WATER QUALITY OF THE 

POTOMAC RIVER ESTUARY 

AT WASHINGTON, D.C. 

I 

Water Quality of the 

Potomac River Estuary 

at Washington, D.C. 

By W. H. Durum and W. B. Langbein 

POTOMAC RIVER STUDIES 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 529-A 
United States Department of the Interior 

STEWART l. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page 

The setting__________________________ 1 

Riverflow and quality characteristics _ _ 1 

Effects of wastes ------------------ 4 

Dissolved oxygen------------------ 4 

SummarY---------------------------

References-------------------------

ILLUSTRATIONS 

Figure 1. Graph showing seasonal distribution of flow of the Potomac River, at Point 

Page 

8 

9 

Page 

of Rocks, Md __________________ ---- ____________ -------- ____ ---- _ -------- 2 

2. Graph showing suspended sediment-load at Point of Rocks, Md_________________ 3 

3. Ratio of river to tidal flows------------------------------------------------ 3 

4. Map showing estimated daily biochemical oxygen demand reaching the estuary _ _ 5 

5. Diagram showing pollution index of Potomac River estuarY-------------------- 6 

6. Graph showing relation of dissolved oxygen content to distance for various 

rates of discharge, 1963-65- ________ -- ___ --- ____ --- _____ - ___ --- __ -- __ ---- 8 

TABLES 

Page 

Table 1. Characteristics of water in Potomac River estuary___________________________ 7 

2. Most prevalent quality class, October 1963 to September 1964----------------- 7 

III 

Potomac River Studies 

Warer Q.uality of the Potomac River Estuary 

at Washington, D.C. 

By W. H. Durum and W. B. Langbein 

THE SETTING 

Along the Atlantic seaboard several prom-

_nent communities lie at or near the heads of 

~stuaries. These cities~ such as the TrentonPhiladelphia-Camden complex and Washington and Richmond~ lie along the Fall Line that 

marks the boundary between the drowned 

valleys and the higher rocks of the Piedmont. 

The drowned valleys form the estuaries~ 

where ocean and river waters mix. The estuaries provide a resource and a problem 

for the communities. At first, the cities 

looked to the estuaries for navigation, which 

was essential for commerce and growth. With 

growth, the hinterland and the cities became 

less dependent on navigation and "turned their 

backs" on the estuaries. Waste disposal became the dominant use~ creating problems of 

water supply and concern for the neglected 

recreational potential. Today~ "quality" is 

the key word as the cities again look toward 

the estuaries. 

An estuary is neither river nor ocean. The 

mechanical action of the tides reaches to the 

very head of an estuary, where the river 

tumbles over the fall-line rocks. The tide 

in the' Potomac River reaches just above 

Chain Bridge near the western limit of the 

District of Columbia. There is sharp contrast between the swift~ turbulent~ narrow 

river seen looking upstream from Chain 

Bridge and the gentle currents of the wide 

estuary just downstream at Georgetown. The 

quality of water in the estuary is a composite 

of the effects of the tide~ the river discharge~ 

and the waste disposal. 

The estuary begins at about Chain Bridge 

and extends southeastward to the Chesapeake 

Bay at lat 38°. The estuary is several hundred feet in width at the head and broadens in 

its length of 117 miles to nearly 6 miles at 

its mouth. A channel with a minimum depth 

of 24 feet is maintained in the estuary as far 

upstream as the upper end of the Washington 

channel on the west side of Potomac Park in 

Washington. Except for the channel and depths 

of 80 feet crt Chain Bridge and at Three Sisters 

Island, the estuary is relatively shallow; 

depths are predominantly less than 5 feet 

throughout most of the estuarine reach. 

The upper reach of the tidal Potomac extends from Chain Bridge in the District of 

Columbia to Hallowing Point~ about 25 miles 

downstream. Hallowing Point (where colonials 

hallooed for the ferry on the Maryland shore) 

is about 3 miles below Fort Belvoir between 

Gunston Cove and Occoquan Bay. 

