Variations in soil properties with depth strongly influence the degradation and persistence of herbicides, underlining the importance of studying these processes in soil horizons with distinctively different properties. The persistence of the herbicides acetochlor, atrazine, and S-metolachlor was measured in samples of the A, B, and C horizons of 2 Typic Argiudolls from Argentina under no-till management. The soils studied differed in soil organic carbon (OC) content, pH, particle size distribution, and structure. Quantification of herbicides in soil was done through high-performance liquid chromatography with diode array detector. There were interactions of herbicidexhorizon (p < 0.01) that resulted in degradation rates (k) of all herbicides decreasing, and their corresponding dissipation half-life (DT50) values increasing, with soil depth. Herbicide persistence across all soils and horizons ranged from 15 to 73 d for acetochlor, 13 to 29 d for atrazine, and 82 to 141 d for S-metolachlor, which had significantly (p < 0.01) greater persistence than atrazine and acetochlor. The DT50 values of herbicides were negatively correlated with the contents of OC (correlation coefficients ranging from -0.496 to -0.773), phosphorus (-0.427 to -0.564), and nitrogen-nitrate (-0.507 to -0.662), and with microbial activity (-0.454 to -0.687) and the adsorption coefficient (-0.530 to -0.595); DT50s were positively correlated with pH (0.366 to 0.648). Adsorption was likely the most influential process in determining persistence of these herbicides in surface and subsurface horizons. The present study can potentially improve the prediction of the fate of acetochlor, atrazine, and S-metolachlor in soils because it includes much needed information on the degradation of the herbicides in subsurface horizons. Environ Toxicol Chem 2017;36:3065-3073. (c) 2017 SETAC

La determinación de los parámetros de adsorción y disipación de plaguicidas en el suelo permite un mejor pronóstico de los procesos y del potencial de contaminación del agua subterránea. El objetivo fue comparar dos métodos de cálculo de los índices factor de retardo y de atenuación log-transformado (AFT), a fin de estimar el riesgo de lixiviación potencial de los herbicidas atrazina, acetoclor y s-metolacloro en los horizontes A, B, C y en todo el perfil de dos suelos de la provincia de Buenos Aires (Argentina). Se elaboraron los índices a partir de los parámetros de adsorción y persistencia de los herbicidas obtenidos mediante experimentos con enfoque multicapa, respecto de cálculos mediante bases de datos con enfoque monocapa. Se determinó el índice de adsorción y el tiempo de vida media para los tres horizontes, a partir de experimentos bajo condiciones controladas. De acuerdo al AFT, calculado para diferentes recargas (0,3; 1,1 y 2,3 mm/día) mediante información de los experimentos multicapa, se estimó un potencial de lixiviación muy improbable de los tres herbicidas en todos los horizontes y perfiles. La utilización de índices tabulados determinó un mayor riesgo de lixiviación, que tendió a incrementarse a mayor profundidad según el orden C>B>A>perfil.

Understanding herbicide sorption within soil profiles is the first step to predicting their behavior and leaching potential. Laboratory studies were conducted to determine the influence of surface and subsurface soil properties on acetochlor, atrazine, and S-metolachlor sorption. Soil samples were taken from horizons A, B, and C of two loamy soils of the humid pampas of Argentina under no-till management; horizon A was divided into two layers, A(0) (0-5 cm) and A(1) (5 cm to the full thickness of an A horizon). Sorption isotherms were determined from each sampled horizon using the batch equilibrium method and seven concentrations (0, 0.1, 0.5, 2.0, 5.0, 10.0, and 20.0 mg L-1). Sorption affinity of herbicides was approximated by the Freundlich equation. The sorption strength K-f (mg(1 - 1/n) kg(-1) L-1/n) over the soils and horizons studied followed the order S-metolachlor (16.51-29.19) > atrazine (4.85-12.34) >= acetochlor (5.17-11.97), which was closely related to the hydrophobicity of herbicides expressed as octanol-water partition coefficient (K-OW). The K-f values of the three herbicides were positively correlated with soil organic carbon, with a significance of p < 0.01. Values of K-f for the three herbicides decreased with depth in the two soils, indicating greater sorption onto surficial soil horizons and possibly a delayed transport toward subsurface soils and subsequent pollution of groundwater. Environ. Toxicol. Chem. 2011;30:1990-1996. (C) 2011 SETAC

