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Evidence for Declining Herbicide Concentrations

Introduction

Herbicide concentrations in Midwestern streams are affected by a number of factors, including climate, soils, agricultural practices, and the rate and types of herbicides used within the associated basins. Pre-emergence herbicide concentrations are likely to be near their annual maximum in streams during the first major runoff event after herbicide application (post-application runoff). The USGS sampled 52 sites on Midwestern streams during post-application runoff in 1989, 1990, 1994, 1995, and 1998 with the intent of measuring peak pre-emergence herbicides concentrations (fig. 1).

Figure 1 showing sampling sites and associated drainage basins
Figure 1. Herbicide reconnaissance sampling sites
and drainage basins.

(Click on image for a larger version)

Objectives and Methods

The objective of this study was to determine if reductions in the use of atrazine and alachlor between 1989 and 1995 resulted in a reduction in their concentrations during post-application runoff events in Midwestern streams. The results were also to be used to determine if concentrations of other major herbicides in post-application runoff had changed significantly since 1989/90 and if acetochlor, a newly registered herbicide, would be detected in rivers. Estimates of herbicide use in 8 Midwestern States are shown on fig. 2. In 1989, 1990, 1994, 1995, and 1998, post-application water samples were collected by USGS personnel in glass or Teflon sampling bottles using a depth integrating sampler from three or more verticals. Samples were filtered at the time of collection to remove suspended materials. All samples were analyzed for herbicides and herbicide degradation products by gas chromatography/mass spectrometry.

Figure 2 showing herbicide use on corn, soybeans, and potatoes in 8 midwestern states
Figure 2. Herbicide use on corn, soybeans, and
potatoes in 8 Midwestern States.
(Click on image for a larger version)

Discussion and Results

The 1998 samples contained the highest concentrations in a single sample for acetochlor, atrazine, cyanazine, metolachlor, and simazine measured at these sites. However, the median concentrations of atrazine, alachlor, cyanazine, and metolachlor during post-application runoff events in Midwestern rivers all were less in 1998 than in 1989 (figs. 4-7). Only the median acetochlor concentrations were greater in 1998 than in previous years (fig 3). The median atrazine concentration in the sampled streams was 10.9 micrograms per liter (ug/L) in 1989, 5.5 ug/L in 1995, and 4.3 ug/L in 1998. The median alachlor concentration was 1.9 ug/L in 1989, 0.13 ug/L in 1995, and less than the detection limit (0.05 ug/L) in 1998. The median cyanazine concentration was 2.6 ug/L in 1989, 1.3 ug/L in 1995, and 0.44 ug/L in 1998. The median metolachlor concentration was 2.5 ug/L in 1989, 1.7 ug/L in 1995, and 1.4 ug/L in 1998. The median acetochlor concentration increased, from less than the detection limit in its first year of use, 1994 (33% detections) to 0.4 ug/L in 1995, and 0.7 ug/L in 1998. The decreases in herbicide concentrations do not always parallel changes in herbicide use amounts. This suggests that a change in other factors such as reductions to herbicide application rates, changes to the timing of herbicide application, changes to other herbicide products, and better use of herbicide best management practices may affect herbicide concentrations.


Figure 3 showing acetochlor concentrations in post-application runoff samples from midwestern rivers
Figure 3. Acetochlor concentrations in post-application runoff samples from Midwestern Rivers.


Figure 4 showing alachlor concentrations in post-application runoff samples from midwestern rivers
Figure 4. Alachor concentrations in post-application runoff samples from Midwestern Rivers.

Figure 5 showing atrazine concentrations in post-application runoff samples from midwestern rivers

Figure 5. Atrazine concentrations in post-application runoff samples from Midwestern Rivers.

Figure 6 showing cyanazine concentrations in post-application runoff samples from Midwestern Rivers

Figure 6. Cyanazine concentrations in post-application runoff samples from Midwestern Rivers.
Figure 7 showing metolachlor concentrations in post-application runoff samples from midwestern rivers

Figure 7. Metolachlor concentrations in post-application runoff samples from Midwestern Rivers.
Figure 8 showing total herbicide concentrations in post-application runoff samples from midwestern rivers.

Figure 8. Total herbicide concentrations in post- application runoff samples from Midwestern Rivers.

References:

Thurman, E.M.; Goolsby, D.A.; Meyer, M.T.; Mills, M.S.; Pomes, M.L.; Kolpin, D.W., Environ. Sci. Tech. 1992, 12:2440-2447.

Goolsby, D.A.; Battaglin, W.A, in Agrochemical Environmental Fate: State of the Art, Leng, M.L.; Leovey, E.K.; Zubkoff, P.L., eds., CRC Press, 1995.

Larson, S.J.; Capel, P.D.; Majewski, M.S., Pesticides in Surface Water: Distribution, trends, and governing factors. Ann Arbor Press, Inc., Chelsa, MI, 1997.

Scribner, E.A.; Thurman, E.M.; Goolsby, D.A.; Meyer, M.T.; Mills, M.S.; Pomes, M.L., U.S. Geol. Surv. OFR 93-457, 1993.

Scribner, E.A.; Goolsby, D.A.; Thurman, E.M.; Battaglin, W.A., U.S. Geol. Surv. OFR 98-181, 1998.

 

 

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