Eur. J. Agron.• 1995, 4(4), 473-484
Modelling pesticide leaching in soils
main aspects and main difficulties R. Calvet
Institut National Agronomique Paris-Grignon, France, Centre de Grignon, 78850 Thiverval-Grignon Accepted: 14 September 1995.
Abstract
The paper gives a review of the main models which have been proposed to describe the fate of pesticides in soils. Descriptions of processes taken into account by the models are compared and briefly discussed. Difficulties for modelling pesticide leaching are examined in relation to the pos sible improvement of the description of involved processes and to model sensitivity and validation. Key-words : pesticides, leaching, soil, models.
INTRODUCTION Urban, industrial and agricultural activities produce increasing amounts of potential pollutants which are introduced to cropped and uncropped areas. There is growing concern about the fate of these chemicals, their consequences for the long-term soil health and quality of surface and ground-water. Water monitoring in many countries has revealed contamination by vari ous toxic chemicals, particularly pesticides. A number of pesticides has recently been detected in groundwa ters in western Europe and in the USA in the past years. Thus, nonpoint pollution due to pesticides may represent a threat to groundwater and to public water supply. As the need for water increases and the amount of potable water in the world is limited, people are increasingly conscious of the need to pro tect water resources. The extent of leaching and the resulting alteration in water quality depend on soil and pesticide properties, climatic conditions, crop type and cropping practices, and water management methods. Considering the need of pesticide use in agriculture, the only acceptable solution for the prevention of groundwater contamination is improved pesticide management, leading to acceptable and safe applica tion rates. Knowledge of the fate of pesticides in soil, in terms of basic phenomena such as transport, reten tion and transformation, is essential to the develop ment of predictive tools. One way to proceed is by numerical modelling of chemical transport, coupled with evaluation of sources and sinks. Models may be useful because they accomodate environmental and hydrological conditions, chemical properties, alterna tive agronomic and management practices and the spa tial variability of soil and chemical/media interactions. ISSN 1161-0301195/041$ 4.001© Gauthier-Villars - ESAg
There have been many attempts to model solute transport in soils and other porous media. Determinis tic and mechanistic models based on miscible dis placement theory (Nielsen and Biggar, 1962) have been widely used in transport modelling. Many math ematical developments and field experiments have been reported in the literature (Wagenet and Rao, 1990 ; Wagenet, 1993). Transport models are classified according to several criteria (Addiscott and Wagenet, 1985). Modelling approaches can be classified as deterministic or stochastic models, with two subcat egories, mechanistic and functional, according to the description of the processes. Closely related is the dis tinction between rate and capacity models. From the point of view of their use, models have been devel oped from various perspectives : research, manage ment, regulation, screening and educational purposes. They present descriptions which vary greatly accord ing to the phenomena considered and the nature of their mathematical formulation. These models are very different regarding the number of input data and dis tinctions between different categories may appear arti ficial. The general trend is to increase the complexity of models by increasing input parameters when going from educational to research models. However, research models may prove useful teaching tools. In principle, management models can relate the output results to cropping systems and cropping techniques. The aim of this paper is to provide a general pre sentation of some of the published pesticide leaching models and to discuss briefly some important difficul ties encountered when using them. Mathematical and numerical aspects are beyond the scope of the paper and the reader is referred to publications of the model developers.
R. Calvet
474
them applied strictly to homogeneous media, others can be used for describing transport in heterogeneous soil profiles. Transport of pesticides can take place through run off, erosion, leaching and volatilization. Runoff and erosion models are not described here, the paper being limited to the most recent and frequently used leach ing models (Table 1). Relationships describing water and solute transport and water balance in soil constitute the master set of equations. These equations are coupled with other equations representing sorption/desorption, transfor mation/degradation, plant uptake and volatilization.
GENERAL CHARACTERISTICS OF PESTI CIDE LEACHING MODELS Several methods are available for modelling the fate of pesticides in the environment, depending on the global character of the description and on the pro cesses taken into account. Global or multimedia mod els calculate the distribution of pesticides among vari ous environmental compartments (soil, air, water, fauna, plants, etc ..). Basically, these models use pesti cide fugacity taken as representative of the ability of a compound to escape from a given compartment (Mackay, 1979; Wania et al., 1993). Thus, they allow simulation of the fate of chemicals in large and com plex ecosystems. Others models are limited to the description of pesticide transport with or without cou pling to sinks and sources. These models can be used in water saturated and unsaturated media, some of
Transport Phenomena Table 2 gives an overview of transport phenomena included in various models. In addition to water and
Table 1. Some pesticide leaching models. Reference
Category
Model
Acronym
Jury et al., 1983, 1987
BAM
Behaviour Assessment Model
screening
CMLS
Chemical Movement in Layer Soils
educational
Nofziger and Hornsby, 1986
PRZM2
Pesticide Root Zone Model
management
Carse! et al., 1984, 1992*
VULPEST
Vulnerability to Pesticides
management
Villeneuve et al., 1990
VARLEACH
Model for describing transport and degradation of pesticides
research/management
Nicholls et al., 1982a, ! 982b Walker and Barnes, 1981 Walker and Welch, 1989
LEACHMP2
Leaching Estimation and Chemistry Model
research
Wagenet and Hutson, 1989, 1992*
Mathematical model for describing transport in the unsaturated zone of soils
research
Piver and Lindstrom, 1990
y
Modeling the influence of sorption and transformation on pesticide leaching and persistence
research
Boesten and van der Linden, 1991
PESTFADE
Pesticide fate and transport model
research
Clemente et al.. 1993
x
*
Date of the last published version.
