Single and dual-permeability modelsof chlorotoluron transport in the soil profile
R. Kodešová1, J. Kozák1, J. Šimůnek2, O. Vacek1
1Czech University of Agriculture in Prague, Czech Republic
2University of California Riverside, CA, USA

ABSTRACTThis study presents the transport of chlorotoluron in the soil profile under field conditions. The herbicide Syncuran
was applied on a plot (4 m²) using an application rate of 2.5 kg/ha of active ingredient. Soil samples were taken a�er
119 days to study the residual chlorotoluron distribution in the soil profile. The single and dual-permeability models
in HYDRUS-1D (Šimůnek et al. 2003) were used to simulate water movement and herbicide transport in the soil profile. Soil hydraulic properties and their variability were previously studied by Kutílek et al. (1989). The solute transport parameters, such as the adsorption isotherm and the degradation rate, were determined in the laboratory. Since
the solute transport in the field was probably affected by preferential flow, the chlorotoluron distribution in the soil
profile calculated using the single-permeability model had a different character than observed chlorotoluron concentrations. The chlorotoluron distribution within depth calculated using the dual-permeability model was closer to theobserved behavior of chlorotoluron. While the herbicide did not reach a depth of 8 cm for the single-porosity system,in the case of the dual-permeability model the solute moved to the depth of 60 cm. The dual-permeability model significantly improved correspondence between calculated and observed herbicide concentrations.
Keywords: herbicide; chlorotoluron; solute transport; preferential flow; single pore system; dual-permeability; fieldand numerical studies

Soil and groundwater contamination by pesticides from agricultural activities is a worldwide
environmental problem. Although pesticide andother contaminant concentrations can be monitored, such monitoring is quite expensive and time
consuming. Various simulation models have beendeveloped for assessment of groundwater vulnerability to contamination, resource management, anddesign of monitoring programs. The BPS (Kozák
and Vacek 1996) and HYDRUS-1D (Šimůnek et
al. 1998) models have been developed recently,
among many others, to simulate water movement
and solute transport in soils. The chlorotolurontransport in several soil types of the Czech Republic
was studied experimentally and described with the
BPS code (Kočárek et al. 2005). HYDRUS-1D wasused to simulate chlorotoluron transport that was
experimentally studied in chernozem (Kodešová etal. 2004). Both models can describe relatively wellchlorotoluron behavior in soil profiles. However,simulated and observed herbicide distributions
are often different, probably due to the effects ofthe preferential flow of water and solute in thesoil profile and the spatial variability of transportparameters. In addition, some transport properties

obtained under laboratory and field conditions may
be quite different. Streck et al. (1995) presented apparent inconsistency between sorption isotherms
determined from laboratory and field lysimeter
experimental data. Poletika et al. (1995) used linear
and nonlinear one- and two-stage sorption models to fit the sorption and desorption isotherms.
Kamra et al. (2001) studied pesticides transport
in small soil columns applying non-equilibrium
two-region/mobile-immobile model. Flury et al.
(1995) investigated preferential flow in the field.
In this study herbicide was only partly sorbed by
the soil matrix. A fraction of chemicals was transported with or without minor adsorption along
cracks or fissures. Kočárek et al. (2005) observed
chlorotoluron transport affected by preferential
flow in 3 soil profiles from 5 studied soil types.
Jorgensen et al. (2002) experimentally studied pesticides transport through preferential paths. For
numerical simulation they used code FRAC3Dvs(Therrier and Sudicky 1996) that simulated water and solute flow in fractured porous system.
Gerke and van Genuchten (1993, 1996) proposed
the dual-permeability model that solves flow andtransport equations in both matrix and fracture

Supported by the Ministry of Agriculture of the Czech Republic, Project No. QF3250.

