MANAGING POTOMAC WATER QUALITY: EVOLVING APPROACHES1
By Austan S. Librach, Cameron Wiegand, Stuart Freudberg, Michael Sullivan
and Robert N. Magill2
INTRODUCTION
The upper Potomac River Estuary is the principal focus of water quality management
planning in the Washington, D.C. metropolitan area. Its drainage area of over 12,000
square miles (31,000 km2) embraces sizeable portions of four states and the District
of Columbia, and includes 87 percent of the 24-00 square mile (6200 km2) metropolitan
Washington area (Figure 1). Like other estuaries that occupy submerged river valleys
along the eastern seaboard of the United States, the Potomac is neavily taxed
in its effort to assimilate the large pollutant loads it receives from urban, suburban
and rural sources. Major problems within the Estuary are oxygen deficits related
to organic wastes, and eutrophication caused by an over-enrichment of the Estuary
with phosphorus and nitrogen.
The management program for the Potomac Estuary over the past several decades
has been guided by the principle that, within economic limits, human impact upon
beneficial uses of the Estuary should be minimized. This control program has concen-
trated almost exclusively on implementation of increasingly higher level of treatment
at sewage treatment plants. Overall, the program has been quite successful.
Significant improvements in water quality in the upper estuary have occurred
during the last decade. Examination of dissolved oxygen (DO) trends over the
past twelve years shows a considerable amount of improvement. Nuisance algal
blooms have also not occurred since the late 1960's.
Much of the improvements observed result from the elimination of severe discharges
of raw sewage from the sewer system, and an ambitious program instituted by
the area's local jurisdictions in 1969 to upgrade treatment at Washington area
sewage treatment plants. By 1980, 12 treatment plants, with an aggregate treat-
ment capacity of 500 million gallons per day (mgd) (1900 X 106 L per day) were
operating at advanced secondary treatment levels or above, with further reductions
scheduled to be phased-in according to NPDES permit requirements. The total
1Presented at the March 15-20,1981 ASCE Spring Convention, held at New York, N.Y.
2Mr. Librach is Director of the Metropolitan Washington Council of Governments'
Department of Environmental Programs (DEP). Mr. Wiegand is DEPs Chief
of Water Programs. Messrs. Sullivan and Freudberg are Water Resources Engineers,
and Mr. Magill is an Environmental Planner within the same Department.
-------
Soviet: COO
CHAIN BRIDGE.
HEAD OF POTOMAC ESTUARY
MAINS POINT
BLUE PLAINS
EMBAVMENTS
O.C.
POTOMAC niVEH BASIN
SHOWING WASHINGTON
METROPOLITAN ATtEA
Figure 1-The Washington Metropolitan Area
and Potomac River Basin
-------
capital cost of improvements when all the improvements based on the 1969 program
are completed will be over a billion dollars. Resultant annual O&M costs have
approximately tripled to date and are expected to increase another fourfold by
the Year 2000.
FIG. 1 The Washington Metropolitan Area and the Potomac River Basin
Despite the huge investments cited to upgrade water quality, problems remain.
Water quality standards are still frequently violated, and portions of the upper
Estuary have not achieved the 1983 goal of "fishable-swimmable" waters established
by the Federal Clean Water Act (P.L. 92-500, as amended). Contributing to the
problem are significant uncontrolled loadings of oxygen demanding material and
nutrients originating from natural sources, unregulated agricultural activities,
urban stormwater and other nonpoint pollution sources generated locally and
upstream of the Washington region.
In view of the continued persistence of water quality problems in the Estuary
and the rapidly rising costs of providing advanced wastewater treatment (AWT),
there is growing interest among Washington area local governments for a reexamina-
tion of the existing management program to determine what is required to achieve
Potomac River water quality goals in an economically feasible manner.
This paper presents an overview of the evolution of water quality management
philosophy in the Washington area, and describes an emerging approach to compre-
hensive water quality management. To define the problem, the paper first assesses
the total pollutant loadings to the Estuary from all sources. This is followed by
an historical overview of water quality trends in the Estuary and the management
response to perceived water quality problems. The estimated costs of providing
increasingly higher levels of sewage treatment are then presented. And finally,
the paper describes an evolving approach, based on an analysis of trade-offs between
the various pollution control options available, to determine the most cost effective and
practical way of achieving specific and realistic water quality objectives.
