U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5076

Prepared in cooperation with the Virginia Department of Environmental Quality and the

U.S. Environmental Protection Agency

Mercury Loads in the South River and Simulation of Mercury 

Total Maximum Daily Loads (TMDLs) for the South River, 

South Fork Shenandoah River, and Shenandoah River: 

Shenandoah Valley, Virginia
Cover. South River. View looking downstream from the old Route 611 bridge near Dooms, Virginia. Water-quality monitoring equipment in 

foreground. (Photograph by Jack Eggleston, U.S. Geological Survey, June 2005.)
Mercury Loads in the South River and 

Simulation of Mercury Total Maximum Daily 

Loads (TMDLs) for the South River, South 

Fork Shenandoah River, and Shenandoah 

River: Shenandoah Valley, Virginia

By Jack Eggleston

Prepared in cooperation with the Virginia Department of Environmental Quality 

and the U.S. Environmental Protection Agency

Scientific Investigations Report 2009–5076

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Eggleston, Jack, 2009, Mercury loads in the South River and simulation of mercury total maximum daily loads (TMDLs) 

for the South River, South Fork Shenandoah River, and Shenandoah River—Shenandoah Valley, Virginia: U.S. Geological Survey Scientific Investigations Report 2009–5076, 80 p., available online at http://pubs.usgs.gov/sir/2009/5076/

ISBN 978-1-4113-2599-9
iii

Contents

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

Introduction.....................................................................................................................................................1

Background and History......................................................................................................................3

Regulatory Approach and Total Maximum Daily Load Scope.......................................................3

Description of the Study Area ............................................................................................................4

Previous Studies ...................................................................................................................................6

Modeling Approach..............................................................................................................................6

Data Collected by the U.S. Geological Survey for This Study ................................................................7

Streamflow.............................................................................................................................................7

Water Quality ........................................................................................................................................7

Methods..................................................................................................................................................8

Suspended Sediment ..........................................................................................................................9

Turbidity ...............................................................................................................................................10

Mercury ...............................................................................................................................................10

Units and Terms..........................................................................................................................10

Surface Water............................................................................................................................10

Groundwater...............................................................................................................................12

Sediment......................................................................................................................................12

Mercury Sorption to Suspended Solids.................................................................................16

Data Compiled From Other Sources .........................................................................................................16

Suspended Sediment .........................................................................................................................16

Mercury in Surface Water ................................................................................................................16

Atmospheric Mercury Deposition....................................................................................................16

Mercury in Fish Tissue.......................................................................................................................18

Mercury in Soil....................................................................................................................................18

Mercury in Groundwater...................................................................................................................19

Development of Time-Series Data ............................................................................................................19

Streamflow Time Series.....................................................................................................................19

Suspended Sediment Concentration Time Series.........................................................................20

Meteorological and Climatic Data ...................................................................................................21

Point Sources ......................................................................................................................................23

Additional Model Flows to the River ...............................................................................................23

Conceptual Model of Mercury Fate and Transport in the Watershed ...............................................23

Watershed Model Development ...............................................................................................................24

Functional Description of Hydrological Simulation Program–FORTRAN (HSPF) ....................24

Representation of the Watershed ...................................................................................................26

Development of Hydrologic Response Units (HRUs) ..........................................................27

Stream Reaches ........................................................................................................................29

Hydrologic Model................................................................................................................................29

Point-Source Discharges ..................................................................................................................30

Sediment Transport Model................................................................................................................30

Impervious Land Area Sediment Transport...........................................................................31
iv

Pervious Land Area Sediment Transport...............................................................................31

Sediment Point Sources ...........................................................................................................32

Sediment Transport in the River..............................................................................................32

Mercury Transport Model .................................................................................................................32

Mercury Sources to the River .................................................................................................34

Atmospheric Deposition ..................................................................................................34

Land-Surface Runoff ........................................................................................................34

Groundwater and Interflow.............................................................................................35

Point-Source Discharges ................................................................................................35

Riverbank and Channel Margin Inputs..........................................................................35

Mercury Transport Within the River.......................................................................................35

Watershed Model Results..........................................................................................................................36

Streamflow Model Calibration Results ...........................................................................................36

Sediment Transport Model Calibration Results ............................................................................45

Mercury Transport Model Calibration Results...............................................................................52

Limitations of the South River Watershed Model and 

Suggestions for Future Investigations...............................................................................58

Simulation of Mercury Total Maximum Daily Load (TMDL)..................................................................58

Designation of Endpoints...................................................................................................................58

Existing Conditions .............................................................................................................................59

Scenarios for Mercury Load Reductions .......................................................................................61

Future Conditions.......................................................................................................................61

Single Source Reductions........................................................................................................62

Multiple Source Reductions and Total Maximum Daily Load (TMDL) Scenario.............65

Sensitivity of Total Maximum Daily Load (TMDL) Results to Model Parameter Values..........67

Implications for the Downstream South Fork Shenandoah River 

and Shenandoah River.........................................................................................................69

Summary........................................................................................................................................................72

Acknowledgments.......................................................................................................................................72

References Cited..........................................................................................................................................73

Appendix 1. Data sources used to define point-source loading in the model ...............................78

Appendix 2. User’s Control Input File for the Calibrated South River 

Watershed Model Under Existing Conditions (Available only online 

at http://pubs.usgs.gov/sir/2009/5076/)
v

Figures

1–2. Maps showing locations of—

 1. Rivers in the Shenandoah Valley, Virginia, on the 303d list of 

contaminated water for elevated concentrations of 

methylmercury in fish tissue................................................................................................2

 2. The South River watershed study area in the Shenandoah Valley, Virginia ...............5

3. Timeline for data collection and model simulation, South River, Virginia ...........................6

4–10. Graphs showing—

 4. Annual streamflow for the South River near Waynesboro, Virginia (USGS 

station number 01626000) and at Harriston, Virginia (USGS station

number 01627500), water years 1926–2007 .......................................................................7

 5. Observed suspended sediment concentrations and streamflow 

in the South River, Virginia, April 1, 2005, through March 31, 2007..............................10

 6. Streamflow and turbidity from June 2005 through April 2007 for the 

South River at Harriston, Virginia (USGS station number 01627500) ..........................11

 7. Observed total unfiltered mercury concentration and concurrent 

instantaneous streamflow in the South River, Virginia, 

April 1, 2005, through March 31, 2007...............................................................................13

 8. Observed aqueous concentration of filterable mercury and 

concurrent instantaneous streamflow, South River, Virginia, 

April 1, 2005, through March 31, 2007...............................................................................13

 9. Observed aqueous concentration of mercury associated with 

particulate matter and concurrent instantaneous streamflow, 

South River, Virginia, April 1, 2005, through March 31, 2007.........................................14

 10. Observed concentration of mercury on suspended particulate 

matter and concurrent instantaneous streamflow, South River, 

Virginia, April 1, 2005, through March 31, 2007...............................................................14

11. Map showing location of mercury deposition and precipitation monitoring sites..........17

12–14. Graphs showing—

 12. Fish tissue mercury concentrations in smallmouth bass, 1999–2007, 

South River, South Fork Shenandoah River, and Shenandoah River, Virginia...........18

 13. Time series of regressed and sampled suspended sediment 

concentrations, South River at Harriston, Virginia (USGS station number 

01627500), April 1, 2005, through March 31, 2007 ...........................................................21

 14. Cumulative precipitation at sites in and near the South River watershed, 

Virginia, for the period April 1, 2005, through March 31, 2007......................................22

15. Conceptual model of total mercury sources, sinks, and transport 

in the South River, Virginia ........................................................................................................23

16. Schematic showing hydrologic routing in the South River 

numerical watershed model .....................................................................................................25

17–18. Maps showing—

 17. Location of South River watershed model subbasins...................................................26

 18. Land use in the South River watershed...........................................................................28
vi

19–22. Diagrams showing—

 19. Major hydrologic components for pervious land areas and river 

reaches simulated in the South River numerical watershed model ..........................29

 20. Sediment transport processes for pervious land areas ...............................................31

 21. Sediment routing processes in the South River numerical watershed model..........32

 22. Mercury routing in the South River numerical watershed model...............................33

23–59. Graphs showing—

 23. Observed mean annual flow, South River at Harriston, Virginia (USGS 

station number 01627500), water years 1926–2007 ........................................................37

 24. Simulated and observed daily streamflow for the calibration period 

water years 1991–2000, calibrated model, South River near 

Waynesboro, Virginia (USGS station number 01626000)...............................................39

 25. Simulated and observed daily streamflow during the verification 

period April 1, 2005, through March 31, 2007, calibrated model, 

South River near Waynesboro, Virginia (USGS station number 01626000)................40

 26. Simulated and observed daily streamflow during the calibration period 

water years 1991–2000, calibrated model, South River at Harriston, Virginia 

(USGS station number 01627500) ......................................................................................40

 27. Simulated and observed daily streamflow for the verification period 

April 1, 2005, through March 31, 2007, calibrated model,

South River at Harriston, Virginia (USGS station number 01627500) ..........................41

 28. Simulated and observed annual runoff during the calibration period 

water years 1991–2000, calibrated model, South River near 

Waynesboro, Virginia (USGS station number 01626000)...............................................41

 29. Simulated and observed total runoff during the verification period 

April 1, 2005, through March 31, 2007, calibrated model, South River 

near Waynesboro, Virginia (USGS station number 01626000) .....................................42

 30. Simulated and observed annual runoff during the calibration period 

water years 1991–2000, calibrated, South River at Harriston, Virginia 

(USGS station number 01627500) ......................................................................................42

 31. Simulated and observed total runoff during the verification period 

April 1, 2005, through March 31, 2007, calibrated model, South River 

at Harriston, Virginia (USGS station number 01627500) ................................................43

 32. Flow duration curves for observed and simulated streamflow, calibration 

period water years 1991–2000, calibrated model, for the South River 

near Waynesboro, Virginia (USGS station number 01626000) and at 

Harriston, Virginia (USGS station number 01627500).....................................................43

 33. Flow duration curves for observed and simulated daily streamflow, 

calibrated model, South River near Waynesboro , Virginia (USGS station 

number 01626000) and at Harriston, Virginia (USGS station number 01627500), 

verification period April 1, 2005, through March 31, 2007.............................................44

 34. 1:1 comparison of simulated and observed daily streamflow, calibrated 

model, South River near Waynesboro, Virginia (USGS station number 

01626000), water years 1991–2000 and April 2005 through March 2007.....................44

 35. 1:1 comparison of simulated and observed daily streamflow, calibrated 

model, South River at Harriston, Virginia (USGS station number 01627500), 

water years 1991–2000 and April 2005 through March 2007 ........................................45

 36. Simulated depth of sediment in model river reaches 1 and 4, 

calibrated South River model, water years 1991–2000 .................................................46
vii

 37. Simulated daily, observed (regressed), and sampled suspended sediment 

concentrations for the calibration period, water years 1991–2000, South 

River near Waynesboro, Virginia (USGS station number 01626000)...........................48

 38. Simulated daily, observed (regressed), and sampled suspended sediment 

concentrations for the verification period April 1, 2005, through 

March 31, 2007, calibrated model, South River near Waynesboro, 

Virginia (USGS station number 01626000) .......................................................................48

 39. Simulated and observed (regressed) suspended sediment concentrations 

for the calibration period water years 1991–2000, calibrated model, 

South River at Harriston, Virginia (USGS station number 01627500) ..........................49

 40. Simulated and observed (regressed) suspended sediment concentrations 

for the verification period April 1, 2005, through March 31, 2007, calibrated 

model, South River at Harriston, Virginia (USGS station number 01627500) .............49

 41. Annual simulated and observed (regressed) suspended sediment loads 

for the calibration period water years 1991–2000, calibrated model, 

South River near Waynesboro, Virginia (USGS station number 01626000)................50

 42. Annual simulated and observed (regressed) suspended sediment 

loads for the verification period April 1, 2005, through March 31, 2007, 

calibrated model, South River near Waynesboro, Virginia 

(USGS station number 01626000) ......................................................................................50

 43. Annual simulated and observed (regressed) suspended sediment loads 

for the calibration period, water years 1991–2000, calibrated model, 

South River at Harriston, Virginia (USGS station number 01627500) ..........................51

 44. Annual simulated and observed (regressed) suspended sediment loads 

for the verification period, April 1, 2005, through March 31, 2007, calibrated 

model, South River at Harriston, Virginia (USGS station number 01627500) .............51

 45. Duration plots for simulated and observed (regressed) daily suspended 

sediment concentrations, calibration period water years 1991–2000, 

calibrated model, South River, Virginia............................................................................52

 46. Duration plots for simulated and observed (regressed) daily suspended 

sediment concentration for the verification period April 1, 2005, 

through March 31, 2007, calibrated model, South River, Virginia................................53

 47. Simulated and observed total unfiltered mercury concentration 

distributions showing poor calibration obtained with no channel margin 

inputs, simulation period April 1, 2005, through March 31, 2007, 

South River at Dooms, Virginia (USGS station number 01626920)...............................54

 48. Simulated total unfiltered mercury concentration time series and 

observed concentrations, calibrated model, existing conditions, 

April 2005 through March 2007, South River near Waynesboro, Virginia 

(USGS station number 01626000), South River at Dooms, Virginia 

(USGS station number 01626920), and South River at Harriston, 

Virginia (USGS station number 01627500) .......................................................................55

 49. Duration curves for simulated and observed (USGS samples only) 

total unfiltered mercury concentrations, calibrated model, existing 

conditions, April 1, 2005, to March 31, 2007, South River near 

Waynesboro, Virginia (USGS station number 01626000), at 

Dooms, Virginia (USGS station number 0162920), and at Harriston, 

Virginia (USGS station number 01627500) .......................................................................57
viii

 50. Percent mercury loading to the South River from sources upstream 

(model reach 1) and downstream (model reaches 2–5) of the plant 

site, calibrated model, existing conditions......................................................................60

 51. Simulated total unfiltered mercury values for the South River 

at Harriston, Virginia (USGS station number 01627500), under 

future conditions, Scenario 2 ............................................................................................62

 52. Simulated total unfiltered mercury values for the South River at 

Harriston, Virginia (USGS station number 01627500), under 

future conditions with point sources cleaned up, Scenario 3A ..................................63

 53. Simulated total unfiltered mercury values for the South River at 

Harriston, Virginia (USGS station number 01627500), under future 

conditions with channel margin mercury sources cleaned up, 

Scenario 3B..........................................................................................................................64

 54. Simulated total unfiltered mercury values for the South River 

at Harriston, Virginia (USGS station number 01627500), under 

future conditions with runoff cleaned up, Scenario 3C ................................................64

 55. Simulated total unfiltered mercury concentrations for Scenario 4A, 

reduced channel margin and runoff mercury loads, South River at 

Harriston, Virginia (USGS station number 01627500).....................................................65

 56. Simulated total unfiltered mercury concentrations for the Total 

Maximum Daily Load Scenario (Scenario 4B), reduced channel 

margin, runoff, and point-source mercury loads, South River at 

Harriston, Virginia (USGS station number 01627500).....................................................66

 57. Comparison of total unfiltered mercury concentrations (Virginia 

Department of Environmental Quality samples only) from the 

South River at Harriston, Virginia (USGS station number 01627500) to 

same-day total unfiltered mercury concentrations from the 

South Fork Shenandoah River near Luray, Virginia (USGS station 

number 01629500) ................................................................................................................69

 58. Simulated total unfiltered mercury concentrations for the Total 

Maximum Daily Load Scenario (Scenario 4B), reduced channel 

margin, runoff, and point-source mercury loads, South Fork 

Shenandoah River near Luray, Virginia (USGS station number 01629500) ................70

 59. Estimated total unfiltered mercury concentrations for the 

Total Maximum Daily Load Scenario (Scenario 4B), reduced 

channel margin, runoff, and point-source mercury loads, 

Shenandoah River at the confluence with Craig Run, 

Clarke County, Virginia........................................................................................................71
ix

Tables

1. Selected characteristics of the North, South, South Fork Shenandoah, 

and Shenandoah Rivers, Virginia...............................................................................................3

2. Streamflow-gaging stations and water-quality monitoring sites used 

in the study, South River, Virginia, 2005–2007 ..........................................................................4

3. Water-quality sample treatments and laboratories used in the study ................................8

4. Detection limits for mercury analyses ......................................................................................8

5. Suspended sediment concentrations and turbidity values, 

South River, Virginia, April 2005 through March 2007.............................................................9

6. Description of units for mercury concentrations used in this report ................................11

7. Aqueous total mercury concentrations for the South River, 

April 1, 2005, through March 31, 2007......................................................................................12

8. Concentrations of total mercury on suspended sediment in the 

South River, April 1, 2005, through March 31, 2007 ...............................................................12

9. Observed flow-weighted average sediment and mercury concentrations 

in the South River, Virginia, April 1, 2005, through March 31, 2007.....................................15

10. Observed flow-weighted average mercury concentrations in rivers 

neighboring the South River, Virginia, January 2002 through March 2006 .......................15

11. Concentrations of filterable total mercury THGF in riverbank pore water, 

South River, Virginia, June 2006 ...............................................................................................15

12. Concentrations of total mercury in riverbank sediment, South River, 

Virginia, June 2006......................................................................................................................15

13. Average concentrations of mercury in soil, South River watershed, 

Virginia, April 2003 through October 2006...............................................................................19

14. Sources of time-series data used in the South River watershed model...........................19

15. Linear multi-regression models for suspended sediment concentration.........................20

16. Organization of dataset numbers in the watershed data management 

file for the South River watershed model, Virginia ...............................................................25

17. Land-use representation in the watershed model, South River, Virginia..........................27

18. River reach characteristics in the calibrated South River watershed model..................29

19. Point sources in the South River watershed model, average flow, sediment 

loads, and mercury loads for the period April 1, 2005, through March 31, 2007 ..............30