310

PLANT SOIL ENVIRON., 51, 2005 (7): 310–315

pore systems. The MACRO model for simulationof water and solute transport in a dual-permeability system was developed by Jarvis (1994).
MACRO was used for instance to simulate waterand isoproturon behavior in a heavy clay soil by
Besien et al. (1997). The MACRO model was also
used for one scenario in the EU risk assessment
program (FOCUS 2000).
Here we discuss the effect of preferential flow
on chlorotoluron transport in the soil profile.
Experimental field data presented in Kodešová etal. (2004) that involved the chlorotoluron transport
in the soil profile were simulated using the modified HYDRUS-1D software package (Šimůnek et
al. 1998, 2003). Preferential flow was evaluated bycomparing results of the single-porosity and dualpermeability models (Gerke and van Genuchten
1993, 1996).
MATERIAL AND METHODSThe transport of chlorotoluron in the soil profile
under field conditions was studied at the Tišice experimental field. The soil was defined as chernozem.
Syncuran, containing 80% of active ingredients,
was applied on a 4 m2 plot on May 21st, 1997 at an
application rate of 2.5 kg/ha of active ingredient.
One liter of Syncuran solution (1.25 g/liter, e.g.
1 g/liter of chlorotoluron) was applied on the soil
surface followed by irrigation with two liters offresh water. Soil samples at depths of 0–2, 2–4, 4–6,
6–8, 8–10, 10–12, 12–14, 14–16, 16–18, and 18–20 cm
were taken after 119 days to study the residual
chlorotoluron distribution in the soil profile. The
chlorotoluron concentrations in soil samples were
determined in laboratory using standard laboratory procedures employing HPLC.
The chlorotoluron transport was numericallysimulated using HYDRUS-1D (Šimůnek et al.
1998). Since the chlorotoluron was not studied
under a depth of 20 cm, the soil profile was described as a one-dimensional flow region (60 cm)
divided into two layers: 0–25 cm and 25–60 cm.
The top boundary condition was defined by daily
precipitation and estimated potential transpiration. Given the root zone depth of 20 cm, a Feddes
model (Feddes et al. 1978) with parameters definedfor wheat (winter barley) was applied to simulate the root water uptake. The bottom boundary
condition was defined as a free drainage. The soil
physical and hydraulic properties were studied
before by Kutílek et al. (1989). The bulk densities
were 1.609 g/cm3 and 1.571 g/cm3 for the first andsecond layers, respectively.
Chlorotoluron transport in the soil profile wassimulated using the single-porosity and dualpermeability models (Gerke and van Genuchten
PLANT SOIL ENVIRON., 51, 2005 (7): 310–315

1993, 1996) implemented in HYDRUS-1D. Review
of models describing non-equilibrium and preferential flow and transport in the vadose zone has
been given recently by Šimůnek et al. (2003). While
the single-porosity model describes uniform flow
in soil porous media, the dual-permeability model
can represent a non-equilibrium/preferential flow
in a complex system of soil aggregates (matrix) andintermediate spaces (fractures or macropores). In
both studied cases is used Richards’ equation describing the flow in a variably saturated rigid porous
medium for one-dimensional isothermal Darcian
flow. While the Richards’ equation is solved for theentire flow domain for the single-porosity system,in the case of the dual-permeability model, it isapplied separately to each of two pore regions– matrix and fracture domains. The soil hydraulic properties are described using van Genuchten(1980) equations for each domain. The analyticalexpressions for the soil water content retention
curve, θ(h), and the hydraulic conductivity curve,k(θ), are:1
, �=� ��(h)����r �=���,� h�<�0�e���1�+�|���h|�n �m�s����r,�e�=�1,��� h���0

�

�

�2

k( � )�=�K s�.��e�l� 1���(1����1/� ��)�� �,� �h�<�0k( � )�=�K s ,� ��h���0

(1)

(2)

where: θ e is the effective soil water content(–), K s is the saturated hydraulic conductivity(LT –1), θ r and θ s are the residual and saturated
soil water contents (L 3L –3), respectively, l is thepore-connectivity parameter (–) (l = 0.5 in thiscase), and α (L–1), n and m (= 1 – 1/n) are empiricalparameters. The soil hydraulic parameters for the
single-porosity system were for the depth from0 to 25 cm as follows: α = 0.078 cm –1, n = 1.29, θ r =0.108, θ s = 0.406, K s = 109 cm/day; and from 25 to
60 cm as follows: α = 0.062 cm–1, n = 1.33, θr = 0.106,
θ s = 0.414, K s = 150 cm/day. Assuming the ratio
between the macropore and matrix regions equal
to 0.1 parameters for the dual-permeability system
for the depth from 0 to 25 cm were: α = 0.052 and0.07 cm–1, n = 1.22 and 3, θr = 0.082 and 0, θs = 0.398
and 0.45, K s = 10 and 1000 cm/day, and for thedepth from 25 to 60 cm: α = 0.043 and 0.06 cm –1,
n = 1.27 and 3, θ r = 0.092 and 0, θ s = 0.407 and
0.45, K s = 55 and 1000 cm/day for the matrix andfracture pore systems, respectively. Soil water
retention curves for both porosity systems areshown in Figures 1 and 2. Following parameters
are defined only for the dual-permeability model.
Parameters describing aggregate shapes are theshape factor β (= 15 for spherical aggregates), thecharacteristic length of aggregate a (= 0.3 cm),and the dimensionless scaling factor γ w (= 0.4).
311

