TL
903R77001
THE POTOMAC ESTUARY
CURRENT ASSESSMENT PAPER NO. 2
December 1977
Orterio Villa, Jr.
Leo J. Clark
Stephen E. Roesch
Susan K. Smith
Annapolis Field Office
Region III
U.S. Environmental Protection Agency
and
Norbert A. Jaworski, Ph.D.
Civil Engineering Department
Oregon State University
U& Environmental Protection Agency
ftoften III Information Resource
Cmtw (3PM52)
841 Chestnut Street
Philadelphia, PA 19107
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TABLE OF CONTENTS
Page
Introduction 1
Section
I. Historical Perspective 4
II. Chlorophyll a_ as an Indicator of the Trophic
State of the Upper Potomac Estuary 11
III. Present Status 14
Pertinent Topics
Addendum I - Zone I Loadings 19
Addendum II - The Dynamic Nature of Technical
Reports and Mathematical Models . . 20
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INTRODUCTION
Since the first recorded observation of the water quality of the
Potomac Estuary in 1608 by John Smith, considerable effort has been made
to understand its numerous physical, chemical, and biological processes.
These efforts are typified by studies of the Potomac tidal system con-
ducted by the Annapolis Field Office (formerly the Chesapeake Technical
Support Laboratory of the U.S. Public Health Service), beginning in 1964.
One of the major milestones of water quality management for the
Potomac Estuary was the agreement resulting from the Potomac enforcement
activities of the 1960's. This was the development of a "Memorandum of
Understanding" limiting the amount of oxygen demanding materials and
nutrients which could be discharged from wastewater facilities in the
Washington, D.C., Metropolitan Area. A second result of the enforcement
activity was an expanded water quality study of the estuary culminating
in the publication of Technical Report 35, "Water Resource-Water Supply
Study of the Potomac Estuary," April 1971.
With the passage of the Federal Water Pollution Control Act Amend-
ment of 1972, the implementation of water quality management programs
was effected mainly through the "National Pollutant Discharge Elimination
System" and planning through Section 208 of the act relating to "areawide
waste treatment management." Partially as a result of technical issues
such as the ultimate disposal of sludges and the projected high cost of
denitrification, considerable interest and debate continues both in the
public and private sectors on the various approaches being developed for
areawide waste treatment management.
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Due to intense interest in the previous and current water quality
studies of the Potomac Estuary, the authors of this report thought it
would be prudent to put together a document which would attempt to
clarify some of the more controversial water quality related issues.
In addition, the Annapolis Field Office of Region III, EPA, con-
ducted water quality surveys in the Potomac Estuary in 1977 for the
first time in several years. The data from these surveys are being
processed and collated and will be utilized to (1) evaluate the current
state of the estuary; (2) update our historical trend information; and
(3) refine and update the Potomac Estuary Model. Interpretation of
that data has begun and will continue for the next several months with
a preliminary report of findings available in early 1978. Tentative
findings of these surveys are presented in this report.
Monitoring of the Potomac will continue on a routine basis through
at least one annual cycle, and hopefully two, in order to update our
baseline data. Additional summer intensives are also envisioned as a
part of this effort.
Section I of this paper presents a brief historical overview of
some of the technical issues addressed in the past. Section II presents
our thoughts on the subject of chlorophyll a^ and its utilization as an
indicator of water quality. Section III is a present status report
including major findings and conclusions extracted from the 1977 data.
The addenda are included for informational purposes and are meant to
serve as aids in unravelling the complex mix of technological terms
and principles being used.
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In publishing this paper, our hope is to clarify many of the
technical issues that have surfaced and, where disagreements exist,
at least allow us to talk in common terms concerning the relative
importance and impact of these differences.
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I. Historical Perspective
Since its inception in 1964, the Annapolis Field Office has been
monitoring water quality conditions in the Potomac Estuary. Surveys
for the acquisition of this data were conducted in order to determine
existing water quality stresses, particularly with regard to standards
violations, but more importantly, to better understand how the estuary
behaves and why it responds the way it does. It became apparent quite
early that the Potomac Estuary was plagued by two separate but inter-
related problems which were (1) extremely low DO concentrations and
(2) accelerated eutrophication, as evidenced by high nutrient levels
and massive algal growth.