Table 2. Modelling of transport phenomena. Solute transport
Water transport Model
BAM CMLS PRZM2 VULPEST VARLEACH LEACHMP2
x y
PESTFADE
Downward flow
x x x x x x x x x
Upward fow
x x x x
x
Transport in the soil Convection
x x x x x
Convection/dispersion
Volatilization
Heat transport in the soil
x x
x
x
x
x x
x x
x
x
x
x x
Eur. J. Agron.
Modelling pesticide leaching in soils
solute transport, PRZM2 and PESTFADE calculate the amounts of soil and pesticide transported by runoff and erosion. Some models, not cited in Table 2, are designed to simulate runoff and erosion (e.g. GLEAMS; Leonard et al., 1987).
Water transport Table 3 indicates how water transport is described in the models. When the Richards equation is resolved for transient flow, hydraulic conductivity-water content relations and matric potential-water content relations are needed and this is often a serious limitation to model application. In contrast, PRZM2, VARLEACH and CMLS demand only some water content values such as field moisture capacity, wilting point and the water content at some specified matric potentials (e.g. : - 10 kPa and - 200 kPa). When a water table is present, capillary rise can be simulated and the corresponding flux calculated only with models using a convection/dispersion equation.
Solute transport Solute transport is calculated by resolution of the dispersion/convection equation (LEACHMP, X, Y, PESTFADE), by multiplying a water flux by a solu tion concentration (VULPEST, BAM) or by piston flow (CMLS, PRZM, VARLEACH). For PRZM2 hydrodynamic dispersion is simulated by a numerical dispersion calculation. Volatilization and losses from
475
the soil to the atmosphere are obtained by coupling the liquid/gas partition of the pesticide (Henry's law) with diffusion into the soil gas phase and into the atmosphere just above the soil surface. All models cited above simulate water and solute transport in the unsaturated zone for given sets of boundary conditions. These boundary conditions (Table 4) allow several field situations to be dealt with but require the Richards equation and the convection/dispersion equatioon to be solved numeri cally. Sink/Source Phenomena Table 5 indicates the various sink/source phenom ena included in models cited in Table I. It is interest ing to note some additional information : 1) Concerning sorption. In the X model, the linear sorption coefficient is expressed as a function of the particle-size composition and the organic matter con tent of the soil. A first order rate irreversible sorption is also introduced. In LEACHMP2, two-site sorption kinetics are included ; a fraction of sites displays a local chemical equilibrium and another is character ized by a kinetically controlled sorption and desorp tion which are described by linear isotherms. In PEST FADE, a two-region, two-site approach is also used (Nkeddi-Kizza et al., 1984); the two types of sites are characterized by a first order rate kinetics but with dif ferent localization, one being available in micropores, the other in macropores. A sorption kinetics model
Table 3. Modelling of water transport.
Model
Modelling water transport
Comments
BAM
The Richards equation is resolved for steady flow
CMLS
piston displacement between field water capacity and wilting point
PRZM2
calculation of water flow based on 'tipping bucket' method between field water capacity and wilting point
a fraction of water held in each layer may be allowed to drain; two empirical drainage rules may be used
VULPEST
no calculation for water transport
amount of water which infiltrates is an average monthly value of the difference between rainfall and evapotranspiration
VARLEACH
mass balance applied to water in soil layer; flow drainage is calculated as the excess of water compared to field water capacity
distinction is made between mobile and immobile water for calculating the flow of the soil solution
LEACHMP2
x
generalized Richards equation
possible choice between steady and transient flow
generalized Richards equation
equation can also be resolved for transport in the vapor phase
y
generalized Richards equation
PESTFADE
generalized Richard equation; water flow is simulated by the model SWACROP
Vol. 4, n° 4 - 1995
the soil profile is assumed to be at a uni vorm water content
macropore flow solute transport
may
be
introduced for
describing
R. Calvet
476
Table 4. Various boundary conditions (B.C.) used in the models. The choice between several B.C. is possible with some models.
