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Simulation of the migration behavior of herbicides in unsaturated soils with a modified LEACHP-version
E[ IIL IIIOOEILIIlG ELSEVIER
Ecological Modelling 75/76 (1994) 523-528
Simulation of the migration behavior of herbicides in unsaturated soils with a modified LEACHP-version Jiirgen Rambow a Bernd Lennartz
b,,
a Ecosystem Research Center "Bornhoeved Lakes Region ", Schauenburger Str. 112, 24118 Kiel, Germany b Institute for Water Management and Landscape Ecology, Olshausenstr. 40, Christian-Albrechts-University, 24118 Kiel, Germany
Abstract
The incorporation of the hysteresis effect into the model LEACHP has led to a remarkable improvement in the prediction of herbicide leaching behavior. This modified version uses non-singular and non-linear isotherms for the description of ad- and desorption processes in contrast to the original version that assumes linear adsorption. In most cases of the laboratory studies the hysteresis version calculated less herbicide discharge from the soil profile than the original version. At the same time the modified model showed a much better agreement with the measured data regarding both the herbicide's first appearance in the soil percolate and the location of its maximum concentration. The hysteresis version also calculated less chemical loss under field conditions. The movement of the herbicidal solutes within the soil profile could be identified as highly dependable on large rainfall events. Key words: Herbicide leaching
I. Introduction
Although there has been early evidence of non-uniformity f o r ad-and desorption processes from organic pesticides on soil particles (e.g. Harris and Warren, 1964; van Genuchten et al., 1974), this so-called hysteresis effect of ad- and desorption has not yet found consideration in any of the pesticide behavior simulation models. In case of non-singular ad- and desorption isotherms more of the chemical is retarded from the sorbent during the desorption process than during adsorption. Fig. 1 gives an example of non-singular ad- and desorption isotherms of chlortoluron and soil material from an Ap-horizon. Possible causes of hysteresis might
* Corresponding author. 0304-3800/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-3800(94)00015-A
J. Rambow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
524
0.9 0.8 0.7 E
.
.
~ ~
0.6
.., ~. ~.,7~. ~ "
E
//,s
,o
0.2
'~
0.1 0
' ~ ¢ ~
0
--O-- s = 0.138 CA0.44
0.1
0.2
0.3
T
0.4
Equilibrium concentration c ( m g / I ) Fig. 1. Non-uniform ad- and desorption isotherms for chlortoluron on soil samples from an Ap-horizon.
be the following which are discussed in detail by, e.g., Koskinen et al. (1979) and Brusseau and Rao (1989): chemical precipitation; variation of the binding mechanisms with time; incorporation of the pesticide into the soil biomass; and physical trapping in porous organic and mineral particles of the sorbent. Besides the above factors that lead to a "true" hysteresis effect various methodological errors during the batch experiments might cause an apparent hysteresis. The soil-chemical suspension may not be in equilibrium during adsorption as well as during the subsequent desorption. Loss of the pesticide that can not be accounted for may occur due to volatilization, degradation and sorption to batch containers or to solved organic matter (Brusseau and Rao, 1989). -
-
-
-
2. Theory Instead of the linear isotherm, the Freundlich isotherm is generally used for the description of non-linear adsorption and desorption of pesticides:
s = K *c,
(1)
S u = KFr,ads * C1 / n a ~ = KFr,des * c 1/n~os,
(2)
where s is the adsorbed amount of chemical at equilibrium, K is the linear partition coefficient, and c is the solution concentration at equilibrium, s u stands for the adsorbed amount of chemical at the point where the sorption direction switches from adsorption to desorption, when d s / d t = 0; KFr is the Freundlich
Z Rambow, B. Lennartz/Ecological Modelling 75/76 (1994) 523-528
525
partition coefficient between the soil solids and the solution. "ads" stands for adsorption and "des" for desorption. Rewriting Eq. 2 in terms of KFr.Oes yields
[ Kvr,ads ] l/¢ldes/l/tlads gFr,des : Su I - -s. )
(3~
Hence, different KFr and 1/n values are obtained for each desorption branch (see also Fig. 1). The following relationship for the sorption isotherm gradients between ad- and desorption in dependence on the sorbed amount (s,,) at the turning-point was found for the herbicide picloram and a sandy loam (van Genuchten et al., 1974):
1/naos
- -
1/nde~
= 2.105 + 0.062 * c,71.076.
