions of pounds of atrazine are applied annually to corn and sorghum in Indiana to control a wide range of broadleaf and grassy weeds, Johnson says. Atrazine is the main ingredient in about 40 name-brand herbicides.

"Atrazine in corn is like Roundup in soybeans," Johnson said. "It's easily the No. 1 herbicide used in corn. It's used on over 80% of the acres. The reason it's used is because it's cheap, it's effective and it controls more weeds per dollar spent on herbicide than any other herbicide that we have in corn."

Most Hoosier farmers apply atrazine to their fields in April and May, when rainfall amounts can be heavy and saturate soils. Because it’s water-soluble and slow to break down when dissolved, atrazine often moves with rainwater from where it was applied to nearby bodies of water, Johnson says.

"Monitoring data has shown that atrazine is not showing up in groundwater but is showing up in surface water, such as reservoirs, ponds and lakes," he says. "That's causing our regulatory agencies to become concerned."

The EPA set a maximum annual average for atrazine in public drinking water of 3 parts per billion (ppb), Hahn said. Annual average levels of more than 3 ppb have been measured in some Indiana drinking water sources, especially during wetter-than-normal years, she says.

In response, the EPA ordered atrazine manufacturers to develop a comprehensive monitoring program for community water systems with high atrazine levels. Chemical companies that do not participate cannot continue to market atrazine.

In turn, farmers within affected watershed areas must implement strategies for reducing -- or mitigating -- atrazine in surface water or face losing the herbicide as a weed control option.

"Think of the atrazine mitigation plan as kind of a 'three strikes and you're out' sort of plan," Johnson says. "One detection and you go into an enhanced monitoring program. If you have another detection within a five-year period, you have to develop a mitigation plan. Once that mitigation plan is developed, it is submitted to the EPA for approval. If you have another detection after that mitigation plan is put into place, then atrazine is banned.

"At this point we have 11 watersheds in the state that have one strike against them. We have one watershed in the state -- Batesville -- that has two strikes against it."

Farmers themselves can minimize atrazine runoff by exercising common sense and good application practices, Johnson says.

"If we follow the setback restrictions as far as how close we can apply atrazine to surface water supplies, streams and standpipes on terraced land, we'll go a long way in minimizing loss of atrazine into surface water," he says.

"Another thing we can do is pay attention to the application rates. If we'll shift the application timing to use more atrazine postemergence as opposed to soil-applied, we can reduce the rates quite significantly and still get equal levels of control. And, we should not spray saturated soils just before a rainfall. But that's a little more difficult to control."

Other steps farmers can take to reduce atrazine runoff include:

  • Establish 66-ft. grass buffer strips along bodies of water and ditches to help filter out atrazine from water flowing across fields.
  • Switch to herbicides that are tank-mixed with atrazine, reducing the amount used.
  • Turn off the sprayer when crossing grass waterways.
  • When planting near water sources, choose crops that do not require the use of atrazine.
  • Follow the directions on herbicide product labels.

Several programs are available to help farmers with the cost of setting up buffers or other systems to reduce atrazine runoff, Johnson said. Contact county Extension or the USDA Farm Service Agency to find out more.

Additional information on atrazine is available in Purdue Extension publications PPP-66, “Atrazine and Drinking Water: Understanding the Needs of Farmers and Citizens," and PPP-67, "Atrazine Use and Weed Management Strategies to Protect Surface Water Quality." The publications are available online at http://btny.purdue.edu/Pubs/PPP/PPP-66.pdf and href="http://btny.purdue.edu/Pubs/PPP/PPP-67.pdf , respectively. (This requires Adobe Acrobat Reader, download at: www.adobe.com.)