The estuary is bounded by the large metropolitan areas of Washington, D.C.~ and of 

Arlington and Alexandria, Va .• in the upper 

part~ and predominantly by forest and farmland in the lower, major part. Many tributaries add water to the estuary, the largest 

being the Anacostia River which enters at 

Washington. 

RIVERFLOW AND QUALITY CHARACTERISTICS 

An average of about 11,000 cfs (cubic feet 

per second) of water enters the estuary from 

the Potomac River. Figure 1 shows the seasonal distribution of flow of the river, as 

measured by the U.S. Geological Survey, at 

Point of Rocks, Md., which is 25 miles above 

Great Falls. During low flow the water has 

less than 260 ppm (parts per million) of dissolved solids due to solution of rocks drained 

1 
2 WATER QUALITY OF THE POTOMAC RIVER ESTUARY AT WASHINGTON, D.C. 

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Figure 1. -Seasonal distribution of flow of Potomac River at Point of Rocks, Md. 

by the river system and residues of wastes 

discharged to streams in the system. At high 

flows sediments from the land are moved in 

suspension and along the bed of the river. 

For example, at Point of Rocks the suspended 

load in tons per day increases about hundredfold, with a tenfold increase in flows greater 

than 10,000 cfs. (Fig. 2.) Nearly 3 million 

tons of sediment is discharged by the Potomac 

River each year; an undetermined amount 

deposits in the estuary. Areas undergoing 

urbanization in the metropolitan area produce 

the largest amount of sediment and have sedimen~discharge rates about 50 times greater 

than do average rural areas (Wark and Keller, 

1963), and thus constitute a serious problem 

for the estuary. 

In addition to the water and materials that 

enter the head of the estuary from the upland 

river, other water is added from direct precipitation, tributary inflow, overland runoff, 

and waste disposal outfalls. The amounts of 

water and of suspended and dissolved substances from these sources have not been 

completely defined. Discharge records for 

Potomac River above Chain Bridge and for 

several tributaries to the estuary are available from the u.s. Geological Survey. Approximate values for runoff from tributaries 

and direct precipitation to the estuary can be 

computed. Measurements of stream runoff, 

including tidal reaches, can be measured 

where more precise data are required. The 

volume of water in the estuary has been computed for low tide and mean tide for the reach 

Chain Bridge to Cornwallis Neck; however, 

measurements of tidal interchange have not 

been made. Wastes from sewer outfalls are 

probably the major immediate concern in estuary quality. 

Flows in the estuary are compounded of 

two sources-river flows that vary seasonally 

in response to rainfall over the 10,000-square-
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RIVERFLOW AHD QUALITY CHARACTERISTICS 3 

10,000 100,000 

WATER DISCHARGE, IN CUBIC FEET PER SECOND 

mile watershed and the twice-daily tidal current inducted by the oceans. 

The relative influence of the river and tide 

in the estuary may be approximated by the 

ratio. Q / ( Q+V /I)# where. Q is the river discharge, V is the intertidal prism (Durfor, 

1961, fig. 14), and T is the time of a tidal 

cycle (13.5 hr). Figure 3 shows this ratio in 

terms of mileage along the estuary. Proceeding downstream, one finds that river discharge becomes a smaller part of the total 

water movement. At low flows ( Q=1,000 cfs) 

the flow in the estuary below Roosevelt !~land 

is dominantly tidal. Even at mean discharge 

(Q =11,000 cfs), t:j.dal movement exceeds river 

discharge at a distance of 12 miles below 

Chain Bridge. The distance at which river 

discharge exceeds tidal flow (that is Q ~ V /l') 

increases only as the square root of the discharge. 

Ftawe 2. --5Uipended-sec:Ument load at Point of Rocks, Md. 

