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Publication #SL369

Aquatic Toxicology Notes: Endothall1

P. Chris Wilson and Jun Wu2


This document is a companion to EDIS publication SL236/SS455, Aquatic Toxicology Notes: Predicting the Fate and Effects of Aquatic and Ditchbank Herbicides (Wilson 2006; This publication introduces users of endothall to the physical, chemical, environmental, and ecological properties of this herbicidal active ingredient relative to the aquatic environment. Readers should refer to SL236/SS455 for a more detailed introduction to the various concepts mentioned. These physical and chemical properties directly influence the fate, bioavailability, and potential toxicity of endothall to target and non-target organisms in the environment. Understanding the toxicological properties of endothall will provide users with an idea of which organisms and organism groups may be at greatest risk of negative impacts in a given treatment situation. Knowledge of these properties will allow users to identify high-risk situations for herbicide use. This information is discussed in detail in the following sections. As always, users should consult the product label for specific restrictions and allowed uses.

Chemical Description

Endothall is the herbicidal active ingredient found in commercial formulations labeled for weed control in aquatic systems and on ditch banks. It is also used as a defoliant and desiccant in some terrestrial situations (e.g., in potato, hops, cotton, clover, and alfalfa production) (U.S. EPA 1992; Sprecher, Getsinger, and Sharp 2002). This herbicidal active ingredient is available as dipotassium salt and mono (N, N-dimethylamine) salt formulations (Table 1). Both formulations dissociate forming endothall-diacid, the herbicidal form (U.S. EPA 2005). The toxicological properties of each form differ greatly.

Table 1. 

Chemical nomenclature and properties of the herbicide endothall.



Common Name


Chemical Name1

7-oxabicyclo[2.2.1]heptanes-2,3-dicarboxylic acid

Chemical Class

Dicarboxylic acid

Trade Names2

Aquathol®, Hydrothol®, Accelerate®, Des-i-cate®, Herbicide 273®

Molecular Formula1

Diacid: C8H10O5

Dipotassium salt: C8H8K2O5

Mono(N,N-dimethylamine) salt: C10H17NO5

Molecular Weight2

Diacid: 186.16

Dipotassium salt: 262.35

Mono(N,N-dimethylamine) salt: 231.25

CAS* Number1

Diacid: 145-73-3

Dipotassium salt: 2164-07-0

Mono(N,N-dimethylamine) salt: 66330-88-9


Diacid Dipotassium salt Mono(N,N-dimethylamine) salt

(see images below)

Physical Appearance2

White, crystalline solid


1.431 g/mL

Melting Point2


Vapor Pressure4

1.57×10-10 mm Hg (24°C)

Solubility (Water) 2

100 g/L at 25°C and pH 7

pKa (20°C)5

pK1 = 3.4, pK2 = 6.7





Sorption (Kd)6


Henry’s law constant4,8

3.8×10-16 atm m3 mol-1 (calculated)

Half-life (T1/2)

4.01 d (aqueous)10

4~9 d (soil)4

*Chemical Abstracts Service, 1U.S. EPA 2005, 2Vencill 2002, 3Bugwood 2011, 4HSDB 2011, 5Wauchope et al. 1992, 6Reinert and Rodgers, Jr. 1984, 7Weber 1994, 8Tomlin 2004, 9Reinert et al. 1985


Dipotassium salt. 

Environmental Fate

Endothall dipotassium salt and N,N-dimethylamine salt formulations are applied as granular or liquid formulations directly to ponds, lakes, and canals to control algae and several other aquatic weeds such as pondweed, burreed, milfoil, hydrilla, and coontail (U.S. EPA 2005; Vencill 2002). Many of the chemical properties listed in Table 1 are useful for predicting (in a generalized manner) the fate of endothall in the environment. These qualitative predictions are useful for understanding what happens to the herbicide once it is applied (i.e., where does it go and does it break down into nontoxic constituents) and how long it may be present in water, sediments, and biota. This information is useful to managers for predicting exposures and possible toxicity to non-target plants and animals within the treated system. The environmental compartments considered here are air, water, sediment, and biota.


Endothall has a very low vapor pressure (Table 1), so losses to the atmosphere are not expected. Dust particles and drift droplets may be temporarily suspended in the air, but should settle quickly and are localized in nature.


Endothall is very soluble in water, having a solubility of 100,000 mg/L. Because of its water solubility, it is prone to following the movement of water in the environment. The pK1 and pK2 values for endothall are 3.4 and 6.7, indicating that endothall salt, endothall acid, and the corresponding cations (potassium or coco-alkylamine) will be present in aquatic systems at most environmental pHs (U.S. EPA 2005). Endothall does not photodegrade and is resistant to hydrolysis under typical environmental conditions (U.S. EPA 2005; Verschueren 2001).

