Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-1 September 24, 2010

Appendix N-1.

Resolution of Segments Failing to Attain the Dissolved Oxygen Criteria 

In the process of determining the target nutrient load allocations, it was observed that in a limited 

number of Chesapeake Bay segments, poor dissolved oxygen conditions appeared to persist even 

under scenarios of dramatically reduced nutrient loads. A series of systematic diagnostic 

analyses were conducted to determine the drivers of these persistent violations. The findings of

these analyses, summarized in Section 6.4.1, are described in more detail here.

For the affected Bay segments, additional lines of evidence were explored to determine whether 

the apparent non-attainment represented an area of real concern, or whether these segments could 

reasonably be expected to show sufficient improvement to attain WQS given the proposed 

nutrient load reductions. Each Bay segment was evaluated to determine: 

1. Whether violations of the dissolved oxygen (DO) criteria were isolated or widespread;

2. Whether the Chesapeake Bay Water Quality Model effectively simulated historical 

conditions and improvement in those conditions with reduced loads; and

3. Whether nearby Bay segments also exhibited persistent and/or widespread hypoxia (low 

to minimal dissolved oxygen levels).

Gunpowder River

The dissolved oxygen criteria non-attainment in the tidal Gunpowder River (GUNOH) was 

driven by two converging factors. First, the historical water quality dissolved oxygen monitoring 

data for this location show that the water in Gunpowder River is generally well-oxygenated in 

the summertime, with only a single instance of hypoxia observed (July 1994) over the course of 

10 consecutive summers from 1991-2000 violated the open-water criterion of 5.0 mg/L (red line

in Figure N1-1). 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-2 September 24, 2010

Source: http://www.chesapeakebay.net

Figure N1-1. measurements taken in summer months (June-September) at water quality 

monitoring station WT2.1 in the Gunpowder River from 1991-2000.

Second, the Bay Water Quality Model’s simulations for this location, which ranged from about 

8-10 mg/L, were only moderately higher than the average historical summertime conditions. 

However the Bay Water Quality Model did not simulate conditions below 8 mg/L in this region. 

Because no simulated hypoxia existed, there was no example of simulated improvement in DO 

concentrations with reduced nutrient inputs for this region. With summertime dissolved oxygen 

concentrations at or above 8 mg/L, the Bay Water Quality Model generally simulated a minimal 

increase in DO concentrations in response to reduced nutrient loads. This is in clear contrast to 

Bay Water Quality Model’s performance when hypoxic conditions are simulated under 

calibration (i.e. historical) conditions—see Figure N1-2 for an example from the middle of the 

Chesapeake Bay. Figure N1-2 is example of a regression plot showing WQM performance 

consistent with historical observations. The pink symbols and line represent DO concentrations 

from the calibration scenario; the blue symbols and line represent DO concentrations under 

reduced nutrient loads of the E3 Scenario. The range of DO concentrations in the calibration 

scenario spans the range of historical observations. Greater increase in DO concentrations is 

observed with reduced loads when the initial (calibration) concentrations are low. In these cases 

the Bay Water Quality Model’s predictions are consistent with empirical findings, namely, that 

hypoxic conditions will improve with reduced loads to a greater degree than will initially high 

DO concentrations. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-3 September 24, 2010

Figure N1-2. Example of a regression plot showing Bay Water Quality Model performance 

consistent with historical water quality monitoring dissolved oxygen observations in the lower 

central Chesapeake Bay segment CB4MH at station CB4.3C. 

The regression equation that is used to scenario-modify dissolved oxygen concentrations (see 

Section 6.2.1 for a description of the scenario-modification procedure) is generated from a 

comparison of DO concentrations simulated in the calibration scenario with those simulated in a 

management scenario such as E3. When little change is observed in DO concentrations between 

the two scenarios, the resulting regression equation reflects this (Figure N1-3). When simulated 

DO concentrations are consistently at or above 8 mg/L in the calibration scenario, the Bay Water 

Quality Model generally does not show dramatic improvements in concentrations with reduced 

pollutant loads. Furthermore, when the resulting regression equation is applied to a DO 

concentration well outside the range of the simulated data, it can cause a ―DO response‖ that 

does not accurately reflect the information provided by the Bay Water Quality Model. 

In the case of Gunpowder River monitoring station WT2.1 for July 1994, the Bay Water Quality 

Model-simulated DO concentrations fell between about 8 and 10 mg/L for the calibration 

scenario as well as the numerous reduced loading ―management‖ scenarios. In Figure N1-3, the 

pink symbols and line represent the calibration scenario DO concentrations; the light blue 

symbols and black line show the change in DO concentrations from the calibration to the E3 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-4 September 24, 2010

scenario. The red arrows show the predicted change in an initial DO concentration of 4.5 mg/L. 