POLLUTANT LOADINGS TO THE ESTUARY
The timing and delivery of pollutant loads and their impact on the river ecosystem
is not easy to predict or quantify. Wastewater from sewage treatment plants
and other point discharges represents a relatively steady day-to-day influx at
many points along both the free flowing and estuarine Potomac. In contrast,
nonpoint loads from stormwater runoff and combined sewer overflow loads are
extremely transient and variable. Both respond directly to runoff produced by
precipitation and snow-melt. The generation of- nonpoint pollutants ranges from
-------
nearly no contribution at all during dry periods to the largest and most important
source of pollutants during major runoff events. Similarly, combined sewer over-
flows typically do not occur unless some type of runoff is generated, but overflows
represent the most severe form of localized pollution when they do occur.
The variability of background loads is also significant in attempting to understand
total loads to the Estuary. On an annual basis, mean monthly Potomac River flow
varies by a factor of six between the spring maximum and fall minimum, and more
than a three order of magnitude variation in river flow has been observed between
such extreme events such as Tropical Storm Agnes in 1972 and the basinwide drought
of the mid-1960's. Nonpoint pollution loadings vary in a manner similar to the
flow fluctuations. For example, recent analysis has shown that annual nonpoint
loads to the Estuary can easily vary by a factor of two between wet and dry meteoro-
logical years (Sullivan, Freudberg, Wiegand, 1980).
Improvements in our understanding of the source and magnitude of all pollutant
loads to the Estuary has enabled a more complete accounting of these loads. This
accounting is critical to selecting the most suitable control programs. Estimated
average annual loads to the upper 50 miles of the Potomac Estuary for various
years projected to the Year 2000 are presented in Figure 2. Point source loadings
represent all permitted discharges to the upper Estuary and its tributaries below
Chain Bridge, the head of the Estuary. Five stages of loadings from point source
discharges are presented to reflect implementation, over time, of NPDES discharge
permits to the Estuary. Nonpoint source loadings represent loads generated in
tributaries below Chain Bridge that drain directly to the Estuary. Tnese estimates
represent average annual loads for a typical or average rainfall year. They are
based on current (1980) and forecasted (2000) land use patterns.
FIG. 2 Average annual pollutant loads to the Upper Potomac Estuary
Potomac loadings at Chain Bridge in Figure 2 are estimates of average annual
pollutants delivered to the Estuary from the middle and upper Potomac Basin
above the Washington, D.C. area. Estimates are based on an analysis of U.S. EPA
and USGS data taken at Chain Bridge during the late 1970's. Primarily nonpoint
in origin, these loadings are delivered to the Estuary by the free-flowing Potomac
River in both dissolved and suspended form. It is estimated that less than twenty
percent of these loads are generated within the metropolitan Washington, D.C. area.
The majority originate upstream of the metropolitan area from a variety of sources,
including municipal and industrial discharges; urban, agricultural, mining and forestry
nonpoint sources; and natural weathering processes at work within the basin.
-------
so
L.
z ™
& -
o
1
M
5_?:*
H?
4->vV-
*i »x -s
..•?%*w
xr*
?*ft£^?
**3
?&$,
t"r--.
~-''~*
a
%
r*^
2000
'"" mo RxT
• >~. Iw"* 'Xv!
; H fi H Pi >w
^ ^ . fxixj m m
.no _221
BM
5
8
^K
W%
tS*^
^S^
S4c'-j*i
^
§P*3
^Sj
:fe?*
•i*^
V«K^
££?
•s:iS
^?ki
:'".-
•X"M
*•*•*•*!
SS
::'-:::
S-S
• * "•'
W:'-
x-x
':>:>
||
:x':::
gijl;
11
X-.-.
.vX-'
fi::?:
o
3)
V
°- x.
I
p "
O I)
=5 10
^O
= 5
e.
fo
V
H.30
i
in
(0 ..
«* "
M
.a
o
o
o^
cT
o
Point Sourc* La*air.(S
Nonpoint Sourc» Lo*oing» Potomac Loaaingi
Below Chain Bridge at Chain Briegt
mo
2000
!»T?
Total Nitrogen
P
&S
1*2
I?SO
&sr..
?*l
Poiftt Sourc* LMdin{>
im
Nonpoint Source Loading* Potomac Loaeinji
Below Chain Bridge at Chain Bridge
2000
Total Phosphorus
2000
If SO
^
^d
Point Source
Nonptunt Source Lo«oinga Potomac
ftciov Owm ftridgt
at Ch«in ftridg*
Figure 2-Average Annual Pollutant Loads
to the Upper Potomac Estuary
-------
The slight increase for the Year 2000 indicates an increase in nonpoint loadings
from metropolitan Washington area watersheds above Chain Bridge. Otherwise,
these loadings were held constant.