(4)
Swanson and Dutt (1973) found an average value of 2.3 for the relationship between the gradients of the adsorption and desorption isotherms with atrazine on sandy and silty loam.
3. Material and methods
A laboratory apparatus was constructed to study the leaching behavior of soil applied herbicides in disturbed as well as undisturbed soil monoliths under vadose-zone conditions. The setup was built in the manner that a broad variety of field conditions regarding rainfall characteristics and soil water tension can be simulated. Long-term transport experiments up to 3 months required an automatic withdrawal of the soil percolate as well as a continuous control and registration of the soil water tension, the applied vacuum and the amount of precipitation. During the sampling of the leachate a newly designed lock guaranteed a constant vacuum at the bottom end of the soil column. For detailed information on the construction of the apparatus the reader is referred to Rainbow (1992). The pesticide simulation model LEACHP (Wagenet and Hutson, 1989) was originally developed for the simulation of water and solute fluxes under field conditions. Besides the water and solute transport in the unsaturated zone, the transformation processes and the plant uptake as well as adsorption and precipitation are simulated. When selecting appropriate boundary conditions L E A C H P offers the possibility to simulate water and pesticide fluxes in water-saturated and unsaturated laboratory soil columns. To model water content, fluxes and potentials L E A C H P applies a numerical solution by finite differencing technique to the Richard's equation. The soil profile is subdivided into a number of horizontal segments and the simulation period is divided into brief intervals. As upper boundary condition evaporation, ponded or non-ponded infiltration or zero flux can be chosen. The bottom boundary condition can be simulated as a lysimeter tank, a free-draining profile, a fixed depth water table or zero flux.
J. Rainbow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
526
4. Results
Since the observed pesticide dissipation in unsaturated soil column studies could not be described satisfyingly with the original version of the pesticide simulation model LEACHP which assumes linear adsorption behavior, both the non-linear and the non-linear/non-singular adsorption description using Eqs. 2, 3 and 4 were incorporated into the original model version.
4.1. Laboratory experiment The comparison of the results from the three model versions with observed data of a chlortoluron breakthrough in an Ap-horizon of a typical Cambic arenosol from Bornhoeved, northern Germany, is presented in Fig. 2. The hysteresis version shows the best agreement with the measured values concerning the herbicide's first appearance in the percolate, the location and the maximum of the breakthrough as well as the course of the desorption branch (tailing), while the other versions fail to describe the behavior of chlortoluron. The measured and simulated values of the total herbicide discharge (discharge in % of application) within the percolate from the soil profile are compared also. Again the hysteresis version (60%) matches the measured value (58%) best, while the other versions overestimate the total chemical amount being leached out of the profile.
4.2. Simulated herbicide behavior under field conditions The migration behavior of the soil applied herbicide atrazine was simulated for 1989 with the three adsorption versions. Application date was Day 160 (10 May), application rate 1 kg/ha. Fig. 3 shows the cumulative precipitation and the
350
~. 300
measured dlscharge: 58%
LI orlglnal verslon discharge: 91%
250 mc
200
---,:~ --~/~. . . . . . . . . . . . . . . . . . . .
o e-
f 150
.
.
.
,e,, '.. ~
~/PT~.~'', X
°'°°
0
Freundllch version
.
...........
hysteresis version discharge: _ . . _ 60%
~
8
16
24
32
40
48
56
64
Pore volumes
Fig. 2. Measured and simulatedbreakthroughcurvesof chlortoluronin an Ap-horizon; length: I0 cm, pore water velocity:13 cm/day.
J. Rainbow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
527
8
80O ~,
Op.,
r-.9
~
4 ~
t~'O
t~ 0
-
160
0
200
240
280
320
Tlme( d ) I pr~K:Ipltatlondrainage linearIsotherm Froundllchisotherm )(
- - 'O- - -
~
i,
360
hysteresis --'1"-
[
I
Fig. 3. Comparison of three different adsorption assumptions for the simulated atrazine discharge from the ApApBv-profile (depth: 0-44 cm) in 1989.
simulated drainage out of the ApApBv-profile (0-44 cm) of a Cambic arenosol. The simulated results of the different model versions for cumulative atrazine loss (in % of the applied amount) from the profile are also presented. The first appearance of atrazine at the end of the considered soil profile occurs in response to a large rainfall event on day 205 (107 mm). The linear adsorption model calculates an atrazine discharge of 2%, while both non-linear versions yield approximately 0.5%. Following two further large events on Day 239 and 240 (97 and 49 mm) the linear version calculates 4% atrazine loss, the other two versions 1.5%. Until the end of the simulation the total atrazine loss for the linear version becomes 7.5% being approximately three times larger than for the Freundlich (2.8%) and for the hysteresis version (2.3%). The simulated behavior of terbuthylazine and chlortoluron revealed similar results. These results demonstrate that careful consideration of the applied ad- and desorption assumption must be taken when assessing the migration behavior of pesticides under laboratory as well as field conditions.
