Water-Quality Assessment of the Potomac River 

Basin: Basin Description and Analysis of 

Available Nutrient Data, 1970-90 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 95-4221 

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 

Water-Quality Assessment of the Potomac 

River Basin: Basin Description and Analysis of 

Available Nutrient Data, 1970-90 

By J.D. BLOMQUIST, G.T. FISHER, J.M. DENIS, 

J.W. BRAKEBILL, and W.H. WERKHEISER 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 95-4221 

Prepared as part of the 

NATIONAL WATER-QUALITY ASSESSMENT 

PROGRAM 

Towson, Maryland 

1996 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITI, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute 

endorsement by the U.S. Geological Survey. 

For additional information write to: 

District Chief 

U.S. Geological Survey 

8600 LaSalle Ad, 206 Carroll Bldg. 

Towson, MD 21286 

Copies of this report can be purchased from: 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286 
CONTENTS 

Abstract.................................................................................................................................................................................. 1 

Introduction .. . .. .. . . . . .. . . .. .. . . .. . . . . . .. . . . . .. . . . . .. .. . . ... . .. .. .. . . .. . . . . .. . .. . .. . . . . .. . . . . .. . . . . . . . . . .. . . .. ... . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . .. . . . . .. . . . . .. . . . . 2 

The National Water-Quality Assessment Program...................................................................................................... 3 

Acknowledgments .......................................... ............. ............................................ .. . ..... .. ...... ....................... ...... ... .... 3 

Description of the Potomac River Basin and Its Subdivision for Water-Quality-Assessment Purposes .............................. 4 

Location and Major Features ....................................... ~............................................................................................... 4 

Climate.......................................................................................................................................................................... 4 

Physiography ........ .............................................................. .. .. . ........................ .................... ............. ........................... 7 

Geology....................................................................................................................................................................... 9 

Land Use and Population............................................................................................................................................. 11 

Water Use and Wastewater Discharges........................................................................................................................ 11 

Hydrology.................................................................................................................................................................... 16 

Surface Water..................................................................................................................................................... 16 

Ground Water..................................................................................................................................................... 19 

Ground-Water/Surface-Water Interactions .. ................................................................ ...................................... 22 

Subunits for Water-Quality Assessment...................................................................................................................... 23 

Analysis of Available Nutrient Data ...................................................................................................................................... 24 

Methods ....................................................................................................................................................................... 25 

Estimates of Nutrient Inputs.............................................................................................................................. 25 

Analysis of Nutrient Concentrations ................................................................................................................ 27 

Nutrient Sources .............................................................................................. ...................... ....... ............ ....... ..... ....... 28 

Basinwide Inputs ........................... .............................................................................. ................ ...................... 29 

Distribution in Subunits..................................................................................................................................... 30 

Distribution in Major Watersheds...................................................................................................................... 34 

Nutrient Concentrations............................................................................................................................................... 38 

Data Availability................................................................................................................................................ 38 

Ground Water ........................................................................................................................................... 40 

Surface Water........................................................................................................................................... 42 

Concentrations in Ground Water....................................................................................................................... 44 

Concentrations in Surface Water....................................................................................................................... 52 

Spatial Variability.................................................................................................................................... 52 

Temporal Variability................................................................................................................................ 55 

Streamflow Relations..................................................................................................................... 55 

Seasonal Patterns........................................................................................................................... 59 

Long-Term Trends ...................................... ....... ............................ .. .. .. . ................................................... 59 

Nutrient Loads and Mass Budget ... .......... ......................... ................................................ .... . ........................... 64 

Loads in Major Tributaries ............. .. .. ......... .............................. .. .. .. ....... ............. ............................ ........ 64 

Mass Budget and Land Use ..................................................................................................................... 67 

Base-Flow Nutrient Loads....................................................................................................................... 71 

Summary ................................................................................................................................................................................ 75 

Selected References............................................................................................................................................................... 79 

FIGURES 

1. Map showing location and major features of the Potomac River Basin................................................................. 5 

2. Map showing average annual precipitation in the Potomac River Basin, 1951-80................................................ 6 

3. Graph showing mean monthly precipitation at two locations in the Potomac River Basin, 1951-80.................... 7 

Contents Ill 
FIGURES-Continued 

4-7. Map showing: 

4. Physiographic provinces and subprovinces in the Potomac River Basin.................................................. 8 

5. Generalized rock types in the Potomac River Basin.................................................................................. 10 

6. Generalized land use in the Potomac River Basin, mid-1970's ............................................................... 12 

7. Generalized distribution of total freshwater withdrawals in the Potomac River Basin, 1990 . .................. 14 

8. Graphs showing surface-water and ground-water withdrawals of freshwater in the Potomac River 

Basin, 1990 ............................................................................................................................................................. 15 

9. Map showing major wastewater discharges to streams in the Potomac River Basin, 1990................................... 16 

10. Map showing mean annual surface-water runoff, 1951-80, and selected streamflow-gaging stations in the 

Potomac River Basin.............................................................................................................................................. 19 

11. Graphs showing mean annual and mean monthly streamflow in the Potomac River at Washington, D.C., 

1970-90 .................................................................................................................................................................. 20 

12. Map showing location of major surface-water impoundments in the Potomac River Basin................................. 21 

13. Map showing division of the Potomac River Basin into subunits for the purpose of water-quality 

assessment ........................... ........................ ........... ................ ............ ............ ........................................................ 24 

14. Pie diagrams showing major inputs of nitrogen and phosphorus to the Potomac River Basin, 1990.................... 29 

15-21. Map showing: 

15. Distribution of nitrogen and phosphorus inputs from nonpoint sources to subunits of the Potomac River 

Basin, 1990........................................................................................................................................................ 33 

16. Location and watershed boundaries for surface-water-quality monitoring sites used for analysis of 

nutrient data....................................................................................................................................................... 36 

17. Distribution of nitrogen and phosphorus inputs to major watersheds of the Potomac River Basin, 

1990................................................................................................................................................................... 39 

18. Location of ground-water wells with nutrient data in the U.S. Geological Survey's WATSTORE data 

base, 1970-90.................................................................................................................................................... 42 

19. Location of ground-water wells with nutrient data in the U.S. Environmental! Protection Agency's 

STORET data base, 1970-90 .... ................................................. ......... .............................................................. 43 

20. Location of surface-water-quality monitoring sites with nutrient data in the U.S. Geological Survey's 

WATSTORE data base, 1970-90 ...................................................................................................................... 46 

21. Location of surface-water-quality monitoring sites with nutrient data collected by Federal and State 

agencies in the U.S. Environmental Protection Agency's STORET data base, 1970-90 ................................. 47 

22. Boxplots of dissolved-nitrate concentrations in ground-water samples from Potomac River Basin subunits, 

1970-90 .................................................................................................................................................................. 48 

23. Boxplots of dissolved-nitrate concentrations in ground-water samples from selected land-use settings in 

selected Potomac River Basin subunits, 1970-90 .................. ................................................................................ 49 

24. Graphs showing relation of dissolved-nitrate concentration with well depth in selected land-use settings in 

selected Potomac River Basin subunits.................................................................................................................. 51 

25. Boxplots of total-nitrogen and dissolved-nitrate concentrations in water samples from selected surfacewater-quality monitoring sites in the Potomac River Basin, 1970-90 ................................................................... 54 

26. Boxplots of total-phosphorus and dissolved-orthophosphate concentrations in water samples from selected 

surface-water-quality monitoring sites in the Potomac River Basin, 1970-90 ...................................................... 56 

27. Graph showing relation of total-nitrogen concentrations to stream discharge at selected surface-waterquality monitoring sites in the Potomac River Basin ............................................................................................. 57 

28. Graph showing relation of total-phosphorus concentrations to stream discharge at selected surface-waterquality monitoring sites in the Potomac River Basin ............................................................................................. 58 

29. Boxplots of seasonal total-nitrogen concentrations in water from selected surface-water-quality monitoring 

sites in the Potomac River Basin, 1970-90 ............................................................................................................ 60 

30. Boxplots of seasonal total-phosphorus concentrations in water from selected surface-water-quality 

monitoring sites in the Potomac River Basin, 1970-90 ......................................................................................... 61 

31-34. Graph showing: 

31. Flow-adjusted, total-nitrogen concentrations through time in water from selected surface-water-quality 

monitoring sites, 1970-90 ........ ................. ... .. . .. . ........ .......... .... .... ... . ... .. ........... ............ .......... .......... ... . . ... ......... 62 

IV Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
FIGURES-Continued 

32. Flow-adjusted, total-phosphorus concentrations through time in water from selected surface-waterquality monitoring sites, 1970-90 .................. ........... .......... .. .. . ................. .. ................. ....... ......... ....... ............. 63 

33. Estimated average annual total-nitrogen and dissolved-nitrate loads in the Potomac River and selected 

major tributaries ........ ............... ... ......................................... ...... ........... ............. ... ...... .. ...... ... . ...... .. ....... .. .. ....... 66 

34. Estimated average annual total-phosphorus loads in the Potomac River and selected major tributaries......... 67 

35. Maps showing mean annual total-nitrogen and dissolved-nitrate yields for selected watersheds in the 

Potomac River Basin, 1970-90 .............................................................................................................................. 69 

36. Map showing mean annual total-phosphorus yield for selected watersheds in the Potomac River Basin, 

1970-90 .................................................................................................................................................................. 70 

37-39. Graph showing: 

37. Relation of total-nitrogen and dissolved-nitrate yields to nitrogen-input rate and land use in the Potomac 

River Basin........................................................................................................................................................ 72 

38. Relation of total-phosphorus yields to phosphorus-input rate and land use in the Potomac River Basin ........ 73 

39. Estimated base-flow nitrogen loads, point-source nitrogen loads, and total-nitrogen loads for selected 

watersheds in the Potomac River Basin ........................................................................................................... 75 

TABLES 

1. Potomac River Basin land use by physiographic province and subprovince, mid-1970's..................................... 13 

2. Streamflow characteristics of the Potomac River at Washington, D.C., and of six major tributaries.................... 17 

3. Comparison of nitrogen input estimates to the Potomac River Basin upstream from Washington, D.C., from 

several studies......................................................................................................................................................... 30 

4. Estimated point-source and nonpoint-source inputs of nitrogen by subunits of the Potomac River Basin, 

1990 ........................................................................................................................................................................ 31 

5. Estimated point-source and nonpoint-source inputs of phosphorus by subunits of the Potomac River 

Basin, 1990 ............................................................................................................................................................. 32 

6. Summary of basin characteristics for selected surface-water-quality monitoring sites in the Potomac River 

Basin ....................................................................................................................................................................... 35 

7. Estimated inputs of nitrogen to selected watersheds in the Potomac River Basin, 1990 ....................................... 37 

8. Estimated inputs of phosphorus to selected watersheds in the Potomac River Basin, 1990.................................. 38 

9. Summary of selected surface-water-quality and acid-precipitation monitoring programs collecting nutrient 

data in the Potomac River Basin, 1970-90............................................................................................................. 41 

10. Availability of ground-water nutrient-concentration data in the U.S. Geological Survey's WATSTORE data 

base, 1970-90 .................................................................................. ............................................. .. ....... ................. 44 

11. Availability of ground-water nutrient-concentration data in the U.S. Environmental Protection Agency's 

STORET data base, 1970-90 .......................................................... ................................... ............ .... . ................... 44 

12. Availability of surface-water nutrient-concentration data in the U.S. Geological Survey's WATSTORE 

data base, 1970-90.................................................................................................................................................. 45 

13. Availability of surface-water nutrient-concentration data in the U.S. Environmental Protection Agency's 

STORET data base, 1970-90 ................................... ... ................................... ..... ............... ............................ ........ 45 

14. Estimated long-term mean annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected sites in the Potomac River Basin ........................... ... . . .............................................. ....... ......................... 65 

15. Estimated base-flow loads of nitrogen and phosphorus for selected watersheds in the Potomac River 

Basin ....................................................................................................................................................................... 74 

16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at selected waterquality-monitoring sites in the Potomac River Basin............................................................................................. 82 

Contents V 
CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM 

Multiply 

acre-foot (acre-ft) 

cubic foot per second (ft3

/s) 

foot (ft) 

gallon (gal) 

gallon per minute (gal/min) 

inch (in.) 

inch per year (in/yr) 

mile (mi) 

million gallons per day (Mgalld) 

pound (lb) 

pound per acre (lb/acre) 

pound per person per year [(lb/person)/yr] 

pound per square mile (lb/mi2) 

square mile (mi2) 

By 

1,233 

0.02832 

0.3048 

3.785 

0.06308 

25 .. 4 

25.4 

1.609 

0.04381 

0.4536 

1.121 

453.6 

1.751 

2.590 

To obtain 

cubic meter 

cubic meter per second 

meter 

liter 

liter per second 

millimeter 

millimeter per year 

kilometer 

cubic meter per second 

kilometer 

kilogram per hectare 

gram per person per year 

kilogram per square kilometer 

square kilometer 

Chemical concentration in water is expressed in milligrams per liter (mg!L). 

Temperatures in degrees Fahrenheit (°F) and degrees Celsius (°C) can be converted using the 

following equations: 

°F = 1.8 (°C) + 32 

°C = 5/9 (Of - 32). 

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929-a 

geodetic datum derived from a general adjustment of the first-order level nets of the United 

States and Canada, formerly called Sea Level Datum of 1929. 

Water year: The 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends. Thus, the year ending September 30, 1990, is called 

the "1990 water year." 

VI Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
Water-Quality Assessment of the Potomac River Basin: 

Basin Description and Analysis of Available Nutrient 

Data, 1970-90 

By J.D. Blomquist, G.T. Fisher, J.M. Denis, J.W. Brakebill, andW.H. Werkheiser 

Abstract 

The Potomac River Basin includes 14,670 

square miles and has a complex environmental 

setting consisting of various combinations of natural and human factors that can affect water quality. The basin is divided into eight subunits on the 

basis of physiography and lithology for the purpose of water-quality assessment. The eight subunits are the Appalachian Plateau, Valley and 

Ridge, Great Valley Carbonate, Great Valley Noncarbonate, Blue Ridge, Piedmont, Triassic Lowlands, and Coastal Plain. Land use in the mid1970's was 51 percent forest, 36 percent agricultural, and 8 percent urban. From 1970 to 1990, the 

population increased 43 percent to about 4.6 million people; two-thirds of this population resided 

in the Washington, D.C., area. About 97 percent 

of the freshwater used in the basin in 1990 was 

from surface-water sources. 

In 1990, commercial fertilizer and animal 

manure comprised about 55 percent of the nitrogen input and 93 percent of the phosphorus input 

to the Potomac River Basin. Municipal and 

industrial wastewater discharges contributed 

about 12 percent of the nitrogen and 4 percent of 

the phosphorus inputs. Municipal wastewater discharges were largest downstream from Washington, D.C., where 88 percent of the nitrogen and 

80 percent of the phosphorus discharges occurred. 

Atmospheric deposition contributed 32 percent of 

the nitrogen inputs. Fertilization rates are highest 

in the Monocacy River watershed with nutrient 

application rates of 15,300 lb/mi2 (pounds per 

square mile) nitrogen and 4,490 lb/mi2 phosphorus, and in the Conococheague Creek watershed 

with nearly 9,370 lb/mi2 nitrogen and 2,940 lb/mi2 

phosphorus. The North Fork Shenandoah River 

has the highest manure production rate at 

20,900 lb/mi2 nitrogen and 4,660 lb/mi2 

phosphorus. 

Dissolved-nitrate concentrations in ground 

water vary widely within the Potomac River 

Basin and range from less than 0.01 mg/L (milligram per liter) to 63 mg/L as nitrogen, with a 

median value of 1.8 mg/L. Dissolved-nitrate concentrations in the Appalachian Plateau, Valley and 

Ridge, and Coastal Plain subunits generally are 

low, with median concentrations of0.10, 0.14, 

and 0.10 mg/L, respectively. Dissolved-nitrate 

concentrations in the Great Valley Carbonate subunit are generally higher (median 4.5 mg!L) than 

in other subunits and exhibit only small differences among land-use settings. Fourteen percent 

of the wells in carbonate rock have concentrations 

greater than or equal to the 10.0-mg/L Maximum 

Contaminant Level for drinking water established 

by the U.S. Environmental Protection Agency. 

Nutrient-concentration data from 25 surfacewater-quality monitoring sites indicate that nitrogen and phosphorus concentrations generally are 

lowest in the sparsely populated, forested watersheds of the Appalachian Plateau, Valley and 

Ridge, and Blue Ridge subunits and are highest in 

the agricultural watersheds of the Great Valley, 

Piedmont, and Triassic Lowlands subunits and 

Abstract 
near urban centers where wastewater-treatment 

inputs are greater. Median concentrations of total 

nitrogen range from 0.42 to 3.9 mg/L at 22 sites, 

and median concentrations of dissolved nitrate 

range from 0.2 to 3.5 mg/L at 25 sites. Seasonal 

fluctuations of total-nitrogen and total-phosphorus concentrations in surface water are generally 

small at most sites. In agricultural and forest settings, nitrogen concentrations are highest during 

winter, and phosphorus concentrations are highest 

during summer. Where wastewater inputs are substantial, concentrations of nitrogen and phosphorus are higher during low streamflow conditions 

of the summer. 

Few of the 25 monitoring sites on the Potomac River and its tributaries show strong longterm trends in total-nitrogen concentrations during 1970-90. Sites on Conococheague Creek and 

Seneca Creek both show an increase in nitrogen 

concentrations. Total-phosphorus and dissolvedorthophosphate concentrations appear to decline 

slightly at many of the monitoring sites. 

The Potomac River near Washington, D.C., 

discharges a long-term average of 60 million 

pounds of nitrogen and 5. 79 million pounds of 

phosphorus per year. The North Branch Potomac, 

South Branch Potomac, and Cacapon Rivers, 

draining three large forested watersheds, contribute only 17.5 percent of the dissolved nitrate and 

7 percent of the phosphorus measured at Washington, D.C., and drain 25.9 percent of the basin. 

Greater nutrient loadings are generated from three 

predominantly agricultural watershedsConococheague Creek, Shenandoah River, and 

Monocacy River. Total nitrogen yields in these 

three tributaries range from 1,870 to 8,330 lb/mi2, 

and dissolved nitrate yields range from 1,650 to 

9,690 lb/mi2, The smallest nitrogen yields occur in 

forest and urban watersheds where nitrogen 

inputs are predominantly from atmospheric 

sources. The largest nitrogen yields occur in 

Conococheague Creek, Antietam Creek, and 

Monocacy River, where agricultural land use is 

widespread and nitrogen inputs are dominated by 

commercial fertilizer and animal manure. The 

transport of phosphorus is also highly variable as 

total-phosphorus yields range from 68 to 

654lb/mi2, whereas the phosphorus yield for the 

Potomac River Basin upstream from Washington, 

D.C., is 496lb/mi2• Ground-water discharge can 

contribute a substantial part of the total-nitrogen 

load to streams in agricultural watersheds underlain by carbonate rock. Base-flow nitrogen loads 

in Conococheague Creek and Antietam Creek 

contribute about 61 and 100 percent of measured 

nitrogen loads. 

INTRODUCTION 

This report describes the results of two components of the U.S. Geological Survey's (USGS) 

National Water-Quality Assessment (NAWQA) Program within the Potomac River Basin. First, major 

natural and human factors affecting water quality in 

the Potomac River Basin are described and used to 

subdivide the basin for the purpose of assessing waterquality conditions. The approach used for basin subdivision is consistent with the approach used by concurrent NAWQA studies in watersheds throughout the 

Nation (Hirsch and others, 1988). Second, information 

available from a variety of sources is used to describe 

water-quality conditions in the basin with respect to 

the nutrients nitrogen and phosphorus. 

The basin description provides the essential background information for the analysis of nutrients in the 

basin and encompasses the conceptual framework 

used to design the multidisciplinary assessment of 

water-quality conditions. This description focuses on 

broad-scale natural conditions and human factors 

affecting water quality. 

The analysis of available nutrient data will be used 

to help describe current water-quality conditions and 

to guide the NAWQA Program in the design of future 

studies of the Potomac River Basin. The major objectives of the analysis of available nutrient data are as 

follows: 

( 1) Document sources of nitrogen and phosphorus to 

the Potomac River Basin and describe the spatial 

distribution of sources in basin subunits and major 

watersheds. 

(2) Document current monitoring programs and available nutrient-concentration data for ground water 

and surface water. 

(3) Describe broad-scale geographic patterns of nutrient concentrations in ground water and surface 

2 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970--90 
water and relate these patterns to sources and major 

controlling factors. 

(4) Describe temporal patterns and long-term trends in 

nutrient concentrations in surface water. 

(5) Describe the mass budget (loads) of nutrients in 

surface water and compare surface-water loads to 

estimates of nutrient inputs. 

The National Water-Quality Assessment 

Program 

The NAWQA Program began in 1986 with the 

appropriation of Congressional funds for the USGS to 

test and refine concepts for the design and conduct of 

the program (Hirsch and others, 1988). Seven pilot 

studies were started in 1986 and were mostly completed by 1991. Using lessons learned during the pilot 

studies and supported by recommendations by the 

National Academy of Science (National Academy of 

Science, 1990), the NAWQA Program began fullscale implementation in fiscal year 1991 (Leahy and 

others, 1990). 

The goals of the NAWQA Program are to: 

(1) Provide a nationally consistent description of current water-quality conditions for a large part of the 

Nation's water resources; 

(2) Define long-term trends (or lack of trends) in water 

quality; and 

(3) Identify, describe, and explain, to the extent possible, the major natural and human factors that affect 

observed water-quality conditions and trends. 

It is anticipated that the data and findings of the 

NAWQA Program will provide a scientific basis for 

major national decisions that affect water-quality policy and regulation. It is important that such decisions 

be based on a sound understanding of the factors that 

affect water quality and that they be based on nationally consistent data and approaches. The NAWQA 

Program is designed to provide these requirements for 

informed decision making on a national level. 

The NAWQA Program consists of two major 

components-national synthesis and study units 

(Leahy and Wilber, 1991). The national-synthesis 

component will address specific water-quality issues 

that are of common concern in most parts of the 

Nation. It is designed to address these issues through 

comparative studies among different hydrologic settings in the Nation, using data that are collected and 

analyzed in a consistent manner. The data needed for 

national-synthesis topics will be provided, in large 

part, by the other major component of the NAWQA 

Program-study units. 

Sixty study units (major river basins and aquifer 

systems) have been identified for inclusion in the fullscale NAWQA Program (Leahy and others, 1990). 

The first 20 study units began operation in fiscal year 

1991. A second group of 20 study units began in 1994, 

and a third group of 20 study units are planned to 

begin operation in 1997. Activity in each study unit is 

designed to be continual, with alternating periods of 

intensive investigation and low-level monitoring. Each 

study unit will address physical, chemical, and biological characteristics of surface-water and ground-water 

quality. 

The Potomac River Basin study unit of the 

NAWQA Program is one of the 20 study units that 

began in fiscal year 1991 (Gerhart, 1991). It was 

selected to be studied in the first group of 20 study 

units because of its national prominence, its large population (mostly in the Washington, D.C., area), and its 

significance to the health of the Chesapeake Bay. The 

first intensive phase of the Potomac River Basin study 

unit is scheduled for completion in fiscal year 1997, at 

which time the study unit will enter its first low-level 

monitoring phase. In fiscal year 2002, the second 

intensive phase of the Potomac River Basin study unit 

is scheduled to begin. 

Acknowledgments 

The authors would like to thank the following 

people for their assistance with various aspects of this 

report. Charles Kanetsky of the U.S. Environmental 

Protection Agency, Region III, provided point-source 

nutrient-input data. Carlton Haywood of the Interstate 

Commission on the Potomac River Basin provided use 

of their library and reference materials. Judith Wheeler 

of the U.S. Geological Survey provided water-use data 

for the State of Maryland and additional information 

on the use of the data. Jane McColloch of the Geological and Economic Survey, West Virginia Department 

of Commerce, Labor, and Environmental Resources, 

provided water-use data for the State of West Virginia. 

Dr. Emery T. Cleaves, Director, Maryland Geological 

Survey, provided technical insight and guidance in 

defining subunits of the Potomac River Basin. The late 

Albert Froelich of the U.S. Geological Survey, Geologic Division, and former Chief of the companion 

Potomac Regolith Study, contributed valuable time 

and assistance in defining the subunits in the basin. 

Introduction 3 
Thomas Mesko of the U.S. Geological Survey, 

Virginia District, provided geologic coverages containing Potomac River Basin information from the 

Appalachian Valleys and Piedmont Regional AquiferSystem Analysis Program. Matthew Ferrari of the 

U.S. Geological Survey, Maryland-Delaware-D.C. 

District, compiled and reviewed the ground-water 

description sections of this report. Scott A tor of the 

U.S. Geological Survey, Maryland-Delaware-D.C. 

District, assisted in preparation of geologic illustrations using a geographic information system (GIS), 

and Stephen Maskol of the U.S. Geological Survey, 

Maryland-Delaware-D.C. District, contributed a significant amount of effort in the compilation of figures, 

plots, charts, and tables. 

DESCRIPTION OF THE POTOMAC RIVER 

BASIN AND ITS SUBDIVISION FOR 

WATER-QUALITY-ASSESSMENT 

PURPOSES 

The quality of water in the Potomac River Basin is 

affected by both natural and human factors. Natural 

factors that affect basin water quality include climate, 

physiography, geology, and hydrology. The natural 

water quality produced by these factors is then modified by human factors such as land use, population, 

water use, and wastewater discharges. Because of its 

large drainage area and diversity, the Potomac River 

Basin has a very complex environmental setting consisting of various combinations of these natural and 

human factors. Knowledge of the primary characteristics that comprise the environmental setting is necessary to understand basin water quality. The Potomac 

River Basin has been divided into eight subunits for 

the purpose of water-quality assessment. The following sections describe this subdivision and the important natural and human factors that affect water quality 

in the Potomac River Basin. 

Location and Major Features 

The Potomac River begins as a small spring in 

West Virginia and flows 383 mi to Point Lookout, 

Md., where it discharges into the Chesapeake Bay 

(fig. 1). Flow of the river increases downstream so that 

when it reaches the Chesapeake Bay, it constitutes 

about 15 percent of the estimated 49,300 Mgal/d total 

inflow to the Bay (J.F. Hornlein, U.S. Geological 

Survey, oral commun., 1991). In all, the Potomac 

River and its many tributaries drain 14,670 mP in four 

States-Virginia (5,723 mi2), Maryland (3,818 mi2), 

West Virginia (3,490 mi2), and Pennsylvania 

(1,570 mi2)-and Washington, D.C. (69 mi2). 

Major tributaries to the Potomac River include the 

North Branch Potomac River, South Branch Potomac 

River, Cacapon River, Shenandoah River, Conococheague Creek, Monocacy River, and Occoquan River 

(fig. 1). The North Branch Potomac River drains the 

rugged northwestern part of the Potomac River Basin 

in Maryland, West Virginia, and Pennsylvania. The 

South Branch Potomac River and Cacapon River drain 

the mountainous West Virginia part of the basin. The 

Shenandoah River, the largest of the Potomac River's 

tributaries, drains the broad, relatively flat Shenandoah 

Valley in Virginia. Conococheague Creek and the 

Monocacy River drain the northern and northeastern 

parts of the basin in Maryland and Pennsylvania. The 

largest tributary in the eastern part of the basin is the 

Occoquan River in Virginia, which enters directly into 

the freshwater tidal Potomac River south of 

Washington, D.C. 

The Potomac River is free flowing and contains 

freshwater upstream from Washington, D.C. At the 

northwestern boundary of Washington, D.C., as it 

flows onto the Atlantic Coastal Plain where it becomes 

tidal. The river contains freshwater and is tidal from 

Washington, D.C., to near Indian Head, Md., where 

the water becomes brackish (fig. 1). From Indian Head 

to Point Lookout, Md., the river water becomes progressively more salty as it approaches the Chesapeake 

Bay. 

The major population center in the Potomac River 

Basin is Washington, D.C., and its suburbs, near the 

eastern boundary of the basin (fig. 1). Other major 

population centers include Cumberland, Hagerstown, 

Frederick, Rockville, and Waldorf in Maryland; 

Staunton, Waynesboro, Harrisonburg, Front Royal, 

Winchester, Leesburg, and Manassas in Virginia; 

Petersburg, Moorefield, and Martinsburg in West Virginia; and Chambersburg, Waynesboro, and Gettysburg in Pennsylvania. 

Climate 

The average annual temperature in the Potomac 

River Basin ranges from about 47 of in the mountainous western part of the basin to just less than 58 op in 

Washington, D.C. (U.S. Department of Commerce, 

4 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
Base from U.S. Geological Survey 

digital data, 1:2,000,000, 1972. 

Albers Equal-Area Conic projection 

Standard parallels 29°30'N and 45°30'N 

cental meridian 78°00' W. 

0 

0 

Index map 

10 20 30 40 50 MILES 

10 20 30 40 50 KILOMETERS 

Figure 1. Location and major features of the Potomac River Basin. 

1991 a,b,c,d). Although temperatures are typically 

lower in the western part of the basin, throughout most 

of the rest of the basin average annual temperature 

ranges from about 51 to 55 of, with no apparent areal 

pattern. Temperature varies considerably throughout 

the year. July tends to be the hottest month and January the coldest. The difference between the average 

monthly temperatures in July and January is about 

45 of, regardless of location in the basin. 

The average annual precipitation in the basin 

ranges from about 32 in. in the South Branch Potomac 

River drainage to about 48 in. near the source of the 

Potomac River in the North Branch Potomac River 

drainage as shown in figure 2 (U.S. Department of 

Commerce, 1991a,b,c,d). Precipitation contours were 

drawn using data extending outside the Potomac basin 

boundary, with greater data density in areas with 

greater variability in precipitation amounts. In general, 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 5 
EXPLANATION 

Precipitation gage-Number is average annual 

precipitation, in inches per year 

43.29. Precipitation gage-Number is average annual 

precipitation, in inches per year, for gages 

used in figure 3 

Data from U.S. Department of 

Commerce (1991a,b,c,d) 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 2. Average annual precipitation in the Potomac River Basin, 1951-80. 

precipitation in the area of the South Branch Potomac 

and Shenandoah Rivers averages less than 40 in/yr, 

and precipitation in the rest of the Potomac River 

Basin averages more than 40 in/yr. The high western 

mountains of Maryland and West Virginia have the 

greatest variability in average annual precipitation, 

with amounts ranging from about 37 to 47 in. The 

rapid decrease in average annual precipitation from 

the high western mountains to the South Branch Potomac and Shenandoah River areas probably is caused 

by orographic effects of the western Appalachian 

Mountains. The eastern part of the basin is affected by 

coastal weather patterns and has average annual precipitation ranging from about 39 to 47 in. 

Precipitation amounts vary seasonally as well as 

areally. Mean monthly precipitation is shown in 

figure 3 for two locations in the Potomac River 

Basin-one in the mountainous Cacapon River drainage of West Virginia, and one in the Washington, D.C., 

area, in northern Virginia (U.S. Department of 

6 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197G-90 
4.0 

(/) 

QJ 

.t:: 

(.) 

c 

c 

c' 3.0 .g 

2 

·c. 

·c::; 

e 

c. 

~ 

£ 

c 2.0 

0 

E 

c 

co 

QJ 

~ 

1.0 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

EXPLANATION 

Precipitation gage (fig.2) 

• Wardensville R M Farm (West Virginia) 

E:l Vienna Dunn Loring (Virginia) 

Figure 3. Mean monthly precipitation at two locations in the Potomac River Basin, 1951--80. 

Commerce, 199lc,d). Although the total average 

annual precipitation at the two locations differs by 

about 9 in., the seasonal patterns are the same at both 

sites. In both locations, as well as in the rest of the 

Potomac River Basin, most precipitation occurs in the 

summer (June-August) and the least in the winter 

(December-February). 

Physiography 

The Potomac River Basin contains parts of seven 

physiographic provinces or subprovinces that extend 

from southwest to northeast along the Atlantic Coast 

of the United States (Fenneman and Johnson, 1946). 

The physiographic provinces include the Appalachian 

Plateau, Valley and Ridge, Blue Ridge, Piedmont, and 

Coastal Plain (fig. 4). The Valley and Ridge is the largest province in the basin and includes the Great Valley 

subprovince. The Piedmont is the second largest 

province and contains the Triassic Lowlands subprovince. The Valley and Ridge and Piedmont Provinces 

are treated as separate, distinct entities from their subprovinces because the subprovinces are topographically and geologically distinct. The varied topography 

of these seven provinces and subprovinces forms a 

complex landscape within the Potomac River Basin 

that includes steep mountains, rolling hills, broad valleys, and plains. 

The Appalachian Plateau is the westernmost province and comprises about 4 percent of the basin in 

Maryland, Pennsylvania, and West Virginia. It is characterized by narrow valleys and steep, rugged ridges 

creating local topographic relief of 500 to 2,000 ft. 

The Appalachian Plateau contains the highest point in 

the basin, Spruce Knob, which rises to an altitude of 

4,860 ft. The North Branch Potomac River drains the 

Appalachian Plateau, which is the only basin province 

where coal is found. The Appalachian Plateau is 

separated from the Valley and Ridge Province to the 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 7 
t 

i.t.."' 

E-

-&VALLEY AND 

~ RIDGE 40 -~· ~~·';. 