Model
upper B.C.
Water transport
lower B.C.
upper B.C.
Solute transport
semi-infinite
BAM
steady state water flux
semi-infinite
flux condition
semi-infinite
CMLS
variable flux given by the difference : rainfall-evapo transpiration
free drainage
given amount of applied solute
free drainage
PRZM2
water balance taking into account the flux in and out the soil surface layer
free drainage
diffusion through a limited film at the soil/atmos phere interface
free drainage
VULPEST
variable flux given by the difference rainfall-evapo transpiration
not specified
constant concentration
zero concentration at infi nite
VARLEACH
water balance taking into account the flux in and out the soil surface layer
free drainage
given amount of applied solute
free drainage
LEACHMP2
ponded, non-ponded infil tration, water evaporation, zero flux at the soil/atmos phere interface
permanent water table, free drainage, zero flux, zero flux + constant potential, fluctuating water table
volatilization, diffusion through a surface soil film and a stagnant atmospheric film; non volatile chemicals : zero flux
water table, unit gradient drainage
as for LEACHMP2
fixed water table
as for LEACHMP2
fixed water table
y
variable flux given by the difference : rainfall-evapo transpiration
fixed water table at I m depth
zero flux
only convection flow is allowed out of the system at 3 m depth
PESTFADE
B.C. of SWACROP
B.C. of SWACROP
zero concentration at the soil surface
zero flux at a given depth
x
Table 5. Characteristics of sink/source phenomena taken into account in the models.
Model
Equil. Model
L
BAM CMLS PRZM2
x
VULPEST VARLEACH LEACHMP
x
x y
PESTFADE
x x
BIOTIC Transformation and Degradation
SORPTION
PU
x x x x x x x x x
Liquid
NE NL
x
x
NE
x x x
Ist.
x x x x x x x x x
8
Gas T
ABIOTIC Transformations* Liq. Sol.
z
x x x x x x x
x x x x x
x x x x
?
x x
x
x
PU : solute plant uptake ; T : temperature dependent rate constant; 0 : water content dependent rate constant ; z : depth dependent rate constant ; liq. : abiotic transforma tion in the liquid phase; sol. : abiotic transformation at the soil constituent surfaces ; Equil. : sorption equilibrium; L : linear isotherm; NL : non linear sorption i&otherm ;
!st: overall first order reaction; *: explicitely described in the model. Eur. J. Agron.
477
Modelling pesticide leaching in soils
formulated by Gamble and Khan (Clemente et al., 1993) may be incorporated. Sorption coefficient is allowed to vary with depth in some models (CMLS, LEACHMP2, VARLEACH, PRZM2). 2) Concerning transformation and degradation. In BAM, the degradation rate constant may vary with depth according to an empirical reduction factor. In Y model the degradation rate constant may also vary with depth according to an empirical numerical func tion proposed by the authors. LEACHMP2 can simu late simultaneously the fate of several chemicals and their daughter products undergoing biotic and abiotic reactions, all being of the first order. The several rate constants are allowed to vary with depth and soil moisture content according to the formulation of Walker and Barnes (1981) and the soil temperature (Arrhenius formulation). Transformations in the gas phase are also introduced. Daughter product degrada tion is also described in PRZM2.
