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Revisiting the Chesapeake Bay Water Quality Criteria

Biological Reference Curves

Briefing for the CBP Scientific and Technical Advisory Committee Review Team

Jeni KeismanUniversity of Maryland Center for Environmental

Science/Chesapeake Bay Program OfficeAnnapolis, Maryland

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BackgroundWhat is a reference curve?

A bioreference curve is a cumulative frequency distribution that is used to determine “unacceptable”exceedance of the criteria.

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Percent of Space

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CFD Curve

Area of Criteria Exceedence

Area of AllowableCriteria

Exceedence

Source: U.S. EPA 2003

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Goal: to identify the amount of spatial and temporal criteria exceedance that can occur without causing significant ecological degradation

• Identify an indicator of biological health.

• Identify “healthy” measurements of this indicator, for use as a “reference”.

• Calculate exceedance rates of the target criteria that correspond, in space and time, with these “healthy” reference measurements. These are your “acceptable exceedances.”

• Formulate a CFD from these “acceptable exceedance” rates. This is your “bioreference curve.”

• Compare exceedance rates for each segment-du-assessment period to this bioreference curve, to determine whether exceedances extendbeyond the “allowable” threshold (represented by the bioreference curve).

BackgroundThe biologically-based reference curve

Source: U.S. EPA 2003

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Benthic community

BackgroundThe Chesapeake Bay benthic index of biotic integrity (B-IBI)

CBP Segment Year

APPTF 2004

BOHOH 2004

CB1TF 2004

CB2OH 2002

CB3MH 1995

CB6PH 1993

CB8PH 1996

FSBMH 2005

NANMH 1987

RPPMH 1988

RPPTF 1996

WICMH 1999

… …

• Minimum B-IBI score ≥ 3.0

• No sample size requirement

• Time period is single year

Source: Weisberg et al. 1997

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BackgroundAcceptable exceedances form the bioreference curve

CBP Segment Year

APPTF 2004

BOHOH 2004

CB1TF 2004

CB2OH 2002

CB3MH 1995

CB6PH 1993

CB8PH 1996

FSBMH 2005

NANMH 1987

RPPMH 1988

RPPTF 1996

WICMH 1999

… …

CBSEG Year Month Violation Rate

CB3MH 1987 6 0.2522068

CB3MH 1987 7 0.208657

CB3MH 1987 8 0.1582181

CB3MH 1987 9 0.0461538

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CB3MH 1987

Source: U.S. EPA 2003

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CB4MH 1998 6 0.5882037

CB4MH 1998 7 0.5375158

CB4MH 1998 8 0.399928

CB4MH 1998 9 0.4234667

CBSEG Year MonthViolation

Rate

CB3MH 1987 6 0.2522068

CB3MH 1987 7 0.208657

CB3MH 1987 8 0.1582181

CB3MH 1987 9 0.0461538

CB6PH 2001 6 0

CB6PH 2001 7 0

CB6PH 2001 8 0.0204819

CB6PH 2001 9 0

Curve of all acceptable violation rates

BackgroundAll acceptable violation rates combined

Source: U.S. EPA 2003

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BackgroundAssessment curves evaluated at discrete points

bioreference curve, continuous

YRKPH 1995

bioreference curve, discrete

YRKPH 1995

Source: U.S. EPA 2007

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BackgroundConcerns raised by reviewers

• By combining violation rates from all healthy areas into one bioreference curve, we create a curve that, theoretically, will represent approximately the median of all curves included.

• Thus approximately half of all segment-years used to create the bioreference curve may fail an assessment conducted with that bioreference curve.

• Issue was raised verbally in discussions around the time of the STAC CFD Review (c. 2005-2006)

• We confirmed this concern.

Source: Secor et al. 2006

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CB4MH 1998 6 0.5882037

CB4MH 1998 7 0.5375158

CB4MH 1998 8 0.399928

CB4MH 1998 9 0.4234667

CBSEG Year MonthViolation

Rate

CB3MH 1987 6 0.2522068

CB3MH 1987 7 0.208657

CB3MH 1987 8 0.1582181

CB3MH 1987 9 0.0461538

CB6PH 2001 6 0

CB6PH 2001 7 0

CB6PH 2001 8 0.0204819

CB6PH 2001 9 0

Curve of all acceptable violation rates

CB3MH 1987

Bioreference Curve ReviewSome acceptable exceedances fail their own bioreference curve

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For deep water, 73% of segment-years with “acceptable” D.O. violation rates fail existing the deep water bioreference curve

Some acceptable exceedances fail their own bioreference curve

Bioreference Curve Review

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Percentage of segment-years failing the bioreference curve

Designated use1

Healthy BIBI

Degraded BIBI

DC 67% 98%

DW 73% 92%

OW 52% 47%

Designated use

Healthy segs failing

Degraded segs passing

DC 67% 2%

DW 73% 8%

OW 52% 53%

Percent error

Bioreference Curve ReviewAre the errors “balanced”?

