SETTING AND ALLOCATING 

THE CHESAPEAKE BAY 

BASIN NUTRIENT AND SEDIMENT LOADS

The Collaborative Process, Technical Tools 

and Innovative Approaches

PRINCIPAL AUTHORS

Robert Koroncai

U.S. Environmental Protection Agency

Region III

Philadelphia, PA

Lewis Linker

U.S. Environmental Protection Agency

Chesapeake Bay Program Office

Annapolis, Maryland

Jeff Sweeney

University of Maryland

Chesapeake Bay Program

Annapolis, Maryland

Richard Batiuk

U.S. Environmental Protection Agency

Chesapeake Bay Program Office

Annapolis, Maryland

U.S. Environmental Protection Agency

Region III

Chesapeake Bay Program Office

Annapolis, Maryland

DECEMBER 2003
For the past twenty years, the Chesapeake Bay Program partners have been committed

to achieving and maintaining water quality conditions necessary to support living

resources throughout the Chesapeake Bay ecosystem. The 1983 Chesapeake Bay

Agreement set the stage for the collaborative multi-state and federal partnership, and

the 1987 Chesapeake Bay Agreement set the first quantitative nutrient reduction goals

(Chesapeake Executive Council 1983, 1987). With the signing of the Chesapeake

2000 agreement (Chesapeake Executive Council 2000), the Chesapeake Bay Program

partners committed to:

Defining the water quality conditions necessary to protect aquatic living

resources and then assigning load reductions for nitrogen and

phosphorus to each major tributary; and

Using a process parallel to that established for nutrients, determining the

sediment load reductions necessary to achieve the water quality conditions that protect aquatic living resources, and assigning load

reductions for sediment to each major tributary.

Through a six-state memorandum of understanding, the headwater states of

Delaware, West Virginia and New York joined Maryland, Virginia, Pennsylvania, the

District of Columbia, the U.S. Environmental Protection Agency (EPA) and the

Chesapeake Bay Commission in committing to restore Chesapeake Bay and river

water quality through the adoption of new cap load allocations for nitrogen, phosphorus and sediment (Chesapeake Bay Watershed Partners 2001). All the watershed

partners understood that these allocations represented loading caps that must be

achieved and maintained, even in the face of increasing anthropogenic activities in

the watershed. 

Using the best scientific information available, Chesapeake Bay Program partners

have agreed to nutrient and sediment cap loading allocations. On March 21, 2003

and April 15, 2003, the Chesapeake Bay Program Principals’ Staff Committee and

representatives of the headwater states convened to adopt the nutrient and sediment

cap load allocations and submerged aquatic vegetation (SAV) restoration goals for

the Chesapeake Bay (Appendix A). The cap loads, allocated by major tributary basin

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chapter i • Background

chapteri

Background
and by state jurisdiction, will serve as a basis for each state’s tributary strategies that,

when completed by April 2004, will describe local implementation actions necessary

to meet the Chesapeake 2000 nutrient and sediment cap load allocations by 2010. 

This document describes the scientific and technical information and policy agreements that formed the basis for the important, comprehensive agreements that the

Chesapeake Bay Program partners made with regard to cap load allocations for

nitrogen, phosphorus and sediments, as well as new baywide and local SAV restoration goals. The assessment tools and techniques evolved significantly over the

allocation decision-making process, therefore, it should be noted that this document

is based on the most recent information and procedures used in support of the cap

load allocation decisions that were made.

FUNDAMENTALS OF DEVELOPING 

CAP LOAD ALLOCATIONS

Cap load allocations can be defined as cumulative pollutant loadings for all point and

non-point sources established and assigned to different tributary basins within a larger

watershed that, when achieved, will allow the receiving water body to attain the

prescribed water quality goals. With the accelerated development of total maximum

daily loads (TMDLs) over the recent years, the development of loading caps has

become commonplace, but the size and complexity of the Chesapeake Bay watershed

has made allocation of the nutrient and sediment cap loads similarly complex. 

Typically, water quality goals are prescribed in state water quality standards.

However, current state water quality standards addressing nutrient- and sedimentrelated impairments for the Chesapeake Bay and its tidal tributaries, which are based

on national criteria first published in the 1960s for freshwater systems, only address

dissolved oxygen. For this reason, the EPA, in direct consultation with the watershed

states, developed comprehensive, Chesapeake Bay regional water quality criteria for

dissolved oxygen, chlorophyll a and clarity, along with SAV restoration goals for

each segment of the Chesapeake Bay and its tidal tributaries (U.S. EPA 2003a,

2003b). While at the time of publication of this document these criteria had not yet

been adopted into state water quality standards, they were used as the water quality

basis for setting and allocating the nutrient and sediment cap loads for the Chesapeake Bay watershed. 

To determine the appropriate cap loads and allocate them to individual tributary

basins, the pollutant sources must be related to impacts on water quality. It is important to quantify the loadings from all significant sources and to track the fate and

transport of those pollutants from the source to the Bay’s tidal waters. In the case of

nutrients and sediments, the fate and transport mechanisms can be quite complex. 

A complementary suite of models was employed to simulate the sources, transport,

fate and ultimate impact on tidal Bay water quality conditions of nutrient and

sediment loads. The airshed model was used to track air sources from the 350,000-

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chapter i • Background
square-mile Chesapeake Bay airshed and the transport and deposition of atmospheric nitrogen loads to the Chesapeake Bay watershed and directly to tidal surface

waters. The watershed model tracked all sources—point, non-point and air deposition—within the watershed and simulated the fate of those pollutants as they were

transported through the free-flowing river systems of the watershed and delivered to

the tidal Bay waters. The water quality model, which is actually a compilation of

several models, then simulated the water quality impacts of those pollutants on the

water quality of the Chesapeake Bay and its tidal tributaries.

Knowing the water quality goals through the water quality criteria as applied within

the refined tidal-water designated uses and the reduced pollutant loading effects on

water quality through the models, it was possible to develop defensible, equitable cap

load allocations. However, good science was not enough to derive the cap load allocations. It was also important to blend the scientific understanding with policy input

to derive cap load allocations that not only could achieve the stated water quality

goals but also could gain considerable support from local stakeholders ultimately

responsible for taking the actions necessary to reduce nutrient and sediment loadings.

Policy input to setting the cap load allocations was most important in determining an

appropriate distribution of the allowable pollutant loads by major tributary basin and

jurisdiction. ‘Fair and equitable’ were the basic principles used by the Chesapeake

Bay Program partners in allocating the cap loads. Such subjective qualities do not

readily lend themselves to technically based solutions without significant policy

direction on how to achieve this desired result. Once the policy direction was established on distributing the cap loads ‘fairly and equitably’, a technical construct

supporting these policy principles was developed.

KEY PLAYERS IN DEVELOPING THE ALLOCATIONS

The Chesapeake Bay Program carries out its restoration and protection functions

through an extensive committee structure led by the original Chesapeake Bay agreement signatories of Pennsylvania, Maryland, Virginia, the District of Columbia, the

EPA and the Chesapeake Bay Commission (Figure I-1). For the development of

water quality criteria, refined tidal-water designated use and cap load allocations,

more than 500 individuals representing state, federal, regional and local government

agencies, academic institutions, businesses, conservation organizations, community

watershed organizations, many other nongovernmental organizations, as well as the

headwater states of New York, West Virginia and Delaware joined as full partners.

Point source representatives and environmental groups also were well-represented

on the committees during this effort. An overview of the roles of each group and the

interplay between groups is described below and illustrated in figures I-1 and I-2. 

A brief review of the primary groups is provided below.

WATER QUALITY TECHNICAL WORKGROUP

This workgroup consisted of technical staff and mid-level managers from the states,

the EPA and stakeholders from point source and environmental group interests and

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chapter i • Background
was expanded to include many of the modeling experts. Based on general policy

direction given by the Water Quality Steering Committee, the Water Quality Technical Workgroup assessed modeling results, explored options, developed the

allocation methodology and made recommendations to the Water Quality Steering

Committee on technical issues with regard to the allocations. The workgroup’s

efforts were supported by several other groups:

• The Modeling Subcommittee maintained and updated the watershed and water

quality models used in the allocation effort and provided for all modeling

analyses, including an assessment of the relative impact of pollutant loadings

from the major basins on Bay water quality; the impact of nitrogen versus phosphorus versus sediment inputs on Bay water quality; and all sensitivity and cap

load allocation production runs leading up to the cap load allocations.

• The Nutrient Subcommittee developed the tiered scenarios and conducted an

assessment of sediment reduction efficiencies for near shore sediment reduction

best management practices.

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chapter i • Background

Chesapeake Executive Council 

Principals’ Staff Committee 

Implementation Committee 

Citizens Advisory

Committee 

Scientific & Technical

Advisory Committee

Water Quality Steering

Committee

Water Quality Technical

Workgroup

Federal Agencies

Committee

Budget Steering

Committee

Nutrient

Toxics Modeling Living

Resources

Monitoring

&

Assessment

Land, Growth

&

Stewardship

Communication

&

Education

Information

Management

Fisheries Steering

Committee 

Subcommittees

Local Government

Advisory Committee

Figure I-1. Chesapeake Bay Program organizational structure. 

Source: Chesapeake Bay Program website http://www.chesapeakebay.net.
• The Living resources Subcommittee developed the baywide and local SAV

restoration goals.

• The Monitoring and Analysis Subcommittee provided technical input and recommendations on monitoring-related issues, including using a three-year averaging

period for all integrated monitoring and modeling results to determine attainability.

• The Dissolved Oxygen Criteria, Water Clarity Criteria, Chlorophyll Criteria and

Water Quality Standards Coordinators teams derived the water quality criteria and

refined tidal-water designated uses that were as the basis for setting and allocating

the cap loads and developed the cumulative frequency distribution biological

reference curve approach to determining criteria attainment.

• The Use Attainability Analysis Workgroup provided input on where to apply the

criteria throughout the Bay tidal waters and provided input on feasibility and cost

of options.

WATER QUALITY STEERING COMMITTEE

This committee consisted of senior water program managers from all states in the

Bay watershed, EPA Headquarters and regions II and III, the Chesapeake Bay

Commission, the Susquehanna River Basin Commission and the Potomac River

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chapter i • Background

Water Quality

Steering Committee

Principals’ Staff

Committee and

Headwater State

Representatives

Water Quality

Technical

Workgroup

Monitoring and

Analysis

Subcommittee

Living Resources

Subcommittee

Nutrient

Subcommittee

Modeling

Subcommittee

Use

Attainability

Analysis

Workgroup

Water

Clarity

Criteria

Team

Chlorophyll

Criteria

Team

Dissolved

Oxygen

Criteria

Team

Water Quality

Standards

Coordinators

Team

Figure I-2. Chesapeake Bay Program partner's organizational structure supporting the

development and adoption of the Chesapeake Bay nutrient and sediment cap load 

allocations.
Basin Commission. In addition, representatives from the point source and environmental group interests attended the meetings. The committee provided critical

direction to the Water Quality Technical Workgroup, which explored policy and

technical issues related to the restoring Bay water quality initiative. The issues

explored included the derivation of the Bay water quality criteria, the refinement of

the tidal-water designated uses, analysis of the attainability of current and the refined

designated uses and the establishment and allocation of nutrient and sediment cap

loads. The committee selected the baywide nutrient cap loads from various options

that the workgroup forwarded. The Water Quality Steering Committee ultimately

forwarded a full package of nutrient and sediment cap load allocation recommendations to the Principals’ Staff Committee for review and formal adoption.

PRINCIPALS’ STAFF COMMITTEE

The Principals’ Staff Committee (PSC) consists of the secretaries of the appropriate

natural resource, agricultural, and regulatory pollution control agencies for the

original signatory states of Pennsylvania, Maryland, Virginia, the District of

Columbia, the Regional Administrator of EPA Region III and the Executive Director

of the Chesapeake Bay Commission. While the headwater states of Delaware, New

York and West Virginia were not members of the committee, representatives of these

states attended PSC meetings and were directly involved in all decisions related to

the cap load allocations. The committee was responsible for approving the allocations, but the PSC’s involvement went well beyond approving the package

recommended by the Water Quality Steering Committee. Since the recommendation

of the Water Quality Steering Committee did not fully allocate the agreed-upon

basinwide cap loads, the PSC and the headwater state representatives were called

upon to negotiate the allocation of the additional 12 milion pounds per year of

nitrogen reduction and 1 million pounds per year of phosphorus reduction necessary

to achieve the baywide loading caps. 

REEVALUATING THE ALLOCATIONS 

The nutrient and sediment cap load allocations adopted by the seven watershed jurisdictions and EPA are the best scientific estimates of the annual load reductions

needed to attain proposed water quality criteria and tidal-water designated uses

described in the Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity

and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional

Criteria Guidance) published by the EPA (U.S. EPA 2003a). Over the next two

years, Maryland, Virginia, Delaware and the District of Columbia will promulgate

new water quality standards based on the regional guidance published by EPA. 

Although the public process for adopting water quality standards varies among the

states, each state’s process will provide opportunities for considering and acquiring

new information at the local level. States may choose to explore a number of issues

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chapter i • Background
during their adoption process, such as the economic impact of water quality standards and specific designated use boundaries. 

While the allocations adopted at this time will provide the basis for tributary strategies, these allocations may need to be adjusted to reflect final state water quality

standards. Furthermore, planned Bay model refinements, designed to estimate water

quality benefits from filter feeding resources (such as oysters and menhaden) and

improve understanding of the sources and effects of sediments, will increase the

partners’ understanding of the relationship between nutrient and sediment reductions

and living resource responses in the Chesapeake Bay. For these reasons, the states

and EPA agreed to a reevaluation of these cap load allocations by no later than 2007. 

As partners, the jurisdictions committed to correcting the nutrient- and sedimentrelated problems in the Chesapeake Bay and its tidal tributaries enough to remove

them from the list of impaired waters by 2010 under the Clean Water Act. The states

recognize, however, that it will be difficult to meet projected water quality standards

in all parts of the Chesapeake Bay tidal waters by that time. A key reason for this difficulty is that once nutrient and sediment reduction practices are installed and

implemented, it may be years or even decades before the Chesapeake Bay benefits

from these reductions. The jurisdictions intend to have programs in place and functioning by 2010. The Chesapeake Bay and its tidal tributaries are expected to become

to be eligible for delisting when nutrient and sediment programs are fully implemented in the basin. 

SUPPORTING DOCUMENTS

In addition to recognizing the need for cap load allocations for nutrients and sediments,

Chesapeake 2000 also acknowledged the need for the development of scientifically

sound water quality criteria for the protection of the Chesapeake Bay’s living resources

from nutrient- and sediment-related impacts. Through extensive scientific research,

partner involvement and stakeholder and scientific review, the EPA has published

regional water quality criteria for the Chesapeake Bay and its tidal tributaries for

dissolved oxygen, water clarity and chlorophyll a. A full description of these water

quality criteria can be found in the Regional Criteria Guidance (U.S. EPA 2003a). 

To support the Regional Criteria Guidance, the EPA has published the Technical

Support Document for the Identification of Chesapeake Bay Designated Uses and

Attainability (Technical Support Document) (U.S. EPA 2003b). The purpose of the

Technical Support Document is to identify new refined tidal habitat zones, or designated uses, to which the Chesapeake Bay criteria will apply. In addition, SAV

restoration goals have been established by the Chesapeake Bay Program partners for

segments throughout the Chesapeake Bay tidal waters. The Technical Support Document also delineates the boundaries of these designated uses and assesses their

technological attainability. The EPA also has published a companion document

entitled Economic Analysis and Impacts of Nutrient and Sediment Reduction Actions

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chapter i • Background
to Restore Chesapeake Bay Water Quality, which provides information on costs and

impacts of various levels of nutrient and sediment technology and best management

practices controls across the Chesapeake Bay watershed that may be necessary to

meet the new criteria and designated uses (U.S. EPA 2003c). 

Collectively, these three documents support the establishment of cap load allocations

for nutrients and sediments by identifying attainable water quality and resource

restoration goals for all habitats within the Chesapeake Bay and its tidal tributaries. 

A memorandum from Secretary Tayloe Murphy (2003), Virginia Natural Resources

Secretary, to the Principals’ Staff Committee members and representatives of the

Chesapeake Bay headwater states formally summarized the decisions regarding the

nutrient and sediment cap load allocations and the new submerged aquatic vegetation restoration goals.

A memorandum from Secretary Tayloe Murphy (2003), Virginia Natural Resources

Secretariate, to the Principals’ Staff Committee members and representatives of the

Chesapeake Bay headwater states formally summarized the decisions regarding the

nutrient and sediment cap load allocations and the new submerged aquatic vegetation restoration goals.

ORGANIZATION OF THE ALLOCATIONS DOCUMENT

The cap load allocations were based largely on a scientific understanding of what

affects the water quality of the Chesapeake Bay and its tidal tributaries. Therefore,

much of this document is dedicated to presenting the scientific tools and issues that

were important to the development of the allocations. Accordingly, Chapter II

reviews the primary tools used in developing the cap load allocations, including the

loading scenarios. These scenarios were used to estimate the nutrient and sediment

loads associated with increased levels of pollution control measures and to gain

insight into the source loadings and associated impacts on water quality. In addition,

this chapter provides a brief technical review of the various models used to simulate

the source loads and their impact on the Bay’s tidal water quality.

Chapter III reviews the major technical difficulties that arose and documents the

innovative solutions created by the partners’ technical staff. Included in this chapter

are issues related to developing the methodology for determining attainment of the

water quality criteria (e.g., applying biological reference curves), results of assessments on the impact of each major basin on the Bay’s tidal water quality, results of

assessments on the water quality impact from nitrogen versus phosphorus inputs to

the Bay (e.g., analyzing relative effectiveness) and a brief technical review of the

attainment simulations for the SAV acreage goals established for the Bay.

However, significant policy guidance was vital in order to arrive at cap load allocations with the highest probability of ‘buy-in’ and, therefore, the greatest assurance of

implementation. Chapter IV describes how science informed the policy decisions

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chapter i • Background
applied during the cap load allocation process. It provides a detailed review of the

methodologies, model results and policy decisions used to derive the cap load allocations for nutrients and sediments. Specifically, it presents the principles applied and

approaches taken to derive baywide loading caps for nutrients and sediment and the

two separate methodologies used to distribute the cap loads to the major tributary

basins and then to jurisdictions within the Bay watershed for nutrients and sediments. 

Finally, appendices A through F provide extensive model results and analyses in

support of the allocations. 

LITERATURE CITED

Chesapeake Bay Watershed Partners. 2001.

Chesapeake Bay Executive Council. 2000. Chesapeake 2000 agreement. Annapolis, Maryland.

Chesapeake Bay Executive Council. 1987. Chesapeake Bay Agreement. Annapolis, Maryland.

Chesapeake Bay Executive Council. 1983. Chesapeake Bay Agreement. Annapolis, Maryland.

Secretary Tayloe Murphy. 2003. “Summary of Decisions Regarding Nutrient and Sediment

Load Allocations and New Submerged Aquatic Vegetation (SAV) Restoration Goals.” April

25, 2003, Memorandum to the Principals’ Staff Committee members and representatives of

the Chesapeake Bay headwater states. Virginia Office of the Governor, Natural Resources

Secretariate, Richmond, Virginia.

U.S. EPA. 2003a. 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. Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. EPA. 2003b. Technical Support Document for the Identification of Chesapeake Bay

Designated Uses and Attainability. EPA 903-R-03-004. Chesapeake Bay Program Office,

Annapolis, Maryland.

U.S. EPA. 2003c. Economic Analysis and Impacts of Nutrient and Sediment Reduction

Actions to Restore Chesapeake Bay Water Quality. Chesapeake Bay Program Office,

Annapolis, Maryland. http://www.chesapeakebay.net/ecoanalyses.htm.

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chapter i • Background
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

CHAPTER I

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Fundamentals of Developing Cap Load Allocations . . . . . . . . . . . . 2

Key Players in Developing the Allocations . . . . . . . . . . . . . . . . . . . . 3

Water Quality Technical Workgroup . . . . . . . . . . . . . . . . . . . . . . . 3

Water Quality Steering Committee . . . . . . . . . . . . . . . . . . . . . . . . 5

Principals’ Staff Committee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Reevaluating the Allocations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Supporting Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Organization of the Allocations Document . . . . . . . . . . . . . . . . . . . 8

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CHAPTER II

Overview of the Technical Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Tiered Management Implementation Scenarios . . . . . . . . . . . . . . . 11

Development of the Tiered Scenarios . . . . . . . . . . . . . . . . . . . . . . 13

Tiered Scenario Estimated Nutrient and Sediment Loads . . . . . . . 15

Chesapeake Bay Program Environmental Models . . . . . . . . . . . . . 17

Chesapeake Bay Airshed Model and Atmospheric Deposition . . . 19

Chesapeake Bay Watershed Model . . . . . . . . . . . . . . . . . . . . . . . . . 23

Chesapeake Bay Water Quality Model . . . . . . . . . . . . . . . . . . . . . . . 29

Hydrodynamic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Water Quality Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Adjustment of Model Dissolved Oxygen, Water Clarity 

and Chlorophyll a Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Management Application of Model Outputs . . . . . . . . . . . . . . . . . 36

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

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CHAPTER III

Technical and Modeling Considerations in Setting the Allocations . . 43

Refining Estimates of Pycnocline Depths . . . . . . . . . . . . . . . . . . . . . . 43

Averaging Period for Determining Criteria Attainment . . . . . . . . . 44

Assessing Monitoring Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Assessing Modeling Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Defining Allowable Frequency and Duration of 

Criteria Exceedances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Establishing the Geographic Influence of the Loads

on Tidal Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Focusing on Mainstem Segments CB3MH, CB4MH 

and CB5MH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Comparing Absolute Versus Relative Effectiveness . . . . . . . . . . . . 48

Normalizing for the Combined Nitrogen and Phosphorus Load . . 49

General Findings from the Geographic Isolation Runs . . . . . . . . . 51

Establishing Wetland Influence on Tidal-Water 

Dissolved Oxygen Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Water Quality Model Runs Isolating Individual 

Pollutant Effects on Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Surface Analysis Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Surface Analysis Utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Influence of Sedimentation on Chesapeake Bay 

Dissolved Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Establishing and Assessing the New SAV Restoration Goals . . . . . 60

Developing the 185,000-Acre SAV Restoration Goal . . . . . . . . . . 61

Water Clarity and SAV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Sediment Loads: Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Calibrating the Water Quality Model for Clarity . . . . . . . . . . . . . . 68

Relating SAV Biomass to Acreage . . . . . . . . . . . . . . . . . . . . . . . . . 70

Spatial Analysis of Effective Shoreline Load Reductions . . . . . . . 74

Monitoring Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Sediment Cap Load Allocation Principles . . . . . . . . . . . . . . . . . . . 78

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

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CHAPTER IV

Setting the Nutrient and Sediment Allocations . . . . . . . . . . . . . . . . . . 81

Establishing Nutrient Cap Load Allocations . . . . . . . . . . . . . . . . . . 82

Geographic Location and Criterion Driving the Allocation . . . . . 82

Baywide Cap Loading Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Distributing Basinwide Allocations to the 

Major Basins and Jurisdictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

The PSC Completes the Allocation Process . . . . . . . . . . . . . . . . . . 99

Cap Load Allocations to Achieve the Chlorophyll a Criteria . . . . 99

Additional Nutrient Cap Load Allocation Considerations . . . . . . . 103

Establishing Sediment Cap Load Allocations . . . . . . . . . . . . . . . . . 103

SAV Restoration as the Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Land-Based (Upland) Sediment Allocation . . . . . . . . . . . . . . . . . . 104

Establishing Near-Shore Sediment Allocation . . . . . . . . . . . . . . . 105

SAV-Based Sediment Cap Load Allocations . . . . . . . . . . . . . . . . . 106

Summary of Nutrient and Sediment Cap Load Allocations . . . . . 106

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

APPENDICES

Appendix A: Summary of Decisions Regarding Nutrient and 

Sediment Load Allocations and New Submerged 

Aquatic Vegetation (SAV) Restoration Goals—

Memorandum from W. Tayloe Murphy, Jr., Chair,

Chesapeake Bay Program Principals’ Staff Committee,

to the Principals’ Staff Committee Members and 

Representatives of Chesapeake Bay “Headwater” States . . . . . . A1

Appendix B: Summary of Water Quality Criteria and Use 

Boundaries Used in Setting the Allocations . . . . . . . . . . . . . B1

Appendix C: Summary of Watershed Model Results 

for All Loading Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . C1

Appendix D: Summary of Key Attainment Scenarios . . . . . . . . . . . . . . . . D1

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Acknowledgments

▼vii

Acknowledgments

Development of the nutrient and sediment allocations for the Chesapeake Bay

involved sifting through a wealth of scientific data and information, constructing

a technically sound decision making process and factoring in an array of policy considerations. The collaborative efforts, collective knowledge and applied expertise of

the following committees, subcommittees, workgroups, and headwater representatives were contributed to the success of the process.

Development of the technical recommendations that met the diverse needs of the

Chesapeake Bay Program partners would not have been possible without the understanding of water quality condition needs of the Bay, energy and enthusiasm, which

was provided by members of and staff to the Chesapeake Bay Program Water

Quality Technical Workgroup: Alan Pollock, Chair, Virginia Department of

Environmental Quality; Richard Batiuk, U.S. EPA Chesapeake Bay Program Office;

Victor Bierman, Jr., LimoTech Inc.; Michael Bowman, Virginia Department of

Conservation & Recreation; Charles Lunsford, Virginia Department of Conservation

& Recreation; William Brown, Pennsylvania Department of Environmental

Protection; Arthur Butt, Virginia Department of Environmental Quality; John

Kennedy, Virginia Department of Environmental Quality; James Collier, District of

Columbia Department of Health; Carol Young, Pennsylvania Department of

Environmental Protection; Richard Draper, New York State Department of

Environmental Conservation; Richard Eskin, Maryland Department of Environment;

Dave Montali, West Virginia Department of Environmental Protection; Will Hunley,

Hampton Roads Sanitation District; Maggie Kerchner, NOAA Chesapeake Bay

Office; Wendy Jastremski, U.S. EPA Chesapeake Bay Program Office; Robert

Koroncai, U.S. EPA Chesapeake Bay Program Office; Norm LeBlanc, Hampton

Roads Sanitation District; Lewis Linker, U.S. EPA Chesapeake Bay Program Office;

Russel Mader, National Resources Conservation Service; Robert Magnien,

Maryland Department of Natural Resources; Lee McDonnell, Pennsylvania

Department of Environmental Protection; Mark Morris, U.S. EPA Office of Water;

Scott Phillips, U.S. Geological Survey; Ana Pomales, U.S. EPA Region III;

Christopher Pomeroy, AquaLaw PLC; John Schneider, Delaware Department of

Natural Resources & Environmental Control; Gary Shenk, U.S. EPA Chesapeake

Bay Program Office; Tom Simpson, University of Maryland; Tanya Spano,

Metropolitan Washington Council of Governments; Peter Tango, Maryland

Department of Natural Resources; Lyle Varnell, Virginia Institute of Marine Science;

Lauren Wenzel, Maryland Department of Natural Resources; Allison Wiedeman,

U.S. EPA Chesapeake Bay Program Office; Clyde Wilbur, Greeley & Hansen; and

Kyle Zieba, U.S. EPA Chesapeake Bay Program Office.
▼viii

Acknowledgements

Development of the nutrient and sediment cap load allocations would not have been

possible without the ability to understand the water quality and living resource

responses to pollutant loadings in the Bay. Such understanding is made possible

through the Chesapeake Bay Airshed, Watershed and Water Quality models, which

are ably managed, maintained, and continually enhanced by members of the

Chesapeake Bay Program’s Modeling Subcommittee: James Collier, Chair, District

of Columbia Department of Health; Lowell Bahner, National Oceanic & Atmospheric Administration; Mark Bennett, U.S. Geological Survey; Peter Bergstrom,

U.S. Fish & Wildlife Service; Michael Bowman, Virginia Department of

Conservation & Recreation; William Brown, Pennsylvania Department of Environmental Protection; Arthur Butt, Virginia Department of Environmental Quality;

Robin Dennis, National Oceanographic and Atmospheric Administration; Lewis

Linker, U.S. EPA Chesapeake Bay Program Office; Charles Lunsford, Virginia

Department of Conservation & Recreation; Robert Magnien, Maryland Department

of Natural Resources; Ross Mandel, Interstate Commission on the Potomac River

Basin; Timothy Murphy, Metropolitan Washington Council of Governments;

Narendra Panday, Maryland Department of Environment; Kenn Pattison,

Pennsylvania Department of Environmental Protection; Jeff Raffensperger, U.S.

Geological Survey-Baltimore; Helen Stewart, Maryland Department of Natural

Resources; Peter Tango, Maryland Department of Natural Resources; and Harry

Wang, Virginia Institute of Marine Science.

While all of the Modeling Subcommittee members made important contributions,

special recognition is appropriately due to Lewis Linker (EPA), Gary Shenk (EPA)

and Jeff Sweeney (University of Maryland) for pioneering efforts in creating new

approaches to difficult problems through relentless dedication to the task at hand.

Special thanks and recognition to Ping Wang (University of Maryland) for his skill,

expertise, and dedication in scenario operation and development on The National

Environmental Super Computer Center, and to Kate Hopkins (University Of

Maryland) for her expert application of GIS modeling support to the allocation

analyses.

The sediment allocation recommendations were driven by the new Submerged

Aquatic Vegetation (SAV) restoration goal. Without the close coordination between

the Chesapeake Bay Program’s SAV Workgroup and the Water Quality Technical

Workgroup, linking the SAV resource to the sediment cap load allocations would not

have been possible. Mike Naylor (Maryland Department of Natural Resources), Ken

Moore (Virginia Institute of Marine Science), Frank Dawson (Maryland Department

of Natural Resources) and Mike Fritz (EPA) in particular, led the effort to assure

integration of the SAV restoration goal with efforts to derive water clarity criteria,

set shallow-water designated use boundaries and establish the sediment cap load

allocations.

The Chesapeake Bay Program’s Scientific and Technical Advisory Committee

(STAC) provided timely and important input to the allocation process during a critical juncture in the deliberations. Special thanks to Scott Phillips for his leadership

in facilitating communication between STAC and the Water Quality Technical

Workgroup.

Cap load allocations, especially as complex as that for the Chesapeake Bay, are a

unique blend of science and policy. The Chesapeake Bay Program’s Water Quality

Steering Committee provided much needed direction to the Water Quality
▼ix

Acknowledgments

Technical Workgroup on difficult matters of equity and process. Furthermore, it was

the Water Quality Steering Committee that forwarded the cap load allocations to 

the Principals’ Staff Committee for approval. The members were: Jon Capacasa,

co-chair, U.S. EPA Region III; Rebecca Hanmer, co-chair, U.S. EPA Chesapeake

Bay Program; Russell Baxter, Chesapeake Bay Commission; Jerusalem Bekele,

District of Columbia Department of Health; Michael Bowman, Virginia Department

of Conservation & Recreation; Edward Brezina, Pennsylvania Department 

of Environmental Protection; Patricia Buckley, Pennsylvania Department of

Environmental Protection; William Brannon, West Virginia Department 

of Environmental Protection; James Collier, District of Columbia Department of

Health; Melanie Davenport, Chesapeake Bay Commission; Kevin Donnelly,

Delaware Department of Natural Resource and Environmental Control; Richard

Draper, New York State Department of Environmental Conservation; Phillip M.

DeGaetano, New York State Department of Environmental Conservation; Mario

DelVicario, U.S. EPA Region II; Diana Esher, U.S. EPA Chesapeake Bay Program

Office; Richard Eskin, Maryland Department of the Environment; Jack Frye,

Virginia Department of Conservation and Recreation; Stuart Gansell, Pennsylvania

Department of Environmental Protection; Carlton Haywood, Interstate Commission

on the Potomac River Basin; David Heicher, Susquehanna River Basin Commission;

James Keating, U.S. EPA Office of Water, Office of Science and Technology; Felix

Locicero, U.S. EPA Region II; Steve Luckman, Maryland Department of the

Environment; Robert Magnien, Maryland Department of Natural Resources; Chris

Miller, U.S. EPA Office of Water, Office of Science and Technology; Matthew

Monroe, West Virginia Department of Agriculture; Kenn Pattison, Pennsylvania

Department of Environmental Protection; Alan Pollock, Virginia Department of

Environmental Quality; John Schneider, Delaware Department of Natural Resources

and Environmental Control; Thomas Simpson, University of Maryland; Robert

Summers, Maryland Department of Environment; Ann Swanson, Chesapeake Bay

Commission; Robert Yowell, Pennsylvania Department of Environmental

Protection; and Robert Zimmerman, Delaware Department of Natural Resources and

Environmental Control. 

Without a unified commitment to restoring the Chesapeake Bay, agreement on allocating the last 12 million pounds of nitrogen and 1 million pounds of phosphorus

that is necessary to achieve the basinwide cap loads would not have been possible.

The Chesapeake Bay Program Principals’ Staff Committee along with its new partner states of Delaware, New York, and West Virginia, provided the leadership that

was necessary to approve the allocation recommendations and to allocate further

remaining loads. 

Principals’ Staff Committee

W. Tayloe Murphy, Jr., Secretary of Natural Resources, Virginia, Office of

Governor (Chair)

Joseph Maroon, Director, Virginia Department of Conservation and Recreation

Robert G. Burnley, Director, Virginia Department of Environmental Quality

Theodore J. Gordon, Chief Operating Officer, District of Columbia Department of

Health 

Elizabeth Berry, Special Assistant, Executive Office of Mayor, District of

Columbia
▼x

Acknowledgments

Lewis R. Riley, Secretary, Maryland Department of Agriculture

Lynn Y. Buhl, Acting Secretary, Maryland Department of the Environment

C. Ronald Franks, Secretary, Maryland Department of Natural Resources

Audrey E. Scott, Secretary, Maryland Department of Planning

Kathleen A. McGinty, Secretary, Pennsylvania Department of Environmental

Protection

Michael DiBerardinis, Secretary, Pennsylvania Department of Conservation &

Natural Resources

Richard G. Sprenkle, Deputy Secretary, Pennsylvania Department of Conservation

and Natural Resources

Ann Swanson, Executive Director, Chesapeake Bay Commission 

Donald S. Welsh, Regional Administrator, U.S. EPA Region III

Rebecca W. Hanmer, Director, U.S. EPA Chesapeake Bay Program Office

Headwater State Representatives

William Brannon, West Virginia Department of Environmental Protection

Matthew Monroe, West Virginia Department of Agriculture

Richard Draper, New York State Department of Environmental Conservation

Kevin Donnelly, Delaware Department of Natural Resources and Environmental

Conservation
▼xi

Executive Summary

The Chesapeake 2000 agreement has been guiding Maryland, Pennsylvania,

Virginia, the District of Columbia, the Chesapeake Bay Commission and the 

U.S. Environmental Protection Agency (EPA) in their combined efforts to restore

and protect the Chesapeake Bay. It defined the goal to “achieve and maintain the

water quality necessary to support the aquatic living resources of the Bay and its tributaries and to protect human health.” Subsequently, Delaware, New York and West

Virginia signed a Memorandum of Understanding committing to implement the

Water Quality Protection and Restoration section of the agreement. 

Chesapeake 2000 committed its signatories to:

continue efforts to achieve and maintain the 40 percent nutrient reduction

goal agreed to in 1987 and correct the nutrient- and sediment-related

problems in the Chesapeake Bay and its tidal tributaries sufficiently to

remove the Bay and the tidal portions of its tributaries from the list of

impaired waters under the Clean Water Act by 2010. 

Defining science-based loading caps for nutrients and sediment and allocating

responsibility by major tributary basin to the jurisdictions were critical steps to

fulfilling the water quality commitments. This document presents the collaborative

process, technical tools and innovative approaches that made possible the successful

allocation of nutrient and sediment cap loads to each jurisdiction by major tributary

basin.

NUTRIENT CAP LOAD ALLOCATIONS

Excessive nutrients in the Chesapeake Bay and its tidal tributaries promote a number

of undesirable water quality conditions such as excessive algal growth, low dissolved

oxygen and reduced water clarity. The effect of nutrient loads on water quality and

living resources tends to vary considerably by season and region. Low dissolved

oxygen problems tend to be more pronounced in the deeper parts of the upper bay

region during the summer months. The allocations for nutrients were developed

primarily to address this problem. 

As a result, New York, Pennsylvania, Maryland, Delaware, Virginia, West Virginia,

the District of Columbia and the U.S. Environmental Protection Agency agreed to

cap annual nitrogen loads delivered to the Bay’s tidal waters at 175 million pounds

Executive Summary
▼xii

Executive Summary

and annual phosphorus loads at 12.8 million pounds. It is estimated that these allocations will require reductions, from 2000 levels, in nitrogen pollution by 110

million pounds and phosphorus pollution by 6.3 million pounds. 

The Chesapeake Bay Program partners agreed to these load reductions based upon

Chesapeake Bay Water Quality Model projections of attainment of published Bay

dissolved oxygen criteria applied to the refined tidal water designated uses. The

model projects these load reductions will significantly reduce the persistent summer

anoxic conditions in the deep bottom waters of the Chesapeake Bay and restore suitable habitat quality conditions throughout the tidal tributaries. Furthermore, these

reductions are projected to eliminate excessive, sometimes harmful, algae conditions

(measured as chlorophyll a) throughout the Chesapeake Bay and its tidal tributaries. 

The jurisdictions agreed to distribute the basinwide cap loads for nitrogen and phosphorus by major tributary basin (Table 1) and jurisdiction (Table 2). This distribution

of responsibility for load reductions was based on three basic principles:

1. Tributary basins with the highest impact on Chesapeake Bay tidal water quality

would be allocated the highest reductions of nutrients. 

2. States without tidal waters—Pennsylvania, New York and West Virginia—

would be provided some relief from Principle 1 since they benefit less directly

from improved water quality in the Chesapeake Bay and its tidal tributaries. 

3. Nutrient reductions prior to 2000 would be credited towards achievement of the

cap load allocations. 

The nine major tributary basins were separated into three categories based upon their

impact on Bay tidal water quality. Each basin within an individual category was

assigned the same percent reduction of anthropogenic, or human-caused, load.

Consequently, basins with the highest impact on tidal water quality were assigned

the highest percentage reduction of anthropogenic load. 

After completing the above calculations and applying Principle 2, New York, Pennsylvania and West Virginia allocations were set at the ‘Tier 3’ scenario nutrient load

levels. The Tier 3 scenario is one of several tiers representing different implementation scenarios of nutrient reduction measures for the Chesapeake Bay watershed

developed by the Chesapeake Bay Program partners. The Tier 3 scenario represented

nutrient and sediment loads of 181 million pounds of nitrogen per year, 13.4 million

pounds of phosphorus per year, 4.14 million tons of land-based sediment per year

and included a 20 percent reduction in tidal shoreline erosion sediment loads. Additionally, allocations for Virginia’s York and James River basins were set at previously

established tributary strategy nutrient cap load levels since these two basins have a

minimal impact on mainstem Bay water quality conditions, and their influence on

tidal water quality is predominantly local. 

Application of these rules resulted in shortfalls of 12 million pounds of nitrogen and

1 million pounds of phosphorus above the basinwide cap loads. However, the EPA

committed to pursue the Clear Skies initiative, which is estimated to reduce the

nitrogen load to Bay tidal waters by 8 million pounds per year. Furthermore, the Bay

watershed states agreed to take responsibility for the remaining 4 million pounds of

nitrogen and 1 million pounds of phosphorus. The nutrient cap load allocations in

tables 1 and 2 reflect these agreements.
▼xiii

Executive Summary

The cap load allocations for nitrogen and phosphorus were adopted as ‘nitrogen

equivalents’. Included was a commitment to explore how actions beyond traditional

best management practices (BMPs) might help meet the Chesapeake Bay water

quality restoration goals. A nitrogen equivalent is an action that results in the same

water quality benefit as removing nitrogen. The Chesapeake Bay Program partners

will evaluate how tidal water quality benefits from continued and expanded living

resource restoration, such as oysters and menhaden, can be accounted for in offsetting the reductions of watershed-based nutrient and sediment loads. Seasonal

fluctuations in implementation of biological nutrient removal, nutrient reduction

from shoreline erosion control, implementation of enhanced nutrient removal technologies at large wastewater treatment plants, and trade-offs between nitrogen and

phosphorus will also be evaluated. 

Also, while the allocations adopted at this time will provide the basis for tributary

strategies, these allocations may need to be adjusted to reflect final state water

quality standards. If the final adopted state water quality standards are different than

the criteria and designated use used to establish these cap load allocations, then the

cap load allocations will need to be amended accordingly. 

SEDIMENT CAP LOAD ALLOCATIONS 

Sediments suspended in the water column reduce the amount of light available to

support healthy and extensive submerged aquatic vegetation (SAV), or underwater

bay grass, communities. The relative contribution of suspended sediment and algae

that cause poor light conditions varies with location in the Bay tidal waters. The

Chesapeake Bay Program partners agreed that a primary reason for reducing sediment

loads to the Bay tidal waters is to provide suitable habitat for restoring SAV. As a

result, the cap load allocations for sediments are linked to the recommended water

clarity criteria and the new SAV restoration goals and recognize that sediment load

reductions are essential to SAV restoration. The jurisdictions also agreed that nutrient

load reductions are critical for restoring SAV as well as improving oxygen levels.

To support the sediment cap load allocations, it became clear that updated SAV

restoration goals were needed. The partners explored various methodologies for

developing a baywide SAV acreage restoration target using the available historical

record. The methodology selected used aerial photography from the 1930s to present

to identify the best year of record (in terms of acres of SAV) for each Chesapeake

Bay Program segment. The acreage determined to be the best year of record was

designated as the SAV acreage goal for that segment. In aggregating all of the single

best year results for each segment, a baywide SAV acreage restoration goal for the

entire Bay of 185,000 acres was established. Table 3 provides the SAV acreage goal

for each Bay segment while Table 4 provides the SAV acreage goal for each major

tributary basin in the Chesapeake Bay watershed adopted by the Chesapeake Bay

Program partners. 

Unlike nutrients, where loads from virtually the entire Chesapeake Bay watershed

affect mainstem Chesapeake Bay water quality, impacts from sediments are predominantly localized. For this reason, local, segment-specific SAV acreage goals

have been established and the sediment cap load allocations are targeted towards

achieving those restoration goals. 
▼xiv

Executive Summary

The partners recognize that the current understanding of sediment sources and their

impact on the Chesapeake Bay is not yet complete. Currently, understanding of landbased sediments that are carried into local waterways through stream bank erosion

and runoff is still basic. Knowledge about nearshore sediments that enter the Bay and

its tidal rivers directly through shoreline erosion or shallow-water suspension is even

more limited. Consequently, the sediment cap load allocations are currently focused

on land-based sediment cap loads by major tributary basin (Table 1) and jurisdiction

(Table 2). 

