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Pesticides in Surface Waters

U.S Geological Survey Fact Sheet FS­039­97

Seasonality of Pesticides in Surface Waters

Seasonal patterns of pesticide occurrence in surface waters vary in different areas of the U.S. In Figure 4,

examples of several relatively well­documented seasonal patterns are shown. Herbicides and insecticides

used on rice in California have a distinct seasonal concentration pattern in streams because of the release

of irrigation water at specific times. This is shown for the herbicide molinate in Figure 4A. In streams

draining the Central Valley of California, there is a seasonal appearance of diazinon and other

organophosphorus insecticides used on orchards in January and February (Figure 4B). Rainfall following an

application results in movement of the insecticides to surface waters, and pulses of elevated

concentrations continue downstream to San Francisco Bay.

Figure 4. Seasonal patterns of pesticide occurrence in surface waters in several different regions of the United States.
10/8/2015 USGS ­ Pesticides in Surface Waters

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In midwestern rivers, the pattern for preemergent herbicides, such as atrazine and alachlor, is relatively

well known (Figure 4C). Concentrations of some herbicides in streams that drain agricultural areas show a

distinct peak in spring and early summer that lasts for a few days to several months, depending on the

timing and number of rain events and the size of the drainage basin. By midsummer, transport of

herbicides to surface waters diminishes and riverine concentrations decline.

The seasonal cycle of herbicide concentrations in midwestern reservoirs is somewhat different than in

rivers. This is shown for atrazine in Figure 4D. Reservoirs in the Midwest receive much of their water

during the spring runoff period, when concentrations of herbicides in tributary streams are relatively high.

Concentrations may remain elevated in reservoirs much longer than in streams, because the herbicides

are not flushed from the system as quickly. Relatively high levels of atrazine, cyanazine, metolachlor, and

several transformation products of atrazine and alachlor have been observed in reservoirs long after

inputs of these compounds from agricultural fields have declined (Goolsby and others, 1993).

The seasonal pattern of diazinon concentrations in the Illinois River (Figure 4E) is different from the

pattern for the agricultural herbicides. Concentrations in 1991 were elevated throughout the summer and

fall, with peak concentrations occurring in the fall. Diazinon is an organophosphorus insecticide used

primarily in non­agricultural applications in the Midwest, including lawn care. The Illinois River drains a

major urban area (Chicago), and the seasonal pattern is probably the result of applications to lawns by

home­owners and commercial applicators.

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