## 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

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# 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

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# EQUILIBRIUM AND TIME

DEPENDENT SORPTION MODELS

 Instantaneous equilibrium adsorption model ( Freundlich model)

 C s = K F,EQ C l N

 Time

- dependent kinetic sorption model (two

- site sorption model)

 dC s2 / dt = k d (K F,NE C l N

- C s2 ) = k d ( f NE K F ,,EQ C l - C s2 )

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# EQUILIBRIUM AND TIME

- DEPENDENT SORPTION MODELS (CONT’D)

Adapted from Beulke 4

# ENVIRONMENTAL FATE PROPERTIES OF CHEMICAL

- X

Fate Parameter Value Water Solubility (mg/L ) 130 Vapor pressure ( kPa ) 4 × 10

- 7 (20 o C )
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 half

life (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

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# ADSORPTION/DESORPTION OF CHEMICAL

- X IN THE SOIL

 Equilibrium adsorption/desorption

 Time

- dependent, non
- equilibrium sorption

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# EQUILIBRIUM AND TIME

DEPENDENT 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 o C to determine the equilibrium and time

- dependent sorption

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# EQUILIBRIUM AND TIME

DEPENDENT SORPTION ( CONT’D )

 The Freundlich equilibrium K F oc 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 f NE = 0.55 (arithmetic mean) and k d = 0.03 d

- 1 (geometric mean )

 The measured time

- dependent K Foc values increased with incubation time by a factor of 1.9 to 2.4 for the four soils.

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# MEASURED K FOC 

# VALUES ( ADAM , 2006)

0.0 100.0 200.0 300.0 400.0 500.0 600.0

0 50 100 150 200 250 300 350MeanKFoc(L/kg)

Time (days) 

### Time

- Dependent Sorption of chemical

- X

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# FIELD STUDIES

 Leaching study

 S ix 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 K Foc = 143 L/kg, N = 0.91

Kinetic s orption parameters K F oc = 143 L/kg, N = 0.91 , f NE = 0.55, k d = 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

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# MEASURED AND PREDICTED CONCENTRATIONS (CONT’D)

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# MEASURED AND PREDICTED CONCENTRATIONS (CONT’D)

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# MEASURED AND PREDICTED CONCENTRATIONS (CONT’D)

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# MEASURED AND PREDICTED CONCENTRATIONS (CONT’D)

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# MEASURED AND PREDICTED CONCENTRATIONS (CONT’D)

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# 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 ( K oc ) 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 time

dependent 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 time

dependent 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 CaCl 2 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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