Most land-based best management practices, which reduce nonpoint sources of

phosphorus, will also reduce sediment runoff. Therefore, the partners agreed to landbased sediment allocations that represent the sediment load reductions likely to

result from implementing management actions required for the allocated phosphorus

reductions. 

The sediment cap load allocations were set at the tier level for the phosphorus cap

load allocation for each jurisdiction-basin. This designation is referred to as the ‘phosphorus equivalent’ land-based sediment reduction. If the ‘phosphorus equivalent’

land-based sediment reductions were found to be more than that which are necessary

to achieve the local SAV restoration goals, then the land-based sediment cap load allocations were lowered to that level necessary to achieve the SAV restoration goal. The

tidal-fresh Susquehanna Flats and tidal-fresh Potomac River are two examples where

this modified approach was applied. If, in the development of their tributary strategies, tributary teams conclude that the land-based sediment allocations need

revisions, the tributary teams may identify an alternate land-based allocation. For

example, a jurisdiction may select different nonpoint source management actions than

those prescribed in the tier approach to reach the phosphorus goal; the jurisdiction

may adjust the sediment cap load allocation accordingly so long as SAV restoration

and protection is not compromised. The tributary teams must work with all the jurisdictions within the affected basin in revising the sediment cap load allocations.

It is likely that reductions in nutrients and land-based sediments alone will not be

sufficient to achieve the local SAV restoration goals for many areas of the Chesapeake Bay and its tidal tributaries. In these areas, tributary teams will be asked to

further assess varied and innovative methods to achieve SAV establishment and

growth. Such methods may include, but are not limited to SAV planting, offshore

breakwaters, shore erosion controls, beach nourishment, establishment of oyster

bars, and other actions as appropriate. 
▼xv

Executive Summary

Table 1. Chesapeake Bay watershed nitrogen, phosphorus and sediment cap load allocations 

by major basin.

Nitrogen Phosphorus Upland Sediment

Cap Load Allocation Cap Load Allocation Cap Load Allocation

Basin/Jurisdiction (million pounds/year) (million pounds/year) (million tons/year)

SUSQUEHANNA

PA 67.58 1.90 0.793

NY 12.58 0.59 0.131

MD 0.83 0.03 0.037

SUSQUEHANNA Total 80.99 2.52 0.962

EASTERN SHORE - MD

MD 10.89 0.81 0.116

DE 2.88 0.30 0.042

PA 0.27 0.03 0.004

VA 0.06 0.01 0.001

EASTERN SHORE - MD Total 14.10 1.14 0.163

WESTERN SHORE

MD 11.27 0.84 0.100

PA 0.02 0.00 0.001

WESTERN SHORE Total 11.29 0.84 0.100

PATUXENT

MD 2.46 0.21 0.095

PATUXENT Total 2.46 0.21 0.095

POTOMAC

VA 12.84 1.40 0.617

MD 11.81 1.04 0.364

WV 4.71 0.36 0.311

PA 4.02 0.33 0.197

DC 2.40 0.34 0.006

POTOMAC Total 35.78 3.48 1.494

RAPPAHANNOCK

VA 5.24 0.62 0.288

RAPPAHANNOCK Total 5.24 0.62 0.288

YORK

VA 5.70 0.48 0.103

YORK Total 5.70 0.48 0.103

JAMES

VA 26.40 3.41 0.925

WV 0.03 0.01 0.010

JAMES Total 26.43 3.42 0.935

EASTERN SHORE - VA

VA 1.16 0.08 0.008

EASTERN SHORE - VA Total 1.16 0.08 0.008

SUBTOTAL 183 12.8 4.15

CLEAR SKIES REDUCTION -8

BASINWIDE TOTAL 175 12.8 4.15
▼xvi

Executive Summary

Table 2. Chesapeake Bay watershed nitrogen, phosphorus and sediment cap load allocations 

by jurisdiction.

Nitrogen Phosphorus Upland Sediment

Cap Load Allocation Cap Load Allocation Cap Load Allocation

Jurisdiction/Basin (million pounds/year) (million pounds/year) (million tons/year)

PENNSYLVANIA

Susquehanna 67.58 1.90 0.793

Potomac 4.02 0.33 0.197

Western Shore 0.02 0.00 0.001

Eastern Shore - MD 0.27 0.03 0.004

PA Total 71.90 2.26 0.995

MARYLAND

Susquehanna 0.83 0.03 0.037

Patuxent 2.46 0.21 0.095

Potomac 11.81 1.04 0.364

Western Shore 11.27 0.84 0.100

Eastern Shore - MD 10.89 0.81 0.116

MD Total 37.25 2.92 0.712

VIRGINIA

Potomac 12.84 1.40 0.617

Rappahannock 5.24 0.62 0.288

York 5.70 0.48 0.103

James 26.40 3.41 0.925

Eastern Shore - MD 0.06 0.01 0.001

Eastern Shore - VA 1.16 0.08 0.008

VA Total 51.40 6.00 1.941

DISTRICT OF COLUMBIA

Potomac 2.40 0.34 0.006

DC Total 2.40 0.34 0.006

NEW YORK

Susquehanna 12.58 0.59 0.131

NY Total 12.58 0.59 0.131

DELAWARE

Eastern Shore - MD 2.88 0.30 0.042

DE Total 2.88 0.30 0.042

WEST VIRGINIA

Potomac 4.71 0.36 0.311

James 0.03 0.01 0.010

WV Total 4.75 0.37 0.320

SUBTOTAL 183 12.8 4.15

CLEAR SKIES REDUCTION –8

BASINWIDE TOTAL 175 12.8 4.15
▼xvii

Executive Summary

Table 3. Chesapeake Bay submerged aquatic vegetation (SAV)

restoration goal acreage by Chesapeake Bay Program (CBP) segment

based on the single best year of record from 1930 to present.

CBP Segment Name Segment Acres 

Northern Chesapeake Bay CB1TF 12,908

Upper Chesapeake Bay CB2OH 302

Upper Central Chesapeake Bay CB3MH 943

Middle Central Chesapeake Bay CB4MH 2,511

Lower Central Chesapeake Bay CB5MH 14,961

Western Lower Chesapeake Bay CB6PH 980

Eastern Lower Chesapeake Bay CB7PH 14,620

Mouth of the Chesapeake Bay CB8PH 6

Bush River BSHOH 158

Gunpowder River GUNOH 2,254

Middle River MIDOH 838

Back River BACOH 0

Patapsco River PATMH 298

Magothy River MAGMH 545

Severn River SEVMH 329

South River SOUMH 459

Rhode River RHDMH 48

West River WSTMH 214

Upper Patuxent River PAXTF 5

Western Branch (Patuxent River) WBRTF 0

Middle Patuxent River PAXOH 68

Lower Patuxent River PAXMH 1,325

Upper Potomac River POTTF 4,368

Anacostia River ANATF 6

Piscataway Creek PISTF 783

Mattawoman Creek MATTF 276

Middle Potomac River POTOH 3,721

Lower Potomac River POTMH 10,173

Upper Rappahannock River RPPTF 20

Middle Rappahannock River RPPOH 0

Lower Rappahannock River RPPMH 5,380

Corrotoman River CRRMH 516

Piankatank River PIAMH 3,256

Upper Mattaponi River MPNTF 75

Lower Mattaponi River MPNOH 0

Upper Pamunkey River PMKTF 155

Lower Pamunkey River PMKOH 0

Middle York River YRKMH 176

Lower York River YRKPH 2,272

Mobjack Bay MOBPH 15,096

continued
▼xviii

Executive Summary

CBP Segment Name Segment Acres 

Upper James River JMSTF 1,600

Appomattox River APPTF 319

Middle James River JMSOH 7

Chickahominy River CHKOH 348

Lower James River JMSMH 531

Mouth of the James River JMSPH 604

Western Branch Elizabeth River WBEMH 0

Southern Branch Elizabeth River SBEMH 0

Eastern Branch Elizabeth River EBEMH 0

Lafayette River LAFMH 0

Mouth to mid-Elizabeth River ELIPH 0

Lynnhaven River LYNPH 69

Northeast River NORTF 88

C&D Canal C&DOH 0

Bohemia River BOHOH 97

Elk River ELKOH 1,648

Sassafras River SASOH 764

Upper Chester River CHSTF 0

Middle Chester River CHSOH 63

Lower Chester River CHSMH 2,724

Eastern Bay EASMH 6,108

Upper Choptank River CHOTF 0

Middle Choptank River CHOOH 63

Lower Choptank River CHOMH2 1,499

Mouth of the Choptank River CHOMH1 8,044

Little Choptank River LCHMH 3,950

Honga River HNGMH 7,686

Fishing Bay FSBMH 193

Upper Nanticoke River NANTF 0

Middle Nanticoke River NANOH 3

Lower Nanticoke River NANMH 3

Wicomico River WICMH 3

Manokin River MANMH 4,359

Big Annemessex River BIGMH 2,014

Upper Pocomoke River POCTF 0

Middle Pocomoke River POCOH 0

Lower Pocomoke River POCMH 4,092

Tangier Sound TANMH 37,965

Total acres 184,889

Table 3. Chesapeake Bay submerged aquatic vegetation (SAV)

restoration goal acreage by Chesapeake Bay Program (CBP) segment

based on the single best year of record from 1930 to present (cont.).
▼xix

Executive Summary

Basin/Jurisdiction SAV Restoration Goal (Acres)

SUSQUEHANNA 12,856

EASTERN SHORE – MD 76,193

WESTERN SHORE – MD 5,651

PATUXENT 1,420

POTOMAC1

MD 12,747

VA 6,320

DC 384

RAPPAHANNOCK 12,798

YORK 21,823

JAMES 3,483

EASTERN SHORE – VA 31,215

TOTAL 184,889

1 Breakdown of Potomac SAV restoration goals by jurisdictions are draft, pending

confirmation of split between Maryland, Virginia and the District of Columbia 

along jurisdictional lines. Due to ongoing refinement, some numbers in this table

differ from the April 25, 2003 version included in Appendix A and previous model

estimates presented in tables III-3 and III-4.

Table 4. Chesapeake Bay submerged aquatic vegetation 

(SAV) restoration goal acreage by major basin by jurisdiction.
Pollutant loading allocations must be based on credible science. It is important to

understand and simulate the source loadings, as well as their impact on the water

quality of the receiving water body. Because the Chesapeake Bay nutrient and

sediment dynamics are so complex, establishing the scientific basis required the

application of several coupled models of the Chesapeake Bay ecosystem:

• The Chesapeake Bay Airshed Model provided simulations of air sources of

nutrients and air deposition onto the Chesapeake Bay watershed and the tidal

surface waters;

• The Chesapeake Bay Watershed Model tracked loadings from all sources of

nutrients and sediments in the watershed and simulated pollutant transport down

to the Chesapeake Bay and its tidal tributaries;

• The Chesapeake Bay Water Quality Model is an aggregate of several models—

hydrodynamic, water quality, bottom sediment, benthic community and SAV

community—which combined effectively, simulated the effects of nutrient and

sediment pollutant loadings on the water quality of the Chesapeake Bay and its

tidal tributaries.

A brief review of these important tools is provided below. In addition, a description

is provided of the development of management control scenarios, called ‘tiers’,

which played a critical role in the process of developing allocations.

TIERED MANAGEMENT IMPLEMENTATION SCENARIOS

A series of watershed model scenarios were designed to estimate the nutrient and

sediment loads associated with increased implementation levels of best management

practices (BMPs), wastewater treatment upgrades and/or other point or non-point

control technologies. The resultant watershed model outputs—various combinations

of nitrogen, phosphorus and sediment delivered loads to tidal Bay waters—were

used as inputs to the Chesapeake Bay Water Quality Model to evaluate the relative

response of key tidal water quality parameters (i.e. dissolved oxygen, water clarity

and chlorophyll a concentrations) to these watershed loading levels. The range of

▼11

chapter ii • Overview of Technical Tools

chapterii

Overview of Technical Tools
water quality responses, in turn, helped define cause (nutrient and sediment loadings) and effect (tidal water quality) relationships and were used in assessing the

attainability of current and refined designated uses (see Tiered Scenario Estimated

Nutrient and Sediment Loads).

These tiered scenarios do not prescribe control measures necessary for the watershed

jurisdictions to meet the Chesapeake 2000 nutrient and sediment cap load allocations. Again, they were developed as a tool to assess relative water quality impacts

from a range of load reductions. The scenarios are theoretical constructs of technological levels of effort and do not represent actual programs that jurisdictions must

implement or required combinations of region-specific BMPs. Cost effective combinations of BMPs will be evaluated by the jurisdictions working directly with their

tributary strategy teams, who will address real issues such as physical limitations and

any potential adverse economic impacts from implementation.

The tiered scenarios characterize the Chesapeake Bay watershed’s nutrient and sediment reduction potential in terms of types of BMPs, extent of implementation and

performance of BMPs for both point and nonpoint sources (Appendix B). Tier definitions were designed to ensure each Tier went beyond the nutrient and sediment load

reductions of the previous tier and, therefore, imply a ‘level of effort’. The scenarios

range from the Tier 1 scenario, representing extensions of current implementation

rates throughout the watershed plus regulatory requirements in place by 2010, to

everything, everywhere by everybody (E3 scenario), which goes beyond any previous

Chesapeake Bay Program definition of ‘limit of technology’ (LOT). Two intermediate

levels of implementation, the Tier 2 and Tier 3 scenarios, were also developed. 

If the only objective of developing the tier scenarios was to relate tidal water quality

response to nutrient and sediment load reductions, it would not have been necessary

to define the scenarios in terms of increased implementation levels of BMPs and

control technologies. This objective could have been accomplished by setting incremental loading reductions from all tributary basins in the Bay watershed. However,

assessments of attainability of the current and refined tidal water designated uses

required the association of load reductions to specific implementation levels of

BMPs or control technologies, their nutrient and sediment reduction efficiencies, and

their feasibility of implementation.

Important Note: Tiers are artificial constructs of technological levels of effort

and were not meant to represent actual programs the jurisdictions will eventually implement to meet water quality standards. In addition, the tiers do not

denote combinations of region-specific BMPs that would best reach the

nitrogen, phosphorus and sediment cap loads allocated to each jurisdiction

within each tributary. They were developed as an assessment tool to determine

relative water quality impacts from a range of load reductions. Tier definitions

were designed to ensure each Tier went beyond the nutrient and sediment load

reductions of the previous Tier.

Appendix A of the Technical Support Document describes the development of 

the tier scenarios and the pollutant control technologies represented in each tier 

(U.S. EPA 2003a). Both Appendix A and Chapter V of this document present the

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chapter ii • Overview of Technical Tools
Chesapeake Bay watershed model estimates of the nitrogen, phosphorus and sediment load reductions associated with each of the tier scenarios.

The tier scenarios were based primarily on BMPs and control technologies directed

toward reductions in nitrogen and phosphorus loads. The model-simulated sediment

reductions were incidental responses to the implementation of nutrient reduction

BMPs. Other sediment reduction management practices are available and may, if

implemented along with nutrient reduction efforts, afford additional water quality

improvements (see Chapter III). This is especially true for BMPs applied in the near

shore areas of the tidal Bay.

DEVELOPMENT OF THE TIERED SCENARIOS

The tiered BMP implementation levels were initially defined by the ‘source’ workgroups of the Chesapeake Bay Program Nutrient Subcommittee. The Agricultural

Nutrient Reduction, Forestry, Point Source and Urban Stormwater workgroups,

which is comprised of representatives of Bay watershed jurisdictions and other technical experts, contributed expertise and information for their assigned ‘source’. In

some cases, the Nutrient Subcommittee’s Tributary Strategy Workgroup edited the

tier scenario definitions so that necessary input decks for the watershed model,

which captured the essence of the definitions, could be developed. 

Projected 2010 Conditions

All tier scenarios were based on 2010 projections of land uses, human populations,

agricultural animal populations, point source flows, and septic systems, as well as

2007/2010 or 2020 air emissions. Land use and human and animal population

projections were developed from an array of national, regional, and state databases

as described in Chesapeake Bay Watershed Model Land Use and Model Linkages to

the Airshed and Estuarine Models (Hopkins et al. 2000).

Agricultural land uses were projected from agricultural census county information

(1982, 1987, 1992 and 1997) according to methodologies chosen by individual states.

The projected animal populations were also based on county agricultural census

trends and information provided by state environmental and agricultural agencies.

Urban land uses for 2010 were projected from a methodology involving human

population changes, as determined by the U.S. Census Bureau for 1990 and 2000, as

well as by some individual state agencies. The population changes were related to

1990 high-resolution satellite imagery of the Chesapeake Bay watershed, which is

the root source of urban and forest land acreages. In the case of Maryland, urban

growth from 2000 to 2010 was determined by Maryland Department of Natural

Resources and Maryland Department of Planning. 

With urban and agricultural land use acreages fixed for 2010, the remaining land was

divided between forest and mixed open in the same proportion that existed in 1990

for New York, Pennsylvania, Delaware, West Virginia, and the District of Columbia.

The 2010 forest acreage was fixed for Maryland and Virginia following methodologies or data submitted by those states with the remaining acreage being mixed open. 

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chapter ii • Overview of Technical Tools
Tier Scenario Components

Point Source

A multi-stakeholder Nutrient Reduction Technology Cost Task Force, which

consisted of federal, state and local representatives as well as municipal authority

representatives and expert consultants, was formed as a temporary extension of the

Nutrient Subcommittee’s Point Source Workgroup. The Task Force defined what

would be logical tiers (break points) for incremental levels of point source control

technology implementation (U.S. EPA 2002). Using wastewater flows projected for

the year 2010, the tier scenarios range from the current (year 2000) treatment levels

to the E3 scenario. 

Future flow projections were developed either from population projections or information obtained directly from the municipal facility operators. The tier and E3

scenario flows for industrial dischargers remained at 2000 levels because these flows

are not necessarily subject to population growth. The point source facilities analyzed

in this effort include all significant facilities (including industrial) as defined by 

New York, Pennsylvania, Maryland, Virginia, Delaware, West Virginia and the

District of Columbia.

Nonpoint Source: Agriculture

In the Tier 1 scenario, nonpoint source agricultural BMP implementation rates

between 1997 and 2000 were continued to the year 2010 with certain limitations.

Since historic BMP data were not available from New York, Delaware and West

Virginia, 2010 Tier 1 projections were determined from watershed-wide implementation rates from states that employed and tracked similar practices from 1997

through 2000.

The Tier 2 and Tier 3 scenario BMP implementation levels were generally determined by increasing BMP implementation by a fixed percentage of the remaining

acreage between Tier 1 and E3 levels. The percentages were specific for each BMP

and applied watershed-wide.

Nonpoint Source: Urban

The Tier 1 scenario represents voluntary and regulatory storm water management

programs that are or will be in place by 2010. These include both federal programs,

such as the EPA’s National Pollutant Discharge Elimination System Phase I and II

storm water regulations, and state erosion control/storm water management

programs. The Tier 2 and 3 scenarios represent progressively increasing levels of

urban nonpoint source BMP implementation beyond Tier 1. 

Atmospheric Deposition

The Chesapeake Bay Program modeled four different nitrogen oxide (NOX) emission reduction scenarios to estimate changes in atmospheric nitrate deposition and

loading to the Chesapeake Bay watershed (U.S. EPA 2003a). The NOX emission

reductions associated with Tier 1 and 2 scenarios are based on Clean Air Act

regulations. In the Tier 3 and E3 scenarios, NOx emission reductions go beyond

current regulations and include aggressive voluntary controls. All scenarios involve

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chapter ii • Overview of Technical Tools
the combined NOX emission reductions from 37 states within the Chesapeake Bay

airshed, well beyond the Chesapeake Bay watershed jurisdictions’ boundaries.

E3 Scenario

To estimate non-attainment caused by human-caused conditions that cannot be

remedied, a boundary scenario had to be defined. In the past, the Chesapeake Bay

Program partners defined this as the ‘limit of technology’. The BMP levels and

control technologies in the E3 scenario are believed by the Chesapeake Bay Program

partners to be beyond feasible. Cost, physical limitations and social/economic

impacts were not taken into account in order to eliminate subjectivity as much as

possible from the E3 definitions.

The E3 scenario represents the maximum theoretical implementation of the best

combination of BMPs or control technologies available to a land use or situation. It

is assumed that nutrient and sediment reductions beyond this level represent

“human-caused conditions that cannot be remedied.” Generally, these are the best

nutrient and sediment reductions possible with current technologies at maximum

BMP implementation levels and include new technologies and management practices that are not currently part of jurisdictional pollutant control strategies or

federal, state or local cost-share programs. Appendix A of the Technical Support

Document details the assumptions and methodologies used to develop each control

technology and BMP-based implementation level in the E3 scenario for all nutrient

and sediment source categories and land uses (U.S. EPA 2003a). 

TIERED SCENARIO ESTIMATED NUTRIENT AND SEDIMENT LOADS

The estimated Watershed Model loads for nitrogen, phosphorus and sediment loads

from simulated implementation of the tiered and E3 scenarios are described below

and graphically summarized in figures II-1, II-2 and II-3, respectively.

These load estimates are compared with modeled 2000 Progress loads of 285 million

pounds of nitrogen per year, 19 million pounds of phosphorus per year, and 5.04

million tons per year of land-based sediment.

Tier 1

The nutrient and sediment loads associated with the Tier 1 scenario are 261 million

pounds of nitrogen per year, 19.1 million pounds of phosphorus per year, and 4.64

million tons of land-based sediment per year. This represents an 8 percent reduction

in nitrogen, a 1 percent reduction in phosphorus, and an 8 percent reduction in sediment loads to the Chesapeake Bay from 2000 progress levels. 

Tier 2

The nutrient and sediment loads associated with the Tier 2 scenario are 221 million

pounds of nitrogen per year, 16.4 million pounds of phosphorus per year, and 

4.14 million tons of land-based sediment per year. Compared to 2000 progress

estimated loads, this represents a 22 percent reduction in nitrogen, a 14 percent

reduction in phosphorus, and an 18 percent reduction in sediment loads. 

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chapter ii • Overview of Technical Tools
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chapter ii • Overview of Technical Tools

Figure II-1. Nitrogen loads delivered to the Chesapeake Bay and its tidal tributaries under the

watershed model-simulated 2000 Progress, tiered and E3 scenarios.

Source: Chesapeake Bay Program website http://www.chesapeakebay.net.

284.76

260.93

221.26

180.76

116.38

175

0

50

100

150

200

250

300

2000 Progress 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

Nitrogen Loads

(million pounds per year)

Chesapeake Bay Watershed 2010 Cap Load Allocation

Figure II-2. Phosphorus loads delivered to the Chesapeake Bay and its tidal tributaries under the

watershed model-simulated 2000 Progress, tiered and E3 scenarios.

Source: Chesapeake Bay Program website http://www.chesapeakebay.net.

19.12 18.96

16.41

13.38

10.10

12.8

0

5

10

15

20

25

Phosphorus Loads

(million pounds per year)

2000 Progress 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

Chesapeake Bay Watershed 2010 Cap Load Allocation
Tier 3

The nutrient and sediment loads associated with the Tier 3 scenario are 181 million

pounds of nitrogen per year, 13.4 million pounds of phosphorus per year, and 

3.62 million tons of land-based sediment per year. This represents a 37 percent

reduction in nitrogen, a 30 percent reduction in phosphorus, and a 28 percent reduction in sediment loads from 2000 levels. 

E3

The combination of aggressive land and air nutrient controls resulted in E3 scenario

loads of about 116 million pounds of nitrogen per year, 10.1 million pounds of phosphorus per year, and 2.95 million tons of land-based sediment per year. Compared to

2000 progress loads, this represents a 59 percent reduction in nitrogen, a 47 percent

reduction in phosphorus, and a 41 percent reduction in land-based sediment loads. 

CHESAPEAKE BAY PROGRAM 

ENVIRONMENTAL MODELS

The Chesapeake Bay Program partners use a series of environmental models to

project changes in the complex Bay ecosystem due to management actions. The

Chesapeake Bay Program has developed what have become standard large watershed estuarine modeling tools, including an airshed model or Regional Acid

Deposition Model (RADM) (Shin and Carmichael 1992; Appleton 1995, 1996), a

watershed model (Donigian et al. 1994; Linker 1996; Linker et al. 2000), an estuarine hydrodynamic model (Wang and Johnson 2000), an estuarine water quality

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chapter ii • Overview of Technical Tools

Figure II-3. Land-based sediment loads delivered to the Chesapeake Bay and its tidal tributaries

under the watershed model-simulated 2000 Progress, tiered and E3 scenarios.

Source: Chesapeake Bay Program website http://www.chesapeakebay.net.

5.04 4.64

4.14

3.62

2.95

4.15

0

1

2

3

4

5

6

Sediment Loads

(million tons per year)

2000 Progress 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

Chesapeake Bay Watershed 2010 Cap Load Allocation
model (Cerco and Cole 1993, 1995a, 1995b; Thomann et al. 1994; Cerco and

Meyers 2000; Cerco 2000; Cerco and Moore 2001; Cerco et al. 2002a), and an

estuarine sediment diagenesis model (Di Toro and Fitzpatrick 2001). The Chesapeake Bay Program has used these environmental models for more than 18 years and

has refined and upgraded each of the models several times. Figure II-4 portrays the

interconnections among these cross-media models.

Results from the integrated airshed, watershed and estuarine models are used to

elucidate complexities like eutrophication of the Chesapeake Bay or to closely

examine sediment sources to assess their impacts on water quality and living

resources in tidal waters. Together, these linked simulations provide a system to

estimate dissolved oxygen, water clarity and chlorophyll a conditions in 35 major

segments of the Chesapeake Bay and its tidal tributaries. The same criteria

attainment assessment process applied to observed data is applied to integrated

modeling/monitoring ‘scenario’ data to determine likely criteria attainment under

management loading scenarios (U.S. EPA 2003b, Linker et al. 2002).

The watershed and airshed models are loading models. As such, they provide estimates of the impacts of management actions through air emission controls,

agricultural and urban best management practices, and point source technologies

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chapter ii • Overview of Technical Tools

Regional Acid Deposition Model

Chesapeake Bay Watershed Model

Submerged Aquatic

Vegetation

Hydrodynamic Model

of the Bay, Tributaries, and Continental Shelf

Sediment Benthic

Model

Water Quality Model

Chesapeake Bay Estuary Model

Figure II-4. Cross-media models of the Chesapeake Bay airshed, watershed and estuary.

Source: Chesapeake Bay Program website http://www.chesapeakebay.net.
that will reduce nutrient or sediment loads to the Chesapeake Bay tidal waters. The

advantage of using loading models is that the full simulation through different

hydrology periods (i.e., wet, dry and average) can be simulated on existing or hypothetical land use patterns. All of the Chesapeake Bay Program models used in this

system simulate the same 10-year period from 1985 to 1994 (Linker et al. 2000).

The models are linked together so that the output of one simulation provides input

data for another model. For example, the nitrogen output from RADM affects the

nitrogen input from atmospheric deposition to the Watershed Model. The Watershed

Model, in turn, transports the total nutrient and sediment loads, including the contributions from atmospheric deposition, to the Chesapeake Bay and its tidal tributaries

through the boundary of the watershed and estuarine domains. The Water Quality

Model examines the effects of the loads generated by the Watershed Model, as well

as the effects of direct atmospheric deposition, on Bay water quality and living

resources. 

The models used by the Chesapeake Bay Program focus on quantifiable outcomes,

such as reductions in estimated nutrient and sediment loads resulting from integrated

point source, nonpoint source and air emission management actions, rather than a

pollutant reduction strategy based on a single medium. For Chesapeake Bay

Program decision-makers, model results are options to be examined, analyzed and

further developed through an iterative process with the model practitioners. This was

the process involved in determining cap load allocations (see section above titled

Tiered Management Implementation Scenarios).

The models produce estimates, not perfect forecasts. Hence, they reduce, but do not

eliminate, uncertainty in environmental decision making. Used properly, the models

assist in developing nutrient and sediment reductions that are most protective of the

environment, while being equitable and achievable.

CHESAPEAKE BAY AIRSHED MODEL AND 

ATMOSPHERIC DEPOSITION

Regional Acid Deposition Model 

The Regional Acid Deposition Model, or RADM, is designed to provide estimates

of atmospheric nitrogen deposition resulting from changes in precursor air emissions

due to management actions or growth, and to predict the influence of source loads

from one region on deposition in other regions (Chang et al. 1987). The current

version of RADM, RADM 2.61, encompasses a geographic domain of 2,800 kilometers by 3,040 kilometers (Dennis 1996). Longitudinal coverage in the eastern

United States is central Texas to Bermuda while latitudinal coverage is from south

of James Bay, Canada to Florida, inclusive (Figure II-5). Grid cells are 80 kilometer

by 80 kilometer with 15 vertically layered cells placed from ground level to the top

of the troposphere, which equals an altitude of 16 kilometers. The total number of

cells in the model domain is 19,950 (Chang et al. 1990). As shown in Figure II-5,

over the regions of the mid-Atlantic states and the Chesapeake Bay watershed, the

RADM contains a finer grid of 20 kilometer by 20 kilometer cells nested into the

larger grid, allowing finer spatial distribution of nitrogen deposition. 

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chapter ii • Overview of Technical Tools
The RADM has been used to estimate the area where nitrogen emission sources have

the greatest potential in depositing nitrogen, both wet and dry, to a watershed. The

area encompassing these sources is referred to as the ‘principal airshed’. Figure II-6

shows the boundaries of the Chesapeake Bay watershed juxtaposed with the

principle airsheds for both reduced (ammonia) and oxidized (NOX) nitrogen. The

Chesapeake Bay’s ammonia airshed is about 688,000 square kilometers (266,000

square miles) in size. This is four times larger than the Chesapeake Bay’s watershed

and two-thirds the size of the NOX airshed which is 418,000 square miles (1,081,600

square kilometers) (Paerl et al. 2002). 

Airsheds are not as firmly defined as watersheds in that there are no clear boundaries

to the flow of chemicals in the atmosphere as there are for the flow of surface and

ground waters in watersheds. The absolute influence that an emission source has on

deposition to an area continuously diminishes with distance. Operationally, modelers

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chapter ii • Overview of Technical Tools

Figure II-5. Regional Acid Deposition Model domain grid and fine-scale nested grid for

the Chesapeake Bay watershed. 

Source: Dennis 1996.

Legend

US States

20 Km RADM grid

80 Km RADM grid

Chesapeake Bay Watershed
have found that a good distance of demarcation for setting the airshed boundary is

the 65 percent contour of the normalized range of influence of a source region. 

It is important to understand this concept of airsheds because the relationships

between emissions and deposition, and subsequently atmospheric loadings into a

water body, are not equal. For example, if 100 pounds of nitrogen were released into

the air from a source, it will not all be deposited at once nor in one area. The annual

deposition will be distributed over space and will be unevenly distributed in time.

Just as emissions and deposition are not in a 1:1 ratio, neither are deposition and

loadings to a water body. The terrestrial landscape will retain much of the deposited

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chapter ii • Overview of Technical Tools

REDUCED

OXIDIZED

Figure II-6. Principle nitrogen airsheds for the Chesapeake Bay and its watershed. 

Source: Chesapeake Bay Program website http://www.chesapeakebay.net/wqcmodeling.htm.

REDUCED
nitrogen. For example, current belief is that approximately 10 percent of nitrogen

deposited to a typical forest ecosystem will be transported into receiving waters.

The three-dimensional RADM solves a series of conservation equations and

considers a complex range of physical and chemical processes and their interactions.

It is an Eulerian model in which the concentrations of gaseous and particulate

species are calculated for the specific grid cells as a function of time. The calculation depends on emission input rates, as well as three-dimensional advective

transport, dry deposition rates, turbulent transport, chemical transformations, scavenging and precipitation.

Meteorological fields used for advective transport and meteorological conditions for

RADM chemistry are from the Pennsylvania State University National Center for

Atmospheric Research Mesoscale Model (MM4). The MM4 is a weather model used

to recreate detailed meteorology (Dennis et al. 1990; Brook et al. 1995a, b).

The chemistry that is simulated by the model consists of 140 reactions among 60

species. Photolysis and oxidant photochemistry is included in the simulation as are

aqueous phase reactions which occur in clouds. Forty-one of the longer-lived chemical species are transported between model cells. 

The key nitrogen species that are simulated and are of concern to coastal watersheds

are: 1) particulate nitrate (pNO3

-

), nitric acid gas (HNO3) and nitrate (NO3) in

precipitation, which all originate from NOX emissions; 2) particulate ammonium

(NH4

+), ammonia gas (NH3) and ammonium in precipitation, which all originate

from ammonia emissions; and 3) dissolved organic nitrogen (DON). Although the

sources of DON are not well identified, it is believed to be a small fraction of the

total nitrogen deposition. 

The nitrogen oxide emissions that are accounted for in the RADM include those

from anthropogenic fuel combustion, soil biological processes and ammonia. These

emissions are input to the completely mixed grid cells of the model on an hourly

time step. The simulation uses dynamically determined time steps of seconds to

minutes to generate model output of wet and dry deposition on an hourly basis for

each surface cell. 

Determination of Atmospheric Flux

While the RADM provides estimates of atmospheric deposition due to growth or

management of atmospheric emissions, a base data set of atmospheric deposition is

needed to provide a continuous 10-year time series of daily atmospheric deposition

loads to the watershed and estuary models. This base condition of deposition establishes a reference to which other atmospheric deposition reduction scenarios are

compared, quantifying the effects of managed reductions in emissions. The reduction scenarios are rooted in RADM results, represent changing levels of both

regulatory and voluntary controls, and are simulated from utility, industrial and

mobile sources. 

Since precipitation is the primary forcing function in the Chesapeake Bay Watershed

Model, great care is taken in developing the time-variable atmospheric flux. A data

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chapter ii • Overview of Technical Tools
set of wet deposition of nitrate and ammonia is formed through concentration data

from a regression model and precipitation data from gauging stations that are

weighted according to a Thiessen polygon method. 

The regression model uses National Atmospheric Deposition Program/National

Trends Network data from monitoring stations in the Chesapeake watershed area to

determine wet inorganic nitrogen concentrations. The regression calculates concentrations from measured precipitation amounts, the month of the year, and latitude.

The concentrations are then applied to the volume of precipitation, for each model

segment, to establish daily deposition of wet nitrate, ammonia, and organic nitrogen

for the 10-year simulation period of the Watershed Model. A rate of dry deposition

of nitrate is determined for each model segment from average proportions of wet-todry deposition calculated by RADM. 

When used for scenarios that have reduced emissions and subsequent deposition in

the Chesapeake watershed, RADM information on nitrogen emission reductions is

applied to the Watershed Model through a proportioning method. It is assumed that

the RADM reference inputs are the same as the calculated atmospheric flux. Fractional changes to the RADM reference deposition are related to the deposition

database for each chemical species and both spatially and temporally. The results are

revised fluxes to the watershed, tidal waters and their respective models that are

used, in part, to determine the effects of emission controls on nutrient loads, water

quality and living resources. 

CHESAPEAKE BAY WATERSHED MODEL

The Chesapeake Bay Watershed Model estimates the delivery of nutrients and sediment from all areas of the watershed to tidal waters under different management

scenarios (Donigian et al. 1994; Linker et al. 1996; Linker 1996). The continuous,

deterministic model has been in operation at the Chesapeake Bay Program since

1982. Since that time, many refinements to the simulation and data used in it have

been made. Phase 4.3 of the Watershed Model, in conjunction with the airshed and

estuarine models, was employed in the development of the nutrient and sediment cap

load allocations.

The Chesapeake Bay Program’s Watershed Model is based on a slightly modified

version of Hydrologic Simulation Program-Fortran (HSPF) release 11 (Bicknell et

al.1996), a widely used public domain model supported by the U.S. Environmental

Protection Agency, U.S. Geological Survey, and U.S. Army Corps of Engineers. The

system is run on personal computers with the Linux operating system. All supporting

programs as well as HSPF are open source and written primarily in Fortran 77. 

Nutrient simulation modules in the Watershed Model are detailed and flexible, and

thus can be used to simulate a variety of land use types with associated applications

of chemical fertilizers and animal manure. The model also takes into account loads

from point sources, atmospheric deposition and onsite wastewater management

systems. In addition, the simulation considers nutrient and sediment reductions due

to BMP implementation as well as attenuation of chemical species as they travel

through the river reaches to tidal waters of the Chesapeake Bay. The Watershed

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chapter ii • Overview of Technical Tools
Model simulates a period of ten years (1985-1994) on a one-hour time step and

results are aggregated into daily loads and flows, to be used as input to the estuary

model or into reported 10-year average loads for comparison among scenarios. 

Watershed Model Segmentation

To simulate the delivery of nutrients and sediment to the Chesapeake Bay, the 64,000

square mile Chesapeake Bay watershed is divided into 94 major hydrologic model

segments that have an average segment area of 680 square miles (177,000 hectares)

(Figure II-7). At the interface of the Watershed Model and Water Quality Model

domains, below-fall-line model segments are further divided into sub-segments to

deliver flow and nutrient and sediment loads to appropriate areas of the tidal waters

(Hopkins et al. 2000). 

Segmentation partitions the watershed into regions of similar characteristics based

on criteria such as topographic areas with similar soil characteristics and slopes, or

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chapter ii • Overview of Technical Tools

Major Tributaries of the Chesapeake Bay

EA STERN SHORE MARY LAND

EA STERN SHORE VIRGINIA

JAMES RIVE R BASIN

PA TUXENT RIVE R BASIN

POTOMAC RIVER BASIN

RAPPAHANNOCK RIVER BASIN

SUSQUEHANNA RI VER BASIN

WESTERN S HORE MARYLAND

YORK RIVER BASIN

Figure II-7. Chesapeake Bay Watershed Model segmentation and major tributary basins. 

Source: Linker et al. 2000. 
similar travel times in river reaches (Hartigan 1983). Another consideration in

defining model segments is the location of reservoirs or monitoring stations. Model

segments are located so that segment outlets are as close as possible to monitoring

stations that collect water quality and discharge data (Langland et al. 1995). 

The proximity of monitoring stations to the outlet of model segments is important

because the model is calibrated at the segment level. It is imperative to have the most

accurate calibration of nutrient and total suspended sediment concentrations and

flows in the river reaches so that the output loads of one segment accurately input

the adjacent downstream segment. 

Overall, the right size for segmentation weighs two factors. If a segment is too large,

meaningful differences of many of the simulation parameters are missed. If a

segment is too small, it could be difficult to acquire all the data for the simulation at

that level, or the computing capacity of the model could be limited. 

Watershed Model Calibration

The Chesapeake Bay Program calibrates the Watershed Model over all available data

and then uses the calibrated model to test management scenarios. In the Phase 4.3

version of the model, flow and water quality data from 1984-1997 were used for

calibration. The calibration was reviewed and approved by Chesapeake Bay Program

Modeling Subcommittee. The subcommittee members and quarterly review participants are recognized academic experts in the field of modeling and

representatives from all Chesapeake Bay Agreement signatory jurisdictions—

Pennsylvania, Maryland, Virginia and the District of Columbia. 

For the calibration of land uses, simulated exports from land uses are compared to

literature values and an analysis of the inputs. For example, the calibration of cropland considers the growth and nutrient uptake of estimated crop types—taking into

account drought, heat stress, and the growing season—as well as considerations of

the estimated nutrient inputs. The simulated cropland exports are, in turn, compared

to export values published in peer-reviewed scientific literature and relevant model

parameters are adjusted, if necessary, to achieve the best match. 

For the calibration of river reaches, simulated results for stream flows, nutrient and

sediment concentrations and loads, as well as other water quality parameters, are

compared to observed data from in-stream monitoring sites. Results for the

hydrology calibration of the Phase 4.3 Watershed Model can be found on the

Chesapeake Bay Program Web site at http://www.chesapeakebay.net/pubs/113.pdf

while water quality calibration information can be accessed at: http://www.

chesapeakebay.net/pubs/238.pdf. 

Calibration results are presented as plots and statistical tables of model information

and monitoring data from calibration stations for the following parameters: flow,

temperature, dissolved oxygen, total suspended sediment, total phosphorus, organic

and particulate phosphorus, phosphate, total nitrogen, nitrate, total ammonia and

organic nitrogen. 

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chapter ii • Overview of Technical Tools
Watershed Model Data Sources 

Since precipitation, in a large part, drives loads to the tidal Bay water, much effort is

spent developing this data base. For the 10-year simulation period of the Watershed

Model, rainfall data from 147 monitoring stations are used. Typically, about six

stations are used to develop the precipitation record for a model segment through a

Thiessen polygon method for spatial distribution. In addition, temperature, solar

radiation, wind speed, snow pack and dew-point temperature data are collected for

the simulation from seven primary meteorological stations in the watershed (Wang

et al. 1997). 

A consistent land use dataset is compiled for the entire Chesapeake basin using a

LANSAT-derived GIS land cover as a base (U.S. EPA 1994). The land cover is

enhanced with detailed information on agricultural lands at the county level from the

U.S. Census Bureau series, Census of Agriculture for 1982, 1987, 1992 and 1997

(Volume 1, Geographic Area Series). County tillage information is acquired for the

conventional and conservation cropland distribution from the Conservation Technology Information Center (Palace et al. 1998). The land or source categories

simulated in the Watershed Model are as follows:

• Conventional-tilled cropland;

• Conservation-tilled cropland;

• Cropland in hay;

• Pasture;

• Animal waste areas;

• Forest;

• Pervious urban;

• Impervious urban land;

• Non-agricultural herbaceous or mixed-open land; and

• Atmospheric deposition directly to water surfaces.

Calculations and allocations of the agricultural land categories follow methods

described in Chesapeake Bay Watershed Model Land Use and Model Linkages to the

Airshed and Estuarine Models (Hopkins et al. 2000). The non-agricultural land use

classifications of ‘forest’, ‘pervious’ and ‘impervious urban’, ‘mixed-open’ and

‘water’ are generally developed through comparisons of the agricultural land acreage

and the GIS land cover database and projections or interpolations of these. Hopkins

et al. (2000) describes these calculations and allocations in detail.

For crop land, state agricultural engineers provide chemical fertilizer and manure

application rates and timing of applications as well as information on crop rotations

and the timing of field operations. The information on manure applications to

cropland is part of a time-varying mass balance of manure nutrients developed

through the Agricultural Census’ animal populations and predominant manure

handling practices (Palace et al. 1998). 

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chapter ii • Overview of Technical Tools
Soil characteristics for both nutrient interactions and hydrology are obtained from the

Soil Conservation Service Soil Interpretation Records (USDA 1984) with information

on soil types and land slope from the National Resources Institute (NRI). Delivery of

sediment from each land use is calibrated to the NRI estimates of annual edge-of-field

sediment loads calculated by the Universal Soil Loss Equation (USLE). 