Although the ocean tides reach to Chain 

Bridge, the ocea~ salt water does not extend 

beyond Indian Head. The inland extent of salt 

water in the estuary varies in relation to 

river discharge and tidal action. During a lowflow period in 1930. a chloride concentration 

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DISTANCE, IN MILES DOWNSTREAM FROM CHAIN BRIDGE 
4 WATER QUALITY OF THE POTOMAC RIVER ESTUARY AT WASHINGTON, D.C. 

of 3, 700 ppm (equivalent to about 20 percent 

sea water) was observed at Indian Head. A 

chloride concentration of 68 ppm-several 

i:imes the probable normal value-was observed at Fort Washington during low flow 

in September 196 3. Although this concentration is relatively low in comparison with that 

in sea water, it does indicate a probable 

maximum point of inland movement of wateJ;' 

from Chesapeake Bay. During high flows the 

inland extent of salt water is considerably 

less. 

The water in the estuary at Washington is 

always fresh, and the main concerns of water quality are sediment content, biologic 

character, and dissolved oxygen. 

EFFECTS OF WASTES 

More than 200 million gallons per day of 

waste effluent is added to the estuary. Locations of waste outfalls are shown on figure 4. Although about 70 percent of settleable solids is removed from the wastes 

before they are discharged to the estuary, 

unknown amounts of sludge deposits have accumulated at the mouth of the Anacostia River and at waste outfalls. Dissolved materials 

in the wastes include minerals normally in 

the water and chemicals used in treatment 

of the wastes. The daily wastes from the 

metropolitan area alone are estimated to 

contain more than 22,000 pounds of phosphorus and more than 68,000 pounds of nitrogen. 

The euthrophication by these chemicalsabetted by favorable hydrologic and climatic 

conditions-caused algal populations in 

1963 to be more than 10 times as large as 

populations generally found in tidal waters 

(Brehemer, 1964). Figure 5 illustrates the 

general conditions of the estuary based on 

biochemical oxygen demand and coliform 

bacteria. 

Because variable amounts and types of 

suspended and dissolved materials are added to ·the estuary, the quality of water below 

Chain Bridge is highly variable with respect 

to location and time, a factor that limits water use. In the upper part of the estuary, near 

Chain Bridge, the contents of the water are 

such that the water can be used most of the 

time for most purposes after standard treatment. Concentrations of dissolved solids and 

specific ions such as chloride and sulfate are 

well within the; maximum limits allowed for 

drinking supplies. Also, specific ions such 

as silica are not present in sufficient quantities at this site to impair the utility of the 

water for industrial processes. Coliform 

bacteria occasionally exceed suggested tolerable limits of 5,000 mpn (most probable 

number per 100 milliliters). Much higher 

quantities, up to 390,000 mpn, have been detected below Blue Plains (table 1). 

Downstream, from Chain Bridge to the vicinity of Washington and Mount Vernon, the 

dissolved oxygen decreases and bacterial content of the water increases at low flow. Phosphorous and nitrogen compounds are conspicuous during the dry season. 

DISSOLVED OXYGEN 

Organic matter in water is decomposed 

through oxidation. If oxygen is not available 

to decompose organic matter, septic conditions and foul odors are created. Oxygen 

content then is a direct indicator of the state 

of organic health of a body of water. 

The oxygen content varies with the oxygen 

demand of the organic load (reported as BOD, 

biochemical oxygen demand), with the ternperature (cold water can contain more oxygen than warm water), and with the river 

discharge. The effect of river discharge on 

oxygen content is complex. 
RIVERFLOW AijD QUALITY CHARACTERISTICS 5 

M 0 N T G 0 M E R Y 

Powmac 

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GEORGES 

Other !II.,Yland ootfalls 740. Military establishments 

and federal installations (estimate) 5000 

Figure 4, -Estimated daily bioc:hemicai oxygen demand to estuary, in pounds per day. 
6 WATER QUALITY OF THE POTOMAC RIVER ESTUARY AT WASHINGTON, D.C. 

