Refinements to the USEPA Tier 2 Drinking 

Water Risk Assessment Guidance: 

Addressing atypical use patterns

Nathan Snyder Aldos Barefoot

EMPM – September 20, 2011
Summary

• Tier 2 Exposure Modeling

• Surface Water PRZM/EXAMS (Standard Scenarios using PE5)

• Index reservoir (Drinking Water Exposure)

• Foliar Processes, Application Methods, Crop Season Representation

• Overview of current USEPA policy on using Percent 

Crop Area (PCA) factors in Human Health Risk 

Assessments

• Limitations in procedure for areas outside of the 

continental United States (PR/HI)

• Options for refinements

• Mixed major/minor use PCA values in combined watershed EEC 

determination 

• Higher resolution land-use data

• Focused watershed assessments

2
Current Practice 

• Clear rules and standard practices for selecting 

environmental fate inputs

• Modeler (registrant or USEPA) judgment in 

selection of scenarios, interpretation of label, 

assignment of scenarios to primary use areas or 

regions

• Core models allow for representation of most 

label use requirements

• Examples come from USEPA assessments and 

proposed refinements and label changes

3
Considerations in Model 

Parameterization

• Mixed Application Types

• Foliar, soil applied...

• Multiple Crop Seasons per year

• Crop Rotations

• Runoff and Spray Buffers

• Biphasic, metabolites, or other atypical environmental 

fate conditions

• Label language preventing use when rain is forecast

• Multiple crops in watersheds

• PCA and drinking water watersheds

4
5

Application Representation

• PRZM allows for variable CAM 

within a single simulation

• Assuming seed treatment, 

incorporated, wetted-in, shank 

applications to occur as soil applied 

or foliar applications overestimates 

exposure

• Many products have foliar uses, 

following pre- and at-plant 

applications (possibly at higher 

rates) 

• Use labels provide application 

guidance including methods, rates, 

timing (intervals)

• PE5 can handle variable intervals 

and rates, but only one CAM type
Sensitivity of CAM Selection

• Example 2 kg/ha (at plant), followed by 2 @ 1 kg/ha on 7-

day intervals

• 1-in-10 Year Annual Maximum Peak values: 

• 204 ppb (PE5 all cam 2) versus 172 ppb (PRZM/EXAMS CAM 4 

then 2) 

PE5 Generated PRZM Input

ACS

*** Record 16

 161061 0 2 0.0 2 .99 .064

 231061 0 2 0.0 1 .99 .064

 301061 0 2 0.0 1 .99 .064

 161062 0 2 0.0 2 .99 .064

 231062 0 2 0.0 1 .99 .064

 301062 0 2 0.0 1 .99 .064

Adjusted PRZM Input

ACS

*** Record 16

 161061 0 4 5.0 2 .99 .064

 231061 0 2 0.0 1 .99 .064

 301061 0 2 0.0 1 .99 .064

 161062 0 4 5.0 2 .99 .064

 231062 0 2 0.0 1 .99 .064

 301062 0 2 0.0 1 .99 .064

6

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0 500 1000 1500 2000 2500 3000 3500 4000

Simulation Day

Concentration (ppm)

PE5 (All CAM 2)

PRZM/EXAMS (CAM 4, then CAM 2)
Crop Scenarios

• Soil, Crop, 

Weather defined 

by Scenario

• Currently there are 

standard and extra 

scenarios created 

for special 

assessments

7
Crop Scenarios

• Conditions requiring additional (or modified) 

scenarios

• Crop rotations

• Multiple seasons

• Representation of multiple seasons without also 

adding cropping periods lead to over estimates 

of EECs

8
Modified Crop Scenarios

• Triple cropped Lettuce in California

• Tomato/Cucumber rotation (double cropped) in Florida

• Washoff, plant decay rate, and application timing in crop 

season

9

0.00E+00

2.00E+01

4.00E+01

6.00E+01

8.00E+01

1.00E+02

1.20E+02

1.40E+02

1.60E+02

Standard CA Letuce,

Assume 3 application

"Seasons"

Modified Standard CA

Lettuce, modified to have 3

crop periods

Standard FL Tomato,

assume 2 application

"Seasons"

FL Tomato then FL

Cucumber (combined as

one scenario)
Drinking Water Assessments

• PRZM/EXAMS Predictions adjusted by Percent 

Crop Areas (PCA) for determining Estimated 

Drinking Water Concentrations (EDWC)

• Established National (87%), Major Crop (20% –

57%), Combined Major Crop (46% - 83%), and 

Regional PCA (7% - 87%) available

10
• National PCA Adjustment Factors

• 8-Digit HUCs intersected with County level Crop (1997 and 1992 

USDA Census of Agriculture) all agriculture

• PCA Adjustment Factors for Major Crops

• Same as above, but cropping specific to target crops

• Available for Corn, Wheat, Soybeans and Cotton

• Method developed to combine EECs if use is on several major crops 

with similar or different timing

• Regional PCA Adjustment Factors

• Same methodology but identified highest PCA in each of 18 regions

• Updates Coming....