DIFFICULTIES ENCOUNTERED WITH PESTICIDE LEACHING MODELS
As briefly described earlier, some models incorpo rate a variety of phenomena, sometimes with detailed descriptions. However, models generally need to be improved to simulate field observations properly and one of the difficulties lies in the introduction of these improvements. Other difficulties are related to model sensitivity to variations in input parameters which generally determine their possible applications and validation procedures. Principal improvements to better represent the fate of pesticides
Preferential flow Many field experimental results are poorly simu lated by pesticide leaching models. Some strongly and weakly adsorbed pesticides are transported to great depths as if they were subject to accelerated move ment. Two examples are given below to iHustrate this observation. Ghodrati and Jury ( 1992) designed a field experi ment on an irrigated loamy sand soil. They studied the transport of atrazine, napropamide and prometryn each with two formulations, two soil surface preparations (undisturbed and repacked), under four flow condi tions obtained by continuous or intermitent sprinkler and flood irrigation. Irrigation was started immediately after pesticide application and the soil was sampled 6 days later. (Figure 1 shows their partial results). The data show transport to depth of the three herbicides Vol. 4, n° 4 - 1995
irrespective of the irrigation method and soil surface preparation. According to their retardation factor val ues, the three compounds should have migrated to shallower depths. The authors have attributed this dis crepancy to preferential flow. The second example concerns an experiment designed to study the influence of sewage sludge application on atrazine leaching behaviour (Barriuso et al., 1993). The soil of the experimental site was an hydromorphic clay soil drained at 0.8-1.0 m depth and cropped with maize treated with atrazine 5 months after sludge application. The distribution of atrazine in the soil profile and the time-variation of atrazine con centration in drainage water are given in Figure 2. In this case, observation of atrazine in drainage water indicated deep transport not predicted by the CMLS, VARLEACH and LEACHMP models. Preferential flow seems to be observed in various soils, often in structured clay soils, but also in sandy soils. Flow mechanisms are not yet completely under stood. Channeling through macropores and cracks in clay soils and through saturated zones of high hydrau lic conductivity in sandy soils have been proposed as possible mechanisms. If such flow patterns occur, sol ute transport may deviate greatly from a piston-like displacement. Attempts have been made to model such a phenomenon, mainly by distinguishing mobile and immobile water, the former being partially or totally responsible for the rapid transport. Several models have been proposed to account for preferential flow in laboratory columns (Coats and Smith, 1964 ; Van Genuchten and Alves, 1982) and in the field (Addis cott, 1977 ; Corwin et al., 1991 ; Jarvis, 1991 ; Hutson and Wagenet, 1993). This is certainly an improvement for modelling pesticide leaching. However, besides the mathematical difficulty due to complex flow patterns, another question remains which does not have a satis factory answer, i.e. how soil heterogeneities can be characterized and how the partition between mobile and immobile water can be assessed to feed the mod els with pertinent input data ? This is a future research challenge. Concerning the transport of pesticides, colloidal materials and hydrosoluble humic substances may play a role. These compounds can bind pesticide mol ecules and make them mobile and readily transport able by water (Ballard, 1971 ; Vin ten et al., 1983). This point merits greater attention, particularly in rela tion to soil organic matter transformations.
Pesticide retention in the soil Retention is the transfer of chemical species from the liquid phase or from the gas phase to a solid phase due to reversible (sorption) and/or irreversible phe nomena. Sorption is always included in models, even in simpler ones, but irreversible retention is pratically
478
R. Calvet
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Soil Depth (cm) Figure 1. Examples of preferential flow observed for three pesticides in irrigated plots. Characteristics of irrigation : C = continuous ; I = intermittent; P = ponding ; S = sprinkling. Applied product : TG = technical grade ; EC = emulsifiable concentrate ; WP = wettable powder. Soil structure: U = undisturbed; D = disturbed (from Ghodrati and Jury, 1990).
never introduced. The only model where it is accounted for is the model developed by Piver and Lindstrom ( 1990). This is not a satisfactory situation since an increasing body of observations shows that this phenomenon does occur. There are many unanswered questions about mol ecules retained by irreversible retention (bound resi dues) as regarding their nature, properties and mobil ity. Modelling the fate of pesticides in soil should incorporate such a phenomenon. Nevertheless, whether the questions are fully answered or not, pesti cide leaching models would be improved with a better description of sorption. Two points of view have to be considered. The first, probably connected with bound residue formation, is the time dependence of the sorp tion coefficient. Estimation of sorption coefficients for modelling is a difficult problem (Green and Karick hoff, 1990 ; Calvet, 1993a), further complicated by its relationship to time. As a matter of fact, sorption coef ficients seem to increase with time (Walker, 1987 ;
Lehmann et al., 1990; Barriuso et al., 1993). Unfortu nately, experimental and theoretical procedures are not yet available to evaluate sorption-time relationships. The second point deals with the coupling of sorp tion to transport and transformation-degrad ation pro cesses. Sorption which is rate-limited by transport is related to the presence of heterogeneous flow domains and particularly to more rapid flow in macropores which prevent pesticide molecules reaching adsorption sites in micropores. Some modelling approaches have been proposed to describe this coupling (Pignatello, 1989). Sorption may also be limited by intra-particle and intra-sorbent diffusion, as described by Brusseau et al., 1991). Sorption may modify biotic transforma tions and degradation in two ways. The first way is by reducing the amount of pesticide capable of being degraded because sorbed molecules are not mobile and thus not accessible to .microorganisms. The second way concerns a kinetic effect due to the necessary dif fusion of molecules out of the micropores. Models Eur. J. Agron.