In this case, a historical observation of 4.5 mg/L was scenario-modified to a concentration of 4.4 

mg/L for the E3 scenario.

Figure N1-3. Bay Water Quality Model scenario DO concentrations and regression for station 

WT2.1 in the Gunpowder River.

As is shown here, even at the ―E3‖ scenario (see Appendix J for a description of management 

scenarios) only a slight increase in DO concentrations is observed across the range of simulated 

concentrations. Typically, a greater response – in the form of higher DO concentrations – is 

observed when the initial (i.e. calibration) DO concentrations are low (i.e. less than 5 mg/L). In 

this case, when the linear regression representing the relationship between the calibration and E3 

DO concentrations is extrapolated far below the range of simulated conditions, the result

suggests that under ―E3‖ conditions, hypoxia could actually get worse rather than better. This 

prediction is not an accurate representation of model simulations; rather it is the effect of 

extrapolating the regression equation well outside the range of the simulations from which it was 

generated. Such was the case for July 1994, when a historical observation of 4.5 mg/L was 

scenario-modified to a concentration of 4.4 mg/L under the dramatically reduced load conditions 

of the E3 scenario.

Examination of nearby segments – the Bush River (BSHOH), the upper Chesapeake Bay 

(CB2OH), and the Middle River (MIDOH) – showed attainment of DO WQS under historical 

loading conditions, as well as under all load reduction scenarios (Figure N1-4).
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-5 September 24, 2010

Figure N1-4. Open-water dissolved oxygen criteria attainment “stoplight plot” of the Gunpowder 

River segment GUNOH and nearby segments.

Source: Appendix N.

In summary, the incidence of hypoxia in the tidal Gunpowder River was isolated. In this single 

isolated case, the Bay Water Quality Model was unable to provide information on the magnitude 

of expected improvement in dissolved oxygen conditions with reduced nutrient loads in this 

region. Examination of nearby segments showed consistent attainment of DO WQS under 

historical (―Base‖) as well as reduced loading scenarios. Therefore, it is reasonable to expect 

that the open-water designated use of GUNOH will attain DO WQS under the basinwide target 

allocation of 190 million pounds per year TN and 12.7 million pounds per year TP.

Manokin River

In the Manokin River (MANMH), violations of the segment’s open-water dissolved oxygen 

WQS for the years 1991-2000 were limited to three measurements, ranging from 4.7-4.9 mg/L, 

taken during one sampling event in the month of July 1995 (Figure N1-5). 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-6 September 24, 2010

Source: http://www.chesapeakebay.net

Figure N1-5. Summertime DO observations (dark blue symbols) at water quality monitoring station 

ET8.1 in the Manokin River from 1991-2000. 

These isolated, marginal violations of the DO WQS under historical conditions were scenariomodified to greater non-attainment under simulated load reductions. At the same time, adjacent 

and nearby segments Tangier Sound (TANMH), Big Annemessex River (BIGMH), and the 

lower Pocomoke River (POCMH) all attained their respective DO WQS under historical 

conditions as well as reduced loading scenarios (Figure N-6).

Source: Appendix N.

Figure N1-4. Open-water dissolved oxygen criteria attainment “stoplight plot” of the Manokin 

River segment MANMH and nearby segments.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-7 September 24, 2010

Further examination of the performance of the Bay Water Quality Model in the vicinity of water 

quality monitoring station ET8.1 (MANMH’s single tidal monitoring station) showed lower –

rather than higher – DO concentrations under reduced loading scenarios (Figure N1-7).

Figure N1-7. Regression plot for the Bay Water Quality Model cell (6705) corresponding to the 

MANMH water quality monitoring station (ET8.1).

The grid location that represents the Manokin River’s single monitoring station is shallow, is 

directly adjacent to the land. The highlighted cell (cell 6705) in Figure N1-8 coincides with the 

location of long-term fixed station ET8.1. In such cases, the Bay Water Quality Model often 

struggles to integrate the multiple, interacting drivers of a parameter such as dissolved oxygen. 

Further investigation showed that chlorophyll a concentrations in cell 6705 decreased to zero (or 

less) at the E3 scenario (data not shown). If chlorophyll a concentrations had increased in 

concert with lower DO concentrations, then a temporal anomaly in pollutant loads to cell 6705 or 

its vicinity would have been suspected. However, the combination of non-existent chlorophyll a

concentrations and low DO concentrations observed here indicates that the Water Quality Model 

struggled to integrate the effect of reduced loads on the feedbacks among multiple drivers of 

dissolved oxygen concentrations. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-8 September 24, 2010

Figure N1-8. Chesapeake Bay Water Quality Model grid for the Manokin River and a portion of 

Tangier Bay. 