PCA Values

11
Other States/Territories and Minor 

Crops

• Specialty Crops grown in areas outside of the 

“lower 48” – Puerto Rico and Hawaii

• Products labeled for uses other than Corn, 

Wheat, Soybeans, or Cotton considered minor

• Applying PCA methodology leads to excessively 

conservative assessments

• Approach using PCA methodology and 

conservative representation of land-use in 

regions of concern can lead to improved 

assessments

12
Puerto Rico – Modeling and Cropping

• One Scenario (PR Coffee), FL Surrogates used 

in other cases, with PR Weather

2007 Major Cropland

(>1000 Cuerdas)

Coffee grown without shade (cuerdas)

Plantains (cuerdas)

Coffee grown in the shade (cuerdas)

Pajón grass (cuerdas)

Pangola grass (cuerdas)

Oranges (cuerdas)

Bananas (cuerdas)

Other grasses and other crops (cuerdas)

Guinea grass (cuerdas)

Nursery, etc.

Pumpkins (cuerdas)

Mangoes (cuerdas)

Star grass (cuerdas)

Paragrass (cuerdas)

Sweet peppers (aji dulce) (cuerdas)

Yams (cuerdas)

Avocados (cuerdas)

Other (<1000 cuerdas)

ITEM 2007 Cuerdas

Coffee grown without shade (cuerdas) 26728

Plantains (cuerdas) 18034

Coffee grown in the shade (cuerdas) 11807

Pajón grass (cuerdas) 11608

Pangola grass (cuerdas) 8855

Oranges (cuerdas) 7133

Bananas (cuerdas) 7015

Other grasses and other crops (cuerdas) 5937

Guinea grass (cuerdas) 5257

Nursery, etc. 3057

Pumpkins (cuerdas) 2305

Mangoes (cuerdas) 2197

Star grass (cuerdas) 2191

Paragrass (cuerdas) 1677

Sweet peppers (aji dulce) (cuerdas) 1412

Yams (cuerdas) 1283

Avocados (cuerdas) 1114

Other (<1000 cuerdas) 11285

13
Puerto Rico - PCA

• PCA of 100% due to lack of an established value

• Watershed Boundaries are available at 8-, 10-, 12- Digit Resolution

• Census of Agriculture data available for municipalities

• Smaller unit than typical US counties

Category 2007 Cuerdas 2002 Cuerdas

Land in Farms 557528 690687

Pastureland and rangeland other than 

cropland and woodland 87951 109597

Woodland, forest, and underbrush 38390 64963

All other land 38461 62694

Total Cropland 392728 453433

Harvested 116198 199225

Used for pasture or grazing 204624 178663

Used for cover crops, legumes, and 

soil-improvement grasses, not 

harvested and not pastured 6176 7107

On which all crops failed 4462 4777

Idle 61266 63661

14
PCA for Puerto Rico

Watershed ID Watershed Name

Area 

(cuerdas) PCA1 PCA2

2101000401

Quebrada Boqueron to Rio Loco 

Watersheds 110183.3873 35.412928 18.307461

2101000301 Rio Culebrinas Watershed 84764.5125 27.354811 11.959209

2101000205

Quebrada Los Cedros to Rio Camuy 

Watersheds 189690.2294 24.967006 9.286670

2101000202 Rio Grande de Manati Watersheds 119892.6556 23.547841 13.089323

2101000302 Rio Grande de Anasco Watershed 138327.2021 23.516325 17.611321

2101000304 Rio Guanajibo Watershed 98707.3515 22.444888 11.666598

2101000204 Rio Grande de Arecibo Watershed 156686.5439 20.657501 15.879800

2101000405 Rio Guamani to Rio Jacaboa Watersheds 56843.9619 19.837519 13.775645

2101000203 Cano Tiburones Coastal Watershed 77187.4971 19.240298 8.212524

2101000501

Rio Maunabo to Rio Humacao 

Watersheds 71468.9511 17.143250 6.319603

2101000201 Rio Cibuco Watershed 102488.2642 16.725546 6.628091

2101000402 Rio Yauco to Rio Tallaboa Watersheds 89651.5771 14.211651 8.889758

2101000404 Rio Coamo to Rio Seco Watersheds 135415.6314 14.124723 9.304299

2101000504 Rio Grande de Loiza Watershed 190999.5868 13.415014 2.870285

2101000502

Rio Anton Ruiz to Rio Fajardo 

Watersheds 78268.1507 13.384683 1.862429