For animal waste areas, the designation of a ‘manure acre’ allows for the simulation

of high nutrient content runoff from animal operations. Manure acres are based on

the population of different animal types in the watershed as given in Agricultural

Census data. The animal types include beef and dairy cattle, swine and three categories of poultry (layers, broilers and turkeys). Nutrient export from animal waste

storage areas is simulated as a concentration applied to the calculated runoff where

the surface area of animal waste storages, or manure acres, changes with the number

of animals and implementation of animal waste management systems. 

Loads from point sources, combined sewer overflows (CSOs), and septic systems are

input directly to river reaches. Point source inputs from municipal and industrial

sources are developed from state National Pollution Discharge Elimination System

(NPDES) records. If no state NPDES data are available, state and year-specific

default data are calculated for each missing parameter and annual estimates of loads

are based on flow from the wastewater treatment plant. 

Several cities in the watershed have a sewer system with CSOs, including Washington, D.C., Richmond, Virginia and Harrisburg, Pennsylvania. Estimates of the

average annual discharge from CSOs are only available for Washington, D.C. and the

annual discharge is evenly distributed over the simulation period of the model. 

Loads from septic systems are calculated using U.S. Census Bureau data of waste

disposal systems associated with households, along with a methodology suggested

in Maizel et al. (1995) where standard engineering assumptions of per capita

nitrogen waste and attenuation of nitrogen are applied. Septic system loads are simulated as nitrate loads discharged to the river reaches. 

Watershed Model Simulation

Each Watershed Model segment contains information generated by a hydrologic

sub-model, a nonpoint source sub-model, and a river or transport sub-model. The

hydrologic sub-model uses rainfall, evaporation and meteorological data to calculate

runoff and subsurface flow for land uses in each model segment. 

The surface and subsurface flows ultimately drive the nonpoint source sub-model,

which simulates soil erosion and pollutant loads from the land to the rivers using, in

part, input data for atmospheric deposition, land use areas, nutrient applications, and

BMP implementation and reduction efficiencies. A river sub-model routes flow and

associated pollutant loads from the land through lakes, rivers and reservoirs to the

Chesapeake Bay.

For nitrogen and phosphorus, the simulation represents a mass balance in the basin,

so that the ultimate fate of the input nutrients is 1) incorporation into crops, forests,

or other vegetation, 2) incorporation into soil, or 3) loss through river runoff or

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chapter ii • Overview of Technical Tools
discharge directly to the Chesapeake Bay. Fate of nitrogen may also include

volatilization to the atmosphere and denitrification. 

Much of the nutrient simulation for pervious lands considers cycling and storages in

the soil and plant mass as well as movement between the storages, thereby making

these land use simulations sensitive to nutrient inputs. Crops are specifically

modeled through a yield-based nutrient uptake algorithm allowing for the direct

simulation of nutrient management practices since exports rely heavily on the

nutrient levels above crop need. Nutrient exports from impervious urban land depend

on storage, which accumulates by a factor equal to atmospheric deposition. Rainfall

washes off this storage and the intensity of the rainfall determines how much is

washed off.

Sediment is modeled as eroded material washed off pervious land surface, eroded

from stream banks and transported to the tidal Bay waters. This simulation is

performed through a module, which represents sediment export as a function of the

amount of detached sediment and the runoff intensity. 

The lumped-parameter HSPF model simulates each land use as an average for the

entire segment. For example, conventionally-tilled cropland is modeled as an

average crop rotation of corn, soybeans, and small grains in a segment with an

average model-segment input of chemical fertilizer and manure loads, and with

average slope, soil conditions, and nutrient cycling characteristics. The simulated

single-acre land use, in turn, is multiplied by the acres of each land use draining to

each river segment.

Each Watershed Model river reach is simulated as completely mixed waters with all

land uses considered in direct hydrologic contact. Of the 44 reaches modeled, the

average length is 106 miles (170 kilometers), the average drainage area is 730 square

miles (1,900 square kilometers), and the average time of travel is one day. Seven of

the reaches are impounded by reservoirs and are simulated as such. 

The riverine simulation includes HSPF modules that consider, in part, sediment

transport, oxygen transformations, ammonification, nitrification, and modeling of

periphyton and phytoplankton. For areas close to the Chesapeake Bay and its tidal

tributaries with a time of travel less than one day, a river reach is not modeled and

terrestrial nutrient and sediment loads are directly loaded to the tidal estuary. 

For all nutrient and sediment reduction scenarios, the Watershed Model is run for a 10-

year hydrologic period, representing 1985 to 1994, inclusive. This time frame matches

the years simulated in the Chesapeake Bay Estuary Model and provides a consistent

10-year hydrology, including wet, dry, and average periods of flow in each basin. 

Nutrient and sediment loads from the Watershed Model are reported as the average

annual load over this 10-year period to make comparisons among model scenarios

without the influence of variable hydrology on loads. For example, any 2010

scenario has land uses, human and animal populations, point source discharges, and

land management projected to the year 2010 but modeled using the same 1985–1994

hydrology used for all other Watershed Model scenarios. Assessing loads for an

average-hydrology year shows how anthropogenic factors, such as changes in land

use and management practice implementation, change average annual nutrient and

sediment loads to the Chesapeake Bay over decadal periods of time.

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chapter ii • Overview of Technical Tools
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chapter ii • Overview of Technical Tools

CHESAPEAKE BAY WATER QUALITY MODEL

The Chesapeake Bay Water Quality Model used to assist in developing the

Chesapeake 2000 nutrient and sediment cap load allocations is a linked hydrodynamic and water quality model which is coupled to a sediment processes, benthic

infaunal community and submerged aquatic vegetation (SAV) model. The Chesapeake Bay water quality model is a ‘third generation’ model with two major

refinements since its debut in 1992 when it was first used to develop the original

nutrient cap load allocations committed to in the 1987 Chesapeake Bay Agreement

(Chesapeake Executive Council 1987; Thomann et al. 1994; Cerco and Cole 1993;

Cerco et al. 2002a, 2002b). In 1998, the model grid was refined in the lower Virginia

tidal tributaries and lower Chesapeake Bay mainstem with the new benthic infauna

and SAV model. In 2002, the upper Chesapeake Bay mainstem and tidal tributaries

grid was refined along with significant enhancements in the model simulation of

primary productivity (Cerco et al. 2002a). During the 2002 model refinements,

particular emphasis was placed on the calibration and analysis of dissolved oxygen,

sediment, water clarity, SAV and chlorophyll a.

HYDRODYNAMIC MODEL

The Chesapeake Bay Hydrodynamic, or CH3D (Curvilinear Hydrodynamics in 3

Dimensions), Model provides advective transport for dissolved and particulate material simulated in the Water Quality Model. The model grid covers the entire

Chesapeake Bay, tidal tributaries and the adjacent ocean boundary with about 13,000

computation model cells.

The complex movement of water within the Chesapeake Bay, particularly the density

driven vertical estuarine stratification, is simulated with a Chesapeake Bay

hydrodynamic model of more than 13,000 cells (Wang and Johnson 2000). Threedimensional equations of the intertidal physical system, including equations of

continuity, momentum, salt balance and heat balance, are employed to provide the

correct simulation of the movement, or the barriers to movement, of the water quality

constituents of dissolved oxygen, water clarity and chlorophyll a. Correct formulation of vertical mixing, including the simulation of vertical eddy diffusion

coefficients in the hydrodynamic model is particularly important for the dissolved

oxygen criteria as the principal barrier to vertical movement of dissolved oxygen

from surface waters to the deep water is the pycnocline simulated by the hydrodynamic model.

The Hydrodynamic Model was applied to generate a 10-year record of hydrodynamic transport for the Water Quality Model. The years that were simulated

(1985–94) cover a wide hydrologic range. The years 1985, 1988 and 1992 are

considered dry years; 1986, 1987, 1990 and 1991 are considered average years; and

1989, 1993 and 1994 are considered wet years. Although 1985 is considered a dry

year overall, in November of that year the track of hurricane Juan swept over the

upper Potomac and James basins and generated hundred-year-storm flows at the fall

lines of these rivers.
As its name implies, the Hydrodynamic Model makes computations on a generalized

curvilinear, or boundary-fitted, horizontal grid, i.e., the grid from the planar view

follows the shape of the Bay’s shoreline. However, to ensure that long-term stratification in the deep channels is maintained, the vertical grid, corresponding to depth,

is Cartesian. Boundary-fitted grids in the horizontal plane allow for a better representation of the shoreline boundaries of the Chesapeake Bay, as well as internal

features such as channels and islands (Figure II-8). 

Mathematical simulations of all physical processes influencing circulation and

mixing in water bodies such as Chesapeake Bay are included in the Hydrodynamic

Model. These include freshwater inflows, tides, wind forcing, Coriolis forces,

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chapter ii • Overview of Technical Tools

Figure II-8. Plan view of the Chesapeake Bay Hydrodynamic or CH3D Model boundary

fitted grid.

Source: Cerco and Meyers 2000.
surface heat exchange and turbulence. The vertical turbulence closure model

computes the eddy viscosity and diffusivity from the kinetic energy and dissipation

of the turbulence. This type of closure model is known as a ‘k-e turbulence model’.

Turbulence is produced by wind stress at the surface, velocity shear in the water

column and bottom friction. Density effects due to salinity and temperature are fully

coupled with the developing flow field. Thus, advection/diffusion equations for the

salinity and temperature are solved along with the conservation of mass and

momentum equations for the flow field.

Complete documentation of the Hydrodynamic Model can be found in “Chapter 2:

Validation and Application of the Second Generation Three-Dimensional Hydrodynamic Model of Chesapeake Bay” of Tributary Refinements of the Chesapeake Bay

Model (Cerco et al. 2002a) available at: http://www.chesapeakebay.net/modsc.htm

under the Documentation tab. 

WATER QUALITY MODEL

The Water Quality Model, based on the CE-QUAL-ICM code, is a threedimensional, time-variable model of eutrophication processes in the water column

and bottom sediments. As applied to the Chesapeake Bay and its tidal tributaries, the

model is part of a package that includes the Chesapeake Bay Watershed Model and

the Chesapeake Bay Airshed Model described previously. 

The water quality model is linked to the hydrodynamic model and uses complex

nonlinear equations describing 26 state variables relevant to the simulation of

dissolved oxygen, water clarity and chlorophyll a (Cerco and Cole 1995a, 1995b,

2000; Thomann et al. 1994; Cerco and Meyers 2000). The state variables include the

full suite of nitrogen parameters (ammonia, nitrate, total nitrogen, dissolved labile

organic nitrogen, dissolved refractory organic nitrogen, particulate labile organic

nitrogen and particulate refractory organic nitrogen) and the equivalent set of

phosphorus and carbon parameters. Dissolved oxygen is simulated as the mass

balance calculation of reaeration at the surface, respiration of algae, benthos and

underwater bay grasses; photosynthesis of algae, benthic algae and underwater bay

grasses; and the diagenesis, or decay of organics, by microbial processes in the water

column and sediment. This mass balance calculation is made for each model cell and

for associated sediment cells at each hourly time step, providing an estimate of

dissolved oxygen from nutrient loads from the watershed and airshed to the waters

of the 35 major segments of the Chesapeake Bay and its tidal tributaries. Water

clarity is estimated from the daily input loads of sediment from the watershed and

shoreline acted on by regionally-calibrated settling rates, as well as estimated advection due to hydrodynamics. Chlorophyll a is estimated based on Monod calculations

of algal growth given resource constraints of light, nitrogen, phosphorous or silica. 

Also, three basic phytoplankton groups, including greens, blue-greens and diatoms,

are simulated. Algal limitation is simulated by Michaelis-Menton kinetics, with the

resource in least supply providing the limitation to growth. Complete diagenesis is

simulated between the water column and sediment as organics settle to the bottom,

are incorporated in the sediment, undergo decomposition, and are ultimately simulated as a return flux of nutrients to the water column, or as deep burial (DiToro and

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chapter ii • Overview of Technical Tools
Fitzpatrick 1993). Lastly, simulation of SAV in shallow waters is coupled with the

model (Cerco and Moore 2001).

Complete documentation of the Water Quality Model can be found in “Chapter 3:

Tributary Refinements to the Chesapeake Bay Model” and “Chapter 4: Phytoplankton Kinetics in the Chesapeake Bay Eutrophication Model” of Tributary

Refinements of the Chesapeake Bay Model (Cerco et al. 2002a) available at:

http://www.chesapeakebay.net/modsc.htm under the Documentation tab. Further

information on the model can be found at the same web site in the document ThreeDimensional Eutrophication Model of the Chesapeake Bay (Cerco and Cole 1994).

Additional detailed documentation of this model is currently being developed and

will be available at the above web site in Spring 2004.

Wetland Sediment Oxygen Demand 

During the most recent refinement and recalibration of the Water Quality Model,

processes simulating the incorporation of oxygen demand by wetland sediment were

built in. In some regions of the Chesapeake Bay tidal tributaries surface waters, a

natural oxygen deficit below saturation levels is typically observed in the summer.

These regions are found adjacent to extensive wetlands and contain comparatively

small volumes of water. The tidal fresh and oligohaline regions of the Mattaponi

(segments MPNTF and MPNOH) and Pamunkey (segments PMKTF and PMKOH)

rivers, respectively, are two specific examples. On the other hand, regions of the Bay

tidal waters where there are extensive tidal wetlands but are bordered by relatively

large bodies of water, such as the Tangier Sound, have sufficient water volumes and

mixing to mask the natural oxygen demand of adjacent wetland sediments.

In the segments with extensive tidal wetlands and small volumes of water, oxygen

demand from wetland sediments is thought to influence surface water dissolved

oxygen concentrations. Recent studies estimate wetland sediment oxygen demand to

range from 1 - 5.3 g O2/m2-day (Neubauer et al. 2000; Cai et al. 1999). In the model,

a uniform oxygen demand of 2g O2/m2-day was used. The wetland sediment oxygen

demand is universally applied in the model based on GIS estimates of tidal wetland

area (Cerco and Noel 2003). 

SAV Model

Three components are required for a systemwide SAV model. The first is a unit-level

model of a plant. The second is a Water Quality Model (described above) that

provides light, temperature, nutrient concentrations and other forcing functions to

the plant component. The third is a coupling algorithm that links the systemwide

environmental model to the local-scale plant model. 

The unit-level plant model incorporates three state variables: shoots (above-ground

biomass), roots (below-ground biomass), and epiphytes (attached growth).

Epiphytes and shoots exchange nutrients with the water-column component of the

eutrophication model while roots exchange nutrients with the diagenetic sediment

component (DiToro and Fitzpatrick 1993). Light available to the shoots and

epiphytes is computed via a series of sequential attenuations by color,

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chapter ii • Overview of Technical Tools
fixed and organic solids in the water column, and self-shading of shoots and

epiphytes. The selection of state variables and basic principles of the model were

based on principles established by Wetzel and Neckles (1986) and Madden et al.

(1996). 

To improve the simulation of SAV, the computation model cell grid was extended

into shallow, littoral zones of depth from 0-2 meters. Following Moore et al. 2000,

three primary SAV communities were simulated: a freshwater community, a mesohaline Ruppia community, and a polyhaline Zostera community. The SAV

simulation was further refined by adding an additional ‘Tidal Fresh Potomac SAV’

group to simulate the canopy-forming (as opposed to meadow-forming) SAV

community of hydrilla (Hydrilla verticillata) and eurasian watermilfoil (Myriophyllum spicatum) in the tidal fresh Potomac (Cerco et al. 2002a).

Additional documentation of the SAV simulation can be found in “Chapter 5:

Systemwide Submerged Aquatic Vegetation Model for Chesapeake Bay” of Tributary Refinements of the Chesapeake Bay Model (Cerco et al. 2002a) available at:

http://www.chesapeakebay.net/modsc.htm under the Documentation tab.

As a part of the 26 state variables that the Water Quality Model simulates by computational model cell, estimates of dissolved oxygen, chlorophyll a, and water clarity

are generated in ten minute time steps. To summarize model information into a

manageable form, the standard output for dissolved oxygen, water clarity and

chlorophyll a is presented as monthly averages for each designated use within each

Chesapeake Bay Program segment. The percent attainment of the parameters is

determined from the adjusted model output.

ADJUSTMENT OF MODEL DISSOLVED OXYGEN, WATER CLARITY

AND CHLOROPHYLL A ESTIMATES

To generate the modified data set for a particular scenario (e.g., 2010 Clean Air Act),

the EPA compared the frequency distribution output from a scenario was compared

with the frequency distribution output of the model calibration. Data were compared

on a month-by-month basis. For example, Figure II-9 illustrates the hypothetical

frequency distribution for dissolved oxygen concentration data in the deep-channel

of Chesapeake Bay mainstem segment CB5MH. The deep-channel dissolved oxygen

criterion is applied May 1 through September 30. From this graph one could infer

that the model was estimating the observed data fairly well, since model-simulated

output matches the mean, approximates the range and has the same characteristic

shape as the frequency distribution of the observed data. However, despite the

acceptable calibration, if the criterion had been set as ‘dissolved oxygen concentration of < 2 mg liter-1 no more than 10 percent of the time’, the model would indicate

a ‘pass’ while the observed data would indicate a ‘fail’.

For each point along the frequency distribution where an observation exists during

the 1985–1994 period, a mathematical relationship between the model scenario and

the model calibration was established by regressing the 30 or so daily values for the

month when the observation occurred in the water quality model cell that contains

the observation. Figure II-10 compares the hypothetical output of a Water Quality

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chapter ii • Overview of Technical Tools
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chapter ii • Overview of Technical Tools

0

1

2

3

4

5

6

7

8

9

Dissolved Oyxgen Concentration 

-1 (mg liter )

Cumulative Frequency of Occurence

Observed Data

Model Calibration

0% 10% 20% 30% 40% 50% 60% 80% 100% 70% 90%

Figure II-9. Frequency distribution of hypothetical observed data and model calibration

for a designated use.

Source: Linker et al. 2002.

0

1

2

3

4

5

6

7

8

9

Dissolved Oyxgen Concentration 

-1 (mg liter )

Cumulative Frequency of Occurence

Observed Data

Model Calibration

Model Scenario

0% 10% 20% 30% 40% 50% 60% 80% 100% 70% 90%

Figure II-10. Frequency distribution of hypothetical observed data, model calibration

and model scenario for a designated use.

Source: Linker et al. 2002.
Model scenario based on a given load reduction to the Water Quality Model output

calibration. These are shown on a frequency plot so that changes in the prediction of

attainment can be seen along with the blue line of the observed data. 

Figure II-11 shows the relationship between the calibration and scenario Water

Quality Model output in more detail. By regressing the scenario output against the

calibration output, one can find a relationship that can be used to transform the

observed data set. The regression generates a unique equation for each point and

month that transforms a calibration value to a scenario value. This relationship is

then applied to the monitored observation as an estimate of what would have been

observed had the Chesapeake Bay watershed been under the scenario management

rather than the management that existed during 1985–1994.

Once the relationship between the calibration and any particular scenario is established, this relationship (applied as a regression equation illustrated in Figure II-12)

is used to generate a ‘scenario-modified’ observed data set for the scenario. The

‘scenario-modified’ values represent an estimate of an observed data set under the

conditions of nutrient and sediment management represented by the scenario. Each

observed value for dissolved oxygen, chlorophyll a and light extinction in the 1985-

1994 data set is replaced with a ‘scenario-modified’ value. 

For a full discussion of this procedure, see A Comparison of Chesapeake Bay

Estuary Model Calibration With 1985-1994 Observed Data and Method of

Application to Water Quality Criteria (Linker et al. 2002) available at:

http://www.chesapeakebay. net/modsc.htm under the ‘Documentation’ tab.

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chapter ii • Overview of Technical Tools

[E3] = 0.8 * [calibration] + 1.5

Calibration

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8

Scenario

Figure II-11. Example of a regression between model calibration and scenario data.

Source: Linker et al. 2002.
MANAGEMENT APPLICATION OF MODEL OUTPUTS

Apart from the adjusted model output that is used for assessing attainment of the

three criteria, it is useful to examine the degree of model calibration in each designated use within each Chesapeake Bay Program segment where the water quality

model will be applied to assess the quality/accuracy of the model calibration. For

this purpose, a strict one-to-one comparison is made between the observed and simulated data. The comparisons are made for the same time (observed and simulated)

and space (real and virtual). 

A set of empirical decision rules were developed for the purpose of assessing the

quality of the calibration for each Chesapeake Bay Program segment designated uses

(Table II-1). The relative performance of the predicted metric (e.g., dissolved oxygen

concentration) compared to the observed metric under the decision rules was rated

as ‘high certainty’, ‘moderate certainty’, or ‘low certainty’ (Linker et al. 2002).

One comparison that was made was the central tendency, the mean or median, of the

data. Another was the dispersion, or standard deviation. Range comparisons of the

minimum or maximum were also employed, as well as examination of the frequency

and scatter plots. A relative confidence estimate of model calibration was determined

from the summary statistics and statistical plots of all the comparisons. Best professional judgement was used in cases where most, but not all, of the criteria were met.

While the open-water dissolved oxygen criteria apply year-round, emphasis was on

the periods critical for the living resources protected by the criteria. Evaluation of the

migratory spawning and nursery dissolved oxygen criteria focused on the late winter

▼36

chapter ii • Overview of Technical Tools Dissolved Oyxgen Concentration -1 (mg liter )

Cumulative Frequency of Occurence

0

0% 10% 20% 30% 40% 50% 60% 80% 100% 70% 90%

1

2

3

4

5

6

7

8

9

Observed

Scenario Modified

Figure II-12. Example of the regression equation applied to the observed.

Source: Linker et al. 2002.
to late spring period, while evaluation of the open-water, deep-water and deepchannel dissolved oxygen criteria focused on June through September. 

A summary of the relative confidence in model calibration by Chesapeake Bay

Program segment by designated use is provided in Table II-2. More detailed

information on the Chesapeake Bay water quality model calibration is available 

at: http://www.chesapeakebay.net/modsc.htm under the publications tab and within

the report A Comparison of Chesapeake Bay Estuary Model Calibration With

1985–1994 Observed Data and Method of Application to Water Quality Criteria

(Linker et al. 2002).

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chapter ii • Overview of Technical Tools

Table II-1. The relative confidence in model calibration findings were used directly by the Water

Quality Technical Workgroup and Water Quality Steering Committee in making judgements as to

exactly where the water quality model outputs could be used for setting the cap load allocations.

Best

Standard Professional

R2 Mean Difference Deviation Judgement

Dissolved Oxygen >0.5 desirable 1.0 mg liter-1 (or roughly 10%); Do not differ by Yes

minimum concentrations do not more than 0.5

differ by more than 2.0 mg liter-1

Chlorophyll a >0.2 desirable Not greater than two times the Do not differ by more Yes

concentration; maximum than three times the 

concentration do not differ observed standard

more than 20.0 mg liter-1 deviation

Water Clarity >0.2 desirable Not greater than two times the Do not differ by more Yes

concentration; maximum than two standard 

concentration do not differ deviations

by more than two times Ke

Source: Linker et al. 2002. 
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chapter ii • Overview of Technical Tools

Table II-2. Relative confidence in Chesapeake Bay water quality model calibration for 35 Chesapeake Bay

Program segments for the three Chesapeake Bay water quality criteria by tidal water designated use.

Dissolved Oxygen Chlorophyll a

Chesapeake Bay Migratory Open Water Deep Deep Water

Program Segment Feb-June All Year Water Channel Spring Summer Clarity

CB1TF a NA NA NA b b a

CB2OH a NA b NA a b a

CB3MH a NA b a a c a

CB4MH NA a b b b a a

CB5MH NA a c c a b a

CB6PH NA a c NA b b a

CB7PH NA a a NA b a a

CB8PH NA a NA NA a a a

PAXTF b NA NA NA a c b

PAXOH b NA NA NA c c b

PAXMH a b b NA a b a

POTTF b NA NA NA a b b

POTOH b NA NA NA b c a

POTMH a a b a a b a

RPPTF a NA NA NA c c a

RPPOH b NA NA NA c b b

RPPMH b a a NA b a b

MPNTF c NA NA NA b a b

MPNOH b NA NA NA a b b

PMKTF a NA NA NA b a c

PMKOH b NA NA NA b c b

YRKMH a a NA NA a b a

YRKPH NA a a NA b a a

PIAMH NA a NA NA a a a

MOBPH NA a a NA a a a

JMSTF b NA NA NA a b b

JMSOH a NA NA NA a c a

JMSMH a a NA NA c c a

JMSPH NA a NA NA b a a

EASMH NA a a NA b c a

CHOOH c NA NA NA b c a

CHOMH2 a NA NA NA c c b

CHOMH1 NA a NA NA b b a

TANMH NA b NA NA a a a

POCMH NA a NA NA b a b

Key: a = High Certainty

b = Moderate Certainty

c = Low Certainty

Source: Linker et al. 2002.
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Bicknell, B., J. Imhoff, J. Kittle, A. Donigian, R. Johanson and T. Barnwell. 1996. Hydrologic Simulation Program - Fortran user’s manual for release 11. U.S. Environmental

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Cerco, C. F., L. Linker, J. Sweeney, G. Shenk and J. Butt. 2002b. Nutrient and solids controls

in Virginia’s Chesapeake Bay tributaries. Journal of Water Resources Planning and Management May/June: 179-189.

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Oceanography 44:3, 639-649. 

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applications of the Regional Acid Deposition Model and Engineering Model, Appendix 5F

(Part 2) of NAPAP SOS/T Report 5. In National Acid Precipitation Assessment Program:

State of Science and Technology, Volume 1. National Acid Precipitation Assessment Program,

Washington, D.C. 

Di Toro, D. M. and J. J. Fitzpatrick. 1993. Chesapeake Bay Sediment Flux Model. Report EL93-2. U.S. Army Corps of Engineers Waterways Experiment Station and U.S. Environmental

Protection Agency Chesapeake Bay Program Office. Pp. 198.

Donigian, Jr., A., B. Bicknell, A. Patwardhan, L. Linker, C. Chang, C. and R. Reynolds. 1994.

Chesapeake Bay Program Watershed Model application to calculate bay nutrient loadings.

Report for the U.S. Environmental Protection Agency, Chesapeake Bay Program Office,

Annapolis, Maryland.

Hartigan, J. 1983. Chesapeake Bay basin model-final report. Report for the U.S. Environmental Protection Agency Chesapeake Bay Program, Annapolis, Maryland.

Hopkins, K., B. Brown, L. Linker and R. Mader. 2000. Chesapeake Bay Watershed Model

land use and model linkages to the airshed and estuarine models. U.S. EPA Chesapeake Bay

Program, Annapolis, Maryland. http://www.chesapeakebay.net/pubs/1127.pdf.

Langland, M., P. Lietman and S. Hoffman. 1995. Synthesis of nutrient and sediment data for

watersheds within the Chesapeake Bay drainage basin. Report 95-4233. USGS WaterResources Investigations.

Linker, L. 1996. Models of the Chesapeake Bay. Sea Technology 37(9):49-55.

Linker, L., G. Shenk, P. Wang, C. Cerco, A. Butt, P. Tango and R. Savidge. 2002. A Comparison of Chesapeake Bay Estuary Model Calibration With 1985–1994 Observed Data and

Method of Application to Water Quality Criteria. Chesapeake Bay Program Modeling

Subcommittee Report. Chesapeake Bay Program Office, Annnapolis, Maryland. 

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chapter ii • Overview of Technical Tools
Linker, L., G. Shenk, R. Dennis and J. Sweeney. 2000. Cross-media models of the Chesapeake Bay watershed and airshed. Water Quality and Ecosystem Modeling 1(1-4):91-122.

Linker, L., C. Stigall, C. Chang and A. Donigian, Jr. 1996. Aquatic accounting: Chesapeake Bay

Watershed Model quantifies nutrient loads. Water Environment and Technology 8(1):48-52.

Madden, C. J., M. Kemp and W. Michael. 1996. Ecosystem model of an estuarine submersed

plant community: calibration and simulation of eutrophication responses. Estuaries

19(2B):457-474.

Maizel, M., G. Muehlbach, P. Baynham, J. Zoerker, D. Monds, T. Iivari, P. Welle, J. Robbin

and J. Wiles. 1995. The potential for nutrient loadings from septic systems to ground and

surface water resources and the Chesapeake Bay. Report for Chesapeake Bay Program

Office, Annapolis, Maryland.

Moore, K. A., D. J. Wilcox and R. J. Orth. 2000. Analysis of the abundance of submersed

aquatic vegetation communities in the Chesapeake Bay. Estuaries 23(1):115-127.

Neubauer, S., W. Miller and I. Anderson. 2000. Carbon cycling in a tidal freshwater marsh

ecosystem: a carbon gas flux study. Marine Ecology Progress Series 199:13-30. 

Paerl, H. W., R. L. Dennis and D. R. Whitall, 2002. Atmospheric Deposition of Nitrogen:

Implications for Nutrient Over-enrichment of Coastal Waters. Estuaries 25(4B):677-693.

Palace, M., J. Hannawald, L. Linker, G. Shenk, J. Storrick and M. Clipper. 1998. Appendix

H: tracking best management practice nutrient reductions in the Chesapeake Bay Program.

In: Chesapeake Bay Watershed Model application and calculation of nutrient and sediment

loadings. EPA 903-R-98-009, CBP/TRS 201/98. Chesapeake Bay Program Office,

Annapolis, Maryland.

Shin, W. C. and G. R. Carmichael. 1992. Sensitivity of acid production/deposition to emission reductions. Environmental Science and Technology 26:4 pp 715-725.

Thomann, R. V., J. R. Collier, A. Butt, E. Casman and L. C. Linker. 1994. Response of the

Chesapeake Bay Water Quality Model to Loading Scenarios. Technology Transfer Report

CBP/TRS 101/94 (April, 1994) Chesapeake Bay Program Office Annapolis, MD.

USDA. 1984. Soil Conservation Service Soil Interpretation Records. Statistical Laboratory,

Iowa State University, Ames, Iowa.

U.S. EPA. 2003a. Technical Support Document for Identification of Chesapeake Bay Designated uses and Attainability. EPA 903-R-03-004. Chesapeake Bay Program Office,

Annapolis, Maryland.

U.S. EPA 2003b. Ambiant Water Quality Criteria for Dissolved Oxygen, Water Clarity and

Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries. EPA 903-R-03-002. Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. EPA. 2002. Nutrient Reduction Technology Cost Estimations for Point Sources in the

Chesapeake Bay Watershed. Nutrient Reduction Technology Cost Task Force. Chesapeake

Bay Program, Annapolis, Maryland.

U.S. EPA. 1994. Chesapeake Bay watershed pilot project. EPA/620/R-94. Environmental

Monitoring and Assessment Program Center, Research Triangle Park, NC. 

Wang, H. V. and B. H. Johnson, 2000. Validation and application of the second generation

three dimensional hydrodynamic model of Chesapeake Bay. Water Quality and Ecosystem

Modeling 1(1-4)51-90.

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chapter ii • Overview of Technical Tools
Wang, P., L. Linker and J. Storrick. 1997. Appendix D: precipitation and meteorological data

development and atmospheric nutrient deposition. In: Chesapeake Bay Watershed Model

application and calculation of nutrient and sediment loadings—Phase IV Chesapeake Bay

Watershed Model. EPA 903-R-97-022, CBP/TRS 181/97. Chesapeake Bay Program Office,

Annapolis, Maryland.

Wetzel, R. and H. A. Neckles. 1986. A model of Zostera marina L. photosynthesis and

growth: simulated effects of selected physical-chemical variables and biological interactions.

Aquatic Botany 26:307-323.

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chapter ii • Overview of Technical Tools
In calculating attainability under various loading scenarios using the Water Quality

Model, it is necessary to apply innovative approaches to address various technical

issues. The most important of these issues are described in this chapter, as follows:

refining estimates of pycnocline depths; setting averaging periods for determining

criteria attainment; defining allowable frequency and duration of criteria exceedances;

establishing the geographic influence of loads on tidal water quality; establishing the

tidal wetland influence on tidal-water dissolved oxygen; analysis of the isolation of

individual pollutant effects on water quality; influence of sediment loads on dissolved

oxygen; and the establishment and assessment of SAV restoration goals.

REFINING ESTIMATES OF PYCNOCLINE DEPTHS

The pycnocline is usually characterized by strong gradients in chemical and biological properties and separates the deep, saltier waters from the less saline, well-mixed

surface waters. In the Chesapeake Bay, another well-mixed layer forms on the

bottom of the estuary due to bottom shear from estuary currents. Vertical stratification of the Chesapeake Bay has implications for use designations and, therefore,

accurate estimates of the pycnocline are important for assessing attainment. The

method for assessing upper and lower mixed layer depths is based on Fisher et al.

(2003). This method differs from the standard field method in that it uses a measured

density gradient based on salinity and temperature rather than on the surrogate,

conductivity. Generally, the upper pycnocline depth is the shallowest occurrence of

a density gradient of 0.1 kg/m4 or greater and a lower mixed layer depth is the

deepest occurrence of a density gradient of 0.2 kg/m4, if a lower mixed layer exists. 

Since pycnocline delineation is based on hydrodynamics and not bathymetry, the

depth of the pycnocline and hence the boundaries of the designated uses changes on

a monthly basis. Details are presented in Appendix D of the Technical Support Document (U.S. EPA 2003b). Since monitoring data is used to adjust model output as

described previously in Chapter II, the upper and lower pycnocline boundary depths

are determined on a monthly average period usually formed from two water quality

monitoring sampling cruises each month over the assessment period. Consequently,

only monthly average water quality criteria were assessed.

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chapteriii

Technical and Modeling

Considerations for Setting

the Cap Load Allocations

chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
AVERAGING PERIOD FOR DETERMINING 

CRITERIA ATTAINMENT 

The method for determining attainment of the Chesapeake Bay dissolved oxygen,

clarity and chlorophyll a criteria became an issue. Chesapeake Bay Program partners

decided that the Bay modeling tools would be used to assist in allocating the nutrient

and sediment cap loads, and the ambient tidal water quality monitoring data would

be used to ultimately determine criteria attainment for listing and delisting purposes.

This course of action required greater coordination between monitoring data and

modeling output assessments and decisions on the method for analyzing monitoring

data. 

ASSESSING MONITORING DATA

Monitoring criteria attainment requires reconciliation of dichotomous needs. Long

averaging periods are needed to obtain the best assessment of criteria attainment

through wet, dry and average years. Data from multiple years averages out other

interannual variability, such as the timing of high flow and load events. The large

nutrient and sediment load reductions called for in the Chesapeake 2000 cap load

allocations will occur in different places and at different rates, thereby adding 

more variability to interannual measurements of dissolved oxygen, clarity and

chlorophyll a. All of these factors address assessing attainment with the longest

monitoring period practicable. 

On the other hand, responsive water quality management requires a reasonable

assessment period, which requires a compromise on the quality of assessment. To

best address the disparate needs of quality and responsiveness, a three-year averaging period was chosen. See Chapter 6 in the Regional Criteria Guidance for a

detailed justification for the selection of this averaging period (U.S. EPA 2003a).

ASSESSING MODELING DATA

Currently, Chesapeake Bay Water Quality Model output is available for 10 simulated

years (1985–1994), which provides eight three-year running averages (1985-1987,

1986–1988, 1987–1989, etc.). To compare the standard 10-year assessment of the

model outputs for dissolved oxygen, clarity and chlorophyll a to the three-year

assessment of monitoring data, the model output was modified to provide estimates

of the highest attainment of the eight three-year assessments, the lowest attainment

of the eight three-year assessment and the average of the eight three-year running

assessments. The modification allows for assessment of model estimated ‘best’ and

‘worst’ cases of attainment using a running three-year assessment period. 

The deep waters of the middle mainstem Chesapeake Bay segments CB3MH,

CB4MH and CB5MH, along with the deep waters of the lower Potomac River

(POTMH) and Eastern Bay (EASMH) form a large contiguous region of deep water

in close contact with the often anoxic waters of the deep channel. Attainment of the

dissolved oxygen criteria for these deep waters is difficult, as Figure III-1 illustrates. 

Seven scenarios are shown: 1985–94 Observed, 2000 Progress, Tier 1, Tier 2, Tier 3,

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
Tier 3+20% Shoreline Sediment Reduction, and Trial 2 (basinwide cap loads). The

10-year average of waters ‘Not in Attainment’ (labeled ‘10-yr avg’) is approximated

by the average of the eight three-year running averages (labeled ‘Total 3-yr avg’).

Moreover, as nutrient and sediment loads move toward the agreed-upon basinwide

cap load values (Trial 2 scenario in Figure III-1), the 10-year average, the average of

the eight three-year averages and the high and low three-year average begin to

approach the same value.

The mainstem CB4MH segment, located in the middle of the deep-channel anoxia

region of the Chesapeake Bay, has the greatest difficulty in attaining the deep-water

dissolved oxygen criteria. Figure III-1 indicates that the 10-year average estimate of

nonattainment (19 percent) is close to the average of the eight three-year running

averages (18 percent) and that the worst three-year average period of nonattainment

(23 percent) is, of course, greater than the best three-year average of nonattainment

(16 percent). As the loads of nutrients and sediment are reduced, the levels of nonattainment decrease, and the range between ‘best’ and ‘worst’ three-year average

nonattainment decreases. Model estimates of the range and average of the three-year

averages of dissolved oxygen, clarity and chlorophyll a provide an estimate of what

attainment may look like using a three-year running average of observations.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

0

5

10

15

20

25

High

Low

10-yr avg

Total 3-yr avg

Observed

2000 ProgressTier1Tier2Tier3Tier3+20% ShorelineTrial2

Observed

2000 ProgressTier1Tier2Tier3Tier3+20% ShorelineTrial2Observed2000 ProgressTier1Tier2Tier3Tier3+20% ShorelineTrial2Observed2000 ProgressTier1Tier2Tier3Tier3+20% ShorelineTrial2Observed2000 ProgressTier1Tier2Tier3Tier3+20% ShorelineTrial2

CB3MH CB4MH CB5MH

DO Deep

POTMH EASMH

Segments & Scenarios

% Not in Attainment

Figure III-1. Assessment of the range and mean of a three-year running average based on model estimates of dissolved

oxygen criteria attainment for the deep waters of the contiguous region of CB3MH, CB4MH, CB5MH, POTMH and EASMH.
Given the findings that 10-year and 3-year averaging periods for determining attainment were very close at loading levels approaching the basinwide cap loads, the

Chesapeake Bay Program partners decided the 10-year averaged modeling output

could be used in making cap load allocation decisions.

DEFINING ALLOWABLE FREQUENCY AND DURATION 

OF CRITERIA EXCEEDANCES

Water quality criteria are established as ‘safe’ levels necessary for the protection of

aquatic life. Continued attainment of these levels should result in a healthy aquatic

community. Typically, the EPA’s national water quality criteria are further defined by

magnitude, maximum duration and frequency of exceedances. Although it is wellestablished that exceedances of water quality criteria within limits still support a

healthy aquatic community, the Chesapeake Bay Program identified the need to go

beyond the simple time metrics of frequency and duration that are usually applied. An

innovative approach was adopted based on allowable exceedances of time (percent of

time exceeded with the criteria application period) and space (percent volume or

surface area of the designated use within a Chesapeake Bay Program segment). 

Monitoring for criteria attainment requires collection of data that are as fully representative as possible of the extent of space and time over which the assessment is to

be performed, but resource limitations inevitably limit data collection. Therefore, an

analytical framework is used to evaluate spatial and temporal criteria exceedance

based on limited data.

As the monitoring program was being designed for criteria assessment, the scientists

involved developed an analytical framework based on a cumulative frequency

diagram (CFD) approach. Monitoring data collected at a limited number of locations

were interpolated over a fixed three-dimensional grid. Criteria values were defined

for each grid cell and combined with the data interpolation to provide a cell-by-cell

estimate of criteria exceedance. Then, for each monitoring event, those grid-cell estimates were aggregated to provide a segment-wide estimate of ‘percent of space’

exceeding the criteria. Multiple monitoring events conducted over an assessment

period provided a temporally defined collection of estimates of ‘percent of space’

exceeding the criteria. Those values were then plotted as a CFD using standard

statistical procedures. The CFD generated using this approach reflects criteria in

both space and time since ‘percent of space’ is represented on the horizontal axis and

‘temporal frequency’ is represented on the vertical axis (Figure III-2; see Chapter 6

of the Regional Criteria Guidance for more details) (U.S. EPA 2003a).

As the CFD approach was developed, it was recognized that some spatial and

temporal criteria exceedance could occur at the same time that the overall segment

was achieving its designated use. For example, some small tidal tributaries might

chronically exceed the criteria simply because they are naturally poorly flushed. It

was decided that these exceedances should be considered ‘allowable’ and should be

accounted for in the CFD analytical framework.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
To account for ‘allowable exceedances’ in the CFD approach, multiple options were

considered. Initially 10-percent of time and/or space was considered to be the best

approach because it was consistent with past EPA guidance. However, strict adherence to a 10-percent rule almost always resulted in a violation because CFDs tend to

exceed 10-percent of time or space at some location on the figure, even when there

are few measured violations. A curved line is more consistent with the CFD and so

a mathematically defined hyperbolic line was developed that encompassed 10-

percent of the CFD plot area. This approach appeared to function well, but was

considered arbitrary because it had no scientific basis with regard to actual achievement of a designated use. As a result, a third option was considered where a CFD

was developed based on data from areas that were already achieving their designated

use. That CFD was defined as a ‘reference curve’ that would be used as a benchmark

against which other CFD assessment curves could be compared. The biologicallydefined ‘reference curve’ was selected as the best alternative of those considered and

adopted for routine use in criteria assessment (see Chapter 6 of the Regional Criteria

Guidance for more details; U.S. EPA 2003a).

Exceedances in time and space for model were determined with cumulative frequency

distributions (CFDs) and biological reference curves. As described in Chapter II, each

model scenario was used to create a modified data set for that scenario. These results

were then interpolated and used to create CFDs of spatial and temporal criteria

exceedance for each segment and designated use. The CFD for the segment and

scenario was compared to the appropriate biological reference curve that defines the

biologically acceptable and protective combinations of frequency and spatial extent

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Attainment Curve

Area of ‘Non-Allowable’

Criteria Exceedance 

Area of ‘Allowable’

Criteria Exceedance 

Reference Curve

Percentage of Time a Specified Percentage or

More of Area/Volume Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure III-2. Non-allowable exceedance illustrated in dark blue.

Source: U.S. EPA 2003a.
of criteria exceedances. The total area below the CFD for a segment, but above the

biological reference curve represents the unallowable exceedance for that segment

and scenario (Figure III-2). These calculations were automatically carried out by a

system of computer programs available from the Chesapeake Bay Program Office.

