A PRELIMINARY DISCUSSION OF THE LOWER POTOMAC STP (LPSTP)
ALLOWABLE EFFLUENT FLOW RATE
June 29, 1973
Norman L. Lovelace
Regional Ccnlerfor hmironnicntal Informntion
US EPA Region III
1650AichSt
PhilartclphLi. P^ 19103
Annapolis Field Office
Environmental Protection Agency
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A PRELIMINARY DISCUSSION OF THE LOWER POTOMAC STP (LPSTP)
ALLOWABLE EFFLUENT FLOW RATE
I. INTRODUCTION
Before any vigorous analysis is done on the effects of the LPSTP
discharge, several things should be discussed. Review of TR 35 and
other related reports indicates that conflicts may exist on the
subject of embayment discharges. Furthermore, the integrity and veracity
of the Potomac Load Allocations, as presented in TR 35, is currently
being questioned by Hydroscience, Inc. and other interested parties.
The implications of these questions and observations are quite
significant when related to the LPSTP investigation. It is beyond the
scope of this discussion to fully explore and analyze the validity
of the questions being raised about TR 35. However, it should be kept
in mind that it was not the primary objective of TR 35 to present
loadings for Potomac embayments. Consequently, detailed analyses
were not performed on the local effects of embayment discharges.
No definitive answer will be given here concerning an allowable
discharge rate for the LPSTP. Instead the criteria within which an
analysis is to be made and the probable results of such an analysis
will be discussed.
II. GUNSTON COVE
Gunston Cove is a Virginia embayment of the Potomac Estuary,
approximately 2 miles upstream of Indian Head, Maryland. The
embayment may be characterized as being relatively wide and shallow.
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It has an average depth of about 4 feet and a surface area of
approximately 8.24x10 sq ft. There are two freshwater inflows at the
of trie einbdyfii^nt '. Puhi c!c Crf-ek drrti AccuLink Creek. The LPSTP
discharges into Pohick Creek.
Gunston Cove, and other Potomac embayments, have been described
as being ideally suited for high primary productivity. For this
reason the proposed nutrient criteria (TR 35) are more stringent for
the embayments than for the main channel.
1970 and 1972 AFO data indicates that there are high levels of
chlorophyll a_ present in the embayment throughout the summer months.
The data also shows that the TP and TIN standards proposed in TR 35
are contravened a majority of the time. The nitrogen criteria are
meet occassionally, however, this is generally during periods of high
chlorophyll a_ levels. Pre and post bloom nutrient levels are always
above the desirable levels. Dissolved oxygen is often above saturation
during the daylight hours due to photosynthetic production. Although
no night time data are available, it is possible that significant DO
depressions may exist at night due to algal respiration. 1972 and 1973
Fairfax County data indicates that the inflows of Pohick (before LPSTP)
and Accotink Creeks may be a significant source of nutrients.
III. PROBLEM STATEMENT
As understood by AFO, the objective of the LPSTP discharge rate
analysis may be stated as follows:
Determine the maximum allowable effluent flow rate for the
LPSTP that will not cause a problem in Gunston Cove. Assume
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the LPSTP effluent will have the following characteristics;
BOD5 <_ 3.0 mg/1
Uno.vidized N < 1.0 mg/1
Total P £0.2 mg/1
Total N (when technology is <_ 1.0 mg/1
available)
UOD <_ 10.0 mg/1
The problem statement is deficient in some areas. "Problem" in
Gunston Cove is not defined, it may be defined in terms of a probable
chlorophyll a_ level, dissolved oxygen standard or a combination of
both. The level of oxidized N (NOo-N) in the effluent is not given.
This could be a significant factor until the 1.0 mg/1 Total N
requirement is feasible. The methods of expression for the nitrogen
and phosphorus concentrations are not given; are they expressed at N0~.
N, P, P0», etc.? For the purposes of this discussion, the nitrogen
concentration will be assumed to be expressed as N and the phosphorus
concentration will be considered to be expressed as P.
The statement also implies that the LPSTP will be the cause of
a "problem" in Gunston Cove. As will be discussed later, this may not
be the case. The LPSTP may be a contributing factor to a possible
"problem" in Gunston Cove, however, it may not be the primary factor.
IV. AVAILABLE INFORMATION AND GOVERNING CRITERIA
There are two things that may be viewed as possible governing
criteria. One is nutrient levels (N, P) and the other is dissolved
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oxygen. DO appears to be the only enforceable standard, however,
excessive algal populations caused by high nutrient levels (and/or
oiner factors) can have a detrimental impact on the DO budget. This
is particularly true during the night time hours and during the
fall die off.
