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SECTION 6. CHESAPEAKE BAY TMDL 

DEVELOPMENT

This section discusses the critical elements of the Chesapeake Bay TMDL, many of which 

benefitted from joint collaboration and decision making by EPA and its partners. The following 

subsections discuss the specific approaches adopted to address specific technical aspects of the 

Chesapeake Bay TMDL:

6.1-Establishing Model Parameters

6.2-Interpreting Model Results

6.3-Establishing Allocation Rules

6.4-Assessing Attainment of Proposed Amended Chesapeake Bay WQS

6.5-Assessing Attainment of Current Chesapeake Bay WQS

6.6-Setting Draft Basin-jurisdiction Allocations

The Chesapeake Bay Program partners initiated discussions related to the technical aspects of the 

Chesapeake Bay TMDL starting at the September 2005 Reevaluation Workshop sponsored by 

what would become the partnership’s Water Quality Steering Committee (Chesapeake Bay 

Reevaluation Steering Committee 2005). Over the next 5 years, EPA and its partners, in

particular members of the Water Quality Steering Committee (2005–2008) and then the Water 

Quality Goal Implementation Team (WQGIT) (2009–present) systematically evaluated and 

agreed on approaches to address multiple technical aspects related to developing the Bay TMDL.

EPA, together with its seven watershed jurisdictional partners, developed approaches and 

methodologies to address a number of factors and then applied those approaches and 

methodologies in developing the Bay TMDL. A multitude of policy, programmatic, technical, 

and model setup/application issues were addressed through this collaborative process.

6.1 Establishing Model Parameters

The first step in the process was to establish the key parameters for the model. Those key 

parameters are (1) the hydrologic period, or the period that is representative of typical conditions 

for the waterbody; (2) the critical conditions, or the selection of a set of years that represent the 

range of conditions affecting attainment of the Bay WQS; (3) the WQS protective of all the Bay 

habitats and the aquatic life inhabiting those habitats; and (4) the seasonal variation in water 

quality conditions and the factors (temperature, precipitation, wind, and such) that directly affect 

those conditions.

6.1.1 Hydrologic Period

The hydrologic period for modeling purposes is the period that represents the long-term 

hydrologic conditions for the waterbody. This is important so that the Bay models can simulate 

local long-term conditions for each area of the Bay watershed and the Bay’s tidal waters so that 

no one area is modeled with a particularly high or low loading, an unrepresentative mix of point 

and nonpoint sources or extremely high or low river flow. The selection of a representative 

hydrologic averaging period ensures that the balance between high and low river flows, the 
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resultant point and nonpoint source loadings areas across the Bay watershed and Bay tidal waters 

are appropriate. That provides the temporal boundaries on the model scenario runs from which 

the critical period is determined.

To identify the appropriate hydrologic period, EPA analyzed decades of historical streamflow 

data. It is important to identify representative hydrology to be able to compare various 

management scenarios through the Bay models. In the course of evaluating options for the 

TMDL, EPA and the partnership ran numerous modeling scenarios through the Bay Watershed 

and the Bay Water Quality Sediment Transport models with varying levels of management 

actions (such as land use, BMPs, wastewater treatment technologies, and so on) held constant 

against an actual record of rainfall and meteorology to examine how those management actions 

perform over a realistic distribution of simulated meteorological conditions. It was important that 

this record of precipitation and meteorology, or hydrologic period be representative of local 

long-term conditions for each area of the watershed so that no one area is modeled with a 

particularly high or low loading or an unrepresentative mix of point and nonpoint sources. 

Because of the long history of monitoring throughout the Chesapeake Bay watershed, the CBP 

partners were in the position of selecting a period for model application representative of typical 

hydrologic conditions of the 21 contiguous model simulation years—1985 to 2005. Two extreme 

conditions occurred during the 21-year model simulation period for the Chesapeake Bay models: 

Tropical Storm Juan in November 1985, and the Susquehanna Big Melt of January 1996. In the 

Chesapeake Bay region, Tropical Storm Juan was a 100-year storm primarily affecting the 

Potomac and James River basins. No significant effect on SAV or DO conditions was reported in 

the aftermath of Tropical Storm Juan. In the case of the Susquehanna Big Melt in January 1996, 

a warm front brought rain to the winter snow pack in the Susquehanna River basin and caused an 

ice dam to form in the lower reaches of the river. No significant effects on SAV or DO were 

reported from this 1996 extreme event, likely because of the time of year when it occurred (late 

winter).

From the 21-year period, EPA selected a contiguous 10-year hydrologic period because a 10-

year period provides enough contrast in different hydrologic regimes to better examine and 

understand water quality response to management actions over a wide range of wet and dry 

years. Further, a 10-year period is long enough to be representative of the long term flow 

(Appendix F). Finally, a 10-year period is not overly burdensome on computational resources, 

particularly for the Bay WQSTM, which required high levels of parallel processing for each 

management scenario. The annualized Bay TMDL allocations are expressed as an average 

annual load over the 10-year hydrologic period.

EPA then determined which 10-year period to use by examining the statistics of long-term flow 

relative to each 10-year period at nine USGS gauging stations that discharge to the Bay 

(Appendix F). All the contiguous 10-year hydrologic periods from 1985 to 2005 appeared to be 

suitable because clear quantifiable assessments showed that all the contiguous 10-year periods 

have relatively similar distributions of river flow.

EPA selected the 10-year hydrologic assessment period from 1991 to 2000 from the 21-year flow 

record for the following reasons:
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It is one of the 10-year periods that is closest to an integrated metric of long-term flow.

Each basin has statistics for this period that were particularly representative of the longterm flow.

It overlaps several years with the previous 2003 tributary strategy allocation assessment 

period (1985–1994), which facilitated comparisons between the two assessments.

It incorporates more recent years than the previous 2003 tributary strategy allocation 

assessment period (1985–1994).

It overlaps with the Bay water quality model calibration period (1993-2000), which is 

important for the accuracy of the model predictions.

It encompasses the 3 year critical period (1993–1995) for the Chesapeake Bay TMDL as 

explained in Section 6.1.2 below.

More detail about the hydrologic period is provided in Appendix F.

6.1.2 Critical Conditions

TMDLs are to identify the loadings necessary to achieve applicable WQS. The allowable loading 

is often dependent on key environmental factors, most notably wind, rainfall, streamflow, 

temperature, and sunlight. Because these environmental factors can be highly variable, EPA 

regulations require that in establishing the TMDL, the critical conditions (mostly environmental 

conditions as listed above) be identified and employed as the design conditions of the TMDL (40 

CFR 130.7(c)(1)).

When TMDLs are developed using supporting watershed models, such as the Chesapeake Bay 

TMDL, selecting a critical period for model simulation is essential for capturing important 

ranges of loading/waterbody conditions and providing the necessary information for calculating 

appropriate TMDL allocations that will meet WQS. Because the WQS applicable to this TMDL 

are assessed over 3-year periods, the critical period is defined as the 3-year period within the 

1991–2000 hydrologic period that meets the above description (USEPA 2003a).

Critical Conditions for DO

In the Chesapeake Bay, as flow and nutrient loads increase, DO and water clarity levels decrease 

(Officer 1984). Therefore, the critical period for evaluation of the DO and water clarity WQS are 

based on identifying high-flow periods. Those periods were identified using statistical analysis of 

flow data as described below and in detail in Appendix G.

For the Bay TMDL, EPA conducted an extensive analysis of streamflow of the major tributaries 

of the Chesapeake Bay as the primary parameter representing critical conditions. In this analysis, 

it was observed that high streamflow most strongly correlated with the worst DO conditions in 

the Bay. This is logical because most of the nutrient loading contributing to low DO comes from 

nonpoint sources, whose source loads are driven by rainfall and correlate well to rainfall and 

higher streamflows.

Because future rainfall conditions cannot be predicted, EPA analyzed rainfall from past decades 

to derive a critical rainfall/streamflow condition that would be used to develop the allowable 

loadings in the TMDL. The initial analysis concluded that the years 1996–1998 represented the 
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6-4 September 24, 2010

highest streamflow period for the Chesapeake Bay drainage during the 1991–2000 hydrology 

period. However, it was later discovered that this 3-year period represented an extreme high-flow 

condition that was inappropriate for the development of the TMDL—the high-flow period would 

generally occur once every 20 years (Appendix G). For that reason, EPA selected the second 

highest flow period of 1993–1995 as the critical period. The 1993–1995 critical period 

experienced streamflows that historically occurred about once every 10 years, which is much 

more typical of the return frequency for hydrological conditions employed in developing 

TMDLs. Thus, while the modeling for the Bay TMDL consists of the entire hydrologic period of 

1991–2000, EPA used the water quality conditions during the 1993–1995 critical period to 

determine attainment with the Bay jurisdictions’ DO WQS.

Critical Conditions for Chlorophyll a

To assess attainment of the numeric chlorophyll a criteria that apply to Virginia’s tidal James 

River and the District of Columbia’s tidal Potomac and Anacostia rivers, EPA conducted a 

similar analysis of streamflow. The analysis showed no strong correlation between streamflow 

and chlorophyll a conditions. As a result, EPA assessed numeric chlorophyll a attainment using 

all eight of the 3-year criteria assessment periods (e.g., 1991–1993, 1992–1994) that occur within 

the hydrologic period of 1991–2000. Detailed technical documentation of this assessment is 

provided in Appendix F.

Critical Conditions for Water Clarity and SAV

In the Chesapeake Bay, the water clarity and SAV WQS are applied Bay-wide. Further, sediment 

has similar loading attributes as does nutrients (higher loads under higher streamflow). 

Therefore, the critical period for evaluating attainment of the SAV and water clarity WQS is 

based on identifying high-flow periods, just as it is for DO.

As discussed above, because the WQS applicable to the Bay TMDL are assessed over 3-year 

periods, the critical period is defined as the 3-year period within the 1991–2000 hydrologic 

period that represents the range of critical conditions affecting attainment of the Bay WQS

(USEPA 2003a). Because the critical period for both DO and water clarity/SAV is based on 

identifying high-flow periods, EPA used the same analysis as it did for nutrients. As a result of 

the analysis, EPA determined that the same critical period used for DO was appropriate for water 

clarity/SAV. As with nutrients, detailed technical documentation is provided in Appendix F.

6.1.3 Water Quality Standards

A TMDL must allocate allowable loads to the contributing point and nonpoint sources so that all 

applicable WQS are attained for each of these segments (CWA section 303(d)(1)(C)). The 

applicable Bay WQS and the proposed amended WQS are summarized here and discussed in 

greater detail in Section 3.

Proposed Amendments to the Jurisdictions’ Bay Water Quality Standards

During the water quality modeling and data analysis process to establish the Chesapeake Bay 

TMDL, it became apparent that a small number of the 92 tidal segments would not attain the 

applicable WQS even when nitrogen and phosphorus allocations consistent with the 

longstanding jurisdictions’ tributary strategies were achieved.
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6-5 September 24, 2010

Using modeling and other informational lines of evidence, EPA concluded that the water quality 

in these few segments did not respond to the nutrient or sediment load reductions as expected 

because of the following:

The influence of pycnoclines, which limit re-aeration of the bottom waters prevent 

attainment of the open-water DO criteria.

Limitations in the ability of the Bay Water Quality Model to adequately simulate water 

quality responses to nutrient reduction in certain, small, narrow segments.

The adoption of SAV restoration acreage criteria that were derived using a methodology 

that was inconsistent with that used in the vast majority of other Chesapeake Bay segments.

Subsequent modeling evaluations of alternative allocation scenarios concluded that for all 92 

segments to meet the applicable Maryland, Virginia, Delaware, and District of Columbia WQS, 

reductions from both point and nonpoint sources throughout the Bay watershed would need to be 

established at the E3 (Everything, Everywhere, Everyone) annual level of 141 million pounds of 

nitrogen and 8.5 million pounds of phosphorus (Appendix J). The E3 scenario represents a best 

case possible situation, where all possible BMPs and available control technologies are applied 

to land, given human and animal populations and wastewater treatment facilities are represented 

at highest technologically achievable levels of treatment regardless of costs. The Bay-wide 

loading target that otherwise would be distributed among all seven jurisdictions would be 187 

million pounds of nitrogen and 12.5 million pounds of phosphorus. Thus, to attain WQS in these 

few tidal segments would require an additional Bay-wide reduction in nitrogen and phosphorus 

of 25 percent and 33 percent, respectively.

To address these needed water quality standards refinements, Maryland, Virginia, and the 

District of Columbia are each proposing amendments to their respective Chesapeake Bay WQS 

regulations directly relevant to the Bay TMDL. Delaware has already adopted the EPApublished 2010 Bay criteria addendum into its WQS regulations by reference.

Current Chesapeake Bay Water Quality Standards

As discussed above, the allocations required to meet currently applicable Chesapeake Bay WQS, 

as required by the CWA and federal regulations, are not reflective of EPAs latest scientific 

assessment of appropriate criteria for the Bay, As a result, Maryland, Virginia, and the District of 

Columbia are in the process of amending their respective WQS regulations.

In the time between the issuance of this draft TMDL and the date of completing the final Bay

TMDL, EPA will closely monitor the progress of the jurisdictions in their WQS adoptions. As 

revisions occur over that time frame, EPA will conduct additional modeling runs necessary to 

establish the allocations that would result in full attainment of the applicable WQS in place as of 

December 31, 2010. It is possible, however, that the amendments will not be effective before 

establishing the final Bay TMDL on December 31, 2010. Therefore, EPA is also providing for 

public comment a Bay TMDL based on the jurisdictions’ current Bay WQS, as required by the 

CWA and federal regulations.

6.1.4 Seasonal Variation

A TMDL analysis must consider the seasonal variations within the watershed (CWA 

303(d)(1)(C); 40 CFR 130.7). The Chesapeake Bay TMDL inherently considers all seasons 
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6-6 September 24, 2010

through the use of a continuous 10-year simulation period that captures seasonal precipitation on 

a year-to-year basis throughout the entire watershed. Furthermore, the critical periods selected 

for this TMDL, being a minimum of 3 consecutive years provide further assurance that the 

seasonality of the bay loading and other dynamics are properly addressed in this TMDL. In this 

way, the TMDL simulations ensure attainment of WQS during all seasons.

Jurisdictions’ Bay Water Quality Standards

In the case of the Chesapeake Bay TMDL, the Chesapeake Bay WQS adopted by the four tidal 

Bay jurisdictions are biologically based and designed to be protective of Chesapeake living 

resources, including full consideration of their unique seasonal-based conditions (see Section 3) 

(USEPA 2003a, 2003c). To assess the degree of WQS achievement using the Bay Water Quality 

Model, an overlay of the time and space dimensions are simulated to develop an assessment that 

is protective of living resources with consideration of all critical periods within the applicable 

seasonal period (USEPA 2007a).

The same approach of considering the time and space of the critical conditions is applied in the 

assessment of the WQS achievement with observed monitoring data. Ultimately, the time and 

space of water quality exceedances are assessed against a reference curve derived from healthy 

living resource communities to determine the degree of WQS achievement (USEPA 2007a).

Model Simulation Supporting Seasonal Variation

The suite of Chesapeake Bay Program models being used to establish the Chesapeake Bay 

TMDL—Bay Airshed, Bay Watershed, Bay Water Quality, Bay Sediment Transport, Bay filter 

feeders—all simulate the 10-year period and account for all storm events, high flows/low flows, 

and resultant nutrient and sediment loads across all four seasons. The full suite of Chesapeake 

Bay models operate on at least an hourly time-step and often at finer time-steps for the Bay 

Airshed Model and the Bay Water Quality Model (see Section 5). Therefore, through proper 

operation of the suite of Bay models, the Chesapeake Bay TMDL considers all seasons and 

within season variations through the use of a continuous 10-year simulation period (see Section 

6.1.1).

Seasonal Variations Known and Addressed through Annual Load Reductions

A key aspect of Chesapeake Bay nutrient dynamics is that annual loads are the most important 

determinant of Chesapeake Bay water quality response (USEPA 2004c). Chesapeake Bay 

physical and biological processes can be viewed as integrating variations in nutrient and 

sediment loads over time. The integration of nitrogen, phosphorus, and sediment loads over time 

reduces load fluctuations in the Chesapeake Bay. Bay water quality responds to overall loads on 

a seasonal to annual scale, while showing little response to daily or monthly variations within an 

annual load.