In relating DO concentrations* in the estuary to wastewater and
other contributions of oxygen demanding material, a real time tidal
mathematical model was expanded to include:
(1) carbonaceous BOD
(2) nitrogenous BOD
(3) benthic uptake
(4) algal respiration and oxygen production.
Reaction rates for the various interrelated oxygen demanding systems
were obtained primarily from field and laboratory observations. Except
for item four, these components of the DO budget have been included in
previous modeling studies of other estuarine and river systems.
* DO concentrations have not only been developed as criteria for pro-
tecting various aquatic organisms but have been developed as a set
of "standards" for regulations.
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The hyper-eutrophic condition in the Potomac Estuary presented
two problems in developing water quality management needs: (1) There
were no standard accepted indicators of eutrophication in tidal
systems and (2) the relationships between nutrient concentrations and
trophic responses were not well-defined for tidal systems. Historically,
most of the efforts in defining the causes and corrective needs of
eutrophication were in freshwater lakes. Even the National Academy of
Sciences was unable to recommend standardized techniques for quantifying
the state of eutrophication in their Water Quality Criteria, 1972
publication.
Since algae impact DO through their diurnal photosynthesis-
respiration cycle, as well as through their death and decomposition,
and furthermore, since they can also create nuisance or aesthetically
undesirable conditions, it was imperative that some measure of their
levels or standing crop in the water column be made on a continuing
basis. Chlorophyll a_, a major pigment required for the conversion of
solar energy into organic cellular material (photosynthesis) in both
aquatic and terrestrial plants, was selected as an appropriate indicator
of algal standing crop. Chlorophyll a^ had two features which were
especially appealing: (1) It is relatively easy to analyze for in
a laboratory and (2) the scientific literature is rich in the use,
interpretation, acceptability, and limitations of this parameter based
upon numerous studies.
The intent of measuring chlorophyll was not only to ascertain
algal standing crop conditions but to: (1) provide an interpretative
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tool that would assist in the analysis of nutrient and DO data,
(2) develop formulations expressing in-stream reactions and (3) establish
interrelationships among these constituents for use in mathematical
models. This latter purpose of chlorophyll data can best be exempli-
fied by the course taken in Technical Report 35 to develop a water
quality management program, including a nutrient control policy for the
Potomac Estuary based on nutrient-phytoplankton-DO relationships.
It should be noted that chlorophyll a_ was not the only indicator
of the hyper-eutrophic state of the upper Potomac Estuary. Others
included the following:
1. A reconstruction of the ecological succession and
nutrient loadings of the estuary from 1910 to the present
2. Transparency, primarily measured by Secchi Disk (this
parameter is widely used by limnologists in lake studies)
3. Dominant algal species
4. Diurnal cycles of DO
5. The dissolved oxygen level in the lower estuary (this
is equivalent to the measurement of DO in the hypolimnion in lakes).
Based upon the various adverse impacts caused by algae, which
threaten compliance with current water quality standards, a subjective
analysis was performed in order to establish an acceptable eutrophic
state using chlorophyll a^. It was termed a subjective analysis because
a very limited amount of "hard" data existed and, consequently,
judgement played an important role. Of these impacts, the one pertaining
to aesthetics or nuisance conditions governed the selection of a
25 yg/1 of chlorophyll a^ as an indication of an acceptable eutrophic state.
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The next step entailed estimating the maximum nitrogen and
phosphorus concentrations that could be present in the water in order
to meet the 25 yg/1 of chlorophyll and other objectives. Six inde-
pendent methods were used to estimate maximum acceptable concentrations
of phosphorus and nitrogen which would maintain a predetermined
eutrophic level. They were:
(1) Algal composition analysis
(2) Analysis of data on an annual cycle and longitudinal
profile basis
(3) Bioassay studies
(4) Nutrient and algal modeling
(5) Comparison with a less-stressed estuary
(6) Review of historical nutrient and ecological trends
in the Potomac Estuary.