Given the isolated nature of DO criteria violations in MANMH under historical conditions, the 

poor performance of the Water Quality Model, and the unimpaired nature of adjacent water 

bodies under historical conditions as well as simulated reduced loadings, the EPA concludes that 

it is reasonable to expect full attainment of the DO WQS in MANMH at the basin-wide target 

allocation of 190 million pounds per year TN and 12.7 million pounds per year TP.

Maryland Portion of the Anacostia River

In the Maryland portion of the tidal Anacostia River (ANATF_MD), substantial violations of the 

segment’s open-water dissolved oxygen WQS were observed historically, with particularly 

serious violations occurring at station ANA01 in August 1993 and July 1994 (Figure N1-9). 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-9 September 24, 2010

Source: http://www.chesapeakebay.net

Figure N1-9. Summertime water quality dissolved oxygen monitoring observations at Maryland’s 

tidal Anacostia River water quality monitoring station ANA01 from 1991-2000. 

The majority of these historical violations were estimated to improve substantially or even reach 

full attainment with further load reductions. However, for the two months during the critical 

period with the most serious violations—August 1993 and July 1994, no improvement in DO 

WQS nonattainment percentage was predicted (Table N1-1). 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-10 September 24, 2010

Table N1-1. Monthly open-water dissolved oxygen criteria nonattainment percentages for 

ANATF_MD in the 1993-1995 critical period. 

For these months, EPA Chesapeake Bay Program Office (CBPO) analysts compared Bay Water 

Quality Model simulated dissolved oxygen concentration with historical water quality 

monitoring observations. For July 1994, model simulated DO concentrations at Bay Water 

Quality Model grid cell 6443 – the location coincident with monitoring station ANA01 – ranged 

from 7.2-13.0 mg/L. In contrast, monitoring observations for the same month ranged from 1.0-

3.8 mg/L. Similar results were found for the month of August 1993, when Bay Water Quality 

Model-simulated DO concentrations for cell 6443 ranged from 7.5-15.5 mg/L while historical 

observations at the same location (ANA01) ranged from 0.5-4.4 mg/L. Because the Bay Water 

Quality Model did not simulate severe hypoxia in this region for these summer months, it was 

not able to provide a sufficient estimate of the magnitude of DO response to be expected with 

nutrient load reductions. 

CBPO analysts also considered the attainment status of the two downstream segments closest to 

ANATF_MD: the District of Columbia’s portion of the Anacostia River (ANATF_DC) and the 

District’s portion of the tidal Potomac River (POTTF_DC) (Figure N1-10). Unlike segment 

ANATF_MD, ANATF_DC and POTTF_DC both attained their respective DO WQS at the 

target basinwide allocation of 190 million pounds per year TN and 12.7 million pounds per year 

TP.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-11 September 24, 2010

Source: Appendix N.

Figure N1-10. Open-water dissolved oxygen criteria nonattainment in ANANTF_MD MDATF and 

nearby Bay segments. 

Given the lack of Bay Water Quality Model fit in this segment and the Bay Water Quality Model 

projected DO WQS attainment of the two segments immediately downstream, EPA concludes 

that it is reasonable to expect attainment of the DO WQS in Maryland’s tidal Anacostia River at 

the basin-wide target allocation of 190 million pounds per year TN and 12.7 million pounds per 

year TP.

West Branch Elizabeth River 

Violations of the DO WQS were not uncommon in the Western Branch of the Elizabeth River 

(WBEMH), particularly in the early half of the 1991-2000 decade. Violations of the 5.0 mg/L 

open-water DO criterion (red line in Figure N1-11) were common during summer months, 

particularly at depths below 0.5 meters.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-12 September 24, 2010

Source: Appendix N.

Figure O1-11. Summertime DO concentrations observed at water quality monitoring station WBE1 

in segment WBEMH from 1991-2000. 

Some of these violations improved with model simulated load reductions such as those 

represented in Table N1-2, however for two months in particular – July 1993 and July 1994 – no 

improvement in monthly violation rate was observed under scenario-modified conditions.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-13 September 24, 2010

Table N1-2. Monthly open-water dissolved oxygen criteria nonattainment percentages for water 

quality monitoring station WBE1 in the 1993-1995 critical period. 

