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Occurrence of Herbicide Degradation Products

Introduction
Our understanding of the potential effects of herbicides on humans and ecosystems is incomplete because in most studies herbicide degradation products (degradates) are ignored. Analytical methods for herbicide degradates are either difficult, expensive, or unavailable. Some degradates are as toxic as their parent compound, but little information is available on degradate occurrence or toxicity. A few recent studies have indicated that herbicide degradates are prevalent in ground water, surface water, and precipitation in the Midwestern U.S. For example, in a study of herbicides occurrence in Iowa municipal wells, 9 of the 10 most frequently detected compounds were herbicide degradates.

Objectives and Methods
In 1989, 1990, 1994, and 1995 post-application water samples were collected at 52 sites on Midwestern rivers. All samples were collected by USGS in glass or Teflon sampling bottles using a depth integrating sampler from three or more verticals. Samples were analyzed for herbicide degradation products by gas chromatography/mass spectrometry (GC/MS), or solid-phase extraction and enzyme-linked immunosorbent assay. In 1998, two samples were collected at 75 Midwestern surface-water sites (streams, large rivers, and reservoirs), and one sample was collected at 25 wells. Surface-water samples were collected during post-application runoff events. These samples were analyzed for 10 herbicide degradates using GC/MS and high performance liquid chromatography/mass spectrometry (HPLC/MS).

Discussion and Results
The concentrations of one cyanazine and two atrazine degradates in post-application runoff samples collected from 52 Midwestern streams are shown on figs. 1-3. The median cyanazine-amide concentration was significantly less in 1998 than in 1994 or 1995. The median deethylatrazine and deisopropylatrazine concentrations were also less in 1998 than in previous years, but the differences were not large.

Figure 1. Cyanazine-amide concentrations in post-application runoff samples from Midwestern Rivers. (Click on image for a larger version)	Figure 2. Deethylatrazine concentrations in post-application runoff samples from Midwestern Rivers. (Click on image for a larger version)
Figure 3. Deisopropylatrazine concentrations in post-application runoff samples from Midwestern Rivers. (Click on image for a larger version)

The herbicide degradate concentrations from surface-water and ground-water samples in 1998 are shown on figs. 4 and 5. In surface water, herbicide degradates occurred frequently, at similar concentrations as the parent compounds. Herbicide degradates occurred less frequently in ground water than in surface water, but detection frequencies and concentrations were often greater than those of the parent compounds.

Over 850 pesticide active ingredients are registered for use in the United States and the number of common pesticide degradation products is unknown. Only a few pesticide parent compounds and degradation products are routinely analyzed for. It is likely that some of the observed changes in herbicide use and concentration are a result of shifts to other compounds. Hence, a more complete set of analytes including pesticide degradates is needed to better determine potential effects of herbicides on humans and ecosystems.

 

Figure 4. Boxplots of herbicide degradate concentrations from 154 surface water samples, 1998. (Click on image for a larger version)

Figure 5. Boxplots of herbicide degradate concentrations from 25 ground water samples, 1998. (Click on image for a larger version)

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