Numerous Chesapeake studies show that annually based wastewater treatment nutrient 

reductions are sufficient to protect Chesapeake Bay water quality (Linker 2003, 2005). The 

seasonal aspects of the jurisdictions’ Chesapeake Bay WQS are due to the presence of the living 

resources being protected, but annual nutrient and sediment load reductions are most important 

to achieve and maintain the seasonal water quality criteria, some of which span multiple 

seasons—open-water, shallow-water bay grass, migratory spawning and nursery (USEPA 2003a, 

2003c).
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6-7 September 24, 2010

6.2 Interpreting Model Results

The WQSTM is used to predict water quality conditions for the various loading scenarios 

explored. It is necessary to compare these model results with the operative WQS to determine 

compliance with the standards. This section describes the process by which model results are 

compared to WQS to determine attainment.

6.2.1 Criteria Assessment Procedures

Determining Attainment of DO and Chlorophyll a Criteria

In general, to determine management scenarios that achieved WQS, EPA ran model scenarios 

representing different nutrient and sediment loading conditions using the Bay Watershed Model. 

EPA then took the resultant model scenario output and provided input into the Bay Water 

Quality Model to evaluate the response of critical water quality parameters: specifically DO, 

water clarity, underwater Bay grasses and chlorophyll a.

To determine whether the different loading scenarios met the Bay DO and chlorophyll a WQS, 

EPA compared the Bay Water Quality Model’s simulated tidal water quality response for each 

variable to the corresponding observed monitoring values collected during the same 1991-2000 

hydrological period. In other words, the Bay Water Quality Model was used primarily to 

estimate the change in water quality that would result from various loading scenarios with the 

model-simulated change in water quality then is applied to the actual observed calibration 

monitoring data. In its simplest terms, the following steps were taken to apply the modeling 

results to predict Bay DO and chlorophyll a WQS attainment:

1. Using the 1991 to 2000 hydrologic period, calibrate the Bay Water Quality Model to Bay 

water quality monitoring data.

2. Run a model simulation for a given loading scenario (usually a management scenario 

resulting in lower loads relative to the calibration scenario) through the Bay Watershed 

Model and Bay Water Quality Model.

3. Determine the model simulated change in water quality from the calibration scenario to 

the given loading scenario.

4. Apply the change in water quality as predicted by the Bay Water Quality Model to the 

actual historical water quality monitoring data used for calibration and evaluate 

attainment based on this scenario modified data set.

5. If WQS are met, then allocations are used for TMDL. If WQS are not met, reduce and 

readjust loads to meet WQS.

For a full discussion of this procedure, see Appendix I and the original report titled 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).

Determining Attainment of Water Clarity and SAV Water Quality Criteria

The Chesapeake Bay SAV restoration acreage and resultant WQS are based on achieving SAV 

acreage goals that were based on the highest SAV acreage ever observed over a 40-year to more 

than 70-year historical record depending on the records available for each basin (USEPA 2003a; 

2003c). Bay-wide, the SAV restoration goal is 185,000 acres.
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6-8 September 24, 2010

The linked SAV and water clarity WQS are unique in some respects. Rather than covering the 

entire Bay as the DO WQS does, the SAV-water clarity WQS applies in only a narrow ribbon of 

shallow water habitat along the shoreline in depths of 2 meters or less. That presents certain 

challenges for the Chesapeake Bay model simulation and monitoring systems, both of which 

have long been more oriented toward the open waters of the Chesapeake Bay and its tidal 

tributaries. Scientific understanding of the transport, dynamics, and fate of sediment in the 

shallow waters of the Chesapeake Bay and understanding and simulating all the factors 

influencing SAV growth continues to develop. Appendix H provides more details of the 

Chesapeake Bay WQSTM-based combined SAV-water clarity attainment assessment procedures 

and developing the sediment allocations.

The combined SAV/water clarity WQS can be achieved in one of three ways (see Section 3.4.3). 

First, as SAV acreage is the primary WQS, the WQS can be achieved by the number of SAV 

acres measured by way of aerial surveys—the method that is primarily used in CWA section 

303(d) assessments. Second, the WQS can be achieved by the number of water clarity acres 

(divided by a factor of 2.5) added to the measured acres of SAV. Third, water clarity criteria 

attainment can be measured on the basis of the cumulative frequency distribution (CFD) 

assessment methodology using shallow-water monitoring data.

Although SAV responds to both nutrient and sediment loads, DO and chlorophyll a primarily 

respond only to nutrient loads. Because of that hierarchy of WQS response, the strategy 

developed to achieve WQS was to first set the nutrient allocation for achieving all the DO and 

chlorophyll a WQS in all 92 segments, and then set additional sediment reductions where needed 

to achieve the SAV/water clarity WQS. That strategy is augmented by management actions in 

the watershed to reduce nutrient and sediment loads.

Just as the SAV resource is responsive to nutrient and sediment loads, many management actions 

in the watershed that reduce nutrients also reduce sediment loads. Examples include conservation 

tillage, farm plans, riparian buffers, and other key practices. The estimated ancillary sediment 

reductions from nutrient reductions needed at the level of the proposed amended WQS-based 

allocation scenario are estimated to be about 40 percent less than 1985 sediment loads and 25 

percent less than current (2009) load estimates. The sediment reductions associated with the 

nutrient controls necessary to achieve the basin-jurisdiction target loads provided on July 1, 

2010, is provided in Table 6-1.

Table 6-1. Tributary strategy and proposed amended Bay WQS-based allocation scenarios TSS

loads (millions of pounds) by jurisdiction

Jurisdiction Tributary strategy Allocation scenario—proposed WQS

Maryland 1,195 1, 118

Pennsylvania 2,004 1,891

Virginia 2,644 2,434

District of Columbia 10 10

New York 310 291

West Virginia 248 240

Delaware 55 55

Total 6,467 6,040
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6-9 September 24, 2010

Using the Bay Water Quality Model, the SAV-water clarity WQS were assessed by starting with 

measured area of SAV in each Bay segment from the 1993–1995 critical period. On the basis of 

regressions of SAV versus load, the estimated SAV area because of a particular nutrient or 

sediment load reduction was estimated as described in Appendix H. Then the estimated water 

clarity acres from the Bay Water Quality Model were added in after adjustment by a factor of 2.5 

to convert to the water clarity acres to water clarity equivalent SAV acres (Appendix H). Finally 

the water clarity equivalent SAV acres were added to the regression-estimated SAV acres and 

compared to the Bay segment-specific SAV WQS.

Note that when assessing attainment using monitoring data, only the SAV acres measurement is 

generally used because the number of Bay segments assessed with shallow-water clarity data are 

still limited. When projecting attainment using the Bay Water Quality model, the extrapolated 

measured SAV acres are added to the model-projected water clarity-equivalent SAV acres to 

determine total SAV acres (Appendix H).

6.2.2 Addressing Reduced Sensitivity to Load Reductions at Low 

Nonattainment Percentages

The Chesapeake Bay water quality criteria that the jurisdictions adopted into their respective 

WQS regulations provide for allowable exceedances of each set of DO, water clarity, SAV, and 

chlorophyll a criteria defined through application of a biological or default reference curve 

(USEPA 2003a). Figure 6-1 depicts that concept in yellow as allowable exceedance of the 

criterion concentration.

To compare model results with the WQS, the Bay Water Quality Model results for each scenario 

and for each modeled segment are analyzed to determine the percent of time and space that the 

modeled DO results exceed the allowable concentration. For any modeled result where the 

exceedance in space and time (shown in Figure 6-1) as the red line) exceeds the allowable 

exceedance (shown in Figure 6-1 as the yellow area), that segment is considered in 

nonattainment. The amount of nonattainment is shown in the figure as the area in white between 

the red line and the yellow area and is typically displayed in model results as percent of 

nonattainment for that segment. The amount of nonattainment is reported to the whole number 

percent.
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6-10 September 24, 2010

Source: USEPA 2003a

Figure 6-1. Graphic comparison of allowable exceedance compared to actual exceedance.

Figure 6-2 below displays Bay Water Quality Model results showing percent nonattainment of 

the 30-day mean open-water DO criterion for various basinwide loading levels of the Maryland 

portion of the lower central Chesapeake Bay segment CB5MH_MD.

Source: Appendix Q.

Figure 6-2. Example of DO criteria nonattainment results from a wide range of nutrient load reduction model 

scenarios.

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9% DISSOLVED OXYGEN NON ATTAINMENT 

324TN

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309TN

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248TN

16.6TP

200TN

15TP

191TN

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190TN

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179TN

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170TN

11.3TP

141TN

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58TN

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85TN

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113TN

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6% MODEL RESULTS CB5 - MD

BASIN WIDE LOAD (MILLION POUNDS PER YEAR)
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As can be seen in Figure 6-2, there is a notable improvement in the percent nonattainment as the 

loads are reduced until approximately 1 percent nonattainment. At a loading level of 190 million 

pounds per year TN and 12.6 million pounds per year TP, the 1 percent nonattainment is 

persistent through consecutive reductions in loading levels and remains consistent until a loading 

level of 58 million pounds per year TN and 4.4 million pounds per year of TP is reached. While 

this is one of the more extreme examples of persistent levels of 1 percent nonattainment, this 

general observation of persistent nonattainment at 1 percent is fairly common to the Bay Water 

Quality Model results (Appendix I).

This empirical observation is likely based on the geometry of the time and space-based 

assessment of the Bay WQS. An initial reduction made in the nutrient loads would be associated 

with an increase in attaining the WQS as shown in the green line in Figure 6-3. As reductions 

move toward attainment, the move toward the area of allowable criteria exceedance as shown by 

the light green line in Figure 6-3. Note that even though the reduced nutrient loads under the 

scenario represented by the light green line continue to reduce the time and space of WQS 

nonattainment, different rates of improvement exist at different portions of the curve. In this 

hypothetical example, the scenario represented by the light green line has reduced the time of 

exceedance well below the area of allowable exceedance, but the space component still showed a 

very low level of nonattainment.

The observation of a small, yet persistent percentage of model projected DO criteria 

nonattainment across a wide range of segments and designated uses, all of which are responding 

to nutrient load reductions, is an outcome of the criteria assessment methodology. Because this 

has been observed in a wide variety of different segments across all three designated uses—

open-water, deep-water, and deep-channel—nonattainment percentages projected by the model 

rounded to 1 percent were considered to be in attainment for a segment’s designated use for 

purposes of developing the Chesapeake Bay TMDL (Appendix I).
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Figure 6-3. A graphical representation of how the persistent 1% nonattainment may arise in the criteria 

assessment of the Chesapeake Bay WQS.

A separate validation of the findings described above was undertaken to confirm that 1 percent 

was the correct percentage below which the designated use segment could be considered in 

attainment and is provided in Appendix L.

6.2.3 Margin of Safety

Under EPA’s regulations, a TMDL is mathematically expressed as

TMDL = ∑ WLA + ∑ LA + MOS

where

TMDL is the total maximum daily load for the water segment

WLA is the wasteload allocation, or the load allocated to point sources

LA is the load allocation, or the load allocated to nonpoint sources

MOS is the margin of safety to account for any uncertainties in the supporting data and the 

model

The margin of safety (MOS) is the portion of the pollutant loading reserved to account for any 

lack of knowledge concerning the relationship between LAs and WLAs and water quality [CWA 

303(d)(1)(c) and 40 CFR 130.7(c)(1)]. For example, knowledge is incomplete regarding the 

exact nature and magnitude of pollutant loads from various sources and the specific impacts of 

those pollutants on the chemical and biological quality of complex, natural waterbodies. The 

MOS is intended to account for such uncertainties in a manner that is conservative from the 

standpoint of environmental protection. On the basis of EPA guidance, the MOS can be achieved 

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40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Percent of Space

Percent of Time

CFD Curve

Area of Criteria 

Exceedence

Area of Allowable

Criteria 

Exceedence
DRAFT Chesapeake Bay TMDL

6-13 September 24, 2010

through two approaches (USEPA 1999): (1) implicitly incorporate the MOS by using 

conservative model assumptions to develop allocations; or (2) explicitly specify a portion of the 

TMDL as the MOS and use the remainder for allocations. Table 6-2 describes different 

approaches that can be taken under the explicit and implicit MOS options.

Table 6-2. Different approaches available under the explicit and implicit MOS types

Type of MOS Available approaches

Explicit Set numeric targets at more conservative levels than analytical results indicate.

Add a safety factor to pollutant loading estimates.

Do not allocate a portion of available loading capacity; reserve for MOS.

Implicit Use conservative assumptions in derivation of numeric targets.

Use conservative assumptions when developing numeric model applications.

Use conservative assumptions when analyzing prospective feasibility of 

practices and restoration activities. 

Source: USEPA 1999

Implicit Margin of Safety for Nutrients

The Chesapeake Bay TMDL analysis is built on a foundation of more than two decades of 

modeling and assessment in the Chesapeake Bay and decades of Bay tidal waters and watershed 

monitoring data. The Bay Airshed, Watershed, and Water Quality models are state-of-thescience models, with several key models in their fourth or fifth generation of management 

applications since the early and mid-1980s. The use of those sophisticated models to develop the 

Bay TMDL, combined with application of specific conservative assumptions, significantly 

reduces EPA’s uncertainty that the model’s predictions of standards attainment is correct and, 

thereby, reduces the need for an explicit MOS for the Chesapeake TMDL.

The Chesapeake Bay TMDL for nutrients applies an implicit MOS in derivation of the DO and 

chlorophyll a-based nutrient allocations through the use of numerous conservative assumptions 

in the modeling framework. The three principal sets of conservative assumptions are as follows.

The basinwide allowable nutrient loads were determined on the basis of achieving a select set of 

deep-water and deep-channel DO standards in the mainstem Bay and adjoining embayments—

middle (CB4MH) and lower (CB5MH) central Chesapeake Bay, Eastern Bay (EASMH), and 

lower Chester River (CHSMH). The Bay DO WQS in all the other 88 Bay segments will be 

achieved with reductions less than (i.e., higher loadings) that needed for attainment of these 

deep-water and deep-channel DO WQS, often much less.

The critical period selected (as described above) was based on a 3-year period that represented 

fairly protective conditions, representing a high-flow condition that is expected approximately 

only once in 10 years. This high-flow period is caused by high rainfall, which in turn causes high 

nonpoint source loads. The combination of requiring achievement of the Bay WQS first across a 

3-year period, not a single year, and the decadal scale return frequency for the hydrological 

conditions represented by the 3-year period, puts in place an important set of conservative 

assumptions supporting an implicit MOS. In other words, because the TMDL identifies loading 

to achieve WQS during the critical period (with high rainfall, high streamflows, and high NPS 
DRAFT Chesapeake Bay TMDL

6-14 September 24, 2010

loading), the TMDL provides even more protection for water quality during less critical (e.g., 

lesser rainfall) years.

The allocation scenario model run assumes that all point sources are discharging at their 

maximum (allocated) load in a given year when, in fact, the facilities will almost always be 

operating and discharging at level below their maximum load limits. For example, when 

assigned a concentration-based limit, municipal wastewater treatment facilities will generally 

seek to operate in a manner to provide themselves a buffer in attaining that limit—i.e., they will 

discharge less than the limit, to avoid being on the edge of noncompliance. That is true of 

regulated limits for many parameters and is easily verified using discharge monitoring report 

(DMR) data. Therefore, each permittee will actually be discharging at loads much less than their 

allocated load, providing an implicit MOS for the TMDL.

Explicit Margin of Safety for Sediment

The Bay TMDL allocations for sediment used a variable explicit MOS. EPA acknowledges that 

the science supporting the estuarine modeling simulation of the transport and resuspension for 

sediments is not as strong as that for nutrients.1 Because of that higher degree of uncertainty, 

EPA determined that an implicit MOS was not appropriate for sediment unlike in the case of 

nutrients. As described in section 6.4.2, the sediment allocations were established at a loading 

level that was at varying levels below the maximum loading levels that the Bay water quality 

model predicted would achieve the SAV WQS for most Bay segments. In other words, EPA 

established the Bay TMDL allocations primarily at levels that were attained as a result of the 

management controls proposed in the state WIPs for controlling nitrogen and phosphorus. 

Therefore, the management controls yield sediment loadings (and allocations) with a variable 

MOS from one Bay segment to another.