The development and application of CFDs is described in greater detail in Chapter 6

of the Regional Criteria Guidance (U.S. EPA 2003a).

ESTABLISHING THE GEOGRAPHIC INFLUENCE 

OF LOADS ON TIDAL WATER QUALITY

In developing the cap load allocations, it was key to understand each major tributary’s influence on tidal Bay water quality. To assist the Water Quality Steering

Committee in isolating the effects of each of the nine major basins, the Chesapeake

Bay Program ran a series of geographic isolation runs with the Water Quality Model.

The model runs helped establish estimates of the influence of loads from each of the

nine major tributary basins on water quality in each segment of the Bay’s tidal

waters. Specifically, isolation scenarios, in which the management controls were set

at Tier 3 levels for the isolated basin and held at year 2000 levels for the rest of the

watershed, were performed for each major basin. Issues in identifying the most

affected segments and estimating the absolute and relative effects were addressed.

Absolute effects were defined as the total effect of a basin on water quality, including

loads, either large or small, and the geographic influence of a basin’s position in the

estuary (i.e., the Susquehanna, with the largest loads and a position at the head of the

estuary, always had the highest estimated absolute effect). Relative loads were an

estimate of the geographic effect alone, irrespective of the amount of the load, so that

for upper Bay regions, the Western Shore and Patuxent were estimated to have about

the same relative effect as the Susquehanna. The cap load allocation assessments

took estimates of both the absolute and relative effects into account.

FOCUSING ON MAINSTEM SEGMENTS 

CB3MH, CB4MH AND CB5MH 

As described above, the Bay tidal-water area that requires the highest level of

nutrient reduction to attain dissolved oxygen criteria is the deep-water and deepchannel designated use in the middle and central Chesapeake Bay (segment

CB4MH). Hence, all other tidal Bay habitats that are not currently in attainment for

dissolved oxygen would come into attainment before the CB4MH deep-water and

deep-channel designated uses were fully in attainment. For this reason, the Water

Quality Technical Workgroup focused on segment CB4MH when comparing the

influence of different basins. This focus was later broadened to include segment

CB3MH and segment CB5MH, respectively, since they were near segment CB4MH

and also required higher levels of nutrient reduction to reach attainment.

COMPARING ABSOLUTE VERSUS RELATIVE EFFECTIVENESS

Absolute effectiveness gives an indication of the total effectiveness of a particular

basin in reducing nonattainment in a given segment by taking into account geography and total load, while relative effectiveness takes into account only geography.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Absolute effectiveness is the change in criteria nonattainment that results from a

single basin changing from year 2000 level management to Tier 3 management. It is

expressed in the same units as the CFD, which is percentage of space and time in

nonattainment. For example, if the lower Potomac River segment POTMH moves

from 35 percent nonattainment to 30 percent nonattainment from the implementation

of Tier 3 in the Potomac basin, then the absolute effectiveness is 5 percent.

Comparing the absolute effectiveness among basins helps to identify basins that can

have the greatest total effect in correcting nonattainment. Figure III-3 shows the

absolute effectiveness of each basin on reducing the deep-water dissolved oxygen

criteria nonattainment in middle central Chesapeake Bay segment CB4MH.

Relative effectiveness is the absolute effectiveness divided by the total load reduction

necessary to gain that water quality response. For example, if the load reduction in the

Potomac basin was 5 million pounds of pollutant to get that 5 percent change in

nonattainment, the relative effectiveness is 1 percent per million pounds. The relative

effectiveness calculation is an attempt to isolate the effect of geography by normalizing by load. Comparing the relative effectiveness among basins shows the resulting

gain in attainment from performing equal reductions among the nine major basins.

NORMALIZING FOR THE COMBINED NITROGEN 

AND PHOSPHORUS LOAD

Since the reductions that cause the absolute effect are taken across the board from

nitrogen, phosphorus and sediment loads, one must determine the pollutant load type

responsible for the increased attainment. The Chesapeake Bay is both nitrogen- and Change in Nonattainment (Red percentage points lost)

Susquehanna

MD Western Shore

Patuxent

Potomac

Rappahannock

York

James

MD Eastern Shore

VA Eastern Shore

Major Tributary Basins

3.50

3.00

2.50

2.00

1.50

1.00

0.50

0.00

Figure III-3. Absolute effect of load reductions from the nine major basins on segment

CB4MH deep-water dissolved oxygen concentrations.
phosphorus-limited in different regions and seasons (D’Elia et al. 1986; Fisher et al.

1992, 1994, 1999, 2001; Boynton et al. 1995; Malone et al. 1996; Conley 1999). The

spring algal bloom near the tidal-fresh and oligohaline regions is generally

controlled by phosphorus. The summer algal bloom, primarily in the mesohaline

regions, is controlled more by nitrogen. Low dissolved oxygen conditions in deep

waters are caused by a combination of these seasonal blooms. Suspended sediment

also has an effect on dissolved oxygen, but its effect is less than the nutrient effect

(see Influence of Sediment on Chesapeake Bay Dissolved Oxygen below). 

Since the effects of nitrogen and phosphorus are inherently connected in the Chesapeake Bay management control scenarios and cannot easily be isolated by separate

management practices, the Water Quality Technical Workgroup agreed that the most

appropriate divisor to convert absolute effectiveness to relative effectiveness is a

combination of the two nutrients. Since nutrients are taken up by algae in roughly a

10:1 N:P ratio (by weight), this ratio was also used in the metric. Therefore, an algal

unit is defined as 1 unit mass of phosphorus and 10 units mass of nitrogen. 

Figure III-4 shows the relative effectiveness of the nine different major basins on

deep water in segment CB4MH, normalized by algal units.

To test if the water quality response was further separable by basin, the Susquehanna

River basin was run with a reduction in phosphorus only, since the tidal-fresh waters

that it empties into are typically phosphorus-limited. Figure III-5 shows the results

of this test in the three mid-Bay segments. From this graph, it is clear that both

nitrogen and phosphorus coming from the Susquehanna River basin both have large

effects on water quality in the mid-Bay region.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations Change in Nonattainment (Red percentage points lost)

Major Tributary Basins

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Susquehanna

MD Western Shore

Patuxent

Potomac

Rappahannock

York

James

MD Eastern Shore

VA Eastern Shore

Figure III-4. Relative effect of load reductions from the nine major basins on segment

CB4MH deep-water dissolved oxygen concentrations normalized by algal units.
GENERAL FINDINGS FROM THE GEOGRAPHIC ISOLATION RUNS

Figure III-6 shows the absolute and relative effectiveness of each major tributary

basin on mainstem Chesapeake Bay segments CB3MH and CB5MH and suggests

some generalizations. Northern tributary basins have a greater relative influence than

southern tributary basins, due to the general circulation patterns of the Chesapeake

Bay. Water and nutrients from the southern tributaries of the James and the York

rivers have relatively less influence on the mainstem Bay due to their proximity to

the mouth of the Bay, and the counter-clockwise circulation of the lower Bay also

tends to wash nutrient loads from these southern tributaries out of the Bay mouth,

since they are situated on the western side of the Bay. This same counter-clockwise

circulation tends to sweep loads from the lower Eastern Shore northward. 

Basins whose loads discharge directly to the mainstem Bay, like the Susquehanna,

tend to have more impact on the mainstem Bay segments than basins with river estuaries (e.g., the Patuxent and Rappahannock), due to nutrient attenuation (burial and

denitrification) within the river estuaries prior to the waters reaching the mainstem

Chesapeake Bay. The size of a basin is uncorrelated to its relative influence, though

larger basins, with larger loads, have a greater absolute effect. The upper tier of relative effect in the three mainstem segments includes the largest (Susquehanna) and

the smallest (Eastern Shore Virginia) basins, both directly discharging into the Bay

without intervening river estuaries to attenuate loads, and both ‘up current’ to the

deep-channel region of the mainstem Chesapeake Bay (see Chapter IV).

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Chesapeake Bay Program Segment

Change in Nonattainment

(Red percentage points lost)3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

CB3MH CB4MH CB5MH

N&P

P only

Figure III-5. Estimated absolute effectiveness of Susquehanna reductions of nitrogen

and phosphorus loads (N&P) compared to reductions of the phosphorus load only 

(P only) on dissolved oxygen criteria attainment in the deep waters of mid-mainstem

Chesapeake Bay segments CB3MH, CB4MH and CB5MH.
chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Figure III-6. Absolute (a, c) and relative (b, c) effect on mainstem Chesapeake Bay segments CB3MH and

CB5MH deep water dissolved oxygen concentrations, respectively. Change in Nonattainment (Red percentage points lost)

Susquehanna

MD Western Shore

Patuxent

Potomac

Rappahannock

York

James

MD Eastern Shore

VA Eastern Shore

Major Tributary Basins

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

b. 

Major Tributary Basins

Change in Nonattainment

(Red percentage points lost)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Susquehanna

MD Western Shore

Patuxent

Potomac

Rappahannock

York

James

MD Eastern Shore

VA Eastern Shore

b

a

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Figure III-6. (continued)

Major Tributary Basins

Change in Nonattainment

(Red percentage points lost)

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Susquehanna

MD Western Shore

Patuxent

Potomac

Rappahannock

York

James

MD Eastern Shore

VA Eastern Shore

Major Tributary Basins

Change in Nonattainment

(Red percentage points lost)

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

Susquehanna

MD Western Shore

Patuxent

Potomac

Rappahannock

York

James

MD Eastern Shore

VA Eastern Shore

d

c

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

ESTABLISHING WETLAND INFLUENCE ON TIDALWATER DISSOLVED OXYGEN CONCENTRATIONS 

In some regions of the Chesapeake tidal tributaries in the surface waters in the

summer, an oxygen deficit of several mg/L O2 is typically observed. These regions

are found adjacent to extensive tidal wetlands and contain comparatively small

volumes of tidal waters. The tidal-fresh and oligohaline regions of the upper tidal

York River is an example of relatively small volumes of waters fringed by extensive

wetlands—Mattaponi (MPNTF, MPNOH) and Pammkey (PMKTF. PMKOH) rivers.

In these segments, oxygen demand from tidal wetland sediments is thought to influence surface water dissolved oxygen concentrations. Recent studies estimate

wetland sediment oxygen demand to range from 1 - 5.3 g O2/m2-day (Neubauer et

al. 2000; Cai et al. 1999). In the Chesapeake Bay Water Quality Model, a uniform

oxygen demand of 2 g O2/m2-day was used as described in Chapter II. Regions of

the Bay tidal waters where there are extensive tidal wetlands, but border relatively

large bodies of water, such as in the Tangier Sound have sufficient volume and

mixing to mask the oxygen demand of adjacent wetland sediments.

WATER QUALITY MODEL RUNS ISOLATING 

INDIVIDUAL POLLUTANT EFFECTS ON WATER QUALITY

Allocations were developed for nitrogen, phosphorus and sediment loads for each

basin. To achieve the water quality criteria, some reduction of each of the three loads

is needed. Within limits, however, the mix of nitrogen, phosphorus and sediment

load reductions can be altered and still achieve attainment of the criteria; for

example, relatively fewer nitrogen reductions could be played against relatively

more sediment reductions to achieve the same result. To examine these trade-offs, an

analysis tool called a ‘surface analysis’ was used. While this tool was useful to

examine the tradeoffs between reducing nitrogen, phosphorus and sediment loads,

ultimately it will be most useful for tributary strategy development, where different

load strategies that achieve the same level of water quality protection can 

be examined.

SURFACE ANALYSIS PLOTS

In order to examine a particular water quality parameter, such as dissolved oxygen,

with respect to different load inputs of nitrogen, phosphorus and sediment, a surface

analysis was applied. Surface analysis is a statistical method that uses Water Quality

Model output from multiple scenarios and produces three-dimensional plots called

response surface plots. A response surface plot may be produced as a concentration

of dissolved oxygen, or as the percent attainment of a dissolved oxygen criteria for

a particular Chesapeake Bay Program segment or designated use over a particular

attainment period (i.e., June through September for the segment CB4MH deep-water

designated use). Response surface plots facilitate the understanding of interactions

among nutrient and sediment loads in the Chesapeake Bay’s ecosystem and in
developing effective nutrient and sediment management strategies for improving

water quality.

Figure III-7 examines the influence of nitrogen and phosphorus loads on the segment

CB4MH deep-water dissolved oxygen concentrations. Note that the nutrient loads

are expressed as a portion between 0.4 and 1.0 of the 2000 Progress Scenario loads

for nitrogen and phosphorus. A 40 percent reduction in nitrogen and phosphorus

loads, therefore, is represented by the grid position corresponding to 0.6 TN and 0.6

TP. The surface analysis estimate of the dissolved oxygen response in segment

CB4MH deep water shows that a 40 percent reduction in either nitrogen or phosphorus alone would improve dissolved oxygen conditions, but reducing both

nitrogen and phosphorus would bring about greater improvements.

In using this tool, it is important to examine the surface responses with respect to all

the criteria—dissolved oxygen, water clarity and chlorophyll a—and in all regions

or designated uses of the Chesapeake Bay and its tidal tributaries, while keeping in

mind that the surface response plots are a statistical estimate of a series of model

scenarios, which are also estimates of Chesapeake water quality.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

DO

TP

5.26

4.50

3.75

3.00

0.4

0.4

0.6

0.6

0.8

0.8

1.0 1.0

Figure III-7. Surface response of deep-water dissolved oxygen concentrations to nitrogen and

phosphorus loads in segment CB4MH.
SURFACE ANALYSIS UTILITY

Despite the fact that surface analysis is an ‘estimate of an estimate’ and the esoteric

nature of three-dimensional regression plots, it remains a valuable tool. The utility

of surface analysis is threefold. First, one can gain insight from the relative response

of dissolved oxygen, clarity, SAV, chlorophyll a or any other water quality parameter, to any combination of nitrogen, phosphorus and sediment loads. Second, by

interpolating scenario runs, the surface analysis provides an initial estimate of a

water quality response without the need for exhaustive generation of numerous water

quality scenarios. Third, and most important, surface analysis enables the tributary

teams that are developing detailed implementation plans to achieve the nutrient and

sediment cap load allocations to examine possible tradeoffs between nitrogen, phosphorus and sediment cap loads.

INFLUENCE OF SEDIMENT ON CHESAPEAKE BAY 

DISSOLVED OXYGEN

In the Water Quality Model, decreases in sediment loads were accompanied by

increases in water clarity, which resulted in simulated increases in shallow-water

algae, benthic algae and SAV. Further examination of dissolved oxygen responses to

sediment loads uncovered what initially seemed to be unusual simulation results. As

sediment loads were decreased, deep-water dissolved oxygen concentration

increased slightly. Further examination was necessary to explain this response. 

Shallow waters of the Chesapeake Bay and its tidal tributaries occupy a region at the

interface of the land and estuary. Higher sediment concentrations are generally

observed in the shallow-water regions than in deeper open waters due to local watershed inputs, shore erosion and wave resuspension of sediment. As Figure III-8

illustrates, improved water clarity allows more processing of nutrients and organics

in shallow-water regions. Effective interception of nutrients by benthic algae, SAV

and phytoplankton in shallow, aerobic waters leads to nutrient diagenesis and loss in

sediment through denitrification and phosphorus sequestration.

Additional sensitivity runs provided insight into the cascading effect of reducing

sediment loads in shallow regions of the Chesapeake Bay on water quality in deep

waters (Figures III-9 and III-10a-b). Figure III-9 shows the benthic algae summer

biomass for the final calibration. The planar model grid shows benthic algae present

in many of the shallow regions, but rarely exceeding 2 grams meter-2 biomass. Figure

III-10a is a sensitivity scenario that removes shoreline sediment loads and associated

nutrient loads (SENS 146). A related sensitivity scenario (SENS 147) removed only

the shoreline sediment loads, keeping the nutrients associated with the shoreline

sediment loads unchanged (Figure III-10b). The pair of sensitivity scenarios (SENS

146 and 147) demonstrated that the reduced sediment load was the cause of the

benthic algae increase. In reality, complete elimination of shoreline sediment loads

is extreme and infeasible. However, reduced sediment loads will have a relative

effect on nutrients and, ultimately, on dissolved oxygen concentrations.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
▼57

chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

aerobic waters

anaerobic waters

SAV

NH4 + PO4

NH4 + PO4

anaerobic sediment

anaerobic sediment

decay

decay

benthic algae

diffusion to

the sediment

surface

diffusion to

the surface

binding of PO4 into

iron phosphate

at water-sediment

interface

nitrification and

dentrification at

water-sediment

interface

ammonia

released to

Bay waters

phosphate

released to

Bay waters

settling

of algae

settling

of algae

algae

algae

NH4

N2

PO4

Figure III-8. Illustration of the influence sediment load has on shallow-water water quality and living resources.

Figure III-9. Final calibration summer biomass of benthic algae. The units are in grams

biomass meter-2.

Shallow waters are usually aerobic, as

they are located above the pycnocline.

When sediment is reduced in shallow

waters, light limitation of algae is reduced,

allowing more shallow water algal growth,

particularly benthic algae, and more SAV

growth. Overall more nutrients are either

sequestered into the sediments of shallow

waters or dentrified.

If shallow waters have algae sediment

loads, there is greater light limitation of

algae, benthic algae, and SAV. Then

nutrients no longer effectively sequestered

by processes in the shallow waters, fuel 

algal growth in the deeper waters of the

Bay where the presence of a pycnocline

results in low to no dissolved oxygen

waters and ammonia and phosphate 

flux out of the sediment.
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Figure III-10a. Sensitivity scenario of benthic algae summer biomass with no shoreline

sediment or associated nutrient loads. The units are in grams biomass meter-2.

Figure III-10b. Sensitivity scenario of benthic algae summer biomass with no shoreline

sediment, but associated shoreline sediment nutrient loads included. The units are in

grams biomass meter-2.
A modeling study of the Delaware inland bays demonstrated the importance of

benthic algae on sediment diagenesis in shallow waters. Figure III-11 shows the

simulated effect of the presence and absence of benthic algae on nutrient and oxygen

(C. Cerco, unpublished data). The presence of benthic algae results in greater variation in nutrient and oxygen flux from the sediment (positive values out of the

sediment, negative values into the sediment) but an overall net decrease of dissolved

inorganic phosphorus and dissolved inorganic nitrogen flux out of the sediments.

Model findings were substantiated by observed flux of oxygen and nutrients in

Delaware Inland Bay sediments (S. Seitzinger, unpublished data), i.e., shallow

waters with sufficient light for the growth of benthic algae demonstrated greater

retention of nutrients. 

In addition, researchers have shown the role of improved clarity on decreased level

of nutrients. Using a STELLA modeling analysis, Kemp et al. (1994) explained that

the interception of nutrients by “enhanced suspension feeding in the shallow waters

of the Bay ... effect greater improvements for bottom oxygen than comparable action

at deep sites”. The interception of nutrients and increased nutrient processing was

further examined by Newell et al. (2002), who described the potential for greater

denitrification and sequestering of nutrients in aerobic shallow-water sediment.

While these researchers specifically examined shallow-water oyster beds as the

agent for greater interception and processing of nutrients in aerobic shallow waters,

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

With Algae

No Algae

YEARS

1

2

1

0

-1

-2

150

125

100

75

50

25

0

-25

-50

15

10

5

0

-5

-10

25

0

25

-50

-75

-100

234

12 34 12 34

12 34

YEARS

YEARS YEARS

Dissolved Oxygen Flux Phosphate Flux

Ammonium Flux Nitrate Flux

GM O2 M-2 D-1

MG P M-2 D-1 MG N M-2 D-1

MG N M-2 D-1

Figure II-11. Delaware Inland Bay study showing relative range of nutrient and dissolved

oxygen flux resulting from the presence or absence of benthic algae. 
decreased light attenuation was the principal factor in increasing nutrient interception in the model simulations.

Estimates of the influence that sediment loads, particularly to shallow waters, have on

dissolved oxygen indicate an interesting synergy among nutrient/sediment loads and

living resources, but care should be taken not to over-interpret these results. The

current sediment simulation is the most refined model estimate of sediment loads, fate

and effects, and considerable time was spent with the calibration. However, shoreline

loads were approximated everywhere as a daily baywide average input, and sediment

transport and wave resuspension were not simulated. Overall the influence of sediment loads on dissolved oxygen was found to be reasonable and consistent with

current understanding of shallow-water processes, but also proved slight in its effect.

A reduction of 20 percent of the loads from shoreline erosion and resuspension had

the equivalent reduction in deep water dissolved oxygen as about a five million pound

reduction in nitrogen. A 2003 review by the Chesapeake Bay Program’s Scientific and

Technical Advisory Committee (STAC) found that the “. . . mechanisms linking

shoreline erosion reduction to improvements in deep-water dissolved oxygen through

improved water clarity and increased shallow-water microphytobenthos production

were plausible, but unproven” (STAC 2003). One concern the STAC review raised is

the inability of the model structure and mechanisms to scour benthic algae, which

may, after transport of organics to deep water, contribute, to deep-water oxygen

demand.

The Water Quality Model is now being refined with a sediment transport simulation,

refined spatial and temporal inputs of shoreline erosion loads and a simulation of

wave resuspension with enhanced feedbacks between SAV and resuspended sediment. The influence of sediment load reductions on deep-water dissolved oxygen

concentrations will be reexamined during the reevaluation of the nutrient and sediment allocations, which is planned for 2007.

ESTABLISHING AND ASSESSING

NEW SAV RESTORATION GOALS 

During the development of the water clarity criteria, the shallow-water designated use

and the associated sediment cap load allocations, the partners agreed to the alignment

of the Chesapeake 2000 commitment to establishing a new SAV restoration goal with

the commitment to reducing sediment loads to “achieve the water quality conditions

that protect living resources.” Chesapeake 2000 called for a recommitment to the 

long-standing 114,000-acre SAV goal and further called for more ambitious “SAV

restoration goals and strategies to reflect historic abundance, measured as acreage and

density from the 1930s to the present” including the specific levels of water clarity

needed (Chesapeake Executive Council 2000). 

Ultimately the SAV goal established to meet the Chesapeake 2000 commitment was

a 185,000-acre restoration goal, specified as baywide, tributary basin, and segmentspecific restoration goals (Appendix A; Secretary Tayloe Murphy 2003). The

sediment load allocation is closely aligned to this goal. 

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

DEVELOPING THE 185,000-ACRE SAV RESTORATION GOAL

The Chesapeake Bay Program has long committed to protecting and restoring SAV.

In 2001, there were 85,415 acres of SAV in the Chesapeake Bay and its tributaries.

The total potential shallow-water habitat available for SAV in the Chesapeake Bay

out to 2-meters is 640,000 acres. SAV never covers 100 percent of the available

habitat, but on average covers approximately 35 percent of it (U.S. EPA 2003b). On

April 15, 2003, the Chesapeake Bay Program’s Principals’ Staff Committee and

representatives of the headwater states approved the new Bay grass restoration goal

of 185,000 acres by 2010. The partner states of the Bay Program have adopted the

new goal, meeting the Chesapeake 2000 commitment to:

“By 2002, revise SAV restoration goals and strategies to reflect historic

abundance, measured as acreage and density from the 1930s to the

present. The revised goals will include specific levels of water clarity

that are to be met in 2010. Strategies to achieve these goals will 

address specific levels of water clarity, water quality and bottom

disturbance.”

The following principles guided the development of the recommended new SAV

goal:

• Use the best available data;

• Establish a direct link between the new goal and the new water quality criteria;

• Recognize that SAV should not be expected to cover all available shallow-water

habitat;

• Areas expected to contribute to the goal should only include those that have

demonstrated some minimal level of abundance or persistence in the past; and

• Provide segment-specific acreage goals for use by the tributary strategy teams.

The SAV abundance and distribution record includes interpreted historical aerial

photography from the late 1930s to the 1960s, as well as the annual baywide aerial

survey data from 1978 and 1984-2000. The single best year of SAV growth observed

in each CBP segment from the entire record of aerial photographs (1938-2000) is the

best available data on SAV occurrence over the long term. These same data were

used to define, within each Chesapeake Bay segment, the depth to which the

shallow-water Bay grass designated use should be considered. That depth is the

maximum depth at which water clarity criteria would apply in the context of state

water quality standards, and is, therefore, referred to as the ‘application depth’ for

each segment (U.S. EPA 2003b). Table III-1 contains the derived segment-specific

application depths. 

As the first step to setting the SAV goal acreage out to the specific application depth

was determined through bathymetry data and aerial photographs used to slice the

single-best-year SAV acreage in each segment into three depth zones: 0–0.5 meters,

0.5–1.0 meters and 1–2 meters; and aerial photographs used to determine the depth
to which SAV grew in each segment with either a minimum abundance or minimum

persistence. 

Next, the SAV goal for a segment is the portion of the single-best-year acreage that

falls within this determined depth range, which, in turn, was established as follows:

In all segments, the 0–0.5 meter depth interval will be designated for

shallow-water Bay grass use. In addition, the shallow-water Bay grass use

will be designated for deeper depths within a segment if either:

• The single-best-year SAV distribution covered at least 20 percent of the

potential habitat in a deeper depth zone; or

• The single-best-year SAV covered at least 10 percent of the potential

habitat in the segment-depth interval, and at least three of the four fiveyear periods of the record (1978-2000) show at least 10 percent SAV

coverage of potential habitat in the segment-depth interval. 

SAV restoration goals have been established in a manner consistent with the singlebest-year method used to determine the application depths. Within each segment, the

2010 restoration goal for designated use attainment purposes is equal to the acreage

of the single best year on record within the segment’s application depth (U.S. EPA

2003b). These segment-specific SAV restoration goals are listed in Table III-1. New

SAV acreage goals have been established on a segment-specific basis, a baywide

basis and, between those two, a major tributary basin-specific basis (e.g., Potomac

River; see Table III-2).

The new baywide SAV goal, 185,000 acres, is the sum of acreage targets for each of

the 78 Chesapeake Bay segments based on the single-best-year acreage on record for

each segment. The achievement of the baywide goal, as well as the local tributary

basin and segment-specific restoration goals, will be based on the single-best-year

SAV acreage within the most recent three-year record of survey results (U.S. EPA

2003a). 

The Chesapeake Bay Program partners reached the following agreements on the

implementation of the SAV restoration goals:

1. The tidal states of Delaware, Maryland and Virginia and the District of Columbia

will adopt numerical water clarity criteria and consider the adoption of the SAV

acreage restoration goals for each Chesapeake Bay Program segment into their

Water Quality Standards.

2. The SAV acreage restoration goals will be used as the primary metric in the

development and implementation of the tributary strategies with regard to sediment controls. If the SAV acreage restoration goal is achieved in a given segment,

and if the state water quality standards allow, that segment will be considered as

having achieved the shallow-water bay grass designated use even if the sediment

loading caps were not met.

3. Virginia and Maryland will develop comprehensive SAV restoration strategies to

meet the new SAV restoration goal.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Table III-1. The new Chesapeake Bay SAV acreage goal and current SAV acreages by

Chesapeake Bay Program segment.

2001 SAV SAV 

Chesapeake Bay Program Shallow-water SAV Acreage Out to Restoration

Segment Name Application Depth Application Depth Goal

Northern Chesapeake Bay 2 7,979 12,908

Upper Chesapeake Bay 0.5 203 302

Upper Central Chesapeake Bay 0.5 1 943

Middle Central Chesapeake Bay 2 112 2,511

Lower Central Chesapeake Bay 2 4,487 14,961

Western Lower Chesapeake Bay 1 715 980

Eastern Lower Chesapeake Bay 2 9,168 14,620

Mouth of the Chesapeake Bay 0.5 8 6

Bush River 0.5 3 158

Gunpowder River 2 * 2,254

Middle River 2 * 838

Back River 0.5 * 0

Patapsco River 1 * 298

Magothy River 1 * 545

Severn River 1 120 329

South River 1 27 459

Rhode River 0.5 * 48

West River 0.5 * 214

Upper Patuxent River 0.5 205 5

Western Branch (Patuxent River) 0.5 * 0

Middle Patuxent River 0.5 104 68

Lower Patuxent River 1 22 1,325

Upper Potomac River 2 1,964 4,368

Anacostia River 0.5 4 6

Piscataway Creek 2 * 783

Mattawoman Creek 1 * 276

Middle Potomac River 2 3,070 3,721

Lower Potomac River 1 1,739 10,173

Upper Rappahannock River 0.5 66 20

Middle Rappahannock River 0.5 * 0

Lower Rappahannock River 1 478 5,380

Corrotoman River 1 389 516

Piankatank River 2 539 3,256

Upper Mattaponi River 0.5 * 75

Lower Mattaponi River 0.5 * 0

Upper Pamunkey River 0.5 140 155

Lower Pamunkey River 0.5 * 0

Middle York River 0.5 * 176

Lower York River 1 801 2,272

Mobjack Bay 2 9,508 15,096

Upper James River 0.5 95 1,600

Appomattox River 0.5 * 319

Middle James River 0.5 15 7

continued
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Chickahominy River 0.5 268 348

Lower James River 0.5 2 531

Mouth of the James River 1 232 604

Western Branch Elizabeth River * * 0

Southern Branch Elizabeth River * * 0

Eastern Branch Elizabeth River * * 0

Lafayette River * * 0

Mouth of the Elizabeth River * * 0

Lynnhaven River 0.5 43 69

Northeast River 0.5 * 88

C&D Canal 0.5 7 0

Bohemia River 0.5 354 97

Elk River 2 2,034 1,648

Sassafras River 1 1,169 764

Upper Chester River 0.5 * 0

Middle Chester River 0.5 * 63

Lower Chester River 1 205 2,724

Eastern Bay 2 2,886 6,108

Upper Choptank River * * 0

Middle Choptank River 0.5 * 63

Lower Choptank River 1 148 1,499

Mouth of the Choptank River 2 5,257 8,044

Little Choptank River 2 2,377 3,950

Honga River 2 4,945 7,686

Fishing Bay 0.5 6 193

Upper Nanticoke River * * 0

Middle Nanticoke River 0.5 * 3

Lower Nanticoke River 0.5 * 3

Wicomico River 0.5 * 3

Manokin River 2 404 4,359

Big Annemessex River 2 721 2,014

Upper Pocomoke River * * 0

Middle Pocomoke River 0.5 * 0

Lower Pocomoke River 1 1,528 4,092

Tangier Sound 2 13,310 37,965

Totals 77,858 184,889

+7,561 acres of 

SAV not included in 

the survey due to 

heightened security

85,419

*Denotes no data available or no SAV present.

Source: U.S. EPA 2003b.

Table III-1. (continued)

2001 SAV SAV 

Chesapeake Bay Program Shallow-water SAV Acreage Out to Restoration

Segment Name Application Depth Application Depth Goal
WATER CLARITY AND SAV

The water quality criteria distinguish two types of SAV

communities with respect to light

requirements (Batiuk et al. 2000;

U.S. EPA 2003a). The tidal-fresh

and oligohaline communities are

estimated to require greater than 13

percent light-through-water while

the more light-sensitive SAV

communities of the mesohaline and

polyhaline areas are estimated to

require 22 percent light-throughwater. These light requirements can

also be expressed as light attenuation (variously symbolized as Kd or

Ke), which represents the rate of

light lost through the water column. 

Representing the equivalent light needs for the tidal-fresh/oligohaline and the mesohaline/polyhaline communities, Figure III-12 graphs the tradeoffs between light

attenuation and depth. Light attenuation is a rate of light lost by passage through a

water column, either by scattering through sediment, absorption by algal chlorophyll, or loss through absorption by dissolved organic material or water. For

tidal-fresh or oligohaline SAV communities, a Kd of 2.0 meter-1 is equivalent to the

13 percent light-through-water light requirement at a 1-meter depth. As depth

increases the light path, less light attenuation (greater water clarity) is required to

achieve the same 13 percent light-through-water, so that at a 2 meter depth, light

attenuation of no less than 1 meter-1 is required to support SAV. At decreased depths

more light attenuation is allowable for SAV, so that at 0.5 meters depth, a Kd of 4.0

meter-1 is still supportive of SAV growth.

To achieve the water clarity levels to support SAV, reductions in light attenuation by

any of the various components (sediment, algae or color) may achieve the light

requirements supportive of SAV, but for much of the Bay’s tidal waters, reductions

solely in nutrients and ultimately chlorophyll and epiphytic growth, are insufficient.

A light attenuation model developed by Gallegos (2001) provides insight into the

various components of light attenuation (see http://www.chesapeakebay.net/cims/

index.htm under “Factors Contributing to Water-Column Light Attenuation: A Diagnostic Tool”). This model is applicable to any monitoring station data for any period

of time. Figure III-13 represents estimated seasonal average light attenuation from

the various components with color and attenuation due to water combined at the

monitoring station EE3.2 in the Tangier Sound for the SAV season of April to

October. At a 1-meter depth plotted here, only an estimated 1 year in 10 provides, on

a seasonal average basis, the light required for a mesohaline SAV community of 22

percent light-through-water (Kd of 1.5 meter-1).

Reductions in light attenuation due to dissolved organic material and the base light

attenuation rate of water are here depicted as ‘Kd color’ and are not possible as they

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Table III-2. Chesapeake Bay SAV restoration

goals by major tributary basin.

SAV Restoration

Jurisdiction-Basin Goal (acres)

Susquehanna 12,856

Eastern Shore – MD 76,193

Western Shore – MD 5,651

Patuxent 1,420

Potomac 19,450

Rappahannock 12,798

York 21,823

James 3,483

Eastern Shore – VA 31,215

TOTAL 184,894
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Application Depth (meters)

Application Depth vs allowable Kd

Allowable Kd (1/m)

7

6

5

4

3

2

1

0

TF and OH

MH and PH

0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9

Figure III-12. Plots of equivalent 13 percent light-through-water for tidal-fresh (TF) and oligohaline 

(OH) SAV communities and 22 percent light-through-water for mesohaline (MH) and polyhaline (PH) 

SAV communities at depths between 0.3 and 2.0 meters expressed as light attenuation coefficient, or Kd.

1985

1.14

1.36

0.87

1.52

2.50

2.61

1.79 1.68

1.47

2.28

0.16 0.15 0.15

0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37

0.16 0.10 0.19 0.10 0.14 0.16 0.15

1986 1987 1988 1989 1990 1991 1992 1993 1994

kd (1/m)

3.50

3.00

2.50

2.00

1.50

1.00

0.50

0.00

Kd Sediment

Kd Chlorophyll

Kd Color

Figure III-13. Estimated average components of light attenuation (Kd) for the Tangier Sound monitoring

station EE3.2 for the clarity criteria season (April-October) using a light attenuation model developed by

Gallegos (2001).

Source: Chesapeake Bay water quality output http://www.chesapeakebay.net.
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

are natural conditions. Management reductions in reducing algal nutrients will

reduce light attenuation from phytoplankton and epiphytes, but in Tangier Sound, as

in many regions of the Bay’s tidal waters, reductions in nutrients are estimated to be

insufficient to improve water clarity to levels necessary to support SAV growth. 

SEDIMENT LOADS: LOCAL EFFECTS

Sediment loads and allocations are different from nutrient loads and allocations

previously done by the Chesapeake Bay Program partners in that sediments are more

local in their effect. Nutrient loads affect large regions of the Bay’s tidal waters, as

discussed above. Sediment loads in this water quality model simulation have a

higher settling rate than organic material and, without simulated sediment transport,

do not resuspend from the bottom. 

Figure III-14 illustrates the local effect of sediment model behavior through the use

of a simulated sediment load in a particular surface water quality model cell (#1720)

in the Eastern Bay of the Chesapeake Bay. The simulated sediment tracer concentration was highest in the cell into which the tracer was loaded, with the concentration

quickly dropping in adjacent cells. At a distance of 10 cells from the cell into which

the tracer was loaded (about 10 kilometers), the tracer was no longer detectable. 

These findings are consistent with monitoring and research findings of sediment

behavior, with one important difference. With the current lack of a sediment

transport model in the water quality simulation, once suspended sediment reaches

0.00

0.50

1.00

1.50

2.00

2.50

1690 1700 1710 1720 1730 1740 1750 1760

Water Quality Model Cell Numbers

Sediment Concentration

(mg liter-1)

Down-Bay from Cell 1720 Up-Bay from Cell 1720

<=========== ===========>

Figure III-14. Local effect of a conservative sediment tracer loaded to a single shoreline

surface model cell (#1720) in Eastern Bay.
the bottom of the water column and is incorporated into the bottom sediments, it is

lost from the system, as no resuspension is simulated. Work is under way to incorporate sediment transport into the simulation. A complete description of the

Chesapeake Bay Water Quality Model’s sediment simulation can be found in Cerco

and Noel (2003).

CALIBRATING THE WATER QUALITY MODEL FOR CLARITY

To support the development of the sediment cap load allocation, much attention was

given to calibrating various components of light attenuation in the Water Quality

Model. In the absence of a full sediment transport model, calibration entailed reconciling the daily Chesapeake Bay Watershed Model (Phase 4.3) sediment input loads

with monitoring program estimates of suspended sediment concentrations by regionally adjusting the sediment settling rates. Further adjustment of the sediment

concentrations, particularly useful for the calibration of sediment in bottom waters

and in turbidity maximum zones, used differential settling rates in the water column

and in the sediment bed. The settling rate of sediment in the water column was set

at a higher rate than that of sediment bed incorporation to factor in the potential for

resuspension of accumulated sediment in the bottom model cells. Figure III-15 is an

example of the resulting calibration in upper Chesapeake Bay mainstem segment

CB2OH surface waters and is representative of a single monitoring station calibration in comparison with model estimates of the few associated model cells at the

monitoring station CB2.2 location. Full calibration results for sediment, chlorophyll

and light attenuation are documented in Cerco and Noel (2003) and further documented in http://www.chesapeakebay.net/modsc.htm.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

0 2 4 6 8 10 0

1

2

3

4

5

6

7

8

9

10

11

12

13

Figure III-15. Time series plot of model simulated light attenuation (solid line) and modeling program observed (circles) for a single monitoring station, CB2.2, compared to a single

surface model cell. The time series extends from January 1985 to December 1994.
While calibration of light attenuation was necessarily at the location of the long-term

monitoring stations, these stations are typically located in deeper waters and away

from shallow-water SAV habitats. Shallow regions, located at the interface of the

watershed and the tidal Bay, are at the ‘point of discharge’ of all sediment loads from

the adjacent watershed, shore erosion and shallow-water resuspension and generally

have less clarity than that found in the traditional deep-water monitoring stations.

Figure III-16 shows the relative difference of Kd model estimates for the shallows

and deep waters of the Chesapeake Bay. Typically, the simulation estimates that the

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Figure III-16. Chesapeake Bay Water Quality Model estimated light attenuation (Kd)

showing greater light attenuation in the shallows than in the deeper waters of the

Chesapeake Bay and its tidal tributaries due to the local influence of sediment loads 

from the adjacent watershed and shoreline (units are meter-1).
shallows have an attenuation rate two times greater than that of deeper waters, a

finding consistent with monitoring program observations.

To address the difference between light attenuation estimates in shallow and deep

waters, the monitoring program initiated a long-term shallow-water monitoring

assessment in 2003. With information from new and existing monitoring assessments and research into sediment processes and transport, the Water Quality Model

will be refined with an explicit sediment transport simulation by 2007, in time to

support the reevaluation of the nutrient and sediment allocations.

RELATING SAV BIOMASS TO ACREAGE

SAV abundance has been assessed by aerial surveys from 1984-2000, usually flown

once annually at the time of estimated peak abundance (Moore et al. 2000). Area is

related to abundance of SAV through density and biomass, that is, the area of SAV

coverage increases as either density deceases or biomass increases, all else being the

same. Further, changes in area may not be linear with changes in biomass. For these

reasons, biomass has been described as a better measure of abundance than area, and

biomass is the key SAV metric simulated by the Chesapeake Bay Water Quality

Model. On the other hand, aerial surveys of SAV area are cost-effective, and the

public goal of SAV restoration to an established acreage is a straightforward message

to communicate to the public. For these reasons alone the aerial surveys of SAV have

been, and will continue to be, the basis for abundance estimates of SAV in the Chesapeake Bay, augmented by associated estimates of SAV biomass.

The model structure of simulating a ‘unit plant’ of SAV within the larger water

quality context of the Water Quality Model (Cerco et al. 2002; Cerco and Moore

2001) generates biomass estimates of SAV. Obviously, the first step in relating the

SAV restoration goal and the sediment cap load allocations estimated in the model

is the reconciliation of the acreage and biomass estimates. Fundamental to this

reconciliation is the work done by Moore et al. (2000). Building on this approach,

the Chesapeake Bay Program partners used the Water Quality Model estimates of

SAV biomass to derive SAV acreage estimates (Cerco et al. 2003). The essence of

using the model estimates of biomass relies on the simple assumption that SAV

density does not change in the model scenarios. Then model estimates of SAV area

can be determined by the simple relationship:

SAV Acres = (SAV biomass scenario/SAV biomass calibration) * acres SAV observed 1985-1994

As the calibration relates the SAV biomass to estimated SAV area over the 10-year

period of 1985-1994, and further as the scenarios relate a relative change in SAV due

to changes in nutrient or sediment loads, the relationship above provides a reasonable estimate of SAV area for any scenario.

SAV biomass varies over the growing season as shown in figures III-17 through 

III-19. Aerial estimates of SAV abundance are taken once during the year at about the

time of peak biomass, generally in the late summer for oligohaline and mesohaline

regions, and in the late spring or early summer for the polyhaline regions (Moore et al.

2000). To coordinate the observed peak abundance estimates with model-simulated

estimates of SAV, July averaged simulated SAV biomass was used throughout, except

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

200

180

160

140

120

100

80

60

40

20

0

freshwater community

observed

modeled (CB1, run 178)

g C/m2

month

1 2 3 4 5 6 7 8 9 10 11 12

Figure III-17. Modeled (mean [solid line] and interval encompassing 95 percent of computations [dashed line]) and observed (mean [dot] and 95 percent confidence interval

[vertical line through dot]) freshwater SAV community (above ground shoot biomass

only). Observations from Moore et al. (2000). Model simulation from the Susquehanna

Flats (Segment CB1TF) using the 10,000-cell 1998 version of the Water Quality Model.

Source: Cerco et al. 2002.

for tidal-fresh regions, which use a September averaged biomass. The monthly peak

average is typically about three times higher than the annual average biomass.

In relating the SAV restoration goal to the sediment cap load allocations, two factors

needed to be considered in determining the scale at which SAV estimates will be

made. In assessing SAV biomass: 1) sediment cap loads, like nutrients, are allocated

at the major basin level; and 2) sediment is not transported far from its discharge

point, which is usually a fall line or shoreline, due to relatively rapid settling.