BIOCHEMICAL OXYGEN DEMAND 

1 Year May-Sept 

0 50 100 

I II II I • I I I I 

PERCENTAGE OF TIME WATER 

IS OF CLASS SHOWN 

CLASS OF WATER 

• A, Excellent 

~ B,Good 

CJTI] C,Fair 

0 D, Poor 

BOD 

(ppm) 

< 1.5 

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COLIFORM BACTERIA 

1 Year May-Sept 

PERCENTAGE OF TIME WATER 

IS OF CLASS SHOWN 

COLIFORM 

(mpn} 

< 50 

50-500 

500-5000 

Figure 5. -Pollution index of Potomac River estuary. 
RIVERFLOW AND QUALITY CHARACTERISTICS 7 

Table 1.-Characteristics of water in Potomac I<.iver estuary 

Range in monthly av.erag~ values, Conductance values2 

water year 19641 (micromhos) 

Parts per million Coliform 9-10-63 9-30-64 

Bio- bacteria Location Dissolved chemical (most probable Discharge Discharge 

Turbidity oxygen oxygen number 

demand per 

100 ml) 

Fletchers Boat House,D.C .• 11-101 7.1-14.8 1.4-3.0 620- 13,000 

Roosevelt Island, D.C.----- 24-178 6.~14.5 1. 7-4.4 1 .. 700- 62 .. 000 

Hains Point, D.C. --------- 32-153 4. 7-14.6 1.3-8.6 1,460- 62;000 

Below Sewage Treatment 23-118 1.9-14.0 1.4-12.0 62.,000-390,000 

Plant .. D.C. 

Fort Washington, Md ------ 23-115 3.3-13.8 1.8-12.5 56~240.,000 

Mount Vernon, Va --------- 32- 93 3,6-13.9 2.8-10.0 230- 24,000 

Marshall Hall, Md -------- 32- 66 5.0-12.2 1.5- 7.2 121- 24,000 

Hallowing Point., Va ------- 34-102 6.8-12.3 2.3- 8.1 69- 24,000 

1 From Interstate Commission on the Potomac River basin. 

2From U.S. Geological Survey. 

Location 

-

Table 2.-Most prevalent quality class, October 1963 to September 1964 

[A., excellent; B, good; C., fair; D., poor] 

Dis- BioTur- solved chemical Coliform 

bidity oxygen oxygen bacteria 

demand 

at USGS 

gage: 

1 .. 220 cfs 

381 

392 

342 

352 

470 

820 

850 

1.,680 

Remarks 

Fletchers Boat House, D. C._ B A c c Quality satisfactory. 

at USGS 

gage: 

1, 830 cfs 

400 

340 

390 

400 

400 

500 

600 

800 

Roosevelt Island, D.C. ----- B A c D Occasional high coliform probably 

local drainage. Hains Point, D.C. __________ B B D D Excess BOD and coliform from 

Blue Plains Plant. 

Effect of Blue Plains Treatment Plant 

Below Sewage Treatment 

Plant, D. C. ___ ----------- B D D D DO low; BOD and coliform usually 

excessive. 

Fort Washington, Md _______ B c D D Occasionally low DO; generally 

high BOD and coliform. 

Mount Vernon .. Va --------- B c D c Occasionally low DO; excess BOD, 

coliform. 

Good recovery of quality of water generally 

Marshall Hall, Md _________ B B D B Excess BOD, coliform becoming 

lower. 

Hallowing Point, Va ------- B A D B 
8 WATER QUALITY OF THE POTOMAC RIVER ESTUARY AT WASHINGTON, D.C. 

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DISTANCE, IN MILES DOWNSTREAM FROM CHAIN BRIDGE 

Figure 6. -Relations of dissolved-oxygen content to distance for various rates of discharge, 1963-65. 

River water carries with it a considerable 

amount of oxygen in solution; it increases 

the turbulence and mixing in the estuary and 

thus acts to increase the re-aeration of the 

water; and it serves to transport and thus to 

distribute the organic load downstreamward 

over a greater volume of water. 