The explicit MOS is appropriate for sediment because the Bay Water Quality Model projected 

that many Bay segments would be in attainment with the SAV/water clarity standards at the 

current (2009) loading levels. In contrast, recent data from the Bay-wide SAV aerial survey and 

limited, shallow-water quality monitoring data showed that most Bay segments were not in 

attainment with the SAV restoration acreages goals or water clarity criteria. That observation 

demonstrates that the Bay Water Quality Model was overly optimistic in its simulation of SAV 

acreages and water clarity in the shallows and, therefore, promotes the need for an explicit MOS 

to ensure the sediment allocations would achieve the Bay jurisdictions’ SAV/water clarity WQS.

6.2.4 Temporary Reserve

EPA has included a separate Temporary Reserve, for both nitrogen and phosphorus, of 5 percent 

of the allocated load for each jurisdiction that will be applied for purposes of WIP development 

and incorporating contingency actions (USEPA 2010f). EPA requested the jurisdictions 

incorporate contingency actions into their WIPs as a separate suite of actions to be undertaken if 

the 2011 refinements to the Phase 5.3 Chesapeake Bay Watershed Model result in draft 

allocations lower than those provided with EPA’s July 1, 2010, letter (USEPA 2010f). 

Contingency actions were to be described in similar detail to implementation actions included in 

 

1 Copies of the Chesapeake Bay Water Quality Sediment Transport Model Review Panel’s (convened by the CBP’s 

Scientific and Technical Advisory Committee) reports are at 

http://www.chesapeakebay.net/committee_msc_projects.aspx?menuitem=16525#peer.
DRAFT Chesapeake Bay TMDL

6-15 September 24, 2010

each jurisdiction’s WIPs for the 2017–2025 time frame. EPA identified the Temporary Reserve 

to lessen the effect of any potential revisions to draft nutrient allocations (resulting from the two 

model refinements) that may be lower than the draft allocations assigned within the July 1, 2010, 

letter (including the Temporary Reserve). No jurisdiction has requested a temporary reserve 

allocation in their draft WIP. EPA has considered this and has not included a temporary reserve 

in any of the allocation scenarios set forth in Section 9. EPA is seeking comment on whether to 

include such a temporary reserve in the final TMDL allocations.

The additional 5 percent Temporary Reserve was derived on the basis of two main factors. The 

basinwide nitrogen draft allocation changed approximately 5 percent when transitioning from 

Phase 5.2 of the Chesapeake Bay Watershed Model (approximately 200 million pounds in fall 

2009) to Phase 5.3 (approximately 190 million pounds currently), and therefore, the additional 

model revisions are not expected to result in changes to draft allocations that are any greater than 

that extent. Very preliminary analyses suggest that the two forthcoming refinements to the Bay 

Watershed Model will alter basinwide nutrient draft allocations by 5 percent or less. 

Depending on the results of the 2011 Phase 5.3 Watershed Model refinements, the Temporary 

Reserve will be revised or removed as appropriate during the 2011 Phase II WIP development 

process (USEPA 2010g). In parallel, if needed, jurisdictions can submit for public comment and 

EPA approval any proposed modifications to the Chesapeake Bay TMDL draft allocations 

(USEPA 2010f). No jurisdiction draft WIPs has reserved such an allocation. The temporary 

reserves are identified in Table 6-3 below. 

Table 6-3. Nitrogen and phosphorus temporary reserves by Chesapeake Bay watershed 

jurisdiction.

Jurisdiction Nitrogen temporary reserve

(million pounds per year)

Phosphorus temporary reserve

(million pounds per year)

Pennsylvania 3.84 0.14

Maryland 1.95 0.14

Virginia 2.67 0.27

District of Columbia 0.12 0.01

New York 0.41 0.03

Delaware 0.15 0.01

West Virginia 0.23 0.04

Total temporary reserve 9.37 0.63

Source: USEPA2010g.

6.2.5 Daily Loads

Consistent with the D.C. Circuit Court of Appeals decision in Friends of the Earth, Inc. v. EPA, 

EPA is expressing its draft Chesapeake Bay TMDL in terms of daily time increments (446 F.3d 

140 (D.C. Cir. 2006)). Specifically, the Chesapeake Bay TMDL has developed a maximum daily 

and seasonal load calculation for nitrogen, phosphorus and sediment for each of the 92 

Chesapeake Bay main-stem and tidal segments. However, EPA also recognizes that it is 

appropriate and necessary to identify non-daily allocations in TMDL development despite the 

need to also identify daily loads. In an effort to fully understand the physical and chemical 

dynamics of a waterbody, many TMDLs are developed using methodologies that result in the 
DRAFT Chesapeake Bay TMDL

6-16 September 24, 2010

development of pollutant allocations expressed in monthly, seasonal or annual time periods 

consistent with the applicable WQS. 

EPA encourages TMDL developers to continue to apply accepted and reasonable methodologies 

when calculating TMDLs for impaired waterbodies, and to use the most appropriate averaging 

period for developing allocations based on factors such as available data, watershed and 

waterbody characteristics, pollutant loading considerations, applicable standards, and the TMDL 

development methodology. Consistent with this policy, the Chesapeake Bay TMDL was 

developed to reflect a statistical expression of a maximum daily load applicable to each day of 

the year and as a seasonal representation based on daily maximum values. While only the daily 

maximum loads are provided for each tidal segment using the output of the Bay TMDL models, 

the methodology is described here for deriving the seasonal daily maximum loadings.

The process for deriving daily loads for TMDLs is often based on non-daily allocations, such as 

the annual expression in the Chesapeake Bay TMDL. It builds on the data and information used 

in the non-daily TMDL analysis, supplementing that data as necessary and identifying a daily 

load dataset—a population of continuous or frequent allowable daily loads that meet the loading

capacity and therefore represent maintenance of WQS. In the Chesapeake Bay TMDL, watershed 

and water quality dynamic models were used that generated daily load datasets as routine model 

output. 

Approach for Expressing the Maximum Daily Loads

The methodology applied to calculate the expression of the maximum daily loads and associated 

wasteload and load allocations in the Chesapeake Bay TMDL is consistent with the approach 

contained in EPA's published guidance. Establishing TMDL "Daily" Loads in Light of the 

Decision by the U.S. Court of Appeals for the D.C. Circuit in Friends of the Earth, Inc. v. EPA, 

et al., No. 05-5015, (April 25, 2006) and Implications for NPDES Permits, dated November 15, 

2006 (USEPA 2006). Additionally, the analytical approach selected in the Bay TMDL is similar 

to the wide range of technically sound approaches and the guiding principles and assumption 

described in the technical document Options for the Expression of Daily Loads in TMDLs

(USEPA 2007c). Those principles and assumptions are: 

1. Methods and information used to develop the daily load should be consistent with the 

approach used to develop the loading analysis. 

2. The analysis should avoid added analytical burden without providing added benefit. 

3. The daily load expression should incorporate terms that address acceptable variability in 

loading under the long-term loading allocation. Because many TMDLs are developed for 

precipitation-driven parameters, it may be appropriate to represent the daily load with a range 

to account for allowable differences in loading due to seasonal or flow-related conditions 

(e.g., daily maximum and daily median).

4. The specific application (e.g., data used, values selected) should be based on knowledge and 

consideration of site-specific characteristics and priorities. 

5. The TMDL analysis on which the daily load expression is based should fully meet the EPA 

requirements for approval, be appropriate for the specific pollutant and waterbody type, and 

result in attainment of water quality criteria.
DRAFT Chesapeake Bay TMDL

6-17 September 24, 2010

Computing the Daily Maximum Loads and the Seasonal Daily Maximum Loads

Daily loads are derived for each of the 92 tidal segments and for each of the three pollutants as a 

direct product of the Chesapeake Bay TMDL and associated modeling. This modeling output 

serves as the starting point for the maximum daily load expression and the maximum seasonal 

load expression. These daily maximum loads and seasonal daily maximum loads are a function 

of the ten-year continuous simulation produced by the paired Bay Watershed-Bay Water Quality 

models. This modeling approach allows for the daily maximum load expression to be taken 

directly from the output of the TMDL itself, assuring a degree of consistency between the daily 

maximum load calculation and the loads necessary to meet water quality standards included in 

the final TMDL. That is, this methodology uses the annual allocations derived through the 

modeling/TMDL analysis, and converts those annual loads to daily maximum loadings.

Both the Chesapeake Bay TMDL daily maximum load and seasonal daily maximum load 

represents the 95th percentile of the distribution to protect against the presence of anomalous 

outliers. This expression implies a 5 percent probability that a daily or seasonal daily maximum 

load will exceed the specified value under the TMDL condition. The steps employed to compute 

the Daily and Seasonal Maximum Load for each segment are:

1. Calculate the annual average loading for each of the 92 tidal segments, (this would be the 

annual loading under the TMDL/allocation condition)

2. Calculate the 95th percentile of the daily loads delivered to each of the 92 tidal segments

(using the same loading condition as step 1)

3. Calculate the Annual/Daily Maximum ratio (ADM) for each of the 92 tidal segments by 

dividing the annual average load by the daily maximum load, 

4. Calculate a Baywide ADM by computing a load weighted average of all of the 92 tidal 

segments ADM ratios,

5. Apply the Baywide ADM to all of the annual TMDLs, WLAs and LAs in each of the 92 

tidal segments contained in the TMDL to calculate the daily maximum loads,

6. Using the approach described in 1-5 above, calculate a Baywide ADM for each season 

for each of the 92 tidal segments.

Using this method, the Annual/Daily Maximum Loading ratios listed in Table 6-4were 

developed.

Table 6-4. Annual/Daily Maximum (ADMs) for calculating daily maximum loadsWinter Spring Summer Fall All Year

TN 123.7 80.9 337.1 210.9 123.6

TP 95.8 60.1 260.7 141.2 98.2

TSS 96.5 58.0 384.7 158.1 100.3

It should be noted that a statistical expression of a daily load is just that, an expression of the 

probability that a specific maximum daily load will occur in a given segment for a specific 

pollutant. There will be situations where the maximum daily load allocation for some segments 

will exceed the TMDL allocation, and in other segments the maximum daily load allocation will 

be less than the TMDL allocation. However, the magnitude of the TMDL allocations was 
DRAFT Chesapeake Bay TMDL

6-18 September 24, 2010

established to assure the attainment of all applicable water quality standards in each of the 92 

tidal segments. 

In addition to the maximum daily load provided for each of the 92 tidal segments in Section 9, 

the reader can readily calculate a daily maximum load expressed in seasonal terms for any 

segment, WLA, or LA of interest. This seasonal expression reflects a temporally variable target 

because the various pollutant sources (point and nonpoint) vary significantly by month and by 

season. Additionally, a daily maximum load expressed in seasonal terms for each segment is 

also informative because the recently adopted water quality standards are also expressed with a 

degree of temporal specificity. For example, the Migratory Fish Spawning and Nursery 

designated uses require a 7 day mean dissolved oxygen value of 6 mg/L, with an instantaneous

minimum of 5 mg/L in the time period February 1 through May 31. 

The expression of maximum daily loads for individual wasteload and load allocations proposed 

in this draft TMDL represent EPA's best efforts to date to calculate nitrogen, phosphorus, and 

sediment allocations, informed by the jurisdictions' watershed implementation plans and other 

elements of the TMDL accountability framework, necessary to implement all applicable Bay 

water quality standards with seasonal variations, considering critical conditions and with a 

margin of safety. EPA invites comment on this approach or alternative approaches for 

calculating daily maximum load values. 

6.3 Establishing Allocation Rules

An early step in the process for developing the Bay TMDL, especially for nutrients, is to 

determine the allowable loading from jurisdictions and major basins draining to the Bay. There 

are limitless combinations of loadings from the various jurisdictions and basins that would 

achieve this objective. As a result, an equitable approach must be employed to apportion the 

allowable loading among the jurisdictions. This subsection describes the process used for this 

purpose in the Bay TMDL.

6.3.1 Nutrient Allocation Methodology

Nutrients from sources well up within the Chesapeake Bay watershed affect the condition of 

local receiving waters and affect tidal water quality conditions far downstream, hundreds of 

miles away in some cases. For example, the middle part of the mainstem Chesapeake Bay is 

affected by nutrients from all parts of the Bay watershed. A key objective of the nutrient LA 

methodology was to find a process, based on some expression of an equitable distribution of 

loads for which the basinwide load for nutrients could be distributed among the basinjurisdictions. This section describes the specific processes involved in allocating the nutrients 

loads necessary to meet the jurisdictions’ Chesapeake Bay DO and chlorophyll a WQS. While 

many alternative processes were explored (see Appendix K), only the processes determined to be 

appropriate by EPA and agreed upon by five of the seven Bay watershed jurisdictional partners 

are described here.
DRAFT Chesapeake Bay TMDL

6-19 September 24, 2010

Principles and Guidelines

The nutrient basin-jurisdiction allocation methodology was developed to be consistent with the 

following guidelines adopted by the partnership: 

The allocated loads should protect the living resources of the Bay and its tidal tributaries 

and result in all segments of the Bay mainstem, tidal tributaries, and embayments meeting 

WQS for DO, chlorophyll a, and water clarity.

Major river basins that contribute the most to the Bay water quality problems must do the 

most to resolve those problems (on a pound per pound basis).

All tracked and reported reductions in nutrient loads are credited toward achieving final 

assigned loads.

A number of critical concepts are important in understanding the major river basin by 

jurisdiction nutrient allocation methodology. They include the following:

Accounting for the geographic and source loading influence of individual major river 

basins on tidal water quality termed relative effectiveness

Determining the controllable load

Relating controllable load to relative effectiveness to determine the allocations of the 

basinwide loads to the basin-jurisdictions

The following subsections further describe the above concepts and how they directly affect the 

Chesapeake Bay TMDL.

Accounting for Relative Effectiveness of the Major River Basins on Tidal Water Quality

Relative effectiveness accounts for the role of geography on nutrient load changes and, in turn, 

Bay water quality. Because of various factors such as in-stream transport and nutrient cycling in 

the watershed, a given management measure will have a different level of effect on water quality 

in the Bay depending on the location of its implementation (USEPA 2003b). For example, the 

same control applied in Williamsport, Pennsylvania, will have less of an effect than one applied 

in Baltimore, Maryland.

A relative effectiveness assessment evaluates the effects of both estuarine transport (location of 

discharge/runoff loading to the Bay) and riverine transport (location of the discharge/runoff 

loading in the watershed). EPA determined the relative effectiveness of each contributing river 

basin in the overall Bay watershed on DO in several mainstem Bay segments and the lower 

Potomac River by using the Bay Water Quality Model to run a series of isolation runs and using 

the watershed model to estimate attenuation of load through the watershed.

From the relative estuarine effectiveness analysis, several things are apparent. Northern, major 

river basins have a greater relative influence than southern major river basins, because of the 

general circulation patterns of the Chesapeake Bay (up the eastern shore, down the western 

shore). Water and nutrients from the most southern river basins of the James and York rivers 

have relatively less influence on mainstem Bay water quality because of their proximity to the 

mouth of the Bay. The counter-clockwise circulation of the lower Bay also tends to wash nutrient 

loads from these larger southern river basins out of the Bay mouth, because they are on the 
DRAFT Chesapeake Bay TMDL

6-20 September 24, 2010

western side of the Bay. That same counter-clockwise circulation tends to sweep loads from the 

lower Eastern Shore northward.

River basins whose loads discharge directly to the mainstem Bay, like the Susquehanna, tend to 

have more effect on the mainstem Bay segments than basins with long riverine estuaries (e.g., 

the Patuxent and Rappahannock rivers). The long riverine estuaries provide nutrient attenuation 

(burial and denitrification) before the waters reaching the mainstem Chesapeake Bay. The size of 

a river basin is uncorrelated to its relative influence, though larger river 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) river 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, again, given the Bay 

circulation pattern that moves water up the Eastern Shore, and down the Western Shore.

The estuarine effectiveness is estimated by running a series of Bay Water Quality Model 

scenarios holding one major river basin at E3 loads and all other major river basins at calibration 

levels. For each scenario, the increase in the 25th percentile DO concentration during the summer 

criteria assessment period in the critical segments CB3MH, CB4MH, and CB5MH for deepchannel and CB3MH, CB4MH, CB5MH, and POTMH for deep-water was recorded. The 25th

percentile was selected as the appropriate metric as indicative of a change in low DO. The 

riverine effectiveness is calculated as the fraction of load produced in the watershed that is 

delivered to the estuary. It is estimated as an output of the watershed model. For more details on 

this method, see Appendix M.