20. Modules in Hydrological Simulation Program–FORTRAN used to 

simulate mercury transport.......................................................................................................33

21. Mercury sources to the South River in the watershed model............................................34

22. Primary model transport parameters changed from Chesapeake Bay Model 

Phase 5 values during calibration of the South River watershed model ..........................38

23. Simulation results for the calibration period water years 1991 

through 2000, calibrated model, South River, Virginia..........................................................38

24. Simulation results for the verification period April 1, 2005, through 

March 31, 2007, calibrated model, South River, Virginia......................................................39

25. Comparison of simulated and observed (regressed) daily sediment 

concentrations and loads, calibration period water years 1991 

through 2000, calibrated model, South River, Virginia..........................................................47
x

26. Comparison of simulated and observed (regressed) daily sediment 

concentrations and loads, verification period April 1, 2005, 

through March 31, 2007, calibrated model, South River, Virginia.......................................47

27. Input mercury concentrations to the calibrated watershed model...................................54

28. Statistics for hourly simulated and observed total mercury concentrations, 

calibrated watershed model, South River, Virginia, calibration and 

verification periods combined, April 1, 2005, through March 31, 2007 ..............................56

29. Simulated annual total mercury loads to the South River, calibrated 

model, existing conditions, April 1, 2005, through March 31, 2007 .....................................56

30. Target total mercury water column concentrations for rivers in the study......................59

31. Simulated total mercury concentrations for the South River, Virginia, 

existing conditions, median hourly concentrations for the period 

April 1, 2005, through March 31, 2007......................................................................................59

32. Simulated annual mercury loads by reach in the South River, existing 

conditions, April 1, 2005, through March 31, 2007 .................................................................59

33. Mercury loading rates to watersheds and subwatersheds by 

model reach, calibrated model, existing conditions, South River, 

Virginia, April 1, 2005, through March 31, 2007 ......................................................................60

34. Mercury load-reduction simulation scenarios......................................................................61

35. Changes in median simulated total mercury concentrations, 

relative to existing conditions, April 1, 2005, through March 31, 2007, 

South River, Virginia ...................................................................................................................61

36. Percentage change to in-stream total mercury loads, relative to existing 

conditions, April 1, 2005, through March 31, 2007, South River, Virginia...........................62

37. Percentage of time that simulated 90-day median total mercury concentrations 

 exceed the 3.8 nanogram per liter target concentration, April 1, 2005, 

through March 31, 2007, South River, Virginia .......................................................................63

38. Annual mercury loads under Total Maximum Daily Load

conditions (Scenario 4B), South River, Virginia.....................................................................66

39. Model input parameters changed during sensitivity analysis............................................67

40. Results of sensitivity analysis for existing conditions, changes to median 

concentrations for the period April 1, 2005, through March 31, 2007, resulting 

from + or –50 percent in model parameter values, South River at Harriston, 

Virginia (U.S. Geological Survey station number 01627500) ................................................68

41. Results of sensitivity analysis under the Total Maximum Daily Load Scenario, 

changes to percent of time that 90-day median concentrations exceed the 

3.8 ng/L target concentration, resulting from + or –50 percent in model 

parameter values, South River at Harriston, Virginia (U.S. Geological Survey 

station number 01627500), April 1, 2005, through March 31, 2005.......................................68

42. Estimated total maximum daily loads (TMDLs) for mercury for 

listed waters in the Shenandoah Valley, Virginia..................................................................71
xi

Conversion Factors and Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

gallon (gal) 0.003785 cubic meter (m3)

million gallons (Mgal) 3,785 cubic meter (m3)

cubic foot (ft3) 0.02832 cubic meter (m3)

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Million gallons per day (Mgal/d) 1.5472 cubic foot per second (ft3/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

pound (lb) 453.592 gram (g)

ton 0.9072 megagram (Mg)

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). 

The term “water year” is defined as the 12-month period from October 1 for any given year through 

September 30 of the following year. The water year is designated by the calendar year in which it ends 

and which includes 9 of the 12 months. Thus, the year ending September 30, 1999, is called the “1999” 

water year. 

Abbreviations and Acronyms

CBM5 Chesapeake Bay Watershed Model, Phase 5

HRU Hydrologic Response Unit

HSPEXP Expert System for the Calibration of the Hydrological Simulation Program–FORTRAN 

HSPF Hydrological Simulation Program–FORTRAN 

TMDL Total Maximum Daily Load

USGS U.S. Geological Survey

Abstract

Due to elevated levels of methylmercury in fish, three 

streams in the Shenandoah Valley of Virginia have been 

placed on the State’s 303d list of contaminated waters. These 

streams, the South River, the South Fork Shenandoah River, 

and parts of the Shenandoah River, are downstream from the 

city of Waynesboro, where mercury waste was discharged 

from 1929–1950 at an industrial site. To evaluate mercury 

contamination in fish, this total maximum daily load (TMDL) 

study was performed in a cooperative effort between the U.S. 

Geological Survey, the Virginia Department of Environmental 

Quality, and the U.S. Environmental Protection Agency. The 

investigation focused on the South River watershed, a headwater of the South Fork Shenandoah River, and extrapolated 

findings to the other affected downstream rivers. A numerical 

model of the watershed, based on Hydrological Simulation 

Program–FORTRAN (HSPF) software, was developed to 

simulate flows of water, sediment, and total mercury. Results 

from the investigation and numerical model indicate that 

contaminated flood-plain soils along the riverbank are the 

largest source of mercury to the river. Mercury associated with 

sediment accounts for 96 percent of the annual downstream 

mercury load (181 of 189 kilograms per year) at the mouth of 

the South River. Atmospherically deposited mercury contributes a smaller load (less than 1 percent) as do point sources, 

including current discharge from the historic industrial source 

area. In order to determine how reductions of mercury loading 

to the stream could reduce methylmercury concentrations in 

fish tissue below the U.S. Environmental Protection Agency 

criterion of 0.3 milligrams per kilogram, multiple scenarios 

were simulated. Bioaccumulation of mercury was expressed 

with a site-specific exponential relation between aqueous total 

mercury and methylmercury in smallmouth bass, the indicator fish species. Simulations indicate that if mercury loading 

were to decrease by 98.9 percent from 189 to 2 kilograms per 

year, fish tissue methylmercury concentrations would drop 

below 0.3 milligrams per kilogram. Based on the simulations, 

the estimated maximum load of total mercury that can enter 

the South River without causing fish tissue methylmercury 

concentrations to rise above 0.3 milligrams per kilogram is 

2.03 kilograms per year for the South River, and 4.12 and 

6.06 kilograms per year for the South Fork Shenandoah River 

and Shenandoah River, respectively.

Introduction

Three rivers in the Shenandoah Valley of Virginia are 

contaminated with mercury and have been designated as 

“impaired” on Virginia’s 303d list of contaminated waters due 

to fish consumption advisories issued by the Virginia Department of Health. These rivers, the South River, South Fork 

Shenandoah River, and the Shenandoah River between Front 

Royal and the confluence with Craig Run (fig. 1), are regulated 

by the Virginia Department of Environmental Quality (VDEQ) 

under the Total Maximum Daily Load (TMDL) Program, 

which develops plans to restore and maintain water quality for 

impaired waters.

This study by the U.S. Geological Survey (USGS), performed in cooperation with the U.S. Environmental Protection 

Agency (USEPA), VDEQ, and the South River Science Team, 

an interdisciplinary scientific group studying mercury in the 

South River, provides a scientific foundation for the VDEQ 

to establish mercury TMDLs for the three rivers. Results of 

this study will be used by VDEQ to develop an implementation plan to restore water quality in the three rivers so that fish 

tissue methylmercury concentrations are below 0.3 mg/kg 

(milligrams mercury per kilogram of fish tissue). The watershed modeling approach used to develop a mercury TMDL 

for the South River could be applied in other watersheds with 

comparable legacy mercury contamination.

Mercury Loads in the South River and Simulation of 

Mercury Total Maximum Daily Loads (TMDLs) for 

the South River, South Fork Shenandoah River, and 

Shenandoah River: Shenandoah Valley, Virginia

By Jack Eggleston
2 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 1. Location of rivers in the Shenandoah Valley, Virginia, on the 303d list of contaminated water for elevated 

concentrations of methylmercury in fish tissue.

AUGUSTA

ALBEMARLE

ROCKINGHAM

PAGE

LOUISA

FREDERICK

SHENANDOAH

MADISON

NELSON

ORANGE

CULPEPER

WARREN

CLARKE

FLUVANNA

FAUQUIER

ROCKBRIDGE

GREENE

RAPPAHANNOCK

HIGHLAND

GOOCHLAND

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

South River watershed

Shenandoah River watershed

WEST

VIRGINIA

VIRGINIA

Washington, D.C.

Staunton

Harrisonburg

Waynesboro

Charlottesville

Winchester

Figure 1. Location of rivers in the Shenandoah Valley, Virginia, on the 303d list of contaminated water for elevated concentrations

of methylmercury in fish tissue.

79°

38°

38°30’

39°

78°30’ 78°

VA08-Culpeper

VA28-Shenandoah National Park

Big Meadows

North

River

Middle

River

South

River

River

Shenandoah

River

Shenandoah

River

North 

Fork

Shenandoah

South 

Fork

WEST VIRGINIA

VIRGINIA

01626920

01626850

01627500

01626000

01629500

01631000

EXPLANATION

SHENANDOAH RIVER WATERSHED

SOUTH RIVER WATERSHED

SOUTH RIVER MERCURY IMPAIRMENT

SOUTH FORK SHENANDOAH RIVER MERCURY IMPAIRMENT

SHENANDOAH RIVER MERCURY IMPAIRMENT

STREAMFLOW-GAGING STATION AND NUMBER

U.S. ENVIRONMENTAL PROTECTION AGENCY

MERCURY DEPOSITION NETWORK SITE

01626920

Port Republic B L U E R I D G E M O U N T A I N S

Confluence with Craig Run

Base from U.S. Geological Survey Digital Line Graph, 1987, 1:2,000,000

Universal Transverse Mercator 17 projection, NAD 83, Central Meridian 81o

00'W

County boundaries from U.S. Geological Survey National Atlas coverage, 1995, 1:2,000,000

Front Royal
Introduction 3

Background and History

Elevated levels of methylmercury in fish tissue have 

caused parts of the South River, the South Fork of the 

Shenandoah River, and the Shenandoah River to be placed 

on Virginia’s 303(d) list of impaired waters, and the Virginia 

Department of Health has restricted fish consumption from 

these rivers (fig. 1). The affected rivers are: 24.63 mi (miles) 

of the South River from the DuPont foot bridge in Waynesboro downstream to the headwaters of the South Fork Shenandoah River; the entire 100.96 mi of the South Fork Shenandoah River; 0.67 mi of the North Fork Shenandoah River from 

its mouth upstream to the Riverton Dam; and 29.83 mi of the 

Shenandoah River from the confluence of the North Fork and 

South Fork Shenandoah Rivers downstream to the confluence 

with Craig Run. Selected characteristics of the three rivers and 

the uncontaminated North River are listed in table 1.

A textile plant in Waynesboro is known to have discharged mercury to the South River from 1929 to 1950 (Bolgiano, 1980), when it was owned and operated by DuPont. 

Mercury released during that period has spread downstream, 

with the highest concentrations found within the 24 mi of the 

South River from the plant site downstream to its confluence 

with the South Fork Shenandoah River. DuPont has performed 

extensive site assessment and investigations at the plant site 

under the USEPA Resource Conservation and Recovery 

Program (DuPont Corporate Remediation Group, 2003a, b, 

2006a, b). The State of Virginia began regular monitoring of 

mercury in the late 1970s and is scheduled to continue monitoring through the year 2092. Other studies of mercury in the 

South River watershed are being conducted by members of the 

South River Science Team, a group of scientists and representatives from local universities, conservation groups, state 

and federal government agencies, DuPont, and its consultants. 

Data collected for these other studies provided an important 

foundation for this study.

Mercury concentrations remain elevated above background levels in soil and groundwater at the plant site, 

and some mercury still enters the river from the plant site 

through surface runoff, groundwater, and permitted pointsource discharges (DuPont Corporate Remediation Group, 

2006a, b). Atmospheric mercury is deposited on the watershed 

by wet and dry deposition (U.S. Environmental Protection 

Agency, 2007). The largest current (2009) source of mercury to the river may be erosion of contaminated flood-plain 

and channel margin sediments, which have elevated mercury concentrations and were estimated to contain at least 

57,000 lbs (pounds) of mercury along the South River alone 

(Bolgiano, 1980). 

The present study was performed to quantify sources 

of mercury to the river and develop a simulation model that 

could be used to examine relations between mercury loading 

to the river and mercury concentrations in the water column. 

The model was then used to estimate mercury TMDLs for the 

listed rivers.

Regulatory Approach and Total Maximum 

Daily Load Scope

When a water body is placed on the 303d list of contaminated waters, a regulatory requirement is triggered for a 

cleanup plan to be developed for the water body. The TMDL 

approach is based on the idea that if contaminant concentrations in a water body need to be below a specified maximum 

level, then only a limited amount of the contaminant can be 

allowed to enter the water body. A study is typically performed to determine a daily total maximum contaminant load 

(TMDL) so that contaminant concentrations remain below 

the maximum level. This study estimates the maximum daily 

loads of mercury to the South River, South Fork Shenandoah 

River, and Shenandoah River so that methylmercury concentrations in fish tissue can be kept below the USEPA ambient 

water-quality criterion of 0.3 mg methylmercury/kg fish (U.S. 

Environmental Protection Agency, 2001).

Fish tissue total mercury and methylmercury concentrations have been monitored in the South River, the South Fork 

Shenandoah River, and the Shenandoah River by the VDEQ 

since 1990. Methylmercury typically makes up about 90 percent of the total mercury present in South River fish (Virginia 

Department of Environmental Quality, 1999, 2008a). Total 

mercury concentrations in smallmouth bass, the indicator fish, 

have been consistently elevated in the South River since at 

least 1990, averaging above 1.0 ppm (parts per million), and 

individual fish have had concentrations higher than 3.0 ppm. 

In the South Fork Shenandoah River and Shenandoah River, 

average methylmercury concentrations have been somewhat 

lower, but in 2007 were still above 0.3 mg/kg at all monitoring stations. Although mercury concentrations in the water 

column itself have not exceeded any regulatory standards set 

by the USEPA or the VDEQ, high concentrations of mercury 

have been observed in fish because mercury bioaccumulates 

Table 1. Selected characteristics of the North, South, South Fork 

Shenandoah, and Shenandoah Rivers, Virginia. 

[mi2, square miles; Avg., average; R., river]

Name of

river Drains to Drainagearea (mi2)

On 

303(d) 

list for

mercury

Known

 industrial

mercury 

sources

North South Fork 

Shenandoah

818 No No

South South Fork 

Shenandoah

235 Yes Yes

South Fork 

Shenandoah

Shenandoah 1,649 Yes Yes, from 

South R.

Shenandoah Potomac 2,899 

(to Craig Run)

Yes Yes, from 

South R.
4 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

as it moves up the food chain. How fish tissue methylmercury 

concentrations correlate with water column concentrations in 

the South River and how a site-specific bioaccumulation factor 

(BAF) is used in this study are described in the VDEQ companion report to this study (Virginia Department of Environmental Quality, 2008b).

The purpose of the present report is to describe the current understanding of mercury transport in the South River 

watershed and to provide estimates of the mercury loading 

reductions needed to protect human health from risks posed 

by consumption of fish from the river. The area of investigation focused on the South River because the original mercury 

source was located there and the South River has had the highest mercury concentrations in the Shenandoah River watershed. This focus permitted a spatially intensive data-collection 

effort. Results from the South River are extrapolated downstream to estimate loading reductions needed to meet methylmercury fish tissue targets for the South Fork Shenandoah and 

Shenandoah Rivers. 

Description of the Study Area

The 234.6 mi2 (square mile) South River watershed in 

the Shenandoah Valley of Virginia comprises the study area 

(fig. 1). The downstream (northern) end of the study area is 

at the town of Port Republic, where the South River joins the 

South Fork Shenandoah River. The southern and southeastern 

boundaries of the watershed are defined by the Blue Ridge 

Mountains, whereas the northwestern boundary is a low rise of 

hills. Elevations range from 1,037 ft (feet) at the mouth of the 

South River to 3,848 ft at the peak of the Blue Ridge Mountains. Land use is primarily forested (58 percent) or agricultural (31 percent) with developed (8 percent) land accounting 

for most of the remainder (Chesapeake Bay Program, 2002). 

The largest population center in the study area is Waynesboro, 

with a 2000 census population of 19,520. The entire study area 

had an estimated 2000 population of 34,184 (U.S. Census, 

2000). The South Fork Shenandoah River and Shenandoah 

Rivers are included in the TMDL part of the study. 

Precipitation in the study area averaged 43.0 in/yr (inches 

per year) from October 2000 through March 2005, on the 

basis of precipitation data from the Waynesboro sewage treatment plant. Average evapotranspiration (ET) for the city of 

Waynesboro is 29.6 in/yr, on the basis of spatially averaged 

data from multiple weather-monitoring stations outside the 

watershed from January 1984 through March 2007 (Chesapeake Bay Program, 2006). Annual streamflow at Harriston 

(01627500), the most downstream streamflow-gaging station 

in the study area, averaged 261.3 ft3/s (cubic feet per second) 

for the full period of record (fig. 2 and table 2). This flow is 

equivalent to 16.7 in/yr over the 212-mi2 watershed above the 

Harriston gage.

Table 2. Streamflow-gaging stations and water-quality monitoring sites used in the study, South River, 

Virginia, 2005–2007. 