2101000507 Rio de la Plata Watershed 158887.8815 13.150088 5.046601

2101000403

Rio Matilde to Rio Descalabrado 

Watersheds 154177.5865 11.365773 5.813997

2101000506

Rio de Bayamon to Rio Hondo 

Watersheds 59249.2104 8.766588 2.838660

2101000503

Rio Herrera to Las Cabezas de San Juan 

Coastal Watersheds 66624.7556 8.324192 1.989393

2101000303 Rio Yaguez Watershed 10661.5805 7.332743 3.859651

2101000603 Isla de Mona 13896.8083 7.331300 3.858100

2101000505 San Juan Bay Estuary Watershed 57048.0700 4.648563 2.069634

2101000601 Isla de Vieques 31226.5047 4.056300 1.665000

2101000602 Isla de Culebra 5241.7498 0.000000 0.000000

PCA1 = 100*Total Cropland/ Land Area

PCA2 = 100*(Total Cropland - Pasture/Grazing)/Land Area

15
Hawaii – Modeling and Cropping

• No existing scenarios

• Use FIRST or Surrogates with HI Weather

Category Annual Total in acres

1987 1992 1997 2002 2007

Total 

Cropland

327, 398 293, 371 292, 107 211, 120 177, 626

Harvested 

Cropland

152, 719 136, 431 100, 094 109, 461 103, 120

Pineapples 22, 262 15, 500 12, 992 10, 211 Withheld

Vegetables 

Harvested

5, 587 5, 129 6, 549 6, 554 6,845

Land in 

Orchards

33, 564 38, 590 37, 906 34, 908 36, 6521

12007 Total Macadamia nut acreage was 17, 628 acres

Source: 

http://www.agcensus.usda.gov/Publications/2007/Full_Report/Volume_1,_Chapter_1_State_Level/Hawaii/st15_1_001_001.

pdf

16
Hawaii - PCA

• PCA of 100% due to lack of an established value

• Watershed Boundaries are available

• Census of Agriculture data available for Counties (Islands)

Hawaii - 2007 Census of Agriculture Summary (County Summary)

Hawaii Honolulu Kauai Maui

Total Cropland (ac) 81837 18896 22336 54557

(percent of county) 3.2% 4.8% 5.5% 7.3%

Harvested 56310 9518 10764 26528

Used for pasture or grazing 17072 (D) 2366 (D)

Used for cover crops, legumes, and 

soil-improvement grasses, not 

harvested and not pastured 6696 (D) 8990 (D)

On which all crops failed 1249 (D) 53 (D)

Summer Fallow 510 (D) 163 (D)

County Total Area 2580175 394004 406084 751912

Pineapples harvested (acres) 19 (D) 16 (D)

Pineapples not harvested (acres) 17 (D) (D) (D)

Ginger (acres), 80 acres total 51 (D) (D) -

(D) Data witheld to protect individual farmer data

Pineapple - 2002 state total 10,211 ac, 2007 not reported

Category Crop Acreage

17

Hawaii – Drinking Water Supplies

• Hawaii has 128 Public 

Water Supplies, 10 use 

Surface Water as their 

source

PWSID PWSName PCounty

HI0000130 SOUTH KOHALA Hawaii

HI0000348 WAIAWA CORRECTIONAL FACIL Honolulu

HI0000320 MILILANI MEMORIAL PARK Honolulu

HI0000231 MAUNALOA-KALUAKOI Honolulu

HI0000400 LIHUE-KAPAA Kauai

HI0000255 KULA NANI Maui

HI0000247 LOWER KULA Maui

HI0000215 UPPER KULA Maui

HI0000214 LAHAINA Maui

HI0000212 WAILUKU Maui

19
State/Local Source Water Assessment 

Information

• Specific PCA calculations difficult (Modern Land-use data limited 

and specific intake locations PCA determination not within current 

USEPA Methodology)

• Pragmatic review of local information and target crops reveal PCA 

resulting in a failing assessment not realistic

• Example Descriptions:

• “The upstream watersheds for the sources lies within a Forest Preserve 

which is non-agricultural ”

• “To further complicate the network, water can be shared from all 3 

WTFs, Several Reservoirs were identified but individual sources were 

not, however the combining of the Maui hydrology reports reveal that 

nearly all of the upstream land from the intakes is in conservation and 

non agricultural uses. “

• “The stream is in government owned non agriculture land.”

