Aquatic Herbicide Testing, Toxicity, and EPA Registration

Herbicides are pesticides that target plants. Aquatic herbicides are a class of herbicides used to manage various aquatic weeds, either by killing them or severely interrupting their normal growth processes. Aquatic herbicides are placed directly in water. This can cause great concern for the public because of their use of water for recreational activities such as swimming and fishing, and for irrigation and drinking.

Events in the history of pesticide use prior to regulation cause the public to view current, regulated uses of pesticides and other synthetic chemicals with concern and skepticism. On December 2, 1970, the Environmental Protection Agency (EPA) was established to consolidate a variety of federal research, monitoring, standards and enforcement activities to ensure environmental protection. EPA's mission is to protect human health and to safeguard the natural environment—air, water, and land—upon which life depends. Once established, the EPA developed a Pesticides Program that requires stringent testing and strict regulation of pesticides prior to their commercial release for use in the environment. Pesticide registration assures that proper use of the pesticide will not result in unwanted impacts to human health or the environment. A pesticide cannot legally be used if it is not registered with the EPA Office of Pesticide Programs. In order to release a pesticide for legal use, the EPA evaluates the chemical in the pesticide in relation to the location where it is to be used; the amount, frequency and timing of application; and storage and disposal methods. In addition, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) requires pesticides to be re-registered on a continuing basis. Learn more about the EPA regulation and registration of pesticides at their website

After EPA approval, a herbicide must also be registered in each state where it will be used. State registration may require additional testing for specific environmental or agricultural conditions. Registration in Florida is regulated by the Florida Department of Agriculture and Consumer Services (FDACS), AES Bureau of Pesticides.  

Registration of Aquatic Herbicides

To achieve EPA registration, aquatic herbicides must be effective in controlling target weeds, and also meet the rigid environmental and toxicology criteria required by the EPA. Aquatic herbicides require testing over and above that needed for non-aquatic herbicides, and take many years of research before a new aquatic herbicide can be approved by the EPA. Data from nearly 150 tests must be submitted and evaluated, including toxicity, environmental persistence and other factors.

Herbicide Toxicity and Breakdown

Toxicity is a measure of the danger of chemicals to different species. Toxicity varies based on many factors including the dose (the quantity of chemical exposure per unit of time). The dose can be influenced by route of entry into the body (exposure). Exposure can occur by contact with skin (dermal exposure), through inhalation, or by ingestion (oral exposure).

Herbicides kill plants by acting on biochemical pathways that do not occur in animals. Therefore, while they are toxic to plants, they have low toxicity in animals, even if ingested. The LD50 (lethal dose 50%) is the amount of a chemical required to kill 50 percent of the test population acutely exposed through ingestion. This test is performed on different types of animals including mammals (rats), birds (quail and ducks), and fish, shrimp and zooplankton. The screening-level assessment process used by the EPA to evaluate the potential impact of pesticides to non-target organisms is called an Ecological Risk Assessment.

Data collected is used to determine the acceptable toxicity level, defined as the level of pesticide present in food or water in which a lifetime of human consumption is not likely to cause adverse health effects in humans.

Herbicide testing and registration costs are borne by the pesticide manufacturer (registrant). Studies include, but are not limited to

  • potential residue in potable water, fish, shellfish, and crops that may be irrigated
  • environmental fate 
  • how the compound breaks down
  • whether or not it is absorbed by test animals
  • short-term or acute toxicity to test animals
  • whether or not it causes birth defects, tumors, or other abnormalities after long-term exposure
  • toxicity to aquatic organisms such as waterfowl, fish, or invertebrates.

Aquatic Herbicides Used in Florida

Fourteen herbicide active ingredients are currently registered for use in Florida waters. They are listed in Table 2 with maximum use rate, toxicity levels, half-life in water, and breakdown pathways. These aquatic herbicides are approved for use by both the EPA and the FDACS.

Herbicide Maximum Use Rate for Submersed Treatments* Acute Toxicity Lab Rat – LD50 (mg/kg body wt) Acute Toxicity Fish (bluegill) (96 hr LC50) Half-Life in Water** Method of Transformation (breakdown pathway) to Non-toxic***
Bispyribac-sodium 45 ppb 2,635 mg/kg (female); 4,111 mg/kg (male) > 100 ppm approx. 30 days microbial
Carfentrazone not used in submersed treatments 4,000 mg/kg 2 mg/L 3-9 days hydrolysis, microbial
Copper 1 mg/L 470 mg/kg 0.884 mg/L in soft water 7.3 mg/L in hard water 2-8 days adsorption, precipitation
Diquat .37 mg/L 230 mg/kg 245 mg/L 1-7 days photolysis, microbial, adsorption
Endothall 5 mg/L 182 mg/kg (as potassium salt) 233 mg/kg (as alkylamine salt) 343 mg/L (as potassium salt) 0.96 mg/L (as alkylamine salt) 4-7 days microbial
Flumioxazin 400 ppb >5,000 mg/kg >21 mg/L pH 5: 3.4-5.1 days; pH 7: 21.4-24.6 hours; pH 9: 14.6-22 minutes alkaline hydrolysis
Fluridone .15 mg/L >10,000 mg/kg 14.3 mg/L 20-82 days photolysis, microbial
Glyphosate 0.5 mg/L not applied to water; only to foliage of emergent vegetation >5,600 mg/kg 120 mg/L 1-7 days (water) 47 days (soil) microbial
Sodium carbonate peroxyhydrate 10.2 mg/L 1,034 mg/kg 71 mg/L (fathead minnow) immediate hydrolysis
Imazamox .5 mg/L >5,000 mg/kg 122 mg/L 7-14 days photolysis, microbial
Imazapyr not used in submersed treatments >5,000 mg/kg 82 mg/L 2-3 days hydrolysis
Penoxsulam .15 mg/L >5,000 mg/kg >103 mg/L 25 days photolysis, microbial
Triclopyr 2.5 mg/L 730mg/kg 148 mg/L 6-8 days photolysis
2, 4-D 4.0 mg/L >1,000 mg/kg 263 mg/L 7-48 days microbial

Table 2: Aquatic herbicide maximum use rates, toxicity levels, half-life in water, and breakdown pathways. *Concentrations observed are often much lower than the maximum. **Half-life is the time it takes for the concentration of a herbicide to be reduced by half because of breakdown of the molecule. ***Transformation processes include:

Adsorption – binding to the soil
Hydrolysis – breaking down in water
Microbial – broken down by microbes
Photolysis – breaking down in sunlight
Precipitation – falling out of solution and sinking to the bottom

Note: Many different units of measure are used in discussions of pesticides.
1 part per million (ppm) = 1 mg/L = 1mg/kg.

Additional resources explaining toxicity

Toxicity of Pesticides by O.N. Nesheim, F.M. Fishel and M.A. Mossler, (EDIS publication #PI-13, 2014).

Aquatic Toxicology Notes: Predicting the Fate and Effects of Aquatic and Ditchbank Herbicides by C. Wilson, (EDIS publication #SL236, 2013).

The Environmental Protection Agency Pesticides website.

Aquatic Pest Control SM 3 by K. Langeland and F. Fishel, University of Florida-IFAS, 2010.