[USGS, U.S. Geological Survey; mi2, square miles; “–” before a number indicates upstream; ft3/s, cubic feet per second]

USGS streamflow-gaging stations and water-quality monitoring sites

USGS station name South River near 

Waynesboro

South River near 

Dooms

South River at 

Dooms

South River at 

Harriston

Station number 01626000 01626850 01626920 01627500

Location Waynesboro Hopeman Parkway Dooms Harriston

Streamflow monitoring 

(this study)

yes yes none yes

Water-quality sampling 

(this study)

yes none yes yes

Drainage area (mi2) 127 148 164 212

River miles downstream 

from plant site

–2.8 2.3 5.3 16.5

Streamflow record period 1952–2008 1974–1997, 

2005–2008

none 1926–1951, 

1969–2008

Mean annual flow (ft3/s) 150 214 no data 261
Introduction 5

Figure 2. Location of the South River watershed study area in the Shenandoah Valley, Virginia.

AUGUSTA

NELSON

ALBEMARLE

Waynesboro

01626920

01626850

01627500

01626000

AUGUSTA

AUGUSTA

ROCKINGHAM

South

River

Figure 2. Location of the South River watershed study area in the Shenandoah Valley, Virginia.

01627500

STREAMFLOW-GAGING

STATION AND NUMBER

EXPLANATION

BLUE RIDGE MOUNTAINS

78°45'

79°

79°15'

38°15'

38°

0 5 10 MILES

0 5 10 KILOMETERS

1,173

316

ELEVATION, IN METERS

South River watershed

Shenandoah River watershed

WEST

VIRGINIA

VIRGINIA

Washington, D.C.

Base from U.S. Geological Survey Digital Line Graph, 1987, 1:2,000,000

Universal Transverse Mercator 17 projection, NAD 83,

 Central Meridian 81o

00'W

Elevation from U.S. Geological Survey National Elevation Dataset

 digital elevation model, 2001, 30-meter resolution
6 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Previous Studies

A history of mercury in the South River was obtained 

from studies and reports located in the VDEQ office in Harrisonburg, Virginia. Since DuPont announced in the fall of 1977 

that mercury had been found in the soil at its Waynesboro 

plant, numerous studies have documented mercury contamination at the plant site and in the downstream watershed (Paylor, 

1977; Bolgiano, 1980; Todd, 1980; Old Dominion University, 

1996, 1997, 1998; Messing and Winfield, 1998). Mercury sulphate was used by DuPont as a catalyst in fabric manufacturing from 1929 to 1950. Although the majority of the mercury 

catalyst was captured and reused, losses to the river resulted 

in widespread mercury contamination downstream (Bolgiano, 

1980). Other potential sources of mercury in the watershed, 

including agricultural fungicides, mercury in precipitation, 

and hydraulic seals in industrial equipment, have been documented, but appear insignificant relative to the large mass of 

mercury released from the plant site. 

The Bolgiano study (1980) estimated that there were 

57,000 lbs of mercury in the South River and the adjacent 

flood plain and a further 20,000 lbs in the South Fork Shenandoah River and the adjacent flood plain. A later study estimated 1,800 lbs of mercury in river sediments downstream 

from the plant site and 97,200 lbs of mercury in flood-plain 

soils (Lawler, Matusky, & Skelly Engineers, 1989).

 Environmental concentrations of mercury in the South 

River have not changed appreciably since they were first 

measured in the late 1970s (Bolgiano, 1980; Old Dominion 

University 1996, 1997, 1998; Virginia Department of Environmental Quality, 1999, 2008a). Mercury concentrations in 

water, sediment, and biota vary with time and location, but do 

not show an obvious temporal trend, either positive or negative. Previous studies used different sampling and analytical 

methods, which makes comparison of results more difficult. 

The development of low-concentration analytical methods for 

mercury in the 1980s that lowered detection limits by a factor 

of 1,000, from about 0.1 mg/L (milligrams per liter) to about 

0.1 ng/L (nanograms per liter), has made it possible to detect 

aqueous mercury in the South River. 

Modeling Approach

This study used the numerical model Hydrological Simulation Program–FORTRAN (HSPF) to simulate the transport 

of water, sediment, and mercury in the South River watershed. 

The model allows mass-balance calculations of all three media 

and captures the transient fluctuations in flows and concentrations that occur in the watershed. After calibrating model 

parameters to match observed existing conditions, the model 

was then used to simulate hypothetical future conditions such 

as reductions in mercury loads to the river. The model can 

be modified in the future to incorporate new observations or 

additional processes that are found to affect mercury transport 

and fish tissue concentrations in the South River.

The choice of modeling software was guided by needed 

capabilities and potential regulatory acceptance. HSPF was 

chosen primarily because of its ability to simulate all media 

of interest in the South River watershed and stream system 

at the desired time scales. HSPF is capable of simulating the 

transport of water, sediment, and mercury as well as phase 

exchange of mercury, all of which are important to mercury 

transport in the South River watershed. The time-series based 

simulations performed by HSPF allow for a 1-hour simulation 

period; this is fast enough to simulate changing river conditions during floods while still allowing long-term simulations 

that reflect average conditions. HSPF is also readily accepted 

by the regulatory community for TMDL purposes, and many 

TMDL studies approved by the USEPA have used it. 

The timeline for this study is presented in figure 3, along 

with the timeframes used for the modeling. Data collection in 

the South River watershed by the VDEQ and other groups has 

been ongoing since the 1970s. USGS streamflow monitoring 

has been ongoing since 1926, and data were collected specifically for this study from April 2005 through March 2007. The 

three components of the watershed model were calibrated and 

verified separately using the time periods shown in figure 3. 

Jan. 1

1985

Oct. 1

1990

Sept. 30

2000

Apr. 1

2005

Apr. 1

2006

Mar. 31

2007

Hydrologic verification

Mercury

calibration

Mercury

verification

Hydrologic model calibration

Sediment transport model calibration Sediment verification

Full simulation period–hourly time steps

Virginia Department of Environmental Quality–Mercury data collection

U.S. Geological Survey–Streamflow data collection

1970s

1926

USGS mercury

data collection

} Model

simulation

periods

Data

collection } periods

Apr. 1

2005

Mar. 31

2007

Figure 3. Timeline for data collection and model simulation, South River, Virginia.

USGS Streamflow Data Collection

Figure 3. Timeline for data collection and model simulation, South River, Virginia.
Data Collected by the U.S. Geological Survey for This Study 7

Data Collected by the U.S. Geological 

Survey for This Study

Data were collected by USGS personnel over a 2-year 

(April 2005 through March 2007) field program. Goals of 

the field sampling and monitoring program were to collect 

data that would (1) characterize the locations and concentrations of mercury, (2) improve understanding of mercury loads 

in the watershed, and (3) allow calibration of a watershed 

mercury transport model. Most of the data were collected from 

three USGS monitoring stations along the South River near 

Waynesboro (01626000), at Dooms (01626920), and at Harriston (01627500) (fig. 2; table 2). Samples also were collected 

periodically from other sites along the river and at other locations such as pipe outfalls, groundwater wells, and riverbanks 

to guide model parameterization. 

Data were also compiled from other organizations and 

from previous studies. These additional data help in understanding mercury in other media, such as fish and groundwater, and provide an independent measure for checking 

model calibration.

Streamflow

At three streamflow-gaging stations (01626000, 01626850, 

and 01627500) on the South River, 15-minute and daily 

average streamflow values were collected using standard 

USGS methods (Rantz and others, 1982; Kennedy, 1983). 

The daily streamflow data are available online and at the 

USGS National Water Information System (NWIS) website 

http://waterdata.usgs.gov/nwis/. Streamflow data were used to 

calibrate the watershed model and, in combination with concentration values, used to calculate sediment and mercury loads. 

The average of annual mean streamflow for the period 

of record increased from 150 ft3/s at Waynesboro 2.8 mi 

upstream from the plant to 261 ft3/s at Harriston 16.5 mi 

downstream from the plant site (table 2). Annual average flows 

varied widely, with a lowest measured annual average flow at 

Harriston of 70 ft3/s and highest of 516 ft3/s (fig. 4).

Water Quality 

At the beginning of the project, it was not clear which 

water-quality parameters would control or correlate with fish 

tissue methylmercury concentrations and therefore be important to study. Therefore data collection was designed to measure water-quality parameters that had been shown at other 

mercury contaminated sites to correlate with mercury transport 

and mercury concentrations in fish (Gilmour and others, 1998; 

Yin and Balogh, 2002). The following water-quality parameters were selected for monitoring in the South River: mercury (particulate and filtered concentrations of total mercury 

and methylmercury in various media), suspended sediment 

concentration, turbidity, dissolved organic carbon, chloride, 

sulfate, pH, and temperature. The last five parameters were 

Figure 4. Annual streamflow for the South River near Waynesboro, Virginia (USGS station number 01626000) and at 

Harriston, Virginia (USGS station number 01627500), water years 1926–2007.

0

100

200

300

400

500

600

19261931193619411946195119561961196619711976198119861991199620012006

ANNUAL STREAMFLOW, IN CUBIC FEET PER SECOND

Harriston

Waynesboro

YEAR

Figure 4. Annual streamflow for the South River near Waynesboro (USGS station number 01626000) and at Harriston (USGS station

number 01627500), Virginia, water years 1926-2007.
8 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

selected with the specific intent of discovering correlations to 

methylmercury concentrations in water. Near the end of the 

data-collection program, it was decided that the study should 

focus only on total mercury because total mercury had the 

strongest correlation with fish tissue methylmercury concentrations in the South River. For this reason, only water-quality 

results for suspended sediment concentration, turbidity, and 

mercury concentrations are presented. The other parameters 

(dissolved organic carbon, chloride, sulfate, pH, and temperature) are not presented because they did not show a strong 

correlation with methylmercury concentrations.

Methods

At each stream monitoring site, vertically integrated 

grab samples were collected from a single lateral location at 

the centroid of flow under base-flow conditions. All waterquality samples were taken as single vertically integrated 

grab samples. Continuous water-quality and grab sample data 

from this study can be accessed at the USGS NWIS website http://waterdata.usgs.gov/nwis/. Surface-water samples 

were collected from bridges at the monitoring locations: 

Waynesboro (01626000), Dooms (01626920), and Harriston 

(01627500) (fig. 2). After each sampling event, bottles were 

sent for analysis to the USGS Eastern Region Sediment Laboratory, the USGS National Water Quality Laboratory, and the 

USGS Mercury Laboratory (tables 3, 4).

A large percentage of mercury in the water column is 

typically bound to suspended particulate matter (Hem, 1989), 

so suspended sediment concentrations were measured in this 

study. Suspended sediment data were used to calibrate the 

numerical model for sediment transport as discussed in a later 

section, Sediment Model Calibration Results. Raw water 

samples were collected in 1-pint glass bottles and filtered with 

a 1.5-μm (micrometer) glass fiber filter during analysis of 

suspended sediment concentration (Guy, 1969). (Sampling and 

processing details are available online at http://ky.water.usgs.

gov/technical_info/dist_sedlab_files/sed_lab.htm.)

Horizontal variability in water-quality constituent concentrations is not reflected in a single vertically integrated 

sample, unlike a full representative cross-sectional sample. 

The decision to collect single vertically integrated samples 

was made to decrease the possibility of contaminating 

trace-level mercury concentrations due to the extra handling 

and equipment involved. A full concurrent cross-sectional 

Table 3. Water-quality sample treatments and laboratories used in the study. 

[mg/L, milligrams per liter; mL, milliliter; oC, degrees Celsius; USGS, U.S. Geological Survey]

Analyte Laboratory Sample container Field treatment Detection limit 

(mg/L)

Dissolved organic 

carbon

National Water Quality 

Laboratory

125-mL amber glass Filter immediately, 

acidify with H2SO4, 

and preserve at 4 oC

0.33

Sulfate 250-mL plastic Filter immediately and 

preserve at 4 oC

0.18

Chloride 0.20

Suspended sediment 

concentration 

USGS Eastern Region 

Kentucky Sediment Lab

1-pint glass Preserve at 4 oC 1

Table 4. Detection limits for mercury analyses. 

[USGSML, U.S. Geological Survey Mercury Laboratory; mL, milliliters; ng/L, nanograms per liter; μg/g, micrograms per gram; 

oC, degrees Celsius; see table 6 for description of analyte abbreviations]

Analyte Laboratory Sample container Field 

treatment

Detection 

limit Units

Total filterable mercury (THGF), 

aqueous methylmercury (MHG)

USGSML Precleaned Teflon from 

USGSML (250, 500, 

1,000, and 2,000 mL)

Preserve at 

4 oC

0.04 ng/L

Aqueous total mercury associated with 

non-filterable particulates (THGP)

0.06 ng/L

Total mercury on soils or surface 

sediment (THGSED)

0.30 μg/g
Data Collected by the U.S. Geological Survey for This Study 9

sampling event was performed at the Harriston station in 

August 2005 under base-flow conditions to test the representativeness of a single vertically integrated sample. The results 

indicated no consistent patterns of horizontal variation in the 

water-quality parameters tested (THG, THGF, THGP, THGSS, 

MHG, chloride, sulfate, pH, suspended sediment, and specific 

conductivity). Data from other studies have shown higher filtered mercury (THGF

) concentrations in the South River closer 

to riverbanks and to sediment-water interfaces (Turner and 

Jensen, 2007) under base-flow conditions, however.

Continuous monitoring of in-stream water quality was 

performed by hanging probes from bridges into the river near 

Waynesboro, at Dooms, and at Harriston (table 2). Probes 

were located close to the centroid of flow at deep points in 

the river so that they would remain submerged under low 

water conditions. Continuous water-quality data for the three 

South River monitoring stations are available on the USGS 

NWIS website at http://waterdata.usgs.gov/nwis/. Probes were 

calibrated and serviced on a monthly basis. The continuous 

parameters were collected using a YSI multi-parameter field 

meter (model 6920 or similar) following standard USGS protocol (Wagner and others, 2000).

Water-quality samples analyzed for mercury were collected according to established sampling protocol for ultratrace metals; aqueous and sediment samples were collected 

using the “clean hands – dirty hands” technique (Ward and 

Harr, 1990; Horowitz, 1991; Horowitz and others, 1994; 

U.S. Environmental Protection Agency, 1996). Surface-water 

samples were collected in 2-L (liter) Teflon bottles precleaned 

by the USGS Mercury Laboratory. The precleaned 2-L Teflon 

bottles were placed into a stainless steel bottle 

holder, and then lowered into the river from a 

bridge. A single vertically integrated sample was 

collected at each monitoring site and capped, 

placed on ice, and shipped to arrive at the USGS 

Mercury Laboratory within 24 hours. Laboratory 

personnel then processed the sample using techniques based on USEPA Method 1631 (Olson 

and DeWild, 1999; DeWild and others, 2002; 

U.S. Environmental Protection Agency, 2002; 

Olund and others, 2004). Filtering of mercury 

samples through a 0.7-μm filter was performed 

in the laboratory; no filtering of mercury samples 

was performed in the field. Detection limits for 

laboratory analyses of mercury are shown in 

table 4.

During a base-flow period in June 2006, 

pore-water samples were collected from the 

riverbank along the river’s edge at one location upstream from the plant site at Waynesboro 

(01626000), and at three locations downstream 

from the plant site: Basic Park, 0.2 mi upstream 

from monitoring station 01626850; Steeles Run 

confluence, 0.4 mi upstream from monitoring 

station 01626850; and at Dooms (01626920). 

Pore-water samples were collected using a Teflon 

drivepoint connected to Teflon tubing driven by a peristaltic pump. Samples were drawn from depths of 5 and 15 cm 

(centimeters) below land surface into precleaned Teflon bottles 

and shipped to the USGS Mercury Laboratory for analysis as 

previously described. Sediment samples were collected from 

the same 5- and 15-cm depths using precleaned stainless steel 

implements, placed into precleaned Teflon bottles, and shipped 

on ice to the USGS Mercury Laboratory.

Suspended Sediment 

Results from the USGS suspended sediment concentration data collected for this study are summarized in table 5. 

Suspended sediment concentration was strongly affected 

by streamflow, generally increasing with increasing flows 

(fig. 5). Although the raw data show that Waynesboro had 

higher suspended sediment concentration values than Dooms 

and Harriston, this result is biased due to a greater proportion of stormflow samples collected at Waynesboro. When 

flow-corrected mean suspended sediment concentrations are 

calculated, Waynesboro exhibited a lower mean suspended 

sediment concentration than Dooms (table 5). Flow correction 

is performed by taking a weighted average that accounts for 

the magnitude of flow, assessed by streamflow duration at the 

time of sampling, and removes bias towards either low-flow 

or stormflow sampling. Results from the Harriston site show 

slightly lower mean and flow-weighted mean suspended 

sediment concentration values than the values from either the 

Waynesboro or Dooms sites.

Table 5. Suspended sediment concentrations and turbidity values, South River, 

Virginia, April 2005 through March 2007. 

[U.S. Geological Survey samples only; FNU, Formazin nephelometric units; SSC, suspended 

sediment concentration; mg/L, milligrams per liter; %, percent]

Statistic

Monitoring Site

Waynesboro Dooms Harriston

(01626000) (01626920) (01627500)

Suspended sediment 

concentration 

(mg/L)

Count 29 28 36

Mean 56.4 39.4 38.2

Median 8.0 6.5 7.5

Standard deviation 112.0 99.7 83.9

Range 1–434 1–433 1–377

Flow weighted 

SSC* (mg/L)

Mean 37.2 38.7 26

Turbidity (FNU) Period 5/1/2005 to 

4/1/07

4/21/2005 to 

4/1/2007

6/3/2005 to 

4/1/2007

Count 62,588 63,052 58,595

Data coverage 93% 93% 92%

Mean 9.1 8.2 7.1

Standard deviation 39.9 32.1 38.1

*

 Corrected for flow bias as described in the Mercury/Surface Water section.
10 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Turbidity 

Turbidity indicates the ability of a fluid to transmit light 

without scattering or absorption (Gray and Glysson, 2003). 