TR 35 sets desirable upper bounds for the nitrogen and phosphorus
pre bloom concentrations in Gunston Cove, they are;
Inorganic N as N £0.3 mg/1
Total Phosphorus as P £ 0.03 mg/1
These levels are claimed to be necessary to prevent nuisance
algal growths, which are defined as being greater than 25 yg/1 of
chlorophyll a_. TR 35 does not claim that this chlorophyll a_ level
(25 yg/1) is an absolute figure given the above nutrient levels,
however, it does imply that nuisance conditions will not exist at these
levels.
TR 35 sets allowable mass loading rates for N, P, and UOD into
Gunston Cove. If it is assumed that these loadings are based on the
above criteria, then a possible conflict arises. The loadings are
given in terms of a wastewater flow rate and a corresponding Ibs/day
mass loading. These figures have been further reduced to include an
effluent concentration: The TR 35 figures are:
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Waste Flow
mgd
50
103
170
TP as P
Ibs/day
35
60
140
mg/1
0.084
0.070
0.099
Total N as N
Ibs/day
130
270
460
mg/1
0.31
0.31
0.32
UOD
Ibs/day
7000
11000
16000
mg/1
16.8
12.8
11.3
These figures are based on a warm weather condition. The
following observations can be made:
1. The LPSTP proposed effluent concentrations for N and P
are above the TR 35 proposed effluent concentrations.
2. The LPSTP effluent UOD concentration is below the TR 35
limits for the flow range shown.
3. The TR 35 allowable P effluent concentrations are
substantially higher than the proposed receiving water
upper limits.
4. The TR 35 allowable effluent Total N concentrations
are at the receiving water upper limit for inorganic
N. The reasoning behind this is that the total N will
be converted to inorganic N (hydrolysis, oxidation).
This seems to be a reasonable assumption because of
the warm weather conditions and long retention time of
the embayment.
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In another section^TR 35 (pp. 11-18, XII-9) proposes effluent
concentrations for embayment discharges, these are:
•
uon
,
Total N £ 1.0 mg/1
Total P ± 0.2 mg/1
These concentrations appear to conflict with the mass loadings
presented for Gunston Cove. The UOD concentration is more stringent
while the N and P concentrations are less stringent.
The only waters available for the dilution of the LPSTP effluent
are:
1. Pohick Creek
2. Accotink Creek
3. Potomac Estuary
Review of the historical data for these 3 waters indicate they
have N and P concentrations that are regularly above the proposed limits,
The proposed upper limits for TIN and TP in the main Potomac, in this
area, are 0.4 mg/1 and 0.067 mg/1, respectively. It it is assumed
that these constituents act conservatively or nearly conservatively
then it appears as though there is virtually no water available for
dilution of the LPSTP effluent nutrients, even at a "cleaned up"
condition.
The UOD concentration in the LPSTP effluent appears to be well
within the limits stated in TR 35. However, the TR 35 UOD loading
was determined assuming there was not a large standing crop of
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phytoplankton. The question arises; are the TR 35 UOD loadings valid
if the nutrient criteria are violated? Unfortunately there is no
v* r-1 IT-
The question of whether the nutrient levels established in TR 35
will, in fact, meet the stated chlorophyll ^objective will not be
considered here. This is a question of much debate and of great
importance. The fact that it is not considered in detail here should
not diminish the importance of this question.
V. PRELIMINARY MODEL APPLICATIONS
Some preliminary model runs were made in an attempt to quantify
some of the intuitive observations already made in this discussion.
Due to the preliminary nature of this analysis, strict adherence to
normal verification and calibration procedures was not observed.
Therefore, the results cannot be considered as final predictions, but
rather as general indications.
The objectives of the model runs are:
1. To demonstrate the effects of the Potomac Estuary on
Gunston Cove at various LPSTP flows and Potomac flows.
2. To demonstrate the effects of the LPSTP at various
f1ows.
3. To check the UOD loading for possible DO standard
violations.
Five hydraulic conditions were specified for the model runs, they
are:
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1. Potomac flow above Gunston Cove 6542 cfs.