" 

EXPLANATION 

Physiographic provinces 

E:;S;3 Appalachian Plateau 

B Valley and Ridge 

c:::::J Great Valley subprovince 

IIJ Blue Ridge 

[Ill Piedmont 

Physiography modified from 

Fenneman and Johnson (1946) 

[2Lj Triassic Lowlands 

subprovince 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS D Coastal Plain 

Figure 4. Physiographic provinces and subprovinces in the Potomac River Basin. 

east by the Allegheny Front, a major escarpment with 

as much as 3,000 ft of local relief that trends northeast 

through the basin. 

The Valley and Ridge Province is the most extensive province in the basin and occurs in Virginia, West 

Virginia, Maryland, and Pennsylvania (fig. 4). This 

province comprises 55 percent of the basin and 34 percent of the basin excluding the Great Valley subprovince. The rocks in this province have been intensely 

folded and faulted, producing long, narrow, northeasttrending structures. Subsequent erosion and 

weathering have resulted in the distinctive topographic 

grain of this province, with ridges capped by resistant 

sandstone and valleys underlain by less-resistant shale 

and carbonate rocks. Topographic relief in the Valley 

and Ridge Province is considerable, ranging to as 

much as 1,800 ft. The trend of the ridges substantially 

affects surface drainage in the basin, so that the principal tributary streams, the South Branch Potomac and 

Cacapon Rivers, flow northeast to the Potomac River. 

The Great Valley is an important subprovince of 

the Valley and Ridge Province. It occupies the eastern 

8 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970--90 
part of the Valley and Ridge Province and is a broad 

valley, 15 to 20 mi wide, with minor relief over extensive areas (fig. 4). It covers 21 percent of the basin, 

and agricultural activities are the predominant land 

use. Shale and siltstone underlie the central part of the 

Great Valley and are bordered by areas underlain by 

carbonate rocks. The carbonate rocks that make this 

subprovince favorable for farming also are susceptible 

to dissolution, resulting in numerous caves and karstic 

features throughout most of the area. The Shenandoah 

River and Conococheague Creek are the major 

tributaries to the Potomac River that drain this subprovince. 

Bordering the Great Valley on the east is the Blue 

Ridge Province (fig. 4). It covers 6 percent of the basin 

and consists of a mass of crystalline rocks that rises 

about 1,500 to 2,000 ft above the lowlands on either 

side. In Virginia, the Blue Ridge forms the southeastem boundary of the Potomac River Basin. In Maryland and Pennsylvania, it forms a major drainage 

divide within the basin, with all streamflow from west 

of the Blue Ridge flowing through the gap at Harpers 

Ferry, West Virginia. Because of relief and narrowness, most of the streams that drain the Blue Ridge are 

headwater tributaries of larger streams in the Great 

Valley or Piedmont. 

The Piedmont Province lies to the east of the Blue 

Ridge and comprises about 12 percent of the basin in 

Virginia, Maryland, Pennsylvania, and Washington, 

D.C. (fig. 4). It is an area of gently rolling terrain with 

low to moderate relief. The eastern and western parts 

of the Piedmont are underlain by resistant crystalline 

rocks, whereas the central part, the Triassic Low lands 

subprovince, 7 percent of the basin is underlain by 

less-resistant sedimentary rocks of primarily Triassic 

age. The Triassic Lowlands subprovince is generally 

flatter than the surrounding Piedmont. The principal 

tributary to the Potomac River in the Piedmont Province is the Monocacy River, which also drains agriculturallands in the Triassic Lowlands subprovince. 

The Fall Line separates the Piedmont from the 

Coastal Plain Province to the east. At the Fall Line, the 

rolling hills of the Piedmont drop in elevation to meet 

the gently sloping Coastal Plain. Stream gradients 

abruptly steepen through the Fall Line as they enter 

the Coastal Plain. The Potomac River drops nearly 

150ft to near sea level as it flows through Great Falls 

and Little Falls near Washington, D.C. The Fall Line 

also marks the terminus of the upper Potomac River, 

which, at this point, drains about 11,670 mi2 of the six 

provinces and subprovinces upstream. Smaller tributaries enter the tidal Potomac River after flowing from 

the Triassic Lowlands and Piedmont through the Fall 

Line onto the Coastal Plain. 

The Coastal Plain Province is fundamentally different from the other physiographic provinces in that it 

is underlain by unconsolidated sediments that form a 

gentle seaward-sloping plain of low relief. Much of 

the population of the Potomac River Basin resides in 

this province, which covers about 15 percent of the 

basin in Virginia, Maryland, and Washington, D.C. 

(fig. 4). It is in this province that the Potomac River is 

tidally effected, eventually becoming a broad estuary 

before entering the Chesapeake Bay. The estuary is 

typically flanked by broad lowlands that mark the sinuous and deep course of an ancestral Potomac River 

Valley now filled by unconsolidated sediments. 

Geology 

The geology of the Potomac River Basin is complex and diverse, ranging from relatively undisturbed, 

unconsolidated sediments to intensely deformed crystalline rocks (Milici, 1963; Cardwell, 1968; Cleaves, 

1968; Berg, 1980) (fig. 5). The most intensely 

deformed and the oldest rocks in the basin are crystalline rocks in the Piedmont and Blue Ridge Provinces. 

These rocks are primarily metamorphic and igneous 

rocks of Precambrian to Ordovician age that are 

deformed as a result of at least four episodes of 

increased tectonic activity. Predominant rock types are 

massive granite and layered gneiss, foliated phyllite 

and schist, quartzite, marble, and metadolomite. The 

folding and faulting associated with the tectonic 

stresses have produced structures too complex to show 

in figure 5. Instead, crystalline rocks are undifferentiated on the map. 

Sedimentary rocks of Cambrian through Pennsylvanian age underlie the Valley and Ridge and Appalachian Plateau Provinces (fig. 5). The rocks of the 

Valley and Ridge have been deformed by folding and 

thrust faulting into a series of plunging folds that create the topographic grain of the province. Although 

folded and faulted, these rocks were farther away from 

the center of tectonic activity than the crystalline rocks 

of the Piedmont and Blue Ridge. Therefore, the rocks 

in the Valley and Ridge were not recrystallized during 

orogenic events. In the Valley and Ridge Province, the 

rocks can be broadly categorized into siliciclastic and 

carbonate types. Siliciclastic rocks are prevalent in the 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 9 
.)_ 

> 

·\. 

z. 

I COASTAL 

(PLAIN 

EXPLANATION ··""'· 

Rock types ·"-. 

D Unconsolidated sediment, sand, silt, and clay 

• Carbonate and siliciclastic 

sedimentary rocks, undivided 

D Siliciclastic sedimentary rock 

Carbonate sedimentary rock 

Ill Crystalline rock, undifferentiated 

Rock types modified from digital geology data, 

1:500,000, provided by Thomas 0. Mesko, 

U.S. Geological Survey, Richmond, Va. 

Figure 5. Generalized rock types in the Potomac River Basin. 

Silurian, Devonian, Mississippian, and Pennsylvanian 

Systems, and carbonate rocks are prevalent in the 

Cambrian and Ordovician Systems. The largest extent 

of carbonate rocks is in the Great Valley subprovince 

where karstic features associated with dissolution of 

carbonate rocks are common. Rocks in the Appalachian Plateau Province are similar lithologically to the 

siliciclastic types in the Valley and Ridge with the 

exception that coal-bearing rocks are found in the 

Appalachian Plateau. Although major thrust faults 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

occur at depth, rocks in the Appalachian Plateau Province are relatively flat-lying and less deformed than 

rocks in the Valley and Ridge. 

The youngest consolidated rocks in the basin are 

found to the east of the Blue Ridge Province, in the 

part of the Piedmont Province underlain by sedimentary rocks (fig. 5). These rocks were formed during the 

Late Triassic and Early Jurassic by streams discharging sediment into down-faulted rift basins. In the 

Potomac River Basin, the faulted margin of these 

10 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197o-90 
basins borders the Blue Ridge, and other faults are 

present throughout the sequence. The sedimentary 

sequence is interrupted locally by igneous intrusive 

and extrusive rocks. 

The youngest geologic units in the basin are in the 

Coastal Plain Province (fig. 5). These units consist of 

unconsolidated sediment of Cretaceous through 

Holocene age that form a southeastward-thickening 

wedge of interbedded sand, silt, and clay. 

Regolith, consisting of soil, terrace deposits, colluvium, residuum, tufa, travertine, alluvium, saprolite, 

and chemically and physically weathered rock, overlies most of the Potomac River Basin. The thickness of 

the regolith is variable, ranging from 0 to more than 

150ft over relatively short distances. In addition to 

areal variability, the character of the regolith may differ considerably over short vertical intervals. 

Although the geology of the Potomac River Basin 

is very complex, the geologic units may be broadly 

categorized into four groups-unconsolidated sediment, carbonate sedimentary rocks, siliciclastic sedimentary rocks, and crystalline rocks (fig. 5). 

Unconsolidated sediments underlie about 15 percent 

of the basin, carbonate sedimentary rocks underlie 

about 17 percent, siliciclastic sedimentary rocks 

underlie about 42 percent, and crystalline rocks underlie about 19 percent of the basin. The remaining 7 percent consists of geologic units that contain significant 

proportions of both carbonate and siliciclastic 

sedimentary rocks. 

Land Use and Population 

Land-use patterns were fairly stable throughout 

the basin during 1970-85, with most of the development occurring in the Washington, D.C., area and it 

suburbs. In the mid-1970's, 51 percent of the land in 

the Potomac River Basin was in forest, 36 percent was 

used for agricultural activities, and 8 percent was 

urban (U.S. Geological Survey, 1979a,b,c,d; 

1980a,b,c,d) (fig. 6). By 1985, 52 percent of the land 

in the basin was forested, 32 percent was used for agricultural activities, and 12 percent was urban (Camacho, 1989). Land-use data for the 1970's are from a 

digital geographic data base available for the entire 

United States. The 1985 land-use percentages were 

calculated using nondigital, county-based, land-use 

data, so the differences in land use from the mid1970's to 1985 could reflect the different analysis techniques as well as changing land-use patterns. The data 

suggest a shift from agricultural land use to urban land 

use from the mid-1970's to 1985, with 4 percent of the 

basin that was agricultural land in the mid-1970's 

being converted to urban land use by 1985. 

Mid-1970's land use differed considerably among 

physiographic provinces (table 1). Agriculture comprised 50 percent or more of the land use in the Great 

Valley, Piedmont, and Triassic Lowlands. Forests covered more than 75 percent of the land in the Appalachian Plateau, Valley and Ridge, and Blue Ridge. No 

individual land use comprised more than 40 percent of 

the land in the Coastal Plain, where the single largest 

land use was forest (38 percent), followed by water 

(23 percent), urban ( 19 percent), and agriculture 

(16 percent). Provinces having the most urban land 

use (19 percent) were the Piedmont and Coastal Plain. 

Less than 5 percent of land use was urban in the Appalachian Plateau, Valley and Ridge, and Blue Ridge 

Provinces. 

The population of the Potomac River Basin 

increased from about 3.2 million people in 1970 to 

about 4.6 million people in 1990 (Carlton Haywood, 

Interstate Commission on the Potomac River Basin, 

written commun., 1995). Most of the 4.6 million people who lived in the Potomac River Basin in 1990 

resided in the urban land-use settings (U.S. Department of Commerce, 1991e) (fig. 6). An estimated twothirds of the basin's 1990 population lived in the 

Washington, D.C., metropolitan area. Population density exceeded 10,000 people per square mile in nearly 

all of Washington, D.C., and in nearby parts of Maryland and northern Virginia (K.J. Hitt, U.S. Geological 

Survey, written commun., 1992). Other population 

centers where 1990 population density exceeded 

10,000 people per square mile included Cumberland, 

Hagerstown, Frederick, Rockville, and Waldorf, Md.; 

Harrisonburg, Leesburg, and Manassas, Va.; and 

Chambersburg, Pa. Because of the location of Washington, D.C., on their common boundary, the Piedmont and Coastal Plain were the most populous 

physiographic provinces in the Potomac River Basin 

in 1990, each having about 36 percent of the total 

basin population. The Great Valley and Triassic Lowlands subprovinces were next, with about 14 and 

8 percent of the basin population, respectively. The 

Appalachian Plateau, Valley and Ridge, and Blue 

Ridge Provinces shared the remaining 6 percent of the 

1990 population. 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 11 
EXPLANATION ·--··"'(_. 

·'"'1-·· Land use 

.Urban 

D Agricultural 

0 Forest 

. Water Land use from U.S. Geological Survey 

(1979a,b,c,d; 1980a,b,c,d) 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 6. Generalized land use in the Potomac River Basin, mid-1970's. 

Water Use and Wastewater Discharges 

In 1990, about 6,374 MgaVd of freshwater (about 

85 percent of the average flow of the Potomac River at 

Washington, D.C.) was used for human-related activities in the Potomac River Basin. Most of the water 

used is returned to streams within the basin. The largest use of water, 86 percent of total use, was for power 

generation. Public water supply was the next largest 

use in the basin, accounting for about 10 percent of the 

total, followed by industrial, domestic, mining, agricultural, and commercial uses. (All data in this section 

are from H.A. Perlman (U.S. Geological Survey, written comrnun., 1993) and are for counties contained 

wholly or partially in the basin. Totals for water use in 

the basin may be slightly high because they include 

water use for those parts of counties that are outside 

the basin.) 

If power-generation uses are ignored, the distribution of freshwater withdrawals in the Potomac River 

12 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197o-90 
Table 1. Potomac River Basin land use by physiographic province and subprovince, mid-1970's 

[Data from U.S. Geological Survey, 1979a,b,c,d; 1980a,b,c,d] 

Percentage of physiographic province or subprovince 

Area in each land use 

Province or (square 

subprovince miles) Forest Agricultural Urban Water Other 

Appalachian Plateau 660 82 

Valley and Ridge 5,062 80 

Great Valley 3,070 22 

Blue Ridge 919 78 

Piedmont 1,851 29 

Triassic Lowlands 1,018 23 

Coastal Plain 2,090 38 

Potomac River Basin 14,670 51 

Basin in 1990 (fig. 7) closely follows the distribution 

of population. The counties shown in figure 7 having 

the largest total freshwater withdrawals are Montgomery County in Maryland and Prince William County in 

Virginia, where the water supply for the Washington, 

D.C., metropolitan area is derived. Potomac River 

water was withdrawn in Montgomery County, Maryland, just upstream from Washington, D.C., at an average rate of about 387 MgaVd in 1990, and the 

Occoquan River in Prince William County, Virginia, 

supplied about 58 MgaVd to the Washington, D.C., 

metropolitan area in 1990. No major freshwater withdrawals were located in Washington, D.C., or in very 

populated Fairfax County, Virginia, as they were 

served by these Potomac River and Occoquan River 

withdrawals in neighboring counties. 

Seven additional counties had total freshwater 

withdrawals exceeding 20 Mgalld or more in 1990. 

These counties are located in parts of the Piedmont 

Province, and Triassic Lowlands and Great Valley 

subprovinces, as well as the northern part of the Valley 

and Ridge Province, where urban areas are served by 

public water supplies (fig. 7). Most of the counties 

with less than 5 MgaVd of total freshwater use in 1990 

are located in rural parts of the Valley and Ridge in 

West Virginia, the central part of the Great Valley, and 

the Coastal Plain Province in Virginia. 

Although freshwater use for thermoelectric and 

hydroelectric power generation are not included in the 

county data shown in figure 7, they constitute an 

important water use at 14 locations in the Potomac 

River Basin (fig. 7). A total of about 2,130 MgaVd of 

freshwater were withdrawn in 1990 for thermoelectric 

power generation at six locations throughout the basin. 

Only about 31 Mgal/d, or 1.5 percent, of this water 

13 1 0 4 

18 0 

68 8 I 

17 3 0 2 

50 19 

66 9 1 

16 19 23 4 

36 8 4 1 

were consumed. The rest was returned to surface 

water in the basin. Similarly, a total of about 

3,377 MgaVd of freshwater was used in 1990 for 

hydroelectric power generation at eight locations 

throughout the basin. All of this water was returned to 

streams in the basin. 

About 6,170 of the 6,374 Mgal/d (97 percent) of 

total freshwater used in the Potomac River Basin in 

1990 was from surface-water sources (fig. 8). Excluding the 5,508 Mgal/d of power-generation use, about 

662 Mgal/d (76 percent) of freshwater withdrawals 

were from surface-water sources. Besides power-generation uses, which were essentially all from surfacewater sources, public-supply withdrawals of freshwater were the next largest type of use in 1990, about 

618 Mgal/d. About 93 percent of the public-supply 

withdrawals, or 577 Mgalld, was from surface-water 

sources. Surface-water sources also supplied about 

65 percent of the 86.3 Mgal/d used for industrial purposes and about 54 percent of the 34.5 MgaVd used for 

livestock watering and irrigation in 1990. 

Total ground-water withdrawals in the basin in 

1990 were about 204 Mgalld (fig. 8). Ground water 

supplied essentially all of the 58.3 Mgal/d withdrawn 

for domestic purposes, about 72 percent of the 

17.4 Mgal/d used for commercial purposes, and about 

87 percent of the 51.5 MgaVd withdrawn for mining 

purposes. 

Excluding freshwater withdrawals for power generation, about 64 percent, or 551 MgaVd, of the freshwater withdrawn in 1990 was returned to surface 

water in the basin by wastewater-treatment facilities. 

The distribution and relative magnitude of the major 

discharges from wastewater-treatment facilities are 

shown in figure 9. The largest municipal discharge in 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 13 
79° 

,r , ~___ --/'' , -J, '----, \ A . 

I I "" 78° ·/.{ 

/' / /""-'~ /'' 

40oJ I Bedford /r} .. .A..:l l/'·.~.',A--- -./\. ~ Somerset •• ........._ {( li ''? .: \ 

~L (:~'"-'"j ?f1v:t,_,:J ~ulton Franklin \ V~~~\. ~ 

r------.+~~4-- -------f---f--ii.~- 1 Garrett ./I .~ &11....-ny .,.;. ... ~ Washington r' '('_' Carroll I I • I ''• ~ ,..!;;, t"" ~ ,.., 

I ( j /~ )-...__t Morgan / '"\ J \ . I 

: f' ~ ',/ / 1 J-, / Berkeley ) f, } Frederick 

I . .r I I '<.. I • I 

/ .;:___ Mineral _/ J , ( ~ 

:./ ~_.1 - '--. ,/' Hampshire , 'v'Jeffersoif ._ 

dc::...'?'Y /'' "-, < Frederick /' J'-..' 'j,' 

G ""--.r 1 

(, rant j (__ /'"" ' Clark 1 , f ( , Loudoun 

39°-i \ Hardy 1 ~ _ __} , ~ _ 

rA , · /" \ • I --........ r ' J / • I Warren •• Fauquier 

I "" , f ' 1 Shenandoah / j\ "\. """\ y ( ,A , ,

1 ·· 

Pendleton #J , ~ f ~) ~ 

(-\ /// [ •"-) \L. Yv- \__£/~, Rockingham ~ r \, ~~tafford ' Ch:rles I ·'-': 

I , ··,n "' I ......_ , _J , ) • " 

( Highta d •• ' ......_ y · EXPLANATION ',--...'-··-C;.~~ ( \ , "\_...-_\ '--. f ' \.._ I ' ( ' I '• 

- / _,.-ti? '--. • j St Marys \ 1 

- •· A gusta Total freshwater withdrawals in Ly f... '"'-!...,_ · \ 

) u in 1990, by county, in million King/ " vi .....,.. ·-""4- - - ') : 

gallons per day, excluding George -1---/', \ / ~ ·':.A thermoelectric and hydroelectric .A ' f 

38o- ~~ power withdrawals Westmoreland __! ~ . "\. rl'- ""-.. ...._ D Less than 5 \..!" 

~ ~~:o orth~'"'""' _'X:::-- \ 

'....___j • Greater than 50 

Location of freshwater withdrawals 

for power generation in 1990 

Freshwater withdrawa ls from 

HA Perlm an, (U.S. Geological 

Survey, written commun., 1993) 

50 MILES 

~~,-~--~--~--

0 10 20 3 0 40 'Y Thermoelectric 0 10 20 30 40 50 KILOMETER S 

e Hydroelectric 

Figure 7. Generalized distribution of total freshwater withdrawals in the Potomac River Basin, 1990. 

1990, about 317 Mgal/d, was into the tidal Potomac 

River from the Blue Plains Wastewater Treatment 

Plant downstream from Washington, D.C. Seven other 

municipal discharges of more than lO Mgalld were 

located in the Washington, D.C., metropolitan area. 

There were nine industrial discharges of more than 

10 Mgal/d in 1990, with most of them located in the 

North Branch Potomac River area or the Great Valley 

subprovince. Two mining discharges of more than 

lO Mgal/d were located in the Piedmont Province in 

1990. 

14 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970--90 
3,500 

1,000 

I I Surface water 

2,500 

- Ground water 

> 

co 

"0 2,000 ,_ 

Q) 

a. 

C/l 

c 

0 1,500 

co 

Cl 

c 

0 1,000 

E 

c 

o' 500 

O'l --z._ O'l 

..... 100 c 

~ 

co 90 ~ co 

,_ 

"0 80 ..c 

+-' '§ 70 ,_ 

Q) 

+-' 

co 60 ~ ..c 

C/l 

Q) 50 ,_ 

LL 

40 

30 

20 

10 

0 

> co (.) co (.) (.) Cl "OC 

a. +-' ·;: ·;: ·;: c co (.) C/l +-' +-' co·.;:; a. ,_ Q) +-' (.) (.) c ::J Q) E C/l Q) ~ ~co 

C/l E ::J ~ (.)Cl 

0 "0 Q) Q) o·;: (.) E c 0 0 +-' ,_ 0 ,_ E Cl)·-

.n 0 "0 Q) 

::J u > ,_ 

Q) > 

0.. I ..c :.:::i 

1-

Figure 8. Surface-water and ground-water withdrawals of freshwater in the Potomac River Basin, 1990 (data from H.A. 

Perlman, U.S. Geological Survey, written commun., 1993). 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 15 
EXPLANATION 

Major discharges of wastewater in 

1990-Discharges greater than 

1 million gallons per day (Mgal/d} 

Municipal 

0 Less than 10 Mgal/d 

e Equal to or greater than 10 Mgal/d 

Industrial 

!:::,. Less than 10 Mgal/d 

A Equal to or greater than 10 Mgal/d 

Mining 

'\1 Less than 10 Mgal/d 

'Y Equal to or greater than 10 Mgal/d 

0 10 

Data from U.S. Environmental 

Protection Agency STORET data file 

20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 9. Major wastewater discharges to streams in the Potomac River Basin, 1990. 

Hydrology 

Surface Water 

The Potomac River Basin (fig. 1) includes six 

major tributaries-North Branch Potomac River, 

South Branch Potomac River, Cacapon River, Conococheague Creek, Shenandoah River, and Monocacy 

River. These six tributaries drain to the upper Potomac 

River upstream from the Fall Line. Selected streamflow characteristics for the upper Potomac River at 

Washington, D.C., and these six tributaries are shown 

in table 2. At the Fall Line, the Potomac River at 

Washington, D.C., maintains a mean annual streamflow of 11,771 fP/s. Additional tributaries drain 

directly to the tidal Potomac River and include the 

Anacostia and Occoquan Rivers. These smaller tributaries are important for water-quality and water-supply 

issues. 

Streams upstream from the Fall Line generally 

have steeper gradients and flow more swiftly than 

streams downstream from the Fall Line. The mainstem Potomac River and major tributaries generally 

16 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1971}-90 
R 

::::!. 

'9. 

0 

:I 

a 

:T 

CD 

"tJ 

a 

~ m 

n 

:a 

i 

.. 

m 

m 

fl) 

5" 

m 

:I 

a. 

:;:; 

.~ 

en 

c 

C" 

a. 

< i" 

0 

:I 

0 

.. 

~ CD 

b 

i 

1 

I 

3 

CD 

a 

"tJ 

c 

-a 

0 

I 

..... 

..... 

Table 2. Streamflow characteristics of the Potomac River near Washington, D.C., and of six major tributaries 

[mi2, square miles; ft3/s, cubic feet per second; in/yr, inches per year; M7,

20, 7-day minimum low-flow discharge, 20-year recurrence interval] 

u.s. 

Geological 

Survey Mean Mean 

gaging Period of annual annual 

station no. Drainage record streamflow runoff 

(fig. 10} Station name area (mi2} (water years} (ftl/s} (in/yr} 

1

01603ooo North Branch Potomac River near Cumberland, Md. 875 1929-90 1,276 19.82 

01608500 South Branch Potomac River at Springfield, W.Va. 1,471 1928-90 1,310 12.1 

01611500 Cacapon river near Great Cacapon, W. Va. 677 1922-90 586 11.76 

01614500 Conococheague Creek near Fairview, Md. 494 1928-90 586 16.13 

01636500 Shenandoah River at Millville, W. Va. 3,040 1928-90 2,697 12.05 

01643000 Monocacy River near Frederick, Md. 817 1929-90 932 15.5 

01646500 Potomac River near Washington, D.C. 11,560 1930-90 11,771 13.83 

1

Streamftow regulated since 1982. 

High-flow 

discharge 

Low-flow (ftl/s, 10-

discharge, year 

M7,2o recurrence 

(ft3/s} interval} 

112 37,100 

62.1 56,600 

34.7 36,000 

41 14,400 

330 82,000 

37 35,000 

862 237,000 
have bedrock bottoms, with alluvial sediments in depositional areas. Stream-bottom materials range from 

bedrock to small cobbles and gravel in upstream areas, 

to eroded fine sediments in agricultural areas, to 

gravel, sand, and silt in Coastal Plain streams. Several 

streams, notably Conococheague Creek and the South 

Fork Shenandoah River, undergo considerable floodplain meandering in their downstream reaches. Coastal 

Plain streams, downstream from the Fall Line, have 

shallow gradients and discharge to tidal creeks or wetlands, which have considerable effect on streamflow, 

stream morphology, and water quality. 

Trainer and Watkins (1975) found that average 

base runoff from tributaries in the upper Potomac 

River Basin was approximately proportional to drainage area. The six major tributaries upstream from the 

Fall Line represent about 64 percent of the drainage 

area and contribute about 63 percent of the mean 

annual streamflow. At lower flows, however, Trainer 

and Watkins (1975) found that the areas underlain by 

carbonate rocks, which are mostly in the Great Valley 

subprovince, contribute a proportionately larger share 

of flow to the Potomac River at Washington, D.C. For 

example, the Shenandoah River contributes about 

38 percent of the streamflow during low-flow conditions yet contains only 26 percent of the upper Potomac River Basin (Trainer and Watkins, 1975). 

Figure 10 shows the distribution of mean annual 

runoff in the Potomac River Basin. The greatest runoff 

occurs in the Appalachian Plateau and westernmost 

parts of the Valley and Ridge Province. The least runoff occurs in the Shenandoah River part of the Great 

Valley. A comparison of the maps of mean annual runoff and average annual precipitation (fig. 2) reveals 

that patterns of runoff generally follow those of precipitation. However, the ratio of runoff to precipitation 

is somewhat higher in the Appalachian Plateau and 

westernmost parts of the Valley and Ridge Province, 

where steep slopes, shallow soils, and less-permeable 

bedrock contribute to the greater total runoff. About 

50 percent of precipitation becomes surface runoff in 

these mountain areas. In the remainder of the Potomac 

River Basin, with generally flatter slopes, deeper soil 

profiles, and more fractured bedrock, karst terrain, or 

Coastal Plain deposits, about 35 percent of precipitation contributes to surface runoff. 

Most streams in the Potomac River Basin generally have good year-round flow and infrequently experience very low or no flow. However, the Potomac 

River Basin has experienced notable hydrologic 

extremes. Floods of record for the Potomac River near 

Washington, D.C., include a maximum discharge of 

484,000 ft3fs in March 1936, resulting from intense 

rainfall and snowmelt, and 359,000 ft3

/s in June 1972, 

resulting from Hurricane Agnes. Although the peak 

discharge from Agnes was large, but not exceptional, 

on the main-stem Potomac River, this flood caused 

peaks of record on many tributaries downstream from 

Conococheague Creek (Bailey and others, 1975). 

Rains from Tropical Storm Juan in November 1985 

produced catastrophic flooding in the South Branch 

Potomac River and parts of the Shenandoah River 

(Carpenter, 1990). The extreme low flow for the Potomac River at Washington, D.C., was only 601 ft3/s on 

September 10, 1966, only about 5 percent of the mean 

annual streamflow. 

Figure 11 shows mean annual and mean monthly 

streamflow for the Potomac River at Washington, 

D.C., for water years 1970-90. During this period, this 

gaging station had a mean annual streamflow of 

11,600 ft3/s, slightly less than the long-term mean. 

Maximum annual streamflows occurred in water years 

1972 and 1984, and the minimum annual streamflow 

occurred in 1981. The last 10 years (1981-90) were 

the driest of the 21-year period as 7 out of 10 years 

were less than the annual mean. Streamflows for the 

Potomac River at Washington, D.C., show a strong 

seasonal pattern as mean monthly streamflows vary 

from less than 5,000 ft3

/s in August to more than 

22,000 ft3/s in April. 

Streamflow regulation in the Potomac River Basin 

is minimal. Figure 12 shows the location of major surface-water impoundments in the Potomac River Basin. 

The four largest impoundments in the basin are Savage River Reservoir, William Jennings Randolph Lake 

on the North Branch Potomac River, Mt. Storm Lake 

on Stony River, and Occoquan Lake on Occoquan 

River. The Stony River dam provides cooling water 

for coal-generated electric power. William Jennings 

Randolph Lake provides flood control on the North 

Branch Potomac River in addition to other uses. William Jennings Randolph Lake and Savage River Reservoir combine to provide water-quality control in an 

area affected by acid-mine drainage, as well as lowflow augmentation on the North Branch Potomac 

River. Occoquan Lake on Occoquan River is a multiple-purpose reservoir that contains only Piedmont 

drainage but discharges to the Potomac River estuary 

downstream from Washington, D.C. There are anumber of low-flow dams on the main-stem Potomac River 

18 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
. 

. 

\.. ''""\.. EXPLANATION 

-20- Line of equal mean annual runoff 

1951-80-Interval is variable, in inches 

01611 500.6,. U.S. Geological Survey streamflowgaging station-Number corresponds to that 

used in table 2 

Modified from Gebert and others ( 1985) 

0 50 MILES ~~~-~~~~-L----

10 20 30 40 

0 10 20 30 40 50 KILOMETERS 

Figure 10. Mean annual surface-water runoff, 1951-80, and selected streamflow-gaging stations in the Potomac River Basin. 

and on the Shenandoah River, but these have negligible effect on general streamflow characteristics. 

Ground Water 

Ground water in the Potomac River Basin is 

present in a variety of hydrogeologic settings, in both 

primary and secondary openings in the rock matrix 

and in local, intermediate, and regional flow systems. 

Principal factors affecting ground water are climate, 

aquifer and regolith characteristics, land use/land 

cover, water use, and topographic relief. The 

hydrogeologic setting that differs the most from the 

other settings in the basin is the Coastal Plain. Ground 

water in this setting resides in and moves through 

interstices between individual mineral grains, whereas 

ground water in the rest of the basin resides in and 

moves through fractures and other secondary openings 

in the rock matrix. The water-transmitting and waterstoring properties of sediment in the Coastal Plain are 

determined by the environment in which the deposits 

were formed. Although these depositional environments varied greatly in space and time, the sediment 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 19 
~"'C oc:: 

'+=8 

EQ) 

cti(J) Q),_ 

~Q) 

en a. 

(ij~ ::JQ) 

c::-

20,000 

15,000 

c:: 0 10,000 cti:.C 

C::::J 

ctiO 

~.!: 

5,000 

0 

A. 

Mean annual 

discharge 

11,771 cubic feet 

per second 

1970 '71 '72 '73 '74 '75 '76 '77 '78 '79 '80 '81 '82 '83 '84 '85 '86 '87 '88 '89 1990 

Water year 

o-c ~ 

-C:: 

-o Eo 

ctiQ) 

Q)(J) 

.......... -Q) 

(I) a. 

~­_c::Q) -Q) 

c::-

00 

E:.C 

c::::J 

ctlo Q)C:: 

~·-

20,000 

15,000 

10,000 

5,000 

0 

B. 

Mean annual 

discharge 

11,771 cubic feet 

per second 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

Figure 11. (A) Mean annual and (B) mean monthly streamflow in the Potomac River at Washington, D.C., 

197Q-90. 

associated with them is conceptualized as a series of 

alternating aquifers and confining units. The uppermost sediment, which mantles the Coastal Plain, forms 

generally unconfined aquifers characterized by local 

ground-water flow systems and short flow paths. 

Regional confined aquifers subcrop beneath this surficial aquifer and are characterized by much longer flow 

paths. The Coastal Plain aquifers are capable of yielding large quantities of ground water, and many communities use ground water as their primary source of 

water. 

The remainder of the basin may be divided into 

four hydrogeologic settings-Piedmont and Blue 

Ridge crystalline rocks, Piedmont sedimentary rocks, 

Valley and Ridge sedimentary rocks, and Appalachian 

Plateau sedimentary rocks. In all four settings water is 

stored primarily in and moves through secondary 

openings in consolidated rocks. Types of secondary 

openings that contain water include faults, joints, bedding-plane partings, stress-relief fractures, cleavage, 

and schistosity. In carbonate rocks, these openings 

may be enlarged further by dissolution of the rock 

matrix. In rocks that contain carbonate cementing 

material, the carbonate cement may be weathered 

away, resulting in intergranular porosity. The Appalachian Valleys-Piedmont Regional Aquifer-System 

Analysis Program (Swain and Holly, 1991) identified 

local and intermediate ground-water flow systems for 

20 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
Savage River 

Reservoir 

Mt. Storm 

Lake 

/Spruce 1{. 

