Project
title / Factors controlling pesticide leaching from structured soils
/ DEFRA
project code / PL0541

Department for Environment, Food and Rural Affairs CSG 15

Research and Development

Final Project Report

(Not to be used for LINK projects)

Two hard copies of this form should be returned to:
Research Policy and International Division, Final Reports Unit
DEFRA, Area 301
Cromwell House, Dean Stanley Street, London, SW1P 3JH.
An electronic version should be e-mailed to
Project title / Factors controlling pesticide leaching from structured soils
DEFRA project code / PL0541
Contractor organisation and location / Cranfield Centre for EcoChemistry
Cranfield University
Silsoe, Bedford, MK45 4DT
Total DEFRA project costs / £ 209,100
Project start date / 01/06/00 / Project end date / 31/05/03
Executive summary (maximum 2 sides A4)
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CSG 15 (Rev. 6/02) 2

Project
title / Factors controlling pesticide leaching from structured soils
/ DEFRA
project code / PL0541

Reduction of pesticide losses to surface waters from diffuse sources is central to the Pesticide Safety Directorate’s Policy Objective to develop strategies to reduce contamination from agricultural pesticide pollution to below acceptable limits. Previous DEFRA research has demonstrated that the primary route for such losses is rapid movement through clay-rich soils via preferential flow which is intercepted by drains. Work at Brimstone Farm indicated that losses of some, but not all, pesticides from a cracking clay soil decreased rapidly with increasing time from application to first drainflow event. Losses of the moderately sorbed isoproturon in drainflow initiated 10 days after application were four orders of magnitude larger than those in drainflow initiated 110 days after application. In contrast, the effect of residence time on leaching of the weakly sorbed triasulfuron was not significant. There is a clear need to understand the factors which limit availability of pesticide residues in soil for leaching. The starting premise for this research was that only by generating this basic understanding can further measures for reducing pesticide losses to drains be formulated, tested and applied.

The aim of the research was to elucidate and characterise the processes which control availability of pesticide for leaching with particular emphasis on separating chemical adsorption from diffusion into soil aggregates. Three series of experiments were undertaken to address the research objectives: (i) lysimeter experiments with four contrasting soil types to investigate temporal changes in the availability of pesticide residues for leaching; (ii) work with artificial soil aggregates to implement and evaluate a theoretical model to describe diffusion of pesticides in aggregated soils; and (iii) laboratory experiments to investigate the influence of soil temperature and moisture content on changes in availability for leaching. Three refereed journal papers have been written to describe the three sets of experiments.

Leaching of three pesticides (isoproturon, chlorotoluron and triasulfuron) and a tracer (bromide) were determined in four contrasting soils ranging in texture from sandy loam to clay. The compounds were applied to undisturbed columns of soil and four columns for each soil were randomly selected and leached with 24-mm equivalent of water at prescribed time intervals (3, 9, 24, 37 and 57 d after application). A rapid decline in leached loads of isoproturon and chlorotoluron as time from application to irrigation increased was observed in all soils and the decline was much faster than that for total residues in soil. In contrast, triasulfuron and bromide loads only decreased rapidly in the clay soil. Magnitudes of bromide losses decreased with decreasing clay contents of the soil and therefore with a decrease in structural development. Pesticide losses varied from soil to soil, depending on structural development and the organic carbon content.

Work was undertaken to implement and test a diffusion-based model to describe time-dependent sorption. Two clay soils were used to create artificial aggregates of 0.8, 1.4 and 1.7 cm diameter when dry. After saturation, the aggregates were immersed in solutions containing one or more pesticides. The decline with time of the pesticide concentrations in the bathing solution was monitored and the results were compared with predictions of a diffusion-based model. The diffusion coefficients of the compounds were obtained by either fitting the non-linear model to the data (Def) or by independent calculations based on the properties of the compounds and of the aggregates (Dec). The diffusion model was able to predict the temporal variation in pesticide concentrations in the bathing solution reasonably well whether Def or Dec values were used. The ratio between Def and Dec ranged from 0.35 to 0.95 which was a relatively small variation when compared to the range of aggregate sizes used in the experiments and of the Kd values of the compounds. Effective partition coefficients were 1.05 to 1.4 times larger than those derived in batch experiments.

