Screening approaches for 

predicting pesticide 

concentrations in groundwater

Dazhi Mao1, W. Martin Williams1, J. Mark Cheplick1, and 

Robert Morris2

1Waterborne Environmental, Inc., Leesburg, Virginia, USA

2FMC Corporation, Ewing, New Jersey, USA
2

Objective

• Use the results of prospective ground-water 

monitoring (PGW) studies to evaluate the ability 

of three screening models to predict the fate and 

transport of pesticide residues through the soil 

profile and into shallow groundwater 
3

Modeling Approaches

Modeling approaches evaluated:

– SCI-GROW (Screening Concentration in Ground 

Water)

– “NMC” (N-Methyl Carbamate Cumulative Analysis)

– PRZM-ADAM (Pesticide Root Zone Model – Aquifer 

Dilution Assessment Model)
4

Screening Concentration in Ground 

Water (SCI-GROW)

• EPA Tier I model

• Empirical model for predicting 

“conservative high-end concentrations” 

of pesticides in groundwater

• Regression based on prospective 

groundwater monitoring (PGW) studies 

available at the time

• Simple user-input 
5

N-Methyl Carbamate (NMC) 

Cumulative Risk Assessment

• Based on PRZM model

• Field capacity set to porosity to 

simulate a saturated zone

• Soil horizons divided uniformly in 

the top 1 m

• Environmental fate linearly 

declining with depth until the water 

table

• Concentration averaged vertically 

1 m below water table (assuming 

1 m well screen)

USEPA (2005). Preliminary N-methyl Carbamate Pesticides Cumulative Risk Assessment, Appendix IID: Drinking 

water exposure, 35pp. 
6

PRZM-ADAM

• PRZM provides water and chemical 

flux into aquifer model, ADAM 

(aquifer dilution assessment model)

• The water balance algorithm 

includes displacement from recharge 

and lateral transport using Darcy’s 

law: 

• Chemical algorithms account for 

degradation, adsorption, and 

advection.

• Ability to simulate a single parent 

chemical and up to four degradation 

products Qlat = Kh

* I* A

Kh = hydraulic conductivity (m/day) I = the hydraulic gradient (m/m) 

A i= cross-sectional area (5 m depth)
7

Methods and Materials

• Set up a model representation of five prospective 

groundwater monitoring (PGW) studies

– SCI-GROW: application rates, number of applications

– PRZM: application rates, dates, soil properties, 

meteorology, irrigation, soil properties

– ADAM: bulk density, porosity, hydraulic gradient, 

hydraulic conductivity, 5-m mixing depth

• Compare predicted concentrations (without 

calibration) of bromide and pesticide residues to 

values measured in field studies
8

Prospective Ground-Water Monitoring Study

• 1-2 hectare plot

• Apply pesticide and tracer

• 120% normal monthly rainfall or 

crop water demand

• Sample soil, soil-pore water, 

and ground water for 2-3 year 

period
9

Study Sites 

Location IN95 IN85 IN75 NC4L GA1L

Crop Soybean Soybean Soybean Soybean Sweet corn

Soil Sand Sand Sand Loamy sand Sand loam

GW depth (m) 5.7 9.7 6.4 3.1 4.3

Bromide rate 

(kg/ha)

168 159 168 150 150

IN85 NC4L GA1L

IN75 

IN95
10

Results (IN95)

Bromide Concentration in Groundwater

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800

DAT (Days after Treatment)

Concentration (ppm)

PRZM-ADAM

NMC

Max

Avg

Median

Min

SCI-GROW: 2140 ppm

Bromide Concentration in Soil (0-6 in)

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min

Bromide Concentration in Soil-pore Water (6 ft)

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800 900 1000

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min
11

Results (IN85)

Bromide Concentration in Groundwater

0

10

20

30

40

50

0 100 200 300 400 500 600 700 800 900 1000

DAT (Days after Treatment)

Concentration (ppm)

PRZM-ADAM

NMC

Max

Avg

Median

Min

SCI-GROW: 2140 ppm

Bromide Concentration in Soil (0-6 in)

