Drinking Water Intake PCA 

Project Update

Briefing for the EMPM

March 19, 2013

James N. Carleton, PhD

Rochelle F. H. Bohaty, PhD

Tanja Crk

Marietta Echeverria, Management Lead

Mohamed Ruhman, PhD

Shelly Thawley

Nelson Thurman, Senior Science Advisor

Phil Villanueva

Katrina White, PhD

1
PCA History

• Following passage of FQPA (1996), EFED developed two modifications of 

existing aquatic exposure modeling methodologies to apply them to 

surface drinking water sources:

– Index Reservoir

– Percent Cropped Areas (PCA) adjustment factors

• First PCAs were based on 8-digit cataloguing units (HUC-8) as surrogates 

for watersheds

– PCA-calculation methodology vetted through a 1999 Scientific Advisory Panel 

(SAP), and use of PCAs subsequently adopted into routine practice by EFED

– In early 2012, WQTT finalized update of PCAs based on more current crop and 

land use data. Included PCAs for new crop groups and turf.

• The current effort updates PCAs using Community Water System (CWS) 

Drinking Water Intake (DWI) watersheds, rather than HUCs

2
DWI-PCA Project History

• Quality-Assured DWI location dataset developed by USGS 

in 2007 under IAG with OPP

– Based on data from then-current SDWIS supplemented with 

data obtained directly from States

• DWIs indexed to NHD, and watersheds subsequently 

delineated by contractors

• Watersheds Quality-Assured by USGS in 2011

– 4,840 “pass” the QA check, out of 6,432 DWIs in the 48 

conterminous states

• PCAs developed by EFED in 2012 for 4,840 QA’d 

watersheds, for:

– 8 crops

– 9 crop combinations

– All-Agriculture

3
DWI Watersheds

4
DWI Locations

5
How DWI watersheds differ from HUCs:

‘nested’, and range widely in size

• Median HUC8 area = 1315 mi2

• 4,840 QA’d watersheds:

– 10th percentile = 1 mi2

– 50th percentile = 44 mi2

– 95th percentile = 27,887 mi2

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0.00

0.25

0.50

0.75

1.00

0.001 0.100 10.000 1,000.000 100,000.000

Rel. Frequency

Watershed Area (sq. mi.)
Higher PCAs Are Associated with 

Smaller Watersheds

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– 10th percentile = 150 people

– 50th percentile = 4,610 people

– 95th percentile = 120,000 people

8

Apparent distribution of CWS populationserved is also highly skewed

0.0

0.2

0.4

0.6

0.8

1.0

1 10 100 1,000 10,000 100,000 1,000,000 10,000,000

Relative Frequency

Population Served
0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Percent Cropped Area

HUC-2 Region

Results: Regional Maximum All-Ag PCAs

New (DWI-watershed based) Current (HUC-8 based)

9
0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Percent Cropped Area

Results: National Maximum PCAs for Various Crops

New (DWI-watershed based) Current (HUC-8 based)

10
0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Percent Cropped Area

Results: National Maximum PCAs for Various Crops

New (DWI-watershed & HUC-12 surrogate-based) Current (HUC-8 based)

11
12

Comparisons with Monitoring Data: 

Max. Concentrations in CWS Sources 

(USGS)

0.001

0.01

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100 1000 10000

Max measured conc.(ppb)

national maximum PCAs

Modeled ‘peak’ conc. (ppb)

0.001

0.01

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100 1000 10000

Max measured conc.(ppb)

regional maximum PCAs

Modeled ‘peak’ conc. (ppb)
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Comparisons with Monitoring Data: 

Max. Concentrations in Surface Water 

(NAWQA)

0.001

0.01

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100 1000 10000

Max measured conc.(ppb)

national maximum PCAs

Modeled ‘peak’ conc. (ppb)

0.001

0.01

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100 1000 10000

Max measured conc.(ppb)

Modeled ‘peak’ conc. (ppb)

regional use-appropriate PCAs
Current Status

• Draft white paper developed, documenting DWI 

watershed and PCA-development technical 

details.

• Targeted peer review of white paper completed;

comments received from all reviewers.

• Responses to reviewer comments under 

development

• Opportunity for public comment will be 

forthcoming

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Why DWI-PCAs Represent an 

Improvement Over HUC8-PCAs

• Represent uses within actual drinking water 

watersheds

– This is in accordance with the recommendations of 

the original 1999 SAP.

– DWI-PCAs focus our attention on areas where use 

may impact drinking water.

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