Fate of Aquatic Herbicides in the Environment


We refer to herbicides disappearing and dissipating from the environment. Disappearance refers to the removal of the herbicide from a certain part of the environment. Aquatic herbicides can disappear from treated water by dilution, adsorption to bottom sediments, volatilization, absorption by plants and animals or by dissipation. Dissipation refers to the breaking down of a herbicide into simpler chemical compounds. Herbicides can dissipate by photolysis (broken down by light), microbial degradation, or metabolism by plants and animals. Both dissipation and disappearance are important considerations to the fate of herbicides in the environment because even if dissipation is slow, disappearance due to processes such as adsorption to bottom sediments makes a herbicide biologically unavailable.

Aquatic herbicides are non persistent in treated water, that is, they disappear rapidly. Disappearance is greatest when spot treatments are made in large bodies of water because the dominant effect is dilution. Aquatic herbicides are water soluble and quickly dilute to non-detectable concentrations. They disappear at different rates and by different methods.


Copper is a naturally occurring element and essential at low concentrations for plant growth. It does not break down in the environment, but it forms insoluble compounds with other elements and is bound to charged particles in the water. It rapidly disappears from water after application as a herbicide. Because it is not broken down, it can accumulate in bottom sediments after repeated high application rates. Accumulation rarely reaches levels that are toxic to organisms or significantly above background concentrations in the sediment. Copper can be slowly transported out of bottom sediments after it is taken up by plants and moved out of the system through the food chain.


Dissipation of 2,4-D is mostly by microbial degradation. A small amount of photo-decomposition and breakdown by tolerant plants also occurs. Volatile forms of 2,4-D are not used for aquatic weed control and therefore volatilization is not an important route of disappearance after aquatic weed control applications. Complete decomposition usually takes about 3 weeks in water and can be as short as 1 week. 2,4-D breaks down into naturally occurring compounds. Two pounds of 2,4-D amine will break down into 1 pound carbon dioxide, 1/4 lb water, 1/4 pound ammonia, and 1/2 pound chlorine.


After application to enclosed ponds for submersed weed control, diquat is rarely found longer than 10 days after application and is often below detection 3 days after application. The most important reason for the rapid disappearance of diquat from water is that it is rapidly taken up by aquatic vegetation and bound tightly to particles in the water and bottom sediments. When bound to certain types of clay particles diquat is not biologically available. When it is bound to organic matter it can be slowly degraded by microorganisms. When diquat is applied foliarly it is degraded to some extent on the leaf surfaces by photodegradation, and because it is bound in the plant tissue a proportion is probably degraded by microorganisms as the plant tissue decays.


Like 2,4-D, endothall is rapidly and completely broken down into naturally occurring compounds by microorganisms. The by-products of endothall dissipation are carbon dioxide and water. Complete breakdown usually occurs in about 2 weeks in water and 1 week in bottom sediments.


Dissipation of fluridone from water occurs mainly by photodegradation. Metabolism by tolerant organisms and microbial breakdown also occurs, and microbial breakdown is probably the most important method of breakdown in bottom sediments. The rate of breakdown of fluridone is variable and may be related to time of application. Applications made in the fall or winter when the suns rays are less direct and days are shorter result in longer half-lives. Fluridone usually disappears from pond water after about 3 months but can remain up to 9 months. It may remain in bottom sediment between 4 months and 1 year.


Glyphosate is not applied directly to water for weed control, but when it does enter the water it is bound tightly to dissolved and suspended particles and to bottom sediments and becomes inactive. Glyphosate is broken down into carbon dioxide, water, nitrogen, and phosphorus over a period of several months.