Further investigation of model performance in WBEMH showed that the Bay Water Quality 

Model failed to simulate the range of DO concentrations observed at WBE1 for either of these 

months. While the Bay Water Quality Model consistently simulated concentrations greater than 

7 mg/L for the Bay Water Quality Model cell located at station WBE1, monitoring observations 

for the same month and year were below 5.0 mg/L. In Figure O1-12, the pink symbols represent 

DO concentrations for the calibration scenario; blue symbols and line represent DO 

concentrations and linear regression for the 179 TN, 12TP load reduction scenario. Dark blue 

symbols represent DO observations for July 1994 at depths ranging from 0.5-3 meters.

Figure O1-12: Chesapeake Bay Water Quality Model simulations at WQM cell 257 and 

observations at water quality monitoring station WBE1 for July 1994. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-14 September 24, 2010

As described for previous segments, when the range of Bay Water Quality Model simulations 

falls in this range, the Bay Water Quality Model fails to provide an estimate of improvement in 

hypoxic conditions with load reductions.

When Bay Water Quality Model simulations do not span the range of hypoxic conditions 

observed, additional lines of evidence such as the attainment of nearby segments are considered 

in determining the necessity for further load reductions. In the case of WBEMH, adjacent and 

nearby segments attained their respective open-water DO WQS at or before the basinwide target 

nutrient allocations (Figure O1-13).

Source: Appendix N.

Figure O1-13. Attainment of the open-water DO WQS for WBEMH and nearby Bay segments under 

progressively stringent load reduction scenarios.

While the periodic occurrence of hypoxia in the Western Branch of the Elizabeth River remains 

a matter of concern, in this case the WQM provided no information on the magnitude of 

response in DO concentrations to be expected with load reductions. Considering the attainment 

of DO WQS observed in adjacent segments well before the target basinwide allocation, EPA 

concludes that it is reasonable to expect attainment of the DO WQS in Western Branch of the 

Elizabeth River at the basin-wide target allocation of 190 million pounds per year TN and 12.7 

million pounds per year TP.

Upper Pamunkey River

Dissolved oxygen concentrations at station TF4.2 in the upper Pamunkey River (PMKTF) 

occasionally violated this segment’s open-water DO criterion of 4.0 mg/L (Figure O1-14). 

Violations during the 1993-1995 critical period were moderate and limited to the summer of 

1995.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-15 September 24, 2010

Source: http://www.chesapeakebay.net

Figure O1-14. Summertime monitored dissolved oxygen concentrations (mg/L) at station TF4.2 in 

segment PMKTF. 

A closer look at DO violations occurring in July and August of 1995 (Table N1-3) showed that 

while DO concentrations in August improved sufficiently to attain WQS with simulated load 

reductions, no improvement was observed in the July 1995 violation rate.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-16 September 24, 2010

Table N1-3. Monthly open-water dissolved oxygen criteria nonattainment percentages for water 

quality monitoring station TF4.2 in segment PMKTF in the summer months of 1993-1995 critical 

period. 

Investigation of the Bay Water Quality Model-derived regression for July 1995 revealed that as 

with other small tidal tributaries discussed in this section, simulated DO concentrations for the 

calibration scenario did not match historical observations for the same month and location in the 

upper Pamunkey River. In Figure O1-15, DO concentrations for the 190 TN, 12.7 TP load 

reduction scenario (blue symbols and linear regression line) showed little or no improvement 

compared with those of the calibration scenario (pink symbols). DO concentrations for both 

scenarios were greater than those observed at station TF4.2
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-17 September 24, 2010

Figure O1-15. Simulated DO concentrations for cell 1803, the Bay Water Quality Model grid cell 

coincident with monitoring station TF4.2 in segment PMKTF. 

It is also worth noting that the observed violations were only marginally lower than the 4.0 mg/L 

criterion. Furthermore, the two segments immediately downstream from PMKTF – the lower 

Pamunkey River (PMKOH) and the mesohaline York River (YRKMH) – attained their 

respective open-water DO WQS at or before the target load allocation (Figure O1-16).

Source: Appendix N.

Figure O1-16. Attainment of the open-water DO WQS for PMKTF and nearby Bay segments under 

progressively stringent load reduction scenarios.

Given the mismatch between historical water quality monitoring observations and the Bay Water 

Quality Model simulations in this segment, the complete lack of response in DO concentrations 

with simulated load reductions, the moderate nature of violations observed in PMKTF for the 

critical period, and the attainment of the two nearest downstream segments at or before the target 

basinwide allocation, EPA concludes that it is reasonable to expect attainment of the DO WQS in 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-18 September 24, 2010

upper Pamunkey River at the basin-wide target allocation of 190 million pounds per year TN and 

12.7 million pounds per year TP.