The interaction between the matrix and fracture
regions is defined by the effective saturated hydraulic conductivity, Ka, equal to 0.01 cm/day. The
mass exchange between the matrix and macropore
region, Γw, is then calculated as follows:

�w �

�

a

2

K a� w � h f � hm �(3)where: hf and hm are pressure heads in the macropore and matrix domains, respectively.
Similarly the governing solute transport equation is solved for the entire flow domain (single-porosity system) and separately for both themacropore and matrix regions (dual-permeability
system). Sorption parameters and degradation
rates are assumed in this study to be the same inboth regions. The adsorption isotherm relating
adsorbed concentration of solute on soil particles(s) and solution concentration (c) is described byFreundlich equation:s = kF c

1
nF

(4)

where: k F (L 3/nF M –1/nF ) and n F (–) are empiricalcoefficients. Parameters of the Freundlich adsorption isotherm were: kF = 3.48 and 1.02, 1/nF = 0.632
and 0.9 for the top and bottom layers, respectively.
Herbicide degradation was assumed in both liquidand solid phases. Degradation rates were 0.02 per
day and 0.002 per day in the top and bottom layers, respectively. Since the herbicide sorption anddegradation are major processes, the molecular
diffusion was neglected and longitudinal dispersivity was set to 1 cm.
RESULTS AND DISCUSSION

surface water flux (evaporation – precipitation)at the top of the soil profile and the cumulative
actual root water uptake are shown in Figure 3. The
cumulative actual water outflow at the bottom of
the soil profile is shown in Figure 4. The cumulative fluxes at the surface are 22.56 and 22.30 cm
for the single- and dual-permeability models,
respectively. The cumulative root water uptakes
are 17.65 and 17.50 cm for the single- and dualpermeability models, respectively. The cumulative
bottom fluxes are 8.54 and 9.14 cm for the singleand dual-permeability models, respectively. The
fracture and matrix cumulative bottom fluxes forthe dual-permeability model were 1.28 and 7.86 cm,
respectively. While the surface flux and the root
water uptake are driven mainly by the meteorological forcings, the bottom flux is determined
mainly by the soil profile itself. The cumulative
fluxes at surface and the cumulative root water are
similar. The cumulative fluxes at the bottom are
different. At the beginning of the simulation thebottom outflow is lower for the dual-permeability
model than the single-permeability model. This isdue to water being present initially mainly in thematrix pore system of the dual-permeability system
and its lower hydraulic conductivity, comparedto the matrix of the single-permeability system.
Approximately in the middle of the simulated time
period, the soil profile became almost saturated
and considerable downward flow took place inboth cases. Water was drained also through themacropores of the dual-permeability model andas a result, the cumulative outflow for the dualpermeability model is higher than the outflow for
the single-permeability model.
Solute transport

Water regime

The water regime of the soil profile is documented in Figures 3 and 4. The cumulative actual

Simulated chlorotoluron concentrations in waterin the soil profile 119 days after the application of
Syncuran in the matrix pore systems of both models
1000

Pressure Head [cm]Pressure Head [cm]

1000

100
Single-Perm eability

10Dual-perm eability: Matrix Pores

Dual-perm eability: Macropores

100

Single-Perm eability

10Dual-perm eability: Matrix Pores
Dual-perm eability: Macropores

Dual-perm eability: Both

Dual-perm eability: Both

1

1
0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Soil W ater Content [cm 3/cm 3]

Figure 1. Soil water retention curves for the depth from0 to 25 cm for both single-permeability and dual-permeability models

312

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Soil W ater Content [cm 3/cm 3]