Pohick Creek = Accotink Creek = 10 cfs.
i nr-rn _ inn ~^^A
U . I- I ~> I I ~ i •'J'-f !!'• ^ *-»
b. LPSTP = 200 mgd
c. LPSTP = 300 mgd
2. Potomac flow above Gunston Cove = 820 cfs
Pohick Creek = Accotink Creek = 10 cfs
a. LPSTP = 100 mgd
b. LPSTP - 200 mgd
These Potomac flows correspond to flows at the fall line of
roughly 6000 cfs and 300 cfs.
All quality runs were made with a steady state model using net
flows computed from a dynamic hydraulic model. Figure 1 shows a
portion of the computational network used.
Inorganic nitrogen was treated as a conservative parameter. This
assumption was made for the following reasons:
1. No phytoplankton uptake rate was assumed because the
objective was to evaluate pre-bloom conditions. To
assume a uptake rate could give a false impression
because a loss would correspond to an increase in the
algal standing crop. It could otherwise be possible
to meet nutrient criteria if high enough loss rates
were assumed, however, these high loss rates would
correspond to large algal populations (blooms).
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2. Adsorption and deposition of the various nitrogen forms
was assumed to be insignificant.
3 D^nitrificatiOP to elemental N was assumed to he
insignificant.
4. Uptake by rooted aquatics was assumed to be insignificant.
This may not be a completely valid assumption, particularly
in the marsh areas of the cove. However, since no
reliable uptake rates were available they were neglected
in the interest of safety.
Phosphorus was not modelled. However, information concerning
TP can be deduced from the TIN results if one makes the conservative
assumption.
UOD and DO were run with the following conditions:
1. rUOD decay rate (deoxygenation rate) = 0.35 (I/day).
base e.
2
2. Sediment uptake rate - 1.0 gr 0?/m /day.
3. Reaeration rates were computed using the O'Connor-
Dobbins formula.
4. The effects of phytoplankton were neglected.
5. The UOD and DO in the Potomac just above Gunston Cove
were assumed at 2.0 mg/1 and 6.8 mg/1, respectively.
These values also correspond to a "cleaned up" condition
in the Potomac.
6. UOD and DO in Accotink Creek and Pohick Creek were
assigned values of 5.0 mg/1 and 7.0 mg/1, respectively.
7. A DO saturation value of 7.8 mg/1 was assumed.
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10
8. LPSTP UOD was specified at 10.0 mg/1 .
9. LPSTP effluent DO was specified at 5.0 mg/1.
conditions are similar tn the ones used in TR 35,
VI. MODEL RESULTS
The results of the model runs are shown at the end of this
discussion.
A. Dissolved Oxygen
The model runs of dissolved oxygen indicate that minimum
level of 5.0 mg/1 probably can be maintained in the majority of Gunston
Cove with LPSTP flows of up to 300 mgd. This prediction is, of course,
dependent on the validity of the assumptions made in the DO model.
Complete faith should not be put into these DO predictions because of
the assumptions made. They should be viewed as indications and not
absolutes. The TR 35 figures tend to support the DO predictions made
here. However, the predictions are sufficiently close to the 5.0 mg/1
DO standard to warrant a more detailed analysis, should the need arise.
B. Nitrogen
The model runs for inorganic nitrogen indicate that the
proposed nitrogen criteria in TR 35 are not feasible. The conclusion
is based primarily on the observation that intrusion of Potomac water,
at the TR 35 proposed TIN level, into Gunston Cove can violate the TIN
criteria for Gunston Cove. The fact that the LPSTP effluent will have
a TIN concentration of about 1.0 mg/1 will act to further exceed the
TR 35 proposed levels.
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11
TIN runs were made under the five different flow regimes.
The following observations are made:
1. The degree of Intrusion of the Potomac water into
Gunston Cove is significantly effected by the LPSTP
flow.
2. The degree of intrusion of the Potomac water into
Gunston Cove does not appear to be greatly affected
by the Potomac flow, at least for the two flows that
were run (6000 cfs and 300 cfs).
The runs were made by considering only one source of nitrogen
at a time. The combined effects of the sources can be obtained by
simply adding together the responses to the individual sources. To
consider the effect of the Potomac, the Potomac was set at 0.4 mg/1 TIN
and the LPSTP, Pohick and Accotink inflow concentrations were set at
zero. This was done for the various LPSTP flows with the two Potomac
flows. The resulting profiles show the effect of the Potomac alone.
For the other sources the Potomac concentration was set to zero and
the particular inflow was assigned a concentration (1.0 mg/1).