··'< .. JJ 

~. 

"""' ·. 

. EXPLANATION '""\_. 

Main use of surface-water 

impoundments 

• Water supply 

T Recreation 

..A. Hydroelectric power generation 

+ Flood control 

12.000 Upper number is normal 

resetVoir capacity, in acre- feet 

28,000 Lower number is maximum resetVoir capacity, in acre- feet 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 12. Location of major surface-water impoundments in the Potomac River Basin. 

the Piedmont and Blue Ridge crystalline rocks, Piedmont sedimentary rocks, and the Valley and Ridge 

sedimentary rocks and described conceptual flow 

systems for each. The following section summarizes 

those flow systems. 

In the Piedmont and Blue Ridge crystalline rock 

setting, well yields commonly range from 5 to 

35 gal/min. The upper limit of well yields is relatively 

low compared with the sedimentary rock settings 

because secondary openings in crystalline rocks are 

not as likely to be enlarged by dissolution. Regolith, 

which may have a porosity several orders of magnitude greater than the underlying competent rock, is a 

critical component of the ground-water flow system 

because much of the ground water in this setting is 

stored in it. This stored water maintains base flow in 

streams during dry periods and is available to fractures 

in the underlying rocks to maintain water levels and 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 21 
ground-water flow in crystalline aquifers (Trainer and 

Watkins, 1975). Because of the small storage capacities of crystalline aquifers, regolith acts to sustain 

yields when recharge from precipitation is not adequate to meet demand in areas where wells yield moderate to large quantities of water. In areas with little or 

no overlying regolith, well yields can only be sustained if extensive fracture networks are present. 

Crystalline aquifers often are anisotropic because fractures, bedding-plane partings, and foliation tend to be 

preferentially aligned, creating heterogeneity in the 

permeability distribution of the rocks. 

In the Piedmont sedimentary setting, ground water 

is stored and transmitted in both primary and secondary openings, with most water present in secondary 

openings. Well yields range from 5 to 1,500 gal/min, 

with the highest yields being from relatively deep 

wells that are located to obtain maximum quantities of 

water. Preferential ground-water flow along bedding 

strike is typical of these rocks and probably is caused 

by the fracturing characteristics of individual beds 

within the sedimentary sequence. Intrusive and extrusive igneous rocks within the sedimentary sequence 

yield little water and locally may impede groundwater flow. 

In the Valley and Ridge, fracturing and dissolution 

have produced substantial secondary permeability in 

the upper 300 ft of rocks. Wells in this setting yield 

from 5 to 500 gal/min, with wells that encounter solution cavities having the greatest yield. Ground-water 

flow in the region is restricted by the parallel ridges 

common in this setting. The relation of streams to 

topography causes many adjacent, but hydraulically 

isolated, shallow flow systems. Ground water flows 

from ridge to adjacent valley until it either discharges 

to local streams or is intercepted and directed downvalley by a layer of rocks with well-developed secondary permeability. Such interception is probably 

common in the Great Valley subprovince where there 

are extensive carbonate aquifers. 

The combination of geology and topography produces both local and regional ground-water flow systems in the Appalachian Plateau (Carswell and 

Bennett, 1963). Rock units in this setting are relatively 

flat-lying and consist of alternating beds of sandstone, 

shale, and coal. Topographically, the area is a relatively uniform upland that has been dissected by a network of stream valleys that act as local discharge areas 

for ground water. As ground water is recharged in the 

uplands and moves through the system, it tends to 

move vertically in fine-grained units and laterally 

towards valleys in coarse-grained units. As water 

moves progressively deeper in the system, lateral 

movement in the coarse-grained units may cause water 

to bypass local streams to discharge to larger, higher 

order streams. Past and present mining operations alter 

the ground-water flow system by dewatering sections 

of aquifers in the vicinity of mines and changing 

ground-water flow directions in parts of the aquifer not 

dewatered (Hobba and others, 1972; Duigon and Smigaj, 1985). Well yields in the Appalachian Plateau 

range from 2 to 250 gal/min. 

Ground-Water/Surface-Water Interactions 

Precipitation in the Potomac River Basin eventually becomes streamflow by overland runoff, which is 

controlled by surface and near-surface processes, and 

by base flow, which is controlled by subsurface processes. Overland runoff is delivered to streams over 

short time periods and may constitute the majority of 

streamflow during and shortly after storms. Base flow 

is delivered to streams by ground-water discharge 

through the streambed and maintains streamflow 

between storms. The time it takes for precipitation to 

flow through the ground-water system and become 

base flow may range from days to years, depending on 

conditions in the aquifer and the location of recharge. 

Ground-water discharge usually is highest in the 

spring, declines through the growing season, and is at 

a minimum in the fall. 

In headwater areas, the ground-water flow system 

may not be able to maintain streamflow through 

extended dry periods, with the result that some headwater streams may cease flowing during the summer 

and fall. Even in higher order streams in the basin 

there may be reaches where the ground-water flow 

system ceases to contribute water to the streams. In 

these areas, hydraulic potential in the surface-water 

system is greater than that in the ground-water system, 

and water flows from the stream to the ground-water 

flow system. These losing stream reaches typically 

occur in areas underlain by carbonate rocks and at 

topographic and structural discontinuities. Perhaps the 

most dramatic example of this phenomenon is the Lost 

River, which loses its flow entirely to the subsurface 

and emerges again 2 mi downstream as the Cacapon 

River. Other examples occur along the western slopes 

of the Blue Ridge Province. As streams flow from the 

steep crystalline terrain of the Blue Ridge Province 

onto the flat carbonate rocks of the Great Valley 

22 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197~90 
subprovince, they lose large quantities of streamflow 

to the carbonate aquifers (Nutter, 1973). 

Although losing reaches of streams are important 

locally, most of the streams in the Potomac River 

Basin are gaining streams in that they receive water 

from the ground-water flow system. Total streamflow 

and the relative proportions of overland runoff and 

base flow vary somewhat throughout the basin. In general, streams in the Appalachian Plateau and Valley 

and Ridge Provinces and in the part of the Piedmont 

Province underlain by crystalline rocks receive about 

one-half their flow from base flow, with overland runoff constituting the other ha.ll.f (Hobba and others, 

1972; Duigon and Dine, 1987, 1991). In basins containing carbonate rocks and in basins with substantial 

regolith, base flow may constitute as much as 65 percent or more of the total streamflow (Nutter and Otton, 

1969; Trombley and Zynjuk, 1985; Duigon and Dine, 

1991). Total streamflow per unit area is highest in the 

Appalachian Plateau Province and lowest in the Valley 

and Ridge Province, reflecting precipitation patterns in 

the region (Hobba and others, 1972). In basins underlain almost entirely by carbonate rocks, such as in the 

Great Valley subprovince, and in basins in the Coastal 

Plain Province, streams derive more than 80 percent of 

their flow from base flow, reflecting the high storage 

capacity of these materials (Johnston, 1976; Becher 

and Taylor, 1982; Duigon and Dine, 1987). In contrast, 

base flow comprises only about 35 percent of total 

streamflow in the part of the Piedmont underlain by 

sedimentary rocks of Triassic and Jurassic age (Wood, 

1980; Duigon and Dine, 1987). 

Subunits for Water-Quality Assessment 

Because of the diverse physical characteristics of 

the Potomac River Basin, it has been divided into 

eight subunits for the purposes of water-quality assessment. This subdivision was done using a hierachical 

process considering basin physiography and geology 

(fig. 13). Physical and geologic characteristics provide the primary factors affecting the hydrologic properties of the ground-water and surface-water systems 

and, thus, form the primary natural factors affecting 

water quality within the basin. Physiographic provinces and subprovinces are used as the primary units 

of the subdivision because of their structural effects on 

the hydrologic systems. Lithology is used as the secondary basis for subdivision due to its effects on 

ground-water flow and water quality. The eight 

subunits will be referred to throughout this report and 

will serve as the primary basis for areal comparisons 

of nutrient concentrations and loads in later sections. 

Additionally, these subunits will be used throughout 

the course of the NA WQA Program's assessment of 

water quality in the Potomac River Basin. 

The Appalachian Plateau subunit, comprising 

4 percent of the total area of the Potomac River Basin, 

is composed primarily of siliciclastic rocks, so it is 

considered to be one subunit. 

The Valley and Ridge subunit is underlain by both 

siliciclastic and carbonate rocks and occupies 34 percent of the total basin area. Some of its carbonate 

rocks are interbedded with siliciclastic rocks, and 

some of its siliciclastic rocks are cemented by carbonate material. This interbedding is characteristic of the 

region. In these settings, ground water moves among 

beds of differing rock types, and surface water flows 

through both rock types. Thus, the Valley and Ridge 

subunit is not differentiated with respect to rock type. 

The Valley and Ridge subunit also includes an outlier, 

Massanutten Mountain, in the Great Valley subprovince, which is similar in topography and lithology to 

that of the Valley and Ridge. 

The Great Valley subprovince is divided into a 

carbonate subunit (17 percent of the basin) and a noncarbonate subunit (5 percent of the basin) because carbonate regions have unique hydrologic properties and 

because the carbonate units are areally contiguous and 

extensive. The carbonate subunit is composed of limestone and dolomite of Cambrian and Ordovician age. 

An outlier of the Great Valley Carbonate subunit is in 

the Piedmont, in the vicinity of Frederick, Maryland. 

This small region is more similar to the Great Valley 

Carbonate subunit than the surrounding Piedmont. 

The Great Valley Noncarbonate subunit is underlain 

by shale of the Martinsburg Formation in the central 

part of the Great Valley subprovince. 

The Blue Ridge subunit, comprising 6 percent of 

the basin, is formed by crystalline rocks. It is a separate subunit based on its high topographic position, 

which affects the ecological characteristics of the subunit. 

The Piedmont subunit (12 percent of the basin) 

comprises the crystalline rock part of the Piedmont 

Province. As in the Blue Ridge subunit, the rocks 

underlying this subunit are crystalline. However, the 

rolling topography of this subunit provides a distinct 

hydrologic setting. 

Description of the Potomac River Basin and It's Subdivision for Water-Quality-Assessment Purposes 23 
~ .9 1.{ 

t- & VALLEY AND 

~ RIDGE 4oo '< 

EXPLANATION 

Subunits 

/·\. 

~._/. WALDORF <.. I INDIAN HEAD. ! COASTAL 

: (PLAIN 

. \ __//1 ""·· 

D Appalachian Plateau 

g Valley and Ridge 

·-- ·· ~ -, """· •• ~Po ac R. •\ ........ , __ ~--~ ~ \ : 

c=J Great Valley Carbonate •• POINT LOOKOU\ 

~- ""-- • Great Valley Noncarbonate 

·"\.. .. ~~~ Blue Ridge 

[ll] Piedmont 0 10 20 30 40 50 MILES 

IZZJ Triassic Lowlands 

D Coastal Plain 

0 10 20 30 40 50 KILOMETERS 

Figure 13. Division of the Potomac River Basin into subunits for the purpose of water-quality assessment. 

The Triassic Lowlands subprovince is underlain 

by poorly indurated siliciclastic rocks and intrusive 

and extrusive igneous rocks. The igneous rocks are not 

used extensively for ground-water supply, and the 

effect of these rocks on surface-water chemistry probably is insignificant at the basin scale. Therefore, the 

Triassic Lowlands are considered to be one subunit 

(7 percent of the basin). 

The Coastal Plain subunit (15 percent ofthe basin) 

is underlain by unconsolidated sediment. The hydrologic factors affecting ground-water and surface-water 

flow in this subunit are unique within the Potomac 

River Basin. 

ANALYSIS OF AVAILABLE NUTRIENT 

DATA 

Data on concentrations and sources of nitrogen 

and phosphorus in the Potomac River Basin have been 

compiled for this report from numerous references and 

data sources. The analysis of these data provides an 

24 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197G-90 
understanding of the status of water quality in the 

basin with respect to these nutrients. Also, the compilation and analysis on the basin scale identify the data 

and information gaps that exist. The information 

included here will be used by the NAWQA Program in 

two ways-(1) to contribute to understanding conditions throughout the Nation and (2) to provide guidance for data collection and analysis in the study of the 

Potomac River Basin. 

Nitrogen and phosphorus are two nutrients essential to the growth of plants and animals; however, high 

concentrations of these nutrients in ground water and 

surface water are of concern throughout the Potomac 

River Basin due to potential adverse effects on human 

health and aquatic life. Eutrophication caused by high 

nitrogen and phosphorus concentrations has been a 

problem in the tidal reaches of the Potomac River. The 

nitrogen and phosphorus causing this problem are 

delivered to the tidal Potomac from its network of tributaries that transects the eight diverse subunits of the 

Potomac River Basin. Sources of nitrogen and phosphorus differ across the basin, and available data are 

used to describe the spatial patterns in nutrient 

sources. These sources are used to explain and understand the patterns of nutrient concentrations found in 

ground water and surface water throughout the basin. 

Methods 

Estimates of Nutrient Inputs 

Estimates of nitrogen and phosphorus inputs to the 

Potomac River Basin were made for point-source 

inputs to surface water from municipal and industrial 

discharges and for nonpoint -source inputs to the land 

surface from atmospheric deposition, commercial fertilizer, animal manure, and septic systems. Computer 

data bases with information on point-source discharges, population, land use, and agricultural practices were used with a geographic information system 

to make these estimates for the eight subunits of the 

Potomac River Basin and selected watersheds within 

the basin. 

Information on municipal and industrial pointsource inputs to streams was obtained from two 

national computerized data bases and from information provided by State and regional agencies. The 

computer data bases included the National Pollutant 

Discharge Elimination System/Permit Compliance 

System (NPDES/PCS) and the U.S. Environmental 

Protection Agency (USEPA) STORET data base. The 

PCS data retrieval included 271 facilities. Data on 

geographic locations were available for only 90 of the 

facilities. The STORET data retrieval included design 

flows and locations for 634 wastewater-treatment 

facilities, of which 70 also were included in the PCS 

data base. Only 242 municipal water-treatment facilities and 175 industrial facilities were included in the 

analysis of point sources. Facilities that contribute 

negligible amounts of nitrogen and phosphorus to 

effluent, such as powerplants and coal mines, were not 

included in the analysis. Discharges for the facilities in 

the PCS that could not be located are relatively minor 

and were not included in estimates of point-source 

loads. Lugbill (1990) was used to verify major point 

sources in the basin. Additional point-source information was provided by the Interstate Commission on the 

Potomac River Basin (ICPRB) (Carlton Haywood, 

Interstate Commission on the Potomac River Basin, 

written commun., 1994) and Maryland Department of 

the Environment (MDE) (Peter Legg, Maryland 

Department of the Environment, written commun., 

1994). 

Information on nutrient discharges was insufficient in any one data base to compute nutrient inputs 

for the entire basin. When actual concentrations and 

loads data in the PCS were insufficient, point-source 

inputs were determined by relating Standard Industrial 

Codes (SIC) to average concentrations of nitrogen and 

phosphorus in effluent. Lug bill ( 1990) compiled average concentrations of nutrients by SIC from previous 

work by the National Oceanic and Atmospheric 

Administration. Average concentrations for SIC's not 

included in Lugbill's tabulation were estimated by 

referring to similar industries. Total input was 

obtained by multiplying average concentrations and 

best estimates of facility flow rates. For 83 facilities, 

measured flows were available from the PCS and were 

used to estimate nutrient inputs. A comparison of 

these measured flows with design flows for the facilities showed that design flows are generally a good 

estimate of plant discharge, except for certain industries that are particularly susceptible to economic 

cycles, such as processing of primary metals. For 

504 facilities, design flows were the best estimate 

available and were used to estimate nutrient loadings. 

No flow information was available for 56 facilities; 

flow rates for these facilities were considered to be 

minor and were assumed to be zero. The information 

provided by MDE and ICPRB was used to adjust 

Analysis of Available Nutrient Data 25 
concentration and flow data where computer data were 

missing, outdated, or unreasonable. 

Nonpoint-source nutrient inputs to the land surface in the basin were computed and apportioned to 

basin subunits and selected watersheds using a geographic-information-system data base with the distribution of land use and population within subunit and 

basin boundaries. Land-use and land-cover data were 

obtained from the Geographic Information Retrieval 

and Analysis System (GIRAS) (Mitchell and others, 

1977), which mapped land-use and land-cover data 

obtained from aerial photographs taken between 1972 

and 1974. Although these data are not up to date, they 

do provide a good approximation of the general distribution of land use and land cover. The 1990 Census of 

Population and Housing (U.S. Department of Commerce, 1991 e) and the 1987 Census of Agriculture 

(U.S. Department of Commerce, 1989) were used to 

identify areas of land-use and land-cover change. 

The extent of orchard, forest, and water areas in 

the GIRAS data was assumed to be similar to 1990 

conditions. It was also assumed that no net changes 

have occurred in the distribution of industrial land 

uses as these changes would have a relatively small 

effect on nonpoint-source inputs. Residential and commercial land uses were assumed to increase proportional to population but not to decrease in the case of 

population decrease. Other urban land uses were 

assumed to not change significantly over the time 

period considered. 

Determining the distribution of agricultural lands 

used for crop and animal production, however, 

required a more complex approach. The 1987 Census 

of Agriculture (U.S. Department of Commerce, 1989) 

provided estimates of crop and pasture acreage and 

animal populations on a total county basis. GIRAS 

data were assumed to be representative of the areal 

distribution of cropland and pasture within counties, 

and it was assumed that crops, pasture, and animals 

were uniformly distributed within a county. Animal 

distributions were assumed to follow the distribution 

of cropland and pasture. Thus, the acreage of particular crops and the number of animals were based on 

actual 1987 census data, whereas the geographic distribution within counties and among subunits and 

selected watersheds was based on GIRAS mapping. 

Inputs of nitrogen and phosphorus from application of commercial fertilizer to agricultural lands were 

estimated directly from county sales data, weighted by 

the percentage of a county's cropland in subunits and 

watersheds. 

Nutrient inputs from manure were estimated by 

multiplying average concentrations for different animals by the number of animals from the 1987 Census 

of Agriculture. Nutrient concentrations in manure differ with different animals, as does mass production of 

nutrients. Animal manure may undergo several processes that may result in significant changes in nitrogen and phosphorus between the time it is excreted by 

animals and the time that it is applied to land surfaces. 

These processes are not addressed in this report but 

should be considered further in a more detailed analysis of nutrient cycling. 

Nutrient inputs from septic systems were estimated using 1990 population data because the number 

and distribution of septic systems in the Potomac 

River Basin had not been mapped at the time of this 

analysis. The geographic distribution and number of 

people using septic systems were estimated by visually clustering 1990 census tracts around municipal 

centers and likely adjoining utility corridors. All population within these census-tract clusters was assumed 

to be serviced by municipal wastewater-treatment 

plants. Any other population was assumed to be on 

septic systems. 

Reckhow and others ( 1980) compiled nutrient 

loadings for household wastewater discharged into 

septic tanks. In a review of seven studies, total nitrogen loadings ranged from 1.92 to 7.31 (lb/person)/yr, 

with a median value of 4.11 (lb/person)/yr. In eight 

studies, total phosphorus loadings ranged from 0.66 to 

2.68 (lb/person)/yr, with a median of 1.33 (lb/person)/yr. To estimate septic inputs, these median values 

were multiplied by the population determined to be on 

septic systems. 

Atmospheric inputs of nitrogen were estimated 

using data from the five USGS National Trends Network (NTN) monitoring sites located nearest to the 

Potomac River Basin. Total wetfall data available for 

water years 1980, 1985, and 1990 at these sites are 

tabulated below. Nitrogen deposition rates may differ 

significantly from year to year. The 1990 estimates 

used for estimation were the median of the three years 

at Parsons, West Virginia, and Leading Ridge, Pennsylvanian. The 1990 inputs were slightly higher than 

1985 inputs at the three other sites. Also, the 1990 

annual streamflow in Potomac River at Washington, 

D.C., was below average. Thus, precipitation and wet 

26 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
deposition in 1990 may be sHghtly less than the longterm average. 

Nitrate and ammonia in wet 

National Trends deposition as nitrogen 

Network (pounds per acre) 

monitoring site 1980 1985 1990 

Parsons, W. Va. 7.05 2.16 3.35 

Shenandoah National 

2.83 3.36 Park, Va. 

Charlottesville, Va. 3.12 3.30 

White Rock, Md. 3.23 3.24 

Leading Ridge, Pa. 5.91 3.85 4.01 

Dryfallloadings of nitrogen are difficult to determine, and there are little data available from monitoring programs. Estimates of nitrogen dryfall were 

made using a procedure suggested by Sisterson ( 1990) 

in which average ratios of dryfall to wetfall were 

determined for each State. These ratios were used to 

estimate a dryfallloading at each NTN site. 

Adjustments were made to wetfall estimates 

because Sisterson (1990) found that wetfall in some 

areas exceeds that which would be indicated by interpolation from the NTN sites. In forested areas above 

about 2,600 ft in elevation in the eastern United States, 

Sisterson ( 1990) determined that direct contact with 

low clouds (fog) accounts for about three times as 

much deposition of nitrogen as monitored wetfall. In 

urban areas, the proximity of atmospheric-source generators, such as fossil-fuel powerplants, results in 

increases in loadings through wetfall and dryfall by 

factors of 1.75 and 5.0, respectively. In estimating 

total-nitrogen loadings from atmospheric deposition 

for areas in the Potomac River Basin, adjustments 

were made for both the extent of urban area and the 

area in forest above 2,600 ft in elevation. Urban areas 

were determined using the 1972-74land-use and landcover data, which were updated using 1990 Census of 

Population data (U.S. Department of Commerce, 

1991e). Wetfall and dryfall nitrogen inputs were 

apportioned to subunits and watersheds by weighting 

data from the monitoring sites by the distance to a 

point in the center of the subunit or watershed segment. Adjustments were made on the basis of percentage of the area higher than 2,600 ft and the percentage 

of urban area. 

Analysis of Nutrient Concentrations 

Patterns in nutrient concentrations in the Potomac 

River Basin are presented in this report using graphical and statistical summaries of data compiled from 

the USGS WATer STOrage and REtrieval system 

(WATSTORE) and the U.S. Environmental Protection 

Agency STORET data system. Five forms of nitrogen 

and phosphorus are discussed, including total nitrogen, dissolved nitrate, dissolved ammonia, total phosphorus, and dissolved orthophosphate. Concentrations 

are in equivalent weights of nitrogen and phosphorus. 

The analysis of nutrient concentrations in ground 

water utilizes only data from the USGS WATSTORE 

system because well-depth and water-quality records 

were more complete. The analysis of concentrations in 

surface water relies on data from both WATSTORE 

and STORET data systems. 

Twenty-five surface-water-quality monitoring 

sites are used to describe the spatial and temporal 

variability in nutrient concentrations throughout the 

basin. These sites were selected from a large number 

of sites that were located near continuous streamflowmonitoring stations and had a minimum of 50 samples. The 25 sites were chosen to represent a broad 

range of physiographic and land-use settings. At several sites both USGS and State monitoring programs 

are co-located, and the data from both programs are 

analyzed separately at these sites. 

In general, sample collection at the State sites is 

done by collecting a single-vertical surface grab sample, whereas most samples collected by the USGS utilize depth- and width-integrated sampling methods 

using isokinetic sampling devices. Surface grab samples have been shown to underrepresent average instream concentrations of total nitrogen and total phosphorus because particulate matter is not evenly distributed throughout the water column. However, dissolved 

species of both nitrogen and phosphorus should be 

comparable from either sampling method (Martin and 

others, 1992). 

The data-analysis procedures used in this report 

generally rely on graphical descriptions and robust statistical tests. Graphical analysis used in this report 

includes maps, histograms, X-Y plots, smoothed 

curves, and boxplots. Graphics are used to show the 

general pattern in the observed data and are not rigorous statistical tests. Nonparametric statistical tests are 

used to discern statistical significance in the observed 

patterns in order to minimize the need to satisfy distributional parameters of parametric tests. Estimates of 

Analysis of Available Nutrient Data 27 
nutrient loads, however, are determined using a fully 

parametric regression method. Therefore, data used 

for load estimation were screened rigorously before 

inclusion in this report. 

The primary graphical method used to display patterns in the distribution of data is the boxplot. Boxplots used in this report display the 25th, 50th, and 

75th percentiles as lines joined to form a box. Whiskers are drawn to the lOth and 90th percentiles. The 

size, shape, and central tendency of groups of data can 

be readily compared when plotted against the same 

vertical axis. Truncated boxplots, where whiskers are 

not shown, indicate clustering of values within the 

25th to 75th percentile range. Also included in all boxplots is the number of data points used to construct the 

box and, where applicable, the number of censored 

values within the distribution. However, 20 percent of 

the distribution is not shown, and any outlier values, 

whether accurate or in error, are excluded. 

A locally weighted sum of squares (LOWESS) is 

used to display the patterns of nutrient concentrations 

with respect to continuous variables. The LOWESS 

curve is used to show the relation of nutrient concentrations to well depth and streamflow. LOWESS is 

used to adjust concentrations for the effects of streamflow when presenting long-term trends in nutrient concentrations. 

Streamflow values used throughout this report are 

mean daily discharge values as published in the annual 

data reports of the USGS. Mean daily discharge is 

used instead of instantaneous measurements because it 

is readily available for all stations, calculation of flow 

duration is simplified, and load-estimation programs 

use mean daily discharge values. Mean daily flowduration percentages were determined for water years 

1970 to 1990 and are used to graphically display the 

concentration-streamflow relation at many stations 

using a single coordinate system. Higher streamflow 

values correspond with lower frequency percentages, 

which indicate rarer events. 

Annual loads of total nitrogen, dissolved nitrate, 

and total phosphorus were estimated where possible 

using the Minimum Variance Unbiased Estimator 

(MVUE) program as described in Cohn and others 

(1992). To achieve a linear model, both explanatory 

and response variables were transformed. The MVUE 

program estimates the log of load using the following 

variables: constant, log of discharge, log of discharge 

squared, time, time squared, sine of time, and cosine 

of time. In the load-estimation model, the discharge 

variables account for the effect of streamflow, the time 

and time squared variables account for the occurrence 

of trends, and the sine and cosine variables account for 

seasonal fluctuations in loads. All seven parameters 

were used for all estimates of load with no attempt to 

exclude parameters because of lack of significance. In 

most cases, probability values for discharge and time 

were less than 0.20 for the regression model. Using 

less significant parameters in the model, however, only 

serves to increase the error of the estimate and does 

not significantly affect the predicted load (T.A. Cohn, 

U.S. Geological Survey, oral commun., 1992). The 

MVUE program estimates daily loads on the basis of 

daily values of discharge data. Daily loads are 

summed to estimate monthly and annual estimates of 

load. Standard errors for daily load estimates are quite 

high; however, the cumulative error of monthly and 

annual estimates yields a fair degree of confidence in 

the estimated values. This report presents only 

estimates of annual loads. Average annual loads are 

calculated for the period of load estimation, which differs from site to site. 

Prior to load estimation, all linear-regression models were first evaluated through independent model 

testing and development. Each observation used in all 

models was tested for measures of influence and leverage, as described in Helsel and Hirsch (1992). Insufficient data and outlier values proved to be the greatest 

obstacles to load estimation. Because of the distribution of samples with respect to time for all waterquality monitoring sites, a core period of water years, 

1979-89, was chosen for load estimation. However, 

data were insufficient during this period for many 

sites, and they were dropped from the analysis. Loads 

were estimated at four additional sites for different 

periods. Outlier values were reviewed individually for 

reasonableness. Some values were obviously entered 

in error and could be excluded from the data set. Other 

values were not easily rejected, and where the model 

was still unstable and highly sensitive to these values, 

no load estimation was done. 

Nutrient Sources 

Estimates of nutrient sources to the entire Potomac River Basin in 1990 show the magnitude and relative importance of different sources to the basin. 

However, patterns in land use, land cover, and topography cause nutrient sources to differ among major 

subunits and watersheds in the basin. Nonpoint 

28 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
sources of nitrogen and phosphorus account for nutrients reaching the land surface with the potential to 

enter the ground-water system. Thus, patterns in nonpoint sources in subunits may be related to patterns in 

nutrient concentrations in ground water within basin 

subunits. Point-source nutrient inputs are discharged 

directly to surface water, and nutrient concentrations 

in surface water are dependent upon point and nonpoint sources of nutrients. 

Basinwide Inputs 

The occurrence of nutrients in surface and ground 

water throughout the Potomac River Basin can be 

attributed to several major sources, including atmospheric deposition, commercial fertilizer, animal 

manure, municipal and industrial wastewater discharges, and septic systems. Figure 14 shows the 

nitrogen and phosphorus inputs from major sources 

for 1990. Atmospheric deposition is not considered a 

significant source of phosphorus. 

Nitrogen inputs in the Potomac River Basin in 

1990 were dominated by atmospheric deposition, animal manure, and commercial fertilizer application. 

Agricultural inputs comprised about 55 percent of the 

nitrogen input, and atmospheric inputs contributed 

32.1 percent. Municipal and industrial wastewater discharges contributed about 12 percent of the nitrogen 

inputs. This percentage is significant, however, 

because discharges are made directly to streams and 

rivers. Nitrogen inputs from other sources are distributed throughout the watershed and are subject to 

numerous processes affecting their potential for entering the ground-water or surface-water systems. 

Phosphorus inputs were dominated by agricultural 

sources throughout the basin, with about 93 percent of 

the phosphorus inputs corning from commercial fertilizer and animal manure. Municipal and industrial discharges contributed only 3.8 percent of the phosphorus, and as with nitrogen, these inputs were made 

directly to surface-water bodies. 

The major sources of nitrogen and phosphorus differ significantly in the watershed upstream and downstream from Washington, D.C. The watershed draining 

to the streamflow-gaging station on the Potomac River 

at Washington, D.C., located at the Fall Line, generally is referred to as the upper Potomac River Basin. In 

1990, atmospheric and agricultural inputs were the 

largest sources of nitrogen to the entire basin and the 

primary inputs in the upper Potomac. Commercial fertilizer and animal manure inputs occurred mostly in 

Commercial 

fertilizer 

(26 percent) 

A. Nitrogen 

Animal manure 

(29 percent) 

B. Phosphorus 

Commercial 

fertilizer 

(48 percent) 

Municipal and 

industrial wastewater discharges 

( 12 percent) 

Municipal and 

industrial wastewater discharges r,...., ___ -,..::--_..c.---j (4 percent) 

Animal manure 

(45 percent) 

Figure 14. Major inputs of (A) nitrogen and (B) phosphorus 

to the Potomac River Basin, 1990. 

the agricultural regions of the upper Potomac Basin. 

Municipal wastewater discharge was the largest nutrient source downstream from the Fall Line, and 88 percent of the basin discharges of nitrogen and 80 percent 

of the discharges of phosphorus occur downstream 

from Washington, D.C. 

Several studies have estimated nutrient inputs to 

the Potomac River and Chesapeake Bay Basins. 

Smullen and others (1982) estimated point-source 

inputs of nitrogen from wastewater treatment and 

industrial processes. Lug bill ( 1990) prepared a comprehensive inventory of nutrient sources in the Potomac River Basin using published statistics. Fisher and 

Oppenheimer (1991) studied the relative contribution 

of atmospheric sources to nitrogen loadings to the 

Analysis of Available Nutrient Data 29 
Chesapeake Bay. Their estimations of nitrogen loadings to the Chesapeake Bay for various sources have 

been simply multiplied by the percentage of the Bay 

basin that is included in the Potomac River Basin 

upstream from the Fall Line for comparison with this 

study. This approach probably overestimates nitrogen 

from municipal wastewater because most of the wastewater discharges in the Potomac River Basin occur 

downstream from the Fall Line. Jaworski and others 

( 1992) expanded on both of the previous studies and 

prepared a nitrogen balance for the Potomac River 

Basin upstream from the Fall Line. The results of 

these investigations are compared with the results of 

this study in table 3. 

Total nitrogen inputs estimated in this study are 

less than the total inputs from the three other comprehensive studies. Jaworski and others (1992) estimates 

for 1983-86 were highest and 15 percent higher than 

this study. There are notable differences among 

studies in the magnitudes of input from atmospheric 

deposition and manure in table 3. These studies compiled data from different sources and made different 

assumptions pertinent to their objectives. Although the 

studies used different approaches to estimating atmospheric deposition, the controlling factor for differences appears to be that nitrogen concentrations in 

wetfall were higher during the time periods used by 

earlier studies. Jaworski and others (1992) used 

estimates of raw manure inputs from Lug bill ( 1990), 

and Lugbill further reduced raw inputs to account for 

losses of nitrogen due to environmental factors. 

Manure-input estimates presented in this report are not 

reduced for losses, and atmospheric-deposition inputs 

are based on more recent data. All studies concluded 

that atmospheric deposition, commercial fertilizer, and 

animal manure account for most of the nitrogen input 

to the Potomac River Basin. 

Distribution in Subunits 

The magnitude and relative contributions of nitrogen and phosphorus sources differ considerably 

among the eight subunits of the Potomac River Basin. 