Laboratory studies investigated the influence of soil temperature and initial moisture status on the leaching behaviour of contrasting solutes over time. Aggregates of a heavy clay soil were treated with either bromide or the herbicides chlorotoluron, isoproturon and triasulfuron. The soil was incubated at 90% field capacity and either 5oC or 15oC. The influence of application on initially dry and initially wet aggregate was also investigated. At intervals, parallel samples were either leached in small columns, centrifuged to extract pore water to characterise the fraction of chemical available for leaching or extracted to measure total residue in soil. Bromide concentrations in leachate and in pore water extracted by centrifugation were constant with time. In contrast, concentrations of all three herbicides decreased with increasing time from application in soil incubated at 15oC. The effect of residence time on availability for leaching and extraction in pore was much smaller at 5oC than at 15oC. At the higher temperature, pesticide concentrations in leachate and pore water declined faster than would be expected from degradation alone. The faster decline in availability for leaching at 15oC than at 5oC was attributed to a faster degradation of the readily available fraction. There was no significant influence of soil moisture conditions at the time of application on the leaching of isoproturon or its availability in soil water.

1.  Sorption partitition coefficients (Kd values) measured in standard batch slurry experiments may not be reliable predictors of pesticide mobility in soil. Availability for leaching is likely to be larger than expected from batch Kd measurements soon after application. This is because the approach to equilibrium will be limited by diffusion across water films and into larger pores to reach readily accessible sorption sites.

2.  Availability of pesticide for leaching decreases with increasing time from application. The rate of decline is faster than can be explained by degradation alone. The decrease in availability has now been demonstrated for a range of soil types and pesticides and at different scales ranging from a few grams of disturbed soil in the laboratory up to field experiments at Brimstone Farm and elsewhere.

3.  The shift towards the non-available state is due to slow mass-transfer into and out of soil aggregates and/or slow desorption. The extent of the decline in availability depends on the rate of depletion of the readily available fraction by degradation and leaching events in relation to the rate of slow mass-transfer. It is thus influenced by pesticide properties and environmental conditions and may be greater at higher temperatures. It is expected that the decrease in availability for leaching with time will be faster for: a) conditions that favour degradation; b) well-structured clay soils; c) soils with large organic content; and d) (probably) more strongly sorbed compounds.

4.  At a larger scale, inter-aggregate advective and diffusive transport may physically remove a portion of pesticide residues away from areas interacting with flowing water. The effect will be greatest in strongly structured soils and under conditions of preferential (or bypass) flow. Our results indicate that this process was only a significant control on leaching in the soil with the largest clay content (Lawford), but it is this and similar soils that contribute most to diffuse pollution via drainflow.

5.  A diffusion-based model was developed to describe the approach to sorption equilibrium with time under idealised conditions. The model was able to reproduce the data based either on fitting to observations or using diffusion coefficients derived independently. This confirms that time-dependent sorption can be explained by a diffusion process. Further work is required to evaluate the model at realistic soil moisture contents.

6.  The study has furthered our understanding of time-dependent processes influencing pesticide fate in soil and our ability to predict these processes. However, the new information has not led to the formulation of additional management practices to control leaching of mobile herbicides from structured soils.

7.  Output from this research was used by Cranfield University and the Crop Protection Association in developing a model-based “traffic light” system to guide application of key herbicides (including isoproturon, chlorotoluron, mecoprop, MCPA, atrazine and simazine) in relation to likely time to onset of drainflow. A soil hydrological model predicts the likelihood of drainflow up to 14 days into the future and farmers receive weekly text messages advising them on whether or not to apply pesticide.

CSG 15 (Rev. 6/02) 2

Project
title / Factors controlling pesticide leaching from structured soils
/ DEFRA
project code / PL0541
Scientific report (maximum 20 sides A4)
To tab in this section press the tab key and the Control key together
Press the DOWN arrow once to move to the next question.