Wicomico River

Moderate excursions below the Open Water criterion for Wicomico (WICMH) of 5.0 mg/L were 

not uncommon in summer months (Figure O1-17) between 1991-2000, however, few were 

extensive enough to cause high percentages of WQS non-attainment. For the 1993-1995 critical 

period, two months – June and July 1994—had extensive violations of the DO criterion. 

Source: http://www.chesapeakebay.net

Figure O1-17: Dissolved oxygen concentrations observed at station ET7.1 (WICMH) in the 

summers months from 1991-2000.

While the historical violations present in July 1994 were resolved under scenario-modified 

conditions of the target basinwide allocation (190 TN, 12.7 TP Loading Scenario), DO 

concentrations in June 1994 showed no improvement in violation rate, even under the extensive 

load reductions of the E3 Scenario (Table N1-4).
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-19 September 24, 2010

Table N1-4. Monthly open-water dissolved oxygen criteria nonattainment percentages for water 

quality monitoring station ET7.1 in segment WICMH in the summer months of 1993-1995 critical 

period. 

Further investigation of the conditions causing this persistent violation revealed that DO 

concentrations simulated by the Bay Water Quality Model’s Calibration Scenario for grid cell 

7658 are higher than those observed at station ET7.1 for June 1994. Figure O1-18, the DO 

concentrations observed at station ET7.1 (dark blue symbols) are shown for June 1994. The E3 

linear regression falls below these monitoring observations, illustrating the predicted decrease in 

scenario-modified DO concentrations. Furthermore, DO concentrations in this location were 

generally similar to (or sometimes even lower than) calibration conditions. In other words, no 

improvement in DO concentrations was observed at this location when even dramatically 

reduced loads were simulated. As a result, the mildly hypoxic conditions observed in June 1994 

were scenario-modified to lower, rather than higher, values with reduced nutrient loads. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-20 September 24, 2010

Figure O1-18: Simulated DO concentrations for the Calibration Scenario (pink symbols with 1:1

linear regression line) compared to those for the E3 Scenario (blue symbols and blue linear 

regression line). 

In contrast with predictions for WICMH, adjacent Tangier Sound (TANMH) and other nearby 

segments attained DO WQS at or before the target basinwide load allocation (Figure O1-19). 

Source: Appendix N.

Figure O1-19. Attainment of the open-water DO WQS for WICMH and nearby Bay segments under 

progressively stringent load reduction scenarios.

As with other segments described herein, the Bay Water Quality Model effectively simulated 

neither the observed historical conditions nor the expected improvement in those conditions with 

reduced nutrient loads in this small, shallow region of the Wicomico River. Given the moderate 

nature of the observed violations the unimpaired condition of adjacent and nearby segments, and 

the considerable level of effort already required of this river basin with the current target load 

allocation, the EPA considers that it is reasonable to expect WICMH to attain WQS at the 

proposed target load allocations.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-21 September 24, 2010

Upper and Middle Portions of the Pocomoke River 

The Upper (POCTF) and Middle (POCOH) portions of the Pocomoke River are represented by 

the same, single monitoring station (ET10.1) for the time period of 1991-2000, therefore DO 

criteria violations are the same for both segments. Descriptions and data described here apply to 

all segments in this portion of the tributary. This portion of the Pocomoke River also constitutes 

a relatively narrow tributary; the Bay Water Quality Model grid is only one cell wide in both 

POCTF and POCOH (Figure O1-20). As mentioned previously, the Bay Water Quality Model 

often struggles to integrate multiple drivers of DO concentrations in narrow, shallow waters –

especially in model cells that directly abut land.

Figure O1-20: Chesapeake Bay Water Quality Model grid cell (a) coincident with long-term fixed 

station ET10.1 (b) in the Pocomoke River.

Violations of the open-water DO criterion of 5.0 mg/L occurred rather frequently in the summer 

months from 1991-2000 at station ET10.1 (Figure O-21); in fact DO concentrations often 

decreased to levels below 4.0 mg/L. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-22 September 24, 2010

Source: http://www.chesapeakebay.net

Figure O1-21. Summer dissolved oxygen concentrations observed at water quality monitoring 

station ET10.1 for the 1991-2000 time period.

In spite of this, considerable improvement was observed for most cases of hypoxia in the 1993-

1995 critical period under scenario-modified conditions. However, there was no decrease in 

violation rates of the DO criterion for June 1993 even when nutrient loads were reduced to ―E3‖ 

levels (Table N1-5).
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-23 September 24, 2010

Table N1-2. Monthly open-water dissolved oxygen criteria nonattainment percentages for water 

quality monitoring station ET10.1 in segments POCTF and POCOH in the summer months of 1993-

1995 critical period.