Carbon was not considered as a possible limiting nutrient because it
could not be removed to the degree necessary to control algal growth.
The nutrient criteria decided upon for Zones I, II, and III, which
were expressed as ranges because of differences in growth potential
among the various zones of the estuary, were:
Inorg N - 0.3 - 0.5 mg/1
Total P - 0.03 - 0.1 mg/1
In that the upper reach of the Potomac Estuary is usually light
limited, the use of a linear acceptable concentration by zones is a
practicable approach. Inherent in this approach is the possibility
(mainly under low-flow and high temperature conditions) that for a
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short period of time undesired algal growth could occur in this upper
reach because of exceptionally good water clarity. This means that
the 25 yg/1 of chlorophyll a^ could be exceeded. The main emphasis was
placed on those reaches of the upper estuary which were suspected or
proven to be conducive to algal growth.
The final step in the operation was to translate the above
nutrient criteria, as well as oxygen demanding characteristics for DO
enhancement, into allowable zonal loadings or maximum daily loads.
This should not be confused with individual wasteload discharge allo-
cations, which actually specify the source and location of individual
waste inputs on a mass emission rate basis. Allowable zonal loadings
strictly reflect the assimilative capacity of the particular zone
commensurate with pre-selected water quality objectives.
A previously calibrated and verified mathematical model was used
to establish the maximum allowable ultimate oxygen demand (UOD), nitrogen
(N), and phosphorus (P) loadings under low-flow conditions. Attention is
focused toward these particular parameters (UOD, N, and P) because of
their direct or indirect influence on DO and the eutrophic state of
the upper estuary. It can be implied from the foregoing discussion
that chlorophyll served as a valuable indicator in the process of
establishing a wastewater treatment policy, but that the ultimate
aim was to comply with both specific water quality standards (DO)
and general aesthetic (absence of nuisance conditions) water quality
conditions which are legally enforceable.
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It should be noted that Technical Report 35, while refining the
loading allocations of the "Memorandum of Understanding," firmly
established the following:
1. The upper Potomac Estuary has a limited capacity to
receive oxygen demanding waste (finite limits were developed per zone
in the report).
2. The upper estuary was experiencing serious eutrophication
problems caused mainly by the discharge of nutrients from the waste-
water facilities in the Washington Metropolitan Area.
3. Similar to the limits on oxygen demanding materials,
there is a limit to the nutrient loadings that can be converted to
biomass in the upper Potomac Estuary if a healthy ecosystem is to be
maintained.
While refinements will continue to be made to the loading allocations
and criteria values as more data are obtained, the basic conclusions,
as stated above, are still valid today.
Another important point which should be made is that Technical
Report 35 documented a scientific study that attempted to set forth
technical information on water quality requirements for developing an
achievable and sound wastewater management policy for the Potomac
Estuary. It utilized the best available data base at that time (1970-71)
but, like any scientific study, a strenuous effort must continually
be made to fill data gaps, learn more about the natural system, re-
evaluate certain assumptions, refine certain inputs that are known or
suspected to be sensitive to the final results, and maintain the ability
to temper previous results and conclusions accordingly. If current
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technology (advanced waste treatment) cannot deliver the effluent quality
concentrations as predicted at the time Technical Report 35 was prepared
and published, the feasibility of the wastewater management alternatives
proposed in Technical Report 35 must be re-evaluated. An example of this
re-evaluation was made in 1976 by EPA during the testimony of Dr. Jaworski
at the Blue Plains Adjudicatory Hearing. The major conclusion of the
re-evaluation was to delay the denitrification requirement for two years
until the effect of phosphorus removal alone was evaluated.