Turbidity was measured using the multiparameter probes and 

is reported in formazin nephelometric units (FNU) (fig. 6). 

Turbidity is used to develop a suspended sediment concentration time series. Turbidity results for the three monitoring 

stations in the study are summarized in table 5. Mean turbidity 

decreased from Waynesboro downstream to Harriston. Turbidity data collection from each probe was periodically interrupted due to conditions such as high water velocity and algae 

growth. During high-flow events, interruptions in turbidity 

data were common and, because turbidity typically rose during storms by one to two orders of magnitude, the statistics in 

table 5 are almost certainly affected by the missing data.

Mercury 

Data were collected during this study to describe mercury 

concentrations in the South River, in piped discharges to the 

river, in groundwater, and in soils. Collection of mercury data 

was made using standard USGS sample-collection techniques 

and followed a quality assurance plan to ensure that data were 

comparable, complete, and representative. Mercury analyses 

were performed by the USGS Mercury Laboratory in Middleton, Wisconsin.

Units and Terms

Mercury concentrations are expressed in per mass or per 

volume units that depend on the medium being considered. 

The various mercury concentration units used in this report are 

defined in table 6. 

Surface Water

Mercury concentrations for the monitoring stations on the 

South River are shown in tables 7 and 8. Downstream from the 

plant site, mean THG concentrations were more than 70 times 

higher than at the Waynesboro monitoring station. Concentrations of mercury on suspended sediment (THGSS) increased by 

a factor of more than 100 downstream from the plant site.

During most sampling events, the majority of mercury 

in the water column was associated with suspended particulate matter (Meybeck and Helmer, 1989). At the background 

reference station (Waynesboro), about 78 percent of aqueous 

mercury was particulate-bound, whereas downstream from 

the plant site, 98 percent and 96 percent of the mercury was 

particulate-bound, at Dooms and Harriston, respectively.

Total mercury (THG) concentrations increased with 

increasing streamflow (fig. 7). Filterable mercury THGF

concentrations in nanograms per liter showed a slight positive correlation with streamflow (fig. 8), particularly at the 

Waynesboro reference station. The increase of filterable 

Figure 5. Observed suspended sediment concentrations and streamflow in the South River, Virginia, 

April 1, 2005, through March 31, 2007.

0.1

1

10

100

1,000

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

(USGS SAMPLES ONLY) 

Waynesboro (USGS station number 01626000)

Dooms (USGS station number 01626920)

Harriston (USGS station number 01627500)

Figure 5. Observed suspended sediment concentrations and streamflow in the South River, Virginia, April 1, 2005, through

March 31, 2007.
Data Collected by the U.S. Geological Survey for This Study 11

Figure 6. Streamflow and turbidity from June 2005 through April 2007 for the South River at Harriston, Virginia (USGS 

station number 01627500). (Data recorded at 15-minute intervals.)

0.1

1

10

100

1,000

10,000

100,000

APR

2005

JULY

2005

NOV

2005

FEB

2006

MAY

2006

AUG

2006

DEC

2006

MAR

2007

DATE

STREAMFLOW, IN CUBIC FEET PER SECOND

0.1

1

10

100

1,000

10,000

100,000

TURBIDITY, IN FORMAZIN NEPHELOMETRIC UNITS (FNU) 

Laboratory measured turbidity, in nephelometric turbidity units (NTU)

Observed streamflow, in cubic feet per second

Observed turbidity, in formazin nephelometric units (FNU)

Figure 6. Streamflow and turbidity from June 2005 through April 2007 for the South River at Harriston (USGS station no. 01627500), Virginia.

[Data recorded at 15-minute intervals.]

Table 6. Description of units for mercury concentrations used in this report. 

[ng/L, nanograms per liter; μm, micrometer; μg/g, micrograms per gram]

Acronym Description of concentration Units

THG Aqueous total mercury, typically calculated 

as the sum of THGF and THGP. 

Nanograms total Hg per liter of water (ng/L).

THGF Aqueous filterable total mercury. Nanograms total Hg passing a 0.7-μm filter per 

liter of water (ng/L).

THGP Aqueous total mercury associated with 

non-filterable particulates.

Nanograms total Hg not passing a 0.7-μm filter 

per liter of water (ng/L).

THGSS Total mercury on solids suspended in water, 

calculated as THGP/suspended sediment 

concentration.

Micrograms total Hg per gram of dry suspended 

solids (μg/g).

THGSed Total mercury on soils or surface sediment. Micrograms total Hg per gram of dry soil (μg/g).

MHG Aqueous methylmercury. Nanograms MeHg per liter of water (ng/L).
12 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Table 7. Aqueous total mercury concentrations for the South River, April 1, 2005, through March 31, 2007. 

[U.S. Geological Survey samples only; THGF, filterable total mercury; THGP, particulate total mercury; THG, unfiltered mercury; ng/L, nanograms per 

liter; n, sample size]

 Station ID

Aqueous total mercury

n

THGF (ng/L) THGP (ng/L) THG = THGF + THGP

Mean Median Range Mean Median Range Mean Median Range

Waynesboro 

(01626000)

30 1.0 0.4 0.1–3.7 3.3 0.8 0.3–19.8 4.3 1.3 0.5–23.5

Dooms 

(01626920)

28 7.4 7.2 2.7–14.5 291.9 100.3 14–2,730 299.3 103.6 17–2,740

Harriston 

(01627500)

36 13.6 12.9 4.0–33.2 319.2 99.6 13–4,020 332.8 115.0 18–4,042

Table 8. Concentrations of total mercury on suspended sediment 

in the South River, April 1, 2005, through March 31, 2007. 

[U.S. Geological Survey samples only; THGSS, mercury on suspended 

sediment; μg/g, micrograms per gram; n, sample size] 

 Station ID n

Mercury on suspended sediment 

THGSS (μg/g)

Mean Median Range

Waynesboro 

(01626000)

30 0.1 0.1 0.0–0.7

Dooms 

(01626920)

28 17.4 13.1 2–66

Harriston 

(01627500)

36 13.5 10.8 2–48

mercury concentrations with streamflow may be due to 

desorption of mercury from contaminated sediment entering 

the stream and(or) from higher inflows of precipitation and 

interflow, both of which have average THGF concentrations 

above 1.0 ng/L, or possibly from an increase in the concentration of colloidal particles passing the laboratory filter. THGP

concentrations, the aqueous concentration of mercury associated with suspended particulates, showed a strong positive 

correlation with streamflow at all monitoring stations (fig. 9). 

THGSS concentration, the concentration of mercury on suspended particulates, showed a slight negative correlation with 

streamflow (fig. 10). Therefore, it can be concluded that the 

large increase in THG seen during high flows was driven by 

the large increase in suspended sediment concentration (fig. 5).

The concentrations of mercury and suspended sediment based on sample data from the South River are listed 

in table 9. Mercury concentrations for two other rivers—the 

North River near Burketown (01622000), which is an uncontaminated tributary to the South Fork Shenandoah River, and 

the South Fork Shenandoah River near Luray (01629500), 

which is located 69 mi downstream from the mouth of the 

South River—are listed in table 10. Concentrations listed in 

tables 9 and 10 are flow-weighted to remove sampling bias 

towards either stormflow or base-flow periods. Concentration 

values were grouped into 10 bins according to streamflow 

magnitude at time of sampling, defined by flow duration 

deciles of 0–10 percent, 10–20 percent, 20–30 percent, 

and so forth. To calculate the mean concentrations shown 

in tables 9–10, mean concentrations were calculated for 

each decile and then these 10 decile mean concentrations 

were averaged.

Of the surface-water samples collected from the South 

River, 17 percent were either field blanks or replicate samples 

used for quality control and assurance. Of the 22 THGF and 

THGP analytical values from the field blank samples, one was 

above 0.2 ng/L and was determined to be a laboratory clerical 

error and was dropped from the dataset. Of the 26 THGF and 

THGP analytical values from replicate samples, the average 

paired concentration difference was 0.8 ng/L with no sampling 

biases found, and the data were determined to be comparable 

and reproducible.

Groundwater

In June 2006, pore-water samples were collected at 

multiple locations at the edge of the South River and analyzed 

for THGF concentrations. Results are shown in table 11. Porewater THGF

 concentrations were higher than those found in 

either groundwater from a contaminated flood plain (described 

later) or from the South River. Sediment collected during the 

same sampling events at these locations had high levels of 

mercury, and presumably, mercury in the soil is the source 

of the high dissolved mercury concentrations. No beads of 

elemental mercury were observed at the sampling locations.

Sediment

Sediment samples were collected from the river’s edge 

during the June 2006 pore-water sampling event. These 

concentrations, shown in table 12, are similar to concentrations seen on suspended sediment in the South river (fig. 10) 

and to mean THGSed concentration values for flood-plain and 

riverbank soils compiled from the South River Science Team 

database, described in a later section, Mercury in Soil.
Data Collected by the U.S. Geological Survey for This Study 13

Figure 7. Observed total unfiltered mercury concentration and concurrent instantaneous 

streamflow in the South River, Virginia, April 1, 2005, through March 31, 2007.

Figure 8. Observed aqueous concentration of filterable mercury and concurrent 

instantaneous streamflow, South River, Virginia, April 1, 2005, through March 31, 2007.

0.1

1

10

100

1,000

10,000

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND

TOTAL UNFILTERED MERCURY, IN NANOGRAMS PER LITER

(USGS SAMPLES ONLY) 

Waynesboro (USGS station number 01626000)

Dooms (USGS station number 01626920)

Harriston (USGS station number 01627500)

Figure 7. Observed total unfiltered mercury concentration and concurrent instantaneous streamflow in the South River,

Virginia, April 1, 2005, through March 31, 2007.

0.01

0.10

1

10

100

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND

FILTERABLE MERCURY, IN NANOGRAMS PER LITER

(USGS SAMPLES ONLY) 

Waynesboro (USGS station number 01626000)

Dooms (USGS station number 01626920)

Harriston (USGS station number 01627500)

Figure 8. Observed aqueous concentration of filterable mercury and concurrent instantaneous streamflow, South River,

Virginia, April 1, 2005, through March 31, 2007.
14 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 9. Observed aqueous concentration of mercury associated with particulate matter and 

concurrent instantaneous streamflow, South River, Virginia, April 1, 2005, through March 31, 2007.

0.1

1

10

100

1,000

10,000

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND

AQUEOUS CONCENTRATION OF PARTICULATE

MERCURY, IN NANOGRAMS PER LITER

(USGS SAMPLES ONLY) 

Waynesboro (USGS station number 01626000)

Dooms (USGS station number 01626920)

Harriston (USGS station number 01627500)

Figure 9. Observed aqueous concentration of mercury associated with particulate matter and concurrent instantaneous streamflow,

South River, Virginia, April 1, 2005, through March 31, 2007.

Figure 10. Observed concentration of mercury on suspended particulate matter 

and concurrent instantaneous streamflow, South River, Virginia, April 1, 2005, through 

March 31, 2007.

0

0.1

1

10

100

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND

TOTAL MERCURY CONCENTRATION ON SUSPENDED

SEDIMENT, IN NANOGRAMS PER GRAM

(USGS SAMPLES ONLY) 

Waynesboro (USGS station number 01626000)

Dooms (USGS station number 01626920)

Harriston (USGS station number 01627500)

Figure 10. Observed concentration of mercury on suspended particulate matter and concurrent instantaneous streamflow,

South River, Virginia, April 1, 2005, through March 31, 2007.
Data Collected by the U.S. Geological Survey for This Study 15

Table 9. Observed flow-weighted average sediment and mercury concentrations in the 

South River, Virginia, April 1, 2005, through March 31, 2007. 

[U.S. Geological Survey samples only; ft3/s, cubic feet per second; ng/L, nanograms per liter; mg/L, 

milligrams per liter]

USGS monitoring station

Observed average 

daily streamflow 

 (ft3/s)

Flow-weighted average concentration

Suspended 

sediment 

(mg/L)

Total mercury 

concentration

(ng/L)

Waynesboro (01626000) 167 37.2 3.2

Dooms (01626920) 225 38.7 336

Harriston (01627500) 276 26.0 237

Table 10. Observed flow-weighted average mercury concentrations in rivers neighboring the South River, Virginia, 

January 2002 through March 2006. 

[Virginia Department of Environmental Quality samples only; ft3/s, cubic feet per second; ng/L, nanograms per liter; n, sample size]

U.S. Geological Survey 

monitoring station ID Drains to Type Average daily streamflow (ft3/s)

Total mercury

 concentration

n (ng/L)

North River near Burketown, VA 

(01622000)

South Fork Shenandoah 

River

Reference site 387 25 1.9

South Fork Shenandoah near 

Luray, VA (01629500)

Shenandoah River Mercury contaminated site1,422 25 10.8

Table 11. Concentrations of filterable total mercury THGF in riverbank pore water, South River, Virginia, June 2006. 

[U.S. Geological Survey samples only; cm, centimeters; BLS, below land surface; mi, miles; n, sample size; nd, no data] 

Depth of 

sample

Filterable total mercury concentration (nanograms mercury per liter of water)

Waynesboro

(01626000)

Steeles Run

(0.4 mi upstream 

from station 01626850)

Basic Park

(0.2 mi upstream 

from station 01626850)

Dooms

(01626920)

5 cm BLS nd 158.0 (n=2) 35.6 (n=2) 164.0 (n=1)

15 cm BLS 1.4 (n=1) 326.5 (n=2) 217.7 (n=2) nd

Table 12. Concentrations of total mercury in riverbank sediment, South River, Virginia, June 2006. 

[U.S. Geological Survey samples only; cm, centimeters; BLS, below land surface; mi, miles; n, sample size; nd, no data]

Depth of 

sample

Total mercury concentration on sediment (micrograms mercury per gram dry sediment)

Waynesboro

(01626000)

Steeles Run

(0.4 mi upstream 

from station 01626850)

Basic Park

(0.2 mi upstream 

from station 01626850)

Dooms

(01626920)

5 cm BLS nd 11.2 (n=2) 4.2 (n=2) nd

15 cm BLS nd 11.3 (n=2) 3.4 (n=2) nd
16 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Mercury Sorption to Suspended Solids

Mercury (THG) in the South River partitions between 

sorbed and dissolved phases. The data in tables 7 and 8 indicate a distribution coefficient (Kd) for total mercury in the 

South River of about 1,000,000 L/kg (liters/kilogram), or a 

log Kd of about 6, assuming that sorption was at equilibrium 

in the samples at the time of analysis. This Kd value of about 

6 is seen both in the ratio of sampled mean THGF and THGSS

concentrations (tables 7 and 8) and in laboratory batch tests by 

Mason (2006). A lot of spatial and temporal variation is present in this distribution coefficient, however, some of which 

corresponds to the location relative to the plant site. Average 

log Kd was lowest (5.1) upstream from the former DuPont 

plant site at the Waynesboro monitoring station (01626000), 

highest (log Kd = 6.4) downstream at the Dooms monitoring 

station (01626920), and lower again (log Kd = 6.0) further 

downstream at the Harriston monitoring station (01627500). 

Partitioning ratios also correlated negatively with streamflow. 

Log Kd values, determined from the ratio of sampled THGF

and THGSS concentrations, were higher during low flow than 

high flow at all three monitoring stations. At monitoring 

station 01626920, for example, mean log Kd was 6.25 for 

base-flow samples and 5.80 for stormflow samples. This variation may be due to various causes; non-equilibrium sorption 

(time delay to reach equilibrium) in combination with loads 

having varying proportions of dissolved and sorbed mercury, 

runoff sediment characteristics varying with river location, 

different mixing in the water column according to river location, chemical variations such as pH along the river, or other 

possible variables whose effects on mercury partitioning are 

currently not well understood for the South River. A constant 

and uniform Kd value of 1,000,000 was used in the mercury 

transport model.

Data Compiled From Other Sources

The South River is the subject of many past and ongoing 

studies, which provided valuable supplemental data to this 

study. Selection of data from other sources to include in this 

study was prioritized according to need, reliability, and public 

availability. Many other datasets not used or mentioned in this 

study are available from the VDEQ in Harrisonburg, Virginia, 

or by request from the South River Science Team website 

(http://www.southriverscienceteam.org).

Suspended Sediment

Suspended sediment data previously collected by the 

VDEQ were compiled. The VDEQ data were reported in units 

of milligrams per liter and, like USGS suspended sediment 

samples, are filtered with a 1.5-μm filter. However, they were 

reported as “total suspended sediment” rather than “suspended 

sediment concentration” because of differences in laboratory 

methods, such as a sample split rather than a whole sample 

being analyzed. An additional difference between USGS and 

VDEQ suspended sediment data is that concentrations below 

detection limits were generally reported as 3 mg/L. This 

3 mg/L value for VDEQ data was maintained for analysis in 

this study. The USGS suspended sediment concentration data 

had a reporting limit of 1.0 mg/L, and reporting limit values 

were maintained at 1.0 for data analysis. In later sections of 

this report, the VDEQ total suspended sediment data and the 

USGS suspended sediment concentration data are treated as 

equivalent for purposes of discussion and illustration.

Mercury in Surface Water

Mercury concentrations in surface water measured by the 

VDEQ were compiled to provide representation of time periods before April 2005, and coverage of rivers other than the 

South River. The VDEQ has had an extensive sampling program for mercury in the Shenandoah River watershed in place 

since the 1980s. Mercury concentration values used in this 

report were drawn from the VDEQ’s database in August 2008. 

These data were not used in calibrating the watershed model, 

but provided an independent basis for checking model results. 

These data were also used to estimate mercury loads in the 

South Fork Shenandoah and Shenandoah Rivers.

Mercury concentrations in runoff and wastewater discharge from the plant site are measured by DuPont. These 

mercury data were made available to this study by DuPont, 

DuPont’s contractors, and the USEPA.