Estimated inputs of nitrogen and phosphorus to subunits are shown in tables 4 and 5. Within each subunit, 

nonpoint-source inputs are distributed in proportion to 

land use and account for about 88 percent of nitrogen 

input and 96 percent of phosphorus input to the entire 

basin. Figure 15 indicates the proportions and magnitudes of principal nonpoint -source nutrient inputs to 

the eight subunits in the Potomac River Basin. The 

largest nitrogen inputs occur in the Valley and Ridge 

and Great Valley Carbonate subunits. The largest 

phosphorus inputs occur in the Great Valley Carbonate 

subunit. The figure also shows that the proportional 

contribution by input sources differs considerably 

among subunits. With reference to tables 3 and 4 and 

Table 3. Comparison of nitrogen input estimates to the Potomac River Basin upstream from Washington, 

D.C., from several studies 

[--,inputs not estimated] 

Nitrogen input estimates by principal investigator and base year(s), in thousands 

Source of inputs 

Point sources: 

Sewage treatment 

Industrial processes 

Nonpoint sources: 

Atmospheric deposition 

Commercial fertilizer 

Animal manure 

Septic systems 

Total 

Smullen and 

others (1982) 

1980 

4,777 

1,911 

Fisher and 

Oppenheimer 

(1991) 

1984 

16,493 

*1 

94,340 

63,334 

78,507 

252,674 

1 Sewage treatment and industrial processes were combined. 

of pounds 

Jaworski 

Lug bill and others 

(1990) (1992) This study 

1986 1983-86 1990 

3,185 6,836 5,621 

3,304 *1 1,590 

122,774 87,098 76,403 

55,790 50,715 66,574 

72,252 134,946 83,907 

3,938 

257,305 279,595 238,033 

30 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
Table 4A. Estimated point-source inputs of nitrogen by subunits 

of the Potomac River Basin, 1990 

[All point-source inputs are in thousands of pounds] 

Point sources 

Municipal 

sewage Industrial 

Subunit (fig. 13) treatment processes Total 

Appalachian Plateau 78 179 257 

Valley and Ridge 854 82 936 

Great Valley Carbonate 2,052 611 2,663 

Great Valley Noncarbonate 1,259 703 1,962 

Blue Ridge 62 0 62 

Piedmont 2,120 118 2,238 

Triassic Lowlands 1,731 9 1,740 

Coastal Plain 25,687 355 26,042 

Potomac River Basin, total 33,843 2,057 35,900 

Table 48. Estimated nonpoint-source inputs of nitrogen by subunits of the Potomac River Basin, 1990 

[All nonpoiint-sources inputs are in thousands of pounds. Agricultural estimates are based on 1987 statistics (U.S. Department of Commerce, 1989)] 

Area 

(square Atmospheric 

Subunit (fig. 13) miles) deposition 

Appalachian Plateau 660 7,403 

Valley and Ridge 5,062 35,155 

Great Valley Carbonate 2,220 11,327 

Great Valley Noncarbonate 930 4,699 

Blue Ridge 919 5,669 

Piedmont 1,771 12,485 

Triassic Lowlands 1,018 5,869 

Coastal Plain 2,090 13,952 

Potomac River Basin, total 14,670 96,559 

figure 15, the distribution of nutrient sources and 

potential for water-quality effects from the principal 

input sources are discussed in the following 

paragraphs. 

In the Potomac River Basin in 1990, 73 percent of 

the population resided in the Piedmont and Coastal 

Plain subunits. Municipal wastewater-treatment discharges in these subunits comprised 82 percent of the 

nitrogen and 49 percent of the phosphorus from wastewater in the Potomac River Basin. Because the largest 

treatment facilities are downstream from the Fall Line, 

most of the treated sewage discharges were directly to 

the tidal reach of the Potomac River in the Coastal 

Plain subunit. These treated discharges provided 

11 percent of the total nitrogen input and 3 percent of 

the total phosphorus input to the Potomac River Basin. 

Wastewater discharges are particularly significant to 

Nonpoint sources 

Commercial Animal Septic 

fertilizer manure Systems Total 

895 799 68 9,165 

8,523 16,327 1,047 61,052 

25,139 43,232 1,516 81,214 

6,581 10,658 581 22,519 

2,597 1,937 395 10,598 

15,729 6,224 293 34,731 

10,157 6,363 77 22,466 

7,424 1,265 691 23,332 

77,045 86,805 4,668 265,077 

the tidal Potomac River and estuary because they are 

mostly direct discharges to the estuary or tidal streams 

where the potential for eutrophic conditions to develop 

is high. 

In most subunits, industrial discharges provide 

proportionally small inputs of nitrogen and phosphorus. Industrial inputs are largest in the two Great Valley subunits. Industrial discharges are small in the 

Blue Ridge and Triassic Lowlands subunits. 

Nitrogen inputs from atmospheric deposition are 

distributed throughout the Potomac River Basin, with 

an estimated basinwide deposition rate of 6,582lb/mi2 

for 1990. However, because of increased capture of 

fog droplets by trees at higher elevations, those areas 

with extensive forest cover account for much of the 

nitrogen input from atmospheric deposition. Atmospheric deposition rates are highest in the Appalachian 

Analysis of Available Nutrient Data 31 
Figure 5A. Estimated point-source inputs of phosphorus by subunits 

of the Potomac River Basin, 1990 

[All point-source inputs are in thousands of pounds] 

Point sources 

Municipal 

sewage Industrial 

Subunit (fig. 13) treatment processes Total 

Appalachian Plateau 19 54 73 

Valley and Ridge 80 47 127 

Great Valley Carbonate 292 189 481 

Great Valley Noncarbonate 243 103 346 

Blue Ridge 10 0 10 

Piedmont 237 17 254 

Triassic Lowlands 63 8 71 

Coastal Plain 439 27 466 

Potomac River Basin, total 1,383 445 1,828 

Table 58. Estimated nonpoint-source inputs of phosphorus by subunits of the Potomac River Basin, 

1990 

[All nonpoint-sources inputs are in thousands of pounds. Agricultural estimates are based on 1987 statistics (U.S. Department of 

Commerce, 1989)] 

Nonpoint sources 

Area 

(square Commercial Animal Septic 

Subunit (fig. 13) miles) fertilizer manure Systems Total 

Appalachian Plateau 660 276 

Valley and Ridge 5,062 2,929 

Great Valley Carbonate 2,220 7,775 

Great Valley Noncarbonate 930 2,044 

Blue Ridge 919 758 

Piedmont 1,771 4,575 

Triassic Lowlands 1,018 2,993 

Coastal Plain 2,090 2,162 

Potomac River Basin, total 14,670 23,512 

Plateau at 11,217 lb/mi2 because much of the forests 

are at elevations higher than 2,600 ft. Atmospheric 

inputs comprise 58 percent of the nonpoint sources of 

nitrogen to the Valley and Ridge subunit, although the 

average deposition rate is lower (6,904lb/mi2) as forest elevations are lower than in the Appalachian Plateau. The Piedmont subunit has the second highest 

deposition rate (7 ,050 lb/mi2) due to the presence of 

larger urban areas. Atmospheric inputs of nonpointsource nitrogen were also the predominant source of 

nitrogen in the Blue Ridge and Coastal Plain subunits. 

Commercial fertilizer was an important source of 

nitrogen in subunits with extensive agriculture, including the Great Valley Carbonate, Great Valley Noncarbonate, Piedmont, Triassic Lowlands, and Coastal 

Plain subunits (fig. 15), and a major source of phosphorus in all subunits. Average basinwide fertilizer 

221 22 519 

4,233 339 7,501 

10,531 490 18,796 

2,699 188 4,931 

520 128 1,406 

1,759 95 6,429 

1,727 25 4,745 

402 223 2,787 

22,092 1,510 47,114 

sales rates were 5,250 lb/mi2 of nitrogen (table 4B) and 

1,600 lb/mi2 of phosphorus (table 5B). In the 

Appalachian Plateau, Blue Ridge, Piedmont, Triassic 

Lowlands, and Coastal Plain subunits, fertilizer contributed more than 50 percent of nonpoint-source 

phosphorus inputs as fewer nonagricultural sources of 

phosphorus exist. On a unit-area basis, however, those 

subunits with the highest nutrient-input rates from fertilizer were the Great Valley Carbonate, Triassic Lowlands, Piedmont, and Great Valley N oncarbonate, with 

average fertilizer sales rates of 11,300, 9,980, 8,880, 

and 7,080 lb/mi2 of nitrogen and 3,500, 2,940, 2,580, 

and 2,200 lb/mi2 of phosphorus, respectively. 

Manure from animal-production activities was an 

important source of nitrogen and phosphorus in many 

subunits (fig. 15). Basinwide rates of manure input 

were 5,920 lb/n1i2 of nitrogen and 1,500 lb/mi2 of 

32 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
> :I 

Dl 

-< Ill 

ur 

a 

> 

~ iii 

cr 

iii 

z 

s 

.. 

iii" 

a 

c 

Dl 

iii 

GREAT VALLCY CARBONATE 

GREAT VALLEY NONCARBONATE 

A. Nitrogen inputs B. Phosphorus inputs 

VALLCY AND RIDGE 

TRIASSIC LOWLANDS 

10 30 40 

0 10 20 30 40 50 KILOMETERS 

BLUE RIDGE EXPLANATION 

Area of diagram indicates magnitude 

of nitrogen and phosphorous inputs, 

in thousands of pounds 

Nitrogen and phosphorous 

sources 

f!jJJ;> Atmospheric deposition 

C> Commercial fertilizer 

Animal manure 

C> Septic systems 

GREAT VALLEY CARBONATE 

10 20 30 

0 10 20 30 40 50 KILOMETERS 

w Figure 15. Distribution of (A) nitrogen and (B) phosphorus inputs from nonpoint sources to subunits of the Potomac River Basin, 1990. w 
phosphorus in 1990. Manure was the largest single 

source of nitrogen and phosphorus to the Great Valley 

subunits and contributed more than 50 percent of the 

phosphorus to the Valley and Ridge subunit. Manure 

inputs to the Great Valley Carbonate and Great Valley 

N oncarbonate subunits were highest, with average 

rates of inputs from manure of 19,500 and 

11,500 lb/mi2 of nitrogen and 4,740 and 2,900 lb/mi2 

of phosphorus, respectively. 

Septic-system inputs of nutrients to the entire 

Potomac River Basin were estimated to be less than 

2 percent for nitrogen and 4 percent for phosphorus. 

However, particular areas of concern include Coastal 

Plain and carbonate settings where pathways might 

occur for rapid transport of nutrients and associated 

septic contaminants to ground water. Much of the population in the Coastal Plain subunit is serviced by 

municipal wastewater treatment, so water-quality 

effects from septic systems, if any, probably are limited to rural areas with high water tables or near water 

bodies. The Great Valley Carbonate subunit contains a 

higher percentage of population relying on septic systems than other areas of the basin and has the highest 

input of nitrogen and phosphorus from septic systems. 

Another potential area of concern is the Valley and 

Ridge subunit where carbonate valleys and shallow 

soils increase the potential for ground-water contamination. In the Valley and Ridge subunit, septic inputs 

contribute 1,000,000 lb of nitrogen and 300,000 lb of 

phosphorus. 

Distribution in Major Watersheds 

The distribution of nitrogen and phosphorus 

inputs in major watersheds of the Potomac River 

Basin reflects the nutrient-input patterns of the eight 

major subunits. Basin characteristics of contributing 

watersheds are presented in table 6 for the 25 surfacewater-quality monitoring sites used in this study. The 

location and watershed boundaries for these sites are 

shown in figure 16. Estimates of nitrogen and phosphorus inputs to these watersheds are presented in 

tables 7 and 8. The nitrogen and phosphorus inputs to 

these 25 watersheds are used to explain patterns in 

nutrient concentrations in surface water in later sections of this report. The magnitude and distribution of 

the principal sources of nitrogen and phosphorus to 

10 major watersheds with drainage basins greater than 

490 mi2 are shown in figure 17. In figure 17, the cumulative effect of nutrient inputs to nested watersheds is 

apparent as total nutrient inputs increase downstream 

as drainage area increases. Watershed boundaries 

often transect several subunits; thus, the proportion of 

individual nutrient sources does not differ as widely as 

among subunits. The potential for water-quality 

effects from the principal nutrient sources is discussed 

in the following paragraphs. 

In 1990, municipal wastewater discharges were a 

relatively small component of total nutrient inputs to 

the 10 major watersheds. As previously indicated, 

most of the nitrogen and phosphorus from municipal 

wastewater is discharged downstream from Washington, D.C. Upstream from Washington, D.C., municipal 

discharges accounted for 2. 7 percent of the nitrogen 

and 1.9 percent of the phosphorus input (table SA). Of 

the 10 sites in figure 17, the largest fraction of total 

nitrogen input from wastewater discharges was 

3.5 percent at Monocacy River (site 19, table 7). 

Municipal discharges accounted for 3.2 percent of the 

phosphorus input to the North Branch Potomac River 

(site 2, table 8). 

Nutrient inputs from industrial sources are small 

compared to most other sources; however, these inputs 

directly affect the surface-water system. The highest 

percentage of nitrogen inputs from industrial sources 

was to the North Branch Potomac (site 2, 1.6 percent) 

and Conococheague subbasins (site 7, 2.5 percent). 

Relatively high nitrogen inputs from industrial sources 

occurred in the South Fork Shenandoah (site 11), 

North Fork Shenandoah (site 14), Shenandoah (site 

15), and Potomac River (sites 8 and 23) subbasins; 

however, the nitrogen input from industrial sources 

was less than 1 percent in these subbasins. Industrial 

inputs of phosphorus comprised 8. 7 percent of total 

inputs to the North Branch Potomac River (site 2), the 

highest percentage for the 10 major watershed sites. 

Industrial inputs of phosphorus were also relatively 

high at sites 3, 8, 11, 14, 15, and 23; however, the 

maximum percentage of phosphorus inputs was 

1.4 percent for these sites. Industrial sources generally 

contributed a smaller proportion of total nutrient 

inputs in the Shenandoah subbasins as inputs from 

other sources were larger. 

All10 major watersheds had significant inputs of 

nitrogen from atmospheric deposition. However, as 

discussed earlier, those areas with extensive forest 

cover at higher elevations account for much of the 

nitrogen inputs from atmospheric deposition. The 

Potomac River at Shepherdstown (site 8) accounted 

for about 44 percent of the nitrogen inputs from atmospheric deposition to the entire Potomac River Basin 

34 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
Table 6. Summary of basin characteristics for selected surface-water-quality monitoring sites in the Potomac River Basin 

[<,less than] 

u.s. 

Site Geological Popula- Land use (percent) 

no.1 Survey Drainage tion Physiographic 

(fig. station Sampling area (square (thous- Agri- province or 

16) Stream name and location Identifier agency(s)2 miles) ands) Forest culture Urban subprovince (fig. 4) 

1 North Branch Potomac River at Kitzmiller. Md. 01595500 MD 225 4 80 13 <1 Appalachian Plateau 

2 North Branch Potomac River near Cumberland, Md. 01603000 USGS 875 86 82 13 3 Appalachian Plateau 

3 South Branch Potomac River at Springfield, W.Va. 01608500 wv 31,470 29 78 22 <1 Valley and Ridge 

4 Town Creek at Oldtown, Md. 01609000 MD 148 1.5 85 15 <1 Valley and Ridge 

5 Lost River at McCauley, W. Va. 01610200 USGS 155 2.2 78 22 <1 Valley and Ridge 

6 Cacapon River at Great Cacapon, W. Va. 01611500 wv 3

677 11 82 18 <1 Valley and Ridge 

1AIB Conococheague Creek at Fairview, Md. 01614500 USGS/MD 3

494 78 36 60 4 Great Valley 

8A/B Potomac River at Shepherdstown, W. Va. 01618000 USGS/MD 35,936 424 69 28 2 Mixed 

9AIB Antietam Creek near Sharpsburg, Md. 01619500 USGS/MD 281 115 24 69 7 Great Valley 

10 Christians Creek at Fishersville, Va. 01624800 VA 70 8.6 25 67 8 Great Valley 

11A/B South Fork Shenandoah River at Front Royal, Va. 01631000 USGSNA 31,642 188 51 40 8 Great Valley 

12 North Fork Shenandoah River at Cootes Store, Va. 01632000 VA 213 2.7 88 10 2 Valley and Ridge 

13 Smith Creek near New Market, Va. 01632900 VA 93 7.5 40 52 8 Great Valley 

14A/B North Fork Shenandoah River at Strasburg, Va. 01634000 USGSNA 3

768 48 54 40 6 Great Valley 

15 Shenandoah River at Millville, W. Va. 01636500 USGS 33,040 297 51 41 7 Great Valley 

16 Catoctin Creek near Middletown, Md. 01637500 MD 67 8.3 38 61 1 Piedmont 

17 Monocacy River at Bridgeport, Md. 01639000 USGS 173 29 20 78 2 Triassic Lowlands 

l> 18 Hunting Creek tributary near Foxville, Md. 01640970 USGS 4 4o 76 23 1 Blue Ridge ::::s 

I» 19 Monocacy River near Frederick, Va. 01643000 USGS 3

817 181 23 73 3 Mixed -< tl) 

i" 20 

9. 

Goose Creek near Leesburg, Va. 01644000 VA 322 18 29 66 4 Triassic Low lands 

)> 

< 21A/B Seneca Creek at Daowsonville, Md. 01645000 USGS/MONT 101 137 24 65 9 Piedmont !. 

i» 22 Difficult Run near Great Falls, Va. 01646000 VA 58 92 31 17 50 Piedmont D" 

i" 23 Potomac River at Washington, D.C. 01646500 USGS 11,670 1,655 55 40 4 Mixed 

z 

s. 24 Cameron Run at Alexandria, Va. 01653000 VA 33.7 155 8 2 90 Piedmont ::::!. 

CD 25 Bull Run near Manassas, Va. 01657000 VA 147 112 34 48 16 Piedmont ::::s 

-

c 1

Site identifier: A=U.S. Geological Survey sampling agency, B=non-U.S. Geological Survey sampling agency. g 2Sampling agency codes: MD=Maryland Department of the Environment; MONT=Montgomery County government; USGS-U.S. Geological Survey; VA=Virginia Department of the Environmental 

Quality; WV=West Virginia Department of Natural Resources. 

(,) 3Considered major watershed for purposes of this report (greater than 490 square miles). Cll 4No census-tract centers located in the watershed. 
EXPLANATION 

- Watershed boundary 

11 .A. Surface-water-quality monitoring 

site-Number corresponds to that 

used in table 6 

0 10 20 30 40 50 MILES ~,_~,_~~~~--~--~~ 

0 10 20 3'0 40 50 KILOMETERS 

Figure 16. Location and watershed boundaries for surface-water-quality monitoring sites used for analysis of nutrient data. 

(96,559,000 lb). Atmospheric inputs were most important in the North Branch Potomac River watershed 

(site 2) with 79 percent of the total nitrogen input from 

atmospheric deposition. 

As can be expected, the greatest inputs of nutrients 

from commercial fertilizer and animal manure 

occurred in watersheds with extensive agricultural 

land use. However, the proportion of fertilizer and 

manure inputs differed considerably among four agricultural watersheds, sites 7, 11, 14, and 19. Commercial fertilizer comprised 51 percent of the nitrogen and 

63 percent of the phosphorus inputs to the Monocacy 

River watershed (site 19) and fertilization rates there 

were highest, with nitrogen and phosphorus applied at 

15,300 and 4,490 lb/rni2, respectively. In the Conococheague Creek watershed (site 7), fertilizer contributed 35 percent of the nitrogen and 47 percent of the 

phosphorus inputs, and application rates were 

somewhat lower at nearly 9,370 lb/rni2 nitrogen and 

2,940 lb/mi2 phosphorus. In the North and South Fork 

Shenandoah Rivers (sites 14 and 11), fertilizer contributed about 17 percent of the nitrogen and about 27 percent of the phosphorus inputs, although the extent of 

agricultural land use is similar to the other basins. 

36 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197G-90 
Table 7. Estimated inputs of nitrogen to selected watersheds in the Potomac River Basin, 1990 

[All numbers are in thousands of pounds. Agricultural estimates are based on 1987 statistics (U.S. Department of Commerce, 1989)) 

Point sources Nonpoint sources 

Site no. 

(fig. 17} Stream name 

Municipal 

sewage 

treatment 

Industrial processes 

Atmospheric 

deposition 

Commercial 

fertilizer 

Animal Septic 

manure systems Total 

North Branch Potomac River 

2 North Branch Potomac River 

3 South Branch Potomac River 

4 Town Creek 

5 Lost River 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

Cacapon River 

Conococheague Creek 

Potomac River 

Antietam Creek 

Christians Creek 

South Fork Shenandoah River 

North Fork Shenandoah River 

Smith Creek 

North Fork Shenandoah River 

Shenandoah River 

Catoctin Creek 

Monocacy River 

Hunting Creek tributary 

Monocacy River 

Goose Creek 

Seneca Creek 

Difficult Run 

Potomac River 

Cameron Run 

Bull Run 

14 

130 

61 

49 

0 

6 

282 

1,539 

378 

46 

1,258 

0 

25 

94 

1,704 

39 

128 

0 

864 

25 

272 

0 

5,621 

0 

159 

18 

203 

57 

0 

0 

0 

329 

729 

3 

0 

394 

0 

0 

230 

630 

0 

9 

0 

15 

0 

0 

0 

1,590 

0 

0 

Fertilizer application rates in these watersheds were 

much smaller at about 5,300 lb/mi2 nitrogen and nearly 

1,600 lb/mi2 phosphorus. 

Manure-production rates were quite different than 

fertilizer use as animal populations are largest in the 

southern part of the Great Valley subprovince. Of the 

four agricultural watersheds previously mentioned 

(sites 7, 11, 14, and 19), manure inputs were most 

intense in the North Fork Shenandoah River watershed 

(site 14), where manure contributed 62 percent of the 

nitrogen and 72 percent of the phosphorus input. The 

manure-production rate in this basin was 20,900 lb/mi2 

nitrogen and 4,660 lb/mi2 phosphorus. In the South 

3,425 

9,970 

14,848 

757 

1,483 

4,374 

2,485 

42,649 

1,410 

366 

12,079 

1,877 

535 

5,000 

20,375 

296 

811 

19 

3,785 

1,643 

539 

697 

76,403 

584 

975 

272 

1,1-35 

2,131 

261 

344 

1,156 

4,627 

19,364 

3,588 

573 

8,640 

354 

806 

4,148 

15,789 

1,066 

1,710 

26 

12,508 

2,109 

1,621 

34 

66,574 

0 

512 

322 20 

999 104 

7,404 204 

176 10 

1,262 22 

2,525 

5,080 

23,308 

2,582 

1,020 

25,941 

1,729 

3,834 

16,072 

44,763 

474 

2,471 

11 

7,165 

1,462 

201 

15 

83,907 

0 

290 

44 

286 

1,842 

348 

46 

654 

27 

39 

296 

1,249 

42 

9 

0 

298 

76 

0 

0 

3,938 

0 

0 

4,071 

12,541 

24,705 

1,253 

3,111 

8,105 

13,089 

89,431 

8,309 

2,051 

48,966 

3,987 

5,239 

25,840 

84,510 

1,917 

5,138 

56 

24,635 

5,315 

2,633 

746 

238,033 

584 

1,936 

Fork Shenandoah River watershed (site 11), manure 

contributed 53 percent of the nitrogen and 67 percent 

of the phosphorus inputs. In the Conococheague Creek 

watershed (site 7), manure contributed 39 percent of 

the nitrogen and 47 percent of the phosphorus. Manure 

production and fertilizer applications were nearly 

equal in the Conococheague Creek watershed. 

Manure-production rates were lower in the Monocacy 

watershed (site 19) where manure inputs contributed 

29 percent of the nitrogen and 33 percent of the phosphorus inputs. 

Septic systems were a minor source of nitrogen 

and phosphorus in all 10 major watersheds. A 

Analysis of Available Nutrient Data 37 
Table 8. Estimated inputs of phosphorus to selected watersheds in the Potomac River Basin, 1990 

[All numbers are in thousands of pounds. Agricultural estimates are based on 1987 statistics (U.S. Department of Commerce, 1989)] 

Point sources Nonpoint sources 

Municipal Indus- ComSite no. sewage trial pro- mercia I Animal Septic 

(fig. 17) Stream name treatment cesses fertilizer manure systems Total 

North Branch Potomac River 2 2 87 86 7 184 

2 North Branch Potomac River 24 66 353 278 34 755 

3 South Branch Potomac River 14 33 802 1,964 66 2,879 

4 Town Creek 12 0 78 51 3 144 

5 Lost River 0 0 131 287 7 425 

6 Cacapon River 0 440 600 14 1,055 

7 Conococheague Creek 71 0 1,451 1,466 92 3,080 

8 Potomac River 251 150 6,453 6,397 595 13,846 

9 Antietam Creek 62 7 1,063 745 113 1,990 

10 Christians Creek 6 0 168 270 15 459 

11 South Fork Shenandoah River 158 131 2,530 6,084 211 9,114 

12 North Fork Shenandoah River 0 0 106 380 9 495 

13 Smith Creek 3 0 236 842 13 1,094 

14 North Fork Shenandoah River 10 52 1,216 3,578 96 4,952 

15 Shenandoah River 212 187 4,729 10,406 404 15,938 

16 Catoctin Creek 4 0 309 135 14 462 

17 Monocacy River 6 8 530 635 3 1,182 

18 Hunting Creek tributary 0 0 7 3 0 10 

19 Monocacy River 131 i3 3,671 1,930 96 5,841 

20 Goose Creek 3 0 618 418 25 1,064 

21 Seneca Creek 35 0 470 58 0 563 

22 Difficult Run 0 0 10 4 0 14 

23 Potomac River 792 403 20,459 21,223 1,273 44,150 

24 Cameron Run 0 1 0 0 0 

25 Bull Run 18 0 150 83 0 251 

particular area of concern with septic systems is in Nutrient Concentrations 

watersheds underlain by carbonate rock, as represented by sites 7, 11, 14, and 15, where pathways Data Availability 

occur for rapid transport of contaminants to ground 

water and to the surface-water system. These water- Numerous water-quality-monitoring programs 

sheds also have a high percentage of the population have been operated by governmental and educational 

relying on septic systems. Conococheague Creek (site institutions within the Potomac River Basin. Much of 

7) had the highest number of septic systems per unit this information is available from the USGS 

area of the 10 watersheds included in this analysis, and WATSTORE and USEPA STORET computer sysseptic systems contributed about 2 percent of the nitro- terns. The nutrient-concentration data compiled for 

gen and 3 percent of the phosphorus inputs. this report will be used for several purposes. First, 

38 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
A . Nitrogen inputs 

Area of diagram indicates magnitude of 

nitrogen input, in thousands of pounds 

B. Phosphorus inputs 

EXPLANATION 

11

.6. Surface-water-quality-monitoring site and 

number-Represents a major watershed with 

drainage area greater than 490 square miles. 

Number corresponds to that used in tables 

7 and 8 

Nitrogen and phosphorous 

sources 

Municipal and industrial 

discharges 

Atmospheric deposition 

Commercial fertilizer 

Animal manure 

C> Septic systems 

Area of diagram indicates magnitude of 

phosphorus input, in thousands of pounds 

50,000 

25,000 

10,000 

5,000 

1,000 

Figure 17. Distribution of (A) nitrogen and (B) phosphorus inputs to major watersheds of the Potomac River Basin, 1990. 
some of the data will be used to assess water-quality 

conditions in the Potomac River Basin to guide data 

collection and analysis by the NAWQA Program. Second, other data may be used in conjunction with newly 

collected data to refine our understanding of the occurrence of nutrients in the basin. Using historical data 

sets to assess water-quality conditions, however, is difficult because of problems with data accessibility and 

data comparability. The data summarized in this report 

are limited to computerized data from WATSTORE 

and STORET for the period 1970-90. Data in these 

systems have been collected to meet the specific 

objectives of individual monitoring programs. These 

objectives include specific research or regulatory 

needs and may yield data that are inappropriate for 

broad-scale analysis of water-quality conditions. Also, 

many water-quality-monitoring networks use sampling and chemical-analysis methods that differ 

greatly, which could cause incomparability of nutrientconcentration data. 

Many short- and long-term monitoring programs 

have operated within the Potomac River Basin during 

the past 100 years. The primary long-term, surfacewater-quality and acid-precipitation monitoring networks of State, regional, and Federal agencies operating in the Potomac River Basin for 1970--90 are listed 

in table 9. These programs are identified and described 

in the Chesapeake Bay Basin Monitoring Program 

Atlas (Heasley and others, 1989). The primary Federal 

water-quality-monitoring program operating in the 

basin is the USGS National Stream-Quality Accounting Network (NASQAN), which has operated three 

long-term streamflow-gaging stations and water-quality monitoring sites on the Potomac River at Shepherdstown, W. Va., the Shenandoah River at Millville, 

W. Va., and the Potomac River at Washington, 

D.C.(sites 8, 15, and 23), since 1973. Sites 8 and 15 

were discontinued by the NASQAN program in 1994. 

The Metropolitan Washington Council of Governments operated 50 sites within the basin in the vicinity 

of Washington, D.C., and coordinated the operation of 

41 sites in the Anacostia River watershed. The number 

of surface-water-quality monitoring sites operated 

since 1973 by State programs differs from State to 

State-Maryland (52 sites), Pennsylvania (3 sites), 

Virginia (89 sites), West Virginia (5 sites), and the District of Columbia (76 sites). The data from these programs represent the data most suitable to the regional 

assessment of water-quality conditions in the Potomac 

River Basin because they cover the geographic extent 

of the basin and data have been collected over a range 

of seasonal and hydrologic conditions. Selected sites 

from these programs are used in this report to assess 

water-quality conditions. 

Ground Water 

Nutrient-concentration data are available for water 

samples from 1,158 wells in WATSTORE and 

1,401 wells in STORET. The location of these wells is 

shown in figures 18 and 19. Many of the USGSground-water-quality monitoring sites were sampled 

as part of county ground-water-resource assessments 

of Carroll, Frederick, and Washington Counties, 

Maryland; Berkeley and Jefferson Counties, West Virginia; and Clarke and Prince William Counties, Virginia. Other samples were collected as part of both 

regional and site-specific monitoring, with particularly 

dense sampling distributions in the vicinity of Gettysburg, Pennsylvania, Indian Head, Maryland, and in the 

headwaters of the North Branch Potomac River. Few 

ground-water samples are available from WATSTORE 

for the Shenandoah Valley region of Virginia. A large 

number of ground-water samples from this region and 

the rest of the Virginia part of the basin were collected 

by the Virginia Department of Environmental Quality 

and are available from the STORET system. 

The number of ground-water sites and nutrient 

samples available from the WATSTORE and STORET 

data bases is tabulated in tables 10 and 11 and indicates the nutrient analyses available for the eight subunits in the Potomac River Basin. The number of 

samples for individual nutrient constituents in each 

subunit greatly affects the ability to assess waterquality conditions within and among the subunits. Of 

the eight subunits, the Great Valley Carbonate has the 

largest number of wells with nutrient analyses in both 

data bases. The number of sampled wells in the 

WATSTORE data base ranges from 46 in the Appalachian Plateau subunit to 367 in the Great Valley Carbonate subunit. Dissolved-nitrate-concentration data 

are available for most of these wells. Dissolved-orthophosphate and ammonia concentrations, however, are 

present in a much smaller subset of wells. Thus, 

assessment of water-quality conditions for these constituents will not be as reliable as for nitrate. The number of wells with samples in STORET ranges from 2 

in the Appalachian Plateau subunit to 762 in the Great 

Valley Carbonate subunit. Nearly all of the samples 

have concentration information for both dissolved 

40 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1976-90 
Table 9. Summary of selected surface-water-quality and acid-preciptitation monitoring programs collecting 

nutrient data in the Potomac River Basin, 1970-90 

[Data obtained from Heasley and others ( 1989)] 

Agencies and programs 

Federal 

U.S. Geological Survey, Water Resources Division 

National Stream-Quality Accounting Network 

Regional 

Metropolitan Washington Council of Governments 

Potomac Regional Monitoring Program 

Coordinated Anacostia Monitoring Program 

State 

Number 

First year of samof data piing 

collection sites 

1973 

1982 

1985 

3 

50 

41 

Frequency 

Method of of samples 

collection 1 collected 

a,c 

b,e 

b,e 

Bimonthly 

Monthly 

Monthly 

District of Columbia Department of Consumer and Regulatory Affairs, Environmental Control Division 

District of Columbia Water Quality Monitoring Program 1979 76 f Monthly 

Maryland Department of Environment, Water Management Administration 

Maryland Nontidal Tributary Water Quality Monitoring Program 1974 37 d 

Maryland Chesapeake Bay Water Quality Monitoring Program: 1984 e 

Mainstem Chemical/Physical Component 

Maryland Chesapeake Bay Water Quality Monitoring Program: 1984 11 e 

Tributary Chemical/Physical Component 

Maryland Chesapeake Bay Water Quality Monitoring Program: 1984 a,g 

River Input Component 

Maryland Chesapeake Bay Water Quality Monitoring Program: 1984 2 e 

Ecosystem Processes Component- Sediment Oxygen and Nutrient Exchange 

Maryland Department of Environment, Air Management Administration 

Maryland Acid Precipitation Monitoring Program 1984 h 

Pennsylvania Department of Environmental Resources, Bureau of Water Quality Management 

Pennsylvania Water Quality Network 1962 3 b,c 

Pennsylvania Atmospheric Deposition Monitoring Program 

Virginia Department of Environmental Quality 

VIrginia Ambient Water Quality Monitoring Program (program run 

by Vrrginia Northern Regional and Valley Regional Offices) 

Virginia Northern Regional Ambient Water Quality Monitoring Network 

Virginia Department of Air Pollution Control 

Vrrginia Acid Precipitation Network 

West Virginia Department of Natural Resources 

West VIrginia Ambient Water Quality Network 

1981 

1968 

1973 

1982 

1960 

h 

57 d 

32 b 

2 h 

5 c 

Monthly 

Monthly 

Nov.-Feb. 

Biweekly 

Mar. -Oct. 

