Advances in modeling 

urban/residential pesticide runoff

W. Martin Williams

Amy M. Ritter

Megan L. White

J. Mark Cheplick

Waterborne Environmental, Inc. 

Leesburg, Virginia USA 
2

Status of Regulatory Modeling

• The UK Pesticides Safety Directorate: Excel spreadsheet 

model (HardSPEC ca 2003) -- first tier to predict 

concentrations of herbicides on hard surfaces (e.g. 

concrete, asphalt, railway ballast) in surface water and 

groundwater 

• The U.S. Environmental Protection Agency (USEPA) 

began conducting risk assessments for residential 

pesticide use ca 2004 with PRZM-EXAMS
3

UK Pesticides Safety Directorate 

HardSPEC

• Empirical model based on washoff studies of six herbicides 

(atrazine, diuron, oxadiazon, glyphosate, oryzalin, 

isoxaben) on asphalt, concrete, and railway ballast 

• Four regulatory scenarios:

1. Surface water stream receiving surface drainage from a major road 

in a rural setting 

2. Surface water stream receiving surface drainage from an urban 

catchment

3. A pond receiving surface drainage waters from an urban catchment

4. Ground water aquifer receiving herbicide leached from a double 

railway track

HOLLIS, J.M., RAMWELL, C.T. and HOLMAN, I.P. (2003). HardSPEC: A First-tier Model for Estimating Surface- and 

Ground-Water Exposure resulting from Herbicides applied to Hard Surfaces. NSRI research report No. SR3766E for DEFRA 

PL0531, 79 pp + 3 Appendices. http://www.pesticides.gov.uk/approvals.asp?id=713
4

U.S. Environmental Protection Agency’s 

Residential Scenario

• 10-ha watershed draining into 1-ha by 2-m pond

• Watershed simulated with PRZM model

– 30-year simulation

• Pond simulated with EXAMS

– Pesticide mass added to pond

– Runoff water not added to pond

• Two PRZM simulations (pervious and impervious)

– 70% pervious (turf) -- 50% treated

– 30% impervious (roofs, driveways, sidewalks, roads)

– 1% of application rate on impervious area (over spray)

USEPA ,2007. Pesticides: Science and Policy. Red Legged Frog MetaData.. Last updated 20-Sep-2007. 

http://www.epa.gov/oppefed1/models/water/pe5_rlf.htm
5

CLA Model Evaluation 2006

• Stormwater Management Model (SWMM)

– USEPA urban runoff model 

– Highly sophisticated in its ability to simulate hydraulic 

processes and urban features

– Limited in water quality components, especially in its ability 

to represent pesticide applications and sorption kinetics

• Pesticide Root Zone Model (PRZM)

– USEPA agricultural runoff/leaching model

– Relatively sophisticated in its ability to represent pesticide 

applications and sorption kinetics

– Limited in its ability to simulate urban drainage features; 

unknown ability to simulate pesticide sorption kinetics 

Cheplick, J.M., S. Dasgupta, A.M. Ritter, and W.M. Williams, 2006. Model Review and Scenario Development for 

Urban/Residential Pesticide Runoff Model. Prepared by Waterborne Environmental, Inc. for Crop Life America. 

http://www.epa.gov/oppefed1/models/water/empm_top.htm

• Stormwater Management Model (SWMM)

– USEPA urban runoff model 

– Highly sophisticated in its ability to simulate hydraulic 

processes and urban features

– Limited in water quality components, especially in its ability 

to represent pesticide applications and sorption kinetics

• Pesticide Root Zone Model (PRZM)

– USEPA agricultural runoff/leaching model

– Relatively sophisticated in its ability to represent pesticide 

applications and sorption kinetics

– Limited in its ability to simulate urban drainage features; 

unknown ability to simulate pesticide sorption kinetics 
6

Objective

• Present recent advances in modeling pesticide 

runoff urban/residential landscapes

– Pesticide applications and pesticide washoff from 

hard surfaces with SWMM

– Runoff and pesticide washoff from hard surfaces with 

PRZM
7

Pesticide Application

Time in days

SWMM

PRZM

Time in days 0

-1,000 0

Target Rate

0
Simulation of Storm Water Runoff with 

SWMM and PRZM

Source: Google Maps
9

Portland Oregon Dataset

• 115 single-family residential lots totaling 6.8 hectares 

with 35% impervious area.

• Monitored extensively for runoff from November 1998 

through May 1999 at 5-minute intervals.

• SWMM was used to simulate and connect all source 

water, including runoff from individual roof, driveway, 

and lawn surfaces to lateral and main storm sewers. 

Huber, W.C., Cannon, L., and M. Stouder, 2006. BMP Modeling Concepts and Simulation. EPA/600/R-06/033, 

Office of Research and Development, USEPA, July 2006. 

www.epa.gov/ORD/NRMRL/pubs/600r06033/epa600r06033chp14.pdf
10

Portland Oregon Dataset

Source: Google Maps
11

Detailed

Model

Simple

Model

SWMM Set Up
12

Observed vs. Modeled Discharge (SWMM)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

01/13/99 01/14/99 01/15/99 01/16/99 01/17/99 01/18/99

Date

Flow (cfs)

Monitor Flow SWMM5
13

PRZM Set Up

• Impervious (roof, sidewalk, road)

• Pervious (lawn, ornamentals)

4.4 ha 2.4 ha

Ia = 0.005 cm

CN = 98

BD = 2.2

FC = 0.08

Ia = 0.2 cm

CN = 83

BD = 1.5

FC = 0.47
14

Observed vs. Predicted Discharge 

(SWMM and PRZM)

Observed vs. Predicted Daily Discharge

0

100

200

300

400

500

600

700

800

900

13-Jan 14-Jan 15-Jan 16-Jan 17-Jan 18-Jan

Date

Discharge (m3/day)

Observed SWMM PRZM
Simulation of Herbicide Runoff with 

SWMM and PRZM
16

UK Roadway Study

HEATHER, A.I.J., SHEPHERD, A. & HOLLIS, J.M. (1998). Losses of six herbicides from a kerb-and-gully pot road drain. 