Atmospheric Mercury Deposition

Mercury is deposited from the atmosphere in both wet 

and dry forms. Data describing atmospheric mercury deposition were obtained from the USEPA Mercury Deposition 

Network (MDN) website (http://nadp.sws.uiuc.edu/mdn/). 

Two MDN sites are about 50 mi northeast of Waynesboro: Big 

Meadows (VA28), located on top of the Blue Ridge Mountains at an elevation of 3,524 ft, and Culpeper (VA08), located 

east of the Blue Ridge Mountains at an elevation of 535 ft 

(fig. 11). Annual mercury wet deposition at both stations was 

13.2 μg/m2 during 2003. Actual mercury deposition rates in 

the study area may differ from this value. There is at least one 

coal-fired electric generation plant in the watershed (at the 

Invista/DuPont plant site), which has the potential to locally 

elevate mercury deposition rates. Dry deposition of mercury 

(the transfer of mercury from the atmosphere to the ground 

in the absence of precipitation) likely occurs, but no reliable 

data are available describing rates near the study area. A 1997 

modeling study by the USEPA (U.S. Environmental Protection 

Agency, 1997, table 3–3) found that wet deposition accounts 

for 51 percent of total atmospheric mercury deposition in the 

continental United States, whereas dry deposition accounts for 

49 percent. Data shown in figures from the 1997 U.S. Environmental Protection Agency report indicate that most of the State 

of Virginia, including the South River study area, has a total 

Hg dry deposition rate ranging from 3 to 10 μg/m2/yr. 
Data Compiled From Other Sources 17

Figure 11. Location of mercury deposition and precipitation monitoring sites.

WEST VIRGINIA

VIRGINIA

North

River

Middle

River

South

River

South Fork Shenandoah

River

EXPLANATION

SOUTH RIVER WATERSHED

SHENANDOAH NATIONAL PARK

PRECIPITATION MONITORING SITE

U.S. ENVIRONMENTAL PROTECTION AGENCY

MERCURY DEPOSITION NETWORK SITE

AUGUSTA

ALBEMARLE

ROCKINGHAM

PAGE

MADISON

NELSON

FLUVANNA

GREENE

LOUISA

RAPPAHANNOCK

ORANGE

CULPEPER

ROCKBRIDGE

Staunton

Harrisonburg

Waynesboro

Charlottesville

79° 78°45’ 78°30’ 78°15’

38°30’

38°15’

38°

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

Shenandoah Valley Regional Airport

Waynesboro Sewage Treatment Plant

Invista/DuPont site

VA08-Culpeper

VA28-Shenandoah National Park

Big Meadows

Figure 11. Location of mercury deposition and precipitation monitoring sites.

Base from U.S. Geological Survey Digital Line Graph, 1987, 1:2,000,000

Universal Transverse Mercator 17 projection, NAD 83, Central Meridian 81o

00'W

County boundaries from U.S. Geological Survey National Atlas coverage, 1995, 1:2,000,000
18 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

On the basis of the MDN numbers and the 1997 USEPA 

study, it is assumed that total atmospheric mercury deposition in the study area was 20.0 μg/m2

/yr for the simulation 

period. Multiplying this value by 234 mi2 (6.1×108m2) results 

in estimated atmospheric mercury input to the study area of 

12.1 kg/yr. The modeling efforts described later use average 

atmospheric mercury concentrations that assume all mercury 

deposition takes place in the wet form. To calculate an average 

THGF concentration in precipitation, the total deposition of 

20.0 μg/m2/yr was multiplied by average annual precipitation 

of 1.02 m/yr and by a unit correction factor to yield an average 

THGF concentration in precipitation of 21.79 ng/L. 

Most of the atmospheric mercury deposited on the South 

River watershed binds to surface soils and does not reach 

the South River, as has been found in other watersheds (U.S. 

Environmental Protection Agency, 1997, vol. 3). This is 

evident from a mercury mass balance at the upstream Waynesboro river monitoring site. Using the same decile-weighting 

procedures that were used for the values in table 9 yields an 

estimated mercury load for the South River at the Waynesboro 

monitoring station of 0.7 kg/yr. The estimated annual mass 

of mercury deposited from the atmosphere to the watershed 

above the Waynesboro monitoring station is 6.6 kg/yr, estimated by multiplying the deposition rate (20.0 μg/m2

) by the 

Waynesboro watershed area (127 mi2). Comparing the two 

loads, only 11 percent of the estimated atmospheric mercury 

deposition upstream from the Waynesboro monitoring station 

reaches the South River.

Mercury in Fish Tissue

Virginia State agencies have collected fish from the South 

River for mercury analysis since the late 1970s. The Virginia 

Department of Health has placed fish 

consumption bans or advisories on 

the South River and the South Fork 

Shenandoah River since 1977 (Bolgiano, 1980). The VDEQ currently 

collects fish for mercury analysis, and 

the findings are summarized in their 

reports (Virginia Department of Environmental Quality, 1999, 2000, 2008a). 

Fish tissue mercury concentrations 

have not changed appreciably since 

monitoring started (Virginia Department of Environmental Quality, 2001). 

Fish tissue mercury concentrations 

along the South River from smallmouth bass collected and analyzed by 

the VDEQ from 1999 through 2007 

are shown in figure 12. Upstream from 

the plant site, concentrations were 

generally below the USEPA 0.3 mg/kg 

criterion. Downstream from the plant 

site, fish tissue methylmercury concentrations rose rapidly for about 5 mi, 

and showed the highest concentrations between 5 and 12 mi 

downstream from the plant site.

Compared to fish tissue methylmercury concentrations 

from 20 other regions in the U.S. (Brumbaugh and others, 

2001), the South River had higher fish tissue methylmercury 

concentrations than all water bodies except one, the Nahontan 

Reservoir in Nevada, which has been contaminated as a result 

of mercury mining. Unlike the sites studied by Brumbaugh 

and others (2001), the South River exhibits a strong correlation between fish Hg concentrations and aqueous THG concentrations. This strong correlation is discussed in the VDEQ 

TMDL report accompanying this study (Virginia Department 

of Environmental Quality, 2008b). In the nearby North River 

watershed, where there is no known industrial mercury contamination, fish tissue methylmercury concentrations averaged 

0.2 mg/kg in 2007 (Virginia Department of Environmental 

Quality, 2008a).

Mercury in Soil

Additional data describing mercury concentrations in 

soils were compiled for use as input to the model. The THGSed

values in table 13 were derived from a database of field sample results collected by South River Science Team members 

and maintained by DuPont (http://www.southriverscienceteam.

org) (DuPont, 2008). The mercury concentrations were 

measured in soils collected from depths of 0 to 1.0 m below 

land surface, and from a variety of settings including the 

river channel, riverbanks, and agricultural and forested flood 

plains. Values were grouped according to location along the 

river, and mean values were calculated. In the model, these 

values are multiplied by loading coefficients that control the 

0

1

2

3

4

0 50 100 150

DISTANCE DOWNSTREAM FROM FORMER DUPONT SITE, IN MILES

FISH TISSUE MERCURY CONCENTRATION, IN

MICROGRAMS PER GRAM WET WEIGHT

(VDEQ SAMPLES ONLY) 

Figure 12. Fish tissue mercury concentrations in smallmouth bass 1999-2007, South River, South

Fork Shenandoah River, and Shenandoah River, Virginia.

0.3 criterion

Figure 12. Fish tissue mercury concentrations in smallmouth bass, 1999–2007, 

South River, South Fork Shenandoah River, and Shenandoah River, Virginia.
Development of Time-Series Data 19

amount of mercury entering each model river reach from each 

hydrologic response unit representing contaminated floodplain areas. During calibration, the loading coefficients were 

adjusted to match simulated THG values in the river reaches to 

observed values.

Mercury in Groundwater

Groundwater samples were collected in October 2006, 

from a pastured flood plain 1 mile upstream from the Dooms 

station (01626920) in a joint effort by the U.S. Environmental 

Protection Agency, the Virginia Department of Environmental 

Quality, and the U.S. Geological Survey. The flood plain is 

downstream from the plant site and known to have mercury 

contamination in surface soils. Groundwater samples were 

pumped from 18 wells that had been installed to depths within 

30 ft of land surface. The samples analyzed for total mercury 

were collected and analyzed by the VDEQ following USEPA 

methods 1669 and 1631, Rev. E (U.S. Environmental Protection Agency, 1996, 2002), which have THGF

 detection limits 

of 1.5 ng/L. 

Thirteen of the 18 samples had THGF concentrations 

below the detection limit of 1.5 ng/L. The maximum THGF

concentration was 25.8 ng/L. When the non-detect values 

are set to zero, the mean THGF value of the 18 samples was 

2.9 ng/L, which was later used as an input to the numerical 

watershed model.

Development of Time-Series Data

The complex hydrologic conditions within the South 

River watershed are constantly changing, sometimes rapidly, 

when the river rises in response to a storm, for example. 

Time-series data describing these changing hydrologic conditions are needed to run the numerical watershed model and 

to calibrate the model. Time series were developed containing values for each required model input variable in regular 

intervals, typically 1 hour, to match the time step of the HSPF 

watershed model. These time-series data and their sources are 

summarized in table 14.

Streamflow Time Series

Observed streamflow data were used to calibrate the 

hydrologic part of the numerical watershed model and to 

calculate regressed suspended sediment concentration values. Daily mean streamflow time series for the period October 1, 1990, through September 30, 2007, were developed 

for streamflow-gaging stations 01626000, 01626850, and 

01627500 for calibration and verification of the hydrologic 

model. Hourly streamflow values for the same period were 

used to estimate suspended sediment concentration values as 

described in the next section(s). Both hourly and daily streamflow data were drawn from USGS-NWIS databases.

Table 13. Average concentrations of mercury in soil, South River 

watershed, Virginia, April 2003 through October 2006. 

[Samples from multiple sources compiled in the South River Science Team 

database. Hg, mercury; THGSed, mercury concentration on sediment or soil; 

μg/g, micrograms per gram; negative values indicate upstream from plant site]

Reach

Miles from 

plant site Soil HG samples

Model 

THGSed 

value μg/g 

From To Count

Average 

THGSed

μg/g

Final 

calibrated 

value

1 –30.0 –2.5 2 0.01 0.07

2 

(upstream)

–2.5 0 24 0.27 0.07

2 

(downstream)

0 2.3 145 13.9 13.9

3 2.3 5.3 137 16.2 16.7

4 5.3 16.5 245 17.2 16.7

5 16.5 24.0 41 7.6 7.6

Table 14. Sources of time-series data used in the South River 

watershed model. 

[CBM5, Chesapeake Bay Community Watershed Model, Phase 5, (Chesapeake Bay Program, 2006); NOAA, National Oceanic and Atmospheric 

Administration; VDEQ, Virginia Department of Environmental Quality; 

VPDES, Virginia Pollutant Discharge Elimination System; USGS, U.S. 

Geological Survey]

Time series Source of data

Streamflow Stage monitoring, USGS

Meteorology: rainfall, 

snowfall 

CBM5 datasets, NOAA weather 

station data, and data supplied by Invista and the City 

of Waynesboro 

Climatic conditions: air temperature, wind speed, cloud cover, 

dew point, evapotranspiration, 

solar radiation

CBM5 datasets, NOAA weather 

station data

Suspended sediment 

concentration

Sample analyses and multiple 

linear regression, USGS and 

VDEQ datasets

Mercury concentrations Sample analyses, VDEQ and 

USGS datasets

Point-source discharges VPDES data, permittee records
20 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Suspended Sediment Concentration Time Series

A time series of suspended sediment concentration was 

needed to calibrate the sediment part of the numerical watershed model. Time series allow more detailed and accurate 

model calibration than periodic grab sample values alone. 

Hourly time series of suspended sediment concentration at 

the primary monitoring stations (01626000, 01626920, and 

01627500) were developed to match the hourly time step of the 

watershed model for the periods October 1, 1990, through September 30, 2000, and April 1, 2005, through March 31, 2007.

Suspended sediment concentration values were regressed 

using linear multiple regression models with independent variables of turbidity and various transformations of streamflow. 

Use of turbidity and transformed streamflow as independent 

parameters generally improves predictions of suspended sediment concentration by linear regression models, as has been 

demonstrated in other investigations (Rasmussen and others, 

2005; Jastram, 2007). Suspended sediment concentration 

data available for the regression are from analyses of 78 grab 

samples (USGS only) collected at the three South River waterquality monitoring stations. The samples were collected during 

both base-flow and stormflow conditions at all three stations, 

as described earlier. 

A suitable regression model had to be formulated from 

the many possible explanatory (independent) variables. At the 

start of the regression analysis, a suite of variables was tested 

for correlation with suspended sediment concentration. The 

following variables were then retained as possible explanatory 

variables in the linear regression model: 

Q = Streamflow (ft3/s) measured concurrently 

with water sampling;

log10Q = Log of Q;

Q1⁄2 = Square root of Q;

Qslope = Percent change in Q from 1 hour previous;

Qincrease = Absolute value change in Q (ft3/s) from 

1 hour previous, (no negatives);

QA = Q normalized by watershed area; and

Turb = Turbidity (FNU).

Of the several streamflow parameters, QA exhibited the 

highest correlation with suspended sediment concentration 

(R2 = 0.744). QA was therefore retained in the regression model 

whereas Q, log10Q, and Q1⁄2 were excluded from the model 

because though they each have predictive value on their own, 

they correlate too strongly with QA to be included in the multiple regression model. Qincrease and Qslope were then redefined 

as follows:

Qslope = Percent change in QA from 1 hour 

previous; and

Qincrease = Change in QA (ft3/s) from 1 hour previous, 

negative values = 0.

The remaining four independent variables (Qslope, QA, 

Qincrease, and Turb) were then analyzed to determine the best 

multiple linear regression model for suspended sediment concentration. The best model(s) were selected primarily on the 

basis of three statistics—adjusted r-squared (Ra

2

), Mallow’s 

Cp, and the maximum variance inflation factor (Max VIF). All 

three statistics indicate a regression model’s goodness of fit, 

while also handicapping models that use a greater number of 

explanatory variables (Helsel and Hirsch, 2002). Results of the 

multiple linear regression analysis are shown in table 15.

Table 15. Linear multi-regression models for suspended sediment concentration. 

[U.S. Geological Survey samples only; R2, goodness of fit; Ra

2

, adjusted r-squared; Cp, Mallow’s statistic; Max VIF, maximum variance inflation factor; 

QA, discharge normalized by watershed area; Turb, turbidity; Qincrease, change in QA from 1 hour previous; Qslope, percent change in QA from 1 hour previous]

Model

number

Number of 

variables 

Mean 

squared error R2 Ra

2 Cp Max VIF

Dark gray cells indicate the variable is 

included in the regression model

QA Turb Qincrease Qslope

1 1 1,622.9 0.744 0.744 101.9 —

2 1 888.2 0.896 0.895 22.3 —

3 1 5,002.6 0.434 0.426 468.2 —

4 1 8,239.8 0.035 0.023 819.0 —

5 2 897.2 0.896 0.894 24.0 3.91

6 2 706.3 0.918 0.916 3.5 1.00

7 2 1,115.3 0.871 0.868 47.3 1.00

8 2 857.4 0.901 0.898 19.7 1.69

9 3 804.2 0.908 0.905 14.9 3.91

10 3 1,123.6 0.872 0.867 48.6 1.38

11 3 701.2 0.920 0.917 4.0 3.91

12 4 705.2 0.920 0.916 5.4 3.91
Development of Time-Series Data 21

Multiple regression model no. 6, which uses turbidity and 

the slope of streamflow, is the best predictive model for suspended sediment concentration because it has the lowest Cp

and Max VIF values and the second highest Ra

2 value. During 

periods when turbidity data are not available, such as prior to 

2005 and periodically after 2005, the best regression model is 

no. 7, which has the lowest Cp and Max VIF values, and second highest Ra

2 values of the models that do not use turbidity.

A basis of comparison for the regressed suspended 

sediment concentration values is given by Gellis and others (2004), who used alternate linear estimation methods to 

calculate suspended sediment concentration for the South Fork 

Shenandoah River at Front Royal, Virginia (USGS station no. 

01631000), about 100 mi downstream and to the northeast 

of the South River watershed. Their results show 50th and 

90th percentile suspended sediment concentration values of 

10.5 and 157 mg/L at monitoring station 01631000, which 

has a drainage area of 1,634 mi2. Compared to the regressed 

values in this study, for the combined periods October 1, 

1990, through September 30, 2000, and April 1, 2005, through 

March 31, 2007, Waynesboro (01626000) exhibits 50th and 

90th percentile values of 1.7 and 19.3 mg/L, whereas Harriston 

(01627500) exhibits values of 3.4 and 42.3 mg/L, respectively. 

Because rivers with larger drainage areas have higher suspended sediment concentrations, the statistics for the regressed 

suspended sediment concentration values compare reasonably 

well (Gellis and others, 2004).

The regressed suspended sediment concentration time 

series from model no. 7 is shown in figure 13. There is reasonable agreement between the observed and regressed values, 

except at very low values where analytical detection limits are 

approached. The value of the regression method can be seen 

in the storm peak values of regressed suspended sediment 

concentration, which generally exceed the observed values. 

This is beneficial to the calibration because infrequent large 

storms carry most of the suspended sediment load, but grab 

samples are unlikely to have been collected at the moment of 

peak suspended sediment concentration values during a storm. 

The regression equation provides a means of extrapolating 

beyond the highest observed suspended sediment concentration values, although the extrapolated values have considerably less certainty than regressed values within the range of 

observed data.

Meteorological and Climatic Data

The numerical watershed model requires a variety of 

climatic and meteorological input time-series data. Following Chesapeake Bay Community Watershed Model, Phase 5 

(CBM5) structure, the following data were input to the model: 

precipitation, cloud cover, dew point, wind speed, solar radiation, air temperature, and potential evapotranspiration (Chesapeake Bay Program, 2006). These data all were in the form of 

time series with hourly time steps. HSPF uses these input data 

to calculate actual evapotranspiration, soil moisture storage, 

snow melt, and runoff, among other hydrologic variables. 