Accordingly, the tidal Bay shorelines and adjacent shallow-water habitats were

apportioned into SAV regions associated with major tributary basins—the Susquehanna, Patuxent, Potomac, Rappahannock, York, James, Western Shore, Eastern

Shore Maryland/Delaware and Eastern Shore Virginia. The Potomac basin was

further divided into Maryland, Virginia and the District of Columbia. A major

portion of the Susquehanna Flats (CB1TF) was assigned to the Susquehanna basin,

which is considered the primary influence. The Northeast, Elk, Bohemia and
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Sassafras rivers, though part of CB1TF, were assigned to the Eastern Shore Maryland/Delaware basin (Figure III-20).

Model estimates of the SAV response under different management scenarios were

estimated with two metrics, an SAV single-best-year estimate and an SAV runningthree-year, single-best-year mean estimate. The single-best-year estimate is an

estimate of the highest annual SAV biomass throughout the simulated hydrology

period of the 1985-1994. The single-best-year estimate most closely resembles the

new SAV restoration, in that the highest annual biomass is used for the entire simulation period. The SAV running-three-year, single-best-year mean estimate is a mean

of the best year estimates of the eight three-year periods that the 1985-1994

simulation can be parsed into (1985-1987, 1986-1988, 1987-1989, etc.) The SAV

running-three-year, single-best-year mean and standard deviation is the best a priori

estimate of SAV biomass that will be ultimately be assessed through the monitoring

program as described below. Taken together these two estimates provide an estimate

of the greatest SAV response over a 10-year simulated hydrology and the estimated

variation in SAV biomass over the period (tables III-3 and III-4). To further differentiate the SAV response in the major basins, the major fall line load regions where

split into a tidal-fresh (TF) and a lower tidal river region. The Susquehanna SAV

80

60

40

20

0

g C/m2

month

1 2 3 4 5 6 7 8 9 10 11 12

ruppia community

observed

modeled (EE1, run 178)

Figure III-18. Modeled (mean [solid line] and interval encompassing 95 percent of

computations [dashed line]) and observed (mean [dot] and 95 percent confidence interval

[vertical line through dot]) mesohaline SAV community (Ruppia, above ground shoot

biomass only). Observations from Moore et al. (2000). Model simulation from Eastern 

Bay using the 10,000-cell 1998 version of the Chesapeake Bay Water Quality Model. 

Source: Cerco et al. 2002.
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

response is represented as the SAV response of the Susquehanna Flats in the upper

tidal-fresh Chesapeake Bay.

Under all scenarios, the lower Rappahannock and the tidal-fresh James rivers did not

meet the C2K SAV acreage restoration goal, as the simulated SAV area in response

to the scenario load reductions was much less than the estimates of SAV historic

acreage that forms the basis of the 185,000-acre SAV restoration goal. The Patuxent

and Rappahannock tidal-fresh estimates of SAV area show an initial drop in SAV

acreage between the 2000 Progress and Tier 3 scenarios. In both cases this counterintuitive response is due to a decrease in sediment loads causing an increase in light

through the water column, resulting in greater algal biomass and a simulated poorer

habitat for SAV. Further reductions past Tier 3 show increasing SAV response to

decreasing loads. The standard deviation is in many cases greater than the mean,

particularly in regions of the Bay where SAV acreage is sparse, indicating the high

variability in year-to-year SAV area.

160

140

120

100

80

60

40

20

0

g C/m2

month

1 2 3 4 5 6 7 8 9 10 11 12

zostera community

observed

modeled (WE1, run 178)

Figure III-19. Modeled (mean [solid line] and interval encompassing 95 percent of

computations [dashed line]) and observed (mean [dot] and 95 percent confidence interval

[vertical line through dot]) polyhaline SAV community (Zostera above ground shoot biomass

only). Observations from Moore et al. (2000). Model simulation from the Mobjack Bay 

(segment MOBPH, formerly WE4) using the 10,000-cell 1998 version of the Chesapeake Bay

Water Quality Model. 

Source: Cerco et al. 2002.
SPATIAL ANALYSIS OF EFFECTIVE SHORELINE LOAD REDUCTIONS

Sediment reductions from the watershed alone are estimated to be insufficient for full

SAV recovery to the 185,000-acre SAV restoration goal. Additional reductions in

shoreline loads including shoreline erosion loads and resuspension were simulated for

key scenarios. In tables III-3 and III-4, two scenarios are simulated with an additional

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

SAV Region

James River

Eastern Shore MD/DE

Western Shore MD

Patuxent River

Potomac

Rappahannock River

Susquehanna

Eastern Shore VA

York River

Figure III-20. SAV regions associated with the major Chesapeake Bay tributary basins.
20 percent reduction in the base shoreline loads representing BMPs, which reduce

shoreline erosion or sediment resuspension1. In many regions of the Chesapeake Bay,

particularly those with extensive shorelines, the 20 percent shoreline load reduction

resulted in a significant improvement in shallow-water water clarity and in SAV. 

To further refine where the shoreline loads should be applied, an analysis was

conducted to determine where any historical SAV occurred and map the adjacent

shoreline. The identified shoreline was considered to be that where shoreline reductions would be effective for improving SAV habitat. An arbitrary 0.5-kilometer buffer

was extended along the shoreline beyond the SAV areas as a further protection of SAV

habitat. The combined shoreline adjacent to any historical SAV occurrence and the

0.5-kilometer buffer of shoreline was called the ‘effective shoreline’. 

The percent of effective shoreline varied from basin to basin ranging from a low of 16

percent in the James River to an effective shoreline of 73 percent in the Potomac River

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Table III-3. Single-best-year estimate of SAV area under key management scenarios. Meets CBP goal = 

the original 1992 SAV restoration goal of 113,000 acres. Meets Chesapeake 2000 goal = the new SAV

restoration goal of 185,000 acres. 

SAV Acreage

2000 Progress Tier 3 Tier 3 +20% Scenario 175 CBP Goal C2K Goal

Susquehanna 13171 20400 20400 20400 7620 12856

Western Shore MD 4522 4978 6453 6418 3710 5652

Patuxent TF 142 85 88 198 14 5

Patuxent Lower 209 276 745 772 451 1420

Potomac TF 6278 8768 12618 12842 7374 5438

Potomac Lower 7611 7976 10072 9751 5244 14017

Rappahannock TF 48 29 45 45 0 20

Rappahannock Lower 2927 3203 3539 3527 5376 12778

York TF 1734 2173 4298 4857 0 231

York Lower 15995 17848 20726 20408 18347 21592

James TF 136 313 580 552 0 1944

James Lower 656 746 983 838 441 1593

Eastern Shore MD/DE 33765 40970 54819 54475 42280 77706

Eastern Shore VA 21281 22747 26267 28518 22818 29702

TOTAL 108475 130512 161633 163601 113675 184954

Scenario 175 = nutrient and sediment cap load allocations

Scenario 175 meets original 1992 CBP SAV restoration goal 

Scenario 175 meets Chesapeake 2000 (C2K) SAV restoration goal

1Information was presented to the partners that a 20 percent reduction in base shoreline loads may be

beyond present technical feasibility. However, sufficient information was not available to reach a

definitive conclusion at that time. 
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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations Table III-4. Mean and standard deviation of the estimated running-three-year, single-best-year SAV area under key management scenarios. Meets CBP goal = the original 1992 SAV restoration goal of 113,000 acres. Meets Chesapeake 2000 goal = the new SAV restoration goal of 185,000 acres. SAV Acreage Calculation Using Running-Three-Year SBY Mean and Standard Deviation 2000 Progress Tier 3 T3+20% Scenario 175 Goals Mean Std. Dev. Mean Std. Dev. Mean Std. Dev. Mean Std. Dev. CBP C2K Susquehanna 10,149 1,546 20,400 4,777 20,400 5,348 20,400 5,507 7,620 12,856 Western Shore MD 1,375 1,509 1,514 1,661 1,962 2,153 1,951 2,141 3,710 5,652 Patuxent TF 66 65 40 39 41 40 92 91 14 5 Patuxent Lower 104 67 137 89 369 240 382 249 451 1,420 Potomac TF 4,750 1,197 6,633 1,672 9,546 2,406 9,716 2,449 7,374 5,438 Potomac Lower 5,027 1,697 5,268 1,778 6,652 2,245 6,440 2,173 5,244 14,017 Rappahannock TF 8 16 5 10 8 15 8 16 - 19.7 Rappahannock Lower 2,469 470 2,702 514 2,986 568 2,975 566 5,376 12,778 York TF 372 739 466 925 921 1,830 1,041 2,068 0 231 York Lower 14,271 1,902 15,925 2,122 18,493 2,465 18,209 2,427 18,347 21,592 James TF 29 58 67 133 124 247 118 235 0 1,943.60 James Lower 245 227 279 258 368 340 313 290 441 1,593 Eastern Shore MD/DE 28,284 4,304 34,320 5,222 45,921 6,988 45,633 6,944 42,280 77,706 Eastern Shore VA 18,667 1,951 19,953 2,085 23,041 2,408 25,015 2,614 22,818 29,702 TOTAL 85,816 107,709 130,832 132,293 113,675 184,953 Scenario 175 = nutrient and sediment cap load allocations Scenario 175 meets original 1992 CBP SAV restoration goal Scenario 175 meets Chesapeake 2000 (C2K) SAV restoration goal
(Figure III-21). In scenarios in which reductions in the base shoreline loads were

reduced, these reductions were universally applied to all base shoreline loads. Using

the estimated effective shoreline estimates, the shoreline length to be considered for

BMPs is considerably reduced (Figure III-22). 

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

0.62

Rappahannock James

ESMD/DE

Patuxent ESVA

West Shore Potomac York

0.57

0.73

0.31

0.39

0.62

0.16

0.57

Figure III-21. Estimated effective shoreline for the major Chesapeake Bay basins.

Figure III-22. Effective shoreline regions of the James, York and lower Rappahannock basins.
MONITORING APPROACH

The Chesapeake Bay Program partners agreed to measure the achievement of the

SAV restoration goal at the segment scale based on the single best year of a running

three-year assessment to account for year-to-year fluctuations in water clarity due to

changing hydrology and loads (U.S. EPA 2003a). To ensure consistency between

metrics used to derive the cap load allocations and measure attainment, modelsimulated SAV acreages were assessed using the single best year of a running

three-year assessment period. The model estimates from the 10-year average estimate were matched with running three-year single-best-year estimates and formed

the basis of the final model estimates of SAV under the different scenario assumptions as shown in tables III-3 and III-4.

SEDIMENT CAP LOAD ALLOCATION PRINCIPLES

The principles applied in developing the sediment cap load allocations were as

follows:

1. SAV habitat and the sediment allocation are linked and the primary reason for

reducing sediment loads is to provide suitable habitat for SAV; 

2. Sediment impacts, unlike those of nutrients, are local in their influence; and 

3. The analysis and effects of nutrients on dissolved oxygen is well developed and

understood, but the analysis of sediment loads and water quality effects is incomplete. 

With these principals in mind, the Chesapeake Bay Program partners agreed to include

the SAV goals into water quality standards. The local nature of the effects of sediment

loads is reflected in the assignment of SAV goals to each Chesapeake Bay Program

segment. It was decided that either the SAV acreage goal or the clarity goal would be

used to assess attainment, through detailed monitoring assessments. If the SAV goal

is not achieved with the nutrient and sediment allocations in place, additional innovative methods to achieve SAV regrowth, such as SAV planting, offshore breakwaters,

shore erosion controls and other methods will be applied. The incomplete understanding of shoreline loads from shoreline erosion and resuspension led the partners

to decide to allocate sediment load reductions to the land-based sediment loads. 

LITERATURE CITED

Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.

Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, J. Naylor, D. Wilcox, K. A.

Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation

Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical

Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U. S. EPA Chesapeake Bay Program,

Annapolis, Maryland.

Boynton, R. W., J. H. Garber, R. Summers and W. M. Kemp. 1995. Inputs, transformations,

and transport of nitrogen and phosphorus in Chesapeake Bay and selected tributaries.

Estuaries 18(1B):285-314.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations
Cai, W. J., L. R. Pomeroy, M. A. Moran and Y. Wang. 1999. Oxygen and carbon dioxide mass

balance for the estuarine-intertidal marsh complex of five rivers in the southeastern U.S.

Limnology and Oceanography 44(3):639-649.

Cerco, C. F. and M. Noel. In preparation, 2003. The 2002 Chesapeake Bay Eutrophication

Model. EL-03-XX. U.S. Army Engineering Research and Development Center, Vicksburg,

Mississippi.

Cerco, C. F. and K. Moore. 2001. System-wide submerged aquatic vegetation model for

Chesapeake Bay. Estuaries 24(4):522-534.

Cerco, C. F., M. R. Noel and L. C. Linker. In press, 2003. Managing for water clarity in

Chesapeake Bay. Journal of Environmental Engineering

Cerco, C. F., B. H. Johnson and H. V. Wang. 2002. Tributary Refinements to the Chesapeake

Bay Model. Final Report. U.S. Army Corps of Engineers, Washington, D.C. ERDC TR-02-

4. 201 pp.

Conley, D. J. 1999. Biogeochemical nutrient cycles and nutrient management strategies

Hydrobiologia 410:87-96.

D’Elia, C. F., J. G. Sanders and W. R. Boynton. 1986. Nutrient growth studies in a coastal

plain estuary: Phytoplankton growth in large-scale, continuous cultures. Canadian Journal of

Fisheries and Aquatic Science 43:397-406.

Fisher, T. R., A. B. Gustafson, H. L. Berndt, L. Walstad, L. W. Haas and S. MacIntyre. In

review, 2003. The upper mixed layer of Chesapeake Bay, USA. Estuaries.

Fisher, T. R. and A. B. Gustafson. 2001. Draft report on Bay-wide bioassays to assess P limitation and N distribution in spring. Report to Maryland Department of Natural Resources,

Annapolis, Maryland.

Fisher, T. R., A.B. Gustafson, K. Selner, R. Lacouture, L. W. Haas, R. L. Wetzel, R. Magnien,

D. Everitt, B. Michaels and R. Karrh. 1999. Spatial and temporal variation of resource limitation in Chesapeake Bay. Marine Biology 133:763-778.

Fisher, T. R. and A. J. Butt. 1994. The role of nitrogen and phosphorus in Chesapeake Bay

anoxia. STAC Literature Synthesis. Scientific and Technical Advisory Committee, Chesapeake Research Consortium Edgewater, Maryland.

Fisher, T. R. E. R. Peele, J. W. Ammerman and L. W. Harding Jr. 1992. Nutrient limitation of

phytoplankton in Chesapeake Bay. Marine Ecology Progress Series 82:51-63.

Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect

submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation

coefficient for photosynthetically active radiation. Estuaries 24:381-397.

Kemp, W. M., S. B. Brandt, W. R Boynton, C. J. Madden, J. Luo, J. Hagy and R. Bartleson.

1994. Benthic filtration, nutrient inputs, and hypoxia in mesohaline Chesapeake Bay. In:

Ecosystem Models of the Patuxent River Estuary. Maryland Department of Natural

Resources, Annapolis, Maryland. CBRM-GRF-94-2. Pp. 57-72.

Malone, T. C. D. J. Conley, T. R. Fisher, P. M. Gilbert, L. W. Harding, K. Selner and J. Kevin.

1996. Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay. Estuaries

19(2B):371-385.

Moore, K. A., D. J. Wilcox and R. J. Orth. 2000. Analysis of the abundance of submersed

aquatic vegetation communities in the Chesapeake Bay. Estuaries 23(1):115-127.

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chapter iii • Technical and Modeling Considerations for Setting the Cap Load Allocations

Neubauer, S., W. Miller and I. Anderson. 2000. Carbon cycling in a tidal freshwater marsh

ecosystem: a carbon gas flux study. Marine Ecology Progress Series 199:13-30

Newell, R. I. E., J. C. Cornwell and M. S. Owens. 2002. Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study.

Limnology and Oceanography 47(5):1367-1379.

Secretary Tayloe Murphy. 2003. “Summary of Decisions Regarding Nutrient and Sediment

Load Allocations and New Submerged Aquatic Vegetation (SAV) Restoration Goals.” April

25, 2003 memorandum to the Principals’ Staff Committee members and representatives of

the Chesapeake Bay headwater states. Virginia Office of the Governor, Natural Resources

Secretariate, Richmond, Virginia.

STAC. 2003. Shoreline Erosion and Chesapeake Bay Water Quality: A Scientific Evaluation

of Prediction Uncertainty, Potential for Improvement, and Management Implications. Scientific and Technical Advisory Committee, Chesapeake Research Consortium. Edgewater,

Maryland.

U.S. EPA. 2003a. 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.

Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. EPA. 2003b. Technical Support Document for the Identification of Chesapeake Bay

Designated Uses and Attainability. EPA 903-R-03-004. Chesapeake Bay Program Office,

Annapolis, Maryland.
This chapter describes the specific processes involved in deriving and allocating the

cap loads for nutrients and sediments. While many alternative processes were

explored, only the process ultimately agreed to and followed by the partners is

described. The processes for deriving sediment and nutrient cap load allocations are

distinct and are described separately in this chapter. 

To establish cap load allocations for nutrients, the following steps were taken:

• A basinwide loading cap was established, which required identifying a baywide

load that would meet the dissolved oxygen criteria throughout the Bay. The

sections Geographic Location and Criterion Driving the Allocation and Baywide

Cap Loading Options below, respectively, identifies the parts of the Bay’s tidal

waters and the criteria that were most critical to establishing a baywide allocation

and presents the cap options that the Chesapeake Bay Water Quality Steering

Committee explored. 

• The baywide nutrient cap loads were distributed among the basins and jurisdictions. The section Distributing Basinwide Allocations to Major Basins 

and Jurisdictions below (page 91) describes the principles, decision rules and

processes used to distribute the cap loading to the basins and jurisdictions. 

• The initial process did not result in sufficient nutrient reductions to achieve the

baywide cap loads. As described in the section The PSC Completes the Allocation

Process below (page 99), it was necessary to have input from the Principals’ Staff

Committee and headwater state representatives to complete the allocations.

• The dissolved oxygen-based nutrient allocations were examined to ensure that

they resolved any remaining chlorophyll a problems. The brief section Cap Load

Allocations to Achieve the Chlorophyll a Criteria below (page 99) reviews the

results of this analysis. 

To establish allocations for sediments, the following steps were taken:

• SAV restoration goals were set and used to derive the sediment cap load allocations. The section SAV Restoration as the Goal below (page 103) explains why the

SAV restoration goals were used in the sediment cap load allocation process. 

• Sediment loadings were divided into two major source categories: upland loads

(page 104) and near-shore loads (page 105). 

• Sediment allocations were set. The section SAV-Based Sediment Allocations

below (page 106) describes the scientific and policy bases for that process.

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chapter iv • Setting Nutrient and Sediment Allocations

chapteriv

Setting Nutrient and

Sediment Allocations
The mathematical models described in Chapter II were used to identify the sources

of pollutant loadings and relate reductions in those loading sources to attainment of

the Chesapeake Bay water quality criteria and restoration of underwater bay grasses,

or SAV. While these models provided a good understanding of projected water

quality effects from pollutant source reductions, policy decisions were necessary to

derive an equitable distribution of the allocated load. 

Chapter III describes many of the technical issues that had to be resolved before the

allocations could be derived. The technical methods and policy decisions that led to

the nutrient cap load allocations are discussed below. Because these methods and

decisions differed from those used to develop sediment allocations, the processes for

deriving each are described in separate sections. 

ESTABLISHING NUTRIENT CAP LOAD ALLOCATIONS 

Three steps were involved in setting and allocating allowable caps on nutrient loads

throughout the Chesapeake Bay watershed that, collectively, would restore Chesapeake Bay and tidal tributary water quality:

1. The basinwide caps on nitrogen and phosphorus loads necessary to attain the Bay

criteria for dissolved oxygen through all tidal Bay habitats were determined; 

2. The loading caps were distributed among the major tributary basins by jurisdiction; and

3. Care was taken to ensure that the dissolved oxygen-based nutrient cap load allocations would also bring the Chesapeake Bay and its tidal tributaries into

attainment with the chlorophyll a criteria.

GEOGRAPHIC LOCATION AND CRITERION 

DRIVING THE ALLOCATIONS

Early in the process of developing nutrient allocations, there was a question as to

whether there was a geographic location in the Chesapeake Bay tidal waters and a

criterion (dissolved oxygen or chlorophyll a) that would drive the nutrient cap load

allocations. If such factors converged, then analyses could be focused on that location and that criterion in deriving nutrient cap load allocations. Also, if the cap load

allocations eliminated the water quality impairment at that point, then they would

likely eliminate all other impairments related to nutrients across all other tidal water

habitats. To explore this issue, the modeled water quality predictions were reviewed

(see Appendix D for modeled water quality results of various loading scenarios).

The water quality results from ‘Tier 3 plus 20 percent near-shore reductions’ were

used in this analysis because they constituted the likeliest allocation scenario. The

following observations can be made:

• For dissolved oxygen, the migratory spawning and nursery and open-water

criteria have a high level of attainment under current observed conditions.

• For dissolved oxygen, significant nonattainment is shown for the Pamunkey and

Mattaponi rivers. This nonattainment is observed under all loading scenarios

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chapter iv • Setting Nutrient and Sediment Allocations
because it is due to natural conditions (extensive tidal wetlands resulting in natural

oxygen consuming processes) and for this reason is not considered in establishing

the allocations for nutrients. Chapter 5 of the Technical Support Document (U.S.

EPA 2003b) addresses this issue in greater detail.

• For dissolved oxygen, nonattainment of the water quality criteria was highest in

the deep-water portions of the middle Chesapeake Bay mainstem (segments

CB3MH, CB4MH and CB5MH). It should be noted that the deep-water criteria

only apply in summer, from June through September.

• For chlorophyll a, the Chesapeake Bay and its tidal tributaries achieve a set of

numerical target concentrations except for a limited set of local areas.

These observations suggested that if the nutrient allocations were established to

achieve the dissolved oxygen criteria in the deep-water portions of segments

CB3MH, CB4MH and CB5MH, then all other impairments for dissolved oxygen

and chlorophyll a would most likely be corrected (see Chapter III for details). 

BAYWIDE CAP LOADING OPTIONS

Having established the importance of correcting dissolved oxygen nonattainment in

the deep-water portions of segments CB3MH, CB4MH and CB5MH, it was considered important to explore various baywide nutrient cap load options and their water

quality effects on these segments. 

Chapter III discusses the development of various management control scenarios, or

tiers, that were used for deriving the cap load allocations. The tier scenarios were

based on increasing point and nonpoint source controls throughout the Chesapeake

Bay watershed. The loads delivered to tidal waters as a result of each of these control

scenarios were then derived through the watershed model. This control scenario

approach was considered superior to a straight percent reduction approach because:

• The tier scenarios provide a sense of the actual point and nonpoint technology

necessary to achieve the loadings; and

• Costs for tier scenarios can be estimated for use in use attainability analyses.

Five Options Explored

Ultimately, five bay nutrient cap load options, based largely on the tier approach, were

explored. These options, and the model projections of the water quality response to

them, are explained below and are further described in tables IV-1 and IV-2.

Option 1: Nitrogen load capped at 160 million pounds per year, phosphorus load

capped at 12.8 million pounds per year. This option was actually a model simulation that had been run and resulted in full attainment of the bay dissolved oxygen

criteria.1 Therefore it represented the lowest end of basinwide cap loads under

consideration. While this option resulted in attainment, it was unknown at the time

whether higher loading options would also result in attainment. 

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chapter iv • Setting Nutrient and Sediment Allocations

1The final water quality model results for Option 1 showed a small degree of nonattainment due to

changes (agreed to by the partners) in how the deep-water and deep-channel dissolved oxygen criteria

would be applied.
Option 2: Nitrogen load capped at 175 million pounds per year, phosphorus load

capped at 12.6 million pounds per year. This option, which considers all but the

deep-water portion of segment CB4MH, was developed acknowledging that

attainment of the dissolved oxygen criteria in deep-water portion of segment CB4MH

is difficult. While the total loading under this option is not much different from 

Option 3 (5 million pounds nitrogen and 0.8 million pounds phosphorus less), the

critical difference is the geographic distribution of the cap loadings. The cap loads for

Option 3, compared with this Option, are higher for the northern tributaries (where

additional reductions improve the Bay’s water quality) with lower cap loads in the

southern tributaries (where the impact of loads on Bay water quality is much less).

Option 3: Nitrogen load capped at 181 million pounds per year, phosphorus load

capped at 13.4 million pounds per year. This option represented the application of

the Tier 3 level of controls across all major tributary basins. It was considered viable

because it was perceived at the time as an equitable (all basins at Tier 3) and feasible

allocation by the Water Quality Technical Workgroup and the Water Quality Steering

Committee.

Option 4: Nitrogen load capped at 188 million pounds per year, phosphorus load

capped at 13.3 million pounds per year. This option was created as an alternative to

Option 3, after recognizing that Virginia’s lower western shore tidal tributary basins

had a much lower effect on the dissolved oxygen depletion in the middle mainstem

Chesapeake Bay than the Potomac, eastern shore and northern western shore tidal

tributary basins. The nutrient cap loads for the Rappahannock, York and James

basins were set to their existing tributary strategy levels of nutrient reductions. This

option was established to determine if similar water quality results to Option 3 can

be gained, despite the higher loading, at a lower cost to the lower western shore

Virginia tributaries.

Option 5: Nitrogen load capped at 198 million pounds per year, phosphorus load

capped at 15.7 million pounds per year. Like Option 4, Option 5 is predominantly

based on Tier 3 levels. However, the three lower Virginia western shore tributary

basins’ cap loads were increased to year 2000 progress levels. Again, this option was

explored because it was known from previous Bay water quality model scenarios

that these three tributary basins had less impact on the water quality of the Chesapeake Bay than the eastern shore and northern western shore tributaries. Like Option

4, the partners wanted to explore whether Option 5 would provide water quality

results similar to Option 3, despite the higher loading, at less cost for the lower

western shore Virginia tributaries.

Although the five options focused on assessing the effects of nutrient reductions on

the Chesapeake Bay and its tidal tributaries, it was necessary for them to be applicable to subsequent sediment allocations as well. Therefore, each option included a

20 percent reduction in near-shore sediment loads. 

The five options were intended to derive a basinwide loading cap for nutrients. At

the same time, these options not only consisted of different basinwide cap loadings

but also different geographical distributions of those nutrient cap loadings to the

major tributary basins. For distributing the loading to individual basins, however, a

more deliberate approach was necessary. Discussed later in this chapter, the

approach was based on three underlying principles that sought to assure that the allocation process was both equitable and feasible. 

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chapter iv • Setting Nutrient and Sediment Allocations

Table IV-1. Basinwide nitrogen cap load options (million pounds per year) developed by the

Water Quality Technical Workgroup, broken down by major tributary basin.

Basin Option 1 Option 2 Option 3 Option 4 Option 5

Susquehanna 69.2 (T3.5) 75.9 (T3.25) 82.6 (T3) 82.6 (T3) 82.6 (T3)

Eastern Shore—MD/DE 10.6 (T3.5) 11.9 (T3.25) 13.2 (T3) 13.2 (T3) 13.2 (T3)

Western Shore—MD 8.0 (T3.5) 9.25 (T3.25) 10.5 (T3) 10.5 (T3) 10.5 (T3)

Patuxent 2.5 (T3.5) 2.8 (T3.25) 3.1 (T3) 3.1 (T3) 3.1 (T3)

Potomac 30.5 (T3.5) 34.2 (T3.25) 37.9 (T3) 37.9 (T3) 37.9 (T3) 

Rappahannock 5.0 (T3) 5.0 (T3) 5.0 (T3) 5.0 (T3) 5.0 (T3)

York 5.7 (TS) 5.7 (TS) 5.1 (T3) 5.7 (TS) 8.0 (2000)

James 28.1 (TS) 28.1 (TS) 22.3 (T3) 28.1 (TS) 35.6 (2000)

Eastern Shore—VA 0.7 (T3.5) 1.9 (TS) 0.9 (T3) 1.9 (TS) 2.1 (2000)

Total 160.4 174.8 180.8 188 198.1

Table IV-2. Basinwide phosphorus cap load options (million pounds per year) developed by

the Water Quality Technical Workgroup, broken down by major tributary basin.

Basin Option 1 Option 2 Option 3 Option 4 Option 5

Susquehanna 2.54 (T3.5) 2.69 (T3.25) 2.83 (T3) 2.83 (T3) 2.83 (T3)

Eastern Shore—MD/DE 1.29 (T3) 1.2 (T3.25) 1.29 (T3) 1.29 (T3) 1.29 (T3)

Western Shore—MD 0.62 (T3.5) 0.7 (T3.25) 0.77 (T3) 0.77 (T3) 0.77 (T3)

Patuxent 0.20 (T3.5) 0.22 (T3.25) 0.24 (T3) 0.24 (T3) 0.24 (T3)

Potomac 3.18 (T.3) 2.86 (T3.25) 3.18 (T3) 3.18 (T3) 3.18 (T3) 

Rappahannock 0.66 (T3) 0.66 (T3) 0.66 (T3) 0.66 (T3) 0.66 (T3)

York 0.48 (TS) 0.48 (TS) 0.54 (T3) 0.48 (TS) 0.79 (2000)

James 3.71 (TS) 3.71 (TS) 3.77 (T3) 3.71 (TS) 5.70 (2000)

Eastern Shore—VA 0.10 (T3.5) 0.09 (TS) 0.10 (T3) 0.09 (TS) 0.22 (2000)

Total 12.78 12.61 13.38 13.25 15.68

Key: T3—Tier 3 scenario loading; T3.25—loading one quarter of the way between the Tier 3 and E3 scenarios; 

T3.5—loading halfway between Tier 3 and E3 scenarios; TS—tributary strategy loading; 2000—2000 progress

scenario loading.

Key: T3—Tier 3 scenario loading; T3.25—loading one quarter of the way between the Tier 3 and E3 scenarios; 

T3.5—loading halfway between Tier 3 and E3 scenarios; TS—tributary strategy loading; 2000—2000 progress

scenario loading.
Factors for Selecting the Basinwide Nutrient Cap Loads

In selecting the most appropriate basinwide cap loads from the five options considered above, the Water Quality Steering Committee considered three factors:

1. Basinwide nutrient cap loads should protect the living resources in the Chesapeake Bay and its tidal tributaries. The purpose of the allocation is to identify the

cap loads necessary to achieve water quality standards that conform to the Chesapeake Bay criteria and refined tidal-water designated uses. This goal of protecting

living resources by determining the necessary cap loads to do so is an important

concept to keep in mind during the state-specific development and adoption of

water quality standards. 

2. Basinwide nutrient cap loads should be feasible to achieve. The Water Quality

Steering Committee agreed that the Tier 3 loading levels were feasible to achieve

while achieving the E3 loadings were infeasible. The water quality model results

suggested that loads somewhere between the Tier 3 and E3 levels were necessary

to protect the living resources of the Chesapeake Bay. Obviously, the closer the

loading caps come to E3, the more concern exists as to their feasibility. As states

work toward adopting water quality standards, the issue of feasibility will likely

be subjected to further analysis and may become an important factor requiring

further consideration. 

3. Any nonattainment less than 1 percent is not considered significant. It is important to remember that the results reported here are model-simulated results and

already factor in a defined level of allowable nonattainment. The Water Quality

Steering Committee considered less than 1 percent nonattainment to be an artifact

of the attainment determination methodology and, therefore, insignificant. This

factor did not favor one of the options. Rather it was an important screening

device to identify significant nonattainment in the complex and voluminous set 

of modeling results reviewed throughout the cap load allocation decision 

making process. 

Predicted Water Quality Response to the 

Five Basinwide Cap Load Options

Appendix D contains the predicted water quality response for the Chesapeake Bay

and its tidal tributaries for many loading scenarios, including each of these options

(presented as ‘percent nonattainment’). Table IV-3 and figures IV-1 through IV-4,

below, present the relevant information from Appendix D. Again, the nonattainment

for dissolved oxygen in the Pamunkey and the Mattaponi rivers is not included

because it is due to natural conditions (see Chapter II for details). 

Table IV-3 provides an overview of the five options that were considered as possible

baywide nutrient loading caps and a summary of water quality responses to each of

the loading cap options. 

As Table VI-3 indicates:

• The results of the various control options confirm that the nutrient loads of the

Rappahannock, York and James basins do not have as significant an effect on the

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chapter iv • Setting Nutrient and Sediment Allocations
dissolved oxygen conditions of the mainstem Chesapeake Bay as the Potomac and

Virginia Eastern Shore basins. Therefore, for all options but Option 3, the Potomac,

western shore tributaries north of the Potomac and Eastern Shore basins have higher

levels of nutrient load reductions than the Rappahannock, York and James basins.

• Water quality improves with each increasing level of baywide loading reduction.

• The bay water quality is good for most metrics below under options 1 and 2 but

declines rapidly under options 3 through 5.

• Nonattainment of water quality criteria in the middle central Chesapeake Bay

(segment CB4MH) does increase with increased loading but not dramatically so.

• The number of segments in nonattainment is less under options 1 and 2 (one

segment impaired) than under the other options. However, note that only the deepwater portion of CB4MH is impaired under Option 1, while both the deep-water

and the deep-channel portion of CB4MH is impaired under Option 2 (although the

deep-channel impairment is marginal) (Table IV-3).

Figure IV-1 illustrates the percent of the Chesapeake Bay and its tidal tributaries in

which the Bay bottom surface area is below 1 mg/L dissolved oxygen for various

loading scenarios, including the five cap load allocation options. A low percentage

of bottom surface area with less than 1 mg/L dissolved oxygen is a good indicator of

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chapter iv • Setting Nutrient and Sediment Allocations

Table IV-3. Modeled water quality responses to the five basinwide nutrient cap load options.1

Modeled Responses Option 1 Option 2 Option 3 Option 4 Option 5

Nitrogen Loading 160 175 181 188 198

(million pounds per year)

Phosphorus Loading 12.8 12.6 13.4 13.3 15.7

(million pounds per year)

Potomac and north, Eastern Tier 3.5 Tier 3.25 Tier 3 Tier 3 Tier 3

Shore Tributary Basins

Rappahannock, York and Tributary Tributary Tier 3 Tributary 2000

James Basins Strategy Strategy Strategy Progress

Number of segments with >1% 1 1 3 4 11

dissolved oxygen nonattainment 

(percent time volume)

CB4MH deep-water nonattainment 3.9 6.0 7.6 7.9 3.84

(percent time volume)

Area of Bay bottom with dissolved 4 4 15 15 19

oxygen concentrations <1 mg/L

(percent bay bottom surface area)

Volume of bay in nonattainment 

for dissolved oxygen 4 7 15 19 19

(percent bay volume)

Bay water surface area not meeting 1 15 23 24 25

target chlorophyll a concentrations

(percent surface area)

1The dissolved oxygen response for these options includes a 20 percent reduction of shoreline erosion sediment loads. 

The dissolved oxygen nonattainment results do not include results from the tidal Mattaponi and Pamunkey rivers since 

the lower dissolved oxygen concentrations is a natural condition that was not remedied with nutrient loading reductions.
the extent to which the Bay may be habitable for bottom sediment dwelling worms

and clams. Further, since phosphorus, and to some extent, nitrogen, are released

from bottom sediments predominantly at dissolved oxygen levels below 1 mg/L,

this analysis also indicates the potential for nutrient release from these sediments.

Figure IV-1 shows that:

• Loading reductions represented by the observed through Tier 2 scenarios achieve

little reduction in the percentage of the Bay bottom area that has dissolved oxygen

concentrations of less than 1 mg/L. However, reductions beyond the Tier 2

scenario cause a dramatic reduction in the bottom area with dissolved oxygen

concentrations of less than 1 mg/L.

• Implementation of options 1or 2 achieve dramatic improvement in low dissolved

oxygen concentrations along the Bay bottom in comparison to Option 3. This

improvement is due to the further nutrient reductions realized in the Potomac,

northern western shore tidal tributaries and along the Eastern Shore. These basins

are the ones that have the most impact on the dissolved oxygen levels of the

Chesapeake Bay. Dissolved oxygen concentrations of less than 1 mg/L are almost

eliminated with both options 1 and 2.

Figure IV-2 identifies the model-simulated volume of the Chesapeake Bay and its

tidal tributaries, integrated over the 10-year simulation period, that do not attain the

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chapter iv • Setting Nutrient and Sediment Allocations

60%

50%

40%

30%

20%

10%

0%

(Nitrogen Load/Phosphorus Load in million pounds per year)

(percent of Bay bottom area)

Area of Bay Bottom Layer with Dissolved Oxygen <1 mg liter-1

Scenario

Observed

(336/26.5)

2000

Progress

(285/19.1)

Tier 1

(261/19.0)

Tier 2

(221/16.4)

Option 5

(198/15.7)

Option 4

(188/13.3)

Option 3

(181/13.4)

Option 2

(175/12.6)

Option 1

(160/12.8)

Figure IV-1. Nitrogen and phosphorus load versus percent of the Bay bottom surface area

with dissolved oxygen concentrations less than 1 mg/L.
applicable dissolved oxygen criteria for numerous loading scenarios, including the

five cap load allocation options. This metric shows the fraction of the water column

that provides suitable habitat for living resources with respect to dissolved oxygen.

Figure IV-2 shows that:

• Improvements in water column attainment of the dissolved oxygen criteria are not

dramatic until loading reductions exceed Tier 2 levels. While water quality is

improving with all loading reductions, these necessary nutrient reductions are not

enough to bring many parts of the Chesapeake Bay and its tidal tributaries back

into attainment.

• There is a dramatic improvement in attainment through all five basinwide cap

load allocations stepwise from options 5 through 1.

• While the improvement in attainment of Option 1 over Option 2 looks significant,

further review of the modeling results gives a better perspective. Nonattainment

in the deep-channel portion of segment CB4MH for Option 2 is 1.02 percent, just

barely over the established 1 percent threshold. It is this deep-channel nonattainment that brings the total water column volume nonattainment for Option 2 to 7

percent. Without this deep-channel nonattainment, Option 2 would have the same

water column volume nonattainment as Option 1 (4 percent).

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chapter iv • Setting Nutrient and Sediment Allocations

60%

50%

40%

30%

20%

10%

Volume of Chesapeake Bay and its Tidal Tributaries in Nonattainment 

of the Applicable Dissolved Oxygen Criteria (percent volume and time)

0%

(Nitrogen Load/Phosphorus Load in million pounds per year)

Scenario

Observed

(336/26.5)

2000

Progress

(285/19.1)

Tier 1

(261/19.0)

Tier 2

(221/16.4)

Option 5

(198/15.7)

Option 4

(188/13.3)

Option 3

(181/13.4)

Option 2

(175/12.6)

Option 1

(160/12.8)

Figure IV-2. Percent volume of the Chesapeake Bay and its tidal tributaries in nonattainment for

applicable dissolved oxygen criteria. 
Figure IV-3 shows the model-simulated water surface area of the Chesapeake Bay and

its tidal tributaries that do not meet a set of target chlorophyll a concentrations in

spring (Appendix C). Spring algae concentrations and the timing of the spring 

algal blooms are closely linked to summer low dissolved oxygen levels. Figure IV-3

shows that:

• There are dramatic improvements in the water surface area of chlorophyll a

achievement of target concentrations through the entire range of load reductions,

from 2000 progress through to Option 1. 

• The dramatic improvement in the water surface area of the Chesapeake Bay and

its tidal tributaries in achieving the target chlorophyll a concentrations between

options 2 and 3 requires closer review. That is, under Option 3, the lower Potomac

estuary level of nonattainment is 1.53 percent, while it is less than 1 percent under

Option 2. This small change in the level of achievement of the target concentrations in the Potomac River accounts for the difference in the baywide water

surface area of nonattainment between options 2 and 3.

• The dramatic improvement between options 1 and 2 also needs closer review.

That is, under Option 2, segment CB7PH is 1.52 percent, while it is less than 

1 percent under Option 1. This small change in attainment accounts for the difference in baywide nonattainment between options 1 and 2.

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chapter iv • Setting Nutrient and Sediment Allocations

0%

10%

20%

30%

40%

50%

60%

70%

80%

Water Surface Area of Chesapeake Bay and Its Tidal Tributaries

Not Achieving Target Chlorophyll a Concentrations

(percent volume and time)

(Nitrogen Load/Phosphorus Load in million pounds per year)

Scenario

Observed

(336/26.5)

2000

Progress

(285/19.1)

Tier 1

(261/19.0)

Tier 2

(221/16.4)

Option 5

(198/15.7)

Option 4

(188/13.3)

Option 3

(181/13.4)

Option 2

(175/12.6)

Option 1

(160/12.8)

Figure IV-3. Percent of water surface area of the Chesapeake Bay and its tidal tributaries

achieving target chlorophyll a concentrations in spring.
The Water Quality Steering Committee debated these five cap load allocation

options at length and concluded that:

• Options 1 and 2 provide high protection against low (< 1 mg/L) dissolved oxygen

waters along the Bay bottom and provide significantly better protection from

these effects than options 3 through 5.

• Options 1 and 2 offer a similar level of water quality protection with respect to

dissolved oxygen criteria, superior to that achieved under options 3 through 5.

Although the Water Quality Model did not simulate full criteria attainment, and

since the criteria and designated use will be subject to public review during the

states’ water quality standards adoption process, the marginal level of nonattainment remaining with these options was considered acceptable.

• As characterized by the target concentrations, model-simulated chlorophyll arelated water quality effects were largely addressed with the application of Tier 2

???? loading reductions. While the chlorophyll a improvement through the five

cap load allocation options is significant in Figure IV-2, caution should be taken

not to overestimate the actual improvement, given the localized nature of algalrelated impairments.

• Although the water quality response of Option 1 is similar to that for Option 2,

Option 2 costs less and is more feasible.

Based on this information, the Water Quality Steering Committee recommended

basinwide cap loadings of 175 million pounds of nitrogen per year and 12.8 million

pounds of phosphorus per year. These cap loads combined the best Bay water quality

and living resource protection and feasibility of all the potential options.

DISTRIBUTING BASINWIDE ALLOCATIONS TO MAJOR BASINS 

AND JURISDICTIONS

Once these basinwide cap loads were identified, they needed to be allocated to the

20 major basins, by jurisdiction, in the watershed. To enable states and local stakeholders to develop tributary strategies, it was necessary to divide the basinwide

nitrogen and phosphorus loads into cap loads for each of these 20 areas of the Bay

watershed for which tributary strategies will be developed. States and local stakeholders will identify the actions necessary to achieve the nutrient and sediment cap

load allocations. 

Figure IV-4 delineates each of the major basins in the watershed and identifies the

various jurisdictions for each major basin. In some cases (for example, the Susquehanna River Basin in Pennsylvania), states have chosen to further subdivide their

allocated cap load into smaller watersheds for which tributary strategies will be

developed.