Modelling pesticide leaching in soils
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have been recently developed to account for this kinetic effect by Scow and Hutson (1992), Scow and Alexander (1992) and by Duffs et al. (1993). Introduc tion of such a coupling, at least in research models, would allow more precise simulation of the fate of pesticides. Model Sensitivity
Assessment of model sensitivity to variations in input parameter values is of key importance in appli cation of pesticide leaching models. This is because input parameters are highly variable, however they are measured or estimated. The variability of measured values has two components, one due to uncertainties associated with protocols and analytical methods, the other one being the result of the spatial variability of soil properties. This spatial variability has been described by Rao et al. ( 1986) in terms of factors linked to site pedogenesis (intrinsic factors) and of cultivation and pesticide application practices (extrin Vol. 4, n° 4- 1995
sic factors). As a result, physical, chemical and bio logical properties of the soil display a large in-situ variability. Published results show that spatial variabil ity for adsorption coefficient (Kd) and half life (Tl/2) values determined in surface cropped soils is about 30 per cent (Rao et al., 1986; Wood et al., 1987 ; Allen and Walker, 1987). This is similar to the magni tude observed for physical static characteristics ; hydrodynamic characteristics display a greater vari ability (Vauclin, 1990). It is important to be aware of the variability of esti mated parameter values frequently used for prediction purposes. Some important parameters such as the organic-carbon-normalized sorption coefficient K 0 c and the half-life degradation time T 112 have to be esti mated when measured values are missing. The only way to do this is to refer to published values which are often very variable. Table 6 gives a series of Koc values compiled by Gerstl (1990) together with their associated coefficients of variation. For T 112 the situa tion is more critical because no method exists to esti mate this parameter from soil and pesticide properties.
R. Calvet
480
Table 6. Examples of average K 0 c values and their associated coefficients of variation (from Gerst!, 1990).
Compound atrazine carbofuran diuron lindane napropamide trifluralin n : number of observations ;
K0 c
variation (%).
:
n
Koc
CV
217 52 156 94 36 22
227 78 384 1160 487 1135
158 229 74 176 71 72
average value (I/kg) ; CV : coefficient of
Thus, it is important to know the consequences of such variability on predictions. This is useful for defining the precision of input parameter measure ments and absolutely necessary for interpreting simu lated data. These consequences are given by sensitiv ity analysis which indicates the following general features for pesticide leaching models. The sensitivity of pesticide leaching models to variations of input parameters is often high, particu larly for sorption and degradation parameters (Ville neuve et al., 1988 ; Boesten, 1991 ; Calvet, l 993b ; Walker and Hollis, 1993). Studying the simulation of aldicarb leaching by the PRZM model, Villeneuve et
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From a general point of view, sensitivity assessment must be discussed carefully, taking into account the nature of the parameters and the period corresponding to the simulation. It is also important to be aware of the different possible criteria for assessing model sen sitivity : the distribution of the chemical in the soil profile, the amount remaining in a given soil layer, the leached amounts and the amount transported beyond a given depth. They may lead to different conclusions, so that comparison between performances of different models should be done with the same criteria for each model.
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al. ( 1988) showed that an uncertainty of 15 per cent in degradation rate and 24 per cent in sorption coefficient modify by 100 per cent the predicted cumulative quantity of pesticide reaching the water table after 3 years. Boesten ( 1991) has carefully examined the sensitivity of his model to variations in several param eters. Figure 3 illustrates some of his results concern ing K 0 c and T 112 • He has also observed that non linearity could be critical and that the sensitivity is greater for low leaching rates. Figure 4 shows other sensitivity effects for VARLEACH applied to a drained soil (Calvet, 1993b). As the models are highly sensitive to variations in sorption and degradation parameters, these parameters must be known as pre cisely as possible, including their variation through the soil profile.
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Modelling pesticide leaching in soils
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Model Validation
Model validation is an important step in modelling and is a prerequisite for model application. There are relatively few published papers reporting validation investigations and, generally, they only concern one site and one season. Thus, it is not easy to know pre cisely how various models describe the fate of pesti cides. There are two main difficulties in validation. To properly validate a model, it is necessary to have many experimental results obtained under conditions allowing the determination of all input data. This may seem obvious but complete published experimental data sets are not very numerous. The reason lies prin cipally in the great number of measurements needed and this is always expensive. Nethertheless, any mod elling work should be accompanied by well-designed field experiments allowing observed results and simu lation uncertainties to be correctly estimated. Under these conditions, the validation procedure can be based on sound comparisons between observed and simulated data. Figure 5 gives an example of such a comparison (Cal vet, l 993b ). Variation of the sorption coefficient Koc' of the half-life T 112 and of the applied amount was taken into account for a first estimate of the uncertainty of simulated values. Model users must be aware that calibrating a model against a given field data set is not a validation. This only shows that the model outputs can numerically represent this data set. The second difficulty arises from the interpretation of the comparison between simulated results and observed data. Several theoretical treatments have been published on this subject and examples of appli cations have been given by Pennel et al. (1990). How ever, these treatments do not give an answer to the question : what is the maximum difference between observed and simulated data which is acceptable for Vol. 4, n° 4- 1995
predictive and for regulatory purposes ? A complete and definite answer probably does not exist today, and this is a fundamental problem. A better knowledge of transport, of transport/sink-source phenomena cou pling mechanisms and of their relations to soil struc ture would contribute greatly to answering the ques tion.