For the period January 1985 through August 2005, timeseries data for the above variables were obtained from the 

CBM5 model and used in the South River watershed model 

Figure 13. Time series of regressed and sampled suspended sediment concentrations, South River at 

Harriston, Virginia (USGS station number 01627500), April 1, 2005, through March 31, 2007.

1

10

100

1,000

MAR

2005

JULY

2005

OCT

2005

JAN

2006

MAY

2006

AUG

2006

NOV

2006

FEB

2007

SUSPENDED SEDIMENT CONCENTRATION,

IN MILLIGRAMS PER LITER

(USGS SAMPLES ONLY) 

Regressed

Observed 

DATE

Figure 13. Time series of regressed and sampled suspended sediment concentrations, South River at Harriston (USGS station

number 01627500), Virginia, April 1, 2005, through March 31, 2007.
22 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

without modification, except for scaling potential evapotranspiration time-series data to improve the overall simulated water balance. The CBM5 climatic and meteorological 

time-series data were developed from observational data 

provided by the National Climatic Data Center of the National 

Oceanic and Atmospheric Administration. Observational 

data from over 200 hourly weather monitoring stations in the 

Chesapeake Bay watershed were integrated using statistical 

techniques to account for spatial and temporal gaps in the 

data, and to create continuous hourly time series (Chesapeake 

Bay Program, 2006). Only those time series applicable to 

the five land segments found in the South River watershed 

(A51015, A51165, A51820, B51015, and C51015) were used 

in this study. 

 Hourly climatic and meteorological time series for all 

areas of the model were extended through March 31, 2007, 

using the data from U.S. Air Force meteorological monitoring 

station 724105 at the Shenandoah Valley Regional Airport in 

Weyers Cave, 14 mi north of Waynesboro (fig. 11). Hourly 

surface observations at the Shenandoah Valley Regional 

Airport were obtained from the National Climatic Data Center 

website (http://www.ncdc.noaa.gov/oa/ncdc.html) for precipitation, dew point, wind speed, and air temperature. Cloud 

cover was estimated from hourly observations of sky cover 

(SKC), converting the observation codes as follows: clear 

(CLR) = 0.0, scattered clouds (SKT) = 3.0, partial obscuration 

(POB) = 6.0, broken (BKN) = 7.0, overcast (OVC) = 10.0, 

and obscured (OBS) = 10.0. Hourly solar radiation values 

were calculated from cloud cover and latitude using WDMUtil 

software (Hummel and others, 2001). The Hamon method 

(Hamon, 1961) was applied within WDMUtil to calculate 

potential evapotranspiration using latitude and daily minimum 

and maximum air temperatures.

Additional sources of precipitation data were available 

for 2005–07. Daily precipitation data were provided by Invista 

for its wastewater treatment plant in Waynesboro at the former 

DuPont plant site (Brenda Kennell, Invista, written commun., 

2007), and daily precipitation data were downloaded from the 

National Climatic Data Center website for the City of Waynesboro sewage treatment plant (STP) (cooperative station ID# 

448941). Daily precipitation totals recorded at the Waynesboro 

STP and at the Invista wastewater treatment plant were not 

used in compiling the CBM5 data (Chesapeake Bay Program, 

2006) and differ from precipitation totals recorded at the 

Shenandoah Valley Regional Airport during the same period. 

The calibrated watershed model in this study uses precipitation data from all of these sources, with CBM5 data through 

August 2005 as the base and the additional data included 

where available to modify CBM5 values.

Observed precipitation for the period April 1, 2005, 

through March 31, 2007, at the three monitoring stations in the 

study area, as well as CBM5 estimated precipitation, is shown 

in figure 14. All of the observation data shown were collected 

using different procedures and in the case of the Waynesboro 

Figure 14. Cumulative precipitation at sites in and near the South River watershed, 

Virginia, for the period April 1, 2005, through March 31, 2007. (Chesapeake Bay Watershed 

Model Version 5 data obtained from the U.S. Environmental Protection Agency Chesapeake 

Bay Office, Annapolis, Maryland, 2006).

0

10

20

30

40

50

60

70

80

90

100

FEB

2005

MAY

2005

SEPT

2005

DEC

2005

MAR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

AUG

2007

CUMULATIVE PRECIPITATION, IN INCHES

Waynesboro Sewage Treatment Plant, Waynesboro, Virginia

Shenandoah Valley Regional Airport, Data sav3 station 724105

Invista plant site, Waynesboro, Virginia

Chesapeake Bay Model Version 5, land-county segment A51015

DATE

Figure 14. Cumulative precipitation at sites in and near the South River watershed, Virginia for the period April 1, 2005,

through March 31, 2007. (Chesapeake Bay Watershed Model Version 5 data obtained from the U.S. Environmental

Protection Agency Chesapeake Bay Office, Annapolis, Maryland, 2006).
Conceptual Model of Mercury Fate and Transport in the Watershed 23

STP and the Invista site, were recorded on a daily, rather 

than hourly, basis. In addition to both the Waynesboro STP 

and Invista data showing an additional 20 in. of rain during 

the 2-year period, the storm patterns also show differences 

when compared to the Shenandoah Valley Regional Airport 

data. The most extreme example occurred during a storm on 

October 9, 2005, which shows up in the Invista and Waynesboro STP data as about 6 in. of precipitation, but only about 

0.7 in. are observed in both the Shenandoah Valley Regional 

Airport and CBM-A51015 data. These differences indicate the 

potential for input errors to the HSPF model and are discussed 

in more detail later in the report.

Point Sources

In the South River watershed model, point sources are 

flows of water and associated constituents that discharge 

directly to the river. A typical example is discharge from a 

wastewater treatment plant. 

Data describing point-source discharges to the South 

River were compiled from the Virginia Pollutant Discharge 

Elimination System database that is maintained by the VDEQ 

for discharges in the State of Virginia. Of the 12 individually permitted facilities in the South River, 5 industrial or 

major municipal facilities were included in the model. Other 

smaller discharges were determined to contribute insignificant 

amounts of mercury to the South River. A detailed listing of 

data used to compile point-source discharge time series is 

given in Appendix 1.

Because data in the Virginia Pollutant Discharge Elimination System typically have monthly time steps, whereas the 

time series input to the South River watershed model have 

daily time steps, point-source data were disaggregated assuming constant daily rates within each month. Additional data 

with shorter time steps were collected, where possible, from 

discharge facility operators and used to supplement the Virginia Pollutant Discharge Elimination System data. When only 

annual data were available, these were disaggregated to daily 

intervals assuming seasonal patterns by month and constant 

daily rates within each month.

Additional Model Flows to the River 

Two discharges to the South River, specified in the model 

but not subject to permit regulations, are treated in the model 

as point-source flows to the river. These flows are from Frew 

Pond/Baker Spring and from Loth Spring, which are adjacent to the South River in Waynesboro, and are known to 

have large water flow rates (Brenda Kennell, Invista, written 

commun., 2007). Baker Spring flows into Frew Pond, which 

is a reservoir adjacent to the river at the plant site and managed by Invista. From Frew Pond, water flows over a weir 

and discharges to the South River. Loth Spring is adjacent to 

the river on its north side across from Frew Pond and flows 

directly to the river. Frew Pond/Baker Spring and Loth Spring 

are assigned monthly flow rates in the model on the basis 

of observed flows (DuPont Corporate Remediation Group, 

2006a) and an assumed seasonal variation of 30 percent. 

Conceptual Model of Mercury Fate and 

Transport in the Watershed 

As a foundation for building a numerical watershed 

model, a conceptual model for mercury fate and transport in 

the watershed was developed first. The conceptual model summarizes the primary paths by which mercury enters the South 

River water column and the fate of the mercury once it is in 

the river (fig. 15).

Figure 15. Conceptual model of total mercury sources, sinks, and transport in the South River, Virginia.

TRIBUTARY

MERCURY

INFLUX 

DOWNSTREAM 

MERCURY FLUX 

SEDIMENT MERCURY

DEPOSITION/

SUSPENSION

POINT SOURCES

sorption

desorption

ATMOSPHERIC

DEPOSITION

GROUNDWATER AND

INTERFLOW MERCURY

WATER COLUMN

RUNOFF

CHANNEL MARGIN

INPUTS 

Figure 15. Conceptual model of total mercury sources, sinks, and transport in the South River, Virginia.
24 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Mercury concentrations in all media are markedly higher 

downstream from the plant site than upstream, and it is presumed that most mercury currently (2009) entering the South 

River originated from the plant site at some time since 1929. It 

is known that the flood plain contains legacy mercury from the 

plant site and that contaminated sediment from the flood plain 

is currently entering the river. The Waynesboro monitoring 

station (01626000) is the only one of the three monitoring stations upstream from the plant site, and serves as a background 

reference station in this study.

At all three monitoring stations, filtered mercury concentrations in the water column increased with increasing 

streamflow as shown in figure 8. This observation is consistent 

with the hypothesis that contaminated sediments are the primary source of mercury in the water column. If point-source 

discharges were the primary Hg load, then river concentrations 

would be more likely to decrease with increased streamflow 

due to dilution effects.

Total mercury in the South River exhibits a partitioning 

ratio between dissolved and sorbed phases of about 1,000,000 

(log Kd=6). The importance of partitioning to the TMDL is 

diminished by the fact that fish tissue methylmercury correlates strongly with the sum of filtered and particulate mercury, 

and the sum of the two concentrations is only secondarily 

affected by the exchange of mercury between sorbed and dissolved phases.

Several lines of evidence point to contaminated soil in 

the flood plain and river channel as the current greatest source

of mercury to the river. Downstream surveys have shown that 

mercury concentrations in fish, water, and sediment rise steadily 

from the plant site downstream for about 12 mi (Turner and 

Jensen, 2006; Flanders and others, 2007). The relatively steady 

increase in concentrations points to mercury inputs being dispersed for many miles along the river rather than coming from 

discrete inputs such as point-source discharges or tributaries. 

Except for the sharp increase in water column mercury concentrations at the plant site, there are no abrupt THG increases that 

would indicate a major input of mercury from point sources. 

The accounting of mercury loads to the river also requires that 

a large percentage of mercury come from nonpoint sources to 

achieve a mass balance, as discussed in a later section, Mercury 

Transport Model Calibration Results.

Watershed Model Development

A numerical watershed model of the South River watershed was developed to simulate dynamic streamflow response, 

sediment transport, and mercury transport. The simulation 

model calculates mass balances for water as well as sediment 

and mercury in the South River for the simulation period January 1, 1985, through March 31, 2007. The simulation model 

also permits hypothetical conditions, such as reduced mercury 

source loads, droughts, floods, or long-term mercury mass balances, to be analyzed.

The software used to implement the numerical model 

is Hydrological Simulation Program–FORTRAN (HSPF), a 

watershed-based modeling package widely used for TMDL 

development (Donigian and others, 1995; Bicknell and others, 

2001). The hydrologic component of HSPF generates time 

series of streamflow in response to precipitation, evapotranspiration, and movement of water from the land surface through 

various routes to streams. Simulations are transient and require 

extensive input data describing land use and hydraulic characteristics, climatic conditions, river geometry, and sediment and 

mercury transport characteristics.

The first 6 years of the South River watershed model 

simulation (1985 through 1990) bring the model to relative 

steady state conditions, dampening perturbations from initial 

conditions. A 10-year period, from October 1, 1990, through 

September 30, 2000, was used to calibrate the hydrologic 

and sediment parts of the model. The 5 years from October 1, 2000, through September 30, 2005, were used to verify 

the calibrated hydrologic and sediment parts of the model. 

The mercury transport model was calibrated for the period 

April 2005 through March 2006, and verified for the period 

April 2006 through March 2007.

The South River watershed model has a 1-hour time step, 

which is sufficiently small to represent important hydrologic 

changes, but not so small as to make model run times impractical. The calibrated watershed model has run times of about 

4 minutes. 

Functional Description of Hydrological 

Simulation Program–FORTRAN (HSPF) 

HSPF is a mathematical model designed to simulate the 

hydrology and movement of contaminants in a watershed. 

As applied to the South River watershed, the HSPF model 

simulates streamflow, sediment transport, and mercury transport. HSPF calculates water, sediment, and contaminant loads 

following mass conservation principles of water, with inflow 

equaling outflow plus or minus any change in storage (Bicknell and others, 2001). In HSPF, a watershed is represented 

by a collection of hydrologically similar areas, referred to as 

hydrologic response units (HRUs), which drain into a network 

of stream or lake segments. Each HRU represents land having characteristic hydrologic controls, such as land use, soil, 

subsurface geology, and other factors deemed important in 

controlling hydrology. Each stream segment represents a river 

reach or lake. For each HRU and stream segment, the model 

computes a water budget (inflows, outflows, and changes in 

storage) for each time step. 

HRUs represent either pervious or impervious land areas. 

Both pervious and impervious land areas can retain precipitation on the surface. On pervious land areas, excess precipitation can infiltrate to the subsurface, where storages and fluxes 

are calculated for upper and lower groundwater zones, or can 

run off to a river reach. On impervious area, all water that is 

not evaporated from the surface produces runoff to a river 
Watershed Model Development 25

reach. The downstream end of each river reach is referred to 

as a node. Nodes are typically placed to define channel segments with similar physical properties, or at other locations 

where estimates of streamflow or contaminant concentrations 

are desired. The hydrologic characteristics used for kinematic 

wave routing of water in a river reach are defined in a function 

table that is specified in the model input. The SCHEMATIC 

and MASS-LINK blocks define the physical layout of the 

watershed, linking river reaches together and assigning the 

acres to each pervious and impervious land area.

The inflows to and outflows from a river reach in the 

South River watershed model are shown in figure 16 (modified from Zarriello and Bent, 2004, fig. 9). Surface runoff can 

discharge to a reach from impervious surfaces and pervious 

surfaces. Infiltrated water can discharge to a reach through the 

subsurface as interflow, a fast-responding shallow subsurface 

flow, or from active groundwater, a slow-response base-flow 

component. Inflow to a reach can also come from upstream 

reaches, direct precipitation, and other user-specified sources 

such as treated point-source discharges. 

HSPF requires two primary input files for its operation—

the user control input file and the watershed data management 

file. The user control input file directs the model-process algorithms and sets user-specified input variables. The watershed 

Figure 16. Schematic showing hydrologic routing in the South River numerical watershed model (modified from 

Zarriello and Bent, 2004, figure 9).

RCHRES

Evaporation

Precipitation

Inflow from

upstream

reach

(IVOL) Point-source

wastewater

flow

Outflow to

downstream

reach (OVOL)

SURO

IFWO AGWO

Evapotranspiration

Precipitation

SURI–Surface runoff from impervious areas

SURO–Surface runoff from pervious areas

IFWO–Interflow

AGWO–Active groundwater flow (base flow)

RCHRES–Stream reach or reservoir segment

IVOL–Inflow volume

OVOL–Outflow volume

Impervious

surface

runoff

(SURI)

Figure 16. Schematic showing hydrologic routing in the South River numerical watershed model. [modified from Zarriello

and Bent, 2004, figure 9].

Table 16. Organization of dataset numbers in the watershed data 

management file for the South River watershed model, Virginia. 

[DSN, dataset number; AGWO, active groundwater flow; IFWO, interflow]

DSN Purpose

1101–1523 Simulated daily AGWO output, last 3 digits are 

hydrologic response unit number.

2101–2523 Simulated daily IFWO output, last 3 digits are 

hydrologic response unit number.

3000–3999 Observed/calculated meteorological inputs.

4000–4999 Simulated hydrologic and sediment outputs.

5000–6015 Simulated mercury concentration, storage, and 

load outputs.

6101–6999 Point-source flow and load inputs.

7000–7999 Simulated daily sediment runoff output, last 

3 digits are hydrologic response unit number.

data management file holds a time-series database. Timeseries datasets are organized in the South River watershed 

data management file as shown in table 16. A more complete 

description of the HSPF software is given in the “HSPF User’s 

Manual” (Bicknell and others, 2001).
26 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Representation of the Watershed 

The South River watershed is represented in the HSPF 

model as a combination of HRUs, consisting of pervious and 

impervious land surfaces. Each HRU has an assigned contributing area to each stream reach. The stream reaches are 

linked to each other in downstream order. Basin and subbasin 

boundaries in the model study area were initially obtained 

from the Chesapeake Bay HSPF model Phase 5.14, referred 

to here as “CBM5”, developed by the USEPA, the USGS, and 

other partners (Martucci and others, 2005; Chesapeake Bay 

Program, 2006). 

The South River watershed is divided into five model 

subbasins that each contains a single river reach (fig. 17). The 

nodes defining the subbasins were selected to correspond with 

monitoring locations along the South River so that output 

from the model could be compared to field observations at the 

same locations. Each subbasin is composed of multiple HRUs, 

which send their output (water, sediment, and mercury) to the 

river reaches. The CBM5 model has four subbasins within the 

South River watershed (PS2_6730_6660, PS2_6660_6490, 

PS2_6490_6420, and PS2_6420_6360). For this study, an 

additional subbasin was needed to produce simulation results 

at the location of streamflow-gaging station 01626920. Using 

Figure 17. Location of South River watershed model subbasins.

1

2

3

4

5

AUGUSTA

NELSON

ALBEMARLE

AUGUSTA

AUGUSTA

ROCKINGHAM

78°45'

79°

79°15'

38°15'

38°

0 5 10 MILES

0 5 10 KILOMETERS

Figure 17. Location of South River watershed model sub-basins.

