Pesticides in Midwestern Rivers, 1989-2002

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Small fractions, estimated at <1 to 2% of the pesticides applied to Midwestern cropland are lost from fields and enter nearby streams during rainfall events. In many cases aquatic organisms are exposed to mixtures of chemicals, which may lead to greater non-target risk than that predicted based on traditional risk assessments for single chemicals. Relatively little research has been directed at determining the risk of environmental mixtures of pesticides to non-target aquatic organisms.

The USGS evaluated the potential toxicity of 5 classes of pesticides using concentrations from water samples collected from Midwestern streams during early summer runoff events in 1989 and 1998. Toxicity index values, calculated as the concentration divided by the acute toxicity estimate (LC50 or EC50), were summed by pesticide class. Results indicated that some samples had probable toxicity to duckweed and green algae, but few were suspected of having significant toxicity to bluegill sunfish or frogs.

Herbicide reconnaissance sampling sites and drainage basins.

Herbicide reconnaissance sampling sites
and drainage basins.

Evidence for Declining Herbicide Concentrations

Herbicide concentrations in Midwestern streams are affected by climate, soils, agricultural practices, and herbicide use within the associated basins. The USGS sampled 52 Midwestern streams during post-application runoff in 1989, 1990, 1994, 1995, and 1998 with the intent of measuring peak pre-emergence herbicide concentrations of alachlor, atrazine, cyanazine, and metolachlor which were significantly smaller in 1998 than in 1989/90.

Occurrence of Herbicide Degradation Products

Our understanding of the effects of herbicides on humans and ecosystems is incomplete because most studies ignore herbicide degradation products. Some degradates are as toxic as their parent, but little information is available on herbicide degradate occurrence or toxicity. This study showed that in midwestern streams, herbicide degradates occur frequently and at similar concentrations as the parent herbicides.

Sulfonylurea, Sulfonamide, Imidazolinone, and Other Pesticides

Information on more of the ~875 registered pesticide active ingredients is needed to better quantify the total load carried, and relative abundances of pesticides in Midwestern rivers. Sulfonylurea (SU), Sulfonamide (SA), and imidazoline (IMI) herbicides are relatively new classes of herbicides. Little is known about the occurrence, fate, or transport of these herbicides in surface water or ground water in the United States. In 1998, 210 water samples were collected during post-application runoff events at 75 surface-water and 25 ground-water sites. The samples were analyzed for 16 SU, SA, and IMI herbicides by HPLC/MS and 47 pesticides or degradation products by GC/MS. 

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