A conclusion that may be drawn from Figure 2 is that the overall magnitude of
future point source loadings will be relatively small compared to existing and pro-
jected local nonpoint and upstream loadings, especially if current sewage treatment
plant permit requirements remain in effect.
BASIS OF THE REGION'S PRESENT POINT SOURCE CONTROL PROGRAM
The control of pollutants entering the Potomac Estuary has occupied the attention
of policy makers, government agencies and individuals for many decades. Particular
interest has centered on the upper 50 miles (80 km) of the 115 mile (186 km) long
estuary, where the principal discharges and population centers are located. The
U.S. Public Health Service first surveyed the upper Estuary in 1925 and concluded
that the danger of contracting sewage-borne diseases made swimming unsafe anywhere
in the Washington, D.C. area. Concern regarding high bacteria counts, algal blooms
and noxious odors led to the construction, in 1935, of the Blue Plains primary treat-
ment plant in the District of Columbia. Shortly thereafter, Arlington County
constructed another though much smaller primary plant, and Blue Plains began
to serve other communities in Maryland and Virginia. By 1940, all of the area's
major sewage discharges were receiving primary treatment.
The region experienced rapid population growth during and after World War II.
A metropolitan Washington population which stood at 575,000 in 1938 had by 1943
increased to 1.1 million. Point source loadings also increased sharply to the point
where waste loads entering the Potomac after treatment in 1943 actually exceeded,
by one-third, the loadings experienced prior to construction of the Blue Plains and
Arlington treatment facilities. By 1956, when secondary treatment was introduced
at Blue Plains, treated wasteioads entering the Potomac were double the untreated
level recorded in the early 1930's.
In 1956, Congress passed a second Water Pollution Control Act, which established
the "Enforcement Conference" approach as a means to achieve better pollution
management in interstate waters. The first Federal-State Enforcement Conference
on the Potomac River was convened by the U.S. Public Health Service in August
1957. The Conference met again in 1958 and produced advisory recommendations
for a joint program of remedial action for D.C., Maryland and Virginia pollution
discharges in the Washington region. Principal among the recommendations was
a goal to reduce discharges of biochemical oxygen demand materials (BOD) by .
80 percent and a commitment to disinfect all sewage effluent.
Implementation of the joint Federal-State program recommended by the Conference
was severely impeded by a number of institutional problems. As the region's popula-
tion and sewage generation increased, the Estuary continued to exhibit poor water
quality. Dissolved oxygen values in the upper Estuary were below existing Maryland
standards. The late 1960's also recorded significant phytoplankton blooms and
accompanying nuisance mats of floating blue-green algae, particularly during
-------
periods of low flow. Concerns as to the Estuary's declining water quality prompted
the convening of a second Potomac Enforcement Conference in 1969.
An extremely ambitious program for the removal of phosphorus and nitrogen from
sewage plant effluents was proposed. Conference recommendations included
the imposition of strict effluent load limitations in terms of pounds per day of
BOD, total phosphorus, and total nitrogen for waste treatment facilities discharg-
ing to the upper Estuary, and specific removal levels (96 percent BOD, 96 percent
phosphorus and 85 percent nitrogen) for ten treatment plants discharging to tribu-
taries and embayments of the Potomac.
•
The wasteload allocations for the Potomac plants were determined using early
estuarine water quality models applied to dissolved oxygen data of 1965-66 and
1968-69. Calculations indicated that in order to meet Estuary disso.ved oxygen
standards, the 5-day BOD load from discharges would have to be reduced from
the existing discharge of about 150,000 Ibs/day (68,000 kg/day) to 16,500 Ibs/day
(7500 kg/day). To achieve a phytoplankton chlorophyll-a goal of 25 micrograms
per liter (ug/l)to arrest accelerated Estuary eutrophicaticn (maximum mid-summer
values of 400 jjg/1 were observed in the late 1960's), wastewater loading allocations
were set which called for stringent reductions in effluent concentrations of nitrogen
and phosphorus (Thomann, Fitzpatrick, 1980).
Modeling calculations at the time made no allowance for loadings from nonpoint
sources and combined seu-er overflows. Model assumptions were based simply on
a steady state, minimum stream flow condition (seven-day, ten-year low flow).
This condition was typically used as a representative "worst case" for calculating
allowable loadings. Pollutant contributions from sources other than point sources
were accounted for indirectly by including a margin of safety in assigning wasteload
allocations to individual discharges and subsequently in NPDES permits by including
combined sewer overflows within the original wasteload allocation.