Monthly 

Nov.-Feb. 

Biweekly 

Mar.-Oct. 

1-4 times 

per month 

Quarterly 

Weekly 

Monthly 

Weekly 

Monthly 

Monthly 

with some 

exceptions 

Weekly 

Monthly 

1

Method of collection: a, cross-sectional and depth-integrated sample method; b, grab sample; c, composite laboratory samples with depthintegrated sample method; d, bucket sample method; e, water-column samples collected with submersible pump; f, 1.0-meter profile, April through 

October; g, flow-weighted, storm-sampling automatic sampler; h, wet/dry precipitation collector. 

Analysis of Available Nutrient Data 41 
/ 

\j 

r· 

./. EXPLANATION .· ·-........ .. 

o U.S. Geological Survey groundwater-quality monitoring sites -~-· 

50 MILES 

~~,-~--~--~---

0 10 20 30 40 

0 10 20 30 40 50 KILOMETERS 

Figure 18. Location of ground-water wells with nutrient data in the U.S. Geological Survey's WATSTORE data base, 

1970-90. 

nitrate and dissolved orthophosphate, and none of the 

samples have dissolved-ammonia analyses. 

Surface Water 

Nutrient-concentration data for surface water are 

available for 456 sites in WATSTORE and 1,176 sites 

in STORET. Figures 20 and 21 show the distribution 

of surface-water-quality monitoring sites with nutrient 

data collected by USGS, other Federal programs, and 

States. Many samples are needed to perform a complete analysis of nutrient-concentration data in surface 

water because nutrient concentrations in surface water 

can vary greatly due to seasonal and streamflow conditions. Sites with more than 50 samples have been 

highlighted because these sites are usually long-term 

monitoring sites and are the best candidate sites for 

data analysis with respect to streamflow, seasonality, 

long-term trends, and nutrient loadings. The 

456 USGS surface-water-quality monitoring sites are 

fairly evenly distributed throughout the basin, and 

89 sites have more than 50 samples. Four areas have a 

dense coverage of sites, including the North Branch 

42 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
EXPLANATION 

Ground-water-quality monitoring 

sites 

6 U.S. Forest Service groundwater site 

o Virginia Department of 

Environmental Quality 

\J Pennsylvania Department of 

Environmental Resources 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 19. Location of ground-water wells with nutrient data in the U.S. Environmental Protection Agency's STORET data 

base, 1970-90. 

Potomac River watershed, the Monocacy River watershed, the Blue Ridge subunit, and northern Virginia 

near Washington, D.C. Other Federal surface-water 

sites with nutrient data in the STORET data base are 

fairly evenly distributed throughout the basin. The 

USEPA maintains data from sites throughout the 

basin, with sites concentrated in the Potomac estuary. 

Few of these sites have more than 50 samples. The 

U.S. Forest Service maintains data in STORET from 

13 sites in the southwestern comer of the South 

Branch Potomac River watershed. State and local 

monitoring sites are concentrated on major tributary 

systems, with large numbers of sites in Virginia, 

Washington, D.C., and Montgomery County, Maryland. West Virginia and Pennsylvania have comparatively few long-term monitoring sites. 

A total of 31,567 surface-water samples are stored 

in WATSTORE, and 50,036 surface-water samples are 

stored in STORET (tables 12 and 13). The distribution 

of samples with respect to subunits reflects the areal 

distribution of samples shown in figures 20 and 21 

and, as with the ground-water data, affects the ability 

Analysis of Available Nutrient Data 43 
Table 10. Availability of ground-water nutrient-concentration data in the U.S. Geological 

Survey's WATSTORE data base, 1970-90 

[Data compiled from the U.S. Geological Survey WATSTORE data base; --, no data available] 

Number of nutrient analyses reported 

OrthophosNumber of Number of Ammonia, Nitrate, ph ate, 

Subunits (fig. 13) wells samples dissolved dissolved dissolved 

Appalachian Plateau 46 73 55 24 

Valley and Ridge 154 202 39 ll4 53 

Great Valley Carbonate 367 510 200 497 237 

Great Valley Noncarbonate 71 83 39 83 46 

Blue Ridge 95 286 21 267 200 

Piedmont 152 179 19 166 38 

Triassic Lowlands 182 290 111 233 79 

Coastal Plain 91 223 1 84 18 

Total 1,158 1,846 430 1,499 695 

Table 11. Availability of ground-water nutrient-concentration data in the U.S. Environmental 

Protectior. Agency's STORET data base, 1970-90 

[Data compiled from the U.S. Environmental Protection Agency STORET data base;--, no data available] 

Number of nutrient analyses reported 

OrthophosNumber of Number of Ammonia, Nitrate, phate, 

Subunits (fig. 13) wells samples dissolved dissolved dissolved 

Appalachian Plateau 2 3 

Valley and Ridge 113 211 

Great Valley Carbonate 762 1,115 

Great Valley Noncarbonate 205 257 

Blue Ridge 51 56 

Piedmont 112 175 

Triassic Lowlands 69 188 

Coastal Plain 87 135 

Total 1,401 2,140 

to compare conditions among subunits. In the 

WATSTORE system, the greatest number of samples 

are available for dissolved nitrate, with fewer samples 

available for ammonia, total nitrogen, dissolved orthophosphate, and total phosphorus. In the STORET system, dissolved-nitrate and total-phosphorus samples 

are most prevalent. 

Concentrations in Ground Water 

The analysis of nutrient concentrations in ground 

water for this study relies solely on data from the 

USGS WATSTORE data base because chemical 

analyses were generally complete, well characteristics 

were inventoried, and analytical quality assurance 

could be documented. Also, only one analysis per 

3 

210 150 

1,114 1,026 

256 231 

56 49 

161 145 

106 103 

125 88 

2,031 1,792 

well, the most recent one, was used to assess concentrations in ground water. Potomac River Basin subunits and land-use designations were assigned to each 

ground-water monitoring well on the basis of its geographic location. The GIRAS land-use data from 

about 1972, as shown in figure 6, were used for this 

designation. Only wells in agricultural, urban, or 

forest areas were used to assess the relation between 

nutrient concentrations and land use. 

Dissolved-nitrate concentrations in ground water 

vary widely within the Potomac River Basin, with 

much of the variability related to subunits and nutrient 

inputs from land-use practices. Dissolved-ammonia 

and dissolved-orthophosphate concentrations in 

ground water are generally near the analytical 

44 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
Table 12. Availability of surface-water nutrient-concentration data in the U.S. Geological Survey's WATSTORE 

data base, 1970-90 

[Data compiled from the U.S. Geological Survey WATSTORE data base] 

Number of nutrient analyses reported 

Phos- OrthophosNumber of Number of Nitrate, Ammonia, Nitrogen, phorus, ph ate, 

Subunits (fig. 13) wells samples dissolved dissolved tqtal total dissolved 

Appalachian Plateau 93 2,713 303 56 61 109 65 

Valley and Ridge 67 11,465 643 82 14 160 327 

Great Valley Carbonate 60 5,132 1,922 966 1,291 1,588 883 

Great Valley Noncarbonate 13 2,044 1,471 189 12 261 1,022 

Blue Ridge 46 2,242 1,232 215 8 76 11 

Piedmont 95 3,948 785 450 482 615 557 

Triassic Lowlands 59 3,079 1,818 927 1,460 1,613 1,123 

Coastal Plain 23 944 268 33 28 52 19 

Total 456 31,567 8,442 2,918 3,356 4,474 4,007 

Table 13. Availability of surface-water nutrient-concentration data in the U.S. Environmental Protection 

Agency's STORET data base, 1970-90 

[Data compiled from the U.S. Environmental Protection Agency STORET data base] 

Number of nutrient analyses reported 

Phos- OrthophosNumber of Number of Nitrate, Ammonia, Nitrogen, phorus, phate, 

Subunits (fig. 13) wells samples dissolved dissolved total total dissolved 

Appalachian Plateau 24 1,161 1,091 

Valley and Ridge 143 2,759 2,713 

Great Valley Carbonate 161 6,376 6,302 

Great Valley Noncarbonate 79 3,345 3,292 

Blue Ridge 19 624 610 

Piedmont 328 17,648 16,514 

Triassic Lowlands 129 7,878 

Coastal Plain 293 10,245 

Total 1,176 50,036 

detection level and show only slight spatial patterns 

throughout the basin. 

7,676 

8,613 

46,811 

Dissolved-nitrate concentrations in water from 

1,049 wells in the Potomac River Basin range from 

less than 0.01 to 63 mg/L (milligrams per liter) and 

have a median value of 1.8 mg/L. The boxplots in 

figure 22 show the statistical distribution of dissolved 

nitrate in ground water for wells in the eight subunits. 

Nitrate concentrations are less than the laboratory 

detection level in water from 12 percent of the wells. 

Ground-water samples from all subunits display a 

wide range in dissolved-nitrate concentration. Concentrations are lowest in the aquifers of the Appalachian 

Plateau, Valley and Ridge, and Coastal Plain subunits, 

and highest in the Great Valley Carbonate subunit. 

312 643 1,123 446 

386 1,493 2,338 643 

164 5,920 6,249 2,413 

80 2,945 3,205 1,185 

34 553 570 167 

2,717 5,268 16,486 3,560 

2,148 4,483 7,764 3,462 

3,205 7,388 7,833 6,241 

9,046 28,693 45,568 18,117 

Wells in the Great Valley Noncarbonate, Blue Ridge, 

Piedmont, and Triassic Lowlands subunits show similar median values and a wide range in nitrate concentrations. These patterns in concentration are consistent 

with nonpoint-source nitrogen inputs to the eight subunits and the hydrologic properties of the aquifer systems within these subunits. 

Dissolved-nitrate concentrations in the Appalachian Plateau and the Valley and Ridge subunits are 

among the lowest in the basin, with median concentrations of 0.1 and 0.14 mg/L, respectively (fig. 22). 

These subunits are predominately covered by forest, 

and agriculture is generally limited to the flat, tillable 

areas in the narrow valleys. Thus, nitrogen sources in 

these subunits are fewer than in other subunits and are 

Analysis of Available Nutrient Data 45 
EXPLANATION 

D U.S. Geological Survey surfacewater-quality monitoring site 

with 50 samples or less 

.A. U.S. Geological Survey surfacewater-quality monitoring site 

with more than 50 samples 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 20. Location of surface-water-quality monitoring sites with nutrient data in the U.S. Geological Survey's WATSTORE 

data base, 1970-90. 

dominated by atmospheric deposition in the forested 

highlands and agricultural inputs in the valleys. 

Eighty-six percent of the sampled wells in the Appalachian Plateau are located in forest settings. The 

median nitrate concentration (0.10 mg/L) in this subunit reflects relatively pristine conditions. In the Valley and Ridge subunit, the median nitrate concentration in ground water in agricultural settings (0.41 

mg/L) is significantly higher (p=0.051) than in forest 

settings (0.11 mg/L) as shown in figure 23. 

Dissolved-nitrate concentrations in ground water 

from the Great Valley Carbonate subunit are generally 

higher (median 4.5 mg/L) than in ground water from 

other subunits and exhibit relatively small differences 

among land-use settings (fig. 23). Nonpoint sources of 

nitrogen in the Great Valley Carbonate subunit are 

dominated by agricultural fertilizer and manure, with 

atmospheric deposition contributing only about 

14 percent (table 4B). Fourteen percent of the wells in 

this subunit have concentrations greater than or equal 

46 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
A. Federal agencies 

EXPLANATION 

L::. U.S. Environmental Protection 

Agency surface-water-quality 

monitoring site with 50 samples or less 

A U.S. Environmental Protection 

Agency surface-water-quality 

monitoring site with more than 

50 samples 

\1 U.S. Forest Service surface-waterquality monitoring site 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KI LOMETERS 

B. State agencies 

• State-agency surface-waterquality monitoring site with 

more than 50 samples 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KI LOMETERS 

Figure 21. Location of surface-water-quality monitoring sites with nutrient data collected by (A) Federal and (B) State agencies in the U.S. Environmental Protection 

Agency's STORET data base, 1970-90. 
p=0.0001 

c 

I]) 

O'l 

g (361) 

(151) 

·c: (70) C/l 

ca 

.... (92) (168) 

~ .... 

I]) (94) 0. 

C/l 

E 

~ 

.~ 

(42) 

.E 

c 

2~ 

~ ·c: (71) 

"C 

I]) 

> 0.10 0 

C/l 

C/l 

0 

c :::::l I]) I]) 2 I]) C/l +-' c ·c;; ca O'l +-' O'l "C c ·co I]) "C ca ca "C c 0 ca c c E c:: .c +-' 0: 0: ca ca 0 0 "§ ~ c:: -e -e I]) "C "C I]) yg '.j:; c c ca ca :::::l 0 0: C/l 

c .~ ca u (..) iD ...J ca w > c (..) 0 ..c > 0 ·c;; u (..) 

ca ~ ~ 

~ z C/l 

Cii ~ > ca 

0. ~ iE 0. +-' 

c:x: ~ ca 

I]) 

t'5 +-' 

ca 

~ 

(.!) 

Subunits 

EXPLANATION 

{70) Number of samples 

! 90th percentile 

p=0.0001 Probability of equal medians 

from Kruskal-Wallis analysis 

75th 

50th (median) 

25th 

lOth 

MCL U.S. Environmental Protection 

Agency Maximum Contaminant 

Level for nitrate, 10 milligrams 

per liter 

Figure 22. Dissolved-nitrate concentrations in ground-water samples from 

Potomac River Basin subunits, 1970-90. 

to the 10-mg/L Maximum Contaminant Level (MCL) 

for drinking water established by the USEPA 

(U.S. Environmental Protection Agency, 1991) 

(fig. 22). The wells included in this analysis are 

located primarily in the northern extent of the subunit 

in West Virginia and Mary land and may be spatially 

biased. However, Goodell and LoCastro ( 1989) 

reported a higher median nitrate concentration of 

8.4 mg!L in water from 290 wells in carbonate rock of 

Clarke and Frederick Counties, Virginia. Ground 

water in carbonate rock moves predominantly through 

solution channels, and water is quickly transmitted 

from the land surface to the water table. Thus, nitrogen-enriched water in this subunit is subjected to rela48 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
.01 c: 

Q) 

C> 

-~ c: 

en 0.10 ctl 

Q) 

.'!::::: 

Q) 

0. 

en 0.01 E 

ctl 

~ .E 100 

.!:: 

a) 

+-' 

~ ·c: 10 "C 

Q) 

> 

0 

en 

en 

0 1.0 

0.10 

0.01 c: 

ctl 

..c 

:; 

Valley and Ridge 

Piedmont 

(95) 

(4) 

I 

"§ +-' 

en 

Q) 

::I 

.±= 0 ::I u.. (..) 

·;:: 

O'l 

<( 

(27) Number of samples 

I 90th percentile 

75th 

50th (median) 

25th 

lOth 

Great Valley Carbonate Blue Ridge 

I I I 

(15) 

-

I I 

Triassic Lowlands Coastal Plain 

(100) (23) (44) 

! 1-

1-

1=-

fc: "§ 1i) ctl Q) -e ::I 

.±= 0 :::::> ::I u.. (..) 

'§, 

<( 

Land use 

EXPLANATION 

p=0.051 Probability of equal medians 

from Kruskal-Wallis analysis 

I 

MCL 

(~7) 

0 

c: 

ctl 

-e 

:::::> 

MCL U.S. Environmental Protection Agency 

Maximum Contaminant Level for nitrate, 

10 milligrams per liter 

I 

(8) 

"§ 

::I 

+-' 

"S 

(..) 

·;:: 

C> 

<( 

p=0.016 

MCL 

(50) 

_j_ 

I 

-: 

: 

-

p=0.515 -

-

: 

: 

-

-

-

-: 

(19) 

[ -= 

+-' 

en 

Q) 

0 

u.. 

Figure 23. Dissolved-nitrate concentrations in ground-water samples from selected land-use settings in selected Potomac 

River Basin subunits, 1970-90. 

tively short periods of time in the soil horizon, the 

most biologically active zone where nutrient uptake 

and cycling rates are highest. There appears to be only 

small differences (p=0.094) in nitrate concentration 

among land-use settings (fig. 23), and median 

concentrations are greater than 3.0 mg/L in urban, 

agricultural, and forest settings. The forests within the 

Great Valley Carbonate subunit tend to be smaller 

buffered zones within agricultural areas. Because of 

the nature of ground-water flow in carbonate aquifers, 

Analysis of Available Nutrient Data 49 
some ground water underlying much of the small forests is recharged through upgradient agricultural land. 

Nitrate concentrations in ground water from the 

·Great Valley Noncarbonate subunit are generally 

lower (median 1.6 mg!L) than from adjacent carbonate 

settings, although nitrogen inputs are similar. Goodell 

and LoCastro ( 1989) indicated that lower median 

nitrate concentrations in the noncarbonate region were 

the result of less agricultural land use. Land use and 

nitrogen inputs, however, are not significantly 

different between the two subunits. The differing 

hydrologic properties of the subunits probably best 

explain the observed differences in ground-water 

nitrate concentrations. The ground-water system in the 

noncarbonate subunit generally responds more slowly 

to recharge, and water travels through smaller secondary openings including fractures, joints, and bedding 

planes in these mostly shale aquifers. Thus, larger 

amounts of nitrogen may be exported from the land 

surface as runoff. Biogeochemical processes may also 

play an important role in reducing nitrate concentrations as ground-water movement is slower in noncarbonate rock than in the solution cavities of carbonate 

rock, allowing longer contact with aquifer minerals for 

weathering. Denitrification, the reduction of nitrate to 

nitrogen gas, is a biologically mediated process that 

occurs in anoxic ground water. Anoxic conditions 

favorable for nitrate reduction may be prevalent in the 

noncarbonate subunit because pyrite oxidation in 

saprolite and shale aquifers can remove oxygen from 

ground water (McFarland, 1989). Most sampled wells 

in the Great Valley Noncarbonate subunit are located 

in agricultural settings, and there are insufficient data 

in urban and forest settings to permit land-use 

comparisons. 

Nitrate concentrations in ground water from crystalline rock of the Blue Ridge subunit are moderate in 

relation to the Potomac River Basin as a whole, with a 

median of 1.0 mg!L. Similar to the Valley and Ridge, 

the Blue Ridge is 78 percent forest (table 1), with agriculture confined to valleys and sidehills. Urban areas 

are interspersed, including local communities, the 

tourist industry, and some suburban communities. 

Nitrate-concentration data show significantly different median concentrations (p=O.O 16) for these landuse settings (fig. 23). Water from wells in forest settings has a median value (0.41 mg!L), which is much 

lower than in urban (2.3 mg!L) and agricultural 

(2.0 mg!L) settings. Nitrate concentrations in ground 

water from only 1 of the 92 wells in the Blue Ridge 

subunit exceeded the MCL. 

Nitrate concentrations in ground-water samples 

from the Piedmont subunit vary greatly, with a range 

of less than 0.01 to 37 mg!L. The subunit-wide variability is largely related to differences in land use 

(fig. 23). Land use in the Piedmont varies spatially, 

with agriculture predominating in the northern and 

western parts and mixed agriculture, forest, and urban 

areas in the southeastern parts. Although only four 

samples were collected from forest settings, a 

Kruskal-Wallis analysis of nitrate concentrations by 

land use indicates that median concentrations in water 

from wells on agricultural land (3.3 mg!L) and in forests (0.18 mg!L) are significantly different (p<0.0001). 

Nitrate concentrations in urban settings, including 

northern Washington, D.C., and its suburbs, vary 

greatly and probably reflect residual nitrate concentrations characteristic of forest and agricultural land used 

for development in addition to urban nonpoint sources 

of nitrogen. None of the wells in forests or urban settings yielded water samples that exceeded the MCL; 

however, 14 percent of the agricultural wells had 

water samples that equaled or exceeded the 10-mg/L 

MCL for nitrate. 

Nitrate concentrations in ground water from the 

Triassic Lowlands subunit (median 1.2 mg!L) are 

slightly lower than from the surrounding Piedmont, 

and differences between agricultural and forest settings are much less pronounced (fig. 23). However, 

median nitrate concentrations for water from wells in 

urban (0.75 mg!L) and forest (0.74 mg!L) settings are 

slightly lower than for wells on agricultural land 

(1.8 mg!L). Less than 4 percent of the wells have 

water with nitrate concentrations exceeding the MCL. 

Dissolved-nitrate concentrations for wells in the 

Coastal Plain subunit are among the lowest of the 

basin subunits, with a median of0.10 mg/L. Unlike 

other Potomac River Basin subunits, ground water in 

the Coastal Plain is withdrawn from confined and 

unconfined aquifers formed from fairly distinct unconsolidated layers of sand and gravel. Water in the 

unconfined aquifers generally is recharged locally, 

whereas water in deeper confined aquifers may originate at distant outcrops or through leakage from adjacent aquifers. The nitrate-concentration data used in 

this analysis are from samples from predominantly 

deeper wells in confined aquifers. Therefore, it is difficult to assess land-use effects from these data because 

water-quality characteristics of confined aquifers may 

50 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
Triassic Lowlands 

agricultural land use 15~~~~~~~~~~~~ 

c: 

Q) 

0') g 10 

·c: 

(/) 

co 

Q5 5 

~ ,_ 

Q) 

a. 

(/) 

E 

~ 

~ .E 

.!:: 45 

a) 

.... 

~ ·c: 30 

"'0 

Q) 

> 

~ 15 (/) 

0 

.· 

250 500 

CC=-31 

p=0.006 

750 1,000 0 

Great Valley Carbonate 

agricultural land use 

200 

. 

. 

400 

CC=-22 

p=0.0003 

600 0 

Triassic Lowlands 

urban land use 

CC=-50 

p=0.001 

250 500 750 1,000 

Great Valley Carbonate 

urban land use 

CC= 51 

p=0.050 

. ~ 

. .. . . . 200 400 600 

Well depth, in feet below land surface 

EXPLANATION 

CC=-31 Spearman rank correlation coefficient 

p=0.006 Probabllity of false correlation 

--Inferred trend line drawn using LOWESS 

smoothing techniques 

Figure 24. Relation of dissolved-nitrate concentration with well depth in selected landuse setting in selected Potomac River Basin subunits. 

not be affected by the overlying land use. Median 

nitrate concentrations in water from all land-use settings are low, and water from agricultural wells shows 

the widest range of concentrations. Agricultural chemicals have been shown to have substantial effect on 

water quality in the surficial aquifers of the Coastal 

Plain of the Delmarva Peninsula of Maryland, Delaware, and Virginia (Hamilton and others, 1993). The 

hydrogeologic settings and agricultural practices of 

the Delmarva Peninsula are similar to the Coastal 

Plain subunit of the Potomac River Basin where the 

surficial aquifer may show similar degradation. Additional sampling from unconfined aquifers may be necessary to assess shallow ground-water quality on the 

Coastal Plain. None of the samples from Coastal Plain 

wells exceeded the 10-mg/L MCL for nitrate. 

In the Potomac River Basin, nitrogen inputs have 

increased for hundreds of years since colonization; 

dissolved-nitrate concentrations in deeper, older 

ground water generally are expected to be less than in 

younger, more shallow ground water. A nonparametric Spearman rank correlation (Helsel and Hirsch, 

1992) was used to compare dissolved-nitrate concentration with depth of wells in urban, agricultural, and 

forest, land uses of the eight subunits. The two variables were poorly correlated in 20 of the 24 possible 

cases. The poor correlation may be due to insufficient 

numbers of samples or to poor sample distributions 

with respect to well depth. Most ground-water wells 

in the study unit are constructed with a short surface 

casing and a long interval open to water-bearing 

fracture zones. Thus, well depth may not be directly 

related to the depth of the ground water sampled. In 

four cases, a significant concentration-depth relation 

was detected, and a smoothed estimate of these relations is shown in figure 24 using the LOWESS 

smoothing technique. 

Within the agricultural setting of the Great Valley 

Carbonate subunit, nitrate concentrations decrease 

with depth; however, concentrations greater than the 

Analysis of Available Nutrient Data 51 
10-mg/L MCL occur in wells deeper than 300ft. In 

forest settings, nitrate concentrations increase slightly 

as wells deepen. It is possible that deeper wells in forest settings may intersect longer ground-water flow 

paths conveying water from agricultural settings 

where nitrogen inputs are high. In the Triassic Lowlands subunit, nitrate concentrations in water from 

both urban and agricultural wells decrease as wells 

deepen, and concentrations are less than 5.0 mg!L in 

water from wells deeper than 250 ft. Lower nitrate 

concentrations at depth in this subunit may be due to 

both the age of the ground water and biochemical 

denitrification under anoxic conditions in deeper 

ground water. 

Dissolved-ammonia and dissolved-orthophosphate concentrations in ground water are generally 

less than or near the analytical reporting limits. 

Because of the high number of less-than values for 

ammonia and orthophosphate, only general observations of their spatial patterns in ground water can be 

made for the Potomac River Basin. Ammonia concentrations range from less than 0.01 to 7.7 mg/L in water 

from 345 wells. The median ammonia concentration is 

0.02 mg!L, and 36 percent of the samples are less than 

the reporting limit for ammonia. Dissolved-orthophosphate concentrations range from less than 0.01 to 

4.3 mg/L in water from 460 wells, and 56 percent are 

less than the reporting limit. Ammonia and orthophosphate are generally more chemically reactive than 

nitrate, and the low detection frequency is not surprising. Under most natural conditions, ammonia is biologically converted to more stable forms of nitrogen, 

and orthophosphate is used by plants and sorbs to soil 

and aquifer particles. 

Concentrations in Surface Water 

Patterns in nutrient concentrations in surface water 

of the Potomac River Basin are complex due to differences in physiography, hydrology, land use, and nutrient inputs. Because of this complexity, the analysis of 

nutrients in surface water focuses only on data from 

25 surface-water-quality monitoring sites that encompass a wide range of conditions found within the 

basin, the same sites used to assess patterns in nutrient 

inputs (table 6, fig. 16). These 25 sites were selected 

from the sites with available concentration data in the 

WATSTORE and STORET data bases. The location of 

these sites is shown in figure 16, and a summary of 

land use and physiography of the contributing watersheds is shown in table 6 along with the name of the 

data-collecting agency. In this assessment of nutrient 

concentrations in surface water, the Great Valley is not 

subdivided into carbonate and noncarbonate subunits. 

The 25 sites drain relatively large watersheds, and 

where they intersect the Great Valley, the watersheds 

contain significant portions of both subunits. None of 

the sites used were within the Coastal Plain subunit. 

Six sites (7, 8, 9, 11, 14, and 21) (table 6) have data 

collected by both USGS and State or local programs. 

Because of differences in sampling and chemicalanalysis methods, the data from these sites are 

interpreted separately, with USGS data designated as 

"A" and non-USGS data as "B". Most patterns in 

USGS and non-USGS data are similar, and differences 

are highlighted only where they exist. 

Nutrient concentrations in surface water can 

respond dramatically to changing environmental conditions and show a wide range of spatial and temporal 

variability. Large-scale spatial patterns in nitrogen and 

phosphorus concentrations presented for the 25 sites 

are related to predominant physiographic and land-use 

settings. The temporal variability of nutrient concentrations is demonstrated for key sites and contrasted 

among different site types. Long-term trends in nutrient concentrations are inferred at key sites using existing concentration data in relation to known changes in 

land-use and management practices. The mass movement of nutrients in streams throughout the basin is 

evaluated by comparing calculated nutrient loads for 

selected tributaries and by comparing these loads with 

calculated inputs. Also, relative contributions of pointsource inputs, base-flow nutrient loads, and average 

annual nutrient loads in selected streams are 

compared. 

Spatial Variability 

In surface water of the Potomac River Basin, 

nutrient concentrations mirror the spatial patterns in 

ground water with concentrations related to nutrientinput rates. The patterns in surface water are more 

complex, however, because larger streams drain multiple subunits and more diverse land uses. Also, pointsource inputs of nutrients can have both localized and 

widespread effects on concentrations downstream. In 

general, nitrogen and phosphorus concentrations are 

lowest in the sparsely populated, forested regions of 

the Appalachian Plateau, Valley and Ridge, and Blue 

Ridge subunits and are highest in the agricultural 

region of the Great Valley, Piedmont, and Triassic 

52 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
Lowlands, and near urban centers where wastewatertreatment inputs are greater. 

Nitrogen concentrations differ greatly among 

streams in the basin. Median concentrations of total 

nitrogen range from 0.42 to 3.9 mg!L at 22 sites, and 

median dissolved nitrate ranges from 0.20 to 3.5 mg!L 

at 25 sites (fig. 25). The total-nitrogen analysis 

accounts for particulate and dissolved forms of nitrogen including nitrite, nitrate, ammonia, and organic 

nitrogen. The dissolved-nitrate analysis measures the 

species of nitrogen most predominant and available to 

affect aquatic life. Dissolved-nitrate concentrations are 

generally more temporally stable than total-nitrogen 

concentrations and are less affected by differences in 

sampling methods. Thus, dissolved-nitrate concentrations are often more suitable for site-to-site comparisons. Dissolved-ammonia concentrations are generally 

near the detection level, and no strong spatial patterns 

are evident as fewer sites have ammonia-concentration 

data. The statistical distribution of total-nitrogen and 

dissolved-nitrate concentrations has similar spatial 

patterns for both species. The sites are shown in downstream order in figure 25, and the sites are grouped 

according to the subunits in which the sites are 

located. Sites on larger rivers (sites 8, 15, 19, and 23) 

have watersheds that drain parts of several subunits. 

Site 12 is located in the Great Valley yet drains the 

Valley and Ridge subunit. 

Median total-nitrogen and dissolved-nitrate concentrations are lowest at sites where nitrogen inputs 

are smaller and dominated by atmospheric deposition, 

including sites in the Appalachian Plateau, Valley and 

Ridge, and Blue Ridge subunits. These watersheds 

(sites 1--6, 12, and 18) drain predominantly forest land 

use and areas with less population density. Median dissolved-nitrate concentrations are lowest at sites 4 

(0.3 mg/L), 5 (0.4 mg!L), and 6 (0.2 mg!L), reflecting 

conditions most unaffected by wastewater inputs or 

human land-use practices. Sites 1 and 2 on the North 

Branch Potomac River are affected by mining and 

industrial discharges and have slightly higher median 

nitrate concentrations of 0.56 and 0.53 mg/L, respectively. Nitrate concentrations at site 18 on Hunting 

Creek tributary in a forested part of the Blue Ridge 

subunit show a similar median of 0.58 mg!L. Sites 22 

and 24, however, drain urbanized watersheds of Difficult Run and Cameron Run in Virginia, yet have 

nitrate concentrations similar to forest watersheds. 

Median nitrate concentrations at these sites of 0.70 

and 0.50 mg!L also are dominated by atmospheric 

deposition as a nitrogen source in addition to urban 

nonpoint-source runoff. 

Higher nitrogen concentrations occur in the 

largely agricultural watersheds of the Great Valley, 

Piedmont, and Triassic Lowlands; however, strong 

geographic patterns exist (fig. 25). In the Great Valley, 

there appears to be a difference in nitrogen concentration between northern and southern tributaries. Conococheague Creek (site 7) and Antietam Creek (site 9) 

have the highest median total-nitrogen concentrations 

of all sites at 3.1 and 3.9 mg/L, respectively, and drain 

the northern part of the Great Valley including parts of 

Pennsylvania and Maryland. Christians Creek and 

Smith Creek, sites 10 and 13, drain watersheds with a 

similar percentage of agriculture in the southern part 

of the Great Valley of Virginia and have lower median 

total-nitrogen concentrations of 2.1 and 2.3 mg!L. The 

remainder of the sites in the Great Valley encompass 

significant drainage from forests of the Valley and 

Ridge and Blue Ridge subunits and have median totalnitrogen and dissolved-nitrate concentrations slightly 

lower than the sites with a higher percentage of agriculturalland use. 

Monitoring sites in the agricultural areas of the 

Piedmont and Triassic Lowlands also have total-nitrogen and dissolved-nitrate concentrations greater than 

sites in forest settings. Median nitrogen concentrations 

in these areas differ greatly due to differences in landuse practices and point sources of nitrogen. The 

median dissolved-nitrate concentration of 1.3 mg!L in 

water from the Monocacy River at Bridgeport, Md. 

(site 17), is lower than in water from sites with similar 

agricultural intensity in the Great Valley. Downstream 

on the Monocacy River near Frederick, Md. (site 19), 

a higher median concentration of 2.1 mg!L is likely 

due to the effect of wastewater inputs, increased agricultural intensity, and drainage from both carbonate 

and crystalline rock. Nitrogen concentrations are similar at Seneca Creek (site 21), which drains a diverse 

watershed in the Piedmont including 65 percent agriculture, 24 percent forest, and 9 percent urban area, 

with significant nitrogen inputs from wastewater discharges in its headwaters. Other agricultural watersheds in the Piedmont and Triassic Lowlands, 

including Catoctin Creek (site 16) and Goose Creek 

(site 20), have water with lower median concentrations. 