Further investigation of the DO concentrations underlying this persistent violation revealed that 

conditions in June 1993 were only moderately hypoxic (~ 4.3 mg/L), but that DO concentrations 

simulated by the Bay Water Quality Model were much higher than those observed – ranging 

from about 8 to greater than 10 mg/L The historically observed DO concentration at monitoring 

station ET10.1 (dark blue symbol) fell well below the range of Bay Water Quality Model 

simulated dissolved oxygen concentrations. (Figure O1-22).
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-24 September 24, 2010

Figure O1-22: Simulated dissolved oxygen concentrations in Bay Water Quality Model cell 4399 

for the calibration (pink symbols and regression line) and E3 (light blue symbols and regression 

line) scenarios. 

Because the Bay Water Quality Model failed to simulate the hypoxia observed in the location of 

station ET10.1, it was unable to provide a true estimate of the magnitude of improvement to be 

expected in DO concentrations with reduced loads. In the absence of sufficient information, the 

attainment status of the deeper, wider region of the lower Pocomoke River was considered as an 

alternative line of evidence for predicting attainment of POCTF and POCOH (Figure O1-23). 

While POCTF and POCOH showed persistent non-attainment even under the considerable load 

reductions represented by the E3 Scenario, the adjacent lower Pocomoke River (POCMH) 

attained WQS even under historical conditions.

Source: Appendix N.

Figure O1-23: Attainment of open-water DO WQS in the three tidal Pocomoke River segments 

under numerous scenarios representing progressively more stringent load reductions.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-25 September 24, 2010

Causes of persistent non-attainment for the upper and middle Pocomoke River appear 

sufficiently similar to those described for other segments described here, thus the same rationale 

may apply. However, an additional characteristic of the tidal Pocomoke River likely plays a role 

in the extensive violations observed historically, affecting the ability of DO concentrations to 

rebound as nutrient loads to this river basin are reduced. The headwaters of the Pocomoke River 

originate in the Great Cypress Swamp, which serves as a source of considerable dissolved 

organic matter to the tributary. The combined effect of the Great Cypress Swamp and other 

extensive tidal and non-tidal wetlands occurring along the length of the river is clear; the Upper 

Pocomoke River has recorded some of the highest dissolved organic carbon concentrations seen 

in any tidal fresh tributary in the state of Maryland (Maryland Department of Natural Resources 

2010). 

Because these naturally occurring high concentrations of dissolved organic matter often generate 

high rates of respiration that can in turn lead to significant, natural reductions in dissolved 

oxygen, Maryland has proposed that a site-specific open-water dissolved oxygen criterion of 4.0 

mg/L be applied to the upper and middle Pocomoke River (POCTF and POCOH). The proposed 

site-specific dissolved oxygen criterion is fully consistent with EPA’s amended Chesapeake Bay 

water quality criteria guidance published in 2004 (USEPA 2004).

Magothy River 

The Magothy River (MAGMH) is a small, shallow tidal tributary adjacent to the upper central 

Chesapeake Bay segment CB3MH. The Magothy River is represented by one long-term fixed 

monitoring station, WT6.1. The narrow, embayment-like nature of the Magothy River is evident 

in the portion of the Bay Water Quality Model grid that represents it; the entire tributary is 

represented by only five WQM cells. The grid cell representing station WT6.1 highlighted in 

Figure O1-24.

Figure O1-24: Chesapeake Bay Water Quality Model grid for the Magothy River and the adjoining 

portion of the mainstem Chesapeake Bay.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-26 September 24, 2010

Severely hypoxic conditions are common during the summer months in the Magothy River 

(Figure O1-25). Low dissolved oxygen concentrations are often exacerbated by water column 

stratification, which prevents the vertical mixing that would otherwise re-oxygenate bottom 

waters. Concentrations often fell below the Deep Water criterion of 3.0 mg/L (red line), 

particular at depths greater than 2-3 meters (Figure N1-25). The documented presence of an 

upper pycnocline boundary in the Magothy River recently led the EPA and the State of Maryland 

recommend adding a Summer Deep Water designated use to the Magothy River (USEPA 2010). 

However, even when the deep-water criterion of 3.0 mg/L is applied to stratified bottom waters, 

non-attainment of the DO WQS persists with simulated load reductions at the level of the target 

basin-wide allocation (see Figure O1-27).

Source: http://www.chesapeakebay.net

Figure O1-25: Dissolved oxygen concentrations observed at station WT6.1 in segment MAGMH 

during summer months from 1991-2000. 

