Qingli Ma and Rick Reiss 

Exponent, Inc.

A Better Sorption Model for 

Predicting Pesticide Behavior

Paul Whatling

Cheminova, Inc.

Presented at the EMPM, March 24, 2014

1
OUTLINE OF THE 

PRESENTATION

Pesticide sorption models

Environmental fate properties of chemical-X 

Instantaneous equilibrium sorption and time-dependent 

sorption of chemical-X in the soil

Field dissipation and groundwater monitoring studies

Evaluations of the sorption models on predicted 

environmental concentrations 

Conclusions

2
EQUILIBRIUM AND TIMEDEPENDENT SORPTION 

MODELS

Instantaneous equilibrium adsorption model (Freundlich model) 

Cs = KF,EQ Cl

N

Time-dependent kinetic sorption model (two-site sorption model)

dCs2 / dt = kd(KF,NE Cl

N - Cs2) = kd(fNEKF,,EQ Cl - Cs2)

3
EQUILIBRIUM AND TIME-DEPENDENT 

SORPTION MODELS (CONT’D)

Adapted from Beulke

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ENVIRONMENTAL FATE 

PROPERTIES OF CHEMICAL-X

Fate Parameter Value

Water Solubility (mg/L ) 130

Vapor pressure (kPa) 4 × 10-7 (20 oC)

Aerobic soil metabolism half-life 

(days)

833, 4,878,1,141, 581, 686, 2,478, 1,694, 

749, and 1,205 

Anaerobic aquatic metabolism 

half-life (days)

≥365

Aerobic aquatic metabolism halflife (days)

42.6 and 36.3

Aquatic photolysis half-life (days) Stable

Hydrolysis half-life (days) Stable 

Terrestrial field dissipation half-life 

(days)

392, 596, 308, 788, 316, and 505

5
ADSORPTION/DESORPTION 

OF CHEMICAL-X IN THE SOIL

Equilibrium adsorption/desorption

Time-dependent, non-equilibrium sorption

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EQUILIBRIUM AND TIMEDEPENDENT SORPTION

Data from a lab incubation study with four soils (Adam, 2006) 

The instantaneous equilibrium adsorption/desorption was 

measured with batch equilibrium technique using day 0 samples 

at 5 concentration levels

The time-dependent sorption kinetics was measured at one 

concentration level at 0, 30, 70, 120, 203 and 308 days after the 

treatment. 

All samples were agitated in the dark for 48 hours at 20 oC to 

determine the equilibrium and time-dependent sorption

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EQUILIBRIUM AND TIMEDEPENDENT SORPTION (CONT’D)

The Freundlich equilibrium KFoc ranged from 104 to 214 L/kg, with 

a mean of 143 L/kg. The average Freundlich exponent (N) was 

0.91.

The optimized parameters for the two-site, equilibrium-kinetics 

sorption model were fNE = 0.55 (arithmetic mean) and kd = 0.03 d-1 

(geometric mean)

The measured time-dependent KFoc values increased with 

incubation time by a factor of 1.9 to 2.4 for the four soils. 

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MEASURED KFOC VALUES 

(ADAM, 2006)

0.0

100.0

200.0

300.0

400.0

500.0

600.0

0 50 100 150 200 250 300 350

Mean 

KFoc (L/kg)

Time (days)

Time-Dependent Sorption of chemical-X

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FIELD STUDIES

Leaching study

Six nests of lysimeters horizontally placed at 0.4 m, 0.8 m, and 1.2 m 

below the soil surface. One nest was installed at the control plot and 

five other nests around the treated plot.

A vacuum of 150 to 300 mbar was constantly applied to each 

lysimeter to collect freely available soil water.

Chemical-X was applied at 0.125 kg a.i./ha with an interval of 14 

days.

The study was conducted continuously for five years.

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FIELD STUDIES (CONT’D)

(Courtesy of Zietz et al.) 11
FIELD STUDIES (CONT’D)

(Courtesy of Zietz et al.) 12
FIELD STUDIES (CONT’D)

Ground water monitoring study

Four groundwater wells were installed at the same site where the 

field leaching study was conducted. 

The groundwater monitoring wells were 5 meters down-gradient of 

the treated field.

The ground water levels varied from 3.2 to 4.6 m below the surface.

Chemical-X was applied at 0.125 kg a.i./ha with an interval of 14 

days.

The study was conducted continuously for nine years.

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FIELD STUDIES (CONT’D)

(Courtesy of Zietz et al.) 14
EVALUATIONS OF SORPTION 

MODELS ON PREDICTED 

CONCENTRATIONS

The FOCUS-PRZM3.5.2 model was parameterized with 

equilibrium adsorption parameters as well as time-dependent 

sorption parameters to evaluate the influence of sorption kinetics 

on predicted concentrations. 

The model predictions were compared with measured 

concentrations in lysimeters and ground water to determine the 

performance of the two sorption models. 

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KEY MODEL INPUT PARAMETER 

VALUES

Input Parameter Value

Use pattern (kg a.i./ha) 0.125 kg a.i./ha, two applications with 14-day 

interval

Equilibrium sorption parameters KFoc = 143 L/kg, N = 0.91

Kinetic sorption parameters

KFoc = 143 L/kg, N = 0.91,

fNE = 0.55, kd = 0.03 d-1

Plant uptake (TSCF) 0.7

Aerobic soil half-life (days) 2,222

Effective degradation half-life (days) 1,433.5

Field dissipation half-life(days) 316 (Germany)

Hydrolysis half-life (days) Stable

Anaerobic soil half-life (days) Stable

Foliar dissipation half-life (days) 12.8

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MEASURED AND PREDICTED 

CONCENTRATIONS

17
MEASURED AND PREDICTED 

CONCENTRATIONS (CONT’D)

18
MEASURED AND PREDICTED 

CONCENTRATIONS (CONT’D)

19
MEASURED AND PREDICTED 

CONCENTRATIONS (CONT’D)

20
MEASURED AND PREDICTED 

CONCENTRATIONS (CONT’D)

21
MEASURED AND PREDICTED 

CONCENTRATIONS (CONT’D)

22
MEASURED AND PREDICTED 

CONCENTRATIONS (CONT’D)

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CONCLUSIONS

Sorption of chemical-X in the soil increased with aging. The 

average apparent sorption coefficient (Koc) of aged residues in the 

soil is ~2X that of freshly added residues in the same soil.

Overall, the equilibrium sorption model predicted earlier 

occurrence and higher breakthrough concentrations than the timedependent sorption model at all depths. 

Comparisons of the predicted soil pore-water concentrations by 

FOCUS-PRZM with equilibrium and time-dependent sorption 

parameters indicated that the time-dependent sorption model 

better predicted the observed concentrations over longer 

observation period, although both sorption models predicted 

similar concentrations over a shorter period of time. 

Given the better predictions and the clear evidence of timedependent sorption in the laboratory data, the time-dependent 

sorption model is a better sorption model and should be 

incorporated into the PRZM-GW model. 

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TIME-DEPENDENT 

SORPTION/DESORPTION

For short-term study, it can be done as for the batch equilibrium 

study, but samples are analyzed at various time intervals

For long-term study, the treated soil samples were incubated in 

the dark under controlled temperature and soil moisture, usually 

with continuous ventilation as used in an aerobic soil metabolism 

study 

Samples are collected at different times after treatment and 

desorption from the “aged” soil is determined by sequential 

extraction of soil samples, first with CaCl2 solution and then with a 

stronger (organic) solvent

The sorption coefficient is calculated as the ratio of the sorbed

residue to that in aqueous extract

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