J.J.T.I. Boesten

A European test of pesticide-leaching models: methodology and major recommendations

Testing of pesticide-leaching models is important in view of their increasing use in pesticide registration procedures in the European Union. This paper presents the methodology and major conclusions of a test of pesticide-leaching models. Twelve models simulating the vertical one-dimensional movement of water, solute, heat, and, in particular, pesticides, through the soil profile were used by 36 different modellers. The adopted modelling codes differ in terms of modelling concepts and modelling hypothesis. Modellers were affiliated to industry and to the scientific community as well. Four quality datasets were identified to perform the analysis. The dataset included field and lysimeter data, collected in the Netherlands, Germany, Italy and the UK. As well, non-structured as structured soils were available in the dataset. To elucidate the ability to model correctly water transport, solute transport, heat transport and pesticide transport in soils, a stepwise evaluation approach was followed. Splitting up the experimental dataset enabled us to quantify the calibration capability and the prediction capability of the models. The simulations were performed by different model users enabling us also to characterise output variability in terms of user dependent interpretation of the model input and parameters. Recommendations are formulated for improving the quality of modelling datasets, and the process description of water, solute, and heat transport in a pesticide-leaching model, plus the process description of pesticide fate. Application of the principles of good modelling practice (GMP) is briefly described

Modeller subjectivity in estimating pesticide parameters for leaching models using the same laboratory data set.

User-dependent subjectivity in the process of testing pesticide leaching models is relevant because it may result in wrong interpretation of model tests. About 20 modellers used the same data set to test pesticide leaching models (one or two models per modeller). The data set included laboratory studies on transformation and sorption of ethoprophos and bentazone in soil from the top 25 cm, at two or three temperatures. All modellers received the raw data from these studies without guidance for deriving the model input parameters. The modellers were asked to provide the values of the half-lives and sorption coefficients which the model considered would use for this soil layer at 108C (and at field capacity for the half-lives). The half-life of ethoprophos ranged from 92 to 346 days with an average of 191 days and a coefficient of variation of 29%. The half-life of bentazone ranged from 33 to 204 days with an average of 83 days and a coefficient of variation of 46%. The linear and Freundlich sorption coefficients of ethoprophos ranged from 1.7 to 4.3 dm 3 kg ˇ1 with an average of 3.4 dm 3 kg ˇ1 and a coefficient of variation of 21%. The linear and Freundlich sorption coefficients of bentazone ranged from 0.08 to 0.14 dm 3 kg ˇ1 with an average of 0.11 dm 3 kg ˇ1 and a coefficient of variation of 13%. This variability caused by the interpretation of the modeller is so large that it overrules conceptual differences between models in many cases. The most important cause of the variability in the half-lives was the expert judgement involved in establishing the relationship between transformation rate and soil temperature. Differences in fitting procedures played only a minor role for the half-lives but they were an important cause of the variability in the linear sorption coefficient. Some recommendations are proposed to reduce the effect of usersubjectivity on modelling results in future model tests. # 2000 Elsevier Science B.V. All rights reserved.

MODELING ADSORPTION/DESORPTION KINETICS OF PESTICIDES IN A SOIL SUSPENSION

We studied the adsorption/desorption kinetics (time scale: 0 to 24 h) and the reversibility of the sorption process of the herbicides cyanazine and metribuzin in a suspension of a loamy sand soil. Mathematical models of pesticide sorption kinetics based on Langmuir and Freundlich isotherm equations were considered in attempts to describe measured adsorption kinetics. We found a Freundlich model to be most suitable for description. In this model, the sorption kinetics of two classes of site are considered. The sites of the first class equilibrate at a time scale of minutes, those of the second at a time scale of hours. Measured desorption kinetics and equilibria were found to be explained well, quantitatively, by the Freundlich model whose parameters were based on adsorption kinetics and equilibria. Thus, no hysteresis was found in the isotherms, and desorption kinetics was found to proceed as fast as adsorption kinetics.

Movement of water, bromide and the pesticides ethoprophos and bentazone in a sandy soil: the Vredepeel data set

The aim of this study was to collect a data set suitable for testing pesticide leaching models in the case of a Dutch sandy soil with a shallow groundwater table. The movement of water, bromide ion and the behaviour of the pesticides ethoprophos and bentazone was studied. The substances were applied after sowing winter wheat in autumn 1990. This late application time is unusual for bentazone: it was selected on scientific grounds (without agricultural purpose). Rainfall, groundwater level and soil temperature were monitored continuously at the experimental field (80 m long and 54 m wide) until spring 1992. Soil profiles were sampled at 1, 103, 278 and 474 days after application (16 profiles at each date). In the laboratory, pesticide transformation rates were measured with soil material from 0–25, 50–100 and 100–200 cm depth. Sorption isotherms were measured with material from 0–25 cm depth. Concentration profiles showed that mobility increased in the sequence ethoprophos — bentazone — bromide ion. Ethoprophos movement was limited to the top 25 cm layer whereas bentazone leached to below 1 m depth. At the end of the study, the concentrations of ethoprophos and bentazone were below the detection limit (0.2–2 μg dm−3) in all soil layers between 25 and 120 cm depth. Recommended values for the most important input parameters of pesticide leaching models are presented.