Absolute estuarine effectiveness accounts for the role of both total loads and geography on 

pollutant load changes to the Bay. The absolute estuarine effectiveness of a contributing river 

basin, measured separately both above and below the fall line, is the change in 25th percentile 

DO concentration that results from a single basin changing from calibration conditions to E3. For 

example, if the 25th percentile DO in the deep water of the lower Potomac River segment 

POTMH moves from 5 mg/L to 5.3 mg/L from a change in loads from calibration to E3 in the 

Potomac above fall line basin, the absolute estuarine effectiveness is 0.3 mg/L. Comparing the 

absolute estuarine effectiveness among basins helps to identify which major river basins have the 

greatest effect on WQS.

Relative estuarine effectiveness is defined as absolute estuarine effectiveness divided by the total 

load reduction, delivered to tidal waters, necessary to gain that water quality response. For 

example, if the load reduction in the Potomac above fall line basin was 30 million pounds of 

pollutant to get a 0.3 mg/L change in DO concentration, the relative estuarine effectiveness is 

0.01 mg/L per million pounds. The higher the relative estuarine effectiveness, the less reduction 

required to achieve the change in status. The relative estuarine effectiveness calculation is an 

attempt to isolate the effect of geography by normalizing the load on a per pound basis. 

Comparing the relative estuarine effectiveness among the major river basins shows the resulting 

gain in attainment from performing equal pound reductions among the major river basins.

Riverine attenuation also has an effect on overall effectiveness. Loads are naturally attenuated or 

reduced as they travel through long free-flowing river systems, making edge-of-stream loads in 
DRAFT Chesapeake Bay TMDL

6-21 September 24, 2010

headwater regions less effective on a pound-for-pound basis than edge-of-stream loads that take 

place nearer tidal waters in the same river basin. The watershed model calculates delivery factors 

as the fraction of edge-of-stream loads that are delivered to tidal waters. The units of riverine 

attenuation are delivered pound per edge-of-stream pound.

Multiplying the estuarine relative effectiveness (measured as DO increase per delivered pound 

reduction) by the riverine delivery factor (measured as delivered pound per edge-of-stream 

pound) gives the overall relative effectiveness in DO concentration increase per edge-of-stream 

pound. The relative estuarine effectiveness is the same for nitrogen or phosphorus, while the 

riverine delivery is different, so the overall relative effectiveness is calculated separately for 

nitrogen and phosphorus. Error! Reference source not found. gives the overall relative 

effectiveness for nitrogen and phosphorus for the watershed jurisdictions by major river basin for 

above and below the fall line.

The relative effectiveness numbers are separate for wastewater treatment plants and all other 

sources. The distinction is made because the allocation method treats them separately. The 

difference in relative effectiveness is due to the geographic location of the sources. For example, 

in the Maryland western shore basin, the majority of the wastewater treatment load is discharged 

directly to tidal waters, whereas a significant fraction of all other sources are upstream, including 

areas that are above reservoirs with very low delivery factors.

Table 6-5. Relative effectiveness (measured as DO concentration per edge-of-stream pound 

reduced) for nitrogen and phosphorus for watershed jurisdictions by major river basin and above 

and below the fall line

Jurisdiction Basin

WWTP NitrogenAll Other NitrogenWWTP PhosphorusAll Other Phosphorus

District of Columbia Potomac above Fall Line 6.09 6.09 3.08 3.08

District of Columbia Potomac below Fall Line 6.17 5.15 6.17 5.62

Delaware Lower East Shore 7.93 7.30 7.97 7.46

Delaware Middle East Shore 4.13 4.74 5.51 5.83

Delaware Upper East Shore 6.75 6.75 7.10 7.10

Maryland Lower East Shore 7.88 7.37 7.89 7.55

Maryland Middle East Shore 6.91 6.49 6.92 6.71

Maryland Patuxent above Fall Line 1.89 1.25 1.66 1.58

Maryland Patuxent below Fall Line 6.38 6.20 6.38 6.10

Maryland Potomac above Fall Line 3.32 3.25 2.99 2.99

Maryland Potomac below Fall Line 6.17 4.86 6.12 5.75

Maryland Susquehanna 9.39 8.68 9.11 8.77

Maryland Upper East Shore 7.49 7.27 7.49 7.40

Maryland West Shore 7.83 4.98 7.68 6.13

New York Susquehanna 5.60 4.58 4.25 4.11

Pennsylvania Potomac above Fall Line 2.10 1.98 3.08 3.08

Pennsylvania Susquehanna 6.99 6.44 4.38 4.58

Pennsylvania Upper East Shore 5.50 5.95 6.12 6.47

Pennsylvania West Shore 2.23 2.23 2.61 2.61
DRAFT Chesapeake Bay TMDL

6-22 September 24, 2010

Jurisdiction Basin

WWTP NitrogenAll Other NitrogenWWTP PhosphorusAll Other Phosphorus

Virginia East Shore VA 5.72 5.72 5.72 5.72

Virginia James above Fall Line 0.23 0.25 0.33 0.31

Virginia James below Fall Line 0.79 0.61 0.79 0.70

Virginia Potomac above Fall Line 1.45 1.97 3.08 3.08

Virginia Potomac below Fall Line 5.54 3.54 5.49 4.62

Virginia Rappahannock above Fall Line 1.05 0.83 2.10 2.10

Virginia Rappahannock below Fall Line 4.48 4.41 4.48 4.47

Virginia York above Fall Line 0.37 0.31 0.43 0.40

Virginia York below Fall Line 1.85 1.77 1.85 1.82

West Virginia James above Fall Line 0.06 0.06 0.34 0.34

West Virginia Potomac above Fall Line 1.34 1.72 2.12 2.89

Figure 6-4 illustrates graphically the relative effectiveness scores for nitrogen of the major river 

basins provided in Error! Reference source not found. in descending order.

Source: Table 6-5.

Figure 6-4. Relative effectiveness for nitrogen for the watershed jurisdictions and major rivers basins, above 

and below the fall line, in descending order.

Figure 6-5 and Figure 6-6 provide additional graphical illustration of the relative effectiveness 

concept for all the basins in the watershed related to nitrogen and phosphorus loading, 
DRAFT Chesapeake Bay TMDL

6-23 September 24, 2010

respectively. The figures illustrate that, on a per pound basis, a large disparity exists among basin 

loads on the effect of DO concentrations in the Bay. Generally, the Northern and Eastern river 

basins have a greater effect on water quality.
DRAFT Chesapeake Bay TMDL

6-24 September 24, 2010

Figure 6-5. Relative effectiveness illustrated geographically by subbasins across the Chesapeake Bay 

watershed for nitrogen.
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6-25 September 24, 2010

Figure 6-6. Relative effectiveness for illustrated geographically by subbasins across the Chesapeake Bay 

watershed for phosphorus.
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6-26 September 24, 2010

Determining Controllable Load

Modeling in support of developing the Chesapeake Bay TMDL employs two theoretical 

scenarios that help to illustrate the load reductions in the context of a controllable load.

The No Action scenario is indicative of a theoretical worst case loading situation in which no 

controls exist to mitigate nutrient and sediment loads from any sources. It is specifically 

designed to support equity among basin-jurisdiction allocations in that the levels of all control 

technologies and BMP and program implementation are at baseline conditions.

The E3 scenario represents a best case possible situation, where all possible BMPs and available 

control technologies are applied to land given human and animal populations and wastewater 

treatment facilities are represented at highest technologically achievable levels of treatment 

regardless of costs. Again, it considers equity among the allocations in that the levels of control 

technologies and practice and program implementation are the same across the entire watershed.

The gap between the No Action scenario and the E3 scenario represents the maximum theoretical 

controllable load reduction that is achievable under the control technologies covered under the 

E3 scenario. These and other key reference scenarios are defined and documented in detail in 

Appendix J.

Each scenario can be run with any given year’s land use representation. The year 2010 was 

selected as the base year because it represents conditions at the time the Bay TMDL is 

developed. Thus, the 2010 No Action scenario represents loads resulting from the mix of land 

uses and point sources present in 2010 with no effective controls on loading, while the 2010 E3 

scenario represents the highest technically feasible treatment that could be applied to the mix of 

land use-based sources and permitted point sources in 2010 (Table 6-6).

The anthropogenic, controllable load is determined by subtracting the basinwide E3 load from 

the basinwide No Action load. Model scenarios run to show results of various loading reduction 

management options can be expressed as a percentage of E3 to compare and contrast the relative 

level of effort between scenarios.
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6-27 September 24, 2010

Table 6-6. Pollutant sources as defined for the No Action and E3 model scenarios

Model source

Scenario

No Action

E3 = Everyone Everything 

Everywhere

Land uses No BMPs applied to the land All possible BMPs applied to land 

given current human and animal 

population and land use

Point sources Significant municipal WWTPs

Flow = design flows

TN = 18 mg/L

TP = 6 mg/L

BOD = 30 mg/L

DO = 4.5 mg/L

TSS = 15 mg/L

Significant industrial dischargers

Flow = design flows

TN = highest recorded

TP = highest recorded

BOD = 30 mg/L

DO = 4.5 mg/L

TSS = 15 mg/L

Non-significant municipal 

WWTPs

Flow = existing flows

TN = 18 mg/L

TP = 6 mg/L

BOD = 30 mg/L

DO = 4.5 mg/L 

TSS = 15 mg/L

Significant municipal WWTPs

Flow = design flows

TN = 3 mg/L

TP = 0.1 mg/L

BOD = 3 mg/L

DO = 6 mg/L

TSS = 5 mg/L

Significant industrial dischargers

Flow = design flows

TN = 3 mg/L

TP = 0.1 mg/L

BOD = 3 mg/L

DO = 6 mg/L

TSS = 5 mg/L

Non-significant municipal WWTPs

Flow = existing flows

TN = 8 mg/L

TP = 2 mg TP/l

BOD = 5 mg/L

DO = 5 mg/L 

TSS = 8 mg/L

CSOs Flow = 2003 base condition flow

TN = 18 mg/L

TP = 6 mg/L

BOD = 200 mg/L

DO = 4.5 mg/L

TSS = 45 mg/L

Full storage and treatment of CSOs

Atmospheric 

deposition

1985 Air Scenario 2030 Air Scenario, max reductions

Source: Appendix J

Note: BOD = biological oxygen demand; DO = dissolved oxygen; TN = total nitrogen; TP = total phosphorus; TSS = 

total suspended solids

Relating Relative Impact to Needed Controls (Allocations)

To apply the allocation methodology, loads from each major river basin were divided into two 

categories—wastewater and all other sources (Figure 6-7). The rationale for this separate 

accounting is the higher likelihood of achieving greater load reductions for the wastewater sector 

than for other source sectors (Appendix K). In addition there was a wide disparity between basin 

and jurisdictions on the fraction of the load coming from the wastewater sector as opposed to 

other sectors. So, this disparity is addressed in having a separate accounting for the wastewater 

sector from the other sectors in the allocation methodology. Wastewater loads included all major 

and minor municipal, industrial and CSO discharges. Then lines were drawn for each of the two 
DRAFT Chesapeake Bay TMDL

6-28 September 24, 2010

source categories such that the addition of the two lines would add up to the basinwide nutrient 

loading targets for nitrogen and phosphorus.

Using the general methodology described above, the Bay Program partners considered many 

different combinations of wastewater and other source’ controls and slopes of the lines on that 

allocation graph (Appendix K). After discussing these options at length, the following graph 

specifications were generally accepted by the partners and determined to be appropriate by EPA.

The wastewater line was set first and would be a hockey stick shape with load reductions 

increasing with relative effectiveness until a maximum percent controllable load was reached.

For nitrogen

The maximum percent controllable load was 90 percent, corresponding to an 

effluent concentration of 4.5 mg/L.

The minimum percent controllable load was 67 percent, corresponding to an 

effluent concentration of 8 mg/L.

For phosphorus

The maximum percent controllable load was 96 percent, corresponding to an 

effluent concentration of 0.22 mg/L.

The minimum percent controllable load was 85 percent, corresponding to an 

effluent concentration of 0.54 mg/L.

For the nitrogen and phosphorus wastewater lines, any relative effectiveness value that was 

at least half as large as the maximum was given the maximum percent controllable. The 

minimum value was assigned to a relative effectiveness of zero, and all values of relative 

effectiveness between zero and half of the maximum value were assigned interpolated 

percentages (Figure 6-7).

The other sources line was set at a level that was necessary to achieve the basinwide load needed 

for achieving the DO standards in the middle mainstem Bay and lower tidal Potomac River. This

line was set at a slope such that there was a 20 percent overall slope, ranging from 56 percent of 

controllable loads for basins with low relative effectiveness to 76 percent of controllable loads 

for basins with high relative effectiveness for nitrogen (Figure 6-7).

For each category—wastewater and all other sources—loads are aggregated by major basin and 

reductions are assigned according to the specific river basin’s relative effectiveness. The graph in 

Figure 6-7 illustrates the methodology for the total nitrogen target load of 190 million lbs per 

year.
DRAFT Chesapeake Bay TMDL

6-29 September 24, 2010

Figure 6-7. Allocation methodology example showing the hockey stick and straight line reductions 

approaches, respectively, to wastewater (red line) and all other sources (blue line) for nitrogen.

6.3.2 Sediment Allocation Methodology

The methodology used for allocating sediment loads to major river basins and jurisdictions for 

sediment was much different than the methodology used for nutrients. That is because sediment 

has a much more localized effect than nutrients and, therefore, for sediment, the immediate 

subbasin (i.e., the Chester River) has a large influence on the water clarity and SAV growth in 

that subbasin. So for sediment, the allocated load is driven primarily from the local subbasin that 

is contributing sediment to the local Bay segment and, therefore, a methodology is not needed to 

further suballocate the loading to contributing jurisdictions or neighboring basins.

Building from the basin-jurisdiction nutrient allocations described above, the following key steps 

were taken:

Assessed water clarity/SAV criteria attainment across all Bay segments containing the 

shallow-water bay grass designated use under the proposed basinwide nutrient cap loads 

and the corresponding phosphorus-based sediment loads allocated by major river basin by 

jurisdiction (Note: For most non-point source controls for phosphorus, there is a co-benefit 

of also reducing sediment)

Identified those individual Bay segments still not attaining their applicable water 

clarity/SAV WQS at the allocated basinwide nutrient cap loads and the corresponding 

phosphorus-based sediment loads, and addressed the remaining non-attaining segments

Nitrogen -- Phase 5.3 -- Goal=190

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8 9 10

Relative Effectiveness Percent reduction from 2010 noBMPs to E3 All Other

WWTP
DRAFT Chesapeake Bay TMDL

6-30 September 24, 2010

6.4 Assessing Attainment of Proposed Amended Chesapeake Bay 

Water Quality Standards

This subsection describes the application of all of the processes described earlier in this section. 

EPA identified the draft nutrient allocations to the basin-jurisdictions in a letter of July 1, 2010, 

from EPA Region 3 to the jurisdictions (USEPA 2010f). Furthermore, using a separate 

methodology as described in Section 6.2 for allocating loads for sediment, an August 13, 2010, 

letter from EPA to the jurisdictions identified the draft sediment allocations (USEPA 2010g). 

Note that these draft allocations to the jurisdictions were derived to achieve proposed amended 

Chesapeake Bay WQS anticipated by the jurisdictions.

6.4.1 Establishing Nutrient Load Caps to Attain the Proposed Amended 

Water Quality Standards

The proposed amendments to the Bay jurisdictions’ WQS are described in Section 3.3. The 

allocations in those letters are the allocations on which the jurisdictions based their draft Phase I 

WIPs. The full process for establishing these nutrient basin-jurisdiction allocations is described 

below:

Established the atmospheric deposition allocations on the basis of addressing the 

requirements of the CAA to meet existing national air quality standards

Set the basinwide nutrient loads on the basis of attaining the applicable DO criteria in those 

Bay segments (middle Chesapeake Bay mainstem and the lower tidal Potomac River) 

whose water quality conditions are influenced by major river basins and jurisdictions 

throughout the Bay watershed

Distributed the basinwide nutrient loads by major river basin and jurisdiction following the 

methodology developed by the partnership (see section 6.2)

Made certain discretionary adjustments to the allocations, for example for New York and 

West Virginia

Allowed for individual jurisdictions to exchange nitrogen and phosphorus loads within and 

between their major river basins using specific exchange ratios

Identified those individual Bay segments still not attaining their applicable DO/chlorophyll 

a WQS at the allocated basinwide nutrient loads and addressed the remaining 

nonattainment segments

Derived the final jurisdiction-basin nutrient allocations to achieve the applicable WQS for 

DO and chlorophyll a in all 92 Bay segments

Individual jurisdictions further suballocated their major river basin-jurisdiction allocated loads 

within their draft Phase I WIPs down to their respective Bay segment watersheds within their 

jurisdiction. After in-depth review of the draft Phase I WIPs and resultant proposed allocations, 

EPA made further adjustment to the allocations as described in Section 7.