Further investigation of the persistent non-attainment of DO WQS observed in MAGMH showed 

that while violations occurring in some summer months improved with load reductions, hypoxic 

conditions in other months improved to a much lesser degree or not at all (Table N1-6). In 

particular, violations of the DO criterion that occurred in September 1994 showed no 

improvement, even when loads were reduced to the 179 TN, 12 TP level.
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-27 September 24, 2010

Table N-6: Summer monthly violation rates for MAGMH during the 1993-1995 critical assessment 

period. 

The performance of the Bay Water Quality Model in the location of the MAGMH monitoring 

station was examined. As illustrated in Figure O1-26, simulated DO concentrations in the WQM 

cell representing the bottom depths at station WT6.1 were consistently higher than 5.0 mg/L for 

September 1994. However, historical measurements for the lower depths at station WT6.1 

showed concentrations less than 3.0 mg/L. In Figure O1-26, the Calibration Scenario (pink 

symbols and regression line) is compared with the 179 TN, 12.0 TP Loading Scenario (light blue 

symbols and linear regression). Historical observations (dark blue circles) fall well outside the 

range of simulations. As described previously, the failure of the Bay Water Quality Model to 

simulate hypoxic conditions affects its ability to predict the magnitude of improvement that will 

occur in DO concentrations when nutrient loads are reduced.

Figure O1-26. Simulated dissolved oxygen concentrations in grid cell 19393 of the Bay Water 

Quality Model for September 1994. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-28 September 24, 2010

The inability of the Bay Water Quality Model to simulate the hypoxic conditions observed 

during summer months in the Magothy River reduces its ability to predict the magnitude of 

improvement in DO concentrations that can be expected as nutrient loads are reduced. However, 

the Bay Water Quality Model much more effectively simulates historical conditions and, 

therefore, predicted improvements, in nearby deeper, wider regions of the Chesapeake Bay. 

Thus the predicted attainment of WQS in the deep-water designated use of CB3MH, well before 

the target basin-wide load allocation (see Figure O1-27), can help to inform expectations of 

attainment for the Magothy River.

Source: Appendix N.

Figure N1-27. Predicted attainment of DO WQS for the summer deep-water designated use in 

CB3MH and MAGMH.

While the severely hypoxic conditions commonly observed in the Magothy River during the 

summer months remain a matter of concern, at this time EPA lacks data to effectively predict the 

recovery of the Magothy River in those months when the Bay Water Quality fails to simulate 

historical conditions. However, given attainment of adjacent deep-waters of CB3MH, and the 

extensive load reductions already required of the Magothy River basin for the target basin-wide 

allocation of 190 million pounds per year TN and 12.7 million pounds per year TP, the EPA 

anticipates that the MAGMH Deep Water designated use will attain WQS when the target load 

allocation is achieved.

Lower Chester River

Historical monitoring data show a consistent pattern of summer severe hypoxic to anoxic (<0.2 

mg/L dissolved oxygen concentrations) conditions in the deep-channel region of the lower 

Chester River (CHSMH), in the vicinity of water quality monitoring station ET4.2 (Figure N1-

28).
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-29 September 24, 2010

Figure N1-28. The lower Chester River is characterized by Maryland’s Chesapeake Bay water 

quality monitoring program station ET4.2.

In summer months, observed DO concentrations at monitoring station ET4.2 consistently fell 

below 1.0 mg/L, the instantaneous minimum criterion for the deep-channel designated use 

(Figure N1-29).

ET4.2

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

1/1/917/1/911/1/927/1/921/1/937/1/931/1/947/1/941/1/957/1/951/1/967/1/961/1/977/1/971/1/987/1/981/1/997/1/991/1/007/1/00

Sample Date

Bottom DO (mg/L)

Source: 

http://www.chesapeakebay.net

Figure N1-29. Bottom depth dissolved oxygen concentrations measured at water quality 

monitoring station ET4.2 in the lower Chester River from January 1991 – December 2000. 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-30 September 24, 2010

At model-simulated nutrient load reductions that led to attainment of deep-channel dissolved 

oxygen criteria in all other deep-channel regions of the Chesapeake Bay and tidal tributaries (e.g. 

segments in the Chesapeake Bay mainstem, lower Potomac River, lower Rappahannock River, 

and Patapsco River) (see Appendix M), model simulated improvements in dissolved oxygen 

concentration in the lower Chester River’s deep-channel did not yield full attainment of the 

dissolved oxygen criteria. Whereas other deep-channel regions showed attainment of the deep 

channel dissolved oxygen criterion at or before the 190 TN, 12.7 TP loading scenario, the lower 

Chester River’s deep-channel non-attainment remained at a plateau of approximately 10-14% 

under loading scenarios ranging from 191 TN, 14.4 TP down to approximately 170 TN, 11.3 TP. 