1. DC – deep channel; DW – deep water; OW – open water.

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CB3MH1996

CB3MH1998

CB3MH2000

CB3MH2005

CB3MH2006

CB4MH1998

CB4MH2003

CB5MH1999

CB5MH2001

CB5MH2002

CB5MH2004

CB5MH2006

CB6PH1995

CB6PH1998

CB6PH2000

CB7PH1995

CB7PH1999

CB7PH2006

CHSMH2001

YRKPH2002

CB3MH2002

CB4MH1995

CB4MH1996

CB4MH2000

CB4MH2001

CB6PH2002

POTMH1995

POTMH1998

POTMH2000

POTMH2001

RPPMH2000

RPPMH2002

RPPMH2005

RPPMH2006

YRKPH1996

YRKPH2001

YRKPH2005

“Cloudplot” of deep water reference communities

Bioreference Curve ReviewAre we identifying distinct “healthy” and “degraded”

reference communities?

Answer: no.

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Bioreference Curve ReviewCurrent Method Suggested Revisions

1. Obtain dataset of all Benthic Index of Biotic Integrity (B-IBI) scores for time period 1985-2006

Restrict dataset to 1996-2006 time period; for fixed station samples use grand score only

2. For relevant segments (those with Deep Water (DW) and Deep Channel (DC) DUs), match benthic stations and scores in dataset with monthly open water, deep water, and deep channel designated use boundaries. Boundaries are derived using the standardized, automated method for identifying pycnocline boundaries documented in EPA CBP’s 2008 Technical Support for Criteria Assessment Addendum. Pycnocline boundaries are then interpolated using the CBP interpolator. Interpolator cells are matched with benthic station locations, and interpolated pycnocline boundaries are applied to each benthic station location.

None

3. Benthic stations (and their associated B-IBI scores) are assigned to a DU: OW, DW, or DC. None

4. To define the biological reference community for each designated use, all segment-years for which the minimum B-IBI was ≥ 3.0 are identified

a. Use 3-year rolling time periods rather than single years. This brings the reference community ID method in better alignment with the DO criteria assessment method for which reference communities are being identified.

b. Require a B-IBI score sample size >= 10. This improves the spatial representation of the B-IBI score

c. “Healthy” reference communities are those with an average B-IBI score ≥ 3.0, standard deviation (SD) < 1.0, rather than a minimum. Using the average is consistent with methods used by benthic experts to assess benthic community impairment.

5. For the segment-years identified in step #4, the monthly (in the case of OW and DW) or instantaneous (DC) violation rates are obtained.

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6. These violation rates (e.g. percentage of a segment-DU's volume failing the DO criteria in a given month; thus 4 measures per summer for OW and DW – June thru Sept) are used to define “acceptable” exceedances of the dissolved oxygen criteria, based on the logic that if a healthy benthic community existed in the segment-du in that summer, then the degree of DO criteria violation that occurred did not lead to an impaired benthic community.

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Proposed revisions to methodology

Sources: U.S. EPA 2003, 2007; Weisberg et al. 1997

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Bioreference Curve ReviewProposed revisions to methodology: deep water

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CB6PH_1996_1998CB7PH_1996_1998CB6PH_1997_1999CB7PH_1997_1999CB7PH_1998_2000CB6PH_1999_2001CB7PH_1999_2001CB7PH_2000_2002CB6PH_2004_2006CB7PH_2004_2006PAXMH19961998POTMH19961998PAXMH19971999POTMH19971999POTMH19982000PAXMH19992001POTMH19992001PAXMH20002002RPPMH20002002PAXMH20022004PAXMH20032005PAXMH20042006RPPMH20042006

Cloudplot: new “deep water” reference communities

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“100th percentile” DW bioreference curve

• Adopt revised methodology and apply revised deep water bioreference curve.