• “Map 13 (Water Resource Associates 2004). The ALUM-Maui map 

(ALUM 1980) does not appear to show any agriculture in the upstream 

watershed of the Iao stream. “ 

20
Drinking Water Assessment Tiered 

Process

• Run all EPA scenarios 

applicable to product

• Adjust using National PCA 

(highest predicted scenario)

• Adjust using Regional PCA 

(regionally assigned 

scenario)

• In regions with any uses 

exceeding level of concern, 

explore refined approach

21
22

Applying PCAs in Tier 2 Assessments
PCA Calculation

9/15/2011 23
Crop PCA

Variable Region 8 Region 11 Region 12

NASS 2007 Total Cropland 87.871 76.304 69.283

EPA Value (1997 Data) 85.000 80.000 67.000

Maximum for crops may occur in different watersheds

Variable Region 8 Region 11 Region 12

COTTON 22.669 9.950 30.713

PEANUT 0.085 1.197 2.200

TOBACCO 0.332 0.000 0.000

POTATO 0.001 0.037 0.292

SWEET POTATO 0.976 0.005 0.096

ONION 0.000 0.041 0.368

CARROT 0.000 0.000 0.142

CELERY 0.000 0.000 0.000

EGGPLANT 0.003 0.000 0.000

PEPPER 0.001 0.005 0.068

TOMATO 0.039 0.003 0.024

CUCURBIT 0.110 0.111 0.506

NON-BEARING FRUIT 0.251 1.254 1.247

REGIONAL PCA

Maximum PCA Individual Crops in each Region.

24
Regional Refinement

25

0

0.002

0.004

0.006

0.008

0.01

0.012 01 01 1961 01 30 1961 02 28 1961 03 29 1961 04 27 1961 05 26 1961 06 24 1961 07 23 1961 08 21 1961 09 19 1961 10 18 1961 11 16 1961 12 15 1961 01 13 1962 02 11 1962 03 12 1962 04 10 1962 05 09 1962 06 07 1962 07 06 1962 08 04 1962 09 02 1962 10 01 1962 10 30 1962 11 28 1962 12 27 1962 01 25 1963 02 23 1963 03 24 1963 04 22 1963 05 21 1963 06 19 1963 07 18 1963 08 16 1963 09 14 1963 10 13 1963 11 11 1963 12 10 1963 01 08 1964 02 06 1964 03 06 1964 04 04 1964 05 03 1964 06 01 1964 06 30 1964 07 29 1964 08 27 1964 09 25 1964 10 24 1964 11 22 1964 12 21 1964 01 19 1965 02 17 1965 03 18 1965 04 16 1965

SCENARIO:REGION08

COTT_MSA

CUKE_MOE

EGGPSTXA

GRPF_TXE

PEPO_TXB

PEAN_NCE

POTA_FLA

SWPO_FLE

TOBB_NCE

TOM_STXA

Cotton as calculated from data. Other crops, maximum of 3% or 5*Actual (Rounded up to single percentage)

Region 8 Region 11 Region 12 Region 8 Region 11 Region 12

COTTON 23 10 31 MS Cotton STD TX Cotton OP STX Cotton NMC

PEANUT 3 6 11 NC Peanut STD NC Peanut STD NC Peanut STD

TOBACCO 3 0 0 NC Tobacco STD NA NA

POTATO 3 3 3 FL Potato NMC FL Potato NMC FL Potato NMC

SWEET POTATO 5 3 3 FL Potato NMC FL Potato NMC FL Potato NMC

ONION 0 3 3 NA GA Onion STD GA Onion STD

CARROT 0 3 3 NA FL Carrot STD FL Carrot STD

CELERY 0 0 0 NA NA NA

EGGPLANT 3 3 3 STX Veget NMC STX Veget NMC STX Veget NMC

PEPPER 3 3 3 STX Veget NMC STX Veget NMC STX Veget NMC

TOMATO 3 0* 3 STX Veget NMC NA STX Veget NMC

CUCURBIT 3 3 3 MO Melon STD MO Melon STD STX Melon NMC

NON-BEARING FRUIT 3 7 7 STX Grapferuit NMC Orchard BSS Orchard BSS

Total PCA 52 44 73 Sum of Individual Crop Values

Weighting for Regional Assessment

ASSIGNED SCENARIO CROP/Scenario CROP PCA
• A process to combine localized knowledge of drinking 