Setting the Atmospheric Nitrogen Deposition Allocation

Atmospheric deposition of nitrogen is the major source of nitrogen to the Chesapeake Bay 

watershed, greater than the other sources of fertilizer, manures, or point sources. For that reason, 

it is necessary to allocate an allowable loading of nitrogen from air deposition in the Chesapeake 

Bay TMDL. The nitrogen loadings come from many jurisdictions outside the Chesapeake Bay 
DRAFT Chesapeake Bay TMDL

6-31 September 24, 2010

watershed. Figure 6-8 shows the approximate delineation of the Bay airshed. Of the nitrogen air 

deposition loads in the Chesapeake watershed, 75 percent come from within the Bay airshed. 

That means that a quarter of the nitrogen loads originate beyond the airshed, and in the largest 

sense, the source of atmospheric loads to the Chesapeake Bay watershed are global. That is 

reflected in the Bay Airshed Model, which has a domain of all North America (with boundary 

conditions to quantify global nitrogen sources). About 50 percent of the oxidized nitrogen (NOx) 

atmospheric deposition loads to the Chesapeake watershed and tidal Bay come from the seven 

Bay watershed jurisdictions.

By including air deposition in the Bay TMDL’s LAs, the Bay TMDL accounts for the emission 

reductions that will be achieved by seven watershed jurisdictions and other states in the larger 

Bay airshed. If air deposition and expected reductions in nitrogen loading to the Bay were not 

included in the LAs, other sources would have to reduce nitrogen discharges/runoff even further 

to meet the nutrient loading cap. Because CAA regulations and programs will achieve significant 

decreases in air deposition of nitrogen by 2020, EPA believes the TMDL inclusion of air 

allocations (and reductions) is based on both the best available information with a strong 

reasonable assurance that those reductions will occur. The TMDL being developed for the 

Chesapeake Bay will reflect the expected decreases in nitrogen deposition and the 2-year federal 

milestones will track the progress of CAA regulations and programs.

Source: Dr. Robin Dennis, USEPA/ORD/NERL/AMAD/AEIB

Figure 6-8.Principal areas of nitrogen oxide (blue line) and ammonia (red line) emissions that contribute to 

nitrogen deposition to the Chesapeake Bay and its watershed (dark blue fill).
DRAFT Chesapeake Bay TMDL

6-32 September 24, 2010

In determining the allowable loading from air deposition, EPA separated the nitrogen 

atmospheric deposition into two discreet parcels: (1) atmospheric deposition occurring on the 

land and nontidal waters in the Bay watershed, which is subsequently transported to the bay; and 

(2) atmospheric deposition occurring directly onto the Bay tidal surface waters.

The deposition on the land becomes part of the allocated load to the jurisdictions because the 

atmospheric nitrogen deposited on the land becomes mixed with the nitrogen loadings from the 

land-based sources and, therefore, becomes indistinguishable from land-based sources. 

Furthermore, once the nitrogen is deposited on the land, it would be managed and controlled 

along with other sources of nitrogen that are present on that parcel of land. In contrast, the 

atmospheric nitrogen deposited directly to tidal surface waters is a direct loading with no landbased management controls and, therefore, needs to be linked directly back to the air sources and 

air emission controls.

EPA included an explicit basinwide nitrogen atmospheric deposition allocation in the Bay 

TMDL determined to be 15.7 million pounds of nitrogen atmospheric deposition loads direct to 

Chesapeake Bay and tidal tributary surface waters (Appendix L) (see Section 8.1). Activities 

associated with implementation of CAA regulations by EPA and the jurisdictions through 2020 

will ensure achievement of this allocation and are already accounted for within the major river 

basin by jurisdiction nitrogen allocations. Any additional nitrogen reductions realized through 

more stringent air pollution controls at the jurisdictional level, beyond minimum federal 

requirements to meet air quality standards, may be credited to the individual jurisdictions 

through future revisions to the jurisdictions’ WIPs, 2-year milestones, and the Chesapeake Bay 

TMDL tracking and accounting framework.

In determining the amount of air controls to be used as a basis for the Bay TMDL air allocation, 

EPA relied on current laws and regulations under the CAA. Those requirements, together with 

national air modeling analysis, provided the resulting allocated load to air from direct deposition 

to the tidal surface waters of the Bay and its tidal tributaries (Appendix L).

The air allocation scenario represents emission reductions from regulations implemented through 

the CAA authority to meet National Ambient Air Quality Standards for criteria pollutants in 

2020. The air allocation scenario includes the following:

The Clean Air Interstate Rule (CAIR) with second phase and the Clean Air Mercury Rule 

(CAMR)

The Regional Haze Rule and guidelines for Best Available Retrofit Technology (BART)

The On-Road Light Duty Tier 2 Rule

The Clean Heavy Duty Truck and Bus Rule

The Clean Air Non-Road Diesel Tier 4 Rule

The Locomotive and Marine Diesel Rule

The Non-road Large and Small Spark-Ignition Engines Programs

The Hospital/Medical Waste Incinerator Regulations
DRAFT Chesapeake Bay TMDL

6-33 September 24, 2010

The controls described above were modeled using the Community Multiscale Air Quality 

(CMAQ) national model, which enabled quantification of deposition direct to the Chesapeake 

Bay tidal waters to be determined. That approach is the basis for the previously mentioned 15.7 

million pounds per year.

Appendix L provides a more detailed description of the process for establishing the atmospheric 

deposition allocations for nitrogen.

Determining the Basinwide Nutrient Target Load Based on Dissolved Oxygen

With the air allocated loads being proposed at 15.7 million pounds per year, the next step in the 

process was to determine the basinwide nutrient loadings that would cause the mainstem Bay and 

major tidal river segments—all influenced by nutrient loads from multiple jurisdictions—to 

achieve all the applicable DO WQS. DO WQS were used for this basinwide loading 

determination because the numerical chlorophyll a WQS apply to only the tidal James River and 

the District of Columbia’s tidal waters of the Potomac and the Anacostia rivers and, therefore, 

are not affected by the other basins in the watershed. The principal Bay segments that were most 

important for determining the basinwide nutrient loads were the middle mainstem Bay segments 

CB3MH, CB4MH, and CB5MH (Maryland and Virginia) and the lower tidal Potomac River 

segment POTMH_MD because their water quality conditions are influenced by all river basins 

through the Bay watershed. Therefore, achieving attainment in these segments will necessitate 

nutrient reductions from all basins.

The process used for determining the load that will achieve the DO WQS in these segments was 

to progressively lower the nutrient loadings simulated in the Bay Water Quality Model and then 

assess DO WQS attainment for each loading scenario. Numerous iterations of different load 

scenarios were run until the appropriate nutrient loadings to achieve standards could be 

determined (Appendix M).

Figure 6-9 shows the numerous water quality model runs that were performed at various loading 

levels and the resulting water quality results. The water quality measure on the vertical axis is the 

number of Bay segments that were not attaining the applicable Bay DO WQS. As can be 

expected, as loadings are lowered throughout the Bay watershed, the number of DO WQS nonattaining segments was reduced. At the loading of 190 million pounds per year of nitrogen and 

12.6 million pounds per year of phosphorus, only one Bay segment was in nonattainment for 

DO—part of the Chester River (discussed later in this section). Therefore, the nutrient loadings 

of 190 million pounds per year of nitrogen and 12.6 million pounds per year of phosphorus were 

selected as the basinwide loadings necessary to attain the main Bay DO standards and to 

distribute those loadings among the major river basins and jurisdictions in the Chesapeake Bay 

watershed. Note that Figure 6-9 represents the segments considered to be in nonattainment after 

other lines of evidence, beyond the Bay Water Quality Model, were considered. 
DRAFT Chesapeake Bay TMDL

6-34 September 24, 2010

8

0

5

10

15

20

25

30

35

40

45

1985 Base 2009 Target Tributary Loading Loading Loading E3 All

Scenario Calibration Scenario Load A Strategy Scenario Scenario Scenario Scenario Forest

342TN 309TN 248TN 200TN 191TN 190TN 179TN 170TN 141TN 58TN

24.1TP 19.5TP 16.6TP 15.0TP 14.4TP 12.7TP 12.0TP 11.3TP 8.5TP 4.4TP

Number of Segments in DO Violation

Open Water Violations

Deep Water Violations

Deep Channel Violations

Dissolved Oxygen Criteria Attainment

Basin-wide load is

190 N and 12.7 P (MPY)

Note: This graph expands some of the 92 TMDL segments into separate jurisdiction-segments so that the total 

number of Open Water, Deep Water, and Deep Channel is 98, 14, and 11 respectively

Figure 6-9. Chesapeake Bay water quality model simulated DO criteria attainment under various nutrient 

loading scenarios. 

Allocating Nutrient Loads to Jurisdictions within the Bay Watershed

With the exception of New York and West Virginia, all the watershed jurisdictions agreed to the 

method described above for allocating loadings to the major river basins and jurisdictions. Using 

the methods described above, the relative effectiveness of each of the major river basins in the 

Bay watershed was determined and plotted as dots on the lines in Figures 6-10 and 6-11. To 

determine the basin-jurisdictions represented by each of the points on Figure 6-10 and Figure 

6-11, see Table 6-2. On the vertical axis is the percent of controllable load that would correspond 

to the allocated load for each basin-jurisdiction. For example, 100 percent represents that all 

sources would have all control technologies and practices approved by the partnership installed. 

The horizontal axis represents the relative effectiveness of each of the basin-jurisdictions, a 

measure of the impact that a pound of nutrients has on the DO concentrations in the Chesapeake 

Bay. The wastewater (WWTP) line (red line in each figure) was first constructed based on the 

removal efficiencies of established treatment technologies.

The other sources line (blue line in both figures) was then constructed by having a difference of 

20 percent of controllable load when comparing facilities/lands in the basin-jurisdiction with the 
DRAFT Chesapeake Bay TMDL

6-35 September 24, 2010

highest relative effectiveness with the facilities/lands in the basin-jurisdiction with the lowest 

relative effectiveness. As can be seen in Figure 6-10 and Figure 6-11, facilities/lands in those 

basin-jurisdictions that have the highest effectiveness (or impact on the Bay) on a per pound 

basis must install the most controls (the basin-jurisdictions on the right of the graph). Because 

the dots represent the various basin-jurisdictions in the watershed, the percent of controllable 

load can be converted to the actual allocated load to achieve the Bay DO WQS. Finally, the 

allocated load for wastewater (WWTP) is added to the allocated load for other sources to 

determine the total allocated load for each basin-jurisdiction. It must be noted that although the 

graph separates wastewater and other sources, this does not necessarily require the jurisdictions 

to use that separate wastewater or other sources loading in their WIPs for suballocating the loads.

Phosphorus -- phase 5.3 -- Goal=12.67 million lbs

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8 9 10

Relative Effectiveness Percent reduction from 2010 noBMPs to E3

All Other

WWTP

Figure 6-10. Example allocation methodology application for phosphorus.
DRAFT Chesapeake Bay TMDL

6-36 September 24, 2010

Figure 6-11. Example allocation methodology application for nitrogen.

Resolving Dissolved Oxygen and Chlorophyll a Nonattaining Bay Segments

After determining the target basinwide allocation and distributing that loading to the major 

basins river and jurisdictions using the methodology illustrated above, 11 designated use 

segments remained for which the Bay Water Quality Model was predicting nonattainment of the 

applicable Bay DO WQS (Error! Reference source not found.). Note that the nine segments 

out of attainment for the open-water designated use represent only about 1 percent of the total 

volume of open-water habitats in entire Chesapeake Bay.

Nitrogen -- Phase 5.3 -- Goal=190

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8 9 10

Relative Effectiveness Percent reduction from 2010 noBMPs to E3 All Other

WWTP
DRAFT Chesapeake Bay TMDL

6-37 September 24, 2010

Table 6-7. Chesapeake Bay designated use segments showing percent nonattainment of the applicable Bay DO WQS under the 

proposed basinwide nutrient target loadings (shaded column)

CBSEG

309TN, 

19.5TP, 

8950TSS

248TN, 

16.6TP, 

8110TSS

200TN, 

15TP, 

6390TSS

191TN 

14.4TP, 

6462 

TSS

190TN, 

13TP, 

6123TSS 

190TN 

12.7TP, 

6030TSS

179TN 

12.0TP, 

5510TSS

170TN 

11.3TP, 

5650TSS

141TN 

8.5TP, 

5060TSS

All 

Forest

'93-'95 '93-'95 '93-'95 '93-'95 '93-'95 '93-'95 '93-'95 '93-'95 '93-'95 '93-'95

Open Water Summer Monthly

GUNOH 5% 5% 5% 5% 5% 5% 5% 5% 5% 5%

MANMH 1% 5% 5% 5% 5% 5% 5% 5% 5% 0%

MDATF 39% 19% 18% 12% 12% 12% 11% 11% 0% 0%

MPCOH 42% 31% 25% 25% 18% 18% 5% 5% 5% 0%

PMKTF 11% 5% 5% 5% 5% 5% 5% 2% 1% 1%

POCTF 43% 32% 25% 25% 18% 18% 5% 5% 5% 0%

VPCOH 41% 28% 25% 25% 18% 18% 5% 5% 5% 0%

WBEMH 11% 15% 8% 8% 8% 8% 8% 8% 0% 0%

WICMH 11% 11% 15% 5% 5% 5% 5% 5% 5% 4%

Deep Water

MAGMH 35% 35% 16% 16% 16% 3% 3% 1% 1% 0%

Deep Channel

CHSMH 38% 27% 14% 14% 14% 14% 14% 9% 4% 0%

Source: Appendix M

The model also predicted nonattainment for chlorophyll a- all five Bay segments of the tidal James River in Virginia and the two Bay 

segments in the District of Columbia (tidal Potomac and Anacostia rivers) were also predicted to be in nonattainment of each 

jurisdictions’ respective chlorophyll a WQS based on model runs at the basinwide nutrient loading of 190 million pounds per year 

nitrogen and 12.6 million pounds per year phosphorus allocated by major river by jurisdiction. This section and the supporting 

Appendix N explore the process by which the persistent nonattainment at reduced loading levels was resolved for each of these Bay 

segments.
DRAFT Chesapeake Bay TMDL

6-38 September 24, 2010

Dissolved Oxygen Nonattaining Segments

The drivers of persistent nonattainment in these segments were examined. With the notable 

exception of the lower Chester River segment (CHSMH), it was generally found that 

nonattainment in a Bay segment resulted from two or more of the following factors:

1. Less-than-expected change in DO concentrations from the calibration to a given reduced 

nutrient load scenario

2. Poor agreement between model-simulated and historically observed DO concentrations 

for a particular location and historical period

3. Unusually or very low DO concentrations, which were very difficult to bring into 

attainment of the open-water DO criteria even with dramatically reduced loads

The majority of those segments are in small and relatively narrow regions of the Bay’s smallest 

tidal tributaries. Such conditions constrain the Bay Water Quality Model’s ability to effectively 

integrate multiple drivers of DO concentrations. As a result, the Bay Water Quality Model’s 

ability to simulate the water quality changes in response to dramatically reduced loads was also 

limited. In such cases, additional lines of evidence were used to determine whether a segment 

could be expected to achieve the applicable WQS under the reduced nutrient loads.

Each Bay segment was evaluated to determine (1) whether violations of the DO criteria were 

isolated or widespread; (2) whether nearby segments also exhibited persistent or widespread 

hypoxia or both; and (3) whether the Bay Water Quality Model predicted sufficient 

improvements in DO concentrations to achieve DO WQS in nearby deeper, wider segments. 

Results of the evaluations, documented in detail in Appendix N, are summarized as follows.

Gunpowder River (GUNOH)

Monitored DO concentrations over the 10-year period of 1991–2000 were almost universally 

well above the open-water criterion of 5 mg/L. A single instance of moderate hypoxia, combined 

with poor model agreement and an almost complete lack of response by the Bay Water Quality 

Model to load reductions in the monitored location for the relevant month, resulted in persistent 

nonattainment across all reduced loading scenarios for the month in question. In contrast, nearby 

Bay segments—Bush River (BSHOH), Middle River (MIDOH), and upper Chesapeake Bay 

(CB2OH)—all attained their respective DO WQS when loads were reduced to the target 

basinwide allocation of 190 million pounds per year TN and 12.6 million pounds per year TP.