Guiding Principles of Allocation Decisions

The discussions and analyses of options for equitable allocation of the basinwide cap

loads were extensive. The policy decisions for allocating the basinwide cap loads

were driven by an overall desire for equity and achievability. Achieving equity is a

complex and somewhat subjective undertaking. In addition to equity, the Chesapeake

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chapter iv • Setting Nutrient and Sediment Allocations

70 0 70 35 Miles

Chesapeake Bay Basins

EASTERN SHORE MARYLAND/DELAWARE

EASTERN SHORE VIRGINIA

JAMES RIVER BASIN

PATUXENT RIVER BASIN

POTOMAC RIVER BASIN

RAPPAHANNOCK RIVER BASIN

SUSQUEHANNA RIVER BASIN

WESTERN SHORE MARYLAND

YORK RIVER BASIN

Figure IV-4. The major tributary basins and jurisdictions in the Chesapeake Bay watershed. 
Bay Program partners factored in the feasibility of achieving reductions when

distributing the cap loadings. All the partners agreed that the E3 scenario was not

feasible. Similarly, the partners agreed that Tier 3 was feasible. Hence, the Chesapeake Bay Program partners limited allocations options to below E3.

The Chesapeake Bay Program partners succeeded in framing an equitable and

feasible approach by applying three underlying principles to the allocation process.

The specific process that was used to distribute the allocation based upon these principles is provided later in this chapter. The underlying principles were:

1. Basins that contribute the most to the problem must do the most to resolve the

problem. The Chesapeake Bay water quality model allowed the partners to

determine the relative impact of each tributary basin on the dissolved oxygen

problems experienced in the middle mainstem Bay and lower tidal Potomac River. 

Figure IV-5 shows the relative influence from each basin. Basins that have the

greatest influence on the Bay water quality will generally be required to achieve

the highest percent reduction of nutrient loads.2

2. States that benefit most from the Chesapeake Bay recovery must do more. States

that encompass the Chesapeake Bay and its tidal tributaries in its state boundaries,

e.g., Maryland, Virginia, Delaware and the District of Columbia, will realize

greater benefits, such as tourism dollars, than others. This principle was applied

by capping the reductions for nontidal states (New York, Pennsylvania and West

Virginia) at a lower level than reduction targets suggested through application of

Principle 1.

3. All reductions in nutrient loads are credited toward achieving final assigned

loads. This principle was adopted to avoid penalizing states that have achieved

significant nutrient reductions. It was applied by establishing a ‘baseline’ load

using 2010 land use and population, but no point or nonpoint source treatment in

place (2010 anthropogenic load). Since all reductions were from this baseline, all

past and existing best management practices and treatment upgrades were credited toward the needed reductions.

The above principles guided the Water Quality Technical Workgroup in allocating

the loadings among the major tributary basins, but a more detailed approach was

needed in order to divide the load among the major basins by jurisdiction. After

exploring alternatives to allocating the basinwide cap loads based on these principles, the Water Quality Technical Workgroup recommended a more detailed

approach to the Water Quality Steering Committee. The Chesapeake Bay Program

partners agreed to the following decision rules for dividing the cap load allocation:

1. Basins with the greatest impact on the Bay must achieve the highest controls.

Although segment CB4MH was considered the critical area of focus relative to

dissolved oxygen in establishing the basinwide nutrient cap loads, a broader look

at water quality effects than the original focus on CB4MH was preferred for determining which tributary basins has the greatest impact on the Bay water quality. To

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chapter iv • Setting Nutrient and Sediment Allocations

2Earlier modeling studies indicated ocean inputs were responsible for 29 to 36 percent of the total nitrogen loading (approximately 131 million pounds per year) to the Chesapeake Bay (Thomann et al.

1994). These ocean loads were a constant factor in the model-based analysis of relative impact on

water illustrated in Figure IV-5.
avoid an overly narrow assessment, basin impacts on water quality conditions in

segments CB3MH, CB4MH and CB5MH were assessed. 

‘Relative impact’ (i.e., comparative water quality impact of the major basins) was

used to determine the impact on the Bay (see Chapter III for more details). To

derive the ‘relative impact’ of each basin the water quality model was run with all

basins but one (isolated) basin set at their existing loading (2000 progress). The

isolated basin was set at the Tier 3 level of controls. The water quality model was

run nine times, thereby ‘isolating’ each of the nine major basins. In this way, the

water quality improvement from reductions from each basin could be modeled.

‘Relative impact’ was chosen over ‘absolute impact’ because the latter, which

measures the impact of the total loading and the hydrologic proximity of the basin

with the depressed dissolved oxygen concentrations in the middle Bay, was determined to be arbitrary and unfair to the larger basins and subsequently dismissed.

To calculate ‘relative impact’, ‘absolute impact’ was ‘normalized’ by dividing it

by the nitrogen and phosphorus loading for each basin. Hence, the approach

measures the impact from each basin for each unit of nutrient load delivered to the

Bay’s tidal waters and approximates only the hydrologic proximity of each basin.

Figure IV-5 shows the relative influence, determined from a ‘normalized’ impact

assessment, of each basin on the dissolved oxygen concentrations of Bay

segments CB3MH through CB5MH.

Using the Chesapeake Bay water quality model, basins were grouped based on

their relative influence (per pound nutrient loading) on the dissolved oxygen

concentration in the identified segments of the mainstem Chesapeake Bay. In this

analysis, the Chesapeake Bay Program partners decided to assess the combined

nitrogen and phosphorus load impacts on the middle mainstem Chesapeake Bay

(CB3MH through CB5MH). These loads were ‘normalized’ by assuming that 10

pounds of nitrogen had the impact of 1 pound of phosphorus. The potential load

impacts from both nitrogen and phosphorus were thus combined into the term

‘algal units’ (see Chapter III for more details).

Finally, basins were grouped based on their influence on improving the dissolved

oxygen concentrations in segments CB3MH through CB5MH (Figure IV-6). The

Virginia Eastern Shore, Susquehanna River, Patuxent River and Maryland

Western Shore were found to have the greatest influence on the dissolved oxygen

levels of the middle mainstem Chesapeake Bay and, therefore, grouped as basins

having high impact on the Bay tidal water quality. The Potomac River and the

Maryland/Delaware Eastern Shore basins were found to have moderate impact on

the middle mainstem Chesapeake Bay water quality. The Rappahannock, York

and James river basins were found to have a low impact on the middle mainstem

Chesapake Bay water quality.

2. An ‘equal percent reduction’ of the 2010 ‘anthropogenic load’ would be used as a

baseline. Using the grouping of basins identified above, the Chesapeake Bay

Program partners agreed that load reductions would be based on an equal percent

reduction of anthropogenic load for all basins within a single group. 

Using 2010 population and land use estimates and assuming no point or nonpoint

controls in place, watershed model runs were conducted to calculate the total

loading delivered to Bay tidal waters from each basin. The watershed model was

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chapter iv • Setting Nutrient and Sediment Allocations
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chapter iv • Setting Nutrient and Sediment Allocations

SUSQUEHANNA RIVER BASIN

JAMES RIVER BASIN

POTOMAC RIVER

BASIN

YORK RIVER BASIN

RAPPAHANNOCK

RIVER BASIN

EASTERN SHORE

MARYLAND

WESTERN SHORE

MARYLAND

PATUXENT RIVER

BASIN

EASTERN SHORE

VIRGINIA

70 0 70 35 Miles

1

Relative Impact

High

Medium

Low

Figure IV-5. Relative impact of major tributary basins on the dissolved oxygen concentration 

in the mainstem Chesapeake Bay that was considered when allocating the basinwide nutrient 

cap loads.
also used to calculate the ‘forested’ loads. The difference between these loads

became the anthropogenic load. That is:

Anthropogenic load (2010) = Loadings without controls (2010) - Forest Load (2010)

The Bay partners agreed to equal percent reduction of the anthropogenic load as

the best expression of equity. It is important to reiterate that the basins had been

grouped as to their impact on the Bay water quality. Therefore, to implement the

first principle above, the basins within each group were required to achieve the

same percent reduction of their anthropogenic load.

After exploring options for dividing the reductions among the three basin groups,

Bay Program partners agreed that while the percent reduction of loading within

each group was the same, there would necessarily be a smaller percent reduction

required for basins that had less impact on Bay water quality. The issue then

focused on how much smaller a reduction between groups was appropriate. A

difference in percent reduction between groups of 3 percent and 5 percent was

examined. For example, if the group of tributary basins that affected the Bay the

most was required to achieve a 70 percent reduction in anthropogenic load, should

the group of basins that had the next highest impact be required to reduce their

anthropogenic load by 67 percent or 65 percent, or some other reduction? After

exploring these and other options, the partners selected a difference of 3 percent

between the basin groups. This option kept reductions for all basins within a more

feasible range. That is, the 5 percent option, which was considered, placed a

higher burden, necessitated high reductions bordering on infeasible, for the most

impacting basins.

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0.2

0.3

0.4

0.5

0.6

0.7

0.8

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Eastern Shore

VA

Susquehanna

Patuxent

Western Shore

MD

Potomac

Eastern Shore

MD/DE

Rappahannock

York

James

Chesapeake Bay Tributary Basin

Dissolved Oyxgen Improvements

(algal units)

Figure IV-6. The major Chesapeake Bay tributary basins grouped according to their relative

influence on dissolved oxygen concentrations in segments CB3MH, CB4MH and CB5MH. 
To achieve the basinwide target load of 175 million pounds of nitrogen and 12.8

million pounds of phosphorus, the 3-percent difference needed to be translated

into actual percent reduction for each jurisdictional basin. The resulting percent

reductions for nitrogen and phosphorus are shown in Figure IV-6.

Based on application of the “basins with the greatest impact on the Bay need to

achieve the highest controls” and the “equal percent reduction” decision rules

above, preliminary allocations were developed using the equation:

Preliminary = 2010 Anthropogenic Load * 

Load Allocation [(100 – basin group percent reduction)/100] + Forest Land

Table IV-4 summarizes the results of this analysis.

The above decision rules were applicable to principles 1 and 3. The Chesapeake Bay

Program partners agreed that further decision rules were necessary to better address

Principle 1 and Principle 2. After applying the above decision rules, the Bay partners

agreed to the following additional decision rules. 

3. The Virginia tributary basins of the York and James rivers would be set at their

current tributary strategy cap loads. The York River and James River tributary

strategies called for loading targets that were slightly higher for nitrogen and

slightly lower for phosphorus than the cap loads proposed in Table IV-4. Because

the differences are small and offset each other, the Bay Program partners agreed

that the current tributary strategy cap load goals for nitrogen and phosphorus for

the York River and the James River basins would be applied in the proposed cap

load allocations.

4. All nontidal states with nitrogen or phosphorus reductions greater than Tier 3 from

decision rules 1 and 2 above would be adjusted to a Tier 3 levels. As further expression of equity, the Chesapeake Bay Program partners agreed that the states that

benefited most directly from the recovery of the Bay would take on a greater burden

for the cleanup. Obviously, the jurisdictions with Bay tidal waters benefit most from

nutrient reductions directed toward Bay water quality restoration. Hence, they

agreed to limit the loading reductions of the nontidal states—New York, Pennsylvania and West Virginia—to Tier 3 levels for nitrogen and phosphorus.

5. For phosphorus, any tidal jurisdiction with an allocation of reductions greater

than Tier 3.4 would be adjusted to a Tier 3.4 level. When decision rules 1 and 2

were applied for phosphorus, the resultant cap loads allocated to several basins—

Eastern Shore (VA), Susquehanna (MD), Eastern Shore (MD) and Rappahannock

(VA)—were at or beyond E3 levels. The partners expressed concern that such

high levels of controls were not achievable. Therefore, the allocations for these

basins would be set at a Tier 3.4 level, which is a loading equal to Tier 3 plus 40

percent (0.4) of the difference between the Tier 3 and the E3 loadings. In this way

the tidal states were ‘capped’ at a greater loading reduction (Tier 3.4) than the

nontidal states (Tier 3), thus requiring greater reductions for those states that

benefit more from the reductions. 

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NITROGEN PHOSPHORUS

(Million pounds per year) (Million pounds per year)

Preliminary Preliminary

2010 2010 2010 Load 2010 2010 2010 Load

Jurisdiction-Basin No BMPs All-Forest Anthropogenic Distribution No BMPs All-Forest Anthropogenic DistributionEastern Shore VA - VA 2.84 0.24 2.61 1.16 0.29 0.01 0.29 0.08Susquehanna - MD 1.80 0.32 1.48 0.85 0.1021 0.0046 0.09750.00295Susquehanna - PA 117.19 30.35 86.85 61.06 4.89 0.28 4.62 1.46Susquehanna - NY 16.99 7.64 9.35 10.95 0.91 0.08 0.83 0.29Western Shore MD - MD 30.52 1.05 29.47 11.47 3.61 0.02 3.59 0.94Western Shore MD - PA 0.05 0.00 0.04 0.02 0.00 0.00 0.00 0.00Patuxent - MD 6.07 0.55 5.53 2.50 0.84 0.02 0.82 0.23HIGH IMPACT BASINS LOADING 175.47 40.14 135.32 88.00 10.64 0.41 10.24 3.03Eastern Shore MD - MD 24.86 2.45 22.41 11.05 2.96 0.05 2.90 0.88Eastern Shore MD - VA 0.14 0.02 0.12 0.06 0.02 0.00 0.02 0.01Eastern Shore MD - DE 6.64 0.53 6.11 2.88 1.20 0.01 1.19 0.35Eastern Shore MD - PA 0.54 0.05 0.49 0.24 0.05 0.00 0.05 0.01Potomac - D.C. 7.22 0.05 7.18 2.80 1.38 0.00 1.37 0.39Potomac - MD 27.63 2.26 25.37 11.99 3.79 0.08 3.71 1.14Potomac - VA 28.68 2.98 25.69 12.84 4.64 0.10 4.54 1.4Potomac - PA 8.09 1.02 7.08 3.73 0.66 0.04 0.62 0.21Potomac - WV 7.97 1.83 6.14 4.18 0.63 0.09 0.54 0.24MODERATE IMPACT BASINS LOADING 111.77 11.18 100.60 49.77 15.32 0.38 14.94 4.65Rappahannock - VA 9.87 1.98 7.89 5.24 1.40 0.05 1.34 0.48York - VA 10.16 1.66 8.50 5.18 2.03 0.05 1.98 0.6791James - VA 56.83 5.59 51.24 26.79 11.89 0.25 11.64 3.9228James - WV 0.04 0.02 0.01 0.03 0.01 0.00 0.01 0.0055LOW IMPACT BASINS LOADING 76.90 9.26 67.64 37.23 15.33 0.36 14.97 5.08BASINWIDE TOTAL 364.13 60.58 303.56 175.00 41.29 1.14 40.15 12.76Notes: 2010 No BMPs = 2010 land use, no BMPs or wastewater treatment controls; 2010 All-Forest = fully forested watershed; 2010 Anthropogenic = 2010 All-Forest –

Table IV-4.

2010 No BMPs. 

Preliminary nitrogen and phosphorus cap load allocations based on the application of decision rules 1 and 2 with 

jurisdictional-basins grouped according to their relative influence on mainstem Chesapeake Bay water quality.
The preliminary allocations presented in Table IV-4 were modified to reflect decision rules 3 through 5. Table IV-5 shows the allocations that the Water Quality

Steering Committee presented to the Chesapeake Bay Program’s Principals Staff

Committee and headwater state representatives. Note that the basinwide cap loads of

187 million pounds for nitrogen and 13.8 million pounds for phosphorus after rules

3 through 5 were applied. Specifically, the Water Quality Steering Committee’s 

cap load allocations fell short of the agreed upon basinwide cap loads of 175 and

12.8 million pounds by 12 million pounds of nitrogen and 1 million pounds of phosphorus, respectively. These shortfalls were called ‘orphaned loads’.

THE PSC COMPLETES THE ALLOCATION PROCESS

In the spring of 2003, the Principals’ Staff Committee and the headwater state representatives promptly approved the basinwide cap loadings of 175 million pounds per

year of nitrogen and 12.8 million pounds per year of phosphorus, which the Water

Quality Steering Committee had recommended. However, as Table IV-5 shows,

recommended jurisdiction-basin allocations fell short of the nutrient reductions

needed. Twelve million pounds of nitrogen reductions and 1 million pounds of phosphorus reductions still needed to be assigned to the jurisdiction-basins. The

Principals’ Staff Committee and headwater state representatives succeeded in

addressing the ‘orphaned loads’ by sharing the burden. The EPA committed to the

pursuit of adopting the proposed Clear Skies initiative, which is estimated to bring

about an 8 million pound reduction of nitrogen loads delivered to the Chesapeake

Bay and its tidal tributaries per year beyond the Clean Air Act controls. In a genuine

gesture of partnership to restore Bay water quality, Pennsylvania, Virginia, Maryland, Delaware and the District of Columbia promptly accepted further reductions in

their respective nitrogen and/or phosphorus cap loads. These additional reductions in

individual basin/jurisdiction cap loads were sufficient to achieve the nutrient basinwide cap loads. These cap load allocations are presented in Table IV-6 by major

tributary basin by jurisdiction and in Table IV-7 by jurisdiction.

CAP LOAD ALLOCATIONS TO ACHIEVE 

THE CHLOROPHYLL A CRITERIA

As the previous discussion demonstrates, nutrients not only depress the amount of

dissolved oxygen in the water but also promote excessive algae growth. Therefore,

along with numeric criteria for dissolved oxygen for the Chesapeake Bay, the EPA

also recommended narrative chlorophyll a criteria to protect against excessive algal

growth (U.S. EPA 2003a). Early in the process of developing the cap load allocations, it appeared likely that nutrient controls designed to achieve the dissolved

oxygen criteria would also be adequate to achieve the chlorophyll a criteria. Therefore, the analysis of nutrient allocations for chlorophyll a was secondary to that of

nutrient allocations for dissolved oxygen.

After developing the nutrient cap load allocations, the resulting chlorophyll a

concentrations were assessed to determine if further reductions were necessary. After

reviewing the model-simulated chlorophyll a concentrations under the cap load

allocation scenario (see Appendix C), further adjustments to the nutrient cap load

allocations were deemed unnecessary.

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NITROGEN PHOSPORUS

(Million pounds per year) (Million pounds per year)

Preliminary Percent Preliminary Percent

Load Reduction of Load Reduction of

Distribution Anthropogenic Distribution Anthropogenic

Jurisdiction-Basin (from Table IV-4) Allocation Load Tiers (from Table IV-4) Allocation Load Tiers

Eastern Shore VA - VA 1.16 1.16 64.6 2.40 0.08 0.08 74.42 3.40Susquehanna - MD 0.85 0.85 64.6 3.30 0.0295 0.03 74.42 3.40Susquehanna - PA 61.06 69.08 55.4 3.00 1.46 2.20 58.25 3.00Susquehanna - NY 10.95 12.58 47.2 3.00 0.29 0.59 39.11 3.00Western Shore MD - MD 11.47 11.47 64.6 2.90 0.94 0.94 74.42 2.40Western Shore MD - PA 0.02 0.0192 64.6 n/a 0.00 0.0000 n/a n/aPatuxent - MD 2.50 2.50 64.6 3.55 0.23 0.23 74.42 3.10TOTAL BASIN LOADING 88.00 97.65 – – 3.03 4.06 – –Eastern Shore MD - MD 11.05 11.05 61.6 2.60 0.88 0.88 71.42 3.40Eastern Shore MD - VA 0.06 0.0642 61.6 3.10 0.01 0.0100 71.42 3.10Eastern Shore MD - DE 2.88 2.88 61.6 3.00 0.35 0.35 71.42 1.00Eastern Shore MD - PA 0.24 0.27 54.5 3.00 0.01 0.03 49.93 3.00Potomac - D.C. 2.80 2.80 61.6 2.90 0.39 0.39 71.42 2000 ProgressPotomac - MD 11.99 11.99 61.6 3.20 1.14 1.14 71.42 1.90Potomac - VA 12.84 12.84 61.6 3.20 1.40 1.40 71.42 3.00Potomac - PA 3.73 4.02 57.5 3.00 0.21 0.33 52.80 3.00Potomac - WV 4.18 4.71 53.0 3.00 0.24 0.36 49.29 3.00TOTAL BASIN LOADING 49.77 50.62 – – 4.65 4.90 – –Rappahannock - VA 5.24 5.24 58.6 2.70 0.48 0.62 57.79 3.40York - VA 5.18 5.70 52.5 TS/2.4 0.6791 0.48 78.47 TS/3.5James - VA 26.79 27.90 56.5 TS/2.0 3.9228 3.71 70.25 TS/3.0James - WV 0.03 0.03 31.1 3.00 0.0055 0.01 n/a n/a

Table IV-5.

TOTAL BASIN LOADING 37.23 38.87 – – 5.08 4.82 – –BASINWIDE TOTAL 175.00 187.15 – – 12.76 13.78 – –Note: Bolded numbers in this table resulted from application of decision rules 3 through 5 to Table IV-4.

The revised draft nitrogen and phosphorus cap load allocations based on application of decision rules 1 through 5 that the

Water Quality Steering Committee presented to the Principals’ Staff Committee and headwater state representatives. 
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NITROGEN PHOSPORUS

(Million pounds per year) (Million pounds per year)

WQSC PSC Cap Clear Skies Delivered WQSC PSC Cap

Table IV-6.

Jurisdiction-Basin Recommendation Reduction Load To 2010 Load Recommendation Reduction Load

Eastern Shore VA - VA 1.16 0.00 1.16 0.05 1.11 0.08 0.00 0.08Susquehanna - MD 0.85 0.02 0.83 0.03 0.80 0.03 0.00 0.03Susquehanna - PA 69.08 1.50 67.58 3.91 63.67 2.20 0.30 1.90Susquehanna - NY 12.58 0.00 12.58 0.80 11.78 0.59 0.00 0.59Western Shore MD - MD 11.47 0.20 11.27 0.19 11.08 0.94 0.10 0.84Western Shore MD - PA 0.02 0.00 0.02 0.00 0.02 0.00 0.00 0.00Patuxent - MD 2.50 0.04 2.46 0.09 2.38 0.23 0.02 0.21TOTAL BASIN LOADING 97.7 1.8 95.9 5.1 90.8 4.1 0.4 3.6Eastern Shore MD - MD 11.05 0.16 10.89 0.30 10.58 0.88 0.08 0.81Eastern Shore MD - VA 0.06 0.00 0.06 0.00 0.06 0.01 0.00 0.01Eastern Shore MD - DE 2.88 0.00 2.88 0.11 2.77 0.35 0.05 0.30Eastern Shore MD - PA 0.27 0.00 0.27 0.01 0.27 0.03 0.00 0.03Potomac - D.C. 2.80 0.40 2.40 0.02 2.38 0.39 0.05 0.34Potomac - MD 11.99 0.18 11.81 0.38 11.42 1.14 0.10 1.04Potomac - VA 12.84 0.00 12.84 0.52 12.32 1.40 0.00 1.40Potomac - PA 4.02 0.00 4.02 0.19 3.83 0.33 0.00 0.33Potomac - WV 4.71 0.00 4.71 0.35 4.37 0.36 0.00 0.36TOTAL BASIN LOADING 50.6 0.7 49.9 1.9 48.0 4.9 0.3 4.6Rappahannock - VA 5.24 0.00 5.24 0.19 5.05 0.62 0.00 0.62York - VA 5.70 0.00 5.70 0.19 5.51 0.48 0.00 0.48James - VA 27.90 1.50 26.40 0.69 25.71 3.71 0.30 3.41James - WV 0.03 0.00 0.03 0.00 0.03 0.01 0.00 0.01TOTAL BASIN LOADING 38.9 1.5 37.4 1.1 36.3 4.8 0.3 4.5BASINWIDE TOTAL 187 4 183 8 175 13.8 1.0 12.8Target Load 175 175 175 12.8 12.8Orphaned Load 12 8 0 1.0 0.0WQSC – Water Quality Steering Committee; PSC – Principals’ Staff Committee.

The final Chesapeake 2000 nitrogen and phosphorus cap load allocations by major tributary basin and jurisdiction. 
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chapter iv • Setting Nutrient and Sediment Allocations NITROGEN PHOSPORUS (Million pounds per year) (Million pounds per year) WQSC PSC Cap Clear Skies Delivered WQSC PSC Cap Table IV-7. Jurisdiction-Basin Recommendation Reduction Load To 2010 Load Recommendation Reduction Load Pennsylvania 73.40 1.50 71.90 4.10 67.79 2.56 0.30 2.26 Maryland 37.85 0.60 37.25 0.99 36.26 3.22 0.30 2.92 Virginia 52.90 1.50 51.40 1.64 49.76 6.30 0.30 6.00 District of Columbia 2.80 0.40 2.40 0.02 2.38 0.39 0.05 0.34 New York 12.58 0.00 12.58 0.80 11.78 0.59 0.00 0.59 Delaware 2.88 0.00 2.88 0.11 2.77 0.35 0.05 0.30 West Virginia 4.75 0.00 4.75 0.35 4.40 0.37 0.00 0.37 TOTAL 187 4 183 8 175 13.8 1.0 12.8 WQSC—Water Quality Steering Committee; PSC—Principals' Staff Committee. The final Chesapeake 2000 nitrogen and phosphorus cap load allocations by jurisdiction. 
Target concentrations of chlorophyll a by season and salinity regime were drawn

from the technical information published in the Regional Criteria Guidance (U.S.

EPA 2003a) (see Appendix C). Comparing these target chlorophyll a concentrations

with the cap load allocations ‘confirmation’ scenario simulated chlorophyll a

concentration yielded the following findings:

• Chlorophyll a levels relative to the target concentrations were very similar

between the ‘confirmation’ scenario and Option 1, indicating additional nutrient

reductions down to the Option 1 level would not yield large chlorophyll a concentration reductions at the Chesapeake Bay Program-wide segment scale.

• Only in very localized tidal habitats were the target chlorophyll a concentrations

not achieved under the ‘confirmation’ scenario.

ADDITIONAL ALLOCATION CONSIDERATIONS 

Recognizing that nutrient effects could be lessened either by reducing nitrogen or

phosphorus, or by other management controls that would alleviate low dissolved

oxygen concentrations in the Bay’s tidal waters, the Chesapeake Bay Program partners agreed to keep the nutrient allocations flexible (Secretary Tayloe Murphy 2003).

Any jurisdiction can exchange loading for a nutrient from one major tributary basin

to another as long as the basins were in the same grouping of relative effectiveness

(see Figure IV-6).

A jurisdiction may exchange or credit nitrogen reductions for phosphorus reductions. It is important to note that both nitrogen and phosphorus have an adverse, but

exchangeable, effect on the water quality of the Chesapeake Bay, so the nutrient allocations could be viewed as ‘nitrogen equivalents’. ‘Nitrogen equivalent’ means any

management action (e.g., phosphorus control, oyster or underwater bay grass

restoration, etc.) that has the similar water quality effect of reducing a known quantity of nitrogen loading.

ESTABLISHING SEDIMENT CAP LOAD ALLOCATIONS 

SAV RESTORATION AS THE GOAL

The cap load allocation process for sediments originally focused on achieving the

water clarity water quality criteria. However, good reasons existed for focusing

management plans and even the establishment of the sediment cap load allocations

on the recovery of the underwater bay grass or SAV beds. This was a positive shift

in emphasis, for the following reasons:

• It directly measures the health of the underwater bay grass living resource;

• Annual aerial surveys enable the partners to record and measure the SAV beds,

whereas, sparse water clarity data exists for Bay tidal- and shallow-water habitats;

• The Chesapeake Bay water quality model cannot at present reliably simulate

reductions in sediment loads or water clarity responses in Bay tidal- and shallowwater habitats at the desired geographic scale with sufficient accuracy; and

• It accommodates the transient nature of SAV growth, which the historical record

clearly documents, whereas water clarity criteria necessitated rigid boundaries.

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While acres of SAV have become the dominant measure to direct management

controls of sediment within the tributary strategies, water clarity still plays an important role. The EPA has published water clarity criteria for the protection of the SAV

in the Regional Criteria Guidance (U.S. EPA 2003a). In the Technical Support

Document the EPA also identified SAV restoration goals for the Chesapeake Bay and

its tidal tributaries (U.S. EPA 2003b). Currently, sediments are discharged into the

Bay’s tidal waters in quantities that cause exceedance of the water clarity criteria and

prevent achievement of the SAV restoration goals. In response, sediment cap load

allocations have been developed along with the SAV restoration goals for each tributary basin. Sediment loads that affect water clarity and SAV growth in the Bay and

its tidal tributaries come from two major areas:

1. Land-based loads originate from land erosion, are discharged in runoff and

include stream bank erosion; and

2. Near-shore erosion is attributable to tidal shoreline erosion and includes resuspension of sediment material from the shoreline.

Model results have indicated that sediments tend to affect areas in the Chesapeake

Bay and its tidal tributaries close to where they are introduced (see Chapter III for

details). For this reason, the Bay and its tidal tributaries were divided into discrete

sections representing areas affected by the local sediment loads that had little to no

impact on other areas (see Figure III-20). These areas roughly correspond with the

nine major tributary basins of the Bay.

LAND-BASED (UPLAND) SEDIMENT ALLOCATION

As the previous section points out, the Water Quality Model is not capable of

providing a reliable understanding of the sediment loads’ effect on water clarity.

Therefore, it was necessary to derive and agree on reasonable sediment cap load

allocations without a similar level of quantitative support from the Water Quality

Model as was used in establishing the nutrient cap load allocations. 

Since no reliable modeling tool existed, neither water clarity criteria attainment nor

SAV-based sediment cap load allocations could be developed. Sediment cap load

allocations, therefore, were based on sediment loads (reductions) that would likely

result from implementing the land-based phosphorus controls necessary to achieve

the dissolved oxygen-based phosphorus cap load allocations. 

It is well-known that for nonpoint sources, most BMPs that reduce phosphorus do so

by reducing the sediment that carries the phosphorus to the stream. Thus, phosphorus

and sediment controls for nonpoint sources are closely related. Figure IV-7 illustrates the strong correlation between phosphorus and sediment load reductions 

from BMPs.

The methodology used to determine the ‘phosphorus equivalent’ sediment loads was

relatively straightforward. Since increasing control scenarios were already defined

(e.g., Tier 1 through E3), the first step involved identifying the tier level represented

by the phosphorus cap load allocations already established to achieve the dissolved

oxygen and chlorophyll a criteria for each major tributary basin. If the phosphorus

cap load allocation was between two loading levels (i.e., two tiers), the exact tier was

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interpolated. Once the tier for the phosphorus allocation was identified, the sediment

cap load allocation was determined by calculating the corresponding sediment load

for that tier. It is important to recognize the tentative nature of these allocations,

which are driven by the anticipated sediment loads after the phosphorus controls are

in place. The tributary teams were, therefore, given latitude in identifying alternate

land-based sediment allocations if it could be shown to be more appropriate for

achieving the SAV restoration goals for that particular tributary basin. 

After the ‘phosphorus equivalent’ sediment loads were established, these land-based

sediment cap load allocations were modified for several major tributary basins (e.g.,

Potomac tidal-fresh and Susquehanna) because the Chesapeake Bay Water Quality

Model and existing data suggested the previously determined land-based sediment

allocations were greater than necessary to achieve the SAV restoration goal for that

particular major tributary basin. For these basins, the sediment cap load allocations

were relaxed to levels estimated to be appropriate for that basin. Table IV-8 provides

the resulting allocations.

ESTABLISHING THE NEAR-SHORE SEDIMENT ALLOCATION

Due to an insufficient technical understanding of near-shore sediment fate and

transport, no specific recommendation for near-shore sediment load reductions was

recommended when the land-based sediment cap load allocations were established.

Tributary teams should acquire local knowledge of near-shore erosion problem areas

and make specific recommendations on reductions in those sediment loadings. 

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0

1

2

3

4

5

6

7

8

2010 No BMPs 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

Sediment Phosphorus

(million tons/year)

(million lbs/year)

Figure IV-7. Chesapeake Bay Watershed Model simulated relationship between sediment and

nonpoint source phosphorus loads.
SAV-BASED SEDIMENT CAP LOAD ALLOCATIONS

Scientific understanding of the quantitative relationship between sediment reductions and SAV recovery is less broad than the current understanding of the

relationship between nutrient loads and dissolved oxygen on Bay tidal waters. Thus,

the tributary teams should note the following when establishing their sediment/SAV

related components of the tributary strategies:

1. The local SAV goal should serve as the primary goal in establishing sediment

control measures by the tributary teams. Attaining more SAV acreage is the most

direct measure of the status of this living resource and can be measured directly.

Therefore, if all management actions prescribed in the tributary strategy are taken,

yet the SAV restoration goal is not attained, further measures should be employed.

Conversely, if the SAV restoration goal is achieved, yet all sediment reduction

measures have not been implemented, then further management actions may not

be necessary.

2. Based on a comprehensive SAV recovery plan by the tributary teams, revisions

to the upland sediment cap load allocations may be recommended to the Principals’ Staff Committee, where appropriate. As noted, the ‘upland’ allocations

are tentative. That is, the sediment cap load allocations represent the sediment

load likely to result from implementing the phosphorus cap load allocations that

have been established by the Principals’ Staff Committee and headwater state

partners. Obviously, these upland sediment loads have only a qualitative relationship to SAV recovery. Thus, these upland sediment allocations should be used as

a basis for sediment controls unless the tributary teams, based on the tributary

strategy development process, conclude that the recommended upland sediment

controls are not appropriate. In such cases, the tributary team shall identify the

upland sediment cap load allocations that are appropriate and also identify the

management actions necessary to achieve those allocations. 

3. The tributary teams should explore a comprehensive suite of management actions

to achieve the local SAV restoration goal. It is apparent that upland sediment

controls alone will be insufficient to achieve the local SAV restoration goals in some

areas. The tributary teams should assess varied and innovative methods to achieve

SAV regrowth in such areas. Methods could include, but not be limited to, streambank stabilization, SAV planting and near-shore erosion control, where appropriate.

SUMMARY OF NUTRIENT AND SEDIMENT 

CAP LOAD ALLOCATIONS

Through extensive modeling and an intricate interplay of technical and policy decisions, the Chesapeake Bay Program partners agreed to load allocations for nitrogen,

phosphorus and sediment for each of the 20 areas of the Chesapeake Bay watershed

delineated by major basin and jurisdiction. These nutrient and sediment cap load allocations are based on achieving the Chesapeake Bay water quality criteria for dissolved

oxygen, chlorophyll a and water clarity, and the baywide and local SAV acreage

restoration goals. Tables IV-9 and IV-10 provide a full tabulation of the nutrient and

sediment cap load allocations by major tributary basin and jurisdiction, respectively. 

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Table IV-8. Land-based ‘phosphorus equivalent’ sediment cap load allocation by

major tributary basin, by jurisdiction.

Land-Based Sediment

Allocation

Basin Jurisdiction (Million tons per year)*

SUSQUEHANNA PA 0.793

NY 0.131

MD 0.037

Basin Total 0.962

EASTERN SHORE – MD MD 0.116

DE 0.042

PA 0.004

VA 0.001

Basin Total 0.163

WESTERN SHORE MD 0.100

PA 0.001

Basin Total 0.100

PATUXENT MD 0.095

Basin Total 0.095

POTOMAC VA 0.617

MD 0.364

WV 0.311

PA 0.197

DC 0.006

Basin Total 1.494

RAPPAHANNOCK VA 0.288

Basin Total 0.288

YORK VA 0.103

Basin Total 0.103

JAMES VA 0.925

WV 0.010

Basin Total 0.935

EASTERN SHORE – VA VA 0.008

Basin Total 0.008

SUBTOTAL 4.15

BASINWIDE TOTAL 4.15

*The tributary teams will assess these upland sediment allocations and, if necessary, revise them as

part of a comprehensive strategy of management actions necessary to achieve the local SAV

restoration goals.
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Table IV-9. Chesapeake Bay watershed nitrogen, phosphorus and sediment cap load allocations by

major tributary basin.

Nitrogen Phosphorus Upland Sediment

Cap Load Allocation Cap Load Allocation Cap Load Allocation

Basin/Jurisdiction (million pounds/year) (million pounds/year) (million tons/year)

SUSQUEHANNA

PA 67.58 1.90 0.793

NY 12.58 0.59 0.131

MD 0.83 0.03 0.037

SUSQUEHANNA Total 80.99 2.52 0.962

EASTERN SHORE - MD

MD 10.89 0.81 0.116

DE 2.88 0.30 0.042

PA 0.27 0.03 0.004

VA 0.06 0.01 0.001

EASTERN SHORE - MD Total 14.10 1.14 0.163

WESTERN SHORE

MD 11.27 0.84 0.100

PA 0.02 0.00 0.001

WESTERN SHORE Total 11.29 0.84 0.100

PATUXENT

MD 2.46 0.21 0.095

PATUXENT Total 2.46 0.21 0.095

POTOMAC

VA 12.84 1.40 0.617

MD 11.81 1.04 0.364

WV 4.71 0.36 0.311

PA 4.02 0.33 0.197

DC 2.40 0.34 0.006

POTOMAC Total 35.78 3.48 1.494

RAPPAHANNOCK

VA 5.24 0.62 0.288

RAPPAHANNOCK Total 5.24 0.62 0.288

YORK

VA 5.70 0.48 0.103

YORK Total 5.70 0.48 0.103

JAMES

VA 26.40 3.41 0.925

WV 0.03 0.01 0.010

JAMES Total 26.43 3.42 0.935

EASTERN SHORE - VA

VA 1.16 0.08 0.008

EASTERN SHORE - VA Total 1.16 0.08 0.008

SUBTOTAL 183 12.8 4.15

CLEAR SKIES REDUCTION -8

BASINWIDE TOTAL 175 12.8 4.15
▼109

chapter iv • Setting Nutrient and Sediment Allocations

Table IV-10. Chesapeake Bay watershed nitrogen, phosphorus and sediment cap load allocations by

jurisdiction.

Nitrogen Phosphorus Upland Sediment

Cap Load Allocation Cap Load Allocation Cap Load Allocation

Jurisdiction/Basin (million pounds/year) (million pounds/year) (million tons/year)

PENNSYLVANIA

Susquehanna 67.58 1.90 0.793

Potomac 4.02 0.33 0.197

Western Shore 0.02 0.00 0.001

Eastern Shore - MD 0.27 0.03 0.004

PA Total 71.90 2.26 0.995

MARYLAND

Susquehanna 0.83 0.03 0.037

Patuxent 2.46 0.21 0.095

Potomac 11.81 1.04 0.364

Western Shore 11.27 0.84 0.100

Eastern Shore - MD 10.89 0.81 0.116

MD Total 37.25 2.92 0.712

VIRGINIA

Potomac 12.84 1.40 0.617

Rappahannock 5.24 0.62 0.288

York 5.70 0.48 0.103

James 26.40 3.41 0.925

Eastern Shore - MD 0.06 0.01 0.001

Eastern Shore - VA 1.16 0.08 0.008

VA Total 51.40 6.00 1.941

DISTRICT OF COLUMBIA

Potomac 2.40 0.34 0.006

DC Total 2.40 0.34 0.006

NEW YORK

Susquehanna 12.58 0.59 0.131

NY Total 12.58 0.59 0.131

DELAWARE

Eastern Shore - MD 2.88 0.30 0.042

DE Total 2.88 0.30 0.042

WEST VIRGINIA

Potomac 4.71 0.36 0.311

James 0.03 0.01 0.010

WV Total 4.75 0.37 0.320

SUBTOTAL 183 12.8 4.15

CLEAR SKIES REDUCTION –8

BASINWIDE TOTAL 175 12.8 4.15
Nutrient and sediment cap load allocations were established based on the EPA’s

recently published Regional Criteria Guidance (U.S. EPA 2003a), which is specific

to the Chesapeake Bay. Therefore, the states will be modifying their water quality

standards based upon the published Bay criteria as well as the refined tidal-water

designated uses. If the final adopted state water quality standards do not rely on the

criteria and designated uses employed to establish these cap load allocations, then

the allocations will need to be amended accordingly.

It will be difficult to achieve these cap load allocations. States will develop tributary

strategies for each of the 20 areas by April 30, 2004. Some states may choose further

to subdivide their tributaries into smaller watershed for the development of tributary

strategies. These tributary strategies will identify specific actions needed to achieve

the cap load nutrient and sediment cap load allocations.

LITERATURE CITED 

Secretary Tayloe Murphy, 2003. “Summary of Decisions Regarding Nutrient and Sediment

Load Allocations and New Submerged Aquatic Vegetation (SAV) Restoration Goals.”

April 25, 2003, memorandum to the Principals’ Staff Committee members and representatives of the Chesapeake Bay headwater states. Virginia Office of the Governor, Natural

Resources Secretariate, Richmond, Virginia.

U.S. Environmental Protection Agency. 2003a. 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. Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. Environmental Protection Agency. 2003b. Technical Support Document for Identification of Chesapeake Bay Designated Uses and Attainability. EPA 903-R-03-004. Chesapeake

Bay Program Office, Annapolis, Maryland.

▼110

chapter iv • Setting Nutrient and Sediment Allocations
appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals

appendixA

Summary of Decisions 

Regarding Nutrient and Sediment Load

Allocations and New Submerged Aquatic

Vegetation (SAV) Restoration Goals

Memorandum from W. Tayloe Murphy, Jr.,

Chair, Chesapeake Bay Program Principals’ Staff Committee,

to the Principals’ Staff Committee Members and 

Representatives of Chesapeake Bay “Headwater” States

▼A1
▼A2

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals
▼A3

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals
▼A4

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals
▼A5

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals
▼A6

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals
▼A7

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals

Table A-1. Chesapeake Bay watershed nitrogen, phosphorus and sediment cap load allocations by

major basin.

Nitrogen Phosphorus Upland Sediment

Cap Load Allocation Cap Load Allocation Cap Load Allocation

Basin/Jurisdiction (million pounds/year) (million pounds/year) (million tons/year)

SUSQUEHANNA

PA 67.58 1.90 0.793

NY 12.58 0.59 0.131

MD 0.83 0.03 0.037

SUSQUEHANNA Total 80.99 2.52 0.962

EASTERN SHORE - MD

MD 10.89 0.81 0.116

DE 2.88 0.30 0.042

PA 0.27 0.03 0.004

VA 0.06 0.01 0.001

EASTERN SHORE - MD Total 14.10 1.14 0.163

WESTERN SHORE

MD 11.27 0.84 0.100

PA 0.02 0.00 0.001

WESTERN SHORE Total 11.29 0.84 0.100

PATUXENT

MD 2.46 0.21 0.095

PATUXENT Total 2.46 0.21 0.095

POTOMAC

VA 12.84 1.40 0.617

MD 11.81 1.04 0.364

WV 4.71 0.36 0.311

PA 4.02 0.33 0.197

DC 2.40 0.34 0.006

POTOMAC Total 35.78 3.48 1.494

RAPPAHANNOCK

VA 5.24 0.62 0.288

RAPPAHANNOCK Total 5.24 0.62 0.288

YORK

VA 5.70 0.48 0.103

YORK Total 5.70 0.48 0.103

JAMES

VA 26.40 3.41 0.925

WV 0.03 0.01 0.010

JAMES Total 26.43 3.42 0.935

EASTERN SHORE - VA

VA 1.16 0.08 0.008

EASTERN SHORE - VA Total 1.16 0.08 0.008

SUBTOTAL 183 12.8 4.15

CLEAR SKIES REDUCTION -8

BASINWIDE TOTAL 175 12.8 4.15
▼A8

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals

Table A-2. Chesapeake Bay watershed nitrogen, phosphorus and sediment cap load allocations by 

jurisdiction.