Testing PESTLA using two modellers for bentazone and ethoprophos in a sandy soil

Two modellers tested the PESTLA model (version 2.3.1) against results of a field study on bentazone and ethoprophos behaviour in a sandy soil. Both modellers achieved an acceptable description of the measured moisture profiles but only after calibration of the soil hydraulic properties. Both could describe the bromide-ion concentration profiles measured at the end of the first winter reasonably well. However, both predicted that practically all bromide had leached out of the top 50 cm of the soil at the end of the second winter, whereas about 10% of the bromide dose remained in this layer. This is attributable to a systematic deviation of bromide transport from the concept assumed in the convection/dispersion equation and/or to the release of bromide from dead root remnants. Both modellers derived pesticide transformation and sorption parameters from laboratory studies with soil from the field. Both described bentazone movement reasonably well. Modeller 1 described the concentration profiles reasonably well, whereas Modeller 2 strongly overestimated the concentrations at the end of the study. This difference was mainly attributable to a difference in interpretation of the temperature dependence of the transformation rate of bentazone. Only Modeller 2 simulated ethoprophos behaviour. He simulated the persistence of ethoprophos in the top 20 cm very well during the first 200 days. However, thereafter the transformation in the field proceeded much faster than simulated. This is probably caused by accelerated transformation resulting from exposure of the top soil layer to about 1 mg kg -1 of ethoprophos over 200 days. Simulated penetration of ethoprophos was deeper than measured. By including accelerated transformation (admittedly on an ad-hoc basis) within the simulations, good agreement was achieved between measured and simulated penetration of ethoprophos. Calculations showed that the effect of calibrating water flow was substantial for bentazone but small for ethoprophos. However, the effect of calibration of water flow for bentazone was much smaller than the effect of the difference between the transformation rate parameters derived by the two modellers. We recommend that the guidance for deriving pesticide-soil input parameters be improved in order to reduce differences between modellers because a large influence of the person of the modeller on the outcome of model tests is unacceptable for methodological reasons

Transformation of 14C-labelled 1,2-dichloropropane in water-saturated subsoil materials

Abstract The transformation of [1- 14 C]-1,2-dichloropropane in three water-saturated sandy subsoil materials (sampled at a depth of between 1.2 and 2.3 m) was studied in laboratory incubations at 10 °C. In the first material (methanogenic with a redox potential of 0.0–0.2 V and a pH of 7) no measurable transformation took place during the first 156 days but after 608 and 712 days more than 96% of the dose had been transformed into one or more highly volatile products. Analysis of the headspace above this subsoil material after 1059 days indicated that propene and propane are probably transformation products and excluded chlorinated hydrocarbons with one, two or three C atoms as significant transformation products. In the two other subsoil materials (with redox potentials of 0.0–0.4 V and 0.3–0.6 V and pH values between 4 and 5) no measurable transformation took place during the whole incubation period (1056 days).

Field Test of The Pestla Model For Ethoprophos on a Sandy Soil

The nematicide ethoprophos was applied to a sandy soil (5% organic matter) in autumn. Concentration profiles in field soil were measured after 103 and 278 days. In the field, rainfall and soil temperature were continuously recorded. In the laboratory, the transformation rate and adsorption of ethoprophos were measured using soil sampled in the field before application of ethoprophos. The results were used to test an existing mathematical model (PESTLA) which is based on the convection/dispersion equation and which assumes equilibrium sorption and first-order transformation kinetics. During the first 22 days about 50% of the dose disappeared whereas the model calculated a loss of 4% in this period. The measured loss is probably the result of volatilisation which is not accounted for by the model. Measured and calculated persistence in soil corresponded well between 22 and 103 days but after 278 days the calculated remaining amount was about 1.5 times the measured amount. Measured and calculated concentration profiles corresponded reasonably well after 103 days. However, after 278 days the model overestimated leaching out off the top 4 cm.

Scenarios for exposure of aquatic organisms to plant protection products in the Netherlands: Part 2: Sideways and upward spraying in Dutch fruit crops (interim report)

Een studie is uitgevoerd naar de impact op de toelaatbaarheid van gewasbeschermingsmiddelen als de nieuwe oppervlaktewater exposure scenario’s voor substraatteelten in Nederlandse kassen worden ingevoerd. Ook is de gevoeligheid van de modeluitkomsten bepaald voor een aantal belangrijke modelparameters. De berekende milieuconcentratie in oppervlaktewater van de 35 bekeken gewasgewasbeschermingsmiddel combinaties lag in 27 gevallen hoger dan het bijbehorende toelatingscriterium. Voor deze combinaties zullen end-of-pipe reductietechnieken nodig zijn om de milieuconcentraties voldoende te verlagen.

An improved soil organic matter map for GeoPEARL_NL: Model description of version 4.4.4 and consequence for the Dutch decision tree on leaching to groundwater

GeoPEARL_NL is used as a higher tier instrument in the leaching assessment of plant protection products in the Netherlands. Because the soil organic matter contents in arable soils in the current version were too high, a new soil organic matter for the Netherlands was needed.