Given those factors, EPA believes that Gunpowder Run can reasonably be expected to attain its 

DO WQS at the target loadings of 190 million pounds per year TN and 12.6 million pounds per 

year TP.

Manokin (MANMH), Maryland Anacostia (MDATF), West Branch Elizabeth (WBEMH), 

Pamunkey (PMKTF), and Wicomoco (WICMH) Rivers

Similar to the Gunpowder River segment, few violations of the open-water DO criteria occurred 

in these five Bay segments, and Bay Water Quality Model simulations did not match well with 

historically observed water quality conditions. The Bay Water Quality Model often failed to 

simulate hypoxia for these locations under observed loads; thus, it was also unable to estimate 

improved DO concentrations when nutrient loads were reduced. Nearby deeper, wider regions 

generally attained DO WQS at or before the target basinwide loadings. Given those factors, EPA 
DRAFT Chesapeake Bay TMDL

6-39 September 24, 2010

believes these Bay segments can reasonably be expected to attain the DO WQS at the target 

loadings of 190 million pounds per year TN and 12.6 million pounds per year TP.

Upper and Middle Portions of the Pocomoke River (POCTF, POCOH_MD, POCOH_VA)

These three Bay segments of the narrow, Eastern Shore tidal Pocomoke River are all represented 

by the same water quality monitoring station and a single Bay Water Quality Model cell. The 

range of DO concentrations simulated by the Bay Water Quality Model did not match well with 

historically observed conditions in this location. The persistent nonattainment shown by the 

model was driven by a single violation of the WQS with the reduced load scenarios. Downstream 

segments (e.g., POCMH) achieved attainment at the target basinwide loading. Furthermore, it 

was confirmed that this river is influenced by natural tidal marshland that naturally depresses DO 

concentrations of the river. Maryland and EPA believe that because of the documented influence 

of wetlands on water chemistry in the Pocomoke River, the current open-water DO 30-day mean 

criterion of 5 mg/L is not appropriate. As discussed in Section 3, Maryland is proposing an 

alternative site-specific criterion for this segment, consistent with EPA’s published guidance 

(USEPA 2004a). The proposed allocated loads for this river system will achieve the proposed 

criterion of 4.0 mg/L. Virginia is proposing to analyze the conditions of the Pocomoke River to 

demonstrate that the lower DO is caused by natural conditions.

Magothy River (MAGMH)

Summer hypoxic conditions were not uncommon in the Magothy River from 1991–2000, 

particularly when episodes of water column stratification prevented mixing of the bottom waters 

with more oxygenated surface waters. An episodic deep-water designated use was added to 

MAGMH to account for periods of water column stratification (USEPA 2010a). However, some 

violations of the deep-water DO 30-day mean criterion of 3.0 mg/L persisted even when nutrient 

loads were reduced to the target basinwide allocation. Because of the small, embayment nature 

of the Magothy River, the Bay Water Quality Model again struggled to simulate observed 

conditions in MAGMH or consistently estimate a response of sufficiently improved DO in 

response to load reductions. However, the deep-water region of the adjacent mainstem segment 

CB3MH attained its DO WQS well before the target basinwide nutrient LAs. Given the poor 

simulation of MAGMH conditions by the Bay Water Quality Model, the significant load 

reductions already required of the Magothy River basin at the target basinwide LAs, the 

considerable influence of the mainstem Chesapeake Bay on MAGMH water quality conditions, 

and the predicted attainment of CB3MH deep-water well before the target basinwide loading, 

EPA determined that MAGMH can reasonably be expected to attain its DO WQS at the target 

loadings of 190 million pounds per year TN and 12.6 million pounds per year TP.

Lower Chester River (CHSMH)

On the basis of further exploration of the modeling results for this segment and for neighboring 

segments, EPA concluded that the projected lower Chester River’s deep-channel DO criterion 

nonattainment as shown in Error! Reference source not found. is valid. What could be 

observed from these Bay Water Quality Model results was that the nonattainment improves but 

persists, even at an E3 scenario, until an All Forest scenario. Also, the basinwide-based 

allocations for the Chester River watershed represent very stringent levels of controls. For those 

reasons, Maryland is proposing in its WQS regulations a variance of 14 percent for the deepchannel portion of the lower Chester River segment. EPA is proposing the primary Bay TMDL 

scenario to use the basinwide nutrient target load-based allocations according to the proposed 
DRAFT Chesapeake Bay TMDL

6-40 September 24, 2010

WQS modifications to the Chester River watershed to ensure WQS. EPA is also proposing 

another Bay TMDL scenario on the basis of current WQS (assuming the Chester River and other 

proposed WQS do not become effective before finalization of this TMDL).

Chlorophyll a Nonattaining Segments

Potomac and Anacostia Rivers in DC

The Bay Water Quality Model projected that the District of Columbia’s portions of the Potomac 

and Anacostia river segments would be in nonattainment of the applicable numeric chlorophyll a

WQS at the proposed basinwide nutrient target loads allocated to these two river basins. 

However, through diagnostic analysis of the modeled chlorophyll a simulations for the Potomac 

and Anacostia rivers in the District of Columbia, EPA determined that the Bay Water Quality 

Model did not reliably simulate measured chlorophyll a levels. Therefore, other lines of evidence 

(i.e., monitoring data) were weighed more heavily by EPA in the attainment determination 

(Appendix N). Through further investigation, EPA analyzed recent chlorophyll a data for the two 

segments. The actual monitoring data show that the Potomac River segment is attaining the 

District’s chlorophyll a WQS and has been attaining that standard for at least the past 7 years 

(Figure 6-12). In the Anacostia River segment, a 4 percent level of nonattainment was found, 

again using current water quality monitoring data (Appendix N). 

Because these two Bay segments will experience significant further lowering of the present 

nutrient levels upon achievement of the nutrient loadings under the proposed allocations 

scenario, EPA has concluded that both of the Bay segments will be in attainment with the 

chlorophyll a WQS under these nutrient allocations (Appendix N). Additionally, a TMDL for 

biochemical oxygen demand and nutrients was approved by EPA in 2008 for the Anacostia River 

Basin Watershed in Montgomery and Prince Georges Counties, Maryland and the District of 

Columbia (USEPA 2008). That TMDL for the Anacostia River requires significant nutrient 

reductions that, when implemented, will result in attainment of the chlorophyll a standard.
DRAFT Chesapeake Bay TMDL

6-41 September 24, 2010

11

POTTF Chla: TF2.1 v PMS44

0

10

20

30

40

50

60

70

80

90

2/9/1999 10/17/1999 6/23/2000

2/28/200111/5/20017/13/20023/20/2003 11/25/2003 8/1/2004

4/8/2005 12/14/2005 8/21/2006

4/28/2007 1/3/2008

9/9/2008 5/17/2009

1/22/2010

sample date chlorophyll a (ug/L)

PMS44 TF2.1

DC (dark blue) and MD (light blue) Chlorophyll a 

Monitoring Data Compared with DC’s WQ 

Chlorophyll a Criterion of 25 μg/L 

DC Chlor. 

Criterion

Source: http://www.chesapeakebay.net

Note: DC station PMS44 is on the Potomac River at the Woodrow Wilson Memorial Bridge (50 meters upstream of 

the draw span). TMD station TF2.1 is on the Potomac River at Buoy 77 off the mouth of Piscataway Creek.

Figure 6-12. Potomac River chlorophyll a monitoring data compared with the District’s chlorophyll a water 

quality criteria.

James River in Virginia

In general, the Bay Water Quality Model is well-calibrated to the tidal James River and 

effectively simulates average seasonal conditions in the five tidal segments of this river. The Bay 

Water Quality Model also consistently estimates improved chlorophyll a conditions with 

increasing nutrient load reductions. At the same time, however, the Bay Water Quality Model 

does not simulate individual algal bloom events, which are highly variable and caused by 

numerous factors, some of which are still not well understood by the scientific community. The 

chlorophyll a WQS adopted in Virginia’s regulation to protect the tidal James River were set at 

numerical limits for spring and summer seasonal averaged conditions, not for addressing 

individual algal bloom events lasting hours to days. Therefore, EPA’s determination of nutrient 

loadings required to attain chlorophyll a WQS in the tidal James River was based on those years 

and Bay (James River) segments for which the Bay Water Quality Model reliably simulated the 

water quality monitoring-based chlorophyll a calibration data. That approach was used to 

determine the James River basin target LA of 23.5 million pounds per year TN and 2.35 million 

pounds per year TP. 

However, at the target James River LA, nonattainment of the summer chlorophyll a WQS 

persisted in the lower tidal fresh James segment (JMSTFL) for the summer periods of 1995–
DRAFT Chesapeake Bay TMDL

6-42 September 24, 2010

2000, and in the James River mouth segment (JMSPH) for the 1997–2000 summer periods 

(Appendix Q). Because these remaining nonattainments represented monitoring algal bloom 

conditions and the Bay Water Quality Model did not reliably simulate the calibration data, the 

nonattainment model results were not used to establish the James River allocations (Appendix 

N). Instead, the allocations were established on the basis of the remaining Bay segments, spring 

and summer seasons, and years where the Bay Water Quality Model simulation was reliable.

Figure 6-13 shows the number of segments and 3-year periods in nonattainment of Virginia’s 

James River chlorophyll a WQS (out of the simulation period of 1991–2000) for the various load 

scenarios simulated, using those model results where the model is reliably simulating the 

calibration data (Appendix N). From the graph it can be seen that the James River does not fully 

attain the chlorophyll a WQS until a loading of 23.5 million pounds per year of nitrogen and 

2.35 million pounds per year of phosphorus was achieved. For that reason, EPA determined the 

allocations to the James River necessary to achieve the chlorophyll a criteria is 23.5 million 

pounds per year of nitrogen and 2.35 million pounds per year of phosphorus.

James River Chlorophyll a Response to Load 

Reductions

0

5

10

15

20

25

30

35

40

45

50

36.8/4.330.4/3.327.5/3.327.1/3.126.6/2.7

26/2.6

25.5/2.523.5/2.421.5/2.216.1/1.5

James River Basin TN/TP Load

Nr of Segment-Periods 

Violating WQS

36.8N

4.3Pe

30.4N

3.3P9

27.5N

3.3Pt

27.1N

3.1PA

26.6N

2.7P2.7

26N

2.6P

25.5N

2.5P

23.5N

2.4P

21.5N

2.2P

16.1N

1.5P

James River Basin TN/TP Load

Figure 6-13. James River nonattainment of the chlorophyll a standards at various load scenarios.

Similar to the EPA analysis of attainment of the District of Columbia’s chlorophyll a criterion 

using upper tidal Potomac and Anacostia River chlorophyll a monitoring data, EPA also assessed 

attainment using chlorophyll a monitoring data for the tidal James River. In contrast to the 

District’s tidal Anacostia and Potomac river segments, EPA found that the past and current 

monitoring data for most of the tidal James River segments showed significant nonattainment of 

Virginia’s chlorophyll a WQS (Appendix N). An example of the comparative analysis of the 

monitored data for the James as compared to Virginia’s segment-season specific chlorophyll a
DRAFT Chesapeake Bay TMDL

6-43 September 24, 2010

criteria is shown in Figure 6-14 EPA therefore has continued to rely on the model results in 

assessing the appropriate allocations of nutrients.

12

Comparable VA James River 

Monitoring Station Chlorophyll 

a Data Time Series

Summer

Criterion

Spring 

Criterion

Source: http://www.chesapeakebay.net

Figure 6-14. Tidal James River monitoring data for chlorophyll a at station TF5.5 (located in the upper tidal 

James River near Hopewell, Virginia) compared to Virginia’s James River segment-season specific 

chlorophyll a criteria.

Allocation Considerations for the Headwater States (New York–West Virginia)

The methodology described above for distributing the basinwide loading was accepted by all 

jurisdictions except New York and West Virginia. From an additional model run, EPA 

determined that additional assimilative capacity was available. EPA used its discretionary 

authority to allocate to New York an additional 700,000 pounds per year of nitrogen (above the 

allocation calculated for New York using the method used to distribute the basinwide loads of 

190 million pounds per year of nitrogen and 12.6 million pounds per year of phosphorus). In 

addition, EPA used its discretionary authority to allocate to West Virginia an additional 200,000 

pounds per year of phosphorus (above the level allocated to West Virginia using the allocation 

methodology to distribute the basinwide load of 190 million pounds per year of nitrogen and 

12.6 million pounds per year of phosphorus). EPA, through model analysis, confirmed that those 

loadings will achieve WQS in the Chesapeake Bay. EPA provided the additional allocations 

because

Following the principles and guidelines as expressed in Section 6.3, tributary basins that 

contribute the most to the Bay water quality problems must do the most to resolve those 

problems (on a pound per pound basis). The headwater jurisdictions of New York and West 

Virginia contribute small portions of the overall nutrient delivered to the Bay (5 percent) 

and therefore are provided some relief in their allocations.

On the basis of information provided by New York, the water quality from the streams and 

rivers coming from the headwaters is generally of better quality than that of downstream 

waters.
DRAFT Chesapeake Bay TMDL

6-44 September 24, 2010

The allocation methodology accommodates to some extent future growth by providing 

WLAs for wastewater treatment facilities at design flow rather than actual flow, thereby 

reserving a load for expansion of the facility.

New York considered the methodology to be biased against Bay watershed jurisdictions 

that are growing relatively slowly, like New York.

A cleaner Bay provides greater benefit (in terms of commercial and recreational benefits of 

a cleaner bay) to the tidal jurisdictions than to the nontidal jurisdictions such as New York 

and West Virginia.

Nitrogen-to-Phosphorus Exchanges

EPA permitted the jurisdictions’ to propose the exchange of nitrogen and phosphorus loads 

within major river basins at a 1:5 ratio for reducing existing allocated phosphorus loads in 

exchange for increased nitrogen loads and a 15:1 ratio for reductions in existing allocated 

nitrogen loads I exchange for increased phosphorus loads. For example, in state allocations, for 

every 1 pound of phosphorus reduced, 5 pounds of nitrogen can be added and for every 15 

pounds of nitrogen reduced, 1 pound of phosphorus can be added. This section documents the 

technical basis for those exchange rates.

Two scientific papers published in recent years specifically address tradeoffs between nitrogen 

and phosphorus. The two analyses were completed with earlier versions of the Bay Watershed 

Model and the Bay Water Quality Model, but the results can still be meaningful if used to put 

bounds on the exchanges on a bay-wide scale.

Wang et al. (2006) published response surface plots for chlorophyll a concentrations and anoxic 

volume days using a matrix of nitrogen and phosphorus load reduction scenarios. The response 

surface plots were generated by applying equations predicting overall chlorophyll a 

concentrations and anoxic volume days as quadratic functions of the nitrogen and phosphorus 

fraction of 2000 loading levels. Applying the Phase 5.3 Bay Watershed Model generated values 

in these same equations to assess the area around the allocation levels of 187.4 million pounds 

total nitrogen (TN) and 12.52 million pounds total phosphorus (TP), one can use the derivatives 

of the original published equations to determine estimated TN:TP exchange relationships.

Figure 6-15 illustrates the TN:TP exchange ratio for different levels of TP based on the Anoxic 

Volume Days metric. At the allocation level of 12.52 million pounds of TP, the calculated 

exchange ratio is about 9:1, but the ratio has a good deal of variability. Considering that these are 

earlier versions of the Bay Watershed and Bay Water Quality models applied to the current 

reduction percentages, the local exchange ratio may vary depending on the location of the basin 

within the Bay. Given the degree of variability in this graph, a conservative approach is 

warranted. Figure 6-16 is the same analysis, except it uses chlorophyll a concentration in place 

of Anoxic Volume Days. The exchange ratios are lower, putting a greater importance on TP 

overall.

Wang and Linker (2009) documented an application of the earlier Bay models to the deep-water 

designated use of the upper central Chesapeake Bay segment CB4MH and determined a TN:TP 

exchange ratio of roughly 5:1 for that region of the mainstem Bay.
DRAFT Chesapeake Bay TMDL

6-45 September 24, 2010

Taking both of those analyses and the two published papers into account, an asymmetrical 

exchange ratio of 5:1 TN:TP when allowing more nitrogen loads and lowering the phosphorus 

load, and a ratio of 15:1 TN:TP when allowing more phosphorus loads and lowering the nitrogen 

load are recommended. All applications of these TN:TP exchanges are confirmed to not affect 

the attainment of the jurisdictions’ Bay WQS through follow-up Bay Water Quality Model 

scenarios.