Some participants in the 1969 Conference warned that there was insufficient under-
standing of the behavior of nitrogen and phosphorus in relation to their impacts
on water quality in the Estuary to warrant such a program. Nonetheless, these
recommendations were confirmed in a 1970 Memorandum of Understanding adopted
by area local jurisdictions, the District of Columbia, and the Virginia and Maryland
pollution control agencies. They became the basis for subsequent expansion and
upgrading of treatment plants in the Washington, D.C. area (Bower, Bandler, 1975).
The strict discharge requirements established by the 1969 Enforcement Conference
were later incorporated into the NPDES discharge permits issued to each local
sewage treatment plant as a result of the Federal Clean Water Act. The primary
focus of abatement activities has been the Washington, D.C. Blue Plains Treatment
Plant, the major single point source input to the Estuary. (Approximately 309
mgd (1160 X 106 L/day) of the current M5 mgd (16SO X 106 L/day) sewage treatment
plant loadings are from Blue.Plains. The plant's NPDES permit issued in 1974
called for a staged reduction in BOD, phosphorus and Total Kjedahl Nitrogen (TKN)
to AWT effluent levels of 5 mg/1 BOD5, 0.22 mg/1 P, and 2.4 mg/1 TKN).
-------
As of 1980, all treatment plants discharging to the upper Estuary had achieved
advanced secondary treatment levels. The Blue Plains plant and five additiond
plants are scheduled to achieve AWT levels in the next few years.
DEVELOPING NEW MANAGEMENT APPROACHES
Figure 3 illustrates the marked reduction in point source loadings of oxygen demanding
materials (BOD5) delivered to the upper Potomac Estuary that has resulted from the
upgrading of area wastewater treatment facilities called for by the 1969 Enforcement
Conference. It can be seen that the BOD5 load has decreased almost 90 percent from
1968-79 to 1981. Although trend data is limited, Figure 4 shows recent improvements
recorded in upper Estuary dissolved oxygen levels under a range of summer fresh water
inflows. This trend can likely be attributed to reductions in wastewater effluent
concentrations achieved in the last ten years.
FIG. 3 Daily BOD Loadings to the Upper Potomac Estuary from Major Wastewater
Treatment Plants
FIG. 4 Dissolved Oxygen Trends in the Upper Potomac Estuary
Though the recent improvements in Estuary DO levels have been welcomed, it
has also been acknowledged that these gains have involved tremendous federal
and local capital expenditures. From a management standpoint and given the
substantial costs incurred thus far, serious questions are now being raised in the
region as to the need to proceed still further down the line to full NPDES permit
implementation when there still has been no clear and prior determination as to the
cost-effectiveness of doing so. These questions are being raised, in part, because
the traditional and rigidly interpreted approach for setting water quality standards
and wasteload allocations called for by federal regulations and adhered to in
establishing effluent requirements for Estuary discharges, provided for very little
flexibility. Left unaddressed with this approach are difficult questions concerning
-------
Q
01
HI
to
CQ
160
IfO.
120
100
80
§ 60
20
1970
Estimated 1970 Loading 141,000 Ibs.
1975
1979 Loading 6!>,000 Ibs.
Current Loading 18,000 Ibs.
1980
1985
1990
YEAR
Figure 3-Total Daily BOD5 Loading to the Potomac by
Major Wastewater Treatment Plant Discharges
-------
Years of Observation
1968-70
1977
1978
1979
Summer
Average
/77
Minimum
0,
V)
id
t-
txo
O
Q
—
4
ll
~~ /77
(2000-
5000 cfs)
4
V
/77
(2000 cfs)
T
'"
(6000 cfs)
t.
X77
(6000 cfs)
•
Dissolved Oxygen .
Standard
• Average Summer Potomac
Flows at Chain Bridge
Source: I lydroQual, Inc.
Figure 4-Dissolved Oxygen Trends
in the Upper Potomac Estuary
-------
the seasonal aspects of certain designated water uses requiring protection, and the
frequency and importance of standards' violation. Questions as to the costs of control
mechanisms to avoid such violations were also unaddressed. Thus while cost considera-
tions were extremely important to decisions related to treatment plant sizing and location,
they were not considered from the standpoint of water quality improvements achieved
per dollar expended.