Average content of dissolved oxygen (reported in percentage of saturation for the 

given temperature) along the estuary in 

terms of river discharge is shown on figure 

6. The curves on this diagram show that upstream from Memorial Bridge (above mile 

5), the river is always nearly saturated. ·As 

one proceeds downstream the effect of sewage effluent becomes more evident. In the 

river directly below Blue Plains (mile 11), 

there is a sag in the dissolved-oxygen content that is particularly marked when the 

river discharge is 1,000 cfs. However, the 

sag in these curves moves downstream as 

river discharge increases. 

Farther downstream, in the Mount VernonHallowing Point reach, the dissolved-oxygen 

content is greater when river discharge is 

low than when the river is at a mean rate of 

about 10,000 cfs. Of course, the range in 

conditions is less, and the average dissolvedoxygen content is higher than in the estuary 

opposite Blue Plains. 

SUMMARY 

The quality of water in the Potomac River 

estuary below Chain Bridge is a composite 

of the river discharge and of waste disposal 

into the estuary. An average of about 11,000 

cfs of water from the Potomac River enters 

the estuary at Chain Bridge. During low flow 

the mineral content of the water is not high, 
REFERENCES 9 

less than 260 ppm of dissolved solids derived 

from rocks drained by the river system and 

residues of wastes discharged to the system. 

Nearly 3 million tons of sediment is discharged annually by the Potomac River. 

Areas in the Washington metropolitan area 

undergoing urbanization have rates of sediment yield that are 50 times greater than 

rates in rural areas. 

The relative influence of the river and 

tide in the estuary can be expressed by the 

ratio Q/(Q+ V /T), where Q is river discharge, 

V is the volume of intertidal prism, and T is 

the time of a tidal cycle. At mean discharge, 

tidal movement exceeds river discharge at a 

distance of 12 miles below Chain Bridge. 

Although the ocean tides reach to Chain 

Bridge, the maximum point of inland movement of salt water is near Fort Washington. 

The water in the estuary at Washington is 

always fresh. The main concerns of water 

quality are sediment content, biologic character, and dissolved oxygen. 

More than 200 million gallons of waste effluent enter the estuary daily. Daily wastes 

from metropolitan area alone contain an estimated 22,000 pounds of phosphorus and 

68,000 pounds of nitrogen. In the upper 

reaches of the estuary, the water quality is 

satisfactory for most uses after standard 

treatment to sterilize, being about the same 

as the quality at the metropolitan intake at 

Little Falls. Downstream, from Chain Bridge 

to the vicinity of Fort Washington and Mount 

Vernon, the dissolved oxygen decreases and 

the bacterial content of the water increases. 

There is a sag in the dissolved-oxygen content directly below Blue Plains that moves 

downstream as river discharge increases 

above 1, 000 cfs. 

REFERENCES 

Brehemer, Morris, 1964, Nutrients, in Problems of the Potomac Estuary: Interstate 

Comm. Potomac River Basin Winter Mtg., 

Arlington, Va., January 1964, Proc., p. 47-

50. 

Durfor, C. N., 1961, Water quality and hydrology in the Fort Belvoir area, Virginia, 

1954-55: U.S. Geol. Survey Water-Supply 

Paper 1586-A, 57 p. 

Interstate Commission on the Potomac River 

basin, 1964, Potomac River water quality 

network: 3 5 p. 

U.S. Army Corps of Engineers, 1962, Potomac River basin report: U.S. Army Enginear Dist., Baltimore, v. 4, Engineering 

studies; v. 5, Water supply and water quality control-uses, requirements, and benefits. 

Wark, J. W., and Keller, F. J., 1963, Preliminary study of sediment sources and transport in the Potomac River basin: Interstate 

Comm. Potomac River Basin Tech. Bull. 

1963-2, 28 p.