Source: Wang et al. 2006

Figure 6-15. Total nitrogen:total phosphorus exchanges based on anoxic volume days and varying total 

phosphorus loads.

TN / TP trade off based on Anoxic Volume Days

0

2

4

6

8

10

12

14

16

18

10 11 12 13 14 15 16

TP Load TN:TP exchange ratio
DRAFT Chesapeake Bay TMDL

6-46 September 24, 2010

Source: Wang et al. 2006.

Figure 6-16. Total nitrogen (TN): total phosphorus (TP) exchanges based on chlorophyll a concentrations 

and varying total phosphorus loads.

Proposed Basin-Jurisdiction Nutrient Allocations

After performing all the analyses described above, EPA determined the allocations needed to 

attain the proposed WQS for DO and chlorophyll a for each basin-jurisdiction (see Section 9). 

The jurisdictions used the allocations to develop their draft Phase I WIPs that further suballocate 

the nutrient loadings to finer geographic scales and to individual sources or aggregate source 

sectors.

6.4.2 Determining the Sediment Load Caps to Achieve the Proposed 

Amended Water Quality Standards

Building from the basin-jurisdiction nutrient allocations described above, the key steps were 

taken:

Assessed water clarity/SAV criteria attainment across all Bay segments containing the 

shallow-water bay grass designated use under the above nutrient loads and the 

corresponding phosphorus-based sediment loads.

Identified those individual Bay segments still not attaining their applicable water 

clarity/SAV WQS at the allocated basinwide nutrient loads and the corresponding 

phosphorus-based sediment loads, and addressed the remaining nonattainment segments.

Of the 92 tidal Bay segments assessed by Maryland, Virginia, Delaware, and the District of 

Columbia, 26 achieve the respective jurisdiction’s SAV/water clarity WQS on the basis of 

available monitoring data (Appendix P). Twenty segments have mapped SAV acreages meeting 

TN / TP exchange based on average Chlorophyll concentration

0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16

TP Load TN:TP exchange ratio
DRAFT Chesapeake Bay TMDL

6-47 September 24, 2010

the segment-specific SAV restoration acreage in the jurisdiction’s WQS (single best year of the 

past 3 years). Of the 12 water clarity acre assessments that were performed, an additional 6 

segments were found to attain the jurisdiction’s water clarity criteria based on an analysis of 

shallow-water monitoring data (Figure 6-17).

However, the Bay Water Quality Model projected widespread attainment at existing loading 

levels, yet the existing SAV water quality data show SAV/water clarity WQS nonattainment in 

66 of 92 segments with only 46 percent of the Bay-wide restoration acreage achieved. The 

existing state of scientific understanding has resulted in the Bay Water Quality Model to be 

optimistic in its simulation of SAV acreage in the Bay under current (2009) pollutant loads.

Sources: DC DOE 2008; DE DNREC 2008; MDE 2008; VA DEQ 2008; Appendix Q.

Figure 6-17. Chesapeake Bay SAV/Water Clarity WQS attainment from monitoring data assessment.
DRAFT Chesapeake Bay TMDL

6-48 September 24, 2010

In a TMDL, where there is uncertainty, an explicit MOS is appropriate. The sediment allocations 

reflect this application of an explicit MOS (see Section 6.2.3). Also by initially expressing the 

draft sediment allocations as a range, EPA allowed the jurisdictions some flexibility in 

developing their draft Phase I WIPs while assuring with confirmation model runs that all the 

WQS would be met.

Source: USEPA 2010g

Figure 6-18. Model simulated sediment loads by scenario compared with the draft range of sediment 

allocations (billions of pounds per year as total suspended sediments).

Addressing Water Clarity/SAV Nonattaining Segments

Ultimately, four segments were in nonattainment of the SAV-water clarity WQS at the nutrient 

and sediment reductions of the allocation scenario, which achieved all other DO, chlorophyll a, 

SAV, and water clarity WQS. Three of those segments, the Back River (BACOH), upper Chester 

River (CHSTF), and the middle Pocomoke River (POCOH), required nutrient and sediment load 

reductions at either the E3 or all forest levels to achieve the SAV-water clarity WQS (Appendix 

R).

The three segments were unique in that their SAV-water clarity WQS were set using a 

hypothetical SAV coverage back-calculated though 100 percent attainment of the water clarity 

criteria (see Table V-2 on page 54 in USEPA 2007a). All other Chesapeake segments had their 

SAV-water clarity WQS set using the maximum SAV ever observed in a record that goes back at 

least 40 years and in some areas back longer than 70 years (USEPA 2003c). EPA included this 

methodology in its 2007a WQS Addendum but is now reconsidering whether a clarity-based 

0

2

4

6

8

10

12

1985 2009 Tributary Strategies Aug 13 Draft Allocation Billion Lbs per Year

9.78

8.09

6.47 6.1-6.7
DRAFT Chesapeake Bay TMDL

6-49 September 24, 2010

approach for setting the SAV goal is advisable. Maryland has proposed changing the SAV-water 

clarity WQS for these three Bay segments to be consistent with the approach uses in all other 

Chesapeake Bay segments and expects to complete the process before the end of 2010. 

Appendix R provides more details of the resolution found for segments that failed to meet the 

SAV-water clarity WQS at nutrient and sediment loads equivalent to the allocation scenario.

Back River

SAV had not historically been observed in the Back River until 2004, when 30 acres were 

observed for the first time. The current SAV goal of 340 acres, based on the estimated area that 

is equal to all of the area of the application depth (0.5 m) divided by 2.5, is unattainable even 

under estimated nutrient and sediment loads of the all forest scenario. However, all the adjacent 

segments to Back River (BACOH), including the Middle River (MIDOH) and the upper 

Chesapeake Bay (CB2OH), achieved the SAV/water clarity WQS on the basis of observed SAV 

acres in 2009. On the basis of all these lines of evidence, the current WQS in the Back River 

might be overly stringent, and if the new standard proposed by Maryland (consistent with the 

approach for setting the SAV-water clarity WQS elsewhere in the Bay) is adopted, the Back 

River is estimated to fully achieve the proposed WQS under the draft allocation scenario 

(Appendix N). If the proposed amended WQS-based allocation scenario is implemented, the 

estimated reduction in sediment loads will be about 22 percent from current loads.

Upper Chester River

As in the Back River, until 2005 no SAV had been observed in the upper Chester

River segment (CHSTF). In 2005 one acre of SAV was observed for the first time in more than 

40 years. The SAV goal of 230 acres is based on the estimated area that is equal to the entire area 

of the application depth (0.5 m) divided by 2.5, and is unachievable even at the E3 level of

nutrient and sediment reduction. The recently proposed WQS by Maryland is based on the 

maximum level of observed SAV would achieve water WQS. Sediment loads at the draft 

allocation scenario are estimated to be about 29 percent below estimated current sediment loads 

in this watershed.

Middle Pocomoke River

SAV had not historically been observed in the middle Pocomoke River (POCOH) (USEPA 

2003c). The SAV goal of 22 acres is based on the estimated area that is less than or equal to the 

entire area of the application depth (0.5 m) divided by 2.5. Maryland is proposing a change in its 

WQS for this segment of the Pocomoke River (POCOH_MD) to reflect a SAV no-grow zone, 

consistent with the adjacent upper Pocomoke River segment (POCTF) consistent with EPA 

guidance (USEPA 2004e). The presence of no observed SAV would result in an SAV or water 

clarity acre goal of zero acres.

That is the case in the Virginia portion of the middle Pocomoke River (POCOH_VA), where no 

SAV or water clarity acre goal is in Virginia’s WQS regulations. The color in the Pocomoke 

River black water system is the major cause of natural light attenuation and the lack of SAV 

presence in these waters.

Virginia’s Lower Potomac River

Virginia’s lower Potomac River segment (POTMH_VA) has an SAV restoration acreage of 

4,250 acres in Virginia’s WQS regulations. The draft allocation scenario has a relatively low 
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6-50 September 24, 2010

level of 10 percent nonattainment. That level of nonattainment is persistent and was estimated to 

be 9 percent at E3 scenario and 6 percent at the all forest scenario nutrient and sediment load 

levels (Appendix J). The reason for this persistence is the lack of shallow-water habitat (less than 

2 meters) in this segment to achieve the clarity criterion and thereby to support SAV growth. The 

POTMH_VA has a high SAV goal, yet with the described assessment methods it is estimated to 

largely achieve WQS the 10,625-acre water clarity criteria based mostly on water clarity alone.

The observed SAV record shows overall improvement in SAV and use of recent years. The use 

of the recent observed SAV area (in 2004–2005) would achieve the SAV-water clarity WQS 

(Figure 6-19) and sediment loads at the Allocation Scenario are 20 percent below estimated 

current sediment loads in this watershed (Appendix N).

Source: http://www.vims.edu/bio/sav

Figure 6-19. Observed SAV acres in Virginia’s lower Potomac River segment.

6.5 Assessing Attainment of Current WQS

As mentioned above, states are proposing amendments to their WQS. This TMDL document 

indentifies the process and results of deriving TMDL allocations to achieve the proposed 

amended WQS. This subsection describes the process and results of deriving the allocations to 

achieve the current WQS.

6.5.1 Establishing Nutrient Basin-Jurisdiction Load Caps

The steps described above are for developing the basin-jurisdiction allocations to achieve 

proposed amended Chesapeake Bay WQS. Some of the same steps were used for developing 

basin-jurisdiction allocations to achieve the Bay jurisdictions’ current Chesapeake Bay WQS. 

Potomac Virginia Mesohaline

0

100

200

300

400

500

600

1971197419781979198019811984198519861987198919901991199219931994199519961997199819992000200120022003200420052006200720082009 SAV Acres
DRAFT Chesapeake Bay TMDL

6-51 September 24, 2010

The overall process for deriving basin-jurisdiction cap loads to achieve current WQS is described 

below.

Atmospheric deposition: To achieve the current water quality standards, the same air deposition 

allocations were used to achieve the current water quality standards as was used for proposed 

water quality standards.

Basinwide load: Again using the same process as described above, the model output for various 

loading model runs were compared to current WQS. From this analysis, WQS were not achieved 

until all basin-jurisdictions were at E3 levels of nitrogen and phosphorus loading. Basinwide E3 

nutrient loading levels are needed to bring all 92 Bay segments into attainment with the current 

WQS. The basinwide loads at E3 equivalent loading levels are 141 million pounds per year of 

nitrogen and 8.65 million pounds per year of phosphorus.

Distribution of the Bay-wide load among the basin-jurisdictions: The methodology described 

above for distributing this basinwide load was not used for achieving current WQS. That is 

because the allocation should not drive any basin-jurisdiction beyond E3 levels unless needed to 

do so to achieve the Bay jurisdictions’ WQS. To achieve current WQS, since E3 was needed 

basinwide, all basin-jurisdictions were allocated E3 level of controls. Therefore, no methodology 

for dividing the Bay-wide load, like that used for the proposed amended Bay WQS cap loads, 

was needed to derive the cap loads to achieve the current WQS.

Nonattaining segments: Even at E3 loading levels the Bay Water Quality Model predicted that 

there would be some Bay segments in nonattainment with the jurisdictions’ current Bay DO 

WQS. However, on further review of the Bay Water Quality Model output with other lines of 

evidence, these segments were found to be in attainment at the E3 loading levels. A brief 

discussion of the analysis and findings for these previously nonattaining segments is provided 

below.

The drivers of persistent nonattainment in these segments were examined. With the notable 

exception of the lower Chester River segment (CHSMH), it was generally found that 

nonattainment in any given segment resulted from two or more of the following factors:

1. Less-than-expected change in DO concentrations from the Bay Water Quality Model 

calibration to a given reduced nutrient load scenario

2. Poor agreement between the Bay Water Quality Model-simulated and historically 

observed DO concentrations for a particular location and historical period (e.g., station 

ET10.1 for June 1993)

3. Unusually or very low DO concentrations, which were very difficult to bring into 

attainment of the open-water DO criteria even with dramatically reduced loads

The majority of these Bay segments are in small and relatively narrow regions of the Bay’s tidal

tributaries. Such conditions constrain the Bay Water Quality Model’s ability to integrate multiple 

drivers of DO concentrations. As a result, the Bay Water Quality Model’s ability to simulate the 

water quality responses to dramatically reduced loads was also limited. In such cases, additional 

lines of evidence were used to determine whether a given segment could be expected to achieve 

the applicable WQS.

Each Bay segment was evaluated to determine whether violations of the DO criteria were 

isolated or widespread; whether nearby segments also exhibited persistent or widespread 
DRAFT Chesapeake Bay TMDL

6-52 September 24, 2010

hypoxia; and whether the Bay Water Quality Model predicted sufficient improvements in DO 

concentrations to achieve DO WQS in nearby deeper, wider segments. Results of these 

evaluations, documented in detail in Appendix R, are summarized as follows.

Gunpowder River (GUNOH)

DO concentrations over the 10-year period 1991–2000 were almost universally well above the 

open-water criterion of 5 mg/L. A single instance of moderate hypoxia, combined with poor 

model agreement and an almost complete lack of response by the Bay Water Quality Model to 

load reductions in the monitored location for the relevant month, resulted in persistent 

nonattainment across all reduced loading scenarios for the month in question. In contrast, nearby 

Bay segments—Bush River (BSHOH), Middle River (MIDOH), and upper Chesapeake Bay 

(CB2OH)—all attained their respective DO WQS when loads were reduced to the target 

basinwide allocation of 190 million pounds per year TN and 12.6 million pounds per year TP.

Manokin (MANMH), Maryland Anacostia (MDATF), West Branch Elizabeth (WBEMH), 

Pamunkey (PMKTF), and Wicomoco (WICMH) Rivers

Similar to the Gunpowder River segment, few violations of the open-water DO criteria occurred 

in these Bay segments and Bay Water Quality Model simulations did not match well with 

historically observed water quality conditions. The Bay Water Quality Model often failed to 

simulate hypoxia for these locations under observed loads; thus, it was also unable to estimate 

improved DO concentrations when nutrient loads were reduced. Nearby deeper, wider regions 

generally attained DO WQS at or before the target basinwide loadings.

Upper and Middle Portions of the Pocomoke River (POCTF, POCOH_MD, POCOH_VA)

These three Bay segments of the narrow, Eastern Shore tidal Pocomoke River are all represented 

by the same water quality monitoring station and a single Bay Water Quality Model cell. The 

range of DO concentrations simulated by the Bay Water Quality Model did not match well with 

historically observed conditions in this location. The persistent nonattainment shown here was 

driven by a single persistent violation of the WQS with the reduced load scenarios, and 

downstream segments (e.g., POCMH) achieved attainment at the target basinwide loading. 

Furthermore, it was confirmed that the river is influenced by natural tidal marshland that 

naturally depresses DO concentrations of the river. Maryland and EPA believe that because of 

the documented influence of wetlands on water chemistry in the Pocomoke River, the current 

open-water DO 30-day mean criterion of 5 mg/L is not appropriate. Maryland is proposing an 

alternative, site-specific criterion for this segment, consistent with EPA’s published guidance 

(USEPA 2004a).

Magothy River (MAGMH)

Summer hypoxic conditions were not uncommon in the Magothy River 1991–2000, particularly 

when episodes of water column stratification prevented mixing of the bottom waters with more 

oxygenated surface waters. An episodic, deep-water designated use was added to MAGMH to 

account for periods of water column stratification (USEPA 2010a). However, some violations of 

the deep-water DO 30-day mean criterion of 3.0 mg/L persisted even when nutrient loads were 

reduced to the target basinwide allocation. Because of the small, embayment nature of the 

Magothy River, the Bay Water Quality Model again struggled to simulate observed conditions in 

MAGMH or consistently estimate a response of sufficiently improved DO in response to load 

reductions. However the deep-water region of the adjacent mainstem segment CB3MH attained 

its DO WQS well before the target basinwide nutrient LAs. Given the poor simulation of 
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6-53 September 24, 2010

MAGMH conditions by the Bay Water Quality Model, the dramatic load reductions already 

required of the Magothy River basin, the considerable influence of the mainstem Chesapeake 

Bay on MAGMH water quality conditions, and the predicted attainment of CB3MH Deep Water, 

it was determined that MAGMH can reasonably be expected to attain its DO WQS at the E3 

loading levels.