Endothall is not persistent in the aquatic environment. Hiltibran (1962) used a flaxseed bioassay to measure the presence of toxic concentrations of endothall and reported disappearance of toxicity after an average of 2.5 days (maximum of 4 days) in pond water treated at 0.3–10 mg/L. While the concentrations decreased to below toxicity thresholds within a few days, endothall residues were likely still present in the water. Several studies have reported the rapid disappearance of endothall from treated water in less than 36 days. Reinert, Hinman, and Rodgers, Jr. (1988) reported the most rapid dissipation, finding that aqueous endothall concentrations in Pat Mayse Lake decreased to below 0.002 mg/L within three days after treatment at 2 ppm. However, others have reported longer dissipation periods. Holmberg and Lee (1976) reported reaching 0.1 mg/L within 18 days following treatment at 5 mg/L. Sikka and Rice (1973) reported that following treatment at 2 ppm, the maximum concentration (1.8 mg/L) in water appeared one day after treatment, decreasing by 55% in three days after treatment, and decreasing to less than 0.01 mg/L within 36 days after application. Langeland and Warner (1986) observed endothall concentrations reached non-detectable concentrations (<0.010 ppm) in a pond 26 days after treatment at a concentration of 2 mg/L. The rate of disappearance is directly related to the treatment concentration as shown by Yeo (1970), who reported dissipation of the dipotassium salt of endothall (initial concentrations of 0.3–1.4 mg/L) to less than 0.03 ppm in 8–20 days. Likewise, Walker (1963) reported that concentrations of the di-N,N'-dimethylococoamine salt of endothall disappeared within eight days after treatment at 0.3 mg/L, and within two weeks after treatment at 0.6 mg/L. However, the herbicide required up to 25 days to disappear at treatment levels of 1–3 mg/L (Walker 1963).

The reduction in endothall concentrations is partly due to microbial metabolism as illustrated by Sikka and Rice (1973) and Sikka and Saxena (1973). Sikka and Rice (1973) found little degradation when pond water was sterilized by autoclave, and Sikka and Saxena (1973) showed complete mineralization (conversion of carbon in the herbicide to carbon dioxide) of about 25% of the initial endothall within 10 days after treatment and 40% of the initial endothall after 30 days. Simsiman, Daniel, and Chesters (1976) reported that 72% of endothall persisted for over 30 days due to oxygen depletion, indicating the importance of aerobic metabolism for endothall dissipation. Situations where high levels of biological oxygen demand persist will likely increase the persistence of endothall.


Despite its ionic structure, some endothall will move into the upper sediments in treated systems. However, partitioning of endothall into sediments is a relatively minor pathway (Reinert and Rodgers, Jr. 1984), and it does not accumulate as indicated by its disappearance from the hydrosoil within 30–60 days, presumably because of microbial metabolism (U.S. EPA 1992). Sikka and Rice (1973) reported that as endothall concentrations decreased in the water column following treatment at 2 ppm (dipotassium salt formulation), they increased in the top inch of the hydrosoil up to 22 days after treatment when the maximum concentration (0.44 mg/kg) was detected. No endothall was detected in the hydrosoil 44 days after treatment. Accumulation in the hydrosoil was rapid during the first three days, becoming more gradual through day 22. Reinert, Hinman, and Rodgers, Jr. (1988) observed that endothall in sediments was below the minimum detectable level (0.01 mg/kg) within 96 hours after treatment. Metabolism of endothall by microbial organisms plays a significant role in its dissipation within the hydrosoil. Sikka and Rice (1973) suggested that the initially rapid decline in endothall concentrations they observed in the water column was because of sorption to the hydrosoil. Following sorption to the sediments, microbial populations capable of degrading endothall were selected and increased in population size and activity. Jenson (1964) isolated bacteria capable of degrading endothall from soil as late as one year after treatment, but not from untreated soil (Sikka and Rice 1973). Within microbes, the tricarboxylic acid cycle and an alternate unknown pathway were primarily responsible for the transformation of endothall into glutamic acid (Sikka and Saxena 1973).

Within the soil/terrestrial environment, endothall is expected to leach and follow the same distribution paths as water. Comes, Bohmont, and Alley (1961) observed that the dissipation of endothall was greater in moist soil than in air-dry soil. Montgomery and Freed (1964) reported that nearly all of the endothall applied to the soil was mineralized to CO2 within two weeks (Simsiman and Chesters 1975).