Rationale

Bioreference Curve ReviewProposed revisions to methodology: deep water (DW)

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• Revised curve is derived from the 100th percentile of healthy D.O. violation rates at each time point.

• Resultant error rate (incorrect classification of a reference community): zero.

Recommendation to CBP WQ Steering Committee:

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Current methodology

New curve example presented at April 2009 WQSC mtg.

Recommended deep water curve

Deep water bioreference curve comparison

Bioreference Curve ReviewProposed revisions to methodology: deep water

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Bioreference Curve ReviewProposed revisions to methodology: open water (OW)

Cloudplot:New “open water” reference communities based on proposed revised methodology

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Conclusion:• Benthic communities do

not provide an appropriate biological reference for violations of the “open water” D.O. criteria.

• Consistent with findings reported in U.S. EPA 2003.

Recommendation to CBP WQ Steering Committee: • Apply the default

bioreference curve for assessing the OW D.O. criteria.

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Curve percentile

good fail

bad pass

0.10 87% 14%

0.20 84% 18%

0.30 76% 21%

0.40 73% 24%

0.50 67% 34%

0.60 62% 41%

0.70 59% 48%

0.77 51% 50%

0.78 49% 50%

0.79 46% 50%

0.80 42% 50%

0.90 25% 58%

1.00 0% 87%

Bioreference Curve ReviewProposed revisions to methodology

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Currently published curve

Revised methodology: 78th percentile

Default 10% curve

Open Water bioreference curve

Sources: U.S. EPA 2003, 2007

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Bioreference Curve ReviewProposed revisions to methodology: deep channel

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Cloudplot:Preliminary “deep channel” reference

communities presented May 2009• May 2009 preliminary

results presented to the CBP Water Quality Steering Committee contained a calculation mistake.

• Corrected analysis yielded ZERO “healthy”deep channel reference communities in the two decade data record.

Findings:

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Current methodology

“Preliminary” curve presented May 2009

Default 10% curve

• Current conditions in the Chesapeake Bay deep channel habitat are not sufficient for identifying “acceptable” rates of D.O. violation.

Conclusion:

Bioreference Curve ReviewProposed revisions to methodology: deep channel

Deep channel bioreference curve

• Until a time when “healthy”reference benthic communities can be identified on the relevant spatiotemporal scalein the deep channel, apply the default 10% curve for assessing D.O. criteria violation in the deep channel designated use.

Recommendation to CBP WQ Steering Committee:

Sources: U.S. EPA 2003, 2007

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Bioreference Curve ReviewImplications of proposed changes: average percent volume failing

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Deep Water Summer

Deep Channel Summer

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Bioreference Curve ReviewImplications of proposed changes: average percent volume failing

Average Volume Failing, All Summer DUs Combined

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Current Status

At its June 22, 2009 conference call, the Chesapeake Bay Program’s Water Quality Steering Committee approved application for the recommended open-water, deep-water and deep-channel reference curves pending the recommendations from the CBP Scientific and Technical Advisory Committee’s sponsored peer review.

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Sources

Secor, D., M. Christman, F. Curriero, D. Jasinski, E. Perry, S. Preston, K. Reckhow and M. Trice. 2006. The Cumulative Frequency Diagram Method for Determining Water Quality Attainment-Report of the Chesapeake Bay Program STAC Panel to Review of Chesapeake Bay Program Analytical Tools STAC Publication 06-9 October 2006. Chesapeake Research Consortium, Edgewater, MD

U.S. Environmental Protection Agency. 2003. Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries. EPA 903-R-03-002. Region III Chesapeake Bay Program Office, Annapolis, Maryland.http://www.chesapeakebay.net/publication.aspx?publicationid=13142

U.S. Environmental Protection Agency. 2007. Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries–2007 Addendum. EPA 903-R-07-003. CBP/TRS 285-07. Region III Chesapeake Bay Program Office, Annapolis, Maryland.http://www.chesapeakebay.net/content/publications/cbp_27849.pdf

Weisberg, S. B., J. A. Ranasinghe, D. M. Dauer, L. C. Schaffner, R. J. Diaz and J. B. Frithsen. 1997. An estuarine benthic index of biotic integrity (B-IBI) for Chesapeake Bay. Estuaries 20:149-158.

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Bioreference Curve ReviewQuestions and comments

Jeni Keisman

University of Maryland Center for Environmental Science/ Chesapeake Bay Program Office

Annapolis Maryland

jkeisman@chesapeakebay.net

410-295-1321