References Brusseau, M.L. and Rap, P.S.C., 1989. Sorption nonideality during organic contaminant transport in porous media. CRC Crit. Rev. Environ. Cont., 19: 33-99. Harris, C.I. and Warren, G.F., 1964. Adsorption and desorption of herbicides by soil. Weeds, 12: 120-126. Koskinen, W.C., O'Connor, G.A. and Cheng, H.H., 1979. Characterization of hysteresis in the desorption of 2,4,6.T from soils. Soil Sci. Soc. Am. J., 43: 871-874.
528
J. Rambow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
Rambow, J., 1992. Modellstudie zum Verbleib herbizider Wirkstoffe in wasserteilges~ittigten Bfden. Schriftenreihe des Institutes fiir Wasserwirtschaft und Landsehafts6kologie der ChristianAlbrechts-Universit~it Kiel, Heft 19. Swanson, R.A. and Dutt, G.R., 1973. Chemical and physical processes that effect atrazine and distribution in soil systems. Soil Sci. Soc. Am. Proc., 37: 872-876. van Genuchten, M.Th., Davidson, J.M. and Wierenga, P.J., 1974. An evaluation of kinetic and equilibrium equations for the prediction of pesticide movement through porous media. Soil Sci. Soc. Am. J., 38: 29-35. Wagenet, R.J. and Hutson, J.L., 1989. LEACHM - Leaching Estimation And Chemistry Model: A process-based model of water and solute movement, transformations, plant uptake and chemical reactions in the unsaturated zone. User's Manual, Continuum 2. Water Resources Institute, Cornell University, Ithaca, NY.
Ecological Modelling 75/76 (1994) 523-528
Simulation of the migration behavior of herbicides in unsaturated soils with a modified LEACHP-version Jiirgen Rambow a Bernd Lennartz
b,,
a Ecosystem Research Center "Bornhoeved Lakes Region ", Schauenburger Str. 112, 24118 Kiel, Germany b Institute for Water Management and Landscape Ecology, Olshausenstr. 40, Christian-Albrechts-University, 24118 Kiel, Germany
Abstract
The incorporation of the hysteresis effect into the model LEACHP has led to a remarkable improvement in the prediction of herbicide leaching behavior. This modified version uses non-singular and non-linear isotherms for the description of ad- and desorption processes in contrast to the original version that assumes linear adsorption. In most cases of the laboratory studies the hysteresis version calculated less herbicide discharge from the soil profile than the original version. At the same time the modified model showed a much better agreement with the measured data regarding both the herbicide's first appearance in the soil percolate and the location of its maximum concentration. The hysteresis version also calculated less chemical loss under field conditions. The movement of the herbicidal solutes within the soil profile could be identified as highly dependable on large rainfall events. Key words: Herbicide leaching
I. Introduction
Although there has been early evidence of non-uniformity f o r ad-and desorption processes from organic pesticides on soil particles (e.g. Harris and Warren, 1964; van Genuchten et al., 1974), this so-called hysteresis effect of ad- and desorption has not yet found consideration in any of the pesticide behavior simulation models. In case of non-singular ad- and desorption isotherms more of the chemical is retarded from the sorbent during the desorption process than during adsorption. Fig. 1 gives an example of non-singular ad- and desorption isotherms of chlortoluron and soil material from an Ap-horizon. Possible causes of hysteresis might
* Corresponding author. 0304-3800/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-3800(94)00015-A
J. Rambow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
524
0.9 0.8 0.7 E
.
.
