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CADDIS Volume 2: Sources, Stressors & Responses


Sources and activities that suggest listing herbicides as a candidate cause

Forestry management practices, agricultural operations, and urban development and maintenance are all sources of herbicides that may enter surface waters and cause impairments. Herbicides are applied to forests after harvesting to suppress brush and noncommercial trees. For that use, the rate of application may be high and exposed streams are more likely to be of higher quality than agricultural or urban streams. Conversely, agricultural operations may contribute large quantities of herbicides because they may apply herbicides multiple times per year and they may be applied by planes, addition to irrigation water, or spraying onto crops (Figure 3). Urban land uses can contribute as homeowners and managers of parks, golf courses and other lawns use herbicides for aesthetic enhancement. Herbicides also are used on rights of way for roads, pipelines, railroads and electrical transmission lines and for control of plants in cracks in pavements. Such urban and suburban uses are likely to contaminate storm waters.

photo of herbicide application
Figure 3. Foliar application of herbicide on row crops.
Courtesy of U.S. EPA Region 9.

Herbicides also are directly applied to waters to control vegetation in ponds, ditches, irrigation canals and recreational waters. Such applications are sources of exposure at the point of application and downstream.

Site evidence that suggests listing herbicides as a candidate cause

photo of an agricultural ditch
Figure 4. Agricultural ditches can transport herbicides from fields to receiving waters.

Evidence of the presence of herbicides at toxic levels includes dead, deformed, chlorotic or necrotic plants, or the absence of plants from a waterbody or the riparian zone (Figure 4). Irrigation ditches and row crop farming near streams provide opportunities for herbicides to enter streams (Figure 4). Lakes and reservoirs used for recreation are often treated for macrophyte control as well.

Although herbicides in general have lower toxicity to animals than other pesticides, fish or invertebrate kills may be a sign of herbicide use. For example, acrolein has been applied to irrigation ditches at levels sufficient to be acutely lethal to fish and invertebrates (see acrolein in U.S. EPA 2009), and if not properly applied to fields it can cause kills in receiving waters. Kills also may be due to low dissolved oxygen (DO) concentrations resulting from plant materials decomposing in water.

Biological effects that suggest listing herbicides as a candidate cause

Because herbicides tend to affect plants more quickly and severely than animals, the most useful biological sign of herbicides is effects on aquatic plants (Kreutzweiser et al. 1995, Van den Brink et al. 1997, Hall et al. 1997). This trait may help distinguish the biological effects of herbicides from those of insecticides and most other toxic chemicals. Secondary effects of herbicides are mediated by low DO concentrations from plant decomposition and changes in trophic structure due to plant community changes.

Herbicides may reduce taxa richness and abundance of fish and benthic macroinvertebrates due to reductions of sensitive species and increased abundance of tolerant species at high concentrations (Daam and Van den Brink 2007, Dewey 1986). It also has been contended that some herbicides, particularly atrazine, have specific mechanisms of action in aquatic frogs and fish, including developmental abnormalities (Hayes et al. 2006, Tillit et al. 2010). However, a review by the U.S. EPA found that evidence for such effects in amphibians was weak and inconsistent (U.S. EPA 2007).

photo of aquatic vegetation
Figure 5. Dense submerged aquatic vegetation.

Site evidence that supports excluding herbicides as a candidate cause

Absence of sources of herbicides such as agricultural or forestry or urban uses in the watershed and absence of upstream waters that might be treated with herbicides would suggest exclusion of herbicides as a candidate cause. Additionally, if abundant, healthy and diverse periphyton and macrophytes are observed in a stream (Figure 5) it is unlikely that herbicides are responsible for the impairment. Discretion should be used when excluding herbicides as a candidate cause, and the specific conditions of the case should be considered.

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