Evidence for Declining Herbicide Concentrations
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. 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. 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
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