~ ~
0.6
.., ~. ~.,7~. ~ "
E
//,s
,o
0.2
'~
0.1 0
' ~ ¢ ~
0
--O-- s = 0.138 CA0.44
0.1
0.2
0.3
T
0.4
Equilibrium concentration c ( m g / I ) Fig. 1. Non-uniform ad- and desorption isotherms for chlortoluron on soil samples from an Ap-horizon.
be the following which are discussed in detail by, e.g., Koskinen et al. (1979) and Brusseau and Rao (1989): chemical precipitation; variation of the binding mechanisms with time; incorporation of the pesticide into the soil biomass; and physical trapping in porous organic and mineral particles of the sorbent. Besides the above factors that lead to a "true" hysteresis effect various methodological errors during the batch experiments might cause an apparent hysteresis. The soil-chemical suspension may not be in equilibrium during adsorption as well as during the subsequent desorption. Loss of the pesticide that can not be accounted for may occur due to volatilization, degradation and sorption to batch containers or to solved organic matter (Brusseau and Rao, 1989). -
-
-
-
2. Theory Instead of the linear isotherm, the Freundlich isotherm is generally used for the description of non-linear adsorption and desorption of pesticides:
s = K *c,
(1)
S u = KFr,ads * C1 / n a ~ = KFr,des * c 1/n~os,
(2)
where s is the adsorbed amount of chemical at equilibrium, K is the linear partition coefficient, and c is the solution concentration at equilibrium, s u stands for the adsorbed amount of chemical at the point where the sorption direction switches from adsorption to desorption, when d s / d t = 0; KFr is the Freundlich
Z Rambow, B. Lennartz/Ecological Modelling 75/76 (1994) 523-528
525
partition coefficient between the soil solids and the solution. "ads" stands for adsorption and "des" for desorption. Rewriting Eq. 2 in terms of KFr.Oes yields
[ Kvr,ads ] l/¢ldes/l/tlads gFr,des : Su I - -s. )
(3~
Hence, different KFr and 1/n values are obtained for each desorption branch (see also Fig. 1). The following relationship for the sorption isotherm gradients between ad- and desorption in dependence on the sorbed amount (s,,) at the turning-point was found for the herbicide picloram and a sandy loam (van Genuchten et al., 1974):
1/naos
- -
1/nde~
= 2.105 + 0.062 * c,71.076.
(4)
Swanson and Dutt (1973) found an average value of 2.3 for the relationship between the gradients of the adsorption and desorption isotherms with atrazine on sandy and silty loam.
3. Material and methods
A laboratory apparatus was constructed to study the leaching behavior of soil applied herbicides in disturbed as well as undisturbed soil monoliths under vadose-zone conditions. The setup was built in the manner that a broad variety of field conditions regarding rainfall characteristics and soil water tension can be simulated. Long-term transport experiments up to 3 months required an automatic withdrawal of the soil percolate as well as a continuous control and registration of the soil water tension, the applied vacuum and the amount of precipitation. During the sampling of the leachate a newly designed lock guaranteed a constant vacuum at the bottom end of the soil column. For detailed information on the construction of the apparatus the reader is referred to Rainbow (1992). The pesticide simulation model LEACHP (Wagenet and Hutson, 1989) was originally developed for the simulation of water and solute fluxes under field conditions. Besides the water and solute transport in the unsaturated zone, the transformation processes and the plant uptake as well as adsorption and precipitation are simulated. When selecting appropriate boundary conditions L E A C H P offers the possibility to simulate water and pesticide fluxes in water-saturated and unsaturated laboratory soil columns. To model water content, fluxes and potentials L E A C H P applies a numerical solution by finite differencing technique to the Richard's equation. The soil profile is subdivided into a number of horizontal segments and the simulation period is divided into brief intervals. As upper boundary condition evaporation, ponded or non-ponded infiltration or zero flux can be chosen. The bottom boundary condition can be simulated as a lysimeter tank, a free-draining profile, a fixed depth water table or zero flux.
J. Rainbow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
526
4. Results
Since the observed pesticide dissipation in unsaturated soil column studies could not be described satisfyingly with the original version of the pesticide simulation model LEACHP which assumes linear adsorption behavior, both the non-linear and the non-linear/non-singular adsorption description using Eqs. 2, 3 and 4 were incorporated into the original model version.
