# 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

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# 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

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# 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

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# DWI Watersheds

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# DWI Locations

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# How DWI watersheds differ from HUCs :

‘ nested ’, and range widely in size

• Median HUC8 area = 1315 mi 2



## • 4,840 QA’d watersheds:

– 10 th 

## percentile = 1 mi 2



## – 50 th 

## percentile = 44 mi 2



## – 95 th 

## percentile = 27,887 mi 2

6 0.00 0.25 0.50 0.75 1.00

0.001 0.100 10.000 1,000.000 100,000.000Rel. Frequency

Watershed Area (sq. mi.)

# Higher PCAs Are Associated with Smaller Watersheds

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## – 10 th 

## percentile = 150 people

– 50 th 

## percentile = 4,610 people

– 95 th 

## percentile = 120,000 people

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# Apparent distribution of CWS population

served 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 10,000,000Relative Frequency



### Population Served0% 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 18Percent Cropped Area



### HUC

- 2 Region 

## Results: Regional Maximum All

- Ag PCAs

New (DWI

- watershed based) Current (HUC

- 8 based)

90% 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)

100% 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)

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# 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 PCAsModeled ‘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 PCAsModeled ‘peak’ conc. (ppb)13 

# 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 PCAsModeled ‘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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