CONCLUDING COMMENTS Whatever their complexity, existing models are not able to provide an accurate description of the fate of pesticides in soils. For a particular situation, some of them seem to predict correctly the center-of-mass position of the solute, the pesticide degradation and the maximum leaching depth. However, none provides a good simulation of the solute distribution in the soil profile and a feasible prediction of the amount of pes ticide transported to groundwater. Thus, model improvements are necessary and this is a challenge for future research. This leads to a question about devel oping more detailed models. As discussed by Decour sey (1992), the answer is not straightforward and the quality of the prediction cannot be related to model complexity. Nonetheless, it is true that improvements in process understanding will lead to improved simu lations. The problem is that each increase in model complexity requires more input information. Whatever the purpose of modelling, users must be aware of model uncertainty due to data quality (sam pling, measurement errors), model structure and parameter estimation methods. The spatial variability of input and observed data and the adequacy of the simulation scale to the problem to be resolved (Wagenet, 1993) are also important. What can be said about modelling the fate of pesti cides and particularly modelling pesticide leaching in
R. Calvet
482
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<>· ....... ·<> .. X·-·······X·-.
800
··<>· QSmax
···--o.
--0-Mmax -llr-Mmin · ·><:· QSmin
··-x
600 400 200 0 0
7
18
36
73
163
340
Time days
Figure 5. Examples of comparisons between observed (full lines) and simulated (dotted lines) values. Mmax and Mmin indicate the range of variation of observed values ; QSmax and QSmin indicate the range of variation of simulated values : a) dissipation kinetics of atrazine in the 0-20 cm layer in a hydromorphic clay soil (at Rambouillet, Ile de France). Simulation with model CMLS. b) dissipation kinetics of atrazine in the 0-10 cm layer in a sandy loam soil (at Mont Saint Michel, France). Simulation with VARLEACH. c) dissipation kinetics of atrazine in the 0-20 cm layer in a hydromorphic clay soil (at Rambouillet, Ile de France). Simulation with model VARLEACH (from Calvet, 1993b).
soils ? Research and educational models are useful because they can serve initially as an evaluation for conducting field experiments. They provide a conve nient way to account for climatic variations (simulat ing many seasons), and soil variability (simulating many sites). For research they allow fictitious experi ments to be performed to evaluate the relative role of various factors, thereby providing a powerful tool for defining protocols for field and laboratory experi ments. For teaching they enable the simulator to illus trate a variety of conditions. The usefulness of management and regulatory mod els are more difficult to assess. At best, a calibrated model for a given site and a given climate may pro vide a basis for management decisions but it is impos sible to safely extrapolate the simulation. Improving simulation quality has beeen discussed by Wauchope (1992). Probabilistic model analysis may be more use ful using Monte Carlo techniques (Villeneuve et al,
1990). It is clear that modelling will only allow classi fication of pesticides according to their potential mobility and comparison of diversity of agro-pedo climatic scenarios. However, quantitative prediction of transport to groundwater using pesticide leaching models, seems impossible at present.
REFERENCES Addiscott T. M. (1977). A simple computer model for leaching in structured soils. J. Soil Sci., 28, 554-563. Addiscott T. M. and Wagenet R. J. (1985). Concepts of solute leaching in soils : a review of modelling approaches. J. Soil Sci., 36, 411-424. Allen R. and Walker A. (1987). The influence of soil properties on the rates of degradation of Metamitron, Metazachlor and Metribuzin. Pestic. Sci., 18, 95-111. Ballard T. M. (1971). Role of humic carrier substances in DDT movement through forest soil. Soil Sci. Soc. Am. Proc. 35, 145-147. Eur. J, Agron.