Lower Chester River (CHSMH)

On the basis of further exploration of the modeling results for this segment and for neighboring 

segments, EPA concluded that the projected lower Chester River’s deep-channel DO criterion 

nonattainment as shown in Table 6-7 above is valid. What could be observed from the Bay 

Water Quality Model results was that the nonattainment improves but persists, even at an E3 

scenario, until an all forest scenario. Also, the basinwide-based allocations for the Chester River 

watershed represent very stringent levels of controls.

Piankatank River (PIAMH)

Under historical conditions, the level of WQS nonattainment in the Piankatank River (PIAMH) 

was low (< 1 percent). However, percent nonattainment actually increased in the load reduction 

scenarios. Further investigation revealed that in this small tributary adjacent to CB6PH, the Bay 

Water Quality Model did not simulate historical conditions well. The range of Bay Water 

Quality Model simulations was outside that of historical observations—particularly at lower 

depths where hypoxic or near-hypoxic conditions tended to occur—resulting in regressions that 

were not appropriate for use in the scenario-modification procedure. Furthermore, the adjacent 

mainstem segment CB6PH attained WQS (assuming the 1 percent rule) at loading levels higher 

than those of the E3 scenario, even with existing WQS. Given the inability of the WQM to 

provide estimates of the response of PIAMH to reduced loads, the attainment of CB6PH at or 

before E3, and the low level of WQS nonattainment under historical conditions, the EPA 

determines that it is reasonable to expect that PIAMH will attain current WQS E3 loading levels.

New York and West Virginia adjustment: No determination was made of additional assimilative 

capacity (beyond the E3 level of control), so no adjustment was made to the New York and West 

Virginia allocations (as was done for the allocations based on proposed WQS). Therefore, the 

allocations to New York and West Virginia remain at the E3 level.

6.5.2 Establishing Sediment Load Caps

As mentioned above, the effects from sediment are much more localized to the Bay segment and 

neighboring tidal segments receiving the sediment loading. In addition, the water clarity and 

SAV WQS are proposed to be amended only for the Back, Chester, and Pocomoke rivers—all in 

Maryland. For those reasons, the sediment proposed allocation is the same as the allocations for 

the proposed amended Chesapeake Bay WQS except for these segments.

For these three segments, more stringent allocations are needed in their respective surrounding 

watersheds. Table 6-8 shows the amount of nonattainment for each segment.
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6-54 September 24, 2010

Table 6-8. Percent nonattainment of the current Chesapeake SAV-water clarity WQS for the Back, 

Chester and middle Pocomoke (Maryland) rivers under a range of nutrient and sediment reduction 

load scenarios

*91_00 L_190 Allocate L_179 L_170 E3_2010 All

*1985 Base 2009 Tributary Loading Loading Loading Loading P based Forest

Scenario Scenario Scenario Strategy Scenario Scenario Scenario Scenario Scenario Scenario

BACOH MD 69.52% 29.31% 24.94% 20.31% 26.86% 23.87% 25.06% 25.06% 23.75% 45.36%

CHSTF MD 95.35% 95.35% 88.66% 79.74% 79.74% 79.74% 66.86% 51.36% 13.24% 0.00%

MPCOH MD 66.83% 66.83% 7.68% 6.32% 6.32% 6.32% 3.08% 0.00% 3.08% 0.00%

Source: Appendix Q

From Table 6-8, EPA concludes that the loadings needed to attain current jurisdictions’ Bay 

WQS for water clarity/SAV from the three Bay segments surrounding major river basin are as 

follows:

Back River—While the water quality model results show that even for an all forest 

scenario, the SAV WQS is not achieved, an all forest loading from this basin is being 

proposed (Appendix N). While more analysis of this assessment is needed, it could be 

argued that implementing an all forest scenario would attain applicable WQS. Furthermore, 

while sediment impacts are local, until model results are available to confirm that an all 

forest loading is needed only in the Back River basin, EPA took a conservative approach to 

establish the TMDL for the entire Maryland Western Shore basin at the all forest loading 

level. Therefore, the cap allocations for the Maryland Western Shore basin to achieve the 

current water clarity/SAV WQS is 84 million pounds per year of sediment.

Chester River (tidal fresh) and Pocomoke River (Oligohaline)—As with the Back River, 

EPA concludes from the Bay Water Quality Model results that all forest loading levels are 

needed to attain the current water clarity/SAV WQS for the Chester and Pocomoke rivers. 

However, the all forest loading cap will be applied to only the Maryland Eastern Shore 

basin. That is because Delaware and Virginia share very small portions of the drainage area 

for the Pocomoke (oligohaline) and Chester rivers. Therefore, EPA has established the 

Maryland Eastern Shore cap loading at an all forest loading level of 51 million pounds per 

year of sediment to achieve the current water clarity/SAV WQS.

6.6 Setting Draft Basin-jurisdiction Allocations

Based on all of the methods and analyses described above, EPA identified allocations for the 

major basins within each jurisdiction called the basin-jurisdiction allocations. These allocations 

are the beginning point for the development of the Bay TMDL and are provided below. 

6.6.1 Basin-jurisdiction Allocations to Achieve the Proposed WQS 

Throughout 2009 up until the summer of 2010, EPA and its watershed jurisdictional partners 

worked together to develop the basinwide and then major river basin/jurisdiction target loads. 

Based on these collaborative efforts, EPA shared an initial set of major river basin/jurisdiction 

nutrient target loads on November 3, 2009 based on decisions at the October 23, 2009 PSC 

meeting (USEPA 2009b). Then following a 2-day PSC meeting on April 29-30, 2010, EPA 

shared an updated Bay TMDL schedule and further outlined a long term commitment to an 

adaptive management approach to the Bay TMDL in a letter to the partners (USEPA 2010e). 
DRAFT Chesapeake Bay TMDL

6-55 September 24, 2010

One set of basin-jurisdiction allocations was based on attaining the proposed amendments to the 

state water quality standards. On July 1, 2010, EPA shared the draft nutrient allocations (USEPA 

2010f) and the draft sediment allocations on August 13, 2010 (USEPA 2010g). These are the 

allocations that states used to develop their WIPs and EPA used to backstop the WIPs. These 

allocations are calculated as delivered loads (the loading that actually reaches tidal waters) and as 

annual loads. These loads are provided below in Table 6-9 and 6-10.

Table 6-9. Chesapeake Bay watershed nutrient and sediment draft allocations by major river basin 

by jurisdiction to achieve the proposed Chesapeake Bay WQS.

Basin Jurisdiction

Nitrogen draft 

allocations

(million 

lbs/year)

Phosphorus 

draft allocations

(million 

lbs/year)

Sediment draft 

allocations

(million

lbs/year)

Susquehanna NY 8.23 0.52 293-322

PA 71.74 2.31 1,660-1,826

MD 1.08 0.05 60-66

Total 81.06 2.88 2,013-2,214

Eastern Shore DE 2.95 0.26 58-64

MD 9.71 1.09 166-182

PA 0.28 0.01 21-23

VA 1.21 0.16 11-12

Total 14.15 1.53 256-281

Western Shore MD 9.74 0.46 155-170

PA 0.02 0.001 0.37-0.41

Total 9.76 0.46 155-171

Patuxent MD 2.85 0.21 82-90

Total 2.85 0.21 82-90

Potomac PA 4.72 0.42 221-243

MD 15.70 0.90 654-719

DC 2.32 0.12 10-11

VA 17.46 1.47 810-891

WV 4.67 0.74 226-248

Total 44.88 3.66 1,920-2,113

Rappahannock VA 5.84 0.90 681-750

Total 5.84 0.90 681-750

York VA 5.41 0.54 107-118

Total 5.41 0.54 107-118

James VA 23.48 2.34 837-920

WV 0.02 0.01 15-17

Total 23.50 2.35 852-937

Total Basin/Jurisdiction Draft Allocation 187.44 12.52 6,066-6,673

Atmospheric Deposition Draft Allocationa

15.70 --

Total Basinwide Draft Allocation 203.14 12.52 6,066-6,673

a. Cap on atmospheric deposition loads direct to Chesapeake Bay and tidal tributary surface waters to be achieved 

by federal air regulations through 2020.
DRAFT Chesapeake Bay TMDL

6-56 September 24, 2010

Table 6-10. Chesapeake Bay watershed nutrient and sediment draft allocations by jurisdiction by 

major river basin to achieve the proposed Chesapeake Bay WQS.

Jurisdiction Basin

Nitrogen draft 

allocations

(million lbs/year)

Phosphorus draft 

allocations

(million lbs/year)

Sediment draft 

allocations

(million lbs/year)

Pennsylvania Susquehanna 71.74 2.31 1,660-1,826

Potomac 4.72 0.42 221-243

Eastern Shore 0.28 0.01 21-23

Western Shore 0.02 0.001 0.37-0.41

PA Total 76.77 2.74 1,903-2,093

Maryland Susquehanna 1.08 0.05 60-66

Eastern Shore 9.71 1.09 166-182

Western Shore 9.74 0.46 155-170

Patuxent 2.85 0.21 82-90

Potomac 15.70 0.90 654-719

MD Total 39.09 2.72 1,116-1,228

Virginia Eastern Shore 1.21 0.16 11-12

Potomac 17.46 1.47 810-891

Rappahannock 5.84 0.90 681-750

York 5.41 0.54 107-118

James 23.48 2.34 837-920

VA Total 53.40 5.41 2,446-2,691

District of Columbia Potomac 2.32 0.12 10-11

DC Total 2.32 0.12 10-11

New York Susquehanna 8.23 0.52 293-322

NY Total 8.23 0.52 293-322

Delaware Eastern Shore 2.95 0.26 58-64

DE Total 2.95 0.26 58-64

West Virginia Potomac 4.67 0.74 226-248

James 0.02 0.01 15-17

WV Total 4.68 0.75 241-265

Total Basin/Jurisdiction Draft Allocation 187.44 12.52 6,066-6,673

Atmospheric Deposition Draft Allocationa

15.70 --

Total Basinwide Draft Allocation 203.14 12.52 6,066-6,673

a. Cap on atmospheric deposition loads direct to Chesapeake Bay and tidal tributary surface waters to be achieved 

by federal air regulations through 2020.

6.6.2 Basin-jurisdiction Allocations to Achieve the Current WQS 

In addition to the basin-state allocations for the attainment of proposed water quality standards, 

EPA also develop basin-jurisdiction allocations to attain existing water quality standards. These 

allocations were not provided to states for the development of their WIPs. But these allocations 

are important in the event that the state water quality standards are not amended in time for the 

establishment of the final TMDL by December 31, 2010. These allocations are also based on 

delivered load and annual load and are provided in Table 6-11 below.

In order to achieve the current water quality standards that are extant in the Chesapeake Bay 

today, the estimated nutrient loads must be lowered to the watershed loadings that would require 
DRAFT Chesapeake Bay TMDL

6-57 September 24, 2010

an E3 level of effort described above. The allocated nutrient loads include atmospheric 

deposition loads equal to that in the proposed standard allocations of 15.7 mpy of nitrogen. 

For the current SAV-clarity water quality standard, achievement is estimated to be achieved by 

the current WIP levels of TSS loads, expressed as sediment loads, except for two basins on the 

Maryland Eastern Shore and one on the Maryland Western Shore. The Eastern Shore basins are 

the Chester Tidal Fresh and the Maryland Pocomoke Oligohaline. Both of these basins require 

All Forest Scenario loads to achieve the SAV-clarity water quality standard. Because of the 

nonattainment in the Chester and the Pocomoke, the entire Maryland Eastern Shore is set at an 

All Forest load level for sediment only. The Back River basin of the Western Shore also requires 

an All Forest Scenario level of sediment reduction and so the All Forest condition is set for the 

entire Maryland Western Shore as well.

For all other jurisdiction-basins in Table 6-11, the following decision rules were applied using 

the State Watershed Implementation Plans (WIPS):

1) If the final approved WIP came in below the sediment range (from the August 13, 

2010 sediment allocation letter) the sediment allocation is set to the low end of the 

sediment range.

2) If the final approved WIP came in within the high and low ends of the sediment range, 

then the sediment allocation is set at the WIP load.

3) If the WIP came in above the high end of the range then the sediment allocation is set 

at the high end of the sediment range.

The combined nutrient and sediment loads in Table 6-11 are estimated to fully achieve all 

current and existing water quality standards.

 
DRAFT Chesapeake Bay TMDL

6-58 September 24, 2010

Table 6-11. Chesapeake Bay Allocations for Existing WQS by Jurisdiction

Jurisdiction/Basin Nitrogen Allocation

(million pounds/year)

Phosphorus Allocation

(million pounds/year)

Sediment Allocation

(million pounds (TSS)/year)

PENNSYLVANIA

Susquehanna 56.89 1.76 1,758.2

Potomac 3.50 0.33 233.9

Eastern Shore 0.20 0.01 21.1

Western Shore 0.01 0.00 0.4

PA Total 60.59 2.10 2013.6

MARYLAND

Susquehanna 0.87 0.04 62.9

Eastern Shore 7.18 0.83 51.1

Western Shore 5.99 0.25 81.8

Patuxent 2.03 0.13 90.1

Potomac 11.42 0.63 682.3

MD Total 27.49 1.88 968.3

Virginia

Eastern Shore 0.79 0.12 10.9

Potomac 13.31 0.98 810.1

Rappahannock 4.39 0.60 688.5

York 3.83 0.35 107.1

James 16.45 1.55 852.8

VA Total 38.78 3.60 2,469.4

District of Columbia

Potomac 1.47 0.05 11.2

DC Total 1.47 0.05 11.2

New York

Susquehanna 6.39 0.43 293.0

NY Total 6.39 0.43 293.0

Delaware

Eastern Shore 2.22 0.19 57.8

DE Total 2.22 0.19 57.8

West Virginia

Potomac 3.61 0.37 248.1

James 0.02 0.01 16.6

WV Total 3.63 0.38 264.8

Basin/Jurisdiction 

Allocation 140.57 8.63

6,078.0

Atmospheric 

Deposition

15.7 - -

Total Allocation 156.27 8.63 6,078.0
DRAFT Chesapeake Bay TMDL

6-59 September 24, 2010

Attainment of the District of Columbia pH Water Quality Standard

Currently, the upper Potomac River Estuary from Key Bridge to Hains Point and the Washington 

Ship Cannel are on the District of Columbia 303(d) list of impaired waters. The cause of these 

impairment is high pH. The pH impairment may result from excess primary productivity or algal 

growth, which might, in turn, result from excess nutrient inputs. A simulation of the Potomac 

River was developed that relates pH to nutrient loading, primary production, and other factors. 

A key underlying assumption in this Potomac modeling framework, called the Potomac River 

Eutrophication Model (PEM), is that calcium carbonate equilibria and solid phase calcium 

carbonate (calcite) formation and precipitation are the primary buffers affecting pH in the tidal 

freshwater Potomac River. This assumption formed the basis for an earlier analysis (HydroQual 

1988) that attempted to explain the development of a large algal bloom in the tidal freshwater 

Potomac in 1983. 

This bloom was dominated by the blue-green alga, Microcystis aaeruginosa. In attempting to 

understand the factors contributing to a bloom of this magnitude (peak chlorophyll 

concentrations of 150 to 200 ug/L), the inter-relationship between pH and release of phosphorus 

from the bottom sediments was investigated by Seitzinger (1986). Seitzinger’s data showed a 

clear relationship between increased pH and increased release of dissolved inorganic phosphorus 

(DIP) from Potomac River sediment cores. Using information from Seitzinger’s data, the 

Potomac River Eutrophication Model was modified to include calcium carbonate equilibria and 

phosphorus release from bottom sediments as a function of overlying water column pH. In 

addition, two new algal classes were added to the three algal classes already in the Water Quality 

and Sediment Transport Model (WQSTM), which is the basis of the PEM.

Since the PEM is a stand-alone model, boundary conditions from the WQSTM from the final 

Allocations need to be provided for the mouth of the Potomac for the PEM. Work is proceeding 

to develop these boundary conditions in order to confirm that the allocation to achieve the 

proposed amendments to the DO and chlorophyll water quality standards will also achieve the 

pH water quality standard in DC waters. Confirmation is in progress, but based on current 

information, EPA expects that current allocations will be sufficient to achieve the District’s pH 

water quality standard.