Full attainment of the applicable dissolved oxygen criterion was not achieved for this deepchannel region until the highly theoretical and unattainable ―All Forest‖ scenario, for which it is 

assumed that all land in the Chesapeake Bay watershed reverts to a forested condition (Figure 

N1-30).

Source: Appendix N.

Figure N1-30. Percent non-attainment of lower Chester River deep-channel dissolved oxygen 

criterion with decreasing total nitrogen (TN) and total phosphorus (TP) loads.

In-depth examination of the Bay water quality model scenario outputs showed stepwise increases 

in DO concentrations with incremental nutrient load reductions in the lower Chester River 

(CHSMH) segment at surface and mid-depths, and consistent simulation of bottom water anoxia. 

However, the response of DO concentrations at lower-depths in the water column—and 

particularly at the bottom of the water column—appeared to be constrained to a degree that 

prevented full attainment of the 1.0 mg/L deep-channel dissolved oxygen criterion under model 

342N

24.1P

(1985)

309N

19.5P

(Base)

248N

16.6P

(2009)

200N

15P

(Targ 

Ld Opt 

A)

191N

14.4P

(Trib 

Strat)

190N

13P

190N

12.7P

179N

12.0P

170N

11.3P

141N

8.5P

(E3)

Chester River Deep Channel

0%

10%

20%

30%

40%

50%

60%

342/24.1309/19.5248/16.6

200/15

191/14.4

190/13

190/12.7

179/12

170/11.3

141/8.5

58/4.4

Global TN/TP Load WQS % Non-Attainment

58N

4.4P

(All 

Forest)

Basinwide TN, TP Loads (Scenario Name)
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-31 September 24, 2010

simulated nutrient load reductions that yield full attainment in all other deep-channel regions of 

the Chesapeake Bay and its tidal tributaries (Figure N1-31). 

Source: 

Appendix N.

Figure N1-31. Dissolved oxygen percent non-attainment for all the deep-channel designated use 

segments in the Chesapeake Bay and its tidal tributaries.

EPA postulates that the bathymetry of the lower Chester River provides a physical barrier to 

complete re-oxygenation of the deepest region of the lower Chester River even under extremely 

high nutrient reductions. A narrow deep channel transects the center of the lower Chester River, 

and exchange of oxygenated deep waters between the mainstem Chesapeake Bay and this deep 

hole may be restricted by the wider, shallower shoal region at the mouth of the river (Figures N1-

32 and O1-33).
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-32 September 24, 2010

Figure N1-32: Chesapeake Bay Water Quality Model grid for the lower Chester River, with total 

depth for each cell labeled (in feet). 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-33 September 24, 2010

Source: EPA Chesapeake Bay Program Office

Figure N1-33. Bathymetry of the lower Chester River. 

The limited response of DO concentrations to reduced nutrient loads (e.g., 30-140 million 

pounds basinwide) in the lower Chester River deep-channel, combined with the physical 

characteristics of the narrow, deep channel in this region, suggest a natural constraint on the reoxygenation of the lower mixed layer by either deep riverine flows or deep estuarine flows from 

the adjacent mainstem Bay. Therefore, given the currently available information, EPA 

recommends a restoration variance of 14% to account for persistent WQS non-attainment in the 

CHSMH deep-channel designated use at the basinwide loads of 190 TN, 12.7 TP. The selection 

of a 14% variance is based on the observation that dropping the basinwide loads by up to 20 

million pounds per year yields relatively little change in the non-attainment percentage, which 

ranges only from 10-14% over this reduction level.

References

MDNR (Maryland Department of Natural Resources). 2010. Justification for Application of a 

Site-Specific Dissolved Oxygen Criterion to the Upper Tidal Fresh and Middle Oligohaline 

Pocomoke River Segments Due to Natural Conditions. Annapolis, MD.

USEPA (U.S. Environmental Protection Agency). 2004a. Ambient Water Quality Criteria for 

Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal 
Draft Appendix N-1 – Chesapeake Bay TMDL for Nutrients and Sediment

N-1-34 September 24, 2010

Tributaries. 2004 Addendum. EPA 903-R-03-002. U.S. Environmental Protection Agency, 

Region 3, Chesapeake Bay Program Office, Annapolis, MD.

USEPA (U.S. Environmental Protection Agency). 2010a. Ambient Water Quality Criteria for 

Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal 

Tributaries: 2010 Technical Support for Criteria Assessment Protocols Addendum. May 2010.

EPA 903-R-10-002. CBP/TRS 301-10. U.S. Environmental Protection Agency, Region 3 

Chesapeake Bay Program Office, Annapolis, MD.