Herbicides leaching

A mobility ranking based on soil thin-layer chromatography fstlci is used to classify the herbicide leaching poiential of various herbicides. Tile... 

Boydston, R.A. (1992). Controlled release starch granule formulations reduce herbicide leaching in soil columns. Weed Technol., 6 317-321. 

Cornejo, L., R. Celis, C. Dominguex, and J. Cornejo (2007). Clay-based formulations of simazine to reduce herbicide leaching in sport turfgrasses. Proceedings of the International Conference on Water Pollution in Natural Porous Media at Different Scales, Barcelona, Spain, April 11-13, pp. 1-5. 

Isensee, A.R. and A.M. Sadeghi (1995). Long-term effect of tillage and rainfall on herbicide leaching to shallow groundwater. Chemosphere, 30 671-685. 

Narayanan, K.S., M. Singh, and R.K. Chaudhuri (1993). The reduction of herbicide leaching using vinyl pyrrolidone copolymers and methyl vinyl ether maleic acid ester copolymers. In P.D. Berger, B.N. Devisetty, and F.R. Hall, eds., Pesticide Formulations and Application Systems, Vol. 13. Philadelphia, PA American Society for Testing and Materials, pp. 57-75. 

Bowman, R.S., and R.C. Rice. 1986. Accelerated herbicide leaching resulting from preferential flow phenomena and its implications for ground water contamination, p. 413-425. In Proc. Conf. on Southwestern Ground Water Issues, Phoenix, AZ. 20-22 Oct. Natl. Water Well Assoc., Dublin, OH. 

CDAA, molinate, EPTC, vernolate, pebulate, CDEC, and cycloate are all relatively mobile in soil systems (123, 385, 386, 387, 388 389, 390, 391). The herbicides leached more readily in coarse textured soils than in fine textured ones and did not significantly leach in peat or muck soils (386, 387, 388, 391), Leachability of the compounds was related to their water solubilities (compounds of higher solubilities moved more than less soluble ones) and to the organic matter and clay contents of the soils (less movement as the organic matter and clay content increased). 

Herbicide use. Some soil textures are more prone to herbicide leaching, particirlarly of some soil applied residrrals. Recommended application rates of these cherrricals are affected by the soil texture in order to avoid any risk of crop damage. On clay soils rates may be higher as some chemicals are adsorbed onto the soil particles and are then unavailable for weed uptake. 

The phenoxyalkanoic herbicides are acidic in nature and thus subject to some degree of ionization. The extent to which the herbicide ionizes is controlled by the acid dissociation constant (fQ of the herbicide in question and the soil solution pH (238). The leaching potential is significantly influenced by these reactions. 

Bipyridiniums. The bipyridinium herbicides (Table 2), paraquat and diquat, ate nonselective contact herbicides and crop desiccants. Diquat is also used as a general aquatic herbicide (2,296). Bipyridinium herbicides are organic cations and are retained ia the soil complex via cation exchange. They are strongly sorbed to most soils and are not readily desorbed (332). Both paraquat and diquat are not readily leached (293). 

Ben2onitrile herbicides tend to possess a high leaching potential dichlohenil is an exception, due to its stronger sorption. The hen onitrile herbicides are also prone to volatilisation losses (340) and off-site deposition (341). 

Considerable research has been conducted to investigate the soil sorption and mobiUty of dinitroaniline herbicides. In general, these herbicides are strongly sorbed by soil (354), and sorption has been correlated to both soil organic matter and clay content (355). Dinitroaniline herbicides are not readily leached in most soils (356), although leaching of triduralin is enhanced by addition of surfactants (357). 

Acid amide herbicides are nonionic and moderately retained by soils. The sorption of several acid amide herbicides has been investigated (369). Acetochlor [34256-82-1] is sorbed more than either alachlor or metolachlor, which are similarly sorbed by a variety of soils. Sorption of all the herbicides is well correlated to soil organic matter content. In a field lysimeter study, metolachlor has been found to be more mobile and persistent than alachlor (370) diphenamid [957-51-7] and napropamide [15299-99-2] have been found to be more readily leached (356). 

Pyridine herbicides are not strongly sorbed to soils and ate readily leached. The mobiUty of flutoxypyt [69377-81-7] has been found to decrease with increasing incubation time (399) this is attributed to entrapment of the herbicide within the soil organic matter. 

Fluridone is a weak base with low water solubiUty. Sorption of fluridone increases with decreasing pH (436). Leaching of fluridone was not significant in field study, and the persistence has been determined to be less than 365 days. The degradation of fluridone appears to be microbial in nature, and accelerated breakdown of the herbicide occurs upon repeated appHcations (437). Fluorochloridone is shown to degrade by hydrolysis at pH 7 and 9, but not at lower pH. The half-Hves for this reaction are 190 and 140 days for pH 7 and 9, respectively. Breakdown by photolysis occurs rapidly with a half-hfe of 4.3 days at pH 7 (438). An HA is available for acifluorfen. 

The soil analysis is presented in Table II. Small amounts of 2,4-D and 2,4,5-T were detected in soil samples receiving these herbicides. Background values from the control soils were subtracted from the observed values in treated soils. The samples were not corrected for recovery since it was better than 80% for the method. Residues decreased with time after application. Leaching and microbial decomposition could account for this observation. 

Owing to its low water solubility and high octanol/water partition coefficients, dinitroaniline herbicides adsorb and bind to soil macromolecules and show minimal leaching potential. Dinitroanilines herbicides show good soil residue activities with soil half-lives ranging from 30 days for benfluralin and oryzalin to 6-7 months for trifluralin. Al-Dealkylation (aerobic conditions) and reduction of the nitro group to an amino moiety (anaerobic conditions) have been reported as major soil degradation pathways. 

The objectives of this study were to (a) determine the mobilities of the herbicides, alachlor (2-chloro-2, 6 -diethyl-N-(me-thoxymethyl)acetanilide), butylate (S-ethyl diisobutylthiocarba-mate), and metolachlor (2-chloro-N-(2-ethyl-6-methyl phenyl)-N-(2-methoxy-l-methyl ethyl) acetamide in the laboratory using soil leaching columns and soil thin-layer vapor diffusion techniques,... 

Soil Column Leaching. The distribution of radioactivity from [ 1 C]butylate applied at 4.5 KG/HA and [1 l C]alachlor and [1 C] — metolachlor applied at 2.25 KG/HA and leached with 15 cm of water in Felton sand, is shown in Figure 4. Although all three herbicides are mobile in this soil type, butylate showed less mobility, with 59.6% of the applied raidoactivity found in the upper 10 cm of the column, while 28.4% and 24.3% of the applied 1 C was found in the upper 10 cm of the alachlor and metolachlor columns, respectively. 

Transport of the herbicides by vapor diffusion on moist soil was shown to be directly related to vapor pressure and inversely related to water solubility. Transport of the herbicides by leaching was shown to be inversely related to the Freundlich adsorption coefficient which in turn was directly related to the octanol/water partition coefficient and inversely related to water solubility (16). 

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