Empirical model to assess leaching of pesticides in soil under a steady-state flow and tropical conditions.

Abstract: An empirical model of leaching of pesticides was developed to simulate the concentration of fungicides throughout unsaturated soil. The model was based on chemical reactions and the travel time of a conservative tracer to represent the travel time required for water to flow between soil layers. The model's performance was then tested using experimental data from dimethomorph and pyrimethanil applied to the soil under field and laboratory conditions. The empirical model simulated fungicide concentration on soil solids and in soil solution at different depths over time (mean square error between 2.9 mg2 kg-2 and 61mg2 kg-2) using sorption percentages and degradation rates under laboratory conditions. The sorption process was affected by the organic carbon, clay, and the effective cation exchange capacity of the soil. The degradation rate values of dimethomorph (0.039 d-1-0.009 d-1) and pyrimethanil (0.053 d-1-0.004 d-1) decreased from 0 to 40 cm and then remained constant in deeper soil layers (60-80 cm). Fungicide degradation was a critical input in the model at subsurface layers. The model was determined to be a reliable mathematical tool to estimate the leachability of pesticides in tropical soil under a steady-state flow. It may be extended to other substances and soils for environmental risk assessment projects. Supplementary Information: The online version contains supplementary material available at 10.1007/s13762-023-05038-w.

LEACHING AND DEGRADATION OF 2,4-DICHLOROPHENOXIACETIC ACID, IN COLOMBIA RICE FLOODED SOIL.

Rice is mostly cultivated on soil held under flooded conditions. Under these conditions pesticides undergo reductive transformations which are characteristic to rice fields and other anaerobic systems. The present study was undertaken to evaluate the mobility and persistence of 2,4-dichlorophenoxy acetic acid (2,4-D) under laboratory conditions for the rice crop in Espinal, Colombia. A displacement study was performed on a hand packed soil column 30 cm length. After leaching experiment, the soil from column was sliced into six successive sections (5 cm). Methanol acidified (H3PO4 0.25%) extraction was used to determine the herbicide residues in each section. 2,4-D experimental breakthrough curve was analyzed using Stanmod program (inverse problem) to obtain transport parameters. The non-equilibrium physical model fitted well the experimental breakthrough curve. The recovery percent of 2,4-D in leachates was 36.44% after 3.4 pore volumes, and retardation factor was 2.1, indicating low adsorption in that conditions. 2,4-D was rapidly degraded, with DT50 = 11.4 days. The results suggest that 2,4-D under flooded conditions have a high potential for leaching through the soil profile, although the elevated rate of degradation reduced the ground water contamination risk.

Impact of rainfall intensity on the transport of two herbicides in undisturbed grassed filter strip soil cores.

Two series of displacement experiments with isoproturon and metribuzin herbicides were performed on two undisturbed grassed filter strip soil cores, under unsaturated steady-state flow conditions. Several rainfall intensities (0.070, 0.147, 0.161, 0.308 and 0.326 cm h(-1)) were used. A water tracer (bromide) was simultaneously injected in each displacement experiment. A descriptive analysis of experimental breakthrough curves of bromide and herbicides combined with a modeling analysis showed an impact of rainfall intensity on the solute transport. Two contrasting physical non-equilibrium transport processes occurred. Multiple (three) porosity domains contributed to flow at the highest rainfall intensities, including preferential flow through macropore pathways. Macropores were not active any longer at intermediate and lowest velocities, and the observed preferential transport was described using dual-porosity-type models with a zero or low flow in the matrix domain. Chemical non-equilibrium transport of herbicides was found at all rainfall intensities. Significantly higher estimated values of degradation rate parameters as compared to batch data were correlated with the degree of non-equilibrium sorption. Experimental breakthrough curves were analyzed using different physical and chemical equilibrium and non-equilibrium transport models: convective-dispersive model (CDE), dual-porosity model (MIM), dual-permeability model (DP), triple-porosity, dual permeability model (DP-MIM); each combined with both chemical instantaneous and kinetic sorption.

Evaluating non-equilibrum herbicides transport in undisturbed grassed buffer strips soil columns.

Possible contamination of water resources by applied pesticides (including insecticides and herbicides) is a problem currently. Grassed buffer strips have been considered to limit pesticide transfer to surface waters by reducing run-off and erosion. As reduction of run-off mainly involves infiltration in the grassed strip soils, possibility of groundwater contamination by rapid transfer of pesticides via the numerous root channels and macropores presents in grassed soils can not be excluded so far. The beneficial impact of high pesticide retention in the superficial horizons of these soils, related to their high organic matter content, could therefore be strongly reduced by the occurrence of preferential flows. Displacement studies on leaching of bromide (water tracer) and two herbicides, metribuzin (weakly adsorbed) and isoproturon (moderately adsorbed) were conducted in two undisturbed soil columns (length 30 cm, diameter 14 cm) collected from La Jaillière (Loire Atlantique, France) under non saturated conditions. The infiltration fluxes, delivered through a rainfall simulator, were 1.5 and 3 mm/h for one of the columns and 0.6, 1.5 and 3 mm/h for the second column. We obtained asymmetrical shapes of the breakthrough curves, with early breakthrough and increased tailing, qualitatively indicating the presence of preferential flow and confirming the risk of pesticide vertical transport through grassed strips superficial soil horizons. The results showed that leaching of both herbicides increased with the intensity of rainfall applied. This could be explained by a decrease of herbicides sorption due to a lower residence time in the soil.