Soil Survey and Land Research Centre report JF4085-1, for the Hard Surfaces Project Consortium. 28 pp. . 

http://www.pesticides.gov.uk

• Natural rainfall study

• Six herbicides applied to curb edge of asphalt road

– Atrazine, glyphosate, diuron (Oct 14 - Nov 11)

– Atrazine, oxadiazon, oryzalin (Dec 9 - Dec 25)

• Spray was applied in a continuous swath 

approximately 0.3 m wide along the full length of the 

section 

• Measurements included rainfall, temperature, runoff 

volume, and herbicide concentration over time.
17

SWMM Water Results

0

500

1000

1500

2000

2500

3000

3500

4000

5-Oct

10-Oct15-Oct20-Oct25-Oct30-Oct

4-Nov9-Nov

Runoff Volume (L)

Date

Cumulative Runoff

Predicted Runoff

Observed Runoff
18

SWMM Washoff Coefficients

• Exponential equation:

• C1 – chemical specific?

• C2 – land cover specific (grass, asphalt, concrete, etc.)?

W = C1 x q C2 x B

C1 = washoff coefficient

C2 = washoff exponent

q = runoff rate per unit area

B = pollutant buildup
19

SWMM Pesticide Results

W = C1 x q C2 x B

Herbicide 1: C1 = 0.20, C2 = 0.95

Herbicide 2: C1 = 0.10, C2 = 0.95

0

500

1000

1500

2000

2500

5-Oct

10-Oct15-Oct20-Oct25-Oct30-Oct

4-Nov9-Nov

Concentration (ppb)

Date

Concentration of Herbicide in Runoff

Predicted Herb1

Obs. Herb1

Predict herb2

Obs. Herb2
20

PRZM Water Results

Ia = 0.005 cm

CN = 98

BD = 2.2

FC = 0.08

0

500

1000

1500

2000

2500

3000

5-Oct 10-Oct 15-Oct 20-Oct 25-Oct 30-Oct

4-Nov9-Nov Runoff Volume (L)

Date

Cumulative Runoff

Observed Runoff

Predicted Runoff
21

PRZM Pesticide Application

 2 cm

Photolysis layer for a rough surface

Photolysis layer for a smooth surface

Application = CAM 4 with 0.1 cm

Application = CAM 1
22

PRZM Pesticide Runoff Algorithm 

Modified for Impervious Surface

0 10 20 30 40 50 60 70 80 90 100 

Percent (%)

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Depth (cm)

Pervious Impervious

WIN-PRZM
23

PRZM Pesticide Results

Application = CAM 4 with 0.1 cm

Pesticide runoff extraction = 0.9

Kd1 = 0.0

Kd2 = 0.1

0

500

1000

1500

2000

2500

3000

5-Oct 10-Oct 15-Oct 20-Oct 25-Oct 30-Oct

4-Nov9-Nov Concentration (ppb)

Date

Concentration of Herbicide in Runoff (ppb)

Observed Herbicide 1

Observed Herbicide 2

Predicted Herbicide 1

Predicted Herbicide 2
24

Washoff Coefficients

What Kd to use with PRZM?

What coefficients to use in 

SWMM?
25

Washoff Database Growing

Agency Study Chemicals

CDPR Formulation effects and off-target 

transport of pyrethroid insecticides from 

urban hard surfaces. UC-Davis – 2009

cyfluthrin, λ cyhalothrin, esfenvalerate,

bifenthrin

CDPR Pesticide hard surface wash-off study. 

UC-Riverside – 2009

bifenthrin, permethrin, cyfluthrin, λcyhalothrin, fipronil,

CDPR Washoff of formulated pyrethroid 

insecticides from concrete, turf, bare soil. 

UC-Davis – 2009

B-cyfluthrin, esfenvalerate

PWG Washoff/runoff of cypermethrin residues 

from slabs of external building material 

surfaces using simulated runoff – 2008

cypermethrin

CDPR Runoff of diazinon from paved plots -

2001

Diazinon

Soil Survey 

and Land 

Research 

Centre

Herbicide partitioning to concrete, 

asphalt and railway ballast – 2002

fuoranthene, phenanthrene, atrazine, 

diuron, oryzalin, isoxaben, isoproturon, 

dichlorophen

Small Catchment report – 1999 atrazine, diuron, oxadiazon, glyphosate, 

oryzalin, isoxaben

Road Wash-off Study – 1999 atrazine, diuron, oxadiazon, glyphosate, 

oryzalin, isoxaben
26

Summary / Conclusions

• SWMM (beta) now has the ability to simulate discrete 

applications of chemicals

• PRZM (WinPRZMwdv) now has the ability to simulate 

hard surface runoff of chemicals

• SWMM and PRZM reasonably simulated hydrologic 

response of UK roadway and Oregon residential area

• SWMM and PRZM reasonably simulated atrazine and 

glyphsate runoff from UK roadway

• Data is becoming available to develop washoff

coefficients for hard surfaces (pesticide 

properties/formulations vs. surface conditions)
27

Acknowledgements

• Paul Hendley (on behalf of Pyrethroid Working Group)

• Wayne Huber (Oregon State University)

• Scott Jackson (on behalf of Crop Life America) 

• Lewis Rossman (USEPA-ORD)
28

Questions?

Photo credit: D. Rosen