The need for a comprehensive evaluation of such considerations has become increasingly
apparent. Capital costs of reaching final NPDES permit advanced treatment
levels at the Blue Plains facility, for example,'will exceed $500 million. As shown
in Figure 5, capital costs increase dramatically as permit levels of 94-98 percent
removal are approached. Operating costs are also expected to increase four-fold as area
facilities proceed from secondary to AWT treatment processes (Sullivan, Freudberg,
Wiegand, 1980). The major and yet unanswered question which must be addressed,
as local governments move to final stages of facility construction, is whether these
additional expenditures are warranted in terms of additional water quality benefits
to be achieved.
FIG. 5 Comparison of Capital Costs for Different Treatment Levels at Blue Plains
Sewage Treatment Plant
Along with these cost issues, some concerns have also been raised as to whether
exclusive emphasis on technology-based point source management solutions will
be sufficient to resolve remaining Estuary eutrophication problems. The significance
of nonpoint source loadings and loadings from sources upstream of the Washington, D.C.
area was earlier identified in Figure 2. Nutrient loadings by source are further presented
in Figure 6. As shown, the magnitude of annual upstream and local nonpoint sources
of nutrient loadings far exceeds annual point source contributions. Such annual
loading comparisons must, of course, be interpreted with some caution since the period
of greatest stress on the Estuary has historically been during mid-to-late summer
when fresh water inflows as well as nonpoint nutrient loads may be at their lowest
levels while, in a relative sense, point source loads are greater. However, there is also
some evidence which suggests that spring nonpoint and upstream nutrient loads
deposited in Estuary sediments and released back to the water column in the
summer during periods of low dissolved oxygen may well be the controlling factors
in producing algal blooms and other associated estuary ecosystem imbalances.
-------
600 —I
5
J
g
o
oo
CT\
U-
O
O
3
200 —
106
91
PRIMARY TREATMENT
SECONDARY TREATMENT
FULL AWT
PERCENT
REMOVAL
BOD5
NITROGEN
PHOSPHORUS
35%
10%
10%
85%
10%
10%
97 %
(Summer)
98%
Figure 5-Comparison of Capital Costs at
Blue Plains Sewage Treatment Plant
'Projected expenditures. Approximately $100 million of this amount represent as yet
• • • ''fnrililios.
-------
FIG. 6 Nutrient Loadings to the Upper Potomac Estuary
(1980 Annual Loadings)
Evidence that the nonpoint load may have significant adverse impacts on
Estuary water quality has led to recent efforts to characterize the extent to
which such loads could be controlled. Nonpoint analysis to date relevant to the
Potomac. Estuary were discussed in detail in a recent paper (Sullivan, Freudberg,
Wiegand, 1981). Six scenarios for controlling nonpoint pollution in metropolitan
Washington were analyzed. Most of the controls investigated focused on those
urban nonpoint controls which could be most practically implemented in concert
with new development in the Washington region. The paper concluded that nonpoint
controls could be more cost-effective on a pollutant per pound removed basis
than point source controls, and that the possibility of a "trade-off" between point
and nonpoint controls merited more detailed analysis.
Cost-benefit and loading source/impact considerations such as described above
have increasingly prompted those responsible for management of water quality
in the Estuary to seriously question the wisdom or practicality of relying on advanced
wastewater treatment to achieve the greatest marginal improvement in Estuary
water quality. Concurrent interest has been stimulated in evaluating whether
a more cost-effective management approach exists which will lead to continued
water quality improvement at less expense for area jurisdictions.
An investigation of possible management pollution approaches is currently under-
way. It involves two main areas of examination: first, a comprehensive look at
exactly what water quality levels can affordably be achieved; and second, the
previously cited "trade-off" analysis of control measures, notably nonpoint source
controls, that could possibly make significant contributions to improved water
quality at less expense than advanced wastewater treatment. The following sections
further describe the management concepts and decision-making tools being factored
into this examination.
-------
50 -i
V)
•o
I
in
O
2 30 -
V)
to
c
'•£
a
o
c
20 -
5-1
3-
l-i
Nitrogen
Phosphorus
Upstream
Local Nonpoint
Point Source
Figure 6-Nutrient Loadings to the
Upper Potomac Estuary (1980 Annual Averages)
-------
New Concepts in the Application and Interpretation of Water Quality Standards
While the adhering to the national goal of achieving fishable and swimmable waters
by 1983, the evolving management approach for the Potomac Estuary will likely
propose a broadened consideration of water quality standards and goals.