It is the role of the scientists and engineers to objectively
describe the water quality implication of various proposed treatment
scenarios. It is the role of the policy maker to balance the predicted
water quality implication against the costs for the various treatment
scenarios and choose that best mix of management techniques that both
meets the imposed regulatory responsibilities and is in the overall
best interest of the public. It is important to incorporate into both
roles the flexibility to take advantage of new data as it is developed.
More recent data and modeling efforts have reaffirmed that the
upper estuary has limited ability to assimilate nutrients and yet maintain
a healthy trophic state. In addition, the need for the control of
nutrients (initially phosphorus and, if needed, nitrogen) to sustain a
more desirable trophic state has been reinforced.
The definition of that desired trophic state and indicators used to
describe it remain elusive to this day. An approach currently being
endorsed by certain groups appears to be the adoption of a fixed chloro-
phyll concentration of 25 pg/1 throughout the entire estuary rather than
setting nutrient concentrations by zones. Some of the shortcomings of
this approach are presented in the next section.
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II. Chlorophyll a^ as an Indicator of the Trophic State of the
Upper Potomac Estuary
A great deal of discussion has recently taken place concerning
the chlorophyll a_ concentration desirable in the Potomac Estuary. It
is vitally important to realize that a specific chlorophyll ^con-
centration or any other trophic state indicator* by itself has to
be used with caution in defining the overall ecological health of a
body of water for several reasons. First, it is difficult to obtain a
representative water sample for chlorophyll a^ or any biomass indicator
which is statistically valid since distribution in the water column
is subject to the whims of wind, wave action, and currents. As a
result, algal growths are not uniformly distributed and tend to con-
centrate in certain areas. Second, the,, identification of the specific
algal species present is important since different species of algae
have:
(1) Varying amounts of chlorophyll a_
(2) Different sizes and behavior patterns
(3) Different grazing potentials
(4) Different growth and nutritional requirements
(5) Various impacts on the food chain.
Chlorophyll a^ is found in all algae but a chlorophyll ^concentration
alone does not discriminate between desirable and nuisance species.
Ideally, a given level of chlorophyll a^ should be used in conjunction
with the identity of the species of algae present at that time so that
* These include: (1) other measures of biomass such as dry weight,
ATP or species counts, (2) diversity indices, (3) productivity,
(4) transparency, and (5) DO levels in hypolimnion.
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a grazing value can be approximated and the effects on the food chain
inferred. A balanced community consisting of algal species which are
utilized in the food chain would be a healthy link in the Potomac's
biological regime.
The use of chlorophyll a_ is valuable as a relative indication of
the algal standing crop, i.e., biomass. It aids in the evaluation of
nutrient data, the assessment of nutrient impacts, and the interpreta-
tion of anomalies in the oxygen balance of receiving waters. It is a
parameter that is useful in the interpretation of analytical data, but
has not been developed to a point where it can be used as the sole
indicator of a trophic state even in lakes.*
More important to the ecosystem, and more indicative of water
quality than a specific chlorophyll level, is the overall health and
balance of the biological community, including phytoplankton, zoo-
plankton, and higher trophic levels. It is this entire biological system
which we seek to improve. While various indicators have been and are
being used to assess the trophic state of the upper estuary, the major
emphases of Technical Report 35 and current activities are directed
toward defining the maximum allowable nutrient concentration which will
produce the desirable trophic state. This approach allows the use of
multiple trophic state indicators and is consistent with the current
state of knowledge for lakes, as summarized in the reference cited below.*
For the reasons outlined above, chlorophyll a_ as a sole indicator
of the trophic state of the estuary cannot be defensibly supported at
* In a recent review of Trophic Status Indices, the readers are referred
to "Summary Analysis of the U.S. Portion of the North American OECD
Eutrophication Study Results Emphasizing Nutrient Loadings-Lake
Responses Relationships and Trophic Status Indices," by Walter Rest
and G. Fred Lee currently being published by EPA.
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at this time. Thus it follows, in the opinion of the authors, that
to consider chlorophyll a_ as a water quality criteria lacks scientific
basis and, moreover, the data and the consequences of having chloro-
phyll a_ as a standard with enforcement implications have not yet been
developed.