Modelling pesticide leaching in soils
Barriuso E., Calvet R. and Houot S. (1993). Field study of the effect of sewage sludge. Application on atrazine behaviour in soil. J. environ. An. Chem., 59, 107-121. Boesten J. J. T. I. (1991). Sensitivity analysis of a mathematical model for pesticide leaching to groundwater. Pestic. Sci., 31, 375-388. Boesten J. J. T. I. and van der Linden A. M.A. (1991). Model ling the influence of sorption and transformation on pesticide leaching and persistence. J. environ. Qua!., 22, 425-435. Brusseau M. L., Jessup E., Suresh P. and Rao C. (1991). Non equilibrium sorption of organic chemicals : Elucidation of rate-limiting processes. Environ. Sci. Technol., Vol. 25, n° I. Calvet R. (1993a). Comments on the characterization of pesti cide sorption in soils. In : Del Re A.A.M., E. Capri, S.P. Evans, P. Natali and M. Trevisan (Eds.). IX Simposium Pesti cide Chemistry - Mobility and degradation of xenobiotics, 11-13 October, Piacenza, Italy. Cal vet R. (1993b). Modelisation du devenir des pesticides dans le sol. Rapport de Recherche, Ministere de I' Agriculture, Paris. Convention de formation par la recherche n° 90132. Carse! R. F., Jones R. L., Hansen J. H., Lamb R. L. and Ander son M. P. (1984). User's manual for the pesticide root zone model (PRZM): release 1. USERA EPA-600/3-84-109. Washington D.C. : U.S. Gov. print Office. Clemente R. S., Prasher S. 0. and Barrington S. F. (1993). Pest fade, a. new pesticide fate and transport model : model devel opment and verification. Soil water, 36 (2), 357-367. Coats K. H. ans Smith B. D. (1964). Dead-end pore volume and dispersion in porous media. AIME Transactions 231, 273 284. Corwin D. L., Waggoner B. L. and Rhoades J. D. (1991) A functional model of solute transport that accounts for bypass. J. environ. Qua/. 20, 647-658. Decoursey G. (1992). Developing models with more detail : Do more algorithms find more truth ? Weed Technol. Vol. 6, 709 715. Duffy M. J ., Carski T. H. and Hanafey M. K. (1993). Conceptu ally and experimentally coupling sulfonylurea herbicide sorp tion and degradation in soil. In : Del Re A.A.M., E. Capri, S.P. Evans, P. Natali and M. Trevisan (Eds.). IX Simposium Pesticide Chemistry - Mobility and degradation of xenobiot ics, 11-13 October, Piacenza, Italy. Gerst! Z. (1990). Estimation of organic chemical sorption by soils. J. contaminant Hydro/., 6, 357-375. Ghodrati M. and Jury W. A.. (1992). A field study of the effects of soil structure and irrigation method on preferential flow of pesticides in unsaturated soil. J. contaminant Hydro!., 11, 101- 125. Green R. E. and Karickhoff S. W. (1990). Sorption estimates for modelling. In H.H. Cheng.(Ed.) Pesticides in the Soil Envi ronment : Processes, Impacts and Modelling Soil Science Society of America Book Series, number 2. Hutson J. L. and Wagenet R. J. (1993). A pragmatic field-scale approach for modelling pesticides. J. environ. Qual. 22, 494 499. Jarvis N. (1991). Macro - A model of water movement and sol ute transport in macroporous soils. Reports and Dissertations. 9. Uppsala. Jury W. A., Spencer W. F. and Farmer W. J. (1983). Behavior assessment model for trace organics in soil : Model descrip tion. J. environ. Qua!., 12, n° 4, 558-564. Jury W. A., Focht D. D. and Farmer W. J. (1987). Evaluation of pesticide groundwater pollution potential from standard indi ces of soil-chemical adsorption and biodegradation. J. envi ron. Qua!., 16, 422-428. Lehmann R. G., Miller J. R. and Laskowski D. A. (1990). Fate of fluroxypyr in soil : II. Desorption as a function of incuba tion time. Weed Res., 30, 383-388. Vol. 4, n° 4 - 1995
483
Leonard R. A., Knisel W. G. and Still D. A. (1987). GLEAMS : Groundwater loading effects of agricultural management sys tems. Soil Water, 30 (5), 1403-1418. Mackay D. (1979). Finding fugacity feasible. Environ. Sci. Technol., 13, 1218-1223. Nicholls P. H., Bromilow R. H., Addiscott T. M. (1982a). Mea sured and simulated behaviour of floumeturon, aldicarb and chloride ion in a fallow structured soil. Pestic. Sci., 13, 484 494. Nicholls P. H., Walker A., Baker R. J. (1982b). Measurement and simulation of the movement and degradation of atrazine and metribuzin in a fallow soil. Pestic. Sci., 13, 484-494. Nielsen, D. R. abd Biggar J. W. (1962). Miscible displacement. III. Theoretical considerations. Soil Sci. Soc. Am. Proc., 26, 216-221. Nkedi-Kizza P., Biggar J. W., Selim H. M., van Genuchten M. Th., Wierenga P. J., Davidson J.M. and Nielsen D.R. (1984). On the equivalence of two conceptual models for describing