4.1. Laboratory experiment The comparison of the results from the three model versions with observed data of a chlortoluron breakthrough in an Ap-horizon of a typical Cambic arenosol from Bornhoeved, northern Germany, is presented in Fig. 2. The hysteresis version shows the best agreement with the measured values concerning the herbicide's first appearance in the percolate, the location and the maximum of the breakthrough as well as the course of the desorption branch (tailing), while the other versions fail to describe the behavior of chlortoluron. The measured and simulated values of the total herbicide discharge (discharge in % of application) within the percolate from the soil profile are compared also. Again the hysteresis version (60%) matches the measured value (58%) best, while the other versions overestimate the total chemical amount being leached out of the profile.
4.2. Simulated herbicide behavior under field conditions The migration behavior of the soil applied herbicide atrazine was simulated for 1989 with the three adsorption versions. Application date was Day 160 (10 May), application rate 1 kg/ha. Fig. 3 shows the cumulative precipitation and the
350
~. 300
measured dlscharge: 58%
LI orlglnal verslon discharge: 91%
250 mc
200
---,:~ --~/~. . . . . . . . . . . . . . . . . . . .
o e-
f 150
.
.
.
,e,, '.. ~
~/PT~.~'', X
°'°°
0
Freundllch version
.
...........
hysteresis version discharge: _ . . _ 60%
~
8
16
24
32
40
48
56
64
Pore volumes
Fig. 2. Measured and simulatedbreakthroughcurvesof chlortoluronin an Ap-horizon; length: I0 cm, pore water velocity:13 cm/day.
J. Rainbow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
527
8
80O ~,
Op.,
r-.9
~
4 ~
t~'O
t~ 0
-
160
0
200
240
280
320
Tlme( d ) I pr~K:Ipltatlondrainage linearIsotherm Froundllchisotherm )(
- - 'O- - -
~
i,
360
hysteresis --'1"-
[
I
Fig. 3. Comparison of three different adsorption assumptions for the simulated atrazine discharge from the ApApBv-profile (depth: 0-44 cm) in 1989.
simulated drainage out of the ApApBv-profile (0-44 cm) of a Cambic arenosol. The simulated results of the different model versions for cumulative atrazine loss (in % of the applied amount) from the profile are also presented. The first appearance of atrazine at the end of the considered soil profile occurs in response to a large rainfall event on day 205 (107 mm). The linear adsorption model calculates an atrazine discharge of 2%, while both non-linear versions yield approximately 0.5%. Following two further large events on Day 239 and 240 (97 and 49 mm) the linear version calculates 4% atrazine loss, the other two versions 1.5%. Until the end of the simulation the total atrazine loss for the linear version becomes 7.5% being approximately three times larger than for the Freundlich (2.8%) and for the hysteresis version (2.3%). The simulated behavior of terbuthylazine and chlortoluron revealed similar results. These results demonstrate that careful consideration of the applied ad- and desorption assumption must be taken when assessing the migration behavior of pesticides under laboratory as well as field conditions.
References Brusseau, M.L. and Rap, P.S.C., 1989. Sorption nonideality during organic contaminant transport in porous media. CRC Crit. Rev. Environ. Cont., 19: 33-99. Harris, C.I. and Warren, G.F., 1964. Adsorption and desorption of herbicides by soil. Weeds, 12: 120-126. Koskinen, W.C., O'Connor, G.A. and Cheng, H.H., 1979. Characterization of hysteresis in the desorption of 2,4,6.T from soils. Soil Sci. Soc. Am. J., 43: 871-874.
528
J. Rambow, B. Lennartz / Ecological Modelling 75/76 (1994) 523-528
Rambow, J., 1992. Modellstudie zum Verbleib herbizider Wirkstoffe in wasserteilges~ittigten Bfden. Schriftenreihe des Institutes fiir Wasserwirtschaft und Landsehafts6kologie der ChristianAlbrechts-Universit~it Kiel, Heft 19. Swanson, R.A. and Dutt, G.R., 1973. Chemical and physical processes that effect atrazine and distribution in soil systems. Soil Sci. Soc. Am. Proc., 37: 872-876. van Genuchten, M.Th., Davidson, J.M. and Wierenga, P.J., 1974. An evaluation of kinetic and equilibrium equations for the prediction of pesticide movement through porous media. Soil Sci. Soc. Am. J., 38: 29-35. Wagenet, R.J. and Hutson, J.L., 1989. LEACHM - Leaching Estimation And Chemistry Model: A process-based model of water and solute movement, transformations, plant uptake and chemical reactions in the unsaturated zone. User's Manual, Continuum 2. Water Resources Institute, Cornell University, Ithaca, NY.