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III. Present Status
There is a vital need for a continuing water quality monitoring
program in the Potomac Estuary. This need is underscored in two
separate but highly important ways: first, a very unique opportunity
exists for establishing the impact of varying levels of advanced
wastewater treatment in an estuarine environment, on a scale never
before attempted; and second, data gaps are present that limit the
degree in which cause and effect relationships in water quality behavior
can be assessed. The second has particular importance in dictating
the degree of confidence one can place in mathematical models and the
manner in which they are calibrated and verified.
The Annapolis Field Office initiated an intensive monitoring program
this past summer, which is expected to be continued and refined in
subsequent summers, for the purpose of updating and expanding the
Potomac Estuary's data base and attempt to provide answers relative to
the denitrification issue. Some of the specific objectives of this
monitoring effort are presented below:
1. To identify the major species of algae inhabiting the
Potomac, their relative concentrations or counts, and to determine
whether or not a relationship exists between dominant algal species
and chlorophyll ^magnitude.
2. To better define or quantitate some of the adverse impacts
of algae such as diurnal DO variability and DO depletion attributable
to algal death and decomposition.
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3. To acquire data relative to the composition of algae
cells and the nutritional status of these cells, with special emphasis
on possible limiting nutrients, and to generally develop better
nutrient-phytoplankton relationships.
4. To quantitate both point source waste discharges and
upper basin loads entering the estuary at Chain Bridge with respect
to nutrients, BOD, and other pertinent parameters.
5. To attempt to determine the impact of a storm event on
the water quality of the estuary.
6. To refine some of the pertinent model rates and to
provide a recent data set that could be used to upgrade and reverify
the present Dynamic Estuary Model (DEM).
As can be seen from the above, a major thrust of the Potomac
monitoring program is to more reliably establish current eutrophication
levels and the implications of such levels in terms of nutrient con-
centrations. A considerable amount of chlorophyll a_ data was collected
as part of this effort. These data are now being analyzed and inform-
ation will be made available as soon as the interpretive phase is
completed.
Some of the more important findings from the preliminary inter-
pretation of the data are given below.
1. Minimum DO concentrations measured during the 12 slack
water runs varied between 2-3 mg/1. With one exception (September 8),
these levels occurred in the immediate vicinity of the Blue Plains
Sewage Treatment Plant.
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2. Chlorophyll levels were highly variable both over time
and space. Maximum concentrations of about 300 yg/1 were recorded
during one week in August between Gunston Cove and Indian Head. Con-
centrations during the study period averaged between 100 to 150 micro-
grams per liter of chlorophyll a_.
3. Phytoplankton counts and species identification were
performed. During the early phase of the survey when chlorophyll
levels were about 100 yg/1 or less, there appeared to be some diversity
in algal populations as both green and blue green varieties were
observed. However, as the study progressed and chlorophyll levels
attained their peak values, the blue green algae, Oscillatoria, became
the dominant form, almost to the complete exclusion of the other forms
observed earlier. Anacystis cyanea, the dominant form of algae inhab-
iting the Potomac Estuary during the 1960's, was not present to any
noticeable degree.
4. Algal mats floating on the surface of the Potomac Estuary
were never observed during the course of this study, as they were during
the late 1960's, but the greenish tint was present in the high bloom
areas extending from about the Woodrow Wilson Bridge to Sandy Point.
The indigenous forms of freshwater algae this past summer appeared to
be almost microscopic in size and well dispersed in the water column.
5. Water clarity of the Potomac Estuary was quite low, as
usual, particularly in the middle reach which supports the major blooms.
Typical Secchi Disk readings were about 20 - 24 inches. Minimum values
during large algal blooms ranged between 7-12 inches, whereas the
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maximum readings in the extreme upper reach (above Hains Point)
ranged between 30 - 35 inches.