Integral to this process is a recognition that the Estuary is a highly dynamic system
with significant spatial and temporal variability. In this context, absolute statements
that a given water quality standard shall never be violated, while perhaps easier
to interpret and enforce, represent an overly simplistic and perhaps misleading
measure of water use protection.
The Estuary's use as a resource also varies according to seasonal and other temporal
factors. Swimming and other forms of water-contact recreation are an example.
However, standards are typically established for strict year round compliance.
Consideration of the seasonal and temporal aspects of water use protection in
the development, application and interpretation of water quality standards is
clearly warranted.
The fact that the water quality standards themselves are somewhat arbitrary
is also an important consideration when evaluating the suitability of a receiving
water for a particular use designation. The standards are usually based on a
series of toxicity experiments in which an indigenous population of fish
is exposed in laboratory tanks to a range of concentrations of a "harmful" constituent
for a somewhat arbitrarily determined duration of time (often 96 hours). An absolute
water quality standard is then set at a concentration value below which some
percentage (usually 50 percent) of the population die during the duration of the
test. Noss and Marks (19S1) among others have recently questioned the technical
assumptions behind selection of the 50 percent lethality figure, and have suggested
that other factors such as morbidity, spawning ability, or growth rates should also
be considered in setting standards. Noss and Marks have also presented a conceptual
model of an improved process for setting standards which would maintain use
of laboratory toxicity or other experiments as a scientific basis, but would also
add equally important cost/benefit and political feasibility considerations as
part of the standard-setting and water quality evaluation process.
Another difficulty with present receiving water quality standards and criteria
and their interpretation is that there usually is not any scientific correlation
between the standards and wastewater treatment levels required in rlfcrhargp
jnermits issued tn individual treatment plants. The 1977 Clean Wnfrr Art Amrnd
tc pr-PcrrihaH gppQJJc levels OJ point SOUTCP control haspd upon tgrhniral
c[hj)ity nf arhjpvprn^Tt ratner tnan on receivine water quaijty impagt. The
Act presumed that the level of control would be high enough to enable standards
to be met. While application of this technology-based approach has likely been
responsible for recent improvements in upper Estuary water quality (see Figure 4),
there have been no corresponding determinations showing that it is also the most
cost effective means of pollution management. Recent thinking suggests an alterna-
tive and more comprehensive approach to future Potomac Estuary pollution management
decisions that establishes a link between treatment levels and water quality impact
-------
over a broad range of naturally occurring flow, rainfall, temperature and loading
conditions. It also broadens the concept of "treatment level" to
include local nonpoint source, combined sewer overflow and upper Potomac Basin
controls in addition to traditional point source controls.
Return Frequency Considerations in Water Use Protection
A key element introduced for consideration would be the notion of frequency
of compliance with a given water quality standard. Traditionally, the analysis
of compliance has been to see whether a standard — the dissolved oxygen standard,
for example, would be violated anywhere at anytime, based on analysis with a
calibrated and verified water quality model. However, due to various natural
and human-induced phenomena — including photosynthesis, respiration, temperature,
river flow, tidal influence, variation in pollution loads — the dissolved oxygen
values measured in the Estuary fluctuate over a wide range on daily, weekly, and
seasonal time scales. Spatial variations in dissolved oxygen longitudinally along
the Estuary and with depth in the water column are also pronounced.
As a new approach, it is proposed that considerations in the selection of the most suitable
management option could be substantially broadened to weigh Estuary responses under a wide
range of seasonal flow, temperature and loading conditions. Managers would be presented with
an array of options constructed with a Potomac water quality model (see later section for
discussion) that simulates Estuary responses under various conditions. Figure 7 depicts the
frequency of compliance for one hypothetical array of controls (Alternatives A-D) under
different river flow regimes. In the example, Curve A might represent the most stringent
set of controls and D the least stringent. Since it is known how frequently a given seven-day
flow occurs, the decision-maker would then have enough information to know that for a certain
expenditure level, the dissolved oxygen standard would be met, say 95 percent of the time
over a specified period of years, or conversely, that violations of the standard would be expected
at a given frequency. The consequence of such violations and the impairment of water quality
uses could then be weighed against such factors as the timing and severity of use impairment
and the cost of the abatement option.
FIG. 7 Hypothetical Frequency of Compliance Curves for Various Treatment
Alternatives
-------
100
C D
to
S
4)
V
o
oo
>.