Given its high water solubility, low Kow, and ionic structure, endothall is not expected to bioconcentrate in aquatic biota significantly. Sikka, Ford, and Lynch (1975) reported the potential for bioconcentration of endothall in bluegill (Lepomis macrochirus) is low based on observations of whole body concentrations of 0.1–0.2 ppm following exposure to 2 ppm for 12–96 hours.


Ecotoxicology refers to the study of the effects of toxic substances and pollutants on plants, animals, and processes in the environment. The effects of endothall on aquatic species can vary, and there are also various formulations of endothall applied. Because endothall is an herbicide, target and non-target plants are especially at risk of negative effects. Data are available indicating that endothall may also be toxic to a variety of fish and invertebrate species. A summary of these effects follows.


Endothall for aquatic weed control is usually applied directly to aquatic vegetation and the water. Target lake concentrations are recommended on the herbicide formulation label. These concentrations generally range from 0.5 to 5 mg/L (Vencill 2002). Because endothall is an herbicide, toxic effects on plants (target and non-target) are expected if concentrations exceed toxicity thresholds. Unfortunately, most toxicity values reported in the literature are not for standard periods of exposure or for non-target plants, so it is not possible to build a species sensitivity distribution for predicting effects on non-target plants at treatment concentrations. However, given the nonspecific mode of action, effects are expected to be similar to target weeds at treatment concentrations.

Endothall is absorbed rapidly by the foliage as the undissociated acid (Vencill 2002). In terrestrial plants, it is also absorbed by the roots and transported in the xylem, but not the phloem (Keckemet and Nelson 1968; MacDonald, Shilling, and Bewick 1993). In contrast, transport is limited to the phloem in aquatic plants (Thomas and Seaman 1968). Uptake into aquatic plants seems to be enhanced by low light conditions and high temperatures (Haller and Sutton 1973; MacDonald, Shilling, and Bewick 1993). Endothall has many modes of action, unlike other herbicides that have only one. One known mode of action is inhibition of plant protein synthesis, though the exact interaction is not well understood (Westerdahl and Getsinger 1988). Additionally, Mann and Pu (1968) reported that endothall inhibits lipid synthesis by inhibiting incorporation of malonic acid into the lipids. They reported that lipid synthesis was inhibited by approximately 38% with an endothall dose of 5 μg/L in hypocotyl segments of hemp plants (Sesbania exaltata) (Mann and Pu 1968). Endothall also affects membrane integrity through disruption of respiratory processes, ultimately resulting in a collapse of the membrane electrical gradient because of a lack of energy, formation of leaky membranes, and rapid desiccation of tissue (MacDonald, Shilling, and Bewick 1993). Endothall has also been shown to reduce the activity of proteolytic enzymes that hydrolyze proteins and dipeptidases that hydrolyze dipeptides during seed germination (Tsay and Ashton 1971).

Aquatic Animals

The toxicity of endothall to aquatic animals differs greatly depending on the formulation applied. Species sensitivity distributions (SSD) were constructed using 50% lethal concentration (LC50) data for fish and aquatic invertebrates exposed to the dipotassium salt (Figure 1) and the dimethylamine salt (Figure 2). The LC50 is the concentration at which 50% of the organisms die following exposure for an adequate period of time. The dimethylamine salt formulation is much more toxic relative to the dipotassium salt formulation. As seen in Figure 1, the LC50 for the most sensitive species is approximately 11 mg/L. At a maximum treatment concentration of 5 mg/L, a margin of safety exists since the treatment concentration is lower than the most sensitive LC50 (i.e., treatment concentration does not overlap with the sensitivity distribution). In contrast, the LC50 for the most sensitive species exposed to the dimethylamine salt is approximately 0.2 mg/L (Figure 2). In this case, a maximum treatment concentration of 5 mg/L would affect 72% of the species. Given the significantly greater toxicity of the dimethylamine formulation, it should not be used where high-value aquatic species are present.

Figure 1. 

Species sensitivity distribution (animals) for determining the potentially affected fraction of species following exposure to endothall (dipotassium salt) for 96 hours.

[Click thumbnail to enlarge.]

Figure 2. 

Species sensitivity distribution (animals) for determining the potentially affected fraction of species following exposure to endothall (mono (N,N-dimethylamine) salt) for 96 hours.

[Click thumbnail to enlarge.]