6. Blue Plains is by far the largest single point source
discharger of oxygen demanding material and nutrients in the Potomac
Estuary. During the study period it contributed an average flow of
276 mgd and the following average loadings:
BODC - 81,000 Ibs/day*
D
TKN - 36,500 Ibs/day
NH3 - 32,500 Ibs/day
N02+N03 - 250 Ibs/day
TP04 - 12,000 Ibs/day
In terms of BODg, TKN, and TPO^, these loadings represent approxi-
mately 85, 75, and 55 percent, respectively, of the total point
source wastewater load generated by the Washington Metropolitan Area.
7. Based upon a statistical analysis of intensive type
data collected in the Potomac Estuary during 1965, 1968, 1969, and
1970, as well as the 1977 data, it can be concluded that DO concen-
trations in the critical reach have, in fact, improved with time.
All of this data was collected at surface stations during high
temperature-low flow periods having somewhat similar algal bloom
intensities.
* On September 8, 1977, a mechanical breakdown occurred at the Blue
Plains treatment plant causing a BODc loading of 345,000 pounds.
If this loading is eliminated from the data, the average BODs loading
becomes 58,000 Ibs/day, representing 78 percent of the total point
source BODs load generated by the Washington Metropolitan Area.
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8. Phosphorus concentrations in the upper Potomac Estuary showed
a substantial decrease in 1977 over previous years. Inorganic nitrogen, on
the other hand, did not exhibit a well defined trend in either direction.
9. An analysis of the spatial distribution of nutrients and
chlorophyll (i.e., phytoplankton densities) in the Potomac Estuary indi-
cates that the nitrogen may be limiting algal growth in the area of
maximum production (downstream of Hallowing Point) since concentrations
of inorganic nitrogen reach nondetectable levels during peak bloom
periods. It is suspected that light may be the limiting factor in the
upper zone (i.e., upstream of Piscataway Creek), where considerably
lower chlorophyll levels are normally found.
10. Two attempts were made to track and monitor a discrete
parcel of water in the upper Potomac Estuary between Rosier Bluff and
Piscataway Creek over a semi-diurnal period extending from 0600 hours to
about 1700 hours. A drogue was used for this purpose. During both
occasions (August 16 and 30) tidal conditions, weather conditions, flows
and water temperatures were very similar.
On August 16 the DO concentration (surface) was 1.5 mg/1 at 0600
hours and increased to about 5.5 mg/1 by 1700 hours. The ambient chloro-
phyll concentration was 80 yg/1. Computed rates of oxygen production were
0.0020 mg 02/yg chloro/hr between 0600 and 1200 hours, and 0.0075 mg
02/yg chloro/hr between 1200 and 1700 hours.
On August 30 the DO concentration (surface) varied from 2.5 mg/1 at
0600 hours to 11 mg/1 at 1700 hours. This variation translated to an
oxygen production rate of 0.0049 mg 02/yg chloro/hr. The ambient chloro-
phyll concentration was 135 yg/1 and the weather was again mostly sunny
and very hot.
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ADDENDUM I
Zone I Loadings
Chesapeake Technical
Support Lab
(1969)*
Annapolis Field
Office
(TR 35 1971)**
BOD5
UOD
Total Inorganic N
Total Unoxidized
Phosphorus (as P)
16,5001
8.0001
740
75.0002
3,4002
900
1969 Conditions:
(1) flow = 705 cfs
(2) temp = 29°C
(3) DO objective =5.0 mg/1
1971 Conditions:
(1) flow = 300 cfs
(2) temp = 29°C
(3) DO objective = 5.0 mg/1
Both 1969 and 1971 results are based on the following assumptions:
(1) all effluents are discharged into the main channel
of the Potomac
(2) all effluents receive equivalent degrees of treatment.
Ultimate Oxygen Demand is calculated as follows:
UOD = 1.45 BOD5 + 4.57 TKN
hn the 1969 zonal loadings BOD5 and Total Kjeldahl Nitrogen (i.e.,
total unoxidized nitrogen) were considered in relation to their
impact on the oxygen balance. No nitrogen criteria were proposed
for nutrient control purposes.