Nitrogen Phosphorus Upland Sediment

Cap Load Allocation Cap Load Allocation Cap Load Allocation

Jurisdiction/Basin (million pounds/year) (million pounds/year) (million tons/year)

PENNSYLVANIA

Susquehanna 67.58 1.90 0.793

Potomac 4.02 0.33 0.197

Western Shore 0.02 0.00 0.001

Eastern Shore - MD 0.27 0.03 0.004

PA Total 71.90 2.26 0.995

MARYLAND

Susquehanna 0.83 0.03 0.037

Patuxent 2.46 0.21 0.095

Potomac 11.81 1.04 0.364

Western Shore 11.27 0.84 0.100

Eastern Shore - MD 10.89 0.81 0.116

MD Total 37.25 2.92 0.712

VIRGINIA

Potomac 12.84 1.40 0.617

Rappahannock 5.24 0.62 0.288

York 5.70 0.48 0.103

James 26.40 3.41 0.925

Eastern Shore - MD 0.06 0.01 0.001

Eastern Shore - VA 1.16 0.08 0.008

VA Total 51.40 6.00 1.941

DISTRICT OF COLUMBIA

Potomac 2.40 0.34 0.006

DC Total 2.40 0.34 0.006

NEW YORK

Susquehanna 12.58 0.59 0.131

NY Total 12.58 0.59 0.131

DELAWARE

Eastern Shore - MD 2.88 0.30 0.042

DE Total 2.88 0.30 0.042

WEST VIRGINIA

Potomac 4.71 0.36 0.311

James 0.03 0.01 0.010

WV Total 4.75 0.37 0.320

SUBTOTAL 183 12.8 4.15

CLEAR SKIES REDUCTION –8

BASINWIDE TOTAL 175 12.8 4.15
▼A9

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals

Table A-3. Chesapeake Bay submerged aquatic (SAV) restoration

goal acreage by Chesapeake Bay Program (CBP) segment based on

the single best year of record from 1930 to present.

Segment Name Segment Acres 

Northern Chesapeake Bay CB1TF 12,908

Upper Chesapeake Bay CB2OH 302

Upper Central Chesapeake Bay CB3MH 943

Middle Central Chesapeake Bay CB4MH 2,511

Lower Central Chesapeake Bay CB5MH 14,961

Western Lower Chesapeake Bay CB6PH 980

Eastern Lower Chesapeake Bay CB7PH 14,620

Mouth of the Chesapeake Bay CB8PH 6

Bush River BSHOH 158

Gunpowder River GUNOH 2,254

Middle River MIDOH 838

Back River BACOH 0

Patapsco River PATMH 298

Magothy River MAGMH 545

Severn River SEVMH 329

South River SOUMH 459

Rhode River RHDMH 48

West River WSTMH 214

Upper Patuxent River PAXTF 5

Western Branch (Patuxent River) WBRTF 0

Middle Patuxent River PAXOH 68

Lower Patuxent River PAXMH 1,325

Upper Potomac River POTTF 4,368

Anacostia River ANATF 6

Piscataway Creek PISTF 783

Mattawoman Creek MATTF 276

Middle Potomac River POTOH 3,721

Lower Potomac River POTMH 10,173

Upper Rappahannock River RPPTF 20

Middle Rappahannock River RPPOH 0

Lower Rappahannock River RPPMH 5,380

Corrotoman River CRRMH 516

Piankatank River PIAMH 3,256

Upper Mattaponi River MPNTF 75

Lower Mattaponi River MPNOH 0

Upper Pamunkey River PMKTF 155

Lower Pamunkey River PMKOH 0

Middle York River YRKMH 176

Lower York River YRKPH 2,272

Mobjack Bay MOBPH 15,096

continued
▼A10

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals

Segment Name Segment Acres 

Upper James River JMSTF 1,600

Appomattox River APPTF 319

Middle James River JMSOH 7

Chickahominy River CHKOH 348

Lower James River JMSMH 531

Mouth of the James River JMSPH 604

Western Branch Elizabeth River WBEMH 0

Southern Branch Elizabeth River SBEMH 0

Eastern Branch Elizabeth River EBEMH 0

Lafayette River LAFMH 0

Mouth to mid-Elizabeth River ELIPH 0

Lynnhaven River LYNPH 69

Northeast River NORTF 88

C&D Canal C&DOH 0

Bohemia River BOHOH 97

Elk River ELKOH 1,648

Sassafras River SASOH 764

Upper Chester River CHSTF 0

Middle Chester River CHSOH 63

Lower Chester River CHSMH 2,724

Eastern Bay EASMH 6,108

Upper Choptank River CHOTF 0

Middle Choptank River CHOOH 63

Lower Choptank River CHOMH2 1,499

Mouth of the Choptank River CHOMH1 8,044

Little Choptank River LCHMH 3,950

Honga River HNGMH 7,686

Fishing Bay FSBMH 193

Upper Nanticoke River NANTF 0

Middle Nanticoke River NANOH 3

Lower Nanticoke River NANMH 3

Wicomico River WICMH 3

Manokin River MANMH 4,359

Big Annemessex River BIGMH 2,014

Upper Pocomoke River POCTF 0

Middle Pocomoke River POCOH 0

Lower Pocomoke River POCMH 4,092

Tangier Sound TANMH 37,965

Total acres 184,889

1Breakdown of Potomac SAV restoration goals by jurisdictions are draft, pending confirmation of split

between Maryland, Virginia and the District of Columbia along jurisdictional lines. Due to ongoing

refinement, some numbers in this table differ from the April 25, 2003, version included in Appendix A

and previous model estimates presented in tables III-3 and III-4.

Table A-3. Chesapeake Bay submerged aquatic (SAV) restoration goal

acreage by Chesapeake Bay Program (CBP) segment based on the single

best year of record from 1930 to present. (cont.).
▼A11

appendix A • Decisions Regarding Nutrient & Sediment Load Allocations and New SAV Restoration Goals

Basin/Jurisdiction SAV Restoration Goal (Acres)

SUSQUEHANNA 12,856

EASTERN SHORE – MD 76,193

WESTERN SHORE – MD 5,651

PATUXENT 1,420

POTOMAC1

MD 12,747

VA 6,320

DC 384

RAPPAHANNOCK 12,798

YORK 21,823

JAMES 3,483

EASTERN SHORE – VA 31,215

TOTAL 184,889

Table A-4. Chesapeake Bay submerged aquatic vegetation 

(SAV) restoration goal acreage by major basin by jurisdiction.

1Breakdown of Potomac SAV restoration goals by jurisdictions are draft, pending

confirmation of split between Maryland, Virginia and the District of Columbia along

jurisdictional lines. Due to ongoing refinement, some numbers in this table differ

from the April 25, 2003, version included in Appendix A and previous model

estimates presented in tables III-3 and III-4.
To better reflect the desired and attainable Chesapeake Bay water quality conditions

called for in the Chesapeake 2000 agreement, Chesapeake Bay Program watershed

partners determined that the underlying tidal-water designated uses needed to be

refined. The Chesapeake Bay watershed partners, thus, proposed five refined subcategories of the current broad aquatic life designated uses contained in the existing

state water quality standards of the four jurisdictions bordering directly on Chesapeake Bay and its tidal tributaries.

Four of the refined designated uses were derived largely to address seasonally

distinct habitats and living resource communities with widely varying dissolved

oxygen requirements:

• Migratory fish spawning and nursery;

• Open-water fish and shellfish;

• Deep-water seasonal fish and shellfish; and

• Deep-channel seasonal refuge.

The fifth refined designated use, the shallow-water bay grass designated use, is a

seasonal overlay on that part of the year-round open-water use which borders the

land along the tidal portions of the Chesapeake Bay and its tributaries.

Table B-1 provides general descriptions of the five designated uses and the aquatic

communities they were established to protect1, while Table B-2 provides the

proposed designated uses for each Chesapeake Bay Program segment. See the Technical Support Document for Identification of Chesapeake Bay Designated uses and

Attainability (U.S. EPA 2003b) for more detailed explanation of the five refined

designated uses.

▼B1

appendix B • Living Resource-Based Refined Designated Uses and Water Quality Criteria

appendixB

Chesapeake Bay Living Resource-Based

Refined Designated Uses and Water

Quality Criteria for Dissolved Oxygen,

Water Clarity and Chlorophyll a

1Note that for brevity, these refined designated uses may be referred to as migratory spawning and

nursery, shallow-water, open-water, deep-water and deep-channel.
The five tidal-water designated uses, in turn, provided the context for deriving

dissolved oxygen, water clarity and chlorophyll a water quality criteria for the

Chesapeake Bay and its tidal tributaries. These criteria, derived to protect each of the

five refined designated uses, were based on effects data from a wide array of biological communities to capture the range of sensitivity of the thousands of aquatic

species inhabiting the Chesapeake Bay and tidal tributary estuarine habitats. See

Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries for more detailed

explanation of the Chesapeake Bay water quality criteria (U.S. EPA 2003a). As

presented in Table B-3, the Chesapeake Bay dissolved oxygen criteria have been

defined for each of the five designated uses. Table B-4 presents the water clarity

criteria applicable to the shallow-water designated use. The narrative chlorophyll a

criteria, which is recommended for all Chesapeake Bay and tidal tributaries, is

presented in Table B-5.

▼B2

appendix B • Living Resource-Based Refined Designated Uses and Water Quality Criteria

Table B-1. General descriptions of the five proposed Chesapeake Bay tidal-water designated uses.

Migratory Fish Spawning and Nursery Designated Use: Aims to protect migratory finfish

during the late winter/spring spawning and nursery season in tidal freshwater to low-salinity

habitats. This habitat zone is primarily found in the upper reaches of many Bay tidal rivers and

creeks and the upper mainstem Chesapeake Bay and will benefit several species including

striped bass, perch, shad, herring and sturgeon.

Shallow-Water Bay Grass Designated Use: Designed to protect underwater bay grasses and the

many fish and crab species that depend on the shallow-water habitat provided by grass beds.

Open-Water Fish and Shellfish Designated Use: Designed to protect water quality in the

surface water habitats within tidal creeks, rivers, embayments and the mainstem Chesapeake

Bay year-round. This use aims to protect diverse populations of sportfish, including striped

bass, bluefish, mackerel and seatrout, bait fish such as menhaden and silversides, as well as the

listed shortnose sturgeon.

Deep-Water Seasonal Fish and Shellfish Designated Use: Aims to protect living resources

inhabiting the deeper transitional water column and bottom habitats between the well-mixed

surface waters and the very deep channels during the summer months. This use protects many

bottom-feeding fish, crabs and oysters, as well as other important species, including the bay

anchovy.

Deep-Channel Seasonal Refuge Designated Use: Designed to protect bottom sedimentdwelling worms and small clams that act as food for bottom-feeding fish and crabs in the very

deep channel in summer. The deep-channel designated use recognizes that low dissolved

oxygen conditions prevail in the deepest portions of this habitat zone and will naturally have

very low to no oxygen during the summer.

Source: U.S. EPA 2003b.
▼B3

appendix B • Living Resource-Based Refined Designated Uses and Water Quality Criteria

Migratory

Spawning

Chesapeake Bay Program (CBP) CBP and Nursery Open-Water Deep-Water Deep-Channel Shallow-Water

Segment Name Segment (2/1–5/31) (Year-Round) (6/1–9/30) (6/1–9/30) (4/1–10/30)

Northern Chesapeake Bay CB1TF x x x 

Upper Chesapeake Bay CB2OH x x x 

Upper Central Chesapeake Bay CB3MH x x x x x 

Middle Central Chesapeake Bay CB4MH x x x x x

Lower Central Chesapeake Bay CB5MH x x x x 

Western Lower Chesapeake Bay CB6PH x x x 

Eastern Lower Chesapeake Bay CB7PH x x x 

Mouth of the Chesapeake Bay CB8PH x x 

Bush River BSHOH x x x 

Gunpowder River GUNOH x x x 

Middle River MIDOH x x x 

Back River BACOH x x x 

Patapsco River PATMH x x x x 

Magothy River MAGMH x x x 

Severn River SEVMH x x x 

South River SOUMH x x x 

Rhode River RHDMH x x x 

West River WSTMH x x x 

Upper Patuxent River PAXTF x x x 

Western Branch (Patuxent River) WBRTF x x x 

Middle Patuxent River PAXOH x x x 

Lower Patuxent River PAXMH x x x x 

Upper Potomac River POTTF x x x 

Anacostia River ANATF x x x 

Piscataway Creek PISTF x x x 

Mattawoman Creek MATTF x x x 

Middle Potomac River POTOH x x x 

Lower Potomac River POTMH x x x x x 

Upper Rappahannock River RPPTF x x x 

Middle Rappahannock River RPPOH x x x 

Lower Rappahannock River RPPMH x x x x x 

Corrotoman River CRRMH x x x 

Piankatank River PIAMH x x x 

Upper Mattaponi River MPNTF x x x 

Lower Mattaponi River MPNOH x x x 

Upper Pamunkey River PMKTF x x x 

Lower Pamunkey River PMKOH x x x 

Middle York River YRKMH x x x 

Lower York River YRKPH x x x 

Table B-2. Recommended tidal-water designated uses by Chesapeake Bay Program segment. 

continued
▼B4

appendix B • Living Resource-Based Refined Designated Uses and Water Quality Criteria

Table B-2. Recommended tidal-water designated uses by Chesapeake Bay Program segment (cont.). 

Migratory

Spawning

Chesapeake Bay Program (CBP) CBP and Nursery Open-Water Deep-Water Deep-Channel Shallow-Water

Segment Name Segment (2/1–5/31) (Year-Round) (6/1–9/30) (6/1–9/30) (4/1–10/30)

Mobjack Bay MOBPH x x x 

Upper James River JMSTF x x x 

Appomattox River APPTF x x x 

Middle James River JMSOH x x x 

Chickahominy River CHKOH x x x 

Lower James River JMSMH x x x 

Mouth of the James River JMSPH x x 

Western Branch Elizabeth River WBEMH x 

Southern Branch Elizabeth River SBEMH x 

Eastern Branch Elizabeth River EBEMH x 

Lafayette River LAFMH x 

Mouth to mid-Elizabeth River ELIPH x x x

Lynnhaven River LYNPH x x 

Northeast River NORTF x x x 

C&D Canal C&DOH x x x 

Bohemia River BOHOH x x x 

Elk River ELKOH x x x 

Sassafras River SASOH x x x 

Upper Chester River CHSTF x x x 

Middle Chester River CHSOH x x x 

Lower Chester River CHSMH x x x x x 

Eastern Bay EASMH x x x x 

Upper Choptank River CHOTF x x 

Middle Choptank River CHOOH x x x 

Lower Choptank River CHOMH2 x x x 

Mouth of the Choptank River CHOMH1 x x x 

Little Choptank River LCHMH x x 

Honga River HNGMH x x 

Fishing Bay FSBMH x x x 

Upper Nanticoke River NANTF x x x 

Middle Nanticoke River NANOH x x x 

Lower Nanticoke River NANMH x x x 

Wicomico River WICMH x x x 

Manokin River MANMH x x x 

Big Annemessex River BIGMH x x x 

Upper Pocomoke River POCTF x x 

Middle Pocomoke River POCOH x x x 

Lower Pocomoke River POCMH x x x 

Tangier Sound TANMH x x 

Source: U.S. EPA 2003b.
▼B5

appendix B • Living Resource-Based Refined Designated Uses and Water Quality Criteria

Table B-3. Chesapeake Bay dissolved oxygen criteria.

1At temperatures considered stressful to shortnose sturgeon (>29°C), dissolved oxygen concentrations above an instantaneous minimum 4.3 mg liter-1 will protect survival of this listed sturgeon species.

Source: U.S. EPA 2003a.
▼B6

appendix B • Living Resource-Based Refined Designated Uses and Water Quality Criteria

Table B-4. Summary of Chesapeake Bay water clarity criteria for application to shallow-water bay

grass designated use habitats.

Table B-5. Recommended Chesapeake Bay chlorophyll a narrative criteria.

Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall not exceed levels that 

result in ecologically undesirable consequences—such as reduced water clarity, low dissolved oxygen, food 

supply imbalances, proliferation of species deemed potentially harmful to aquatic life or humans or aesthetically

objectionable conditions—or otherwise render tidal waters unsuitable for designated uses. 

Source: U.S. EPA 2003b.

1Based on application of the equation, PLW = 100exp(-KdZ), the appropriate PLW criterion value and the selected application depth are

inserted and the equation is solved for Kd. The generated Kd value is then converted to Secchi depth (in meters) using the conversion

factor Kd = 1.45/Secchi depth.

Source: U.S. EPA 2003a.

LITERATURE CITED

U.S. Environmental Protection Agency. 2003a. Ambient Water Quality Criteria for Dissolved

Oxygen, Water Clarity, and Chlorophyll a for Chesapeake Bay and its Tidal Tributaries. EPA

903-R-03-002. Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. EPA. 2003b. Technical Support Document for Identification of Chesapeake Bay Designated Uses and Attainability. EPA 903-R-03-004. Chesapeake Bay Program Office,

Annapolis, Maryland.
This appendix describes general assumptions and methodologies applied for several

model scenario used in the cap load allocation process including the 2010 Tiers,

2010 “Everything, Everywhere, by Everyone” (E3), 2010 No-BMPs and All-Forest

scenarios.

LEVEL-OF-EFFORT AND E3 SCENARIOS

As described in Chapter 4, the Tier and E3 scenarios were developed by the Chesapeake Bay Program Nutrient Subcommittee’s Workgroups to provide reference

points of increasing load reductions of nutrients and sediment that could be associated with increasing levels of BMP implementation for both point and non-point

sources in the Chesapeake Bay watershed. 

The series of ranging scenarios were simulated by the Chesapeake Bay Program’s

Phase 4.3 Watershed Model and the resultant loads for nitrogen, phosphorus and sediment were used as inputs to the Chesapeake Bay Estuary Model. Evaluation of water

clarity, dissolved oxygen and chlorophyll a concentrations from the Estuary Model,

in turn, provided a sense of the response of key water quality parameters to the various

loading levels. For the Tier and E3 scenarios, best management practices (BMP)

implementation levels, the resultant modeled loads and the measured responses in

tidal water quality are informational. They are not intended to prescribe control

measures to meet Chesapeake 2000 nutrient and sediment loading caps.

Implementation levels in all of the Tiers and E3 scenarios are not cost effective. The

most cost effective combinations of BMPs will be evaluated by jurisdictions and

their tributary or watershed teams as their tributary strategies are developed. In addition, and as noted in Chapter 4, E3 levels of BMP implementation are theoretical

since the scenario, generally, did not account for physical limitations or participation

levels in its design. 

▼C1

appendix C • Summary of Watershed Model Results for All Loading Scenarios

appendixC

Summary of 

Watershed Model Results 

for All Loading Scenarios
The Tier and E3 BMP implementation levels were mostly deliberated and set by the

“source” workgroups of the Chesapeake Bay Program’s Nutrient Subcommittee.

These workgroups are made up of representatives of Chesapeake Bay watershed jurisdictions and Chesapeake Bay Program Office personnel. The specific workgroups that

decided BMP implementation levels included the Agricultural Nutrient Reduction

Workgroup, the Forestry Workgroup, the Point Source Workgroup and the Urban

Stormwater Workgroup. The Tributary Strategy Workgroup and Nutrient Subcommittee finalized the E3 scenario definitions after review and further deliberation. 

To conform to Chesapeake 2000 goals, all of the Tier and E3 scenarios were rooted

in 2010 projections of landuses, animals, point source flows and septic systems as

well as 2007/2010 or 2020 air emission controls. Landuses and animal populations

in the Chesapeake Bay Program Watershed Model are developed from an array of

national, regional and state databases as described in Chesapeake Bay Watershed

Model Land Use and Model Linkages to the Airshed and Estuarine Models (CBPO,

2000). The modeled landuses include the following categories:

• Forest

• Conventional-Tilled (High-Till)

• Conservation-Tilled (Low-Till)

• Hay

• Pasture

• Manure Acres (model accounting of runoff from animal feeding operations)

• Pervious Urban

• Impervious Urban

• Mixed Open

2010 agricultural landuses were projected from Agricultural Census information

(1982, 1987, 1992 and 1997) by county and according to methodologies chosen by

individual states. Projected animal populations, to estimate manure applications,

were rooted in county Agricultural Census trends and information from state environmental and agricultural agencies. 

2010 urban landuses were mostly projected from a methodology involving human

population changes as determined by the U.S. Census Bureau for 1990 and 2000 and

by individual state agencies for 2010. The population changes were related to 1990

high-resolution satellite imagery of the Chesapeake Bay watershed which is the root

source of urban and forest acreage. In the case of Maryland, urban growth from 2000

to 2010 was determined by Maryland Department of Natural Resources and the

Department of Planning. 

For all jurisdictions except Maryland and Virginia, 2010 forest and mixed open

landuses were determined by proportioning the net change between 2010 and 1990

agricultural and urban land to 1990 mixed open and 1990 forest. Maryland and

Virginia forest acreage changes followed methodologies or data submitted by 

these states.

▼C2

appendix C • Summary of Watershed Model Results for All Loading Scenarios
Estimates of the number of septic systems in the watershed in 2010 were derived

from human population projections and people per septic system ratios from the

1990 U.S. Census Bureau survey.

Point sources were divided into categories which included: 1) significant municipal

wastewater treatment facilities—discharging flows greater than or equal to 0.5 million gallon per day; 2) significant industrial facilities—discharging flows greater

than or equal to 0.5 million gallon per day; and 3) non-significant municipal wastewater treatment facilities—discharging flows less than 0.5 million gallon per day and

limited to facilities in Maryland and Virginia due to availability of data.

Point source nitrogen and phosphorus loads from significant and non-significant

municipal wastewater treatment facilities were determined using flows projected for

the year 2010 for facilities located in all jurisdictions of the Chesapeake Bay watershed. These future flows were developed mostly from population projections. Tier

and E3 scenario flows for industrial dischargers remained at 2000 levels.

Treatment technologies for municipal facilities varied among the Tier scenarios to

reach and maintain concentrations defined under each Tier scenario description. The

treatment technologies included extended aeration processes and denitrification

zones, chemical additions, additional clarification tanks, deep bed denitrification

filters and micro-filtration. For industrial dischargers, site specific information on

reductions by facility was obtained via phone contacts or site visits.

Atmospheric deposition to the Chesapeake Bay watershed and tidal surface waters

for all Tier and E3 scenarios employed deposition data from the Regional Acid

Deposition Model (RADM), which also provides deposition estimates representing

current conditions used for Progress model runs.

2010 TIER 1 SCENARIO

2010 Tier 1 BMP implementation levels were generally determined by continuing

current levels of effort and cost-share in each Chesapeake Bay watershed jurisdiction. In addition, expected regulatory measures, jurisdictional programs and

construction schedules between 2000 and 2010 were included.

2010 TIER 1 NON-POINT SOURCE BMPs

For most non-point source BMPs, implementation rates between 1997 and 2000

were continued to the year 2010 with limits that levels could not exceed the available or E3 land area to apply the BMPs to. The scale of the calculations is a

county-segment or the intersection of a county political boundary and a Chesapeake

Bay watershed model hydrologic segment. This is the same scale that most jurisdictions report BMP implementation levels to the Bay Program office.

Every effort was made to include BMPs submitted by the jurisdictions for progress

model runs into Tier 1. Since historic BMP data was not available from New York,

Delaware and West Virginia, 2010 Tier 1 projections were determined from watershed-wide implementation rates in states which employ and track the practice.

▼C3

appendix C • Summary of Watershed Model Results for All Loading Scenarios
2010 Tier 1 BMPs were extrapolated from recent implementation rates by the

landuse types submitted by the states for each BMP. For example, if a jurisdiction

submits data for nutrient management on crop land, 2010 Tier 1 crop land was

projected and then split among high-till, low-till and hay according to relative

percentages. If a jurisdiction submits data as nutrient management on high-till, lowtill and hay individually, projections were done for each of these landuse categories.

The 2010 Tier 1 scenario does not include tree planting on tilled land, forest conservation and forest harvesting practices. These practices are tracked by some

jurisdictions and credited in the Watershed Model for progress scenarios, but are not

part of the Tiers and E3. For forest harvesting practices and erosion and sediment

control, the watershed model simulation does not account for additional loads from

disturbed forest and construction areas, respectively. For forest conservation,

planting above what is removed during development is accounted for in the 2010

urban and forest projections. Tree planting on agricultural land is included in Tier 1

for pasture as forest buffers since this BMP is also part of the Tier scenarios and E3

and pasture tree planting and pasture buffers are treated the same in the model.

Table C-1 shows Tier 1 watershed-wide BMP implementation levels for all nonpoint

source BMPs. The table designates the unit of measure for each BMP and the relevant model landuses that BMPs are applied to by four major categories: agricultural,

urban and mixed open, forestry and septic. As references, 2000 nonpoint source

BMP implementation levels are listed as well.

2010 TIER 1 POINT SOURCE TREATMENT TECHNOLOGIES

• Tier 1 significant municipal wastewater treatment facilities

o Nitrogen—Existing municipal facilities with nutrient-removal technologies

(NRT) and those planned to go to NRT by 2010 are at 2010 projected flows and

8 mg/L total nitrogen effluent concentrations (annual average). All remaining

significant facilities are at 2010 projected flows and 2000 total nitrogen effluent

concentrations.

o Phosphorus—2010 projected flows and 2000 total phosphorus effluent concentrations except those targeted in VA which are at 1.5 mg/L total phosphorus

effluent concentrations (annual average).

• Tier 1 significant industrial dischargers 

o 2000 flows and 2000 levels of effluent concentrations for total nitrogen and

total phosphorus or the permit limit effluent concentration, whichever is less.

• Tier 1 Non-significant municipal wastewater treatment facilities

o 2000 total nitrogen and total phosphorus effluent concentrations applied to 2010

projected flows.

▼C4

appendix C • Summary of Watershed Model Results for All Loading Scenarios
▼C5

appendix C • Summary of Watershed Model Results for All Loading Scenarios

Table C-1. Chesapeake Bay watershed-wide nonpoint source best management practice implementation levels for 2000, Tier and E3.

Unit of Applicable 2000 2010 2010 2010 2010

Measure Landuse Progress Tier 1 Tier 2 Tier 3 E3

Conservation Tillage Acres Low-Till 1,994,745 1,962,824 2,340,908 2,300,093 2,312,209Riparian Forest Buffers Acres Row Crop, Hay 9,054 30,588 133,772 206,663 494,450Riparian Forest Buffers 1-side stream miles, Row Crop, Hay 747 2,524 11,036 17,050 40,792

100 foot width

Wetland Restoration Acres Row Crop, Hay 1,277 2,862 10,260 17,659 25,282Land Retirement Acres Row Crop, Hay 87,488 128,510 500,452 742,695 1,090,540Grass Buffers Acres Row Crop 4,294 15,036 71,985 113,800 0Tree Planting Acres Row Crop, Pasture 8,568 4,142 0 0 0Riparian Forest Buffers Acres Pasture 0 0 46,732 63,851 184,081Riparian Forest Buffers 1 side stream miles, Pasture 0 0 3,855 5,268 15,187

100 foot width

Carbon Sequestration/ Acres Row Crop 0 0 0 509,431 770,736

Bio EnergyStandard Nutrient Management Acres Row Crop, Hay 2,283,426 3,023,742 3,850,244 2,967,870 0Plan ImplementationYield Reserve Implementation Acres Row Crop, Hay 0 0 0 1,271,944 4,830,817Total Nutrient Management Acres Row Crop, Hay 2,283,426 3,023,742 3,850,244 4,239,814 4,830,817Plan ImplementationFarm Plans Acres Agriculture 3,666,165 5,075,549 5,860,003 6,854,953 7,202,280Cover Crops Acres Row Crop 220,134 152,766 1,544,635 2,203,196 2,312,209Stream Protection With Fencing Acres Pasture 40,744 69,257 171,739 580,365 712,302Stream Protection Without Fencing Acres Pasture 26,166 27,979 83,584 63,583 0Grazing Land Protection Acres Pasture 134,327 304,868 853,863 1,394,909 2,371,463Animal Waste Management Acres Manure Acres 4,886 6,425 6,953 7,692 8,537

Animal Units Manure Acres 708,498 931,677 1,008,208 1,115,351 1,237,801

Manure Excess Wet Tons As Excreted N/A 1,270,139 1,927,899 2,145,277 1,870,085 8,856,825

AGRICULTURAL BMPs

continued
▼C6

appendix C • Summary of Watershed Model Results for All Loading Scenarios Table C-1. Chesapeake Bay watershed-wide nonpoint source best management practice implementation levels for 2000, Tier and E3. (cont) Unit of Applicable 2000 2010 2010 2010 2010 Measure Landuse Progress Tier 1 Tier 2 Tier 3 E3 Abandoned Mine Reclamation Acres Urban/Exposed 6,062 0 0 0 0 Urban Growth Reduction Acres Urban 38,787 0 26,096 52,192 78,288 Riparian Forest Buffers Acres Pervious Urban 0 364 9,808 28,522 93,643 1 side stream miles, Pervious Urban 0 60 1,618 4,706 15,451 50 foot width Grass Buffers Acres Pervious Urban 0 95,022 84,997 65,702 0 Storm Water Management Acres Urban 0 153,157 207,705 183,440 159,560 on New Development Storm Water Management Acres Urban 165,040 374,357 373,817 373,236 0 on Recent Development Storm Water Management on Acres Urban 0 29,959 187,185 748,488 3,740,806 Recent and Old Development Total Storm Water Management Acres Urban 165,040 557,474 768,707 1,305,164 3,900,366 Erosion and Sediment Control Acres Urban 25,911 0 0 0 0

Urban Nutrient Management Acres Pervious Urban 6,608 28,630 1,055,077 1,964,784 2,601,733Tree Planting Acres Mixed Open 22,596 44,280 0 0 0Riparian Forest Buffers Acres Mixed Open 0 0 54,702 73,757 413,922

1 side stream miles, Mixed Open 0 0 4,513 6,085 34,149

100 ft. width

Mixed Open Nutrient Acres Mixed Open 0 60,791 1,997,497 3,870,252 4,950,621

URBAN AND MIXED OPEN BMPs

continued
▼C7

appendix C • Summary of Watershed Model Results for All Loading Scenarios

Unit of Applicable 2000 2010 2010 2010 2010

Measure Landuse Progress Tier 1 Tier 2 Tier 3 E3

FORESTRY BMPs

Forest Harvesting Practices Acres Forest 67,448 0 0 0 0Septic Connections Systems Septic 31,514 31,514 31,514 31,514 31,514Septic Pumping Systems Septic 2,954 N/A N/A N/A N/ASeptic Denitrification Systems Septic 312 312 N/A N/A N/ASeptic Denitrification/Pumping Systems Septic N/A N/A 8,305 93,014 1,357,026on New and Existing Systems

SEPTIC BMPs

Table C-1. Chesapeake Bay watershed-wide nonpoint source best management practice implementation levels for 2000, Tier and E3. (cont)
2010 TIER 1 ATMOSPHERIC DEPOSITION SOURCE CONTROLS

Tier 1 atmospheric deposition assumes implementation of the 1990 Clean Air 

Act projected for the year 2010 with existing regulations. Air emission source

controls for the Tier 1 scenario include the following:

• 2007 non-utility (industrial) point source and area source emissions.

• 2007 mobile source emissions with “Tier II” tail pipe standards on light duty

vehicles.

• 2010 utility emissions with Title IV (Acid Rain Program) fully implemented and

20-state nitrogen oxides (NOx) state implementation plan (SIP) call reductions at

0.15 lbs/MMbtu during the May to September ozone season only.

The impacts of Tier 1 emissions and deposition to the Chesapeake Bay watershed’s

land area and non-tidal waters are part of the reported nutrient loads from the individual landuse source categories, i.e., agriculture, urban, mixed open, forest and

non-tidal surface waters). The reported Chesapeake Bay Watershed Model loads;

however, usually do not include contributions from atmospheric deposition to tidal

waters although the water quality responses, as measured by the Chesapeake Bay

Estuary Model, account for this source at levels prescribed by Tier 1.

2010 TIER 2 SCENARIO

In the design of the Tier 2 scenario, considerations of the costs of BMP implementation, participation levels and physical limitations are very limited. Tier 2 BMP

levels are considered technically possible and generally described below for each of

the major source category. 

2010 TIER 2 NON-POINT SOURCE BMPs

2010 Tier 2 BMP implementation levels for non-point sources were generally determined by increasing levels above Tier 1 by a percentage of the difference between

Tier 1 and E3 levels for each BMP with the percentages being lower than those used

in Tier 3. These percentages were mostly prescribed by individual source workgroups in the Chesapeake Bay Program Nutrient Subcommittee and were applied

watershed-wide by county-segments or the intersections of county political boundaries and the Watershed Model’s segments.

Table C-1 shows Tier 2 watershed-wide BMP implementation levels for all nonpoint

source BMPs. The table designates the unit of measure for each BMP, the relevant

model landuses BMPs are applied to by four major categories: agricultural, urban

and mixed open, forestry and septic.

▼C8

appendix C • Summary of Watershed Model Results for All Loading Scenarios
2010 TIER 2 POINT SOURCE TREATMENT TECHNOLOGIES

For Tier 2 point source municipal facilities, technologies to achieve 8 mg/L total

nitrogen effluent concentration included extended aeration processes and denitrification zones, along with chemical addition to achieve a total phosphorus effluent

concentration of 1.0 mg/L where facilities are not already achieving these levels.

• Tier 2 significant municipal wastewater treatment facilities

o Nitrogen—All significant municipal facilities are at 2010 projected flows and

reach and maintain effluent concentrations of 8 mg/L (annual average)

including those facilities that planned to go to NRT by 2010.

o Phosphorus—All significant municipal facilities are at 2010 projected flows

and reach and maintain total phosphorus effluent concentrations of 1.0 mg/L

(annual average) or the permit limit, whichever is less.

• Tier 2 significant industrial dischargers

o 2000 flows and generally maintain total nitrogen and phosphorus effluent

concentrations that are 50 percent less than those in Tier 1 or the permit limit,

whichever is less.

• Tier 2 non-significant municipal wastewater treatment facilities

o 2000 total nitrogen and total phosphorus effluent concentrations are applied to

2010 projected flows.

2010 TIER 2 ATMOSPHERIC DEPOSITION SOURCE CONTROLS

Tier 2 atmospheric deposition assumes implementation of the 1990 Clean Air Act

projected for the year 2020 with mobile source controls beyond those in Tier 1. Air

emission source controls for the Tier 2 scenario include the following:

• 2020 non-utility (industrial) point source and area source emissions with no additional controls than Tier 1.

• 2020 mobile source emissions with 2020 mobile source emissions with Tier II tail

pipe standards on light duty vehicles that are more effective than those in the Tier 1

scenario, as well as heavy duty diesel standards to further reduce NOx emissions.

• 2020 utility emissions with Title IV (Acid Rain Program) fully implemented and

20-state NOx SIP call reductions at 0.15 lbs/MMbtu during the May to September

ozone season only—same as Tier 1 controls.

The impacts of Tier 2 emissions and resultant atmospheric deposition to the Chesapeake Bay watershed’s land area and non-tidal waters are part of the reported

nutrient loads from the individual landuse source categories, i.e., agriculture, urban,

mixed open, forest and non-tidal surface waters). The reported Chesapeake Bay

Watershed Model loads, however, usually do not include contributions from atmospheric deposition to tidal waters although the water quality responses, as measured

by the Chesapeake Bay Estuary Model, account for this source at levels prescribed

by Tier 2.

▼C9

appendix C • Summary of Watershed Model Results for All Loading Scenarios
2010 TIER 3 SCENARIO

In the Tier 3 scenario, considerations of the costs of BMP implementation, participation levels and physical limitations are very limited. Tier 3 BMP levels are

considered technically possible and are generally described below for each of the

major source categories.

2010 TIER 3 NON-POINT SOURCE BMPs

2010 Tier 3 BMP implementation levels for non-point sources were generally determined by increasing levels above Tier 1 by a percentage of the difference between

Tier 1 and E3 levels with the percentages being higher than those used in Tier 2. As

with Tier 2, the levels of nonpoint source control were applied watershed-wide by

county-segments or the intersections of county political boundaries and the Chesapeake Bay Watershed Model’s segments.

Table C-1 shows Tier 3 watershed-wide BMP implementation levels for all nonpoint

source BMPs. The table designates the unit of measure for each BMP, the relevant

model landuses BMPs are applied to by four major categories: agricultural, urban

and mixed open, forestry and septic.

2010 TIER 3 POINT SOURCE TREATMENT TECHNOLOGIES

For Tier 3 municipal point source facilities, treatment technologies to achieve 

5 mg/L total nitrogen effluent concentration included extended aeration processes

beyond those in Tier 2, a secondary anoxic zone plus methanol addition, additional

clarification tanks and additional chemicals to achieve a phosphorus effluent concentration of 0.5 mg/L total phosphorus.

• Tier 3 significant municipal wastewater treatment facilities

o Nitrogen—All significant municipal facilities are at 2010 projected flows and

reach and maintain effluent concentrations of 5 mg/L total nitrogen (annual

average) including those facilities that planned to go to NRT by 2010.

o Phosphorus—All significant municipal facilities are at 2010 projected flows

and reach and maintain effluent concentrations of 0.5 mg/L total phosphorus

effluent concentration (annual average) or the permit limit, whichever is less.

• Tier 3 significant industrial dischargers

o 2000 flows and generally maintain total nitrogen and phosphorus effluent

concentrations that are 80 percent less than those in Tier 1 or the permit limit,

whichever is less.

• Tier 3 non-significant municipal wastewater treatment facilities 

o 2000 total nitrogen and total phosphorus effluent concentrations are applied to

2010 projected flows.

▼C10

appendix C • Summary of Watershed Model Results for All Loading Scenarios
2010 TIER 3 ATMOSPHERIC DEPOSITION SOURCE CONTROLS

Atmospheric deposition under the Tier 3 scenario reflects existing regulatory

nitrogen oxide emissions controls under the 1990 Clean Air Act, as well as more

aggressive but voluntary emissions controls on the utility sector, projected for the

year 2020. Estimated changes in deposition for the Tier 3 scenario reflect the

following controls on nitrogen oxide emissions:

• 2020 non-utility (industrial) point source and area source emissions with no additional controls than Tiers 1 and 2.

• 2020 mobile source emissions with the effect of the Tier II tail pipe standards on

light duty vehicles being felt, and the implementation of the heavy duty diesel

standards to further reduce NOx emissions. Same as Tier 2 controls.

• 2020 utility emissions with major (90 percent) reductions in SO2 and aggressive

20-state NOx SIP call reductions through utilities going to 0.10 lbs/MMbtu for the

entire year—no longer just seasonal.

The impacts of emissions and deposition to the Chesapeake Bay watershed’s land

area and non-tidal waters under the Tier 3 scenario are part of the reported nutrient

loads from the individual landuse source categories (i.e., agriculture, urban, mixed

open, forest, and non-tidal surface waters). The reported loads, however, usually do

not include contributions from atmospheric deposition to tidal waters although the

water quality responses, as measured by the Water Quality Model, account for this

source at levels prescribed by the Tier 3 scenario.

2010 E3 SCENARIO

BMP implementation levels in the Tier scenarios were bounded by levels of E3,

which is specifically designed to take out most of the subjectivity surrounding what

can or cannot be achieved in control measures. The particular definitions of E3 BMP

implementation levels are, in part, rooted in earlier work of the Chesapeake Bay

Program when a limit-of-technology condition was assessed by the Tributary

Strategy Workgroup. However, E3 is less subjective than the previous limit-oftechnology scenarios in its determinations of maximum implementation levels.

The BMP levels in E3 are theoretical. There are no cost and few physical limitations

to implementing BMPs for point and non-point sources. As discussed in Chapter 4,

E3 implementation levels and their associated reductions in nutrients and sediment

could not be achieved for many BMPs when considering physical limitations and

participation levels. However, there are some control measures in E3 that physically

could be more aggressive. The E3 conditions for these BMPs were established

because a theoretical maximum implementation level would have been entirely

subjective. Finally, E3 includes new BMP technologies and programs that are not

currently part of jurisdictional pollutant control strategies. BMP implementation

levels for the E3 scenario are generally described below for nonpoint and point

source categories.

▼C11

appendix C • Summary of Watershed Model Results for All Loading Scenarios
2010 E3 NON-POINT SOURCE BMPs

For most non-point source BMPs, it is assumed that the load from every available

acre of the relevant land area is being controlled by a full suite of existing or

innovative practices. In addition, management programs convert landuses from those

with high-yielding nutrient and sediment loads to those with lower yields. Table 

C-1 shows E3 watershed-wide BMP implementation levels for all nonpoint source

BMPs. The table designates the unit of measure for each BMP and the relevant

model landuses that BMPs are applied to by four major categories: agricultural,

urban and mixed open, forestry and septic.

2010 E3 POINT SOURCE TREATMENT TECHNOLOGIES

For point sources in E3, municipal wastewater treatment facilities reach and maintain effluent concentrations of 3 mg/L total nitrogen and at least 0.1 mg/L total

phosphorus through technologies such as deep bed denitrification filters and 

micro-filtration.

• E3 significant municipal wastewater treatment facilities

o Nitrogen—Significant municipal facilities are at 2010 projected flows and

reach and maintain total nitrogen effluent concentrations of 3 mg/L (annual

average) including those facilities that planned to go to NRT by 2010. 

o Phosphorus—Significant municipal facilities are at 2010 projected flows and

reach and maintain total phosphorus effluent concentrations of 0.1 mg/L

(annual average).

• E3 significant industrial dischargers

o Nitrogen—2000 flows and total nitrogen effluent concentrations of 3 mg/L

(annual average).

o Phosphorus—2000 flows and total phosphorus effluent concentrations of 

0.1 mg/L (annual average) or the permit limit, whichever is less.