0

10

20

30

40

50

60

70

80

90

100

-10 40 90 140 190 240

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min

Bromide Concentration in Soil-pore Water (6 ft)

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700 800 900 1000

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min
12

Results (IN75)

Bromide Concentration in Groundwater

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200 1400 1600

DAT (Days after Treatment)

Concentration (ppm)

PRZM-ADAM

NMC

Max

Avg

Median

Min

SCI-GROW: 2140 ppm

Bromide Concentration in Soil (0-6 in)

0

10

20

30

40

50

60

70

80

90

100

-10 40 90 140 190 240

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min

Bromide Concentration in Soil-pore Water (6 ft)

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700 800 900

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min
13

Results (NC4L)

Bromide Concentration in Groundwater

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

DAT (Days after Treatment)

Concentration (ppm)

PRZM-ADAM

NMC

Max

Avg

Median

Min

SCI-GROW: 1910 ppm

Bromide Concentration in Soil (0-6 in)

0

10

20

30

40

50

60

70

80

90

100

-10 10 30 50 70 90 110

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min

Bromide Concentration in Soil-pore Water (5 ft)

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0 40 80 120 160

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min
14

Results (GA1L)

Bromide Concentration in Groundwater

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400 450 500

DAT (Days after Treatment)

Concentration (ppm)

PRZM-ADAM

NMC

Max

Avg

Median

Min

SCI-GROW: 1740 ppm

Bromide Concentration in Soil (0-6 in)

0

10

20

30

40

50

60

0 20 40 60 80 100 120

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min

Bromide Concentration in Soil-pore Water (6 ft)

0

20

40

60

80

100

120

-10 40 90 140 190 240 290 340 390

DAT (Days after Treatment)

Concentration (ppm)

PRZM

Max

Avg

Median

Min
15

Pesticide Concentration in Groundwater

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500

DAT (Days after Treatment)

Concentration (ppb)

PRZM-ADAM

NMC

Max

Avg

Median

Min

Results (GA1L, pesticide)

SCI-GROW: 0.27 ppb

Pesticide Concentration in Soil-pore Water (6 ft)

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500

DAT (Days after Treatment)

Concentration (ppb)

PRZM

Max

Avg

Median

Min

Pesticide Concentration in Soil (0-6 in)

0

100

200

300

400

500

600

700

0 50 100 150 200

DAT (Days after Treatment)

Concentration (ppb)

PRZM

Max

Avg

Median

Min

LOQ = 0.1 ppb
16

Bromide Conclusions

• Surface soil predictions with PRZM

– Concentration magnitude and timing match observed data 

fairly well

• Soil pore-water predictions with PRZM

– Tend to flush chemical through soil profile too quickly

• Groundwater Predictions

– SCI-GROW tends to over-predict maximum concentrations 

by ~2 orders of magnitude

– NMC approach over-predicted observed median 

concentrations by 4-8 times

– ADAM predictions similar to observed median 

concentrations
17

Pesticide Conclusions

• Soil predictions with PRZM:

– Matched maximum concentrations measured in the surface 

soil at GA1L

• Soil-water predictions with PRZM

– Over predicted soil-pore water concentrations

• Groundwater predictions:

– SCI-GROW predictions similar to observed detections 

– NMC under predicted groundwater detections

– ADAM under predicted groundwater detections

But most groundwater samples <LOQ!
18

Further Research

• Examine pesticide performance at additional 

study sites

• Examine pesticide performance after calibrating 

bromide timing

• Sensitivity analysis of PRZM-ADAM and NMC

– Compartment size in PRZM

– Aquifer thickness/mixing zone (ADAM vs NMC)

– Pesticide degradation assumptions
19

ADAM is a component of PLUS

Pesticide Leaching in the U.S.
20

Acknowledgements

• FIFRA Environmental Model Validation Task Force 

(FEMVTF) 

• Amy Ritter, Waterborne Environmental, Inc.

• Katie Betz, Waterborne Environmental, Inc.

• Kristen Umbaugh, Waterborne Environmental, Inc.
21

Questions?