Few monitoring programs routinely sample for 

dissolved-ammonia concentrations; thus, there are 

insufficient data to assess spatial patterns. Eight sites 

Analysis of Available Nutrient Data 53 
Site no. Stream name 

(fig. 16) 

0.10 

1 North Branch Potomac River 

2 North Branch Potomac River 

3 South Branch Potomac River 

4 Town Creek 

5 Lost Creek 

6 Cacapon River 

7 A Conococheague Creek 

7 8 Conococheague Creek 

8A Potomac River 

88 Potomac River 

9A Antietam Creek 

98 Antietam Creek 

10 Christians Creek 

11A South Fork Shenandoah River 

11 8 South Fork Shenandoah River 

12 North Fork Shenandoah River 

13 Smith Creek 

14A North Fork Shenandoah River 

148 North Fork Shenandoah River 

15 Shenandoah River 

16 Catoctin Creek 

17 Monocacy River 

18 Hunting Creek tributary 

19 Monocacy River 

20 Goose Creek 

21A Seneca Creek 

218 Seneca Creek 

22 Difficult Run 

23 Potomac River 

24 Cameron Run 

25 Bull Run 

44 

A. Total nitrogen, 

in milligrams per liter 

1.0 

Appalachian Plateau 

subunit 

Valley and Ridge 

subunit 

93~ 

Great Valley 

subunit 

35 1---{I]-1 

131 1--D:J-i 

111 1---CI:=J-; 

36 1---C[] 

107 1--CIJi 

164 1---CIJ--1 

150 1----C:I:J----1 

106 1---C:D; 

163 1-------C::I:J260 J--C:I:J--1 

59 1---CD---; 

157 

Triassic 

Piedmont, 

BlueRidge, 

Lowlands 

and 

~ 

121 subunits 1-[]]--1 

131 1--------c=c::J 

39(1) f----CI::J--1 

187 1--CI:]-; 

293 1----CI:J---1 

125 1------C:I::J--

60 ~ 

B. Dissolved nitrate, 

in milligrams per liter as nitrogen 

10 0.01 0.10 1.0 

246 

134 

213(10) I ~/ley and Ridge 

145(1) ~ subunit 

94(6) 

64~ 

87 

58 

132 1--ITJ-1 

136(1) 1---CI]--; 

88 I{[J-l 

60 1-------[l}f 

109 Hil-l 

282(1) 1--[I]-1 

168(3) 1---CIJ--1 

152(1) 1----CJ:J-i 

107 1-------D:H 

256(7) 1------CIJ-1 

165(9) 1-------C::IJ-1 

261(4) 1----CJ]-; 

60 f----CIJ--1 

220 

163(2) 

Blue Ridge, 

~ 

1----CIJ-1 

Piedmont, and 

181 Trias;~b~~"ft~ands 1-CIH 

131 (4) 1------C:::I:J82 

152(1) 

189(1) 

325(2) 

127(4) 

60(2) 

10 

I I I I II II I I I II II II 

EXPLANATION 

Percentiles 

(median) 

1Oth 25th 50th 75th 90th 

f-------C=::r:: 

Incomplete box diagrams indicate 

grouping of sample data 

94 Number of samples 

(6) Number of samples reported as less than the detection level 

Sites are shown in downstream order 

and are grouped by physiographic 

province (fig. 4) of the site location. 

"A" following the site number indicates 

U.S. Geological Survey data; "B" indicates 

non-U.S. Geological Survey data 

Figure 25. (A) Total-nitrogen and (B) dissolved-nitrate concentrations in water samples from selected surface-watermonitoring sites in the Potomac River Basin, 1970-90. 

54 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197o-90 
(1, 4, 9, 16, 18, 19, 22, and 24) had more than 10 samples with dissolved-ammonia analyses. Median 

ammonia concentrations at these sites range from 0.02 

to 0.20 mg/L. Only two of these sites had median concentrations greater than 0.06 mg/L. The highest 

median concentration (0.20 mg/L) occurred in water 

from North Branch Potomac River at Kitzmiller, Md. 

(site 1). The elevated ammonia concentrations at this 

site may be due to acid-mine drainage, which occurs 

in many parts of this watershed. The river at this site 

lacks a healthy algal and macrophyte population, 

which in healthy streams would consume the available 

ammonia. The second highest ammonia concentration 

(0.11 mg/L) occurred in water from the Monocacy 

River at Bridgeport, Md. (site 17), which drains an 

agricultural part of the Triassic Low lands subunit. 

The spatial patterns in total-phosphorus and dissolved-orthophosphate concentrations are largely 

related to discharges from wastewater-treatment facilities and runoff from agricultural land. Figure 26 shows 

the distribution of total-phosphorus and dissolvedorthophosphate concentrations for samples from the 

25 surface-water-quality monitoring sites. Lower concentrations occur for both measurements in the forest 

areas of the Appalachian Plateau, Valley and Ridge, 

and Blue Ridge, and higher concentrations occur in 

water from streams in the Great Valley, Piedmont, and 

Triassic Lowlands subunits. Total-phosphorus concentrations can increase substantially during high-flow 

conditions when phosphorus bound in soil and organic 

matter is transported. The median total-phosphorus 

concentration is highest in water from Antietam Creek 

(site 9) at 0.30 mg/L. In water from Christians Creek 

(site 10), Monocacy River (sites 17 and 19), and Bull 

Run (site 25), median total-phosphorus concentrations 

are about 0.20 mg/L. Total-phosphorus concentrations 

at Bull Run are strongly affected by upstream wastewater discharges; the sampling period at this site was 

prior to relocation of a municipal wastewater-treatment facility to a site farther downstream. Each of the 

other three sites encompasses significant agricultural 

drainage and also is affected by wastewater effluent. It 

is difficult to separate the effects of agriculture and 

wastewater at these sites. The lowest median totalphosphorus concentrations occur in water from sites 1 

and 2 on the North Branch Potomac River and in water 

from sites 3, 4, and 6 in the Valley and Ridge subunit. 

In water from the North Branch, median concentrations increase from 0.04 mg/L at Kitzmiller (site 1) to 

0.06 mg/L at Cumberland (site 2), Md. Presumably, 

industrial and municipal discharges near Cumberland 

cause this increase. Median total-phosphorus concentrations range from 0.04 to 0.16 mg/L in water from 

other sites in the Piedmont and Triassic Lowlands, 

including Goose Creek and Seneca Creek where phosphorus inputs to the watersheds are lower. Also, 

median total-phosphorus concentrations are low in 

water from the urban watersheds of Difficult Creek 

and Cameron Run because nonpoint-source inputs in 

urban areas are small. 

Median concentrations of dissolved orthophosphate are higher in streams draining agricultural 

watersheds than in those draining urban or forest 

watersheds. Dissolved-orthophosphate data are available for only 17 of the 25 sites, and concentrations frequently are reported as less than the analytical 

detection level of 0.01 mg/L (fig. 26). Median dissolved-orthophosphate concentrations range from 0.01 

to 0.14 mg/L. Median concentrations were highest in 

water from Christians Creek (site 10), Shenandoah 

River (site 11), and Monocacy River (site 17). All 

three of these sites have substantial agricultural land 

use and wastewater phosphorus sources. Median concentrations in water from sites integrating large drainage areas (sites 8, 15, and 23) are moderate and appear 

to buffer the high-concentration agricultural water 

with more dilute water from forest watersheds. 

Temporal Variability 

Temporal variations in nitrogen and phosphorus 

concentrations in surface water are affected substantially by changes in streamflow conditions and seasonal cycles. In the streams of the Potomac River 

Basin, temporal patterns often differ from site to site, 

yet are similar within subunits and among land-use 

settings. 

Streamflow Relations 

Several different patterns in nitrogen concentration are evident in relation to streamflow conditions at 

the 25 monitoring sites and are shown for 8 key sites 

in figure 27. At all sites where both dissolved-nitrate 

and total-nitrogen concentration data exist, the concentration-streamflow relations are similar; however, 

total-nitrogen concentrations frequently are higher at 

higher streamflows. The relation between streamflow 

and concentration is inferred using the LOWESS 

smoothing technique in figures 27 and 28 using all 

available data for each site. High-streamflow conditions occur infrequently and are characterized by low 

Analysis of Available Nutrient Data 55 
0.001 

Site no. Stream name 

(fig. 16) 

1 North Branch Potomac River 

2 North Branch Potomac River 

3 South Branch Potomac River 

247(37) 

72 

178(1) 

144(29) 

A. Total phosphorus, 

in milligrams per liter 

0.01 0.10 1.0 

r:::::I::J----1 

t--D:J----1 

~ 

~ 

Valley and Ridge 

4 Town Creek subunit 

5 Lost Creek 

6 Cacapon River 

7 A Conococheague Creek 

7 B Conococheague Creek 

8A Potomac River 

88 Potomac River 

9A Antietam Creek 

93( 1 ) 1---CIJ---i 

98 Antietam Creek 

10 Christians Creek 

11A South Fork Shenandoah River 

11 B South Fork Shenandoah River 

12 North Fork Shenandoah River 

13 Smith Creek 

14A North Fork Shenandoah River 

148 North Fork Shenandoah River 

15 Shenandoah River 

16 Catoctin Creek 

17 Monocacy River 

18 Hunting Creek tributary 

19 Monocacy River 

20 Goose Creek 

21A Seneca Creek 

21 B Seneca Creek 

22 Difficult Run 

23 Potomac River 

24 Cameron Run 

25 Bull Run 

64(2) 

132 

139(5) 

43 

66(2) 

106(17) 

185(21) 

152(126) 

105(40) 

34 I 

161(76) 

262 

66 

173 

141 

131(79) 

34 

145(8) 

187(116) 

340(2) 

126(88) 

60(5) 

~ 

, r--r--1_1 Blue Ridge, rL-J,.......C"" Piedmont, and 

Triassic Lowlands 

subunits 

B. Dissolved orthophosphate, 

in milligrams per liter as phosphorus 

10 0.1 1.0 10 

132(91) 

~l--------l13(9) 

1-1-----------175(59) 

H-------1 60(20) 

Appalachian Plateau 

subunit 

Valley and Ridge 

subunit 

Great Valley 

subunit 

1--C::=r::J---- 1 08(1) 

~136(2) 

~109(3) 

H1-___ ....~.1 H 35(1) 

Blue Ridge, 

Piedmont, and 

Triassic Lowlands 

subunits 

k:=::::I=I-------1 41(11) 

11------r-----, 115(30) 

11------r-----, 77( 16) 

EXPLANATION 

Percentiles 

(median) 

1Oth 

~ 

25th 50th 75th 90th 

Incomplete box diagrams indicate 

grouping of sample data 

64 Number of samples 

(2) Number of samples reported as less than the detection level 

Sites are shown in downstream order 

and are grouped by physiographic 

province (fig. 4) of the site location. 

"A" following the site number indicates 

U.S. Geological Survey data; "B" indicates 

non-U.S. Geological Survey data 

Figure 26. (A) Total-phosphorus and (B) dissolved-orthophosphate concentrations in water samples from selected 

surface-water-quality monitoring sites in the Potomac River Basin, 1970-90. 

56 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1976-90 
5~--~--~--~----~--~--~--~--~---~~--~ 

*Dissolved nitrate shown, in 

milligrams per liter as nitrogen I 

§ 

~I 

4 

Inferred relation drawn using Q;;J::,j 

LOWESS smoothing technique !J ~ 

~ 

... 

CD 

c. 

(/) 

\_\_ \ ______ Site 7A, Conococheague Creek '7 '~--------- ---------------------------... ,;(, 

E 3 co 

. 9> 

.E 

', Site 19, Monocacy River -----/< ' ' ' ............. ----- ' Site 73 s . / -~------------~~th£~e~ 

------ -- -~ ~-- ", / -----..................... ·= r::.~ 

-........ ~/ ', ........ ~ ' 