CSG 15 (Rev. 6/02) 2

Project
title / Factors controlling pesticide leaching from structured soils
/ DEFRA
project code / PL0541

1 INTRODUCTION

Reduction of pesticide losses to surface waters from diffuse sources is central to the Pesticide Safety Directorate’s Policy Objective to develop strategies to reduce contamination from agricultural pesticide pollution to below acceptable limits. Previous DEFRA research has demonstrated that the primary route for such losses is rapid movement through clay-rich soils via preferential flow which is intercepted by drains or sub-lateral flow (Brown et al., 1995; Harris & Catt, 1999). Lysimeter experiments at Cranfield University showed that a fine topsoil tilth can reduce pesticide losses to drains from heavy clay soils by ca. 33% relative to a standard agricultural tilth (Brown et al., 2001). Work at Brimstone Farm indicated that losses of some, but not all pesticides from a cracking clay soil decreased with increasing time from application to first drainflow event (Jones et al., 2000). Losses of the moderately mobile, moderately persistent herbicide isoproturon in drainflow initiated 10 days after application were four orders of magnitude larger than those in drainflow initiated 110 days after application. This decline in isoproturon losses was faster than could be explained by degradation alone and cannot be predicted from our current knowledge base. In contrast, the effect of residence time on leaching of the weakly sorbed herbicide triasulfuron was not significant. There is a clear need to understand the factors which limit availability of pesticide residues in soil for leaching and this is the scientific problem which this research addressed. The starting premise was that only by elucidating and characterising the controlling processes can further measures for reducing pesticide losses to drains be formulated, tested and applied. Improved understanding will also enhance risk assessment procedures and increase rigour of regulatory decisions with respect to the use of pesticides.

The declines in pesticide bioavailability and thus in its potential for leaching to depth with increasing time of contact with soil are likely to be caused by slow sorption kinetics and/or diffusion. Diffusion may be a limiting factor during (a) movement of pesticide through bulk aqueous solution to the surfaces of sorbing materials; (b) movement in solution through micropores to areas protected from the main regions of soil water flow; and (c) movement through the three dimensional network of organic material (Pignatello, 2000). Diffusion in soil is slow relative to that in water due to the tortuosity and increased length of the flow path within soil particles, sorption to pore walls, steric interference from pore walls and the viscous nature of water near hydrophilic surfaces. Reactions between the pesticide and the sorbent may also be time-dependent. The distribution between the dissolved and sorbed state may thus require days, weeks or even months to reach equilibrium (Cox and Walker, 1999; Koskinen et al., 2002). A depletion of the readily available fraction of pesticide by degradation, uptake into soil organisms and plants or leaching to depth results in a shift of pesticide distribution towards the unavailable state. Further losses may be limited by the rate of mass-transfer from the unavailable to the available state.

The study contributes directly to DEFRA’s specific research objectives “to establish the sources and dispersal mechanisms of agricultural pesticide pollution” and “to improve advice on reducing the diffuse loss of pesticides (particularly those herbicides which are prone to leaching into water supplies) from agricultural sources”. The work also sought recommendations for improving models of pesticide transport via preferential flow, thus supporting the objective “to develop techniques to model the movement of pesticides in soils and water”. Current models have clearly been shown to be deficient with respect to simulating the behaviour to be investigated.

2 OVERALL AIM

The aim of the research was to elucidate and characterise the processes which control availability of pesticide for leaching with particular emphasis on separating chemical adsorption from diffusion into soil aggregates.

Three series of experiments were undertaken to address the research objective: (i) lysimeter experiments with four contrasting soil types to investigate temporal changes in the availability of pesticide residues for leaching; (ii) work with artificial soil aggregates to implement and evaluate a theoretical model to describe diffusion of pesticides in aggregated soils; and (iii) laboratory experiments to investigate the influence of soil temperature and moisture content on changes in availability for leaching. Three refereed journal papers have been written to describe the three sets of experiments. The experiments are overviewed below with particular emphasis on practical implications for management approaches to control diffuse pollution in the field.