ion exchange during transport through an aggre gated oxisol. Water Resour. Res., 20, 1123-1130. Nofziger D. L. and Hornsby A. G. (1986). A microcomputer based management tool for chemical movement in soil. Appl. agric. Res., 1, 50-56. Pennell K. D., Hornsby A. G., Jessup R. E. and Rao P. S. C. (1990). Evaluation of five simulation models for predicting aldicarb and bromide behavior under field conditions. Water Resour. Res., 26 (11 ), 2679-2693. Pignatello J. J. (1989). Sorption dynamics of organic com pounds in soils and sediments. In : B.L. Sawhney and K. Brown (Eds.) Reactions and Movement of Organic Chemicals in Soils. SSSA Special Publication n° 22. Piver W. T. and Lindstrom F. T. (1990). Mathematical models for describing transport in the unsaturated zone of soils. In : 0. Hutzinger (Ed.) The handbook of Environmental Chemis try. Vol. 5, 125-259. Rao P. S. C., Edwardson K. S. C., Ou L. T., Jessup R. E., Nkeddi-Kizza and Hornsby A.G. (1986). Spatial variability of pesticide sorption and degradation parameters. In : W. Y. Gardner, R. C.Honeycutt & H. N. Nigg (Eds.) Evaluation of pesticides in ground water. ACS Symposium series 315, American Chemical Society. Scow K. M. and Hutson J. (1992). Effect of diffusion and sorp tion on the kinetics of biodegradation: Theoretical consider ations. Soil Sci. Soc. Am. J., 56, 119-127. Scow M. and Alexander M. (1992). Effect of diffusion on the kinetics of biodegradation : Experimental results with syn thetic aggregates. Soil Sci. Soc. Am. J., 56, 128-134. Van Genuchten M. Th. and Alves W. J. (1982). Analytical solu tions of the convective- dispersive solute transport equation. U.S. Department of Agriculture, Technical Bulletin n° 1601. Vauclin M. (1990). Modelisation des transferts dans !es sols non satures : approche deterministe ou stochastiques ? In : R. Cal vet (Ed.) Nitrates Agriculture Eau. International Symposium, Paris-La Defense: Institut National de la Recherche Agronomique, 169-181. Villeneuve J. P., Banton 0. and Lafrance P. (1990). A probabi listic approach for the groundwater vulnerability to contami nation by pesticides : the vulpest model. Ecological model ling, 51, 47-58. Villeneuve J. P., Lafrance P., Banton 0., Frechette P. and Robert C. (1988). A sensitivity analysis of adsorption and degrada tion parameters in the modelling of pesticide transport in soils. J. contaminant Hydro!., 3, 77-96. Vinten A. J. A., Yaron B. and Nye P. H. (1983). Vertical trans port of pesticides into soil when adsorbed on suspended par ticles. J. agric. Food Chem., 31, 662-664. Wagenet R. J. (1993). A review of pesticide leaching models and their, application to field and laboratory data. In : Del Re
484
A. A. M., E. Capri, S. P. Evans, P. Natali and M. Trevisan (Eds.). IX Simposium Pesticide Chemistry - Mobility and deg radation of xenobiotics, 11-13 October, Piacenza, Italy. Wagenet R. J. and Hutson J. L. (1989). LEACHM: a finite dif ference model for simulating water, salt and pesticide move ment in the plant root zone continuum. Vol. 2, version 2.0, New York state water resource Institute, Cornell University, Ithaca, NY. Wagenet R. J. and Rao P. S. C. (1990). Modelling pesticide fate in soils. In : H. H. Cheng (Ed.) Pesticides in in the Soil Envi ronnement. Processes, Impacts and Modelling, Soil Science Society of America Book Series, number 2. Walker A. (1987). Evaluation of a simulation model for predic tion of herbicide movement and persistence in soil. Weed Res., 27, 143-152. Walker A. and Barnes A. (1981). Simulation of herbicide persis tence in soil ; a revised computer model. Pestic. Sci., 12, 123-132.
R. Calvet
Walker A. and Hollis J. (1993). Prediction of pesticide mobility in soil and their potential to contaminate surface and ground water. In British Crop Protection Council Monograph n° 59. Walker A. and Welch S. J. (1989). The relative movement and persistence in soil of chlorsulfuron, metsulfuron-methyl and triasulfuron. Weed Res., 29, 375-383. Wania F., Mackay D., Paterson S., Di Guardo A., Mackay Neil (1993). Compartimental models in environmental science. In : H. H. Cheng (Ed.) Pesticides in the Soil Environnement. Processes, Impacts and Modelling, Soil Science Society of America Book Series, number 2. Wauchope R. (1992). Environmental risk assessment of pesti cides : Improving simulation model credibility. Weed Technol. 6, 753-759. Wood L. S., Scott H. D., Marx D. B. and Lavy T. L. (1987).Variability in sorption coefficients of metolachlor on a captina silt loam. J. environ. Qual., 16, n° 3.
Eur. J. Agron.