X
O
•d
v
>
"o
E
»*«
H
v
o
50
10
100
1000
10,000
100,000
' Seven-Day Mean Flow (in cubic feet per second)
Figure 7-Hypothetical Frequency of
Compliance Curves foFVarjous"
Treatment AlteFhatives
-------
Application of Improved Estuary Models
To be able to incorporate loading source, cost-effectiveness and return frequency
considerations into the assessment of future management options for the Estuary,
it was recognized that new water quality modeling tools were needed to define
and assess an optimal mix of point, nonpoint, combined sewer overflow and upper
basin controls. Financed by a combination of federal and local government support,
a Potomac Estuary model is now under development which represents a
state-of-the-art approach in the area of eutrophication kinetics. Model simula-
tions will supplement the traditional seven-day, ten-year low flow, quasi-steady
event with seasonal, annual, or even multi-year scenarios as considerations in
determining a management strategy.
The rationale for examining longer term events includes recognition of the fact
that the eutrophication process takes place on week-to-week time scale, rather
than hcur-to-hour; that the low flow event does not permit proper analysis of
the impact of nonpoint loads on Estuary water quality; and finally that achieve-
ment of improved water quality in the Estuary is masked by focusing on one
arbitrarily selected worst-case event.
Kinetic functions of the model include both water column and sediment reactions
for dissolved and participate fractions of the important phosphorus, nitrogen,
and mineral nutrients, as well as the capability to model the population dynamics
of two phytoplankton and two zooplankton groups. The model is one-dimensional,
vertically and tidally-averaged, and is designed to be run in a continuous simulation
mode. The model framework is designed specifically to permit determination
of water quality resulting from various management strategies for all loading
sources to the Estuary. With its assistance, it is hoped that a lasting long-term
cost-effective approach to improving and maintaining the water quality in the
Potomac Estuary will be devised.
Figure 8 depicts the conceptual elements of a proposed management approach
for the Estuary. As suggested in the foregoing discussion, the new approach would
replace the traditional one of almost exclusive reliance on point source control.
Introduction of important and heretofore often ignored considerations such as
the timing of water uses needing protection; timing and frequency of water
quality standards' violations and their significance to the uses being protected;
and the costs of meeting or not meeting standards for alternative return periods
are included. The new approach would make possible the identification of
cost-effective management options. As available data suggests, these may well
include an optimized mix of point, nonpoint, combined sewer overflow and
upstream source controls.
FIG. 8 Comparison of Water Quality Management Processes
-------
TRADITIONAL MANAGEMENT PROCESS
USES
Determine Zonal
Removal Needs
by Pollutant Type
STANDARDS
Develop Standards
Based on Desired Uses
LOADS
Assign Point Source
Loading Targets
By Zone
EMERGING POTOMAC ESTUARY MANAGEMENT PROCESS
USES
COST-EFFECTIVENESS
STANDARDS
LOADS
Determine Zonal Load
Removal Needs by:
- Pollutant Type
- Timing of Loading
- Timing of Use
- Loading Source
Develop Cost-Eflective
Management Options
for Each Zone,
Considering All
Pollutant Sources
Develop Frequency
Related Water
Quality Standards
Based on Optimum
Pollution Reduction
at Minimum Cost
Assign Loading Targets:
by Zone
by Season
by Type of Source
Figure 8-Comparison of Water Quality
Management Processes
-------
INTEGRATION WITH RELATED LOCAL AND BASINWIDE ACTIVITIES
Local Watershed Management Activities
Recognizing that nonpoint loadings may have significant impacts, both on important
water uses within individual watersheds and on thePotomac Estuary itself, Washington
area governments established a coordinated approach to local watershed management.
It is directed at setting local nonpoint pollution abatement priorities within the
context of the region's overall water quality control program.
Management priorities were first established by comparing and ranking metropolitan
area watersheds in terms of the magnitude'of their existing and forecasted pollutant
loadings (MWCOG, 1979). The ranking process employed application of quantitiative
criteria to 42 individual watersheds in the region based on simulations of current
0977) and forecasted (2000) average annual total and unit area nonpoint loads, total
and percent change between current and forecast conditions, and stream channel
erosion potential. In addition, qualitative criteria which addressed instream as
well as Estuary receiving water impacts were taken into consideration. Based
upon this analysis, each watershed was assigned a "critical watershed" priority ranking
as shown in Figure 9.
FIG. 9 Ranking of Washington Area Watershed Study Priorities
The ranking system served as a basis for selecting watersheds where more detailed
water quality assessments were warranted. Nonpoint source modeling tools were
also developed and a consistent set of nonpoint source loading rates identified
for local use to ensure that individual studies would be conducted in a reasonably
consistent manner. This approach facilitated the "transferability" of study results
and ensured that resulting local watershed management programs would be sensitive
to broad regional water quality problems.