water supplies and labeled crops should enable the 

development of conservative yet refined PCA values

• Use actual watersheds used for drinking water rather than 

surrogate HUC-8 watersheds

• Use publically available source information

• NHDPlus catchments can be used to hide specific LAT/LON of 

intakes while providing needed watersheds to produce refined 

PCAs 

• Use spatially distributed cropping data rather than “All 

Agriculture” at a county scale

• Additional scenarios/weather stations to address 

regional concerns

• Represent distribution and timing of applications where 

diverse land use exists in watersheds

26

Options for Tier 3
• USEPA Assessment utilized county level NASS 

Census of Agriculture from 1997 (or 1992 if 

missing)

• Used “Cropped Area” for default and “Corn”, 

“Soybean”, “Cotton”, or “Wheat” for major crops

• Distributed county acreage to HUC-8 

Watersheds

• Did not utilize minor crop data 

• Cropland Data Layer (CDL) now seamless for 

Continental US

27

Landuse
NASS CDL Data

28
• CDL offers a complete 

spatial view at a 56m 

resolution • CDL has 130 classes, 

including some from the 

NLCD

• Census of Agriculture is 

summarized at a county 

resolution • Includes only agricultural 

areas

• Includes >200 crop 

categories

• A comparison of the 2 

NASS sources is needed 

to identify groupings 

needed for detailed PCA 

calculations

29

CDL versus Census 

of Agriculture
• County review of CDL versus 

Census of Agriculture

• Neither dataset is perfect 

(omissions, border effects, 

incorrect categorizations, 

under/over reporting, crop 

damage)

30

CDL versus Census of Agriculture

NLCD - Barren Barley 

NLCD - Deciduous Forest Blueberry 

NLCD - Developed/High Intensity Camelina 

NLCD - Developed/Low Intensity Canola 

NLCD - Developed/Medium Intensit Celery 

NLCD - Developed/Open Space Citrus 

NLCD - Evergreen Forest Dry Beans 

NLCD - Grassland Herbaceous Flaxseed 

NLCD - Herbaceous Wetlands Herbs 

NLCD - Mixed Forest Lentils 

NLCD - Open Water Millet 

NLCD - Shrubland Misc. Vegs. & Fruits 

NLCD - Woody Wetlands Oats 

Wetlands Oranges 

Woodland Other Crops 

Corn Other Small Grains 

Cotton Other Tree Fruits 

Durum Wheat Peanuts 

Soybeans Peas 

Spring Wheat Potatoes 

Sweet Corn Rice 

Winter Wheat Rye 

Seed/Sod Grass 

Sorghum 

Strawberries 

Sugarbeets 

Sugarcane 

Sunflowers 

Watermelon 

Alfalfa 

Clover/Wildflowers 

Fallow/Idle Cropland 

NLCD - Pasture/Hay 

Other Hays 

Pasture/Grass 

Florida CDL Groupings 

Pasture/Hay Minor Crop

Non-Ag Major Crops

Category Census of Ag CDL CDL % of Census

Non-Agriculture NA 9,506,731.12

Total Cropland 3,796,669.01 3,673,464.21 96.8%

 -Major Crops 4,733.14 4,995.15 105.5%

 -Forage and Pasture 2,453,258.04 2,434,969.93 99.3%

 -Minor Crops 1,338,677.83 1,233,499.13 92.1%

Florida (Counties with Drinking Water Intakes)
31

Watershed Assessment

• Summarize CDL for each 

Watershed

• Accuracy assessment of 

Counties

• Conservative CDL groupings 

based on County Census of 

Agriculture

• Determine PCA for relevant 

crop groups

• Use as basis for scenario 

creation (intersect with soils) or 

watershed modeling
Conclusions

• Within framework of Tier 2, options exist for 

improving the representation of crop protection 

products 

• Label language accurately represented

• If multi-cropping is typical in a region, scenarios should 

be created to represent cropping practices

• Refinements to regional PCA values for minor crops 

following methodology like mixed major crops

32
• USEPA PCA methodologies can be applied to Puerto 

Rico

• Hawaii is an example where an analysis of limited 

surface water source assessments can be used

• Tier 3 analysis can add refinements not possible in 

currently established guidance

• Refined use pattern representation (and real drinking water 

watershed PCA or modeling)

• Additional soil, weather, environmental fate representation 

(spatial modeling)

• Probabilistic approaches for key parameters

• Use of focused monitoring data

33

Conclusions