CHARGE QUESTIONS: 

FIFRA Scientific Advisory Panel Consultation on the Two-dimensional Exposure RainfallRunoff Assessment (TERRA) Watershed Model and Its Use in the FIFRA Ecological Risk 

Assessment for Antimicrobial Uses of Copper 

October 25-26, 3011 

The purpose of this FIFRA Scientific Advisory Panel (SAP) meeting is to seek consultation on 

the usefulness of the Two-dimensional Exposure Rainfall-Runoff Assessment (TERRA)1 model 

as an aquatic exposure assessment tool – specifically for evaluating the antimicrobial uses of 

copper and potentially for wider use. The EPA is seeking the Panel’s input on the three charge 

questions listed below. Charge question one focuses on the TERRA model’s attributes and the 

general capabilities necessary for modeling pesticides used in urban settings. The second charge 

question focuses on the Goodwin Creek Watershed and the national application of this 

watershed. And lastly, charge question three focuses on model calibration.

Charge Question 1

The three most prevalent antimicrobial uses of copper include antifoulant paints, roofing shingles 

and wood preservatives. The Antimicrobials Division used a basic field scale modeling 

approach, including maximum use rates, realistic heavy rainfall events, the assumption of high 

leaching rates, and storm-water conveyance via impervious surfaces, to estimate potential high 

end aquatic exposure from copper’s use as an antimicrobial pesticide in roofing shingles and 

treated wood. The American Chemistry Council (ACC) has proposed the TERRA Model as an 

alternative and more refined way to assess aquatic exposure due to the two major urban 

antimicrobial uses of copper, in wood preservatives and in roofing shingles, on a watershed scale 

level. 

a) What does the Panel believe are the advantages and/or limitations of assessing 

antimicrobial uses of copper from roofing shingles and wood preservatives with TERRA, a 

spatially-explicit, watershed scale model? Are there other models that the Panel feels 

should be considered for use by OPP in estimating exposure to copper from its use as an 

antimicrobial pesticide, specifically in roofing shingles and wood preservative? 

b) In the opinion of the Panel, what attributes of an urban watershed model are the most 

critical and why does the Panel regard these attributes as critical? Does the TERRA model 

possess these attributes? 

c) Please discuss whether TERRA has the modeling capabilities necessary for it to be applied 

to other urban metal exposure assessments for antimicrobial pesticides as well as what 

modifications, if any, would be needed to assess exposure to other metals used as 

pesticides. 

d) In the opinion of the panel what further development and/or assessment is necessary to 

prepare the TERRA model for use as an exposure assessment tool by the EPA? 

 1

 Please note that the name of the model under review was recently changed from Two-dimensional Runoff, Erosion, and Export 

(TREX) to Two-dimensional Exposure and Rainfall-Runoff Analysis (TERRA). This name change was made to avoid confusion 

with the Terrestrial Residue Exposure (T-REX) model that OPP uses to estimate pesticide concentrations for avian and 

mammalian food items. (United States Environmental Protection Agency. 2008. Terrestrial Residue Exposure Model Version 

1.4.1. www.epa.gov/oppefed1/models/terrestrial/trex). 
Charge Questions 2

EPA employs a tiered strategy to assess aquatic exposure. The first tier facilitates the rapid 

screening of pesticide uses for potential risk issues, while the second tier refines exposure 

estimates by utilizing a field scale model and site specific properties. This process is designed to 

incorporate additional data (e.g., site specific properties) for each progressive tier. EPA has 

historically employed field scale models to assess aquatic exposure in support of national 

pesticide registrations. The TERRA model, unlike other models used by OPP, is a fully 

distributed, spatially-explicit watershed model. It has model capabilities of distributing 

differences in hydrology, meteorology, soil properties, and pesticide uses across a watershed. 

These model capabilities require consideration in terms of the proper scale of the exposure 

assessment. 

a) Are the appropriate pathways (e.g. engineered water conveyance structures) for modeling 

urban antimicrobial uses of copper within the Goodwin Creek scenario currently included 

in the framework of the TERRA model? What, if any, additional pathways are necessary 

to appropriately estimate exposure concentrations? 

b) What are the most important attributes to consider when selecting and designing an urban 

watershed modeling scenario? In the opinion of the Panel, where does the Goodwin 

Creek watershed fall within a distribution of the nation’s urban watersheds? Also, given 

that the model does not account for storm drains, how does this compare to watersheds 

nationwide? 

Charge Question 3

Watershed-scale models require calibration to account for complex watershed-dependent 

hydrology and environmental fate processes. Model calibrations have been conducted by 

altering saturated hydraulic conductivity, Mannings N, soil erodibility factors, land cover factors, 

and chemical partitioning coefficients. In contrast, the OPP field-scale models such as 

PRZM/EXAMS are not calibrated for site-specific hydrology, etc. 

The TERRA model utilizes a simulated urban watershed that was calibrated to the hydrology and 

sediment loads of Goodwin Creek. Because the Goodwin Creek watershed is a predominately 

pastured/forested watershed, the EPA has concerns that the calibration processes may not 

adequately represent urban hydrologic and chemical transport processes. 

a) Please discuss the implications of watershed land use patterns on model calibration. 

What types of impacts could these have on the estimated exposure concentrations of 

copper? 

b) Please indicate any unique calibration issues that should be considered when simulating 

an urban, residential watershed.