To date, five local watershed assessments have been completed and two more
are underway. In composite, watershed study data will provide more detailed
quantification of regional and local nonpoint pollution problems; assist in
identification of loading targets and control needs; and augment nonpoint source
information applied in the Potomac Estuary trade-off analysis previously
described.
-------
Priority I Watersheds
Priority II Watersheds
I I Priority III Watersheds
Jurisdiction Boundary
"——*• Watershed Boundary
Streams
Figure 9-Ranking of Washington Area
Watershed Study Priorities
-------
Upper Potomac Basin Activities
The magnitude of average annual pollutant loadings to the Potomac Estuary from
sources upstream of the Washington, D.C. area is evident in Figures 2 and 6.
These loads are not suprising, however, considering the large size of the Potomac
Basin and the diverse activities it supports. Much of the upstream load is the
result of natural weathering and erosive processes. However, a large portion can
also be attributed to agricultural and extractive activities, while comparatively
small amounts are due to point source discharges upstream of metropolitan
Washington.
The importance of upstream nonpoint source loads underscores the need for effec-
tive and increased upstream use of agricultural runoff controls, basic soil conservation
practices and other nonpoint control practices that minimize the human-induced
portion of this problem. The trend towards no-till farming in the upper Basin
is pronounced and may ultimately help to reduce nutrient and sediment loadings
transported to the upper Estuary. However, there may well be a parallel trend
occurring—away from contour framing and soil conservation practices towards
intensive farming—which could offset the gains of no-till. The success of upstream
abatement programs are a critical concern to the health of the Estuary. Downstream
jurisdictions have emphasized the importance of these efforts and will be closely
watching the implementation process.
-------
CONCLUSIONS
Research indicating the significance of nonpoint source loads delivered to the
upper Potomac Estuary, coupled with a growing recognition that scarce economic
resources must be applied in the most cost-effective manner, has fostered an
interest in broadening the focus of the region's water quality control program.
As suggested in Figure 8, a new management approach is evolving that may
eventually replace the region's traditional and almost exclusive reliance on control
of point source loadings to meet rigidly defined and interpreted water use
definitions and quality standards. This new approach is more flexible and practical
in its consideration of the impacts, controllability, and costs to control all loading
sources. The approach would employ return frequency and economic considera-
tions in tandem with sophisticated water quality model simulations to help identify
the most cost-effective management solutions.
Such techniques include assessment of the timing of water uses needing protection;
the timing and frequency of water quality standards' violations and their signifi-
cance to the uses being protected; and the costs of meeting or not meeting standards
for alternative return periods. The new strategy will make possible identification
of cost-effective management options which, as available data suggests, may
well include some optimized mix of point, nonpoint, combined sewer overflow,
and upstream controls rather than continued singular emphasis on high technology
and costly advanced wastewater treatment.
To support a long-term management strategy, a state-of-the-art eutrophication
model for the Potomac Estuary is under development. Using a frequency of
occurrence approach, variations in future water quality in the Estuary will be
predicted for various levels of pollutant control from all sources for varying
costs. The strategy providing the greatest water quality benefit at the minimum
cost will be selected to guide management of the Estuary into the next century.
-------
APPENDIX 1 - REFERENCES
Bower, B., Blackburn, A., "Dollars and Sense", Interstate Commission on the Potomac
River Basin, Bethesda, Md., Sept. 1975.
Metropolitan Washington Council of Governments, Department of Environmental
Programs, "Analysis of Nonpoint Source Pollutants in the Washington Area and
the Selection of Critical Watersheds", Washington, D.C., 1979.
•
Noss, Richard R., Marks, David H., "Improving the Development of Water Quality
Standards", Journal, Water Pollution Control Federation, Volume 53, Number 4.
Sullivan, M., Freudberg, S., Wiegand, C., "Strategies for Considering Point and
Nonpoint Control Tradeoffs for the Upper Potomac Estuary", Proceedings of a
Technical Symposium on Nonpoint Pollution Control, Tools and Techniques for
the Future, Interstate Commission on the Potomac River Basin, ICPRB Tech.
Pub. 81-1, Jan. 1981, pp. 225-2 W.
Thomann, R.V., Fitzpatrick, 3., "Overview of Potomac Estuary Modeling, Tasks
I and II - Dissolved Oxygen and Eutrophication", HydroQual, Inc., Mahwah, N.J.,
Aug. 1980.
------- |