Figure 2. Soil water retention curves for the depth from25 to 60 cm for both single-permeability and dual-permeability models
PLANT SOIL ENVIRON., 51, 2005 (7): 310–315

Time [days]

Time [days]

40

60

Cumulative Flux - Top [cm]

0

80

100

CF-T: Single-permeability

120
25

CF-T: Dual-permeability-5

CRWU: Single-permeability

20

CRWU: Dual-permeability

-10

15

-15

10

-20

5

-25

0

0

20

40

60

80

100

120

0
Cumulative Flux - Bottom [cm]20

Cumulative Root Water Uptake
[cm]

0

-2
-4
-6

Single-permeability
Dual-permeability-8-10

Figure 3. Cumulative actual water flux at the top of thesoil profile (CF-T) and cumulative actual root water uptake (CRWU) after the herbicide application (+ upward
flux, – downward flux)

Figure 4. Cumulative actual water flux at the bottom of
the soil profile after the herbicide application (+ upward
flux, – downward flux)

are shown in Figure 5. Simulated chlorotoluron
concentrations in soil water for the fracture poresystem of the dual-permeability model are shownin Figure 6. Solute concentrations in the matrix
pore system in the top layer calculated using thesingle-permeability model are higher than concentrations calculated using the dual-permeability
model. On the other hand, the herbicide is present
also in the bottom layer for the dual-permeability
model. The solute which was transported downwards mainly through the fracture pores was thenredistributed into the matrix pores of the bottom
layer. Interestingly, while there is a high concentration of solute in the matrix pore system at thedepth of 0–8 cm, herbicide is no more present at
this depth in the fracture pore system. The highest chlorotoluron concentrations in fracture porewater are in the bottom layer.
Observed concentrations in the soil profile wereexpressed as total amounts of solute per massunit of the soil. The simulated total amounts ofsolute (present in soil water and adsorbed on
soil particles) per mass unit were therefore cal-culated to compare the measured and simulated
chlorotoluron concentrations 119 days after theapplication (Figure 7). Simulated chlorotoluron
concentrations in the soil profile for the singleporosity system are distributed very differently
than the observed chlorotoluron concentrations.
A single-porosity model did not predict presence ofthe herbicide below the depth of 8 cm, though thesolute was observed at significant concentrations
below this depth. The simulated chlorotoluron
distribution in the soil profile obtained using thedual-permeability model is closer to the observed
chlorotoluron distribution. However, concentrations at the top of the soil profile (0–2 cm) are lower
than both observed and simulated concentrations
using the single-permeability model and higher
than observed concentrations at the depths of
12–20 cm. In addition, calculated chlorotoluron
concentrations change abruptly at the boundary
between two layers of the soil profile due to thedifferent sorption properties of those layers. The
sorption and degradation properties (depending
on soil physical and chemical properties) usually

Concentrations in Soil W ater - Macropores [� g/cm 3]

Concentrations in Soil W ater - Matrix Pores [� g/cm 3]
0.005

0.01

0.015

0.02

0

0.025

0

-10

-10

-20

-20

-30
-40

Single-permeability

Depth [cm]Depth [cm]

0
0

0.02

0.04

0.06

0.08

0.1

-30
-40

Dual-permeability

-50

-50

-60

-60

Figure 5. Simulated chlorotoluron concentrations in soil
water in the matrix of the soil profile 119 days after theapplication of Syncuran for both single-permeability
and dual-permeability models
PLANT SOIL ENVIRON., 51, 2005 (7): 310–315

Figure 6. Simulated chlorotoluron concentrations in soil
water in the macropores of the soil profile 119 days
after the application of Syncuran for the dual-permeability model

313

Chlorotoluron Distribution [� g/g]

0

0.05

0.1

0.15

0.2

0.25

0.3

0

0.35

-30
Measured-40-50Single-permeability

Dual-permeability-60

Cumulative Solute Flux - Bottom[� g/cm 2]

Depth [cm]

-20

40

Time [days]
60

80

100

120

0

0
-10

20

-0.0005-0.001-0.0015
Matrix Pores-0.002

Macropores

Both Pore Systems-0.0025

Figure 7. Measured and simulated residual chlorotoluron
concentrations (expressed as total amount of solute permass unit of soil) in the soil profile 119 days after the
herbicide application

