Degradates: A Key to Assessing Herbicide Impacts in Streams

In 1998 the U.S. Geological Survey conducted the first large-scale reconnaissance for herbicide degradates in streams. Degradates are the product of the environmental transformations of the parent pesticide, and they can have similar properties to the parent pesticide. Over 70 Midwestern streams were sampled as part of the reconnaissance. Results from the reconnaissance show that the concentrations of herbicide degradates were relatively high when compared to their parent compounds and that herbicide degradates make up a significant share of the total herbicide load in steams during post-application runoff events. Specific results from the reconnaissance include:

• The median concentrations of most of the herbicide degradates were greater than the median concentrations of their parent pesticide.
• Maximum concentrations of the herbicides acetochlor, atrazine, alachlor, cyanazine, and metolachlor all exceed the maximum concentration of their degradates.
  

The results suggest that studies in which only parent herbicide compounds are measured will not tell the complete story of the potential impacts of herbicide use on aquatic ecosystems. Including herbicide degradates is important because many of the herbicide degradates have similar toxicities as their parent compounds. Degradates are also important in ground-water studies.

These results are part of a much larger study that has sampled the same streams since 1989. The results from the long-term study suggest that since 1989 there has been some lessening of the high herbicide concentrations in Midwestern streams during runoff events after herbicides have been applied to agricultural fields. This good news is tempered by the discovery of relatively high concentrations of herbicide degradates in the 1998 samples. More information on the decline of herbicide concentrations in Midwestern streams is available.

More Information on Pesticides

• Agricultural Chemicals in the Upper Midwest (Corn Belt) Investigation
• The Fate of Cotton Pesticides in the Southern United States Investigation
• Topical Information on Agricultural Chemicals
• Glyphosate Found in Wastewater Discharged to Streams
• A Decade of Research on the Occurrence of Triazine Herbicides in the Environment Leads to a Unique Summary
• Glyphosate Herbicide Found in Many Midwestern Streams, Antibiotics Not Common
• Tracking Atrazine and a Breakdown Product (deethylatrazine) through a Drinking-Water Reservoir
• Measuring Pesticides and How They Transform in the Environment
• Management Practices a Factor in Herbicide Declines
• Where are the Pesticides?
• Science of the Total Environment Special Issue on Agricultural Chemicals

References

Battaglin, W.A., and Goolsby, D.A., 1999. Are shifts in herbicide use reflected in concentration changes in Midwestern Rivers?: Environmental Science and Technology, v. 33, p. 2917-2925.

Scribner, E.A., Battaglin, WA, Goolsby, D.A., and Thurman, E.M., 1999, Changes in herbicide concentrations in Midwestern streams in relation to changes in use, 1989-98, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, p. 313-322.

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