2Technical Report 35 incorporated the BODs and TKN into an Ultimate
Oxygen Demand loading for dissolved oxygen control and also recommended
an inorganic nitrogen guideline which would provide a degree of control
of eutrophication related problems.
**
Adopted by conferees in the "Memorandum of Understanding"
Presented in Technical Report 35
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ADDENDUM II
The Dynamic Nature of Technical Reports and Mathematical Models
Technical Report 35 was one of a series of reports presenting
technical findings and was not meant to be the absolute "final" work on
chlorophyll £ guidelines or any other recommendations. Technical
reports are periodically published and updated as new information becomes
available. Unfortunately, due to higher priority commitments, we have
not had the staff resources which would allow us to update Technical
Report 35 as should have been accomplished by now.
Mathematical models are evolving tools which can and should be
improved with time. One must recognize the limits of a math model or
any other model where one tries to duplicate and eventually anticipate
responses of natural ecosystems. It is presumptuous to try to differ-
entiate between the impacts of 30 and 40 micrograms per liter of chloro-
phyll a^with finality. Such differences are beyond the sensitivities
of most models. The biological reaction rates which are contained in
the model mechanisms are not that well known to provide such a degree
of resolution. Other uncertainties exist which leads one to make some-
what subjective decisions concerning the inputs to a model. The output
of any model run is only as good as the assumptions, rates, and data
that have gone into it.
Mathematical models are predictive tools that operate within
definable limits. Recognizing the sensitivity and accuracy with which
a given model can predict future conditions based on known inputs and
estimated reaction rates is a difficult and delicate process. As
greater understanding of mechanisms and reaction rates is realized,
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models are modified and even recalibrated and reverified to incorporate
this better information. This is a dynamic process. For these reasons
we see no great shift of policy regarding either the recommended chloro-
phyll a^ levels or our general attitude towards the eutrophic condition
of the Potomac. We recognize the need to decrease the magnitude of
bloom events and we are convinced that we are proceeding towards that
end in a reasonable and scientifically valid manner.
It is a known fact that algae need nitrogen and phosphorus, as well
as other nutrients and favorable physical conditions, in order to grow.
We know that at the present time we have an excess of both nitrogen and
phosphorus. By removing a high percentage of the phosphorus at point
sources we should inhibit algal growth under certain conditions, inducing
a phosphorus-limited system. Until advanced waste treatment with a high
degree of phosphorus removal goes on line, we will not know with certainty
the estuarine response to this type of treatment. In that a healthy
ecosystem also contains significant algal growth, the species are usually
grazed by other organisms in the next level of the food chain, thus are
considered to be beneficial to the ecosystem as a whole. What we want to
achieve is a sufficient reduction in phosphorus loads to decrease bloom
intensity and possibly induce a shift back to a utilizable species.
Current model predictions indicate that phosphorus effluent concentrations
as contained in area sewage treatment plant permits should result in a
substantial decrease in algal bloom intensities, although not necessarily
to a level of 25 yg/1 of chlorophyll .a on a continuous basis. There will
be times when the levels will be less and under certain conditions the
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levels will be greater. More important than the 25 yg/1 level is the
enforcement of nutrient limits in the upper estuary in order to promote
biological diversity without producing nuisance conditions.
No one knows with certainty what level of chlorophyll a_will result
in a healthy, ecologically balanced Potomac Estuary. Certainly it is
different for different aquatic systems. The diversity of the algal
species involved in the bloom, as well as the chlorophyll a^ level, are
required to evaluate the biological health and stability of the estuary.
Predictive tools such as methematical models, like other tools, must
be used with care and restraint. Their predictions are not end products
but merely guides that can be consulted in making managerial decisions.
Technical Report 35 did not consider the possibility of inducing
a phosphorus limited system because of predicted low cost nitrogen
control. Current planning makes this alternative more feasible because
of the projected $100 million or more cost for a denitrification
facility. If future studies show the desirability of additional controls,
appropriate recommendations toward that goal will be made. However, they
will not be made without adequate, verified, scientific information
at hand.
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