Figure 8. Simulated cumulative solute outflow at thebottom of the soil profile versus time from matrix pores,
macropores and both pore systems for the dual-permeability model (+ upward flux, – downward flux)

gradually decrease with depth. Assuming sucha gradient decrease of these two properties,
simulated concentrations at depths of 12–25 cm
would be probably lower and without a suddenchange at the depth of 25 cm. Simulated andobserved amounts of solute in the entire flow
domain, CV, are similar: CV Single = 2.294 µg/cm 2,
CV Dual = 2.393 µg/cm 2 (CV Matrix = 2.390 µg/cm 2 ,CVFracture = 0.003 µg/cm2) and CVObserved = 2.49 µg/cm2.
Consideration of larger values of longitudinal
dispersivity and molecular diffusion resulted inwider spread of herbicide in the soil profile (notshown). The resulting chlorotoluron concentrations in the soil profile however differed from theobserved chlorotoluron concentrations even morethan presented simulation results.
Solute moved to a depth of 60 cm in the dual-permeability system, compared to less than 10 cm forthe single-porosity system. The cumulative solute
outflow at the bottom of the soil profile simulatedusing the dual-permeability model is shown in
Figure 8. Solute was drained initially only throughthe macropores and later from the matrix poresystem as well. The macropore, matrix, and total
cumulative solute fluxes were 0.0004, 0.0022 and0.0026 µg/cm2, respectively. Simulated and observed
amounts of solute in the soil profile and the simulated total cumulative solute outflow showed thatthe herbicide was mostly degraded before reaching the bottom layer. Solute infiltrated mainly into
the matrix pore system of the top layer that had
higher sorption and degradation properties thanthe bottom layer. Solute bypassing the top layer
through the macropores and then redistributed
between macro- and matrix pores of the bottom
layer with lower sorption and degradation properties was only slowly degraded and was more easily drained from the bottom layer. Description of
the bypass solute flow and leakage at the bottomof the soil profile into layers underneath, where

sorption and degradation processes are mostly very
low or even nonexistent, is especially importantwith respect to a ground water contamination. The
dual-permeability model is a powerful tool that can
provide an explanation of frequent groundwatercontamination by pesticides in the Czech Republic(Kodeš 2003) and elsewhere, and for the estimationof contaminants leaching into the ground water.

314

Acknowledgement
The authors acknowledge F. Doležal and V. Kuráž
for providing some of the data.
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Received on November 18, 2004

ABSTRAKT
Modely jednoduché a duální propustnosti transportu chlortoluronu v půdním profilu
Transport chlortoluronu v půdním profilu byl studován v terénních podmínkách. Herbicid Syncuran byl aplikovánna ploše 4 m2, přičemž v dávce bylo 2,5 kg/ha účinné látky. Po 119 dnech byly odebrány půdní vzorky pro analýzu
zbytkového obsahu chlortoluronu v půdním profilu. Pro simulaci pohybu vody a transportu herbicidu v půdním
profilu byly použity modely jednoduché a duální propustnosti v HYDRUS-1D (Šimůnek et al. 2003). Hydraulické
vlastnosti a jejich variabilitu studovali Kutílek et al. (1989). Transportní parametry jako adsorpční izoterma a degradační rychlost byly stanoveny v laboratoři. Rozdílný charakter rozdělení koncentrací pozorovaných a simulovaných
modelem jednoduché propustnosti lze zdůvodnit preferenčním prouděním vody a roztoku. Průběh rozdělení koncentrací simulovaných modelem s duální propustností více odpovídá pozorovaným zjištěním. Zatímco se roztok
v případě jednoduchého pórového systému nedostal dál než do hloubky 8 cm, v případě modelu duální propustnosti
pronikl hlouběji pod 60 cm. Model duální propustnosti významně zlepšil řešení.
Klíčová slova: herbicid; chlortoluron; transport roztoku; preferenční proudění; jednoduchý pórový systém; duální
propustnost; terénní a numerická studie

Corresponding author:
Ing. Radka Kodešová, CSc., Česká zemědělská univerzita v Praze, 165 21 Praha 6-Suchdol, Česká republika
phone: + 420 224 382 592, fax: + 420 220 921 643, e-mail: kodesova@af.czu.cz

PLANT SOIL ENVIRON., 51, 2005 (7): 310–315315