01626920

01626850

01627500

01626000

Stuarts Draft Sewage Treatment Plant

Invista-DuPont

Waynesboro STP

Genicom

Alcoa

SOUTH RIVER WATERSHED

SUBBASIN

MODEL STREAM REACH

STREAMFLOW-GAGING

STATION AND NUMBER

POINT-SOURCE DISCHARGE

01626000

EXPLANATION
Watershed Model Development 27

geographic information system (GIS) software, one of the 

CBM5 subbasins (PS2_6490_6420) was divided into subbasins 3 and 4 in the South River watershed model (fig. 17). 

River reach parameters and HRU areas contributing to each 

reach were then recalculated. 

Parameters describing hydrology, sediment transport, and 

mercury transport were assigned to each HRU. Initial parameter values were set equal to those in the calibrated CBM5 

model. The CBM5 model parameters had already been calibrated to streamflow values from 1985 through 2005, including streamflow on three of the South River streamflow-gaging 

stations (01626000, 01626850, and 01627500). Some model 

parameters were then modified in this study to improve the 

match between simulated and observed streamflow, sediment 

concentrations, and mercury concentrations. These changes 

are discussed in the following sections describing calibration 

of the model.

Development of Hydrologic Response 

Units (HRUs) 

The smallest HSPF model component is the HRU. Within 

a single HRU, climatic conditions, hydrologic responses, and 

contaminant transport are assumed to be uniform. The HRUs 

in the South River watershed model are nearly identical to 

those used in the CBM5 model (Martucci and others, 2005). 

These HRUs are based on county boundaries, land use, and 

valley or mountain geography. Five land-county segments 

developed for the CBM5 model are present in the South River 

(fig. 18). Climatic variables and precipitation vary according to land-county segment. There are 25 CBM5 land-use 

types in the watershed, and an additional land use, mercurycontaminated flood plain, was added for this study (table 17). 

The five land-county segments combined with the 26 land uses 

result in 130 different HRUs in the South River watershed. 

Fifteen of the HRUs are impervious land areas, and 115 are 

pervious areas. 

Table 17. Land-use representation in the watershed model, South River, Virginia. 

[HRU, hydrologic response unit; IMPLND, impervious land area; PERLND, pervious land area]

HRU 

type Land use Subbasin area (acres) Total 

1 2 3 4 5 (acres) 

IMPLND animal feeding operations 20 7 1 13 51 92

IMPLND low intensity impervious urban 352 865 53 76 99 1,444

IMPLND high intensity impervious urban 113 263 3 25 32 436

PERLND forest 48,816 5,885 5,862 15,817 6,090 82,470

PERLND harvested forest 493 64 63 175 73 868

PERLND alfalfa 1,221 0 162 520 76 1,979

PERLND natural grass 1,865 1 3 13 11 1,892

PERLND high till without manure 18 1 2 8 5 33

PERLND high till with manure 259 11 36 114 125 544

PERLND hay without nutrients 538 22 59 254 152 1,026

PERLND hay with nutrients 2,371 98 261 1,118 626 4,475

PERLND low till with manure 1,293 54 178 574 439 2,537

PERLND nutrient management alfalfa 262 61 43 137 251 754

PERLND nutrient management high till with manure 176 7 24 78 57 342

PERLND nutrient management high till without manure 12 1 2 5 3 22

PERLND nutrient management hay 1,611 67 232 704 333 2,946

PERLND nutrient management low till 879 36 97 414 217 1,642

PERLND nutrient management pasture 453 380 87 375 1,103 2,397

PERLND pasture 16,042 302 1,727 7,401 2,287 27,759

PERLND bare-construction 369 548 26 101 95 1,138

PERLND extractive 92 3 0 7 2 103

PERLND trampled 83 3 12 34 17 149

PERLND nursery 210 9 32 90 31 372

PERLND high intensity pervious urban 2,541 956 225 606 404 4,732

PERLND low intensity pervious urban 1,230 3,484 513 538 810 6,576

PERLND mercury-contaminated flood plain 0 493 378 1,412 1,105 3,388

Total acres 81,316 13,619 10,082 30,606 14,493 150,115
28 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Pervious areas occupy 98.7 percent of the watershed. 

Forestry and agriculture are the dominant land uses, representing 55.5 percent and 31.2 percent, respectively of the total 

watershed area. Impervious surfaces in the watershed consist 

primarily of developed urban areas with dense building and 

pavement cover and make up 1.3 percent of total watershed 

area. All pervious and impervious areas in the model contribute their outflows directly to a stream reach.

All HRUs were defined according to the CBM5 scheme, 

with one exception. Five pervious land areas were created to 

represent mercury-contaminated flood plains. Areas for the 

five pervious land areas representing mercury-contaminated 

flood-plain areas were calculated from spatial data outlining 

the 62-year flood plain. The 62-year flood plain represents the 

maximum extent of the flood plain inundated since mercury 

was originally released from the former DuPont plant site. The 

largest daily flow recorded since release of mercury was determined by flood frequency analysis to have a return period of 

62 years. Hydraulic analysis was then conducted to delineate 

the flood-plain area inundated by a flood of this magnitude 

(DuPont Corporate Remediation Group, 2007).

Figure 18. Land use in the South River watershed (National Land Cover Dataset, April 27, 1999; DuPont Corporate 

Remediation Group, 2007, South River 62-year flood-plain data layer).

RCHRES 1

RCHRES 4

RCHRES 5

RCHRES 3

B51015

C51165

A51820

MODEL REACH 1 

MODEL REACH 2

MODEL REACH 3

MODEL REACH 4

MODEL REACH 5

LAND-COUNTY SEGMENT

AND NUMBER

LOW INTENSITY RESIDENTIAL

HIGH INTENSITY RESIDENTIAL

INDUSTRIAL/TRANSPORTATION

QUARRIES/MINES

TRANSITIONAL

DECIDUOUS FOREST

EVERGREEN FOREST

MIXED FOREST

PASTURE/HAY

CROPS

URBAN GRASSES

WOODY WETLANDS

EMERGENT WETLANDS

62-YEAR MERCURY CONTAMINATED FLOOD PLAIN ALBEMARLE

AUGUSTA

Figure 18. Land use in the South River watershed [National Land Cover Dataset, April 27,1999; DuPont Corporate Remediation Group,

2007, South River 62-yr floodplain data layer].

78°45'

79°

79°15'

38°15'

38°

0 5 10 MILES

0 5 10 KILOMETERS

Waynesboro

A51015

A51015

A51165

RCHRES 2

LAND USE

EXPLANATION

A51015
Watershed Model Development 29

Stream Reaches 

River reaches receive their input from pervious land 

areas, impervious land areas, atmospheric deposition, and 

upstream reaches, while discharging either to a downstream 

reach or to the model exit (fig. 15). The watershed model 

contains five stream reaches, one for each subbasin. Model 

parameter values for each reach were initially set to be the 

same as those in the CBM5 model. Reach PS2_6490_6420 

in the CBM5 model was divided into reaches 3 and 4 of this 

study’s model, so that simulation results at streamflow-gaging 

station 01626920 could be obtained. 

The physical characteristics of the five South River 

watershed model river reaches are listed in table 18. Parameters were adjusted during the calibration process, including 

the length and elevation drops, to reflect additional observation data and to improve hydrologic and sediment transport 

simulations. These parameters were reset to values determined 

by GIS analysis of 10-m digital elevation model data. The 

FTABLES, which specify channel geometry, were not changed 

from the CBM5 model.

Hydrologic Model

The hydrologic component of the model simulates water 

movement and storage in the South River watershed. Precipitation and point-source discharges to the river are the only 

hydrologic inputs to the model domain whereas actual evapotranspiration and streamflow are the only outputs. Precipitation that falls on the land surface but does not evaporate or 

transpire is routed to the river. Once in the river, water moves 

downstream and exits the model from the last river reach 

(number 5). Major components of the hydrologic cycle simulated by HSPF for pervious land areas and river reaches are 

shown in figure 19. More detailed descriptions of the storage 

and flow terms can be found in Bicknell and others (2001).

Table 18. River reach characteristics in the calibrated South River watershed model. 

[mi, miles; ft, feet; USGS, U.S. Geological Survey]

Model river 

reach number

Extent Upstream

reach number

Length

(mi)

Elevation drop

(ft) Slope From To

1 headwaters USGS station no. 01626000 none 14.3 164 0.0022

2 USGS station no. 01626000 USGS station no. 01626850 1 5.1 46 0.0017

3 USGS station no. 01626850 USGS station no. 01626920 2 3.0 20 0.0013

4 USGS station no. 01626920 USGS station no. 01627500 3 11.3 98 0.0017

5 USGS station no. 01627500 South Fork Shenandoah River 4 7.7 105 0.0026

Figure 19. Major hydrologic components for pervious land areas and river reaches simulated in the South River 

numerical watershed model. 

Interflow

Interception

Evapotranspiration

Precipitation

Groundwater Discharge

Runoff

Groundwater

Recharge

Figure 19. Major hydrologic components for pervious land areas and river reaches simulated in the South River numerical

watershed model.
30 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Point-Source Discharges

Reaches 1, 2, 3, and 5 receive discharges from point 

sources. The discharges come from a variety of permitted facilities, listed in table 19 and Appendix 1. Other small 

facilities discharge to the South River but were not included 

in the model because they have flows of less than 0.5 Mgal/d 

(million gallons per day) and discharge insignificant amounts 

of mercury. Discharge rates, suspended sediment concentrations, and mercury concentrations were assigned outside of 

the model and input to the model as time series with daily 

time steps. The data available to describe each point source 

varied widely and were from a variety of sources. Time series 

describing the point-source discharges were developed in collaboration with Invista, DuPont, and the VDEQ.

Sediment Transport Model

Sediment transport was incorporated into the watershed 

model because most mercury transport occurs in association 

with suspended sediment. At both the Dooms and Harriston 

monitoring stations, water-sample analyses indicate that over 

95 percent of the mercury in the water column is sedimentassociated, with suspended sediment defined as material not 

passing a 1.5-μm filter. 

In the watershed model, sediment moves to the river 

from impervious and pervious land areas during surface 

runoff events (fig. 20). Sediment loads from land surfaces 

vary according to land use and location in the watershed. The 

South River watershed model simulates sediment transport in 

the river using the same parameterization as the CBM5 model 

Table 19. Point sources in the South River watershed model, average flow, sediment loads, and mercury loads for the period 

April 1, 2005, through March 31, 2007. 

[VPDES, Virginia Pollutant Discharge Elimination System; Hg, mercury; g, grams; ft3/s, cubic feet per second; STP, sewage treatment plant; na, not applicable]

Model ID

Point sources in model

Model 

reach

VPDES

permit number

Annual mean (2005–2007) 

Facility name Downstream order Flow (ft3/s) Sediment 

load (ton) Hg load (g)

101 Stuarts Draft STP 1 VA0066877 1.671 1.5 1.0

222 Loth Spring 2 na 1.756 1.5 6.4

— Invista/ former DuPont — —

201 outfall 001 2 VA0002160 6.523 17.2 402.9

203 outfall 003 2 VA0002160 0.066 0.4 2.8

204 outfall 004 2 na 0.014 0.3 0.5

208 outfall 008 2 na 0.374 4.2 89.0

209 outfall 009 2 na 0.111 0.8 8.1

210 outfall 010 2 na 0.040 1.9 8.2

211 outfall 011 (after 08/02) 2 VA0002160 0.020 0.4 21.8

212 outfall 012 2 na 0.000 0.0 0.0

213 outfall 013 2 na 0.007 0.0 0.3

214 outfall 014 2 na 0.005 0.0 0.3

221 Frew Pond, Baker Spring 2 na 7.025 5.9 25.4

231 Plant site groundwater 

discharge

2 na 0.501 0.0 1.5

— Invista/ former DuPont Plant Site Totals 15 31 561

241 Waynesboro STP 2 VA0025151 5.497 42.2 37.1

301 Genicom 3 VA0002402 0.196 0.0 0.0

501 Alcoa 5 VA0001767 2.486 12.7 40.6

Totals 26 89 646
Watershed Model Development 31

(Chesapeake Bay Program, 2006); with a power function 

governing sand transport and critical stress levels controlling 

silt and clay transport. Sediment transport parameter values 

were initially assigned to be the same as those in the CBM5 

model and were then modified during the calibration process. 

Sediment transport is handled differently for impervious land 

areas, pervious land areas, and river reaches as described in 

the next section(s) (fig. 21). 

Impervious Land Area Sediment Transport

Sediment transport from impervious land surfaces to river 

reaches is simulated using the IMPLND-SOLIDS module. At 

each hourly time step, solids accumulate or are removed, by 

street cleaning for example, from the land surface at the userspecified rates listed in Appendix 2 (available only online 

at http://pubs.usgs.gov/sir/2009/5076/). Solids are transported 

from impervious land areas to river reaches, at user-specified 

exponential rates, when overland flow occurs. Parameters 

governing sediment production from each impervious land 

area were initially assigned CBM5 values, and then calibrated 

by matching simulated to observed suspended sediment concentration values, as described in a later section of this report, 

Sediment Transport Model Calibration Results. 

Pervious Land Area Sediment Transport

Simulation of sediment transport to the river from 

each pervious land segment is performed by the PERLNDSEDMNT module. Only detached sediment is available to be 

transported to the river, therefore, no scouring is simulated. 

Sediment can be detached by soil drying, rainfall splash, or 

other processes at detachment rates specified by the user. 

Detached sediment is transported to the river during overland 

flow runoff events at exponential rates controlled by userspecified coefficients. Sediment transport from the five pervious land areas representing mercury-contaminated flood-plain 

areas is simulated using the same processes and simulation 

modules. Final calibrated values for sediment transport parameters are listed in Appendix 2.

Figure 20. Sediment transport processes for pervious land areas (from Bicknell and others, 2001). Figure 20. Sediment transport processes for pervious land areas. (from Bicknell and others, 2001)

MANMADE

INFLUENCES

RAINFALL

SPLASH

COMPACTION

AND SETTLING

SCOUR

SOIL

MATRIX

WASHOFF

DETACHED

SEDIMENT

STORAGE

WIND

ATMOSPHERIC

FALLOUT

DUST COVER
32 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Sediment Point Sources

Municipal and industrial discharges to the river generally 

contain suspended sediment. These point sources are treated 

in the model as direct inputs to the river with sediment loading 

rates specified outside of the model. These rates were determined from data collected by discharge permit owners and 

stored in the Virginia Pollutant Discharge Elimination System 

database. Point sources contribute only a very small percentage (less than 1 percent) to the total sediment load of the South 

River. Summaries of the sediment point-source loads are listed 

in table 19 and Appendix 1.

Sediment Transport in the River

Sediment entering a river reach can be deposited and 

remain stationary on the channel bed or travel downstream in 

suspension. Transport, deposition, and resuspension of sediment within a river reach are handled in HSPF by the modules 

shown in figure 21. At each hourly time step, HSPF recalculates all sediment storage and load terms. Suspended sediment 

present in a river reach can settle to the channel bottom, exit 

downstream, or remain in the reach. Sediment deposited on 

the channel bottom can be resuspended by increased flow 

velocities. Initial conditions are user-specified for initial suspended sediment concentration and for depth of sediment on 

the bed of each river reach.

Sediment within a river reach is divided by HSPF into 

three sediment size classes—sand, silt, and clay. Transport of 

each size class is simulated separately. Non-cohesive particles 

(sand) and cohesive particles (silt and clay) have different 

algorithms controlling transport within the river. There were 

insufficient data from the South River to accurately calibrate 

to suspended sediment size, so sediment size fractions in the 

model were assigned to be 33.3 percent sand, 33.3 percent 

silt, and 33.3 percent clay. The capacity of each river reach to 

transport sand downstream is calculated using an exponential 

equation (Bicknell and others, 2001) with user-specified rates. 

When transport capacity exceeds the rate of sand transport, 

resuspension of bed sand occurs. Conversely, when sand load 

exceeds transport capacity, deposition of sand on the channel 

bed occurs. Silt and clay transport are simulated with a different algorithm that controls scour and deposition according to 

user-specified settling rates, critical stress thresholds for deposition and suspension, and erodibility coefficients. Values for 

sand, silt, and clay transport parameters used in the calibrated 

South River watershed model are listed in Appendix 2. 

Mercury Transport Model

The third component of the South River watershed model 

simulates mercury transport on the basis of the conceptual 

model described earlier. Total mercury is the only form of 

mercury that was simulated. Other forms of mercury such 

as methylmercury were not simulated, not because they are 

absent or unimportant, but because the dynamics of methylmercury cycling and bioaccumulation in the South River 

system are currently not well understood. Modeling of total 

mercury in the South River was performed because fish tissue 

methylmercury concentrations correlate more strongly with 

Figure 21. Sediment routing processes in the South River numerical watershed model.

SLDS

Detached

Solids

Storage

SOSLD

Washoff of

Solids

by Water

ACCSDP

Solids 

Accumulation

IMPLND

SLDS

Detached

Solids

Storage

SOSLD

Washoff of

Solids

by Water

ACCSDP

Solids 

Accumulation

DETS

Detached

Sediment

Storage

NVSI

Net Vertical

Sediment Input

DET AFFIX

Soil Detachment

and Reattachment

WSSD

Washoff of

Solids

by Water

PERLND

Soil Matrix

Unlimited

Sediment

Storage

SuspendedSediment

Storage 

RSED

Sediment

Bed Storage

DEPSCR

Deposition or 

Scour

ROSED

Solids Outflow

ISED

Solids

Point

Sources

RCHRES

RSED

Sediment

Bed Storage

DEPSCR

Deposition or 

Scour

ROSED

Solids Outflow

ISED

Solids

Point

Sources

Figure 21. Sediment routing processes in the South River numerical watershed model.
Watershed Model Development 33

total mercury than with any other form of mercury in the water 

column, including methylmercury (Virginia Department of 

Environmental Quality, 2008b). The model is constructed so 

that future studies can incorporate methylmercury cycling, 

bioaccumulation, or other processes if desired.

