An Ecological Risk Assessment of the Acute and Chronic Effects of the Herbicide Clopyralid to Rainbow Trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) |
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Authors: | J F Fairchild A L Allert K P Feltz K J Nelson and J A Valle |
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Institution: | (1) Columbia Environmental Research Center, U.S. Geological Survey, 4200 New Haven Rd, Columbia, MO 65201, USA;(2) Division of Environmental Quality, U.S. Fish and Wildlife Service, 100 N. Park Suite 320, Helena, MT 59601, USA;(3) U.S. Forest Service, Federal Building, 324 25th St, Ogden, UT 84401, USA |
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Abstract: | Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) is a pyridine herbicide frequently used to control invasive, noxious weeds
in the northwestern United States. Clopyralid exhibits low acute toxicity to fish, including the rainbow trout (Oncorhynchus mykiss) and the threatened bull trout (Salvelinus confluentus). However, there are no published chronic toxicity data for clopyralid and fish that can be used in ecological risk assessments.
We conducted 30-day chronic toxicity studies with juvenile rainbow trout exposed to the acid form of clopyralid. The 30-day
maximum acceptable toxicant concentration (MATC) for growth, calculated as the geometric mean of the no observable effect
concentration (68 mg/L) and the lowest observable effect concentration (136 mg/L), was 96 mg/L. No mortality was measured
at the highest chronic concentration tested (273 mg/L). The acute:chronic ratio, calculated by dividing the previously published
96-h acutely lethal concentration (96-h ALC50; 700 mg/L) by the MATC was 7.3. Toxicity values were compared to a four-tiered exposure assessment profile assuming an application
rate of 1.12 kg/ha. The Tier 1 exposure estimation, based on direct overspray of a 2-m deep pond, was 0.055 mg/L. The Tier
2 maximum exposure estimate, based on the Generic Exposure Estimate Concentration model (GEENEC), was 0.057 mg/L. The Tier
3 maximum exposure estimate, based on previously published results of the Groundwater Loading Effects of Agricultural Management
Systems model (GLEAMS), was 0.073 mg/L. The Tier 4 exposure estimate, based on published edge-of-field monitoring data, was
estimated at 0.008 mg/L. Comparison of toxicity data to estimated environmental concentrations of clopyralid indicates that
the safety factor for rainbow trout exposed to clopyralid at labeled use rates exceeds 1000. Therefore, the herbicide presents
little to no risk to rainbow trout or other salmonids such as the threatened bull trout. |
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