Two sets of curves were developed, from the model runs, to
show the effects of the LPSTP and the Potomac on Gunston Cove. The
Pohick Creek response is included in the LPSTP curves. The effects of
Accotink Creek were not included, however, it was determined that its
effect was overshadowed by the LPSTP at the low flow assigned to the
creek (10 cfs). One set is for a Potomac flow at the fall line of
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12
300 cfs and the other is for a flow of 6000 cfs. From these curves it
is possible to obtain estimates of the TIN concentration at various
1 orations in Gunbton Cove unuer a given set of rondi-Hnns. Boundary
conditions and inflow concentrations other than those run can also be
evaluated. This is possible if one recognizes that the principle of
linear superposition applies here. The quality response to the specified
boundary condition (Potomac concentration) or inflow concentration
(LPSTP) is linear with respect to that condition. For instance if one
wanted to see the influence of the Potomac when its concentration is
0.2 mg/1 rather than 0.4 mg/1, then one would simply reduce the 0.4 mg/1
response at the desired location by 50%. This principle does not
apply to flows, the response to a LPSTP discharge at 100 mgd is not
1/2 the response at 200 mgd. It must be emphasized that these curves
are derived from steady state model runs using the various assumptions
already stated and that the model cannot be considered as fully
calibrated or verified.
VII. CONCLUSIONS AND RECOMMENDATIONS
1. The TR 35 receiving water criteria for nitrogen and phosphorus
in Gunston Cove are not feasible as presented. This conclusion is based
on the fact that the N and P criteria are higher for the main channel
than for Gunston Cove and that the proposed LPSTP effluent concentrations
are higher than the receiving water standards. In addition the inflows
from Pohick and Accotink Creeks have N and P concentrations that are
regularly above the standards.
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13
2. A DO standard of 5.0 mg/1 appears to be feasible in Gunston
Cove with a LPSTP flow rate of up to 300 mgd and an effluent UOD
concentration of 10.0 mg/1. This conclusion may be contingent on
meeting the nutrient criteria, which are not feasible. Further work
is needed to either support or disprove this preliminary conclusion.
3. It appears as though any new discharges and/or expansion of
existing facilities will result in contravention of the TR 35 nutrient
standards, particularly in the Potomac embayments. Thus the problem
of determining allowable loads becomes one of determining the extent
to which standards will be violated. Simply the problem becomes one
of determining the degree of non compliance as opposed to compliance.
Within this framework one has two choices pertaining to allowable
effluent flow rates:
a. If the effluent nutrient concentrations are above the
receiving water nutrient standards then the effluent
flow rate should be kept at a minimum.
b. If the effluent nutrient concentrations are below the
receiving water nutrient standards then the effluent
flow rate should be kept at a maximum.
The second choice is generally not feasible due to treatment
limitations. Both choices are also dependent upon meeting DO standards
and other applicable standards, which may dictate the opposite course
of action depending on UOD levels and other factors.
With respect to the above conclusions, it is recommended that the
TR 35 proposed nutrient standards and allowable loads be reviewed. The
entire tidal Potomac should be treated as an integrated system and
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14
standards should be developed with such an approach. A possible result
of such a review could possibly be two sets of standards. One set
could bp an interim nlannino set which would be used for objective
planning and would be feasible at the present time. The other set
would be absolute standards which may not be feasible at the present
time. In both cas :s consideration should be given to population
projections and the expected levels of BAT. Also consideration should
be given to alternative methods of waste disposal (if any) and a
policy of zero discharge.
The full impact of either meeting or not meeting the proposed
standards should be accessed and quantified. Germane to the process
of developing standards is the determination of a water quality
objective. In the case of the Potomac this objective could be stated
in terms of the extent of the biological response to the various
nutrient inputs, and a DO standard. Obviously, toxic substance and
other parameters would also be included in the standards.
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Model Results:
Following are diagrams showing the results of the various model runs
that were made.
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Influence Curves:
Curve I ; Effect of Potomac, Potomac flow at fall line = 6000 cfs
e 11 ; Effect of LPSTP, Potoniuc flow at fall line ~ 500u cfs.
Curve III; Effect of Potomac, Potomac flow at fall line = 300 cfs.
Curve IV ; Effect of LPSTP, Potomac flow at fall line = 300 cfs.
Directions for use:
To determine the approximate concentration at a location (section);
1. Determine the Potomac influence from the appropriate
curve.
2. Determine the LPSTP influence from the appropriate
curve.
3. Add the two influences together to get the final
result.
Remember that different LPSTP inflow concentrations and Potomac
concentrations can be evaluated by applying a proportionality factor.
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