CD e 2 

~~~---- Site 17 ' -----..t.__Mon ', +-' 

·c: 

Site 23 p -:.:~cv Ft ' , otorn """<..... Iver ', ac River ' " .......... ' ':-... .......... 

co 

;§ 

.................... 

INCREASING STREAMFLOW 

0 L-------~------~------~------~------~---------L-------~------~------~------~ 

0 10 20 30 40 50 60 70 80 90 100 

Mean daily streamflow, as a percentage of occurrence 

Figure 27. Relation of total-nitrogen concentrations to stream discharge at selected surface-waterquality monitoring sites in the Potomac River Basin. Location of sites is shown in figure 16. 

percentages of occurrence. Thus, in figures 27 and 28, 

higher streamflows are drawn to the left and lower 

streamflows to the right side of the horizontal axis. 

Streams draining forest watersheds show a characteristic relation of total-nitrogen concentration to 

streamflow. In water from forest watersheds, nitrogen 

concentrations usually are less than 1.0 mg/L and 

increase only slightly during higher flow conditions as 

shown for total nitrogen at site 3 and for dissolved 

nitrate at site 18 in figure 27. 

Where total-nitrogen concentrations are maintained by wastewater-treatment-plant discharges, as at 

site 25, dilution causes a significant decrease in nitrogen concentration with increasing streamflow. Totalnitrogen concentrations in water from site 25 are 

greater than 3.0 mg/L during low-flow conditions and 

decline to less than 1.5 mg/L during high-flow 

conditions (fig. 27). 

Streams draining agricultural watersheds often 

differ in their response to changing streamflow conditions due to differences in the hydrologic properties of 

the watersheds. Site 17 on the Monocacy River shows 

a concentration-streamflow relation typical of agricultural systems in the Triassic Lowlands subunit. During 

low-flow conditions, concentrations are low, near 

1.0 mg/L, and increase dramatically during higher 

streamflows. The effect of storm runoff of fertilizers 

and animal manure from agricultural areas on totalnitrogen concentrations is substantial at this site. 

Nitrogen concentrations in agricultural settings of 

the Great Valley subunits, however, show a very different relation. At sites 7 and 13, nitrogen concentrations increase only slightly with increased streamflow 

and maintain higher concentrations, 2.0 to 3.0 mg/L, 

throughout all but the lowest streamflow conditions. 

Ground-water discharge, which maintains base flow in 

these carbonate streams, maintains nitrogen concentrations that are higher than at most other sites. Also, 

storm runoff appears to have minimal effect on nitrogen concentration. In the Great Valley, where carbonate rock is predominant, much of the streamflow is 

generated through ground-water discharge, with a relatively small amount from overland and agricultural 

runoff. Extreme low-flow conditions in this region, 

Analysis of Available Nutrient Data 57 
Inferred relation drawn using 

LOWESS smoothing technique 

....... ....... 

Site 3, South Branch Potomac River 

INCREASING STREAMFLOW 

0.01 

0 10 20 30 40 50 60 70 80 90 100 

Mean daily streamflow, as a percentage of occurrence 

Figure 28. Relation of total-phosphorus concentrations to stream discharge at selected surface-waterquality monitoring sites in the Potomac River Basin. Location of sites shown in figure 16. 

which usually occur during late summer, may allow 

in-stream biological processes to slightly reduce instream nitrogen concentrations. 

Total-nitrogen concentrations in water from the 

Monocacy River near Frederick, Md. (site 19), appear 

stable with respect to streamflow. This relation, however, is likely the result of two distinct processes. During stable flow conditions, nitrogen concentrations 

represent the combined effect of ground-water discharge from agricultural areas and wastewater discharge from the City of Frederick, upstream of the 

monitoring site. As streamflow increases, nitrogen 

from wastewater discharges is probably diluted substantially, as at site 25, and agricultural runoff 

increases as seen at site 17. The combined effect of 

these processes likely gives the appearance of stable 

nitrogen concentrations. 

Nitrogen concentrations in water from the Potomac River at Washington, D.C. (site 23), show the 

combined effect of all these patterns and resemble the 

runoff-generated model of nutrient concentrations and 

streamflow most closely. Because the patterns seen in 

contributing watersheds are diverse, it is apparent that 

the dynamics of nutrient concentrations at this site are 

far more complex than the simple relation shows. 

Each of the concentration-streamflow relations discussed previously is typical of large parts of the entire 

Potomac River watershed. Thus, concentrations at 

site 23 are a function of the streamflow relations in all 

physiographic and land-use settings. During low-flow 

conditions, concentrations of nitrogen are small 

because forests, where nitrate concentrations are 

small, cover 55 percent of the upper Potomac River 

Basin's land area. Also, biological uptake of nitrogen 

in the wide, shallow Potomac River is most significant 

during low-flow conditions. During higher flow conditions, however, nitrogen in agricultural runoff from the 

Great Valley, Piedmont, and Triassic Lowlands elevates concentrations significantly. 

Total-phosphorus concentrations also differ 

greatly in response to streamflow in the basin, with the 

most significant differences in patterns occurring 

between forest watersheds and watersheds affected by 

agriculture and wastewater discharges. Because phos58 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
phorus is strongly sorbed to soil particles, concentrations are expected to increase with increasing 

streamflow and suspended-sediment load. This 

expected pattern occurs in the Potomac River at Washington, D.C. (site 23), where concentrations increase 

from about 0.07 to about 2.5 mg/L during highstreamflow conditions, which occur less than 30 percent of the time (fig. 28). Total-phosphorus concentrations in tributaries of the Potomac River, however, 

respond quite differently to changing streamflow conditions. Total-phosphorus concentrations in water 

from forest watersheds of the Appalachian Plateau and 

Valley and Ridge subunits are usually less than 

0.05 mg/L and increase only slightly with increasing 

streamflow. These watersheds encompass a large part 

of the upper Potomac River Basin. An example of 

these sites in shown in figure 28 using data from the 

South Branch Potomac River (site 3). In watersheds 

affected by both agriculture and point-source discharges (sites 1B, 15, 16, 17, and 19), dilution appears 

to control phosphorus concentrations during stable 

streamflow conditions. At these sites, concentrations 

are typically greater than 0.10 mg/L during extreme 

low-flow conditions and decrease slightly with 

increasing streamflow (fig. 28). As streamflow 

increases to durations of less than 20 percent at 

sites 15, 17, and 19, total-phosphorus concentrations 

increase due to runoff from agricultural land. At Bull 

Run (site 25), total-phosphorus concentrations are 

contributed primarily from wastewater discharges and 

are diluted as streamflow increases. 

Seasonal Patterns 

Seasonal fluctuations of nitrogen concentrations in 

surface water are generally small at most sites in the 

Potomac River Basin. Seasonal fluctuations in nutrient 

concentrations are strongly related to seasonal fluctuations in streamflow conditions. Boxplots of seasonal 

nitrogen concentrations are shown in figure 29 for the 

same key sites shown in figure 27. Median nitrogen 

concentrations in water from forest watersheds, sites 3 

and 18, are highest in the winter and lowest during the 

summer months, and overall variability is low. Totalnitrogen concentrations in agricultural settings, 

sites 1B, 13, and 17, also are highest in the winter 

months although fertilizer-application rates are highest 

in the spring and summer. Seasonal patterns at sites 19 

and 25, which receive nitrogen from municipal wastewater discharges, are quite different. In water from the 

Monocacy River near Frederick (site 19), 

concentrations appear stable year-round. At Bull Run 

(site 25), where municipal discharges are the only 

major source of nitrogen, dilution causes lower nitrogen concentrations during the high flows of winter. 

Seasonal fluctuations in nitrogen concentration in 

water from the Potomac River at Washington, D.C. 

(site 23), primarily reflect the patterns seen for agricultural and forest settings, with higher concentrations 

occurring in winter and lower concentrations during 

summer. 

Seasonal fluctuations of total-phosphorus concentrations are nearly opposite those of total-nitrogen 

concentrations (fig. 30). At nearly all sites, the highest 

total-phosphorus concentrations occur during the summer months when dilution is minimal. Seasonal patterns in water from the Potomac River at Washington, 

D.C. (site 23), however, are opposite of patterns in 

water from sites in the contributing watersheds and the 

overall variability is greater. The reason for the difference in seasonal patterns is unknown; however, the 

distribution of samples with respect to both streamflow and season may differ from the other monitoring 

sites. It is possible that larger numbers of high-flow 

and storm-runoff samples during winter months may 

cause higher median concentrations to occur at this 

site. 

Long-Term Trends 

Few of the 25 monitoring sites on the Potomac 

River and its tributaries show strong trends in totalnitrogen concentrations, although improvements in 

wastewater-treatment facilities and agricultural-management practices within the last 20 years would suggest that some decline should have occurred at many 

sites. However, population in the basin grew about 

43 percent during 1970--90, which may affect concentrations due to increased wastewater effluent, septic 

systems, and other land-use changes. An inferred 

trend of flow-adjusted nitrogen concentrations is 

shown in figure 31 for selected sites with long-term 

data collection. To adjust for changes in streamflow, 

the residual of a LOWESS concentration-streamflow 

regression is subtracted from each measured concentration. Previous work by Trombley ( 1989) and Blomquist (1993) applied the nonparametric seasonalKendall trend test to similar data sets for several of the 

sites included in this report. The results of both studies 

are consistent with the visually inferred trends shown 

in this report. Trombley (1989) reported a trend at only 

one of 25 sites, a slight increase in total-nitrogen conAnalysis of Available Nutrient Data 59 
Site 3, South Branch Potomac River 

at Springfield, WV 

Site 7 8, Conococheague Creek 

at Fairview, MD 

Site 13, Smith Creek 

near New Market, VA 6r-~----~----~----~~ I 

5-

4-

3-

2-

- -

- -

- ~ ! -

- -

f- -

f- -

~ -

- -

1 - ~ - - f- -

..... . ~ 0 .___..J....._ ____ _.__ ____ __._ ____ ___.____. 

(i; 

a. Site 17, Monocacy River 0 at Bridgeport, MD § 6r-~----~----~----~~ 

Site 18, Hunting Creek tributary 

near Foxville, MD 

Site 19, Monocacy River 

near Frederick, MD 

I I I 

.~ 51- - - - - - .E 

i 

~ ~ 

:: 

2 fi t.) 

c 8 1 fc 

-

-

~0.___......._ ____ __.__ ____ ___._ ____ ____._____, 

e 

+-' 

-

-

-

-

- -

- - ~ 

- - -

- -

Spring Summer Fall Winter 

·~ Site 23, Potomac River 

co at Washington, D.C. ~ 6r-~----~-------..--------r----. 

Site 25, Bull Run, 

near Manassas, VA 

5f4f3 f- I 

2 f- ~ 

1 f-

-

-

! -

0'----......._ ____ _.__ ____ __._ ____ ___._____, 

- -

-

-

~ : 

EXPLANATION 

! 90th percentile 

75th 

50th (median) 

25th 

lOth 

Spring Summer Fall Winter Spring Summer Fall Winter 

Figure 29. Seasonal total-nitrogen concentrations in water from selected surface-water-quality monitoring sites in the 

Potomac River Basin, 1970-90. Location of sites shown in figure 16. 

centrations in water from the South Fork Shenandoah 

River (site 11). Flow-adjusted concentrations in 

figure 31, however, appear stable throughout the 

period at site 11. Blomquist (1993) reported no significant trend in total-nitrogen concentrations at sites 8, 

15, and 23, the three long-term monitoring stations 

operated by the USGS NASQAN program. 

None of the 25 monitoring sites with sufficient 

data to infer trends showed a large decline in totalnitrogen or dissolved-nitrate concentrations for 

1970-90. Dissolved-nitrate concentrations increased 

at two sites during 1975-80. Site 7A on Conococheague Creek drains a watershed in the Great Valley 

where agricultural inputs of nitrogen predominate. 

Increasing nitrate concentrations in water from site 21 

on Seneca Creek may result from increased wastewater discharge as the watershed has undergone urban 

development during this period. The Conococheague 

Creek watershed has experienced less development 

than Seneca Creek, and increasing nitrate concentrations may be related to changing agricultural practices. 

Specifically, increased fertilization and manure-application rates designed to increase crop yield can 

increase nitrogen concentrations in streams. 

No agricultural watershed showed a declining 

trend for total nitrogen, dissolved nitrate, or dissolved 

ammonia in surface water. Agricultural nutrient bestmanagement practices (BMP's) have been imple60 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
Site 3, South Branch Potomac River 

at Springfield, WV 

Site 7 A, Conococheague Creek 

at Fairview, MD 

Site 15, Shenandoah River 

at Millville, WV 

I I 

~ 0.10 _ ~ I 

=: 0.01 

I 

- l - -ll 

(1) 

c. 

Ul 

E 

~ 0 

Site 16, Catoctin Creek 

near Middletown, MD 

Site 17, Monocacy River 

at Bridgeport, MD 

Site 19, Monocacy River 

near Frederick, MD ~1 . .----~----r------.------,------, 

.E 

c 

c 1 0.10 r ~ c 

0 

CJ 

Ul 

:J 

~ 0.01 L---...1..-----'-------L------L----1 

c. 

Ul 

~ -

Spring Summer Fall Winter 

0 

..r::. Site 23, Potomac River 

'?- at Washington, D.C. 

co 1.0 ~~----.-------r------r----:~ 

Site 25, Bull Run 

near Manassas, VA 

~ 

0.10 

EXPLANATION 

! 90th percentile 

75th 

50th (median) 

25th 

lOth 

Spring Summer Fall Winter Spring Summer Fall Winter 

Figure 30. Seasonal total-phosphorus concentrations in water from selected surface-water-quality monitoring sites in the 

Potomac River Basin, 1970-90. Location of sites shown in figure 16. 

mented at numerous farms throughout the Potomac 

River Basin through cooperative efforts of the Natural 

Resources Conservation Service, Agricultural Stabilization and Conservation Service, and State and county 

agricultural extension agents. These efforts typically 

are designed to minimize runoff of nitrogen, phosphorus, and sediment by use of conservation-tillage practices and improved manure-management procedures. 

In some cases, nitrogen concentrations may be changing but at an imperceptible rate as many monitoring 

programs may have insufficient numbers and types of 

samples to detect slight changes in nutrient concentrations. In settings such as the carbonate regions of the 

Great Valley subunit, chemical nitrogen reduction in 

ground water may be minimal, and BMP's may cause 

long-term increases in nitrate concentrations in ground 

water. This may result in a long-term increase in 

nitrate concentrations in surface water during baseflow conditions. 

Total-phosphorus and dissolved-orthophosphate 

concentrations in surface water appear to decline in 

many watersheds for 1970-90 (fig. 32). The downward trend is most apparent at sites with wastewater 

discharged upstream. These declines are likely due to 

improvements in wastewater-treatment facilities, 

implementation of agricultural BMP's, and the ban of 

phosphate-based detergents first enacted by the State 

of Maryland in 1973 and later enacted in Pennsylvania 

Analysis of Available Nutrient Data 61 
5.--,---.--,---.-~,--.--~--.---~-r---.--,---.-~---.--~--.---.--,---.--. 

4 

3 

2 

6L-~~--~_L~~~~~~~~~~~~~~~~~~~~~~~~ 

5.--,---.--,---.-~,--.--~--.---~-r--~--,---~~------~--.---.--r---.~ 

~&o~~~~~~~~~~~~~-e~~---cr~~~~~~----~r--c 

1 

o~~--~--~--~~--~--~--~--~~--~--~--~~--~--~--~--~~--~~ 

1970 1975 

: ~: ~;~~ : : : : : : : 1 

~08:~0~01 : : : : : 1 

1980 

EXPLANATION 

* Dissolved-nitrate concentration shown 

in milligrams per liter as nitrogen 

: : : : : 1 

1985 1990 

Figure 31. Flow-adjusted, total-nitrogen concentrations through time in water from selected surface-water-quality 

monitoring sites, 1970-90. Location of sites shown in figure 16. 

62 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197G-90 
~H Sit~3,So~thB~anch~ot=~==: 4?= :_ :: ~ 

~:; ~ Sit~7A,~onoc~che~uei~~ > ;

0 ~ : 0 ~OO;O 0; ; : 0: 0: : : : ~ 

: 9: 00~ ~~;~;~:c~: : 0 ;Qe~o 9; ; ; 

0

;90 : : : : ~ 

: : : 

EXPLANATION 

• Value reported less than laboratory reporting 

level 

0 Values exceeds axis range 

* Dissolved-orthophosphate concentration 

shown in milligrams per liter as phosphorus 

1985 

Figure 32. Flow-adjusted total-phosphorus concentrations through time in water from selected surface-water-quality 

monitoring sites, 1970-90. Location of sites shown in figure 16. 

1990 

Analysis of Available Nutrient Data 63 
and Virginia. Because municipal effluent contributes a 

small part of the phosphorus in the rural areas 

upstream from Washington, D.C., no trend was 

apparent in water from the Potomac River (site 23). 

The trends detected in water from other sites are only 

slight because most of the sampling periods began in 

late 1973 following implementation of the phosphatedetergent ban. The greatest decline in total-phosphorus 

concentration occurs in water from Bull Run (site 25) 

where concentrations decline substantially between 

1971 and 1974 as tertiary treatment was brought online at the municipal wastewater-treatment facility. 

Phosphorus concentrations in municipal wastewater 

have declined substantially due to the ban on phosphate detergents; however, most of the wastewater discharges are located downstream of the 25 monitoring 

sites. 

Trombley (1989) reported downward trends in 

total-phosphorus concentrations in water from Antietam Creek (site 9A) and from the Potomac River at 

Point of Rocks, Md., downstream from its confluence 

with the Shenandoah River. Other slight downward 

trends are apparent in water from sites 11A and 17 

(fig. 32) where agricultural practices and wastewater 

discharges are both significant sources of phosphorus. 

Trombley (1989) reported an upward trend in totalphosphorus conce~trations in water from Conococheague Creek (site 7A) and the Potomac River Shepherdstown, W. Va. (site 8). 

Nutrient Loads and Mass Budget 

This report refines current knowledge of nutrient 

movement in the Potomac River Basin by estimating 

loads of dissolved nitrate, total nitrogen, and total 

phosphorus at 20 of the 25 surface-water-quality monitoring sites on the selected tributaries that are used in 

the preceding analyses of nutrient concentrations and 

nutrient inputs. Previous investigations of nutrient 

loadings of the Potomac River focus their analysis at 

the most downstream nontidal site, Potomac River at 

Washington, D.C. (site 23). Although load estimates at 

this site provide a good estimate of nutrients reaching 

the Potomac estuary and Chesapeake Bay from the 

upper Potomac River Basin, the relative contributions 

of major tributaries are not addressed, and processes 

affecting nutrient loads are often oversimplified. 

Long-term mean annual nutrient loads are presented 

for the 20 sites in table 14 along with the period of 

estimation and mean standard error of the model estimate. Annual loads are included in table 16 at the end 

of this report. Mean annual load estimates are used for 

site-to-site comparisons to avoid problems caused by 

comparing years of differing hydrologic conditions. 

Computation of nitrogen and phosphorus loads was 

attempted for dissolved nitrate, total nitrogen, and 

total phosphorus at all 25 sites. Loads are reported 

only for those sites and constituents that met dataquality controls. 

Understanding the mass balance of nutrients in the 

Potomac River and its tributaries is essential when 

assessing relative effects of point sources, nonpoint 

sources, and land use within the watershed. It is particularly important to understand these relative effects 

within watersheds of the Potomac River Basin in order 

to appropriately manage the quality of surface water 

reaching the Potomac estuary and Chesapeake Bay. In 

addition to the estimates of nutrient sources presented 

earlier, several approaches to understanding the mass 

transport of nutrients in the Potomac River Basin are 

used here. Relative contributions of measured nutrient 

loads from the major watersheds draining large 

regions of diverse land use and physiography are compared. Nutrient loads per unit area (yields) are compared with major land-use patterns and source 

estimates within the watersheds. Also, data from 

selected watersheds are used to evaluate base-flow 

nutrient-load contributions and compare those loads 

with total loads and point-source loads. 

Loads In Major Tributaries 

The mass movement of nitrogen and phosphorus 

in surface water of the Potomac River Basin differs 

considerably among the major tributary systems. 

Nitrogen and phosphorus loads previously have been 

determined for the upper Potomac River Basin at 

Washington, D.C., by the USGS (L.D. Zynjuk, 

U.S. Geological Survey, written commun., 1994) as 

part of ongoing efforts by State and Federal agencies 

to monitor and control nutrient loads to the Potomac 

estuary and Chesapeake Bay. In November 1992, the 

USEPA's Chesapeake Bay Program set nutrient-load 

goals for all major tributaries of Chesapeake Bay to 

achieve a 40-percent reduction of nitrogen and phosphorus loads by the year 2000 (Alliance for the Chesapeake Bay, 1993). Targeted nutrient-load reductions 

for the Potomac River are 18.7 and 1.71 million lb of 

nitrogen and phosphorus, respectively. To date, however, the relative contributions of nutrients from major 

tributaries have only been estimated through estimates 

of input and runoff and have not been determined 

64 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
Table 14. Estimated long-term mean annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected sites in the Potomac River Basin 

[Loads and uncertainties reported in thousands of pounds per year; --, not calculated] 

Period of Total nitrogen 

Dissolved nitrate as 

nitrogen Total phosphorus 

Standard 

error of 

Site 

no.1 

record -----=s-ta-n--=d-ar-d=- Standard 

(water error of error of 

(fig. 16) Stream name years) Load estimate Load estimate Load estimate 

1 North Branch Potomac River 1975-85 767 98.1 631 43 101.2 23.5 

2 North Branch Potomac River 1973-83 2,580 276 190 33.8 

3 South Branch Potomac River 1979-89 

1979-89 

1979-86 

2,880 288 2,920 551 168 49.3 

6 Cacapon River 1,270 354 1,220 581 45.8 12.5 

7 A Concocheague Creek 

SA 

SB 

9A 

10 

llB 

12 

13 

14A 

14B 

15 

17 

18 

19 

20 

21B 

22 

23 

Potomac River 

Potomac River 

Antietam Creek 

Christians Creek 

1979-89 

1979-87 

1979-86 

1979-89 

South Fork Shenandoah River 1979-89 

North Fork Shenandoah River 1979-89 

Smith Creek 1979-89 

North Fork Shenandoah River 1979-89 

North Fork Shenandoah River 1979-89 

Shenandoah River 1979-89 

1979-89 

1982-89 

24,100 

20,000 

346 

502 

351 

2,240 

Monocacy River 

Hunting Creek tributary 

Monocacy River 1973-83 6,800 

Goose Creek 

Seneca Creek 

Difficult River 

Potomac River 

1979-87 1,430 

1979-89 

1979-89 151 

1979-89 60,000 

1,650 

2,020 

49.8 

53.4 

22.5 

154 

399 

154 

9.82 

3,230 

4,640 387 

14,000 

2,720 

255 

3,900 

2,370 

2,250 

803 

224 

36.2 

452 

295 

280 

9,050 1,290 

824 224 

9.62 1.06 

4,660 

1,220 

500 

95.7 

38,500 

255 

199 

45.1 

7.82 

3,940 

243 76.6 

1,420 

2,440 

30.1 

654 

34.4 

1,030 

45.4 

535 

75.3 

19.1 

15.3 

5,780 

216 

705 

8.98 

220 

16 

176 

8.47 

66.1 

16.6 

7.45 

4.77 

1,330 

1

Site identifier: A, U.S. Geological Survey data; B, non-U.S. Geological Survey data. 

using in-stream measurements. The load estimates 

presented here should prove useful as specific nutrient-reduction strategies are developed for the Potomac 

River Basin. 

The Potomac River at Washington, D.C. (site 23), 

discharges a long-term average of 60 million lb of 

total nitrogen and 5.79 million lb of total phosphorus 

per year (table 14). Downstream from the Washington, 

D.C., site, an additional28.7 million lb of total 

nitrogen and 0.63 million lb of total phosphorus are 

discharged to the estuary from municipal and industrial sources, and 1.94 million lb of total nitrogen are 

deposited directly into the estuary from atmospheric 

sources. Nitrogen and phosphorus loads from the 

many small tributaries draining the basin downstream 

of site 23 remain unquantified. 

The amount of nitrogen transported differs greatly 

among major Potomac River tributaries. Average 

annual loads of dissolved nitrate and total nitrogen for 

two sites on the Potomac River and eight tributaries 

with drainage areas greater than about 500 mi2 are 

compared in figure 33. Dissolved nitrate constitutes 

the major component of the nitrogen load, and dissolved-nitrate load estimates are available for all 

10 sites. Total-nitrogen loads are not available for all 

sites because insufficient data were available or model 

assumptions could not be met. Because temporal 

variations in total-nitrogen concentrations are greater, 

standard errors of total-nitrogen loads usually are 

Analysis of Available Nutrient Data 65 
Site no. Stream name 

(fig.16) 

2 North Branch Potomac River 

3 South Branch Potomac River 

6 Cacapon River 

7 A Conococheague Creek 

8A Potomac River 

118 South Fork Shenandoah River 

148 North Fork Shenandoah River 

15 Shenandoah River 

19 Monocacy River 

23 Potomac River 

L:=J Total-nitrogen load 

- Dissolved-nitrate load 

0 10,000 20,000 30,000 40,000 50,000 60,000 

Average annual nitrogen load, 

in thousands of pounds per year 

Figure 33. Estimated average annual total-nitrogen and dissolved-nitrate loads in the 

Potomac River and selected major tributaries. 

greater than dissolved-nitrate loads; therefore, 

dissolved-nitrate loads are better suited for site-to-site 

comparison of loads. Dissolved-nitrate and total-nitrogen loads differ greatly at main-stem sites 8A and 23, 

with dissolved nitrate comprising 58 and 64 percent of 

the total load, respectively. At sites 3, 6, and 14, dissolved nitrate and total-nitrogen loads are nearly 

equal. In some cases the estimated dissolved-nitrate 

loads may exceed the estimated total-nitrogen loads. 

This is physically impossible; however, both estimates 

are within the standard errors of the load estimates. 

This indicates that dissolved-nitrate comprises nearly 

all the nitrogen load at these sites. 

The average annual nitrogen load at main-stem 

Potomac River site 8A is the highest of the nine sites 

upstream of the Potomac River at Washington, D.C. 

The 14-million-lb dissolved-nitrate load and 24.2-

million-lb total-nitrogen load at this site comprise 36 

and 40 percent of the loads at site 23, and site 8A represents 51 percent of the watershed upstream of Washington, D.C. The Shenandoah River (site 15), 

Monocacy River (site 19), and Conococheague Creek 

(site 7 A) are the three tributaries with the largest 

annual load of dissolved nitrogen as nitrate. These 

three watersheds have large agricultural regions, and 

each has an annual load greater than 4.5 million lb of 

dissolved nitrate. In the Shenandoah watershed, the 

South Fork Shenandoah River (site 11B) carries a 

larger annual dissolved-nitrate load than the North 

Fork Shenandoah River (site 14A) and contributes 

about 43 percent of the load to site 15 compared to 

26 percent of the load at site 14A. Three major tributaries in the forest headwaters, North Branch Potomac 

River (site 2), South Branch Potomac River (site 3), 

and Cacapon River (site 6), contribute relatively small 

loads of dissolved nitrate to the Potomac River. These 

watersheds drain approximately 25.9 percent of the 

basin (table 6) yet contribute only 17.5 percent of the 

dissolved-nitrate load measured at Washington, D.C. 

(site 23). 

Average annual total-phosphorus loads also differ 

among major tributaries to the Potomac River 

(fig. 34). Standard errors of the estimates of totalphosphorus loads are quite large relative to nitrogen 

loads and range from 12 to 49 percent. However, basinwide patterns in total-phosphorus loads are still 

66 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
Site no. Stream name 

(fig.16) 

2 North Branch Potomac River 

3 South Branch Potomac River 

6 Cacapon River 

7 A Conococheague Creek 

8A Potomac River 

118 South Fork Shenandoah River 

15 Shenandoah River 

19 Monocacy River 

23 Potomac River 

0 1 ,000 2,000 3,000 4,000 5,000 6,000 

Average annual total-phosphorus load, 

in thousands of pounds per year 

Figure 34. Estimated average annual total-phosphorus loads in the Potomac River and selected major 

tributaries. 

discernible. Because phosphorus loads are most 

dependent upon high-flow conditions, more intense 

storm sampling is needed to improve these estimates. 

Potomac River at Washington, D.C. (site 23), has an 

average annual total-phosphorus load of 5.79 million 

lb. Potomac River (site 8B) carried 2.44 million lb of 

phosphorus-42 percent of the load at site 23. Three 

agricultural tributaries, South Fork Shenandoah River 

(site 11B), Shenandoah River (site 15), and Monocacy 

River (site 19), carried annual phosphorus loads 

greater than 0.5 million lb/yr. The three major forest 

tributaries (sites 2, 3, and 6) had phosphorus loads less 

than 0.2 million lb and contributed only 7 percent of 

the load measured at Potomac River at Washington, 

D.C. (site 23). 

Mass Budget and Land Use 

The balance of nutrient inputs and nutrient exports 

(loads) differs among the tributaries of the Potomac 

River Basin. These differences are primarily related to 

the land-use practices within the watersheds and 

resultant nutrient inputs. In general, larger nutrient 

yields (loads per unit area) correspond with larger 

nutrient inputs, and inputs of both nutrients are largest 

in watersheds dominated by agricultural land use. 

The nutrient-input and nutrient-load data compiled 

for this report are used to show the general mass budget of nitrogen and phosphorus in the Potomac River 

Basin. Differences in nitrogen and phosphorus massbudget characteristics occur in watersheds with similar 

land uses, and these differences appear to be related to 

hydrogeologic properties of the watersheds. Several 

recent studies have presented a watershed mass balance of nitrogen and phosphorus in the upper Potomac 

River Basin. Fisher and Oppenheimer ( 1991) used a 

nitrogen-flow model to assess the relative contributions of atmospheric deposition and other inputs to the 

Potomac River Basin. Lug bill ( 1990) presented and 

assessed relative contributions of numerous nutrient 

inputs to the Potomac River Basin. Jaworski and others ( 1992) builds upon these studies by simplifying the 

nitrogen-flow model and by balancing inputs with 

nitrogen and phosphorus loads measured for the Potomac River at Washington, D.C. These approaches 

account for the mass balance of nutrients in the upper 

Potomac River Basin; however, they certainly simplify the numerous and varied processes affecting the 

mass balance in major parts of the Potomac River 

watershed. 

Analysis of Available Nutrient Data 67 
The following discussion attempts to refine the 

knowledge of the sources of nutrients to watersheds 

and the transport of nutrients in surface water. Mass 

budgets of nitrogen and phosphorus in major tributaries indicate large-scale differences in budgets throughout the basin. Mass budgets in smaller watersheds are 

indicative of common land-use practices and hydrogeologic settings that occur throughout the Potomac 

River Basin. 

Nitrogen and phosphorus yields show substantial 

differences in nutrient transport throughout the Potomac River Basin. Dissolved-nitrate and total-nitrogen 

yields from the upper Potomac River (site 23) are 

3,300 and 5,140 lb/mi2, respectively. Dissolved-nitrate 

yields in contributing tributaries range from 1,650 to 

9,690 lb/mi2, and total-nitrogen yields range from 

1,870 to 8,330 lb/mi2. The geographic distribution of 

dissolved-nitrate and total-nitrogen yields is shown in 

figure 35. Both maps show similar patterns, although 

load data for both nitrogen species are unavailable at 

all sites. Dissolved-nitrate yields are smallest, less 

than 2,000 lb/mi2, in three forest and sparsely populated watersheds, sites 3, 6, and 18, and one urban 

watershed, site 22. Dissolved-nitrate yields are slightly 

higher in watersheds draining the Appalachian Plateau, sites 1 and 2; the Shenandoah Valley, sites 10, 11, 

and 14; and the Piedmont in Virginia, site 20. Dissolved-nitrate yields are between 4,000 and 

6,000 lb/mi2 at sites 17, 19, and 21 in the Piedmont 

and Triassic Lowlands of Maryland and Pennsylvania. 

The largest dissolved-nitrate yields occur at Conococheague Creek (site 7) and Antietam Creek (site 9) in 

the northern part of the Great Valley in Mary land and 

Pennsylvania. Conococheague Creek and Antietam 

Creek deliver more than 9,000 lb/mi2 of nitrate to the 

Potomac River, more than double the magnitude of 

sites 11 and 14, which drain similar settings of the 

South Fork and North Fork Shenandoah Rivers. 

Total-nitrogen yields are available for fewer sites 

than dissolved nitrate; however, a similar geographic 

pattern occurs. In most cases, total-nitrogen yields are 

larger than dissolved-nitrate yields. The Monocacy 

River watershed (site 19) yields 8,210 lb/mi2 of total 

nitrogen to the Potomac River, the largest totalnitrogen yield found in the basin. 

The transport of phosphorus in tributaries is also 

highly variable as total-phosphorus yields range from 

68 to 654lb/mi2. The total-phosphorus yield for the 

upper Potomac River Basin (site 23) is 496lb/mi2. 

Small phosphorus yields occur primarily in forest 

watersheds of the Valley and Ridge subunit, including 

sites 3, 6, and 12 (fig. 36). Seneca Creek (site 21) in 

the Piedmont subunit, however, also has a relatively 

small phosphorus yield at 189lb/mP. Larger phosphorus yields occur in watersheds with greater agricultural 

intensity and higher inputs from wastewater-treatment 

facilities. The largest phosphorus yields occur at 

sites 1, 7, 10, and 19, but only Monocacy River 

(site 19) had a phosphorus yield (654lb/mi2) exceeding the downstream yield at site 23. The relatively 

large phosphorus yield of the North Branch Potomac 

River, site 1, may be related to several factors including the effects of acid-mine drainage, erosion from 

mining and silviculture, and wastewater discharges. 

Acid-mine drainage may limit in-stream algal uptake 

of phosphorus, and increased erosion causes elevated 

phosphorus concentrations as phosphate often is 

bound to suspended sediment. Agricultural runoff 

appears to be the primary source of phosphorus at 

other sites, although the combined effect of wastewater discharge also is important to total-phosphorus 

yield. Although wastewater discharges may be the 

primary cause of elevated phosphorus concentrations 

during stable streamflow, the total-phosphorus load 

from wastewater is small when compared to totalphosphorus loads in runoff during high-flow 

conditions. 

The balance of input and export of nitrogen in the 

Potomac River Basin is complex and, as indicated by 

the patterns seen in nutrient concentrations, strongly 

related to physiography and land-use patterns. A simple watershed nutrient budget essentially accounts for 

differences in estimates of nutrient input and measured 

estimates of nutrient loads at monitoring sites. A complete nutrient mass balance quantifies and accounts for 

major input, output, and storage-change terms for 

nitrogen and phosphorus within a watershed. Major 

inputs have been discussed previously. Major watershed output terms include: 

( 1) harvest of grains, lumber, and animal products, 

(2) volatilization, 

(3) denitrification, 

( 4) in-stream uptake and reduction of nutrients, and 

(5) export through surface-water discharge. 

Major storage-change terms include: 

( 1) biomass storage, and 

(2) ground-water storage. 

These mass-balance components are not quantified in 

this report; however, nitrogen and phosphorus export 

coefficients are calculated and indicate the percentage 

68 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197o-90 
> ;:, 

Ql 

-< Ill 

iii" 

a 

> 

< 

!!!. 

iD 

0" 

iD 

z 

s 

... 

iD" 

;:, 

-

~ 

A. Total-nitrogen yield 

10 20 30 40 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

Mean annual nitrogen yield for 

contributing drainage basins, in 

thousands of pounds per square 

miles 

w Less than or equal to 2.0 

~ Greater than 2.0 to 4.0 

- Greater than 4.0 to 6.0 

• Greater than 6.0 

-- Drainage-basin boundary 

10• Location of surface-water-quality 

monitoring site-Number corresponds 

to that in table 16 

Figure 35. Mean annual (A) total-nitrogen and (8) dissolved-nitrate yields for selected watersheds in the Potomac River Basin, 1970-90. 
EXPLANATION 

Mean annual total-phosphorus 

yield for contributing drainage 

basins, in thousands of pounds 

per square miles 

EJ Less than or equal to 0.20 

w Greater than 0.20 to 0.30 

• Greater than 0.30 to 0.40 

• Greater than 0.40 

-- Drainage-basin boundary 

WA. Location of surface-water- quality monitoring siteNumber corresponds to that 

in table 16 

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

Figure 36. Mean annual total-phosphorus yield for selected watersheds in the Potomac River Basin, 197D-90. 

of nutrient inputs that is transported in surface water. 

Major storage-change terms are assumed to be constant. Because the measured loads are a long-term, 

"approximately 10-year" average representing normal, 

wet, and dry climatic cycles, this is probably sufficient. Certainly these storage terms do change from 

year to year and over decades. The implications of the 

changes in these storage terms will be discussed later. 

Nutrient retention and export in Potomac River 

tributaries are closely related to land-use practices and 

sources of nutrients and are affected further by hydrologic processes in the watersheds. Fisher and Oppenheimer's (1991) mass balance of the upper Potomac 

River Basin found that measured nitrogen and 

phosphorus exports agreed with predicted exports on 

the basis of a linear relation of inputs to retention. 

70 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197Q-90 
However, they point out that there are 11 no data to 

determine the key factors regulating nutrient retention 

processes in the upper Potomac watershed. II The nitrogen and phosphorus input and export rates presented 

here demonstrate the complexity of the input-export 

relation in tributaries to the Potomac River. 

The relation of nitrogen yield to nitrogen input is 

shown in figure 37 for urban, agricultural, forest, and 

large watersheds with mixed land use. Simplified 

models suggest that a linear relation of input to export 

could be used for all watersheds. In general, yield 

increases as input increases. However, the proportion 

of nitrogen exported from these watersheds demonstrates considerable variability. The nutrient-export 

rate is the percentage of nutrient input to a watershed 

(tables 7 and 8) that is exported from the watershed as 

surface-water load (table 14). Nitrogen-export rates 

for the Potomac River and tributaries range from 7 to 

35 percent. 

In the Difficult Run (site 22) watershed, the one 

urban watershed with nutrient-load data, both inputs 

and yields are low. Inputs are dominated by atmospheric deposition, and 26 percent of the nitrogen is 

exported. Higher export rates may be expected in 

urban settings; however, land use in the Difficult 

Creek watershed is primarily suburban, with welldeveloped forest buffers. 

Nitrogen inputs to agricultural watersheds are 

higher than the urban watershed and extremely variable, ranging from 16,600 to 56,400 lb/mi2, and nitrogen-export rates range from 7 to 35 percent of the 

nitrogen input. Nitrogen-export rates are highest at 

sites 7, 9, and 19, on Conococheague Creek, Antietam 

Creek, and Monocacy River. Sites 7 and 9 drain 

intense agricultural areas in the Great Valley Carbonate subunit. Site 19 drains an agricultural watershed 

underlain by a combination of siliciclastic, crystalline, 

and carbonate rock. Fertilizer is the predominant nitrogen source in all three watersheds. Nitrogen-export 

rates are lowest at sites 13 and 14 in the southern Great 

Valley, where agricultural sources of nitrogen are 

dominated by manure. 

Nitrogen inputs to forest watersheds (sites 3, 6, 12, 

and 18) are low, less than 19,000 lb/mi2, and are dominated by atmospheric deposition. Nitrogen-export 

rates in most forest watersheds range from 11 to 

13 percent. Nitrogen-export rates from the North 

Branch Potomac River (sites 1 and 2) are 15 to 20 percent higher. The lack of in-stream biological activity 

caused by acid-mine drainage in much of the watershed may inhibit biological uptake of nitrogen in this 

area. Site 8 on the Potomac River at Shepherdstown, 

W. Va., drains a large, primarily forest watershed; 

however, it is located downstream from the confluence 

with the highly agricultural watershed of Conococheague Creek and shows a higher export rate of total 

nitrogen-27 percent. 

The mass budget of phosphorus in watersheds of 

the Potomac River Basin also differs considerably 

among watersheds due to land-use differences and 

phosphorus sources. Phosphorus inputs range from 

600 to 11,900 lb/mi2, and phosphorus yields range 

from 68 to 654 lb/mi2• The relation of phosphorus 

yield to phosphorus input is shown in figure 38. 

The one urban watershed, site 22, exported 26 percent of the phosphorus input; however, the input rate is 

lowest of all watersheds. Phosphorus-export rates in 

agricultural watersheds range from 3.4 to 9 percent, 

and both phosphorus inputs and exports are generally 

higher than in forest watersheds. Phosphorus-input 

rates are lowest in forest watersheds, generally less 

than 2,500 lb/mi2, and export rates range from 4 to 

9 percent. The highest export rates occur in the North 

Branch Potomac River (sites 1 and 2) and in Difficult 

Run (site 22). At site 1, 54 percent of the phosphorus 

is exported in surface water. As stated earlier, 

increased runoff from mined and forest areas may 

cause higher phosphorus loads, or acid-mine drainage 

may limit in-stream biological uptake and storage. 

The phosphorus input/export relation in large tributaries with mixed land uses reflects a combination of 

agricultural and forest land uses. The input/export 

relation at site 8 on the Potomac River is graphically 

similar to the predominantly forest sites (fig. 38), and 

its watershed is 69 percent forest. The South Fork 

Shenandoah River (site 11) and Shenandoah River 

(site 15) are similar to agricultural watersheds. The 

Potomac River at site 23 exports 13 percent of the 

phosphorus input, and phosphorus exports range from 

3.4 to 54 percent. The large standard errors associated 

with the phosphorus-load estimates probably prohibit 

an assessment of difference in phosphorus massbudget characteristics beyond this leveL 

Base-Flow Nutrient Loads 