• E3 non-significant municipal wastewater treatment facilities

o Nitrogen—Non-significant municipal facilities are at 2010 projected flows and

reach and maintain total nitrogen effluent concentrations of 8 mg/L (annual

average).

o Phosphorus—Non-significant municipal facilities are at 2010 projected flows

and reach and maintain total phosphorus effluent concentrations of 2.0 mg/L

(annual average) or 2000 concentrations, whichever is less.

2010 E3 ATMOSPHERIC DEPOSITION SOURCE CONTROLS

E3 atmospheric deposition assumes implementation of the 1990 Clean Air Act

projected for the year 2020 with aggressive controls on utilities, industry and mobile

sources. Air emission source controls for the E3 scenario include the following:

• 2020 non-utility (industrial) point source emissions cut almost in half for both

SO2 and NOx.

• 2020 area source emissions that are the same as Tiers 1-3.

▼C12

appendix C • Summary of Watershed Model Results for All Loading Scenarios
• 2020 mobile source emissions assuming super ultra-low emissions for light duty

vehicles and heavy duty diesel standards to further reduce NOx emissions beyond

Tier 2 and Tier 3.

• 2020 utility emissions with major (90 percent) reductions in SO2 and aggressive

20-state NOx SIP call reductions through utilities going to 0.10 lbs/MMbtu for the

entire year—same as Tier 3 controls.

The impacts of E3 emissions and resultant atmospheric deposition to the Chesapeake

Bay watershed’s land area and non-tidal waters are part of the reported nutrient loads

from the individual landuse source categories, i.e., agriculture, urban, mixed open,

forest and non-tidal surface waters. The reported Chesapeake Bay Watershed Model

loads, however, usually do not include contributions from atmospheric deposition to

tidal waters although the water quality responses, as measured by the Chesapeake

Bay Estuary Model, account for this source at levels prescribed by E3.

BASINWIDE LOADS FOR 2000, TIERS 1-3 AND E3

Figures C-1 through C-3 depict Chesapeake Bay watershed modeled basinwide

nutrient and sediment loads delivered to the Chesapeake Bay and its tidal tributaries

by major source category for each of the Tier scenarios as well as E3. As references,

the model estimated loads for the year 2000 are also portrayed.

▼C13

appendix C • Summary of Watershed Model Results for All Loading Scenarios

101.1

76.0

55.5

25.3

62.8

56.6

45.5

30.6

18.3

39.9

39.7

39.2

40.2

30.3

29.0

25.9

22.9

12.2

18.4

18.8

18.9

18.1

14.9

11.9

12.6

12.6

12.0

3.3

116.3

41.5

3.6

3.0

2.7

2.4

2.2

0

30

60

90

120

150

180

210

240

270

300

330

2000 Progress 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

Nitrogen Loads

(million pounds/year)

Non-Tidal

Water

Atmospheric

Deposition

Septic

Mixed Open

Urban Runoff

Forest

Point Source

Agriculture

Figure C-1. Chesapeake Bay Watershed Model-estimated nitrogen loads delivered to the Chesapeake Bay

and its tidal tributaries by source.
▼C14

appendix C • Summary of Watershed Model Results for All Loading Scenarios

3.188

2.714

2.179

1.669

1.274

0.995

1.000

1.010

1.020

1.067

0.475

0.479

0.461

0.414

0.105

0.386

0.451

0.494

0.522

0.507

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

2000 Progress 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

Sediment Loads

(million tons/year)

Mixed Open

Urban

Runoff

Forest

Agriculture

Figure C-3. Chesapeake Bay Watershed Model-estimated land-based sediment loads delivered to the 

Chesapeake Bay and its tidal tributaries by source.

8.99

7.69

6.71

5.54

3.74

4.26

5.10

3.72

2.21

0.69

0.41

0.36

0.35

0.34

0.37

3.12

3.13

2.89

2.59

2.07

2.18 2.51

2.57

2.52

3.03

0.16 0.16

0.17

0.18

0.19

0

2

4

6

8

10

12

14

16

18

20

22

2000 Progress 2010 Tier 1 2010 Tier 2 2010 Tier 3 2010 E3

Phosphorus Loads 

(million pounds/year)

Mixed Open

Urban

Runoff

Forest

Point Source

Agriculture

Non-Tidal

Water

Atmospheric

Deposition

Figure C-2. Chesapeake Bay Watershed Model-estimated phosphorus loads delivered to the Chesapeake Bay

and its tidal tributaries by source.
As is common for reporting purposes, the model-estimated delivered loads are a

yearly average of loads simulated over a 10-year period (1985-1994). This removes

considerations of the effects of variable precipitation levels or flows on loads. Also,

nutrient loads are reported in units of million pounds per year while sediment fluxes

are in million tons per year.

Load reductions through the Tiers to E3 show the impact of most point and non-point

source BMPs employed in the design of the scenarios. For nonpoint sources, the

influence of generally increasing BMPs listed in Table C-1 is depicted in the nutrient

and sediment load reductions for the three relevant source categories: agriculture,

urban and mixed open and septic. For point sources, the impact of lower effluent

concentrations through the Tier to E3 yields the point source reductions shown in

Figures C-1 and C-2.

Atmospheric deposition to the Chesapeake Bay watershed’s land area and non-tidal

surface waters are part of the reported loads but the loads do not include contributions from atmospheric deposition direct to tidal surface waters. In addition, the

reported loads do not reflect shoreline erosion controls employed in the scenarios.

The water quality responses as measured by the Chesapeake Bay Estuary Model,

however, account for both atmospheric deposition to tidal waters and shoreline

erosion at levels prescribed for the Tiers and E3.

It is important to note that landuses and animal populations change considerably

between 2000 Progress and the Tiers and E3, which are rooted in projected 2010

landuses and populations. Therefore, nutrient applications to agricultural land change

considerably over the decade. Also, the number of septic systems and the flows from

municipal wastewater treatment facilities shift dramatically from 2000 to 2010 based

on an increasing population. For example, point source phosphorus loads increase

from 2000 to 2010 Tier 1 because of increases in municipal facility flows which,

unlike nitrogen, are not offset by technologies to reduce this nutrient in effluents.

In addition to changes between 2000 and the 2010 Tier and E3 scenarios, it is imperative to consider landuse changes among the Tiers and E3 due to increasing

non-point source BMP implementation levels. For example, sediment loads from

forested land increase through the Tiers to E3 because the land area increases as, for

example, more and more riparian buffers are planted on agricultural and urban land.

In addition, increases in loads from mixed open land is attributable to greater acreage

in this category as, for example, agricultural land is retired. 

INFLUENCE OF AIR EMISSION CONTROLS AND 

ATMOSPHERIC DEPOSITION ON LOADS

The impacts of emission controls and the resultant lower atmospheric deposition to

the Chesapeake Bay watershed’s land area and non-tidal surface waters are part of

the reported nutrient loads from the individual landuse source categories in the Tiers

and E3, i.e., agriculture, urban, mixed open, forest and non-tidal surface waters. As

mentioned previously, the reported loads; however, usually do not include contributions from atmospheric deposition to tidal surface waters although the water quality

responses account for this source. 

▼C15

appendix C • Summary of Watershed Model Results for All Loading Scenarios
To estimate the effects of only the Tier and E3 emission controls, i.e., without the

influences of other point and non-point source BMPs, the histograms in Figure C-4

show changes in atmospheric deposition of nitrogen to the watershed’s land area and

non-tidal surface waters and the response in delivered loads. In this model study, all

landuses, fertilizer applications, point sources, septic loads and BMP implementation

levels were held constant at 2000 conditions. Only atmospheric deposition varied.

What the deposition scenarios say, for example, is “If projected emission and deposition reductions associated with the Tiers and E3 were realized today (2000), loads

to the Chesapeake Bay and its tidal tributaries are estimated to be the following.” For

references, reported Tier 1 and Tier 2 loads from the watershed are also shown in the

graphics.

As can be seen in Figure C-4, atmospheric deposition to the watershed progressively

declines from 2000 through the Tiers to E3 as more air emission controls are

included in the model simulation. Loads from the watershed land area and non-tidal

surface waters respond to these progressive emission and deposition reductions, but

to a much smaller degree. 

The most significant reason for the dampened response is that the Chesapeake Bay

watershed is about 57 percent forested, or 57 percent of atmospheric deposition falls

on forests. Among landuses, forests have the greatest potential to uptake nitrogen as,

generally, forests in the Bay basin are not nitrogen-sensitive.

▼C16

appendix C • Summary of Watershed Model Results for All Loading Scenarios

442.2

369.3

328.2

295.7

270.9

284.8 276.5 270.7 265.0 261.1

0

50

100

150

200

250

300

350

400

450

500

2000 Progress

(Baseline)

2000 Base w/

Tier 1 Deposition

2000 Base w/

Tier 2 Deposition

2000 Base w/

Tier 3 Deposition

2000 Base w/ E3

Deposition

(million lbs/year)

Atmospheric Deposition Delivered Load Tier 1 Load Tier 2 Load

Figure C-4. Chesapeake Bay Watershed Model-estimated nitrogen deposition versus delivered loads—

2000 baseline with Tier and E3 emission controls.
It is the impacts of emission controls on delivered loads that are important in the

establishment of tributary strategies—rather than the contribution to loads from

atmospheric deposition. Understanding the loading responses to changes in deposition better addresses to what degree the loads can be controlled. The proportion of

the loads attributed to atmospheric deposition changes dramatically from 2000

through the Tiers and E3 because of both variable air emission controls and changes

in landuses that the atmospheric flux is deposited to.

In the most dramatic case, atmospheric deposition of nitrogen to the watershed

decreases 171 million lbs/year from 2000 to 2010 E3. If this reduction in deposition

were realized today, (i.e., deposition was to 2000 landuses with all other present

conditions), nitrogen loads to the Chesapeake Bay and its tidal tributaries would

decrease 21 million lbs/year or would be at levels associated with the Tier 1 scenario.

It is important to note that E3 levels of emission controls are considered to be the

current limits of technology with aggressive controls on all major sources of technology with aggressive controls on all major sources utilities, mobile and industrial.

E3 emission controls are voluntary, as opposed to regulatory and follow the format

of defining other E3 point and nonpoint source BMPs in that implementation levels

did not consider physical limitations, participation rates and costs. As has been

described previously, the intent of the Tiers is not to establish what can and cannot

be done through management actions, either regulatory or voluntary, as this is the

responsibility of Bay watershed jurisdictions.

LITERATURE CITED

Chesapeake Bay Program Office (CBPO). 2000. Chesapeake Bay Watershed Model Land

Use and Model Linkages to the Airshed and Estuarine Models. Prepared by the Chesapeake

Bay Program Modeling Subcommittee. Annapolis, Maryland.

▼C17

appendix C • Summary of Watershed Model Results for All Loading Scenarios
▼D1

appendix D • Summary of Key Water Quality Attainment Scenarios

appendixD

Summary of 

Key Water Quality 

Attainment Scenarios

Table D-1. Key scenario descriptions.

Observed 1984–1994 Chesapeake Bay water quality monitored conditions.

Progress 2000 Model estimated conditions resulting from implementation of BMPs and treatment technologies

in place in 2000

Tier 1 Model estimated conditions resulting from implementation of Tier 1 BMPs and treatment

technology implementation levels.

Tier 2 Model estimated conditions resulting from implementation of Tier 2 BMPs and treatment

technology implementation levels.

Tier 3 Model estimated conditions resulting from implementation of Tier 3 BMPs and treatment

technology implementation levels; same as Option 3 cap load allocation.

Tier 3 + 20% Tier 3 model estimated conditions plus a 20 percent reduction in shoreline/nearshore 

sediment loads.

Tier 3 + 50% Tier 3 model estimated conditions plus a 50 percent reduction in shoreline/nearshore 

sediment loads.

Option 4 Option 4 cap load allocation model estimated conditions.

Confirm Confirmation of agreed to nutrient and sediment cap load allocations model estimated

conditions.

Confirm + 10 Confirmation scenario model estimated conditions plus a 10 percent reduction in

shoreline/nearshore sediment loads.

Confirm + 20 Confirmation scenario model estimated conditions plus a 20 percent reduction in

shoreline/nearshore sediment loads.

Allocation Selected cap load allocation option (175 million pounds nitrogen/12.8 million pounds

phosphorus) model estimated conditions.

Option 1 Option 1 cap load allocation model estimated conditions.

Option 5 Option 5 cap load allocation model estimated conditions.

E3 E3 scenario model estimated conditions.

All Forest All forested watershed scenario model estimated conditions.

Pristine Pristine watershed scenario model estimated conditions.
▼D2

appendix D • Summary of Key Water Quality Attainment Scenarios Table D-2. Key scenario summary of dissolved oxygen criteria attainment.* Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1 Opt5 Segment DU Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) (198) E3 All Forest Pristine Mainstem Upper Bay (CB1TF) MIG A A A A A A A A A A A A A A A A A OW A A A A A A A A A A A A A A A A A Mainstem Upper Bay (CB2OH) MIG A A A A A A A A A A A A A A A A A OW 1.92 0.88 0.68 0.43 0.17 0.13 0.07 0.14 0.12 0.10 0.10 0.08 0.04 0.14 A A A Mainstem Upper Bay (CB3MH) MIG 0.19 A A A A A A A A A A A A A A A A OW A A A A A A A A A A A A A A A A A DW 4.18 2.52 2.24 1.61 0.73 0.54 0.37 0.61 0.54 0.44 0.40 0.38 0.17 0.67 A A A DC 13.52 8.16 7.21 5.03 1.84 1.24 0.11 1.46 0.67 0.31 0.22 0.12 A 1.68 A A A Mainstem Mid-Bay (CB4MH) OW 0.05 A A A A A A A A A A A A A A A A DW 19.64 15.28 14.28 12.05 8.51 7.57 5.62 7.89 7.08 6.68 6.32 5.96 3.90 8.24 0.69 A A DC 45.19 32.75 28.94 18.81 3.93 2.69 1.00 3.17 1.79 1.44 1.24 1.02 0.33 3.84 A A A Mainstem Mid-Bay (CB5MH) OW A A A A A A A A A A A A A A A A A DW 6.16 4.38 3.75 2.58 1.08 1.00 0.72 1.10 0.82 0.80 0.76 0.72 0.31 1.17 A A A DC 13.79 7.76 6.00 2.59 0.15 0.14 0.11 0.24 0.09 0.09 0.09 0.08 0.01 0.32 A A A Mainstem Lower Bay (CB6PH) OW 5.87 4.26 3.68 2.71 1.30 1.23 0.99 1.39 1.07 1.05 1.01 0.97 0.51 1.58 0.01 A A DW 0.36 0.01 A A A A A A A A A A A A A A A Mainstem Lower Bay (CB7PH) OW 4.55 3.31 2.81 1.82 0.74 0.66 0.49 0.78 0.57 0.55 0.52 0.50 0.22 0.93 A A A DW A A A A A A A A A A A A A A A A A Mainstem Lower Bay (CB8PH) OW A A A A A A A A A A A A A A A A A Patuxent Tidal Fresh (PAXTF) MIG A A A A A A A A A A A A A A A 0.01 0.03 OW A A A A A A A A A A A A A A 0.38 15.14 19.13 Patuxent Mid-Estuary (PAXOH) MIG A A A A A A A A A A A A A A A A A OW 9.79 1.56 1.84 1.62 0.86 0.36 0.11 0.36 0.06 0.05 0.03 0.09 0.01 0.38 A A 0.15 Patuxent Lower Estuary (PAXMH) MIG A A A A A A A A A A A A A A A A A OW 7.40 1.59 1.69 1.04 0.01 A A A A A A A A A A A A DW 5.52 0.85 0.82 0.50 0.07 0.02 A 0.02 A A A A A 0.03 A A A

Potomac Tidal Fresh (POTTF) MIG A A A A A A A A A A A A A A A A A

OW A A A A A A A A A A A A A A A A A

Potomac Mid-Estuary (POTOH) MIG A A A A A A A A A A A A A A A A A

OW 2.10 1.36 1.08 0.63 0.31 0.30 0.25 0.31 0.24 0.17 0.15 0.18 0.08 0.32 0.01 A A

Potomac Lower Estuary (POTMH) MIG A A A A A A A A A A A A A A A A A

OW 0.78 A A A A A A A A A A A A A A A A

DW 6.90 5.03 4.53 3.11 1.12 0.70 0.15 0.87 0.67 0.40 0.33 0.26 A 0.95 A A A

DC 18.89 11.39 8.64 5.07 0.19 0.17 0.16 0.17 0.17 0.17 0.17 0.16 0.11 0.17 A A A

Rappahannock Tidal Fresh (RPPTF) MIG A A A A A A A A A A A A A A A A A

OW A A A A A A A A A A A A A A A A A

Rappahannock Mid-Estuary (RPPOH) MIG A A A A A A A A A A A A A A A A A

OW A A A A A A A A A A A A A A A A A

Rappahanock Lower Estuary (RPPMH) MIG A A A A A A A A A A A A A A A A A

OW 0.44 0.27 0.10 A A A A A A A A A A A A A A

DW 5.58 2.61 1.09 0.01 A A A A A A A A A A A A A

DC 6.39 5.20 3.38 1.65 A A A A A A A A A A A A A

continued
▼D3

appendix D • Summary of Key Water Quality Attainment Scenarios

Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1 Opt5

Segment DU Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) (198) E3 All Forest PristineYork Lower Estuary Piankatank (PIAMH) OW 0.12 A A A A A A A A A A A A A A A AYork Tidal Fresh Mattaponi (MPNTF) MIG A A A A A A A A A A A A A A A A A

OW 33.26 27.37 25.87 27.23 33.73 32.44 30.50 34.44 40.58 39.64 36.53 34.44 34.44 22.51 52.14 63.65 64.72

York Mid-Estuary Mattaponi (MPNOH)MIG A A A 1.72 2.78 2.40 1.79 1.34 2.34 2.34 2.34 1.34 1.34 A 6.08 4.77 3.98

OW 46.88 31.00 28.95 31.86 28.99 26.88 19.11 24.25 29.83 29.85 29.36 24.17 23.72 22.63 48.11 56.07 58.52

York Tidal Fresh Pamunkey (PMKTF) MIG A A A A A A A A A A A A A A 0.10 4.28 4.28

OW 62.25 49.53 42.07 30.35 32.94 21.16 10.32 21.77 36.67 29.48 25.80 21.77 21.77 28.81 54.50 81.08 80.84

York Mid-Estuary Pamunkey (PMKOH) MIG A A A A A A A A A A A A A A A A A

OW 42.15 15.22 12.66 13.86 10.32 4.52 1.06 4.96 11.32 10.21 9.73 4.92 4.88 6.21 11.39 24.27 32.26

York Lower Estuary (YRKMH) MIG A A A A A A A A A A A A A A A A A

OW 18.08 4.85 3.31 2.32 0.42 0.23 0.03 0.42 0.21 0.20 0.20 0.15 A 0.29 A A A

York Lower Estuary (YRKPH) OW 1.48 0.01 A A A A A A A A A A A A A A A

DW 0.01 A A A A A A A A A A A A A A A A

York Lower Estuary Mobjack (MOBPH) OW 2.30 1.78 1.60 1.10 0.34 0.29 0.23 0.38 0.27 0.26 0.25 0.25 0.16 0.56 A A AJames Tidal Fresh (JMSTF) MIG A A A A A A A A A A A A A A A A A

OW 0.66 A A A A A A A A A A A A A A A A

James Mid-Estuary (JMSOH) MIG A A A A A A A A A A A A A A A A A

OW A A A A A A A A A A A A A A A A A

James Lower Estuary (JMSMH) MIG A A A A A A A A A A A A A A A A A

OW A A A A A A A A A A A A A A A A A

James Lower Estuary (JMSPH) OW A A A A A A A A A A A A A A A A AEastern Bay (EASMH) MIG A A A A A A A A A A A A A A A A A

OW A A A A A A A A A A A A A A A A A

DW 3.26 2.18 2.00 0.90 0.36 0.32 0.20 0.32 0.29 0.27 0.28 0.27 0.14 0.34 A A A

DC 20.23 12.87 11.26 6.49 0.67 0.10 0.01 0.18 0.20 0.08 0.02 0.02 A 0.22 A A A

Choptank Mid-Estuary (CHOOH) MIG A A A A A A A A A A A A A A A A A

OW 0.14 A A A A A A A A A A A A A A A A

Choptank Lower Estuary (CHOMH1) MIG A A A A A A A A A A A A A A A A A

OW 2.27 1.83 1.78 1.51 1.08 0.92 0.74 0.97 0.94 0.88 0.83 0.78 0.65 1.00 0.43 A A

Choptank Lower Estuary (CHOMH2) MIG A A A A A A A A A A A A A A A A A

OW 0.33 A A A A A A A A A A A A A A A A

Tangier Sound (TANMH) OW 0.15 0.06 0.06 0.05 0.36 0.31 0.84 0.33 0.31 0.29 0.29 0.31 0.28 0.35 0.22 0.20 0.35Pocomoke (POCMH) OW A A A A A A A A A A A A A A A A AChester Lower (CHSMH)** MIG A A A A A A A A A A A A A A A A A

OW 5.67 4.71 4.56 3.56 2.54 2.42 1.69 2.42 2.13 2.06 1.90 1.74 1.42 2.42 0.86 A A

DW 0.85 A A A A A A A A A A A A A A A A

DC 11.80 3.98 2.89 0.85 A A A A A A A A A A A A A

Elizabeth River (ELIPH)** OW 3.05 A A A A A A A A A A A A A A 2.22 6.48

DW A A A A A A A A A A A A A A A A A

South Branch Elizabeth (SBEMH)** OW 59.51 58.86 58.67 60.37 57.71 56.71 56.39 56.81 58.67 58.58 58.47 56.80 56.80 60.73 56.20 50.31 53.63* 4/1/03, Version 15 — Changes since version 12: SAV Re-calibration, Wetlands Oxygen Demand, No Seasonal Anoxic Zone** for information purposes only, model not sufficiently calibrated for these areas
▼D4

appendix D • Summary of Key Water Quality Attainment Scenarios Table D-3. Key scenario summary of chlorophyll 

a criteria attainment.*

Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1

Segment Season Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) E3 PristineMainstem Upper Bay (CB1TF) Spring 2.51 0.88 0.21/ A/ A A A A A A A A A A A

Summer 0.39 0.36 0.39 0.37 0.35 0.37 0.41 0.37 0.38 0.40 0.40 0.36 0.30 A A

Mainstem Upper Bay (CB2OH) Spring 0.70 0.54 0.57 0.51 0.29 0.02 A 0.02 0.23 0.10 0.01 A A A A

Summer A A A A A A A A A A A A A A A

Mainstem Upper Bay (CB3MH) Spring 1.13 A A A A A A A A A A A A A A

Summer 5.71 A A A A A A A A A A A A A A

Mainstem Mid-Bay (CB4MH) Spring A A A A A A A A A A A A A A A

Summer 3.09 A A A A A A A A A A A A A A

Mainstem Mid-Bay (CB5MH) Spring 4.35 0.16 0.13 0.07 A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

Mainstem Lower Bay (CB6PH) Spring 10.38 6.97 5.50 3.18 0.40 0.22 A 0.30 0.31 0.24 0.14 A A A A

Summer 0.47 A A A A A A A A A A A A A A

Mainstem Lower Bay (CB7PH) Spring 7.81 6.62 5.31 3.31 2.06 1.79 0.18 1.94 1.87 1.75 1.62 1.52 0.03 A A

Summer A A A A A A A A A A A A A A A

Mainstem Lower Bay (CB8PH) Spring 9.28 4.16 3.45 0.01 A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

Patuxent Tidal Fresh (PAXTF) Spring 3.61 3.35 3.15 3.78 3.84 4.26 3.62 4.26 3.35 3.57 3.59 4.01 3.62 0.73 A

Summer 47.01 42.60 42.90 46.93 44.09 48.26 47.20 48.26 44.06 45.93 47.55 47.02 45.49 29.78 A

Patuxent Mid-Estuary (PAXOH) Spring 0.43 0.93 1.57 1.17 0.92 1.55 0.35 1.56 0.24 0.46 0.63 1.10 0.48 0.09 A

Summer 18.49 20.92 20.98 21.32 20.12 23.88 22.04 23.90 19.51 20.74 21.56 22.88 22.15 12.98 A

Patuxent Lower Estuary (PAXMH) Spring 2.93 0.17 0.57 0.21 A A A A A A A A A A A

Summer 11.79 5.36 5.24 2.60 1.06 0.20 A 0.22 A A A A A A A

Potomac Tidal Fresh (POTTF) Spring 0.55 0.54 0.55 0.56 0.55 0.54 0.53 0.54 0.55 0.55 0.55 0.53 0.53 A A

Summer 16.76 11.41 8.76 7.11 1.76 A A A 10.87 5.14 1.82 A A 0.97 A

Potomac Mid-Estuary (POTOH) Spring A A A A A A A A A A A A A A A

Summer 0.09 0.04 0.07 0.05 A A A A 0.01 0.01 A A A A A

Potomac Lower Estuary (POTMH) Spring 7.96 4.20 4.12 3.14 1.53 1.53 0.58 1.53 0.70 0.66 0.62 0.65 A A A

Summer 3.11 1.58 0.49 0.06 A A A A A A A A A A A

Rappahannock Tidal Fresh (RPPTF) Spring 0.66 0.54 0.07 0.12 0.24 0.22 0.10 0.22 0.25 0.25 0.25 0.22 0.22 A A

Summer 36.03 16.06 11.04 2.73 A A A A A A A A A A A

Rappahannock Mid-Estuary (RPPOH) Spring 0.61 A A A A A A A A A A A A A 0.43

Summer 2.01 A A A A A A A A A A A A A A

Rappahanock Lower Estuary (RPPMH) Spring 0.70 A A A A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

York Lower Estuary Piankatank (PIAMH) Spring 4.72 0.49 A A A A A A A A A A A A A

Summer 2.37 0.49 A A A A A A A A A A A A A

York Tidal Fresh Mattaponi (MPNTF) Spring A A A A A A A A A A A A A A A

Summer 0.49 A A A A A A A A A A A A A A

continued
▼D5

appendix D • Summary of Key Water Quality Attainment Scenarios

Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1

Segment Season Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) E3 PristineYork Mid-Estuary Mattaponi (MPNOH) Spring A A A A A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

York Tidal Fresh Pamunkey (PMKTF) Spring A A A A A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

York Mid-Estuary Pamunkey (PMKOH) Spring A A A A A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

York Lower Estuary (YRKMH) Spring 1.99 A A A A A A A A A A A A A A

Summer 2.06 0.50 0.14 A A A A A A A A A A A A

York Lower Estuary (YRKPH) Spring 12.36 2.05 0.70 0.32 0.01 A A 0.04 0.06 0.03 0.01 0.01 0.01 A A

Summer A A A A A A A A A A A A A A A

York Lower Estuary Mobjack (MOBPH) Spring 4.86 2.29 0.96 0.03 A A A A A A A A A A A

Summer 0.17 A A A A A A A A A A A A A A

James Tidal Fresh (JMSTF) Spring 18.40 4.28 5.03 1.37 A A A 0.25 0.06 0.03 0.02 0.25 0.25 A A

Summer 44.31 19.80 28.86 8.88 A A A 3.40 1.45 0.90 0.38 3.40 3.40 A A

James Mid-Estuary (JMSOH) Spring 10.13 0.54 A A A A A A A A A A A A A

Summer 0.33 0.38 0.39 0.31 A A A 0.24 0.19 0.18 0.18 0.24 0.24 A A

James Lower Estuary (JMSMH) Spring 5.20 A A A A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

James Lower Estuary (JMSPH) Spring 26.27 18.87 16.83 8.55 0.07 A A 5.89 2.25 1.92 1.58 2.78 2.67 A A

Summer A A A A A A A A A A A A A A A

Eastern Bay (EASMH) Spring A A A A A A A A A A A A A A A

Summer 9.37 0.71 0.43 0.17 0.05 A A A A A A A A A A

Choptank Mid-Estuary (CHOOH) Spring A A A A A A A A A A A A A A A

Summer 14.34 13.91 11.88 9.30 7.54 7.62 3.21 7.71 7.52 7.43 7.27 6.13 4.87 1.94 A

Choptank Lower Estuary (CHOMH2) Spring 1.01 0.03 0.01 A A A A A A A A A A A A

Summer 9.56 3.02 0.40 0.13 A A A A A A A A A A A

Choptank Lower Estuary (CHOMH1) Spring A A A A A A A A A A A A A A A

Summer 0.30 A A A A A A A A A A A A A A

Tangier Sound (TANMH) Spring 2.05 0.03 0.02 A A A A A A A A A A A A

Summer A A A A A A A A A A A A A A A

Pocomoke (POCMH) Spring 0.23 A A A A A A A A A A A A A A

Summer 3.15 0.30 0.18 A A A A A A A A A A A A

* 4/1/03, Version 15 — Changes since version 12: SAV Re-calibration, Wetlands Oxygen Demand, No Seasonal Anoxic Zone
▼D6

appendix D • Summary of Key Water Quality Attainment Scenarios Table D-4. Key scenario summary of water clarity criteria attainment at maximum depths.* Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1 Segment Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) E3 Pristine Mainstem Upper Bay (CB1TF) 75.37 75.32 75.31 74.18 65.08 54.87 41.09 54.87 73.19 68.43 64.05 52.58 50.16 58.05 0.07 Mainstem Upper Bay (CB2OH) A A A A A A A A A A A A A A A Mainstem Upper Bay (CB3MH) 0.01 A A A A A A A A A A A A A A Mainstem Mid-Bay (CB4MH) 72.64 57.26 54.58 49.45 40.38 26.26 6.86 26.37 42.45 35.89 29.94 23.20 19.97 25.98 A Mainstem Mid-Bay (CB5MH) 60.81 39.64 37.34 31.81 24.69 11.18 0.01 11.26 24.18 17.57 11.67 9.18 6.91 11.33 A Mainstem Lower Bay (CB6PH) A A A A A A A A A A A A A A A Mainstem Lower Bay (CB7PH) 43.87 36.46 35.13 32.23 29.06 12.08 0.10 10.97 28.52 20.83 14.03 10.13 11.12 24.44 A Mainstem Lower Bay (CB8PH) A A A A A A A A A A A A A A A Patuxent Tidal Fresh (PAXTF) 25.77 11.04 14.33 26.15 9.94 2.00 A 2.00 11.66 6.71 2.56 1.13 0.64 4.21 A Patuxent Mid-Estuary (PAXOH) 20.51 7.41 7.18 7.19 6.15 A A A 6.67 2.97 A A A 5.02 A Patuxent Lower Estuary (PAXMH) 36.87 30.29 29.03 27.16 25.57 18.81 6.78 18.87 25.19 21.75 18.60 18.39 17.86 22.20 A Potomac Tidal Fresh (POTTF) 75.37 75.29 75.29 74.79 71.30 46.62 18.54 46.61 75.28 74.80 70.29 44.25 53.46 65.17 0.01 Potomac Mid-Estuary (POTOH) 75.37 74.42 74.20 73.22 72.46 53.29 30.86 53.25 74.89 73.36 68.84 53.43 62.23 75.20 5.13 Potomac Lower Estuary (POTMH) 21.31 10.49 9.84 6.62 6.21 2.13 0.42 2.13 8.09 4.89 3.67 2.10 2.85 7.92 A Rappahannock Tidal Fresh (RPPTF) 1.59 0.02 0.02 0.01 0.01 A A A 0.01 0.01 0.01 A A A A Rappahannock Mid-Estuary (RPPOH) 8.66 0.69 0.69 0.19 0.04 0.02 A 0.02 0.08 0.04 0.04 0.02 0.02 0.03 A Rappahanock Lower Estuary (RPPMH) 39.74 34.67 33.49 32.44 31.11 24.62 7.85 24.67 30.94 29.75 28.07 24.48 24.30 28.82 A York Lower Estuary Piankatank (PIAMH) 70.67 45.30 39.33 20.78 13.17 6.76 2.60 7.72 10.88 8.73 6.76 5.83 4.10 1.35 A York Tidal Fresh Mattaponi (MPNTF) A A A A A A A A A A A A A A A York Mid-Estuary Mattaponi (MPNOH) 11.34 0.77 0.26 0.26 0.14 0.02 A 0.02 0.26 0.26 0.26 0.02 0.02 0.08 A York Tidal Fresh Pamunkey (PMKTF) A A A A A A A A A A A A A A A York Mid-Estuary Pamunkey (PMKOH) 17.63 3.48 2.67 1.94 0.96 A A A 2.20 1.39 0.67 A A 0.30 A York Lower Estuary (YRKMH) 17.09 7.60 6.54 5.42 3.57 A A A 4.32 2.92 1.83 A A 1.60 A York Lower Estuary (YRKPH) 9.10 4.06 3.37 2.69 2.11 0.01 A 0.05 2.31 1.70 1.23 0.03 0.02 1.22 A

York Lower Estuary Mobjack (MOBPH) 62.96 53.25 51.58 48.73 45.18 33.89 10.62 34.31 45.02 40.19 35.19 33.55 32.61 40.34 AJames Tidal Fresh (JMSTF) A A A A A A A A A A A A A A AJames Mid-Estuary (JMSOH) A A A A A A A A A A A A A A AJames Lower Estuary (JMSMH) A A A A A A A A A A A A A A AJames Lower Estuary (JMSPH) 4.95 0.70 0.36 0.02 A A A A A A A A A A AEastern Bay (EASMH) 65.24 35.47 32.16 25.01 9.70 2.03 0.42 2.03 9.43 3.79 2.10 1.96 1.84 2.87 AChoptank Mid-Estuary (CHOOH) 5.47 2.27 2.18 1.92 1.63 0.15 A 0.15 1.63 1.28 1.03 0.15 0.15 1.22 AChoptank Lower Estuary (CHOMH2) 33.64 8.53 5.10 1.46 A A A A A A A A A A AChoptank Lower Estuary (CHOMH1) 67.17 49.00 45.46 39.17 32.68 14.89 1.33 15.13 33.24 26.34 16.45 12.04 9.25 17.62 ATangier Sound (TANMH) 74.55 71.19 70.37 68.17 65.11 46.54 3.44 46.06 64.80 58.65 49.61 45.20 45.08 60.55 APocomoke (POCMH) 41.96 22.15 20.19 14.46 10.02 0.02 A 0.01 9.25 1.43 0.35 0.01 0.02 20.24 A* 4/1/03, Version 15 — Changes since version 12: SAV Re-calibration, Wetlands Oxygen Demand, No Seasonal Anoxic Zone
▼D7

appendix D • Summary of Key Water Quality Attainment Scenarios

Table D-5. Key scenario summary of water clarity criteria attainment at mid depths.*

Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1

Segment Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) E3 PristineMainstem Upper Bay (CB1TF) 71.62 55.46 50.88 40.18 23.13 16.03 10.03 16.03 35.95 31.08 26.85 14.43 12.69 13.42 0.01Mainstem Upper Bay (CB2OH) A A A A A A A A A A A A A A AMainstem Upper Bay (CB3MH) A A A A A A A A A A A A A A AMainstem Mid-Bay (CB4MH) 37.20 16.20 12.48 7.19 2.78 0.30 0.01 0.32 3.78 2.15 1.08 0.04 0.01 0.38 AMainstem Mid-Bay (CB5MH) 16.07 0.01 0.01 A A A A A A A A A A A AMainstem Lower Bay (CB6PH) A A A A A A A A A A A A A A AMainstem Lower Bay (CB7PH) 8.13 1.99 1.33 0.09 0.01 A A A 0.01 0.01 A A A A AMainstem Lower Bay (CB8PH) A A A A A A A A A A A A A A APatuxent Tidal Fresh (PAXTF) 0.20 A A A A A A A A A A A A A APatuxent Mid-Estuary (PAXOH) A A A A A A A A A A A A A A APatuxent Lower Estuary (PAXMH) 17.55 13.63 13.46 12.88 11.77 5.55 A 5.56 11.01 8.58 5.26 4.80 3.69 8.71 APotomac Tidal Fresh (POTTF) 74.97 73.15 73.03 68.56 55.27 25.35 1.25 25.35 72.82 68.13 50.02 23.35 29.40 42.39 APotomac Mid-Estuary (POTOH) 73.51 64.27 63.04 54.36 49.75 31.74 19.24 31.70 65.41 52.99 40.17 31.81 35.88 67.30 APotomac Lower Estuary (POTMH) 4.53 1.02 0.98 0.23 0.09 0.09 A 0.09 0.61 0.09 0.09 0.03 0.03 0.20 ARappahannock Tidal Fresh (RPPTF) A A A A A A A A A A A A A A ARappahannock Mid-Estuary (RPPOH) 0.03 0.01 0.01 A A A A A A A A A A A ARappahanock Lower Estuary (RPPMH) 27.17 17.92 16.50 15.60 14.41 6.64 A 6.69 14.43 13.30 12.22 6.60 6.49 12.79 AYork Lower Estuary Piankatank (PIAMH) 29.01 9.78 3.33 0.87 0.20 0.20 0.06 0.20 0.20 0.20 0.20 0.20 0.06 0.06 AYork Tidal Fresh Mattaponi (MPNTF) A A A A A A A A A A A A A A AYork Mid-Estuary Mattaponi (MPNOH) 0.02 A A A A A A A A A A A A 0.01 AYork Tidal Fresh Pamunkey (PMKTF) A A A A A A A A A A A A A A AYork Mid-Estuary Pamunkey (PMKOH) A A A A A A A A A A A A A A AYork Lower Estuary (YRKMH) A A A A A A A A A A A A A A AYork Lower Estuary (YRKPH) 0.61 A A A A A A A A A A A A A AYork Lower Estuary Mobjack (MOBPH) 32.89 17.42 14.74 12.07 8.83 0.34 A 0.41 8.18 3.74 0.70 0.27 0.22 3.30 AJames Tidal Fresh (JMSTF) A A A A A A A A A A A A A A AJames Mid-Estuary (JMSOH) A A A A A A A A A A A A A A AJames Lower Estuary (JMSMH) A A A A A A A A A A A A A A AJames Lower Estuary (JMSPH) A A A A A A A A A A A A A A AEastern Bay (EASMH) 24.96 2.61 1.91 0.54 A 0.01 0.01 0.01 0.07 0.01 0.01 0.01 0.01 0.01 AChoptank Mid-Estuary (CHOOH) A A A A A A A A A A A A A A AChoptank Lower Estuary (CHOMH2) 0.24 A A A A A A A A A A A A A AChoptank Lower Estuary (CHOMH1) 30.84 11.80 8.79 2.58 0.07 0.01 0.01 0.01 0.57 0.01 0.01 0.01 0.01 0.01 ATangier Sound (TANMH) 49.18 38.38 36.38 32.93 28.44 11.18 A 11.27 28.34 20.62 13.23 11.03 10.45 23.04 APocomoke (POCMH) 7.41 0.43 0.18 A A A A A A A A A A A A* 4/1/03, Version 15 — Changes since version 12: SAV Re-calibration, Wetlands Oxygen Demand, No Seasonal Anoxic Zone
Table D-6. Key scenario summary of water clarity criteria attainment at low depths.*

Progress Tier 3 Tier 3 Tier 3 Opt4 Confirm Confirm Confirm Allocation Opt1

Segment Observed 2000 Tier 1 Tier 2 (181) + 20% + 50% (188) (175) + 10 + 20 (175) (160) E3 PristineMainstem Upper Bay (CB1TF) 21.88 11.93 8.43 1.74 0.04 0.04 0.04 0.04 0.70 0.16 0.07 0.03 0.02 0.01 AMainstem Upper Bay (CB2OH) A A A A A A A A A A A A A A AMainstem Upper Bay (CB3MH) A A A A A A A A A A A A A A AMainstem Mid-Bay (CB4MH) 0.07 A A A A A A A A A A A A A AMainstem Mid-Bay (CB5MH) A A A A A A A A A A A A A A AMainstem Lower Bay (CB6PH) A A A A A A A A A A A A A A AMainstem Lower Bay (CB7PH) A A A A A A A A A A A A A A AMainstem Lower Bay (CB8PH) A A A A A A A A A A A A A A APatuxent Tidal Fresh (PAXTF) A A A A A A A A A A A A A A APatuxent Mid-Estuary (PAXOH) A A A A A A A A A A A A A A APatuxent Lower Estuary (PAXMH) 0.02 A A A A A A A A A A A A A APotomac Tidal Fresh (POTTF) 54.15 30.87 29.00 11.69 3.97 0.01 0.01 0.01 18.35 6.17 2.39 0.01 0.01 0.02 APotomac Mid-Estuary (POTOH) 40.89 26.46 25.09 20.64 17.30 8.73 2.07 8.75 23.85 17.83 10.82 6.64 7.97 14.73 APotomac Lower Estuary (POTMH) A A A A A A A A A A A A A A ARappahannock Tidal Fresh (RPPTF) A A A A A A A A A A A A A A ARappahannock Mid-Estuary (RPPOH) A A A A A A A A A A A A A A ARappahanock Lower Estuary (RPPMH) 2.98 0.13 A A A A A A A A A A A A AYork Lower Estuary Piankatank (PIAMH) 0.87 0.01 0.01 0.01 0.01 A A A A A A A A A AYork Tidal Fresh Mattaponi (MPNTF) A A A A A A A A A A A A A A AYork Mid-Estuary Mattaponi (MPNOH) A A A A A A A A A A A A A A AYork Tidal Fresh Pamunkey (PMKTF) A A A A A A A A A A A A A A AYork Mid-Estuary Pamunkey (PMKOH) A A A A A A A A A A A A A A AYork Lower Estuary (YRKMH) A A A A A A A A A A A A A A AYork Lower Estuary (YRKPH) A A A A A A A A A A A A A A AYork Lower Estuary Mobjack (MOBPH) A A A A A A A A A A A A A A AJames Tidal Fresh (JMSTF) A A A A A A A A A A A A A A AJames Mid-Estuary (JMSOH) A A A A A A A A A A A A A A AJames Lower Estuary (JMSMH) A A A A A A A A A A A A A A AJames Lower Estuary (JMSPH) A A A A A A A A A A A A A A AEastern Bay (EASMH) 0.06 0.01 A A A A A A A A A A A A AChoptank Mid-Estuary (CHOOH) A A A A A A A A A A A A A A AChoptank Lower Estuary (CHOMH2) A A A A A A A A A A A A A A AChoptank Lower Estuary (CHOMH1) 0.01 A A A A A A A A A A A A A ATangier Sound (TANMH) 4.53 1.86 1.32 0.59 0.41 A A A 0.41 A A A A A APocomoke (POCMH) A A A A A A A A A A A A A A A* 4/1/03, Version 15 — Changes since version 12: SAV Re-calibration, Wetlands Oxygen Demand, No Seasonal Anoxic Zone

appendix D • Summary of Key Water Quality Attainment Scenarios

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