Agricultural systems of Argentina have increased herbicides inputs, mostly associated with adoption of no tillage (NT). Several studies have revealed presence of pesticides in groundwater. Therefore, research on the behaviour of herbicides in soils is driven by the need to manage and prevent possible contamination of groundwater. Soil organic carbon (OC) is the main soil component responsible of sorption, and consequently the main tool to reduce the leaching. However, in dynamic systems transport of organic chemicals depends on soil structural and hydraulic properties. Sorption controls the physical and biological availability of chemicals. Physical, heterogeneous flow domain, and chemical, kinetic reactions and molecular diffusion into aggregates, which are nonequilibrium processes that affect solute transport. The main objective of this paper was to evaluate the effects of soil texture and tillage system on atrazine transport through intact soil columns. The study focused on the identification of processes; and determination of parameters that control atrazine transport in the upper layer of soils. Balcarce (BAL, silty clay loam, fine, thermic, illitic, Typic Argiudoll)), Tres Arroyos (TAR, clay loam, fine, thermic, illitic, Typic Argiudoll) and Coronet Dorrego (DOR, loam, fine, thermic, mixed illitic-montmorillonitic, Typic Argiudoll) soils from the southeast of Buenos Aires Province (Argentina) were selected. The soils represent a wide range of OC content (BAL 35.5, TAR 28.8 and DOR 17.3 g kg(-1)). At each site NT and conventional tillage (CT) systems were sampled. Four replicates of intact soils cores (15 x 8 cm) were removed from each combination of soil x tillage (BAL-NT, BAL-CT, TAR-NT, TAR-CT, DOR-NT, DOR-CT). Displacement studies were done using atrazine as the reactive solute and bromide as the nonreactive solute. Equilibrium and nonequilibrium transport models (CXTFIT 2.1) were employed to describe the breakthrough curves (BTCs). The software tool SMART was used to simulate atrazine transport under steady-state flow conditions. Atrazine BTCs were skewed to the right; exhibiting an asymmetric shape and tailing that implied nonequilibrium conditions during transport. Since physical nonequilibrium was assumed to be nearly negligible, the observed nonequilibrium was interpreted as a sorption-related process. The two-site nonequilibrium model showed an acceptable fit with the observed data (72 < R-2 < 86). Recovery percentages of atrazine in effluents were: BAL-CT 54.51 %; BAL-NT 45.10%; TAR-CT 44.28%; TAR-NT 29.70%; DOR-CT 18.60%; DOR-NT 48.95%. The intraparticle diffusion model provided by SMART showed the best fit. In conclusion, intrinsic soil properties were more relevant for atrazine transport than those associated with tillage practices. However, no tillage produced early detection of atrazine in effluents, and favoured atrazine leaching in coarser soils with the lowest OC contents. However, the maximum loss of atrazine in the percolate took place in the soils with the highest OC level; with no effects of tillage practices. These soils had fine texture, and were well structured and aggregated. Intraparticle and intraorganic matter diffusion appear to be responsible for nonequilibrium sorption. Delayed sorption in aggregated soils leads to high concentration of atrazine available for leaching. (c) 2006 Elsevier B.V. All rights reserved.

Laboratory studies were conducted to determine the influence of surface and subsurface properties of three representative soils of the humid pampas of Argentina on atrazine and metribuzin sorption. Atrazine and metribuzin sorption isotherms were constructed for each soil at four depths. Sorption affinity of herbicides was approximated by the Freundlich constant (K-f), distribution coefficient (K-d), and the normalized K-d based on organic carbon content (K-OC). Multiple regression of the sorption constants against selected soil properties indicated that organic carbon content (OC) and silt were related positively and negatively, respectively, to atrazine K-f coefficient (r(2) = 0.93), while K-d coefficient of atrazine was related positively to organic carbon content and negatively to both silt and cation exchange capacity (CEC) (r(2) = 0.96). For metribuzin, only organic matter content was related positively to K-f coefficient (r(2) = 0.51). Lower K-f values for atrazine were obtained for all soils with increasing depth, indicating lesser sorption at greater depths. Metribuzin sorption was quite similar across all depths. Sorption constant K-f of atrazine ranged from 2.06 to 7.82, while metribuzin K-f values ranged from 1.8 to 3.52 and were lower than atrazine for all soils and depths, indicating a greater leaching potential across the soil profile.