Ground-water discharge can contribute large 

amounts of nitrogen to surface water in many tributaries of the Potomac River. It is clear from the analysis 

of nitrogen loads and mass budget that the processes 

affecting the transport of nitrogen in the Potomac 

Analysis of Available Nutrient Data 71 
10 

~ 9 

.E 

8 e 

ro 

::J ~0" 7 "'C(J) 

a:; .... ·- Q) >c. 6 

C:(J) 

Q)"'C 

Ole: 

O::J 5 

.;.o 

·c: c. 4 ...... 

(00 

-(/) 

~"'0 c: 3 Ctl 

(/) 

::J 

0 2 ..c: 

.~ -

0 

10 

~ 9 

.E 

Q) 8 

~ ~::J "'cr 7 Q)(J) 

·:;;.a:; 

Q)C. 6 10(/) 

"-"'0 

:!:: c: C:::J 5 '0 -go. 

>- 4 -o 

0(/) 

~"'0 ·- c: 3 Oro (/) 

::J 

0 2 ..c: 

-c: 

0 

B. 

0 10 

6.7 .6!9 

20 30 40 

Nitrogen input, in thousands of pounds per square mile 

EXPLANATION 

Land use 

0 Urban-Greater than 50 percent 

~ Agricultural-Greater than 60 percent 

0 Forest-Greater than 70 percent 

Q Mixed-Large watershed (greater than 1,500 square miles) 

3 Monitoring site number-Location of site given in figure 16 

~13 

50 

Figure 37. Relation of (A) total-nitrogen and (B) dissolved-nitrate yields to nitrogen-input rate and land use in the 

Potomac River Basin. 

60 

72 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197~90 
e 

ro 

~ 5- 0.5 (!)(/) 

·:;.. .... (/)(!) 

2 ~ 0.4 O"C 

..r::.c 

C. :::::I (1)0 

_g c. 0.3 

9-0 

-(/) !S-c 

~ ~ 0.2 (/) 

::I 

0 

..r::. 

.~ 0.1 

20-percent export 1 0-percent export 

b-19 

5-percent export 

o~~--~--~--~--~--~--~--~~--~---L---L--~--~--~--~~--~--~--~ 

0 2 3 4 5 6 7 8 9 10 

Phosphorus input, in thousands of pounds per square mile 

EXPLANATION 

Land use 

0 Urban-Greater than 50 percent 

b. Agricultural-Greater than 60 percent 

D Forest-Greater than 70 percent 

0 Mixed-Large watershed (greater than 1,500 square miles) 

1 Monitoring site number-Location of site given in figure 16 

Figure 38. Relation of total-phosphorus yields to phosphorus-input rate and land use in the Potomac River Basin. 

River Basin are complex, especially in agricultural 

watersheds. Ground-water discharge maintains 

streamflow during base-flow conditions. Thus, that 

part of the nutrient loads originating in ground water 

can be termed "base-flow loads." Nitrogen and phosphorus loads in surface water increase substantially 

during storms, and a large part of the nutrient load 

generated during storms often is attributed to overland 

runoff carrying nitrogen and phosphorus into streams 

from the land surface. In many settings, base-flow 

loads also increase during storms as ground-water discharge increases. Point-source loads are nearly constant throughout storms as wastewater-treatment 

facilities discharge nutrients into surface water at a 

nearly constant rate. Thus, during stable streamflow 

conditions, surface-water loads are maintained by 

base-flow and point-source loads. During storm-flow 

conditions, surface-water loads are caused by pointsource, base-flow, and overland-runoff loads combined. An analysis of ground-water nutrient concentrations, streamflow data, and point-source inputs enables 

estimates of nutrient loads from base flow and point 

sources to be compared with total-nutrient loads in 

surface water for watersheds with sufficient groundwater data. 

Ground-water discharge may contribute a substantial part of the estimated total-nitrogen load in many 

watersheds and may contribute a small part of the 

phosphorus load. Estimates of base-flow nitrogen 

loads can account for more than 100 percent of measured nitrogen loads in some watersheds. The actual 

load of nutrients discharged to surface water is 

expected to be greater than the nutrient load measured 

at downstream sites because in-stream processes, such 

as plant and algal uptake and denitrification, can 

remove nitrogen from the available fluvial system. At 

a minimum, though, the difference between in-stream 

nutrient load and base-flow loads originates from 

either overland runoff or point-source discharges. 

Base-flow loads are most important to nutrient loads in 

Analysis of Available Nutrient Data 73 
agricultural areas of the Potomac River Basin, particularly in those areas where concentrations of nitrogen in 

ground water are elevated due to agricultural land use 

and where base flow comprises a substantial part of 

annual streamflow. 

Average annual ground-water nitrogen and phosphorus loads were estimated for eight watersheds 

where a sufficient number and geographic distribution 

of ground-water samples were available. Groundwater loads were approximated by using a median 

concentration of all ground-water samples within a 

watershed and by multiplying that concentration by 

that part of ground water in streamflow as determined 

by streamflow hydrograph-separation techniques 

(Pettyjohn and Henning, 1979; Sloto, 1988). This 

method of determining base-flow loads assumes that 

average nutrient concentrations in ground water in 

wells throughout the basin are the same as concentrations in ground water discharging to streams. The 

results of these calculations are tabulated in table 15. 

The contribution of ground water to annual streamflow 

ranges from 31.8 to 81.3 percent in the watersheds 

tested. The largest ground-water contributions occur in 

watersheds underlain by carbonate and crystalline 

rock. Smaller ground-water contributions occur in 

watersheds underlain by sedimentary rock of the Triassic Lowlands subunit, such as Monocacy River at 

Bridgeport (site 17) and Bull Run at Manassas 

(site 25). 

Base-flow loads of nitrogen as nitrate can account 

for between 3 and 100 percent of the measured nitrogen load at six of the eight sites. Base-flow nitrogen 

loads in forest watersheds of the North Branch Potomac River (site 1) and Cacapon River (site 6) only 

contribute 11 and 3 percent of the measured loads. 

Base-flow loads in agricultural watersheds, however, 

contribute between 42 and 100 percent of measured 

loads. The highest percentage base-flow loads occur in 

Conococheague Creek (site 7) and Antietam Creek 

(site 9) (fig. 39). Both of these watersheds are largely 

cropland underlain by carbonate rock where nitrogen 

transport from surficial application down to the 

ground-water system can occur quickly. This effect is 

readily seen as elevated ground-water concentrations 

and high nitrogen loads in surface water. Base-flow 

loads in the Triassic Lowlands subunit (sites 17 and 

19) are substantially lower (42 percent), primarily due 

to low infiltration rates and poor transmissivity of the 

siliciclastic rock. In all cases, ground-water contributions of phosphorus to surface-water loads are minimal. Point-source nitrogen loads are small in relation 

to total loads at the six sites with load estimates and 

are largest at sites 7, 9, and 19, where populations are 

higher. 

Implementation of nutrient-management practices 

in watersheds with large percentages of nitrogen loads 

derived from base flow requires careful consideration 

and planning. Many agricultural BMP's are being 

implemented throughout the basin in an effort to minimize soil erosion and improve water quality by controlling surface runoff and decreasing nutrient 

contributions to surface water. These practices can be 

particularly effective at decreasing phosphorus loads 

to streams as phosphorus is bound to soil particles. 

Table 15. Estimated base-flow loads of nitrogen and phosphorus for selected watersheds in the Potomac River Basin 

[--,data unavailable] 

Ground-water Dissolved orthophosphate as 

discharge Dissolved nitrate as nitrogen phosphorus 

Base- BaseNumber flow load Number flow load 

Site Percent of Median (thou- of Median (thouno. of total ground- (mllli- sands of ground- (mil II- sands of 

(fig. Inches stream- water grams pounds water grams pounds 

16) Stream name per year flow samples per liter) per year) samples per liter) per year) 

I North Branch Potomac River 20.3 63.8 25 0.11 73 15 0 6.63 

6 Cacapon River 6.3 51.9 16 .07 43.3 

7 Conococheague Creek 12 63.6 24 3.3 2,830 17 .01 8.58 

9 Antietam Creek 12.7 81.3 35 5.4 2,800 13 .01 5.19 

16 Catoctin Creek 11 63.7 24 3.7 94 

17 Monocacy River 6.64 39.2 35 2.1 346 

19 Monocacy River 8.61 52.5 161 2.4 2,450 52 .02 20.4 

25 Bull run 6.03 31.8 49 .3 8.6 27 .01 1.28 

74 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
Site no. Stream name 

(fig.16) 

1 North Branch Potomac River 

6 Cacapon River 

7 A Conococheague Creek 

9 Antietam Creek 

17 Monocacy River 

19 Monocacy River 

D Base-flow nitrogen load 

D Point-source nitrogen load 

- Total-nitrogen load 

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 

Nitrogen load, in thousands of pounds per year 

Figure 39. Estimated base-flow nitrogen loads, point-sources nitrogen loads, and totalnitrogen loads for selected watersheds in the Potomac River Basin. 

Many of these practices, however, may increase nitrogen concentrations in ground water. Nitrogen loads in 

surface water may actually increase in areas where 

nitrogen is not chemically reduced in ground water as 

nitrogen-containing ground water eventually discharges to streams. Because most BMP's decrease the 

overland-runoff part of the nutrient loads, they will be 

most effective where overland runoff contributes a 

high percentage of the nitrogen load, such as in the 

Appalachian Plateau, Valley and Ridge, and Triassic 

Lowlands subunits. Also, these BMP's may be effective at decreasing in-stream nitrogen concentrations in 

areas where nitrogen is chemically reduced in ground 

water. 

SUMMARY 

The Potomac River Basin has a complex environmental setting consisting of various combinations of 

natural and human factors that can affect water quality 

in the basin. The 14,670-mi2 watershed is divided into 

eight subunits on the basis of basin physiography and 

geology for the purpose of water-quality assessment. 

The Appalachian Plateau subunit comprises 4 percent 

of the basin and is composed primarily of siliciclastic 

rocks. The Valley and Ridge subunit (34 percent of the 

basin) is underlain by both siliciclastic and carbonate 

rocks. The Great Valley is divided into a carbonate 

subunit ( 17 percent of the basin) and a noncarbonate 

subunit (5 percent of the basin). The Blue Ridge subunit (6 percent of the basin) is formed by crystalline 

rocks. The Piedmont subunit (12 percent of the basin) 

is underlain by crystalline rocks. The Triassic Lowlands subunit (7 percent of the basin) is underlain by 

siliciclastic and igneous rocks; the Coastal Plain subunit (15 percent of the basin) by unconsolidated 

sediments. 

Land use in the Potomac River Basin is diverse 

and generally coincides with physiographic 

boundaries. The population in the basin increased 

from about 3.2 million in 1970 to about 4.6 million in 

1990, with most people residing in urban land-use settings in the Washington, D.C., area. In the mid-1970's, 

the basin was 51 percent forest, 36 percent agricultural, and 8 percent urban. By 1985, agricultural land 

use had decreased to 32 percent and urban land use 

had increased to 12 percent of the basin. Agriculture 

Summary 75 
comprises 50 percent or more of the land use in the 

Great Valley Carbonate and Noncarbonate, Piedmont, 

and Triassic Lowlands subunits, and forests cover 

more than 7 5 percent of the land in the Appalachian 

Plateau, Valley and Ridge, and Blue Ridge subunits. 

In 1990, about 6,374 Mgal/d of freshwater (about 

85 percent of the average flow of the Potomac River at 

Washington, D.C.) was used for human-related activities. Surface-water withdrawals accounted for about 

97 percent of total freshwater use and for about 93 percent of the public-supply withdrawals. Ground-water 

withdrawals were about 204 Mgal/d, with 58.3 Mgal/d 

withdrawn for domestic purposes. 

Ground water in the Potomac River Basin is 

present in a variety of hydrogeologic settings, in both 

primary and secondary openings in the rock matrix, 

and in local, intermediate, and regional flow systems. 

In the Coastal Plain subunit, ground water resides in 

and moves through interstices between individual 

mineral grains. Ground water in the rest of the Potomac River Basin resides in and moves through fractures and other secondary openings in the rock matrix. 

In carbonate rocks, these fractures and openings may 

be enlarged further by dissolution of the rock matrix. 

Wells in Coastal Plain deposits, Piedmont sedimentary 

rocks, and Valley and Ridge sedimentary rocks generally provide the most ground water, whereas wells in 

the Piedmont and Blue Ridge crystalline rocks and 

Appalachian Plateau settings have smaller well yields. 

Major tributaries to the Potomac River include 

North and South Branches Potomac River, Cacapon 

River, Conococheague Creek, Shenandoah River, 

Monocacy River, and Occoquan River. Total streamflow and the relative proportions of overland runoff 

and base flow differ throughout the basin. In general, 

streams in the Appalachian Plateau and Valley and 

Ridge subunits and that part of the Piedmont subunit 

underlain by crystalline rocks receive about one-half 

their flow from base flow. In basins containing carbonate rocks and in basins with substantial regolith, base 

flow may constitute as much as 65 percent or more of 

total streamflow. Total streamflow per unit area is 

highest in the Appalachian Plateau and lowest in the 

Valley and Ridge subunits, reflecting precipitation patterns in the basin. In watersheds underlain almost 

entirely by carbonate rocks and in the Coastal Plain, 

streams derive more than 80 percent of their flow from 

base flow, reflecting high storage capacity. In contrast, 

base flow comprises only about 35 percent of total 

streamflow in that part of the Piedmont underlain by 

sedimentary rocks of Triassic and Jurassic age. 

Nitrogen inputs to the Potomac River Basin in 

1990 were dominated by atmospheric deposition and 

agricultural sources, and phosphorus inputs were dominated by agricultural sources. Commercial fertilizer 

and animal manure comprised about 55 percent of the 

nitrogen input, and atmospheric deposition contributed 

about 32 percent. Municipal and industrial wastewater 

discharges contributed about 12 percent of the nitrogen inputs. Phosphorus inputs were dominated by 

agricultural sources throughout the basin, with about 

93 percent of the phosphorus inputs coming from 

commercial fertilizer and animal manure. Municipal 

and industrial discharges contributed 4 percent of the 

phosphorus. Municipal wastewater discharge was the 

largest nutrient source downstream from the Fall Line, 

and 88 percent of the nitrogen and 80 percent of the 

phosphorus from wastewater were discharged downstream from Washington, D.C. 

The magnitude and relative contributions of nitrogen and phosphorus sources differed considerably 

among the eight subunits in 1990. The largest nitrogen 

inputs occurred in the Valley and Ridge and Great Valley Carbonate subunits, and the largest phosphorus 

inputs occurred in the Great Valley Carbonate subunit. 

Average rates of nitrogen input from atmospheric deposition were highest in the Appalachian Plateau at 

11,217lb/mi2 compared to the basinwide rate of 

6,582lb/mi2• Commercial fertilizer was a significant 

source of nitrogen in intensely farmed subunits, 

including the Great Valley Carbonate, Great Valley 

Noncarbonate, Piedmont, Triassic Lowlands, and 

Coastal Plain subunits, and a major source of phosphorus in all eight subunits. Animal manure was the 

largest single source of nitrogen and phosphorus to 

both Great Valley subunits and contributed more than 

50 percent of the phosphorus to the Valley and Ridge 

subunit. Septic-system inputs were estimated to be 

only about 2 to 4 percent of the total input. 

Estimates of nitrogen and phosphorus inputs to 10 

major watersheds (drainage basins greater than 

490 mi2) indicate that municipal wastewater discharges were a relatively small component of total 

nutrient inputs upstream from the Fall Line. Fertilization rates were highest in the Monocacy River watershed with nitrogen and phosphorus applied at 

15,300 and 4,490 lb/mi2, respectively, and in the 

Conococheague Creek watershed at nearly 

9,370 lb/mi2 nitrogen and 2,940 lb/mi2 phosphorus. 

76 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
The North Fork Shenandoah River had the highest 

manure production rate at 20,900 lb/mP nitrogen and 

4,660 lb/mi2 phosphorus. 

Numerous water-quality-monitoring programs 

have operated within the Potomac River Basin, with 

much of the resulting information available in the 

USGS WATSTORE and USEPA STORET computer 

systems. Nutrient-concentration data are available for 

water samples from 1,158 wells in WATSTORE and 

1,401 wells in STORET. The number of surfacewater-quality monitoring sites operated since 1973 by 

State programs differs from State to State-Maryland 

(52 sites), Pennsylvania (3 sites), Virginia (89 sites), 

West Virginia (5 sites), and the District of Columbia 

(76 sites). Nutrient-concentration data for surface 

water are available for 31,567 samples from 456 sites 

in WATSTORE and 50,036 samples from 1,176 sites 

in STORET. Some of these data are used in the analysis of nutrient-concentration data for this report. Subsets of these data may be used in other analyses of the 

Potomac River Basin. 

Dissolved-nitrate concentrations in ground water 

vary widely within the Potomac River Basin, with 

much of the variability related to basin subunits and 

nutrient inputs from land-use practices. Dissolvednitrate concentrations in water from 1,049 wells in the 

WATSTORE data base range from less than 0.01 to 

63 mg!L as nitrogen and have a median value of 

1.8 mg/L. Dissolved-nitrate concentrations in the 

Appalachian Plateau, Valley and Ridge, and Coastal 

Plain subunits generally are low, with median concentrations of 0.10, 0.14, and 0.10 mg/L, respectively. 

Dissolved-nitrate concentrations were higher in subunits with significant agricultural land use and were 

highest in the Great Valley Carbonate subunit (median 

4.5 mg!L). Fourteen percent of the wells in the Great 

Valley Carbonate subunit have concentrations greater 

than or equal to the 10-mg/L MCL for drinking water 

established by the USEPA. 

Nitrate concentrations in ground water varied due 

to surficial land use, and median concentrations were 

higher in agricultural settings in the Valley and Ridge, 

Blue Ridge, Piedmont, and Triassic Lowlands subunits. There were insufficient data to test for land-use 

effects in the Appalachian Plateau and Great Valley 

Noncarbonate subunits. In the Blue Ridge subunit, 

wells in forest settings had a median value 

(0.41 mg!L) much lower than in urban (2.3 mg!L) and 

agricultural (2.0 mg!L) settings. Nitrate concentrations 

in the Piedmont subunit range from less than 0.01 to 

37 mg!L. Wells in Piedmont agricultural areas have a 

higher median concentration (3.3 mg!L) than wells in 

forest settings (0.18 mg!L). In the Triassic Lowlands, 

median nitrate concentrations in urban (0.75 mg/L) 

and forest (0.74 mg!L) settings are lower than for agricultural land (1.8 mg!L). 

Nutrient-concentration data for 25 surface-waterquality monitoring s-ites in the Potomac River Basin 

show that spatial and temporal patterns in nutrient 

concentrations are complex. Variations in these patterns are due to differences in physiography, hydrology, land use, and nutrient inputs in the contributing 

watersheds. Nitrogen and phosphorus concentrations 

are lowest in the sparsely populated, forest watersheds 

of the Appalachian Plateau, Valley and Ridge, and 

Blue Ridge subunits and are highest in the agricultural 

watersheds of the Great Valley, Piedmont, and Triassic 

Lowlands subunits and near urban centers where 

wastewater-treatment inputs are greater. Median concentrations of total nitrogen range from 0.42 to 

3.9 mg/L at 22 sites, and median concentrations of dissolved nitrate range from 0.20 to 3.5 mg/L at 25 sites. 

In the Great Valley, there appears to be a significant 

difference in nitrogen concentration between northern 

and southern tributaries. Conococheague Creek and 

Antietam Creek have the highest median total-nitrogen concentrations of 3.1 and 3.9 mg!L, respectively, 

and drain the northern part of the Great Valley including parts of Pennsylvania and Maryland. Christians 

Creek and Smith Creek drain agricultural watersheds 

in the southern part of the Great Valley of Virginia and 

have lower median total-nitrogen concentrations of 2.1 

and 2.3 mg/L, respectively. 

The spatial patterns in total-phosphorus and dissolved-orthophosphate concentrations are largely 

related to discharges from wastewater-treatment facilities and runoff from agricultural land. Lower concentrations occur in the forest watersheds of the 

Appalachian Plateau, Valley and Ridge, and Blue 

Ridge subunits, and higher concentrations occur in 

streams in the Great Valley, Piedmont, and Triassic 

Lowlands subunits. Median total-phosphorus concentrations are highest at Antietam Creek (0.30 mg/L), 

followed by Christians Creek, Monocacy River, and 

Bull Run. Total-phosphorus concentrations at Bull 

Run appear to be strongly affected by upstream wastewater discharges, and the other three sites encompass 

significant agricultural drainage in addition to wastewater discharges. 

Summary 77 
Temporal variations in nitrogen and phosphorus 

concentrations in surface water are affected substantially by changes in streamflow conditions and seasonal cycles that often differ from site to site. In forest 

watersheds, nitrogen concentrations are usually less 

than 1.0 mg/L and increase only slightly during higher 

.flow conditions. Streams affected by wastewater-treatment-plant discharges show significant dilution of 

nitrogen and phosphorus concentrations as streamflow 

increases. Streams draining agricultural watersheds 

often differ in their response to changing streamflow 

conditions due to differences in hydrologic properties 

of the watersheds. Runoff-generated nitrogen concentrations occur in the Triassic Lowlands subunit at the 

Monocacy River near Bridgeport, Md. During lowflow conditions, concentrations are small, near 

1.0 mg/L, and increase substantially during higher 

streamftows. In agricultural watersheds of the Great 

Valley subunit, nitrate concentrations increase only 

slightly with increased streamflow and maintain 

higher concentrations, 2.0 to 3.0 mg/L, throughout all 

but the lowest streamflow conditions. Nitrogen concentrations in water from the Potomac River at Washington, D.C., reflect runoff-generated nutrient 

concentrations; however, because the patterns of contributing watersheds are diverse, the relation of nitrogen concentration to streamflow at this site is 

complex. 

Seasonal fluctuations of total-nitrogen and totalphosphorus concentrations in surface water are generally small at most sites. Median nitrogen concentrations in both agricultural and forest watersheds are 

highest in the winter and lowest during the summer, 

and overall variability is slight. Where municipal discharges are substantial, nitrogen concentrations are 

diluted by higher streamflow during winter months. 

At nearly all sites, the highest total-phosphorus concentrations. occur during the summer months when 

dilution is minimal. 

Few of the 25 surface-water-quality monitoring 

sites on the Potomac River and its tributaries show significant trends in total-nitrogen concentrations. Sites 

on Conococheague Creek and Seneca Creek both 

show a slight increase in nitrogen concentration during 

the sampling period, and no agricultural watershed 

shows a declining trend for nitrogen concentration. 

Total-phosphorus and dissolved-orthophosphate concentrations appear to decline slightly in many of the 

watersheds due to the ban of phosphate-based detergents first enacted by the State of Maryland in 1973 

and later enacted in Pennsylvania and Virginia. The 

greatest decline in total-phosphorus concentration 

occurs at Bull Run where concentrations declined substantially between 1971 and 197 4. 

The Potomac River at Washington, D.C., discharges a long-term average of 60 million lb of total 

nitrogen and 5.79 million lb of total phosphorus per 

year. Downstream from the Washington, D.C., site, an 

additional28.7 million lb of total nitrogen and 

0.63 million lb of total phosphorus are discharged to 

the estuary from municipal and industrial sources, and 

1.94 million lb of total nitrogen are deposited directly 

into the estuary from atmospheric sources. Three 

major tributaries, North Branch Potomac, South 

Branch Potomac, and Cacapon Rivers, contribute relatively small loads of dissolved nitrate to the Potomac 

River. These drain approximately 25.9 percent of the 

basin yet contribute only 17.5 percent of 

the dissolved-nitrate and 7 percent of the total-phosphorus loads measured at Washington, D.C. Much 

higher nutrient loadings are generated from three predominantly agricultural watersheds-Conococheague 

Creek, Shenandoah River, and Monocacy River. 

Nitrogen and phosphorus yields (loads per unit 

area) show substantial differences in the amount of 

nutrient transport throughout the Potomac River 

Basin. These difference are primarily related to landuse practices within the watersheds and the magnitude 

of nutrient inputs. Dissolved-nitrate and total-nitrogen 

yields from the upper Potomac River at Washington, 

D.C., are 3,300 and 5,140 lb/mi2, respectively. Dissolved-nitrate yields range from 1,650 to 9,690 lb/mi2, 

and total-nitrogen yields in tributaries range from 

1,870 to 8,330 lb/mi2• The smallest dissolved-nitrate 

yields occur in one urban and three forest watersheds 

where nitrogen inputs are predominantly from atmospheric sources. The largest dissolved-nitrate yields 

occur at Conococheague Creek, Antietam Creek, and 

Monocacy River where agricultural land use is widespread and nitrogen inputs are dominated by commercial fertilizer and animal manure. The transport of 

phosphorus also differs greatly, as total-phosphorus 

yields range from 68 to 654lb/mi2, with the yield at 

the Washington, D.C., site being 496 lb/mi2• Small 

phosphorus yields occur primarily in forest watersheds 

of the Valley and Ridge subunit. Larger phosphorus 

yields occur in watersheds with greater agricultural 

intensity and inputs from wastewater-treatment 

facilities. 

78 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 1970-90 
The proportion of nitrogen inputs retained and 

exported from Potomac River watersheds differs considerably and is affected by hydrologic processes. 

Nitrogen inputs to forest watersheds are low, with 11 

to 13 percent of the nitrogen being exported in surface 

water. Nitrogen inputs to agricultural watersheds are 

higher and more variable, with export rates ranging 

from 7 to 35 percent. Nitrogen-export rates are highest 

in Conococheague Creek, Antietam Creek, and Monocacy River where fertilizer is the predominant nitrogen 

source. 

Ground-water discharge can contribute a substantial part of the total-nitrogen load to streams in many 

watersheds, particularly in those areas where concentrations of nitrogen in ground water are elevated due to 

agricultural land use. The contribution of ground 

water to annual streamflow ranges from 31.8 to 

81.3 percent, and base-flow loads of nitrogen can 

account for between 3 and 100 percent of the 

measured nitrogen load. Base-flow nitrogen loads 

were largest in Conococheague Creek and Antietam 

Creek, two agricultural watersheds where base-flow 

loads contributed about 61 and 100 percent of measured nitrogen loads. Both of these watersheds are 

largely cropland underlain by carbonate rock where 

nitrogen transport from surficial application down to 

the ground-water system can occur quickly. Although 

land use is similar, base-flow loads in the Triassic 

Lowlands subunit at Monocacy River are smaller 

(42 percent), primarily due to low infiltration rates, 

poor transmissivity of the siliciclastic rocks, and lower 

nitrogen concentrations in ground water. 

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Resource Report 49, 87 p. 

Wright, W.G., 1990, Ground-water hydrology and quality in 

the Valley and Ridge and Blue Ridge physiographic 

provinces of Clarke County, Virginia: U.S. Geological 

Survey Water-Resources Investigations Report 

90-4134, 61 p. 

Selected References 81 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

·selected surface-water-quality monitoring sites in the Potomac River Basin 

[Loads and uncertainties reported in thousands of pounds per year;--, loads and uncertainties not estimated. Location of sites shown in figure 16] 

Water 

year 

1979 

1980 

1981 

1982 

1983 

1984 

Mean 

1973 

1974 

1975 

1976 

1977 

1978 

1979 

1980 

1981 

1982 

1983 

Mean 

1979 

1980 

1981 

1982 

1983 

1984 

1985 

1986 

1987 

1988 

1989 

Mean 

1979 

1980 

1981 

1982 

1983 

1984 

1985 

1986 

1987 

Load 

1,357 

862 

575 

577 

452 

776 

767 

3,390 

3,860 

1,560 

2,980 

3,020 

4,280 

2,700 

3,460 

2,720 

1,670 

2,030 

2,880 

1,700 

1,770 

693 

1,380 

1,600 

2,180 

685 

1,190 

1,210 

Total nitrogen Dissolved nitrate as nitrogen Total phosphorus 

Standard Standard 

Standard error of Standard error of 

error estimate Load error estimate Load 

Site 1 North Branch Potomac River at Kitzmiller, Md. 

260 

96.3 

40.9 

41.7 

33.1 

67.4 

89.9 

269 717 45.2 56.6 107.4 

103 623 29.6 38.1 91 

48.2 534 24.3 32.2 76.3 

50 

39.9 

78.3 

98.1 

619 

498 

796 

631 

30.6 

22.7 

46.5 

33.2 

40.5 

31.4 

59.4 

43 

97.5 

76.2 

159.1 

101.2 

Site 2 North Branch Potomac River near Cumberland, Md. 

2,590 194 230 174 

2,270 

3,000 

1,680 

2,320 

3,630 

3,940 

3,060 

1,870 

2,300 

1,750 

2,580 

155 

234 

113 

179 

334 

411 

250 

183 

305 

298 

241 

191 

272 

141 

219 

384 

476 

282 

206 

328 

312 

276 

179 

218 

159 

187 

260 

249 

254 

152 

145 

108 

190 

Site 3 South Branch Potomac River at Springfield, W.Va. 

288 318 3,580 549 656 244 

307 335 4,050 608 715 282 

86 106 1,550 185 256 92.3 

249 282 3,050 471 589 167 

242 

395 

175 

545 

242 

134 

193 

260 

512 

435 

100 

368 

419 

641 

99 

365 

321 

274 

429 

206 

578 

272 

157 

209 

288 

3,180 

4,360 

2,780 

2,980 

2,790 

1,650 

2,100 

2,920 

477 

718 

362 

495 

452 

268 

407 

454 

595 

840 

471 

586 

546 

339 

466 

551 

Site 6 Cacapon River at Great Cacapon, W.Va. 

534 972 441 484 

454 

114 

395 

447 

669 

118 

386 

343 

1,340 

772 

1,380 

1,900 

2,470 

903 

1,220 

1,260 

557 

257 

596 

847 

1,120 

267 

436 

519 

617 

323 

690 

964 

1,230 

347 

517 

600 

154 

228 

119 

304 

114 

65.7 

83.1 

168 

61.1 

73.1 

35 

49.7 

58.4 

74.3 

28 

38.8 

37.4 

Standard 

Standard error of 

error estimate 

19.6 

12.2 

8.98 

13 

9.33 

27.3 

15.1 

33.4 

25.5 

27.8 

18.5 

23.6 

34.5 

33.6 

40.1 

31.4 

40.5 

42.3 

31.9 

41.2 

47.3 

10.8 

28.7 

25.4 

41.6 

15.5 

198 

23.8 

12.2 

16.4 

41.9 

16.8 

20.1 

6.33 

13 

15.8 

20.4 

4.36 

9.96 

9.4 

28.9 

19.1 

14.1 

20.5 

16.4 

42.2 

23.5 

34.6 

27.2 

30.2 

20.6 

26.5 

37.5 

36.2 

42.4 

32.6 

41.2 

42.7 

33.8 

48.9 

55.2 

14.5 

35.9 

31.4 

48 

20 

224 

29.6 

16 

18.4 

49.3 

17.5 

21 

7.1 

14 

16.8 

21.4 

5.06 

10.6 

10.1 

82 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected surface-water-quality monitoring sites in the Potomac River Basin-Continued 

Total nitrogen Dissolved nitrate as nitrogen Total phosphorus 

Standard Standard Standard 

Water Standard error of Standard error of Standard error of 

year Load error estimate Load error estimate Load error estimate 

Site 6 Cacapon River at Great Cacapon, W.Va.-Continued 

1988 874 252 272 707 292 344 26.6 7.33 7.93 

1989 653 148 160 522 233 271 21.2 5.35 5.76 

Mean 1,270 333 354 1,220 506 581 45.8 11.7 12.5 

Site 7A1 Conococheague Creek at Fairview, Md. 

1979 5,200 300 320 330 130 133 

1980 5,020 282 296 231 50.5 51.4 

1981 2,700 140 152 144 23.6 24.7 

1982 3,810 228 237 166 24 24.6 

1983 4,170 304 315 194 35.8 36.7 

1984 7,790 743 758 482 221 227 

1985 3,140 318 323 143 31.4 31.9 

1986 5,300 685 693 252 82.5 83.7 

Mean 4,640 375 387 243 74.8 76.6 

Site 8A Potomac River at Shepherdstown, W.Va. 

1979 25,200 1,680 1,860 14,200 779 841 1,460 172 207 

1980 25,800 1,100 1,250 15,600 600 670 1,310 83.5 99.3 

1981 13,100 454 561 8,670 284 337 515 23.7 33 

1982 22,200 1,090 1,248 13,200 585 656 1,120 81.2 101 

1983 22,600 1,300 1,450 13,100 673 736 1,200 106 129 

1984 38,800 2,670 2,920 20,600 1,200 1,280 2,760 331 394 

1985 18,200 948 1,050 11,900 586 636 694 47.6 56.2 

1986 33,000 2,540 2,910 16,800 962 1,050 4,050 783 1,030 

1987 23,900 1,330 1,500 14,300 709 774 1,000 94.1 121 

1988 20,500 1,340 1,480 12,300 699 750 881 98.4 119 

1989 22,200 1,850 1,920 13,800 1,070 1,100 679 78.2 84.9 

Mean 24,100 1,480 1,650 14,000 741 803 1,420 173 216 

Site SB Potomac River at Shepherdstown, W.Va. 

1979 23,700 2,480 2,670 2,350 584 678 

1980 24,400 2,220 2,380 2,330 424 477 

1981 11,200 767 879 1,050 133 168 

1982 17,400 1,390 1,560 1,890 321 390 

1983 16,600 1,330 1,500 2,060 378 462 

1984 27,800 2,560 2,790 3,940 967 1,116 

1985 14,700 889 1,050 1,500 181 229 

1986 24,300 2,810 3,080 4,450 1,760 2,170 

1987 19,900 2,100 2,250 2,400 551 651 

Mean 20,000 1,840 2,020 2,440 589 705 

Site 9A Antietam Creek near Sharpsburg, Md. 

1979 2,720 176 181 

1980 2,940 176 181 

1981 1,650 81.9 85.8 

1982 2,380 141 146 

1983 2,480 175 180 

Estimated Annual Loads of Total Nitrogen, Dissolved Nitrate, and Total Phosphorus 83 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected surface-water-quality monitoring sites in the Potomac River Basin-Continued 

Total nitrogen Dissolved nitrate as nitrogen Total phosphorus 

Standard Standard Standard 

Water Standard error of Standard error of Standard error of 

year Load error estimate Load error estimate Load error estimate 

Site 9A Antietam Creek near Sharpsburg, Md.-Continued 

1984 4,310 431 436 

1985 2,320 221 224 

1986 2,970 358 361 

Mean 2,720 220 224 

Site 10 Christians Creek at Fishersville, Va. 

1979 335 58.2 60.1 224 40.9 42.2 22.3 7.07 7.32 

1980 379 37.8 39.3 285 33.4 34.7 24.6 4.3 4.49 

1981 96.2 5.17 5.52 71.2 4.83 5.2 12.5 1.2 1.25 

1982 253 19.8 21.8 186 14.9 16.4 23.5 3.48 3.9 

1983 368 44.6 46.8 263 31 32.6 33.5 7.74 8.18 

1984 572 81.5 83.7 410 61.9 63.6 50.3 13.6 14 

1985 275 16.1 18.1 223 15.1 16.9 24.8 2.34 2.63 

1986 435 97.9 102 304 57.6 60 51.2 28 29.4 

1987 494 89.8 92.8 351 56.9 58.8 44.2 17.1 17.8 

1988 184 10.7 11.3 161 12 12.7 16 1.6 1.67 

1989 417 64.1 66.1 326 54.1 55.6 28.2 7.81 8.1 

Mean 346 47.8 49.8 255 34.8 36.2 30.1 8.57 8.98 

Site 11B South Fork Shenandoah River at Front Royal, Va. 

1979 4,740 488 545 804 160 176 

1980 6,450 686 755 712 75.7 82.9 

1981 1,620 131 156 271 18.9 20.9 

1982 3,740 328 383 468 39.2 46.6 

1983 4,450 428 486 611 73.3 81.9 

1984 6,290 649 717 859 156 172 

1985 3,260 268 318 335 21.2 25.2 

1986 3,660 351 410 1,810 1,400 1,510 

1987 3,600 386 428 673 190 213 

1988 2,640 319 349 288 29.9 31.8 

1989 2,480 409 428 363 55 58.4 

Mean 3,900 404 452 654 202 220 

Site 12 North Fork Shenandoah River at Cootes Store, Va. 

1979 579 53.2 59.9 35.4 12.4 13.8 

1980 689 63.2 70 35.3 11.2 12.2 

1981 145 8.9 11.6 6.42 1.38 1.66 

1982 444 32.7 38.1 22.2 5.75 6.68 

1983 486 40.5 46.5 27.8 8.42 9.49 

1984 736 63.1 71 41.7 13.1 14.6 

1985 335 19.7 24.6 14.9 2.87 3.53 

1986 637 89.6 105 80.2 61 70.6 

1987 633 58.2 65.3 49.7 17.2 19.9 

1988 407 36.4 42 30.2 9.68 11.9 

1989 435 50.4 53.8 34.7 11.4 12.2 

Mean 502 46.9 53.4 34.4 14 16 

84 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197G-90 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected surface-water-quality monitoring sites in the Potomac River Basin-Continued 

Total nitrogen Dissolved nitrate as nitrogen Total phosphorus 

Standard Standard Standard 

Water Standard error of Standard error of Standard error of 

year Load error estimate Load error estimate Load error estimate 

Site 13 Smith Creek near New Market, Va. 

1979 393 33.1 34.1 

1980 500 31.7 33.1 

1981 108 4.64 5.17 

1982 329 14.7 16.4 

1983 438 23.2 25 

1984 619 39.9 41.8 

1985 234 10.1 11.5 

1986 343 19.1 20.9 

1987 375 21.5 23.2 

1988 317 17.8 19.1 

1989 208 16.9 17.7 

Mean 351 21.1 22.5 

Site 14A North Fork Shenandoah River at Strasburg, Va. 

1979 3,190 244 294 

1980 3,610 267 317 

1981 607 35.8 54.5 

1982 2,240 163 211 

1983 2,600 220 268 

1984 4,030 380 430 

1985 1,480 143 172 

1986 2,090 259 289 

1987 2,690 427 454 

1988 1,960 359 378 

1989 1,570 366 379 

Mean 2,370 260 295 

Site 14B North Fork Shenandoah River at Strasburg, Va. 

1979 3,040 178 198 3,000 293 335 

1980 3,420 193 215 3,770 383 430 

1981 740 34.6 43.5 654 57.4 79.1 

1982 2,170 112 133 2,220 213 258 

1983 2,420 140 162 2,350 236 281 

1984 3,580 209 233 3,800 382 436 

1985 1,470 71.3 87.3 1,570 148 186 

1986 1,950 Ill 130 2,130 224 266 

1987 2,440 165 183 2,310 274 311 

1988 1,840 136 149 1,770 238 266 

1989 1,590 154 164 1,220 214 230 

Mean 2,240 137 154 2,250 242 280 

Site 15 Shenandoah River at Millville, W.Va. 

1979 13,100 1,400 1,810 1,530 128 193 

1980 15,500 1,520 1,910 1,460 87.2 119 

1981 2,210 173 243 306 13.4 17.6 

1982 6,960 641 887 831 50.8 75.2 

Estimated Annual Loads of Total Nitrogen, Dissolved Nitrate, and Total Phosphorus 85 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected surface-water-quality monitoring sites in the Potomac River Basrn-Continued 

Total nitrogen Dissolved nitrate as nitrogen Total phosphorus 

Standard Standard Standard 

Water Standard error of Standard error of Standard error of 

year Load error estimate Load error estimate Load error estimate 

Site 15 Shenandoah River at Millville, W.Va.-Continued 

1983 8,650 879 1,120 1,170 89.7 118 

1984 14,700 1,680 2,030 1,770 158 216 

1985 5,780 615 810 413 25.5 35.3 

1986 9,540 1,160 1,570 2,390 525 929 

1987 9,280 1,100 1,330 853 91.1 152 

1988 7,130 983 1,130 323 28.9 34.9 

1989 6,720 1,260 1,370 317 40.5 45.1 

Mean 9,050 1,040 1,290 1,030 113 176 

Site 17 Monocacy River at Bridgeport, Md. 

1979 1,120 252 282 93.9 16.4 18.7 

1980 782 137 155 62.8 7.2 8.31 

1981 432 94.2 118 30.3 3.58 4.36 

1982 739 154 175 43.6 5.47 6.18 

1983 852 199 217 49.3 7.88 8.65 

1984 1,460 376 400 78.4 14.4 15.3 

1985 526 119 138 27.3 4.28 4.83 

1986 851 215 235 32.3 5.8 6.22 

1987 892 237 255 31.4 6.32 6.69 

1988 772 234 252 25.8 6.18 6.57 

1989 641 232 241 24.1 7.11 7.38 

Mean 824 204 224 45.4 7.69 8.47 

Site 18 Hunting Creek tributary near Foxville, Md. 

1982 8.71 0.75 0.87 

1983 11 .87 1.1 

1984 17.7 1.77 2.02 

1985 6.65 .44 .56 

1986 9.48 .74 .88 

1987 8.53 .76 .98 

1988 7.35 .79 .97 

1989 7.5 .97 1.06 

Mean 9.62 .89 1.06 

Site 19 Monocacy River near Frederick, Md. 

1973 7,620 483 494 5,900 198 237 580 49.8 53.3 

1974 5,210 213 231 4,000 120 161 401 24.5 30.5 

1975 8,880 419 481 5,860 218 284 1,100 152 214 

1976 6,360 148 179 4,730 155 193 469 22.9 30.4 

1977 6,090 217 269 4,080 146 201 597 57.1 90 

1978 8,360 292 333 5,650 223 277 607 41.8 53.2 

1979 9,290 418 466 5,930 270 328 742 69.7 88.2 

1980 6,890 296 317 4,650 216 246 448 32.4 38.2 

1981 3,610 219 231 2,430 132 153 230 19 21.4 

1982 5,070 442 449 3,420 246 261 283 32.5 34 

1983 7,450 923 935 4,590 440 460 423 69.9 73.6 

Mean 6,800 370 399 4,660 215 255 535 52 66.1 

86 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197D-90 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected surface-water-quality monitoring sites in the Potomac River Basin-Continued 

Water 

year 

1979 

1980 

1981 

1982 

1983 

1984 

1985 

1986 

1987 

Mean 

1979 

1980 

1981 

1982 

1983 

1984 

1985 

1986 

1987 

1988 

1989 

Mean 

1979 

1980 

1981 

1982 

1983 

1984 

1985 

1986 

1987 

1988 

1989 

Mean 

1979 

1980 

1981 

1982 

1983 

1984 

Total nitrogen Dissolved nitrate as nitrogen 

Load 

2,150 

1,670 

334 

1,130 

1,850 

2,840 

645 

1,030 

1,250 

1,430 

196 

193 

74.2 

104 

150 

236 

103 

90.2 

147 

161 

207 

151 

78,500 

75,600 

24,400 

49,600 

59,300 

102,000 

Standard Standard 

Standard error of Standard error of 

error estimate Load error estimate 

Site 20 Goose Creek near Leesburg, Va. 

190 244 1,820 240 309 

118 140 1,410 147 176 

21.7 

80 

154 

249 

54.7 

98.8 

163 

125 

11 

10.8 

2.82 

4.39 

7 

12 

4.17 

3.52 

7.18 

9.49 

15.9 

8.02 

2,740 

2,500 

685 

1,460 

1,910 

3,750 

28.8 

105 

194 

294 

92 

112 

179 

154 

277 

972 

1,600 

2,440 

537 

879 

1,050 

1,220 

28.2 

105 

201 

322 

70 

127 

204 

160 

39.8 

141 

254 

383 

119 

146 

226 

199 

Site 21B Seneca Creek at Dawsonville, Md. 

870 79.6 86.8 

730 

274 

377 

562 

749 

285 

247 

412 

507 

489 

500 

57.7 

18.8 

27 

45 

62.8 

19.8 

17.2 

31.5 

43.3 

45.7 

40.8 

62.7 

21.6 

31.2 

50.2 

67.8 

23.2 

19.4 

35.7 

47.8 

49.5 

45.1 

Site 22 Difficult Run near Great Falls, Va. 

13.2 119 8.13 9.55 

13.3 

3.74 

5.88 

8.7 

14 

5.93 

4.47 

8.77 

11.4 

18.6 

9.82 

117 

51.8 

68.8 

95.1 

143 

69.1 

63.7 

95.8 

103 

126 

95.7 

7.6 

2.82 

3.99 

5.93 

9.7 

3.8 

3.62 

6.52 

8.28 

12.8 

6.65 

8.84 

3.56 

5.04 

7.19 

11.2 

4.95 

4.48 

7.71 

9.5 

14 

7.82 

Site 23 Potomac River at Washington, D.C. 

4,070 51,000 4,980 5,730 

3,230 51,400 3,690 4,330 

1,030 

2,170 

2,760 

5,150 

16,300 

32,100 

34,200 

59,100 

840 

2,140 

2,740 

5,600 

1,260 

2,780 

3,260 

6,310 

Total phosphorus 

Standard 

Standard error of 

Load error estimate 

97.6 

84.4 

13.5 

56.8 

98 

168 

33.7 

56.2 

69.6 

75.3 

48.4 

23.4 

4.12 

7.59 

13 

17.9 

7.26 

5.45 

16.1 

30 

36.9 

19.1 

18 

24.7 

4.44 

7.99 

13 

24 

9.17 

6.19 

13.4 

17.1 

29.9 

15.3 

9,370 

7,220 

1,890 

4,750 

6,560 

12,600 

17.3 

13.4 

1.8 

8.31 

16.4 

28.3 

5.8 

10.6 

18.2 

13.3 

21.8 

7.79 

.5 

1.16 

2.2 

2.93 

1.1 

.62 

2.73 

7.6 

12.5 

5.54 

3.77 

7.38 

.55 

1.15 

1.97 

3.82 

1.36 

.7 

1.94 

3.34 

9.47 

3.22 

1,110 

612 

129 

352 

580 

1,470 

22.6 

16.7 

2.18 

11 

21 

33.6 

9.98 

12 

19.9 

16.6 

27.1 

11.5 

.59 

1.71 

2.81 

3.51 

1.8 

.73 

3.69 

10.5 

18 

7.45 

5.15 

11.5 

.82 

1.89 

2.66 

4.94 

2.46 

.91 

2.63 

4.92 

14.6 

4.77 

2,140 

923 

237 

690 

1,070 

2,650 

Estlma~ed Annual Loads of Total Nitrogen, Dissolved Nitrate, and Total Phosphorus 87 
Table 16. Estimated annual loads of total nitrogen, dissolved nitrate, and total phosphorus at 

selected surface-water-quality monitoring sites in the Potomac River Basin-Continued 

Total nitrogen Dissolved nitrate as nitrogen Total phosphorus 

Standard Standard Standard 

Water Standard error of Standard error of Standard error of 

year Load error estimate Load error estimate Load error estimate 

Site 23 Potomac River at Washington, D.C.-Continued 

1985 38,800 1,270 1,810 27,600 2,460 2,890 2,490 195 385 

1986 72,400 3,430 5,140 43,400 4,200 4,930 10,300 2,070 4,670 

1987 58,600 2,550 3,300 39,200 3,200 3,790 3,820 474 876 

1988 49,000 2,530 3,120 33,900 2,990 3,490 2,690 387 636 

1989 51,500 3,370 3,750 34,900 4,220 4,540 1,940 314 407 

Mean 60,000 2,380 3,230 38,500 3,370 3,940 5,780 699 1,330 

88 Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of Available Nutrient Data, 197G-90 

'A-U.S. GOVERNMENT PRINTING OFFICE: 1996-758-637 

J.D. BLOOMQUIST and others-Water-Quality Assessment of the Potomac River Basin: Basin Description and Analysis of 

Available Nutrient Data, 1970-90-USGS/WRIR 95-4221 

I'