Uncertainty always exists in environmental risk assessments. This analysis is based on several assumptions. First, the SSD assumes that the known species represent the universe of species. Research has shown that this assumption holds for some classes of contaminants, but exhaustive attempts to evaluate it within all classes of contaminants are resource prohibitive. A second assumption is that the acute toxicity EC50 and LC50 measures are protective of species. In fact, 50% of the population would be affected at an organism’s EC50 concentration. This analysis also assumes that laboratory-derived EC50 values are representative of the actual EC50 values that would be observed in the environment. Laboratory-derived values can be more conservative since they must eliminate many of the confounding factors that affect the bioavailability and fate of contaminants.


Endothall does not remain in the water column for long periods of time following application. Plants are most susceptible to injury because of endothall exposures. Endothall is quickly absorbed into plants and exerts its herbicidal activity through a variety of modes of action. The dipotassium salt formulation is least toxic to non-target aquatic animals.

Recommendations for Optimal Use/Environmental Protection

  • Only apply to water bodies following label restrictions for control of algae and aquatic weeds. For dense weed infestations, treat only part of the water body at a time (waiting 5–7 days between treatments) to minimize oxygen loss and fish suffocation due to weed decomposition.

  • Always follow the recommendations printed on the label. Pay special attention to the “Environmental Hazards” section.

  • Since endothall is a contact herbicide, apply only to plant foliage or when plants have emerged. Thorough coverage of the plants will result in optimal control.

  • Use an appropriately sized course nozzle droplet size to minimize drift of fine droplets to non-target areas. Fine droplets are more prone to drift.

  • Apply only to calm water bodies without excessive wind and wave action.

  • Ineffective weed control will result from applications to plants covered with sediment deposits.

  • Only use proper measuring devices and appropriately calibrated application equipment.

Glossary of Terms and Abbreviations

Density: Density of a material is defined as its mass per unit volume (Available on

Melting point: The temperature at which a solid changes state to liquid (Available on

Vapor pressure: The pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system (Available on

Solubility: The property of a solid, liquid, or gaseous chemical substance (called the solute) to dissolve in a liquid solvent to form a homogeneous solution of the solute in the solvent (Available on Water solubility refers to the amount of the pesticide that will completely dissolve in a given volume of water.

Kd: The partitioning coefficient (Kd) is an indicator of the sorptive properties of a pesticide.

Kow: The octanol-water coefficient is defined as the ratio of a chemical’s solubility in n-octanol and water at steady state.

Koc: The organic carbon partition coefficient is derived as: Koc= Kd/Foc, where Foc is the fraction of organic carbon in the soil/sediment.

pKa: The symbol for the acid dissociation constant (or called acidity constant) at logarithmic scale (Available on

Henry’s law constant: A partition coefficient defined as the ration of a chemical’s concentration in air to its concentration in water at steady state.

Half-life: The amount of time needed for half of the chemical to disappear.

LC50: Concentration estimated to cause mortality in 50% of a test population over a specific period of time.

EC50: Concentration estimated to cause a specific effect (e.g., growth reduction) in 50% of a population of test species after a specific period of exposure.


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Comes, R.D., D.W. Bohmont, and H.P. Alley. 1961. “Movement and Persistence of Endothall (3,6-endoxohexahydrophthalic acid) as Influenced by Soil Texture, Temperature, and Moisture Levels.” J. Am. Soc. Sugar Beet Technol. 11:287-93.

Haller, W.T., and D.L. Sutton. 1973. “Factors Affecting the Uptake of Endothall 14C by Hydrilla.” Weed Sci. 21:446-8.

Hiltibran, R.C. 1962. “Duration of Toxicity of Endothall in Water.” Weeds 10:17-19.

Holmberg, D.J., and G.F. Lee. 1976. “Effects and Persistence of Endothall in the Aquatic Environment." Journal (Water Pollution Control Federation) 48:2738-46.

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Jenson, H.L. 1964. “Studies on Soil Bacteria (Arthrobacter globiformis) Capable of Decomposing the Herbicide Endothall.” Acta. Agr. Scand. 14:193-204.

Keckemet, O., and R.T. Nelson. 1968. “Mode of Action, Persistence and Fate of Endothall in the Aquatic Environment.” Proc. South. Weed Sci. Soc. 21:45-6.

Langeland, K.A., and J.P. Warner. 1986. “Persistence of Diquat, Endothall, and Fluridone in Ponds.” J. Aquat. Plant Manage. 24:43-6.

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This document is SL369, one of a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date: October 2012. Please visit the EDIS website at


P. Chris Wilson, associate professor, Soil and Water Science Department, Indian River Research and Education Center, Ft. Pierce, FL; and Jun Wu, visiting scholar, Indian River Research and Education Center, Ft. Pierce, FL; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.