Sorption and leaching potential of acidic herbicides in Brazilian soils.

Leaching of acidic herbicides (2,4-D, flumetsulam, and sulfentrazone) in soils was estimated by comparing the original and modified AF (Attenuation Factor) models for multi-layered soils (AFi). The original AFi model was modified to include the concept of pH-dependence for Kd (sorption coefficient) based on pesticide dissociation and changes in the accessibility of soil organic functional groups able to interact with the pesticide. The original and modified models, considering soil and herbicide properties, were applied to assess the leaching potential of selected herbicides in three Brazilian soils. The pH-dependent Kd values estimated for all three herbicides were observed to be always higher than pH-independent Kd values calculated using average Koc data, and therefore the original AFi model overestimated the overall leaching potential for the soils studied.

Soil sorption of acidic pesticides: modeling pH effects.

A model of acidic pesticide sorption in soils was developed from theoretical modeling and experimental data, which initially considered a combination of a strongly acidic pesticide and a variable-charge soil with high clay content. Contribution of 2,4-D [(2,4-dichlorophenoxy) acetic acid] anionic-form sorption was small when compared with molecular sorption. Dissociation of 2,4-D was not sufficient to explain the variation in Kd as a function of pH. Accessibility of soil organic functional groups able to interact with the pesticide (conformational changes) as a function of organic matter dissociation was proposed to explain the observed differences in sorption. Experimental 2,4-D sorption data and K(oc) values from literature for flumetsulam [N-(2,6-difluorophenyl)-5-methyl [1,2,4] triazolo [1,5-a] pyrimidine-2-sulfonamide] and sulfentrazone [N-[2,4-dichloro-5-[4-(difluromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl] phenyl] methanesulfonamide] in several soils fit the model.

Pesticide leaching potential assessment in multilayered soils

O modelo "Attenuation Factor" (AF) generalizado para solos em multi-camadas foi utilizado para estimar os potenciais de lixiviaçäo de pesticidas näo-i nicos e näo-ionizáveis em solos brasileiros. O modelo aplicado considera as propriedades dos pesticidas e dos solos, assim como a taxa de recarga líquida. O transporte advectivo dos pesticidas e dos solos, assim como a taxa de recarga líquida. O transporte advectivo dos pesticidas e o de primeira ordem dos pesticidas foram assumidos como premissas. Os pesticidas com os maiores potenciais de lixiviaçäo foram Tebutiuron e Hexazinone, os quais apresentaramvalores de tempo de meai-vida (t1/2) maiores que 90 dias e oeficiente de sorçäo (Koc) menores que 80 mL/G. Para todos os pesticidas, nos três solos, os valores de AF aumentaram com a profundidade pelo decréscimo no tempo de percurso (tr) do pesticida. Tal fato se deve ao menor tempo de percurso da água e ao menor fator de retardo (FR). em Neossolo Quartzarêncio )areia quartzosa). o tempo de percursäo mais curto, devido ao menor valor de capacidade de campo 9FC0 resultou em AF mais alto e maior potencial de lixiviaçäo. Os resultados permitem estimar que cerca de 54 por cento de Tebutiuron e 13 por cento de Hexazinone aplicados na superfície do solo passariam através dos primeiros 120 cm do Neossolo Quatzarênico.

Predicted impact of transgenic, herbicidetolerant corn on drinking water quality in vulnerable watersheds of the mid-western USA.

In the intensely farmed corn-growing regions of the mid-western USA, surface waters have often been contaminated by herbicides, principally as a result of rainfall runoff occurring shortly after application of these to corn and other crops. In some vulnerable watersheds, water quality criteria for chronic human exposure through drinking water are occasionally exceeded. We selected three settings representative of vulnerable corn-region watersheds, and used the PRZM-EXAMS model with the Index Reservoir scenario to predict corn herbicide concentrations in the reservoirs as a function of herbicide properties and use pattern, site characteristics and weather in the watersheds. We compared herbicide application scenarios, including broadcast surface pre-plant atrazine and alachlor applications with a glyphosate pre-plant application, scenarios in which losses of herbicides were mitigated by incorporation or banding, and scenarios in which only glyphosate or glufosinate post-emergent herbicides were used with corn genetically modified to be resistant to them. In the absence of drift, in almost all years a single runoff event dominates the input into the reservoir. As a result, annual average pesticide concentrations are highly correlated with annual maximum daily values. The modeled concentrations were generally higher than those derived from monitoring data, even for no-drift model scenarios. Because of their lower post-emergent application rates and greater soil sorptivity, glyphosate and glufosinate loads in runoff were generally one-fifth to one-tenth those of atrazine and alachlor. These model results indicate that the replacement of pre-emergent corn herbicides with the post-emergent herbicides allowed by genetic modification of crops would dramatically reduce herbicide concentrations in vulnerable watersheds. Given the significantly lower chronic mammalian toxicity of these compounds, and their vulnerability to breakdown in the drinking water treatment process, risks to human populations through drinking water would also be reduced.