Aquatic weeds, residues from control

Residues from Control of Submersed Weeds. Recommendations for use of herbicides to control submersed aquatic weeds usually specify treatment concentrations in terms of ppm. Granular formulations of herbicides such as 2,4-D, 2,6-dichlorobenzonitrile (dichlobenil), and 2,3,6-trichlorophenylacetic acid (fenac) are applied at pound-per-acre rates. The herbicides most commonly used to control submersed aquatic weeds are listed in Table I with normal rates of application. The initial residue levels represent the concentrations effective on submersed vegetation. For comparison, treatment rates given in pounds per acre are also shown as ppm in an arbitrarily selected water depth of 4 feet. 

The levels of accuracy, precision, and sensitivity noted for acid and ester recovery from Piedmont and Coastal soils are considered adequate to measure and detect silvex residues associated with the control of aquatic weeds. Similarly, the recovery of ester from Florida muck is judged to be acceptable down to 100 p.p.m. However, free acid recovery from muck is considered inadequate on all of these points. 

Herbicide residues in the aquatic environment usually evoke concerns of toxicity to fish. Acute toxicity data are extensive. In general, the herbicide levels required for weed control are much lower than those which cause acute toxicity in fish. Numerous LC50 values for various fish have been reported 41, 42, 43, 44, 45, 46), Ester derivatives of aquatic-weed herbicides are more toxic than acids, amines, or salts (except for the cocoamine salts of endothall). DiflFerent formulations of the same herbicide may vary widely in toxic effects on fish (42). The toxicity of two formulations of diquat containing the same concentration of herbicide was reported to vary from 64-430 ppm. Water quality, temperature 47, 48), and length of exposure are important factors in assessing acute toxicities. 

The conclusion draum from pesticide monitoring studies carried out in various areas of the United States is that non-purposeful herbicide contamination of natural waters occurs infrequently and at low levels. Residues in water from purposeful use of herbicides for control of weeds in aquatic sites are relatively high initially however, the levels are reduced rapidly, and residues are often not detectable after a few days or weeks. Herbicide residue levels found in aquatic organisms commonly reflect the concentrations of herbicides present. With few exceptions aquatic herbicides do not accumulate and persist in fish or shellfish. Most aquatic organisms cleanse themselves of herbicide residues soon after these disappear from the water. Herbicide use, while controversial, continues until more effective and acceptable control measures evolve. 

The sources of most herbicidal residues are not diflBcult to determine. Residues occur directly from the use of herbicides to control aquatic or marginal weeds or indirectly from and incidental to the control of other types of vegetation. The origins of residues are ... 

Dissipation is a term used when residues disappear, and no distinction is made as to the various avenues by which they are lost. Most of the work done is of this nature. Experimentation under natural conditions is diflBcult and provides reliable data only when the infiuent and effluent water can be controlled. Pilot studies in large bodies of water do not lend themselves to dissipation studies because the water is not confined. The behavior of herbicides in streams and canals is even more diflBcult to establish. Consequently, many of the dissipation data available were derived from artificial ponds or pools, or from irrigation canals where herbicidal residues frequently appear during normal weed-control operations on these systems. Almost all of the herbicides registered for use in the aquatic environment have water-use restrictions which permit partial or complete dissipation of the residues before normal water use is resumed. 

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