Mercury is transported to the river along multiple hydrologic pathways: direct precipitation to the river surface, pointsource discharges, groundwater and interflow, land-surface 

sediment runoff, channel margin inputs, and downstream 

advection. The HSPF modules used to simulate these pathways are listed in table 20 and shown in figure 22. Once mercury enters a river reach, it partitions between dissolved and 

sorbed phases. The model simulates the storage of mercury 

in channel bed sediment and the reintroduction of mercury 

to the water column when bed sediment is resuspended by 

higher flows. 

Silt and clay particles are assigned the same mercury 

transport parameters and initial THGSS concentrations. Sand 

is assumed to have a mercury sorption capacity 1,000 times 

lower than that of silt or clay

Figure 22. Mercury (Hg) routing in the South River numerical watershed model.

Hg Stored on

Suspended

Sediment 

PERLND RCHRES 

Sediment

Deposition or

Scour 

Dissolved

desorption Hg Storage

sorption

IMPLND 

Hg Stored

in Bed

Sediment

Outflow of Hg in

Groundwater

Outflow of Hg in

Interflow

Hg Point

Sources 

Atmospheric

Deposition of

Hg

Runoff of

Sediment

Attached Hg 

Hg Outflow

Channel

Margin Hg

Inputs

Figure 22. Mercury routing in the South River numerical watershed model.

Washoff of

Sediment

Attached Hg

Table 20. Modules in Hydrological Simulation Program–

FORTRAN used to simulate mercury transport. 

[Hg, mercury; PERLND, pervious land area; IMPLND, impervious land area; 

RCHRES, river reach]

Mercury source/Process HSPF module(s) 

Groundwater PERLND>PQUAL>QUALGW

Interflow PERLND>PQUAL>QUALIF

Sediment Hg in runoff PERLND>PQUAL>QUALSD and 

IMPLND>IQUAL>WASHSD

Precipitation Hg on river RCHRES>CONS

Hg point sources EXT SOURCES

Instream sorption/

desorption

RCHRES>GQUAL>ADSDES

Downstream advection RCHRES>GQUAL>ADVECT 

(Dissolved Hg)

RCHRES>GQUAL>ADVQAL 

(Sediment associated Hg)

Channel margin inputs PERLND>PWAT and MASS-LINK
34 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Mercury Sources to the River

Mercury sources to the South River that were known as 

of April 1, 2007, were included in the watershed model. The 

sources are listed in table 21 and discussed separately below. 

For mercury sources that are relatively well described by 

observation data, model concentrations were either held constant or were minimally adjusted during the calibration process. Relatively well-described sources include point sources 

and direct precipitation to the river. Other mercury sources that 

are less well described by data include groundwater, interflow, 

concentrations on runoff sediment, and channel margin inputs, 

all of which had greater adjustments during the model calibration process.

Atmospheric Deposition

It is assumed that precipitation falling directly on the 

river has a dissolved mercury (THGF) concentration equal to 

21.8 ng/L, which is the average HG concentration in precipitation discussed earlier in this report. The model indirectly 

accounts for atmospheric mercury deposited on land surfaces 

by assigning mercury concentrations to hydrologic and sediment loads leaving the land surface. This approach allows 

more accurate mass balancing of mercury, and indirectly 

accounts for the processes of mercury cycling through soils, 

vegetation, animals, and atmospheric evasion.

Land-Surface Runoff

All land surfaces in the model have runoff sediment with 

associated mercury. Runoff sediment THGSed concentrations 

varied by reach and HRU. Sediment from uncontaminated 

areas was initially assigned a THGSed value = 0.127 μg/g, the 

average mercury concentration on suspended sediment at 

the Waynesboro monitoring station. During the calibration 

process, this value was lowered to 0.07 μg/g. Land surfaces 

known to be contaminated with mercury (such as 62-year 

flood-plain areas downstream from the plant site) were 

assigned higher runoff sediment THGSed mercury concentrations, between 7.6 and 16.7 μg/g, to correspond with the 

observational data shown in table 13. These THGSed concentrations for runoff sediment from contaminated areas were not 

changed during the calibration process, but loading coefficients controlling the amount of runoff sediment reaching the 

river were adjusted.

Model river reach 1 receives no runoff from contaminated flood-plain land areas. Model river reaches 2–5 are all 

at least partially downstream from the plant site, and receive 

sediment from both contaminated flood plains (determined 

from the 62-yr flood plain) and uncontaminated land surfaces. 

The watershed model accounts for the acreage of each HRU 

contributing to each model river reach.

Table 21. Mercury sources to the South River in the watershed model. 

[Hg, mercury; THG, total mercury; THGF, aqueous filterable total mercury; THGSed, total mercury on soils or surface sediment; USEPA, U.S. Environmental Protection Agency; VPDES, Virginia Pollutant Discharge Elimination System; ng/L, nanograms per liter; μg/g micrograms per gram; HRU, hydrologic 

response unit]

Hg source to South River Data used to determine 

initial concentrations Model input

Atmospheric deposition on river surface USEPA (U.S. Environmental Protection 

Agency, 2007)

Precipitation Hg concentration = 21.8 ng/L

Groundwater from uncontaminated 

land areas

THGF concentrations at Waynesboro gage 

(01626000)

Groundwater dissolved Hg concentration = 0.7 ng/L

Groundwater from Hg contaminated 

flood plain

Flood-plain groundwater samples, 

plus calibration

Groundwater dissolved Hg concentration = 

1.3–2.9 ng/L 

Interflow Precipitation THGF (U.S. Environmental 

Protection Agency, 2007) and calibration

Calibrated values from 10.0 to 16.7 ng/L

Sediment attached Hg runoff from 

uncontaminated pervious and impervious land surfaces

Sediment samples from uncontaminated areas THGSed concentration = 0.07 μg/g for all uncontaminated HRUs

Sediment attached Hg runoff from contaminated pervious land surfacesSediment samples within respective reaches THGSed concentration varies by reach and HRU 

(from 7.6 to 16.7 μg/g)

Point-source discharges VPDES flow data, grab sample analyses 

for minor sources, routine base-flow and 

stormflow monitoring of former DuPont 

plant site

Point-source flow rates and concentrations to river 

(model river reaches 1, 2, 3, and 5)

Channel margin inputs THG concentrations at Waynesboro 

(01626000), Dooms (01626920), and 

Harriston (01627500)

Input of sediment attached Hg to water column of 

each model river reach, using MASS-LINK block
Watershed Model Development 35

Groundwater and Interflow

Pervious land areas contribute mercury to river reaches 

through groundwater and interflow discharge (AGWO and 

IFWO). All mercury in AGWO and IFWO is assumed to be 

in the dissolved phase (THGF). Impervious land areas have no 

groundwater or interflow discharge. 

Groundwater from all uncontaminated pervious land 

areas was assumed to have the same THGF concentration 

value. This was initially assigned to be 0.49 ng/L, the average 

base-flow THGF concentration at the Waynesboro monitoring 

station above the plant site. In the final calibrated model, this 

value was adjusted to 0.7 ng/L. Groundwater from contaminated pervious land areas was assigned THGF

 concentrations 

of 2.9 ng/L for model river reaches 2, 3, and 4, and 1.3 ng/L 

for model river reach 5. Interflow THGF concentrations 

were assigned initial values between those of precipitation, 

21.8 ng/L, and AGWO THGF concentrations. During calibration these were slightly adjusted, and final IFWO THGF

concentrations ranged from 10.0 to 16.9 ng/L.

Point-Source Discharges

All point-source discharges in the model were assigned 

dissolved mercury concentrations. The actual point sources in 

most cases do carry sediment-associated mercury, but limited 

monitoring data did not permit distinguishing between dissolved and sorbed phases for most of the point sources. Mercury point-source inputs are partitioned by the model between 

dissolved and sorbed phases within a single time step once 

they enter the river. Mercury loads from point sources and the 

data used to assign flow rates and concentrations are listed in 

table 19 and Appendix 1.

There were extensive monitoring data describing discharges from the former DuPont plant site and the other 

discharge facilities (DuPont, 2003a, 2006a, b; Brenda Kennell, 

Invista, written commun., 2007). These data, which were collected at irregular intervals, were disaggregated and extrapolated to produce daily discharge values over the full simulation 

period. These data and the statistical treatments applied are 

summarized in Appendix 1. 

Riverbank and Channel Margin Inputs

During model calibration, it was observed that the model 

could not reproduce high observed THG concentrations during 

low and moderate flow periods, when little or no runoff was 

entering the stream reaches. An additional mercury load of 

roughly 100–200 kg/yr to the river was needed to calibrate the 

model. Model results discussed later in this report indicate that 

groundwater and interflow discharge to the river could not provide 100–200 kg/yr of mercury to the river without assigning 

them unreasonably high concentrations of mercury. The additional mercury entering the South River is most likely coming from contaminated channel margin sediment deposits. A 

variety of possible mechanisms could move mercury in these 

contaminated channel margin sediments to the river: bank 

erosion, bank collapse, disturbance of sediment by animals or 

fishermen or boaters, diffusion of mercury from contaminated 

sediment in contact with the water column, tree falls, ecological extraction of mercury from channel sediment, sediment 

displacement by interflow and groundwater discharge, hyporheic flow, desorption due to changed pH or oxidative state, 

another unknown mechanism, or some combination of these, 

all of which could operate along the length of the river. 

Bank retreat could account for much of the missing 

mercury load. Rhoades and others (2009) found that, on average, 109 kg/yr of mercury enters the South River from bank 

retreat, based on long-term erosion profiling and sampling of 

bank sediment. Bank collapse may work in concert with other 

mechanisms to produce mercury load to the river. Collapsed 

riverbank sediment could release mercury to hyporheic flow 

or groundwater discharge passing through it, for example. 

Sampling of pore water in surface sediment adjacent to the 

river during this study found elevated THGF concentrations 

at several locations downstream from the plant site (table 11). 

Evidence of mercury input to the river from banks was also 

seen by Turner and Jensen (2007), who found river reaches 

where water column mercury concentrations were higher near 

the bank than in the center of the river, implying an active 

source of dissolved mercury close to the banks. 

The exact mechanism responsible for the additional mercury source along the South River is not known. Therefore, 

a relatively simple approach was taken in the model. MASSLINK tables were added to create Hg-contaminated sediment 

inputs to the river reaches. Inputs were scaled with groundwater discharge (AGWO) and interflow (IFWO) from HGcontaminated pervious land areas. Groundwater and interflow 

rates provide signals of hydrologic conditions that are responsible for at least some of the wetting and hydraulic stress 

factors controlling bank collapse (Knighton, 1998). Loading 

coefficients were adjusted during the calibration process so 

that simulated THG concentrations in the river at the Dooms 

and Harriston monitoring stations matched observations. 

Channel margin inputs are treated separately from dissolved 

Hg groundwater and interflow inputs and separately from 

sediment-associated Hg in pervious land area and impervious 

land area runoff.

Mercury Transport Within the River

In the simulations, mercury in the water column moves 

downstream both in the dissolved phase and sorbed to suspended sediment. When suspended sediment with sorbed 

mercury settles out of the water column onto the channel bed, 

the sorbed mercury remains with the sediment until it is resuspended. When sediment exits a reach, the associated mercury 

also exits. 

HSPF partitions mercury in the water column by transferring it between dissolved and sorbed phases so that dissolved 

and sorbed concentrations approach an equilibrium ratio. 

The phase transfer is limited by a user-specified rate coefficient so that equilibrium partitioning is not instantaneous. A 
36 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

finite difference expression of the mercury continuity equation (equation 1) is solved for each reach at each time step to 

calculate the mass transfer between phases. The phase transfer 

is calculated separately for each size fraction (sand, silt, clay) 

of the suspended sediment. No transfer is simulated between 

sediment of different size classes, between bed sediment and 

groundwater, or between bed sediment and the water column. 

Decay or production of mercury from other constituents are 

likely negligible, and therefore they were not simulated. 

 –d(RSED*SQAL)/dt + RSED*KT*(KD*DQAL–SQAL) = 0 (1)

where:

RSED = quantity of sediment in the model river 

reach (mass); 

SQAL = concentration of constituent on sediment 

(mass Hg/mass sediment);

DQAL = concentration of dissolved constituent 

(mass Hg/volume water); 

KD = distribution coefficient; and

KT = rate transfer coefficient. 

A single distribution coefficient of 1,000,000 L/kg (liters 

per kilogram) is used to partition mercury between the aqueous phase and sorbed phase on suspended silt and clay. This 

value is based on ratios of THGSed and THGF concentrations 

listed in tables 7 and 8 and batch tests results from Mason 

(2006). HSPF also requires a distribution coefficient for mercury sorption to sand, and a lower coefficient of 1,000 L/kg 

is assumed. To ensure nearly instantaneous transfer of mercury between phases, a high rate transfer coefficient of 25.0 

was used for partitioning between all suspended sediment 

size fractions and the water column. Mercury partitioning 

between channel bed sediment and the water column was 

slowed to almost zero by assigning a low rate transfer coefficient (0.0001) for all sediment-size fractions. This was done 

because in-situ partitioning of mercury between channel 

bed sediment and pore water is not well understood for the 

South River. 

Watershed Model Results

After compiling input data, the numerical watershed 

model was tested and calibrated. Model calibration, or the 

adjustment of model parameter values to achieve better agreement between observed and simulated values, was performed 

sequentially for streamflow, suspended sediment transport, 

and mercury transport. The streamflow and sediment transport 

calibration covered the period from water year 1991 through 

water year 2000 (October 1, 1990, through September 30, 

2000). For mercury, the model calibration covered the period 

from April 2005 through March 2006, which corresponds with 

the period of intense mercury data collection. Model verification, in which results from the calibrated model are compared 

to observations for a separate period with the same model 

fit targets as used for calibration, was also performed. For 

streamflow and sediment transport, verification covered the 

period from water years 2001 through 2005, whereas for mercury, verification covered the period from April 2006 through 

March 2007. 

As described in a previous section, Representation of 

the Watershed, all hydrologic and sediment model parameters 

were initially assigned values from the calibrated CBM5 

model and were then adjusted to achieve a closer fit between 

simulated and observed values. Most of these changes 

were relatively minor because the CBM5 model parameters 

were previously calibrated. The CBM5 model does not 

simulate mercury; therefore, mercury transport parameters 

were assigned according to observations.

 Improvements to the South River watershed model 

hydrology and sediment parameters were made by using 

additional data that were not available during the CBM5 

calibration effort. CBM5 parameter values were calibrated 

using observed streamflow both outside and within the South 

River watershed. In the CBM5 model, for example, evaporation coefficients for land-county segment A51015 were 

optimized using streamflow observations from the South River 

streamflow-gaging stations (0162600, 01626850, 01627500) 

as well as stations on the Middle River (01624800, 01625000). 

Because this study focuses on the South River, parameter 

values were changed to obtain a better fit for only the South 

River observed streamflow values. Additional justification for 

modifications to the CBM5 parameters include the division 

of CBM5 RCHRES PS2_6490_6360 into reaches 3 and 4 in 

this model, and the availability of more recent streamflow and 

suspended sediment concentration data.

Streamflow Model Calibration Results

The ability of the model to accurately simulate streamflow was evaluated by statistically comparing simulated and 

observed streamflow with respect to annual and seasonal water 

budgets, high-flow and low-flow distribution, and stormflow volumes. These comparisons were performed primarily 

using Expert System for the Calibration of the Hydrological Simulation Program–FORTRAN (HSPEXP) (Lumb and 

others, 1994). 

The hydraulic component of the South River watershed 

model simulates the period January 1, 1985, through March 31, 

2007, using hourly time steps. The 10-year calibration period 

includes the wettest year (1998) and the fourth driest year (1999) 

on record for the Harriston streamflow-gaging station. Observed 

mean annual flows at the Harriston streamflow-gaging station 

for years with complete data are shown in figure 23. A 2-year 

verification period from April 1, 2005, through March 31, 2007, 

was used to verify that the calibrated hydrology model can 

accurately simulate other time periods.

Hydraulic parameter values were adjusted during calibration to match observed and simulated water volumes. Changes 

to initial CBM5 parameter values were made to reduce the 
Watershed Model Results 37

amount of runoff (table 22). Changes were made to the actual 

evapotranspiration coefficients that scale the External Source 

EVAP time series to improve simulated runoff volumes and to 

parameters INFILT and AGWETP to improve the distribution 

of runoff volumes between high- and low-flow periods. Final 

calibrated values for these parameters are listed in Appendix 2. 

The actual evapotranspiration multiplier coefficients used 

to scale evaporation input time series were increased by an 

average of 16 percent to better match observed total runoff. 

Parameter values for INFILT, which controls infiltration 

capacity, were reduced uniformly by 50 percent to reduce runoff in the 40–60 percent streamflow duration range. Values for 

AGWETP, which control how much actual evapotranspiration 

can come from base flow, were set to zero to increase runoff 

during periods of low flow.

Two streamflow-gaging stations were used for calibrating streamflow, South River near Waynesboro (01626000) 

and South River at Harriston (01627500). These two 

streamflow-gaging stations are the only ones on the South 

River with complete daily streamflow data for the calibration period. Station 01626000 is the most upstream stream 

reach node in the model, whereas 01627500 is the penultimate 

downstream node. 

Simulated streamflow exhibits annual, seasonal, and daily 

patterns similar to those in observed streamflow (figs. 24–31). 

Calibrated model simulation results compared to calibration 

goals are shown in tables 23 and 24 (Bicknell and others, 

2001). There is a good overall mass balance for the simulation 

period, and the distribution of high and low simulated flow 

matches well with observed values. The frequency distribution 

of simulated streamflow values matches the observed distribution, as shown by the flow duration curves in figures 32 

and 33. High streamflows caused by storms, which occur 

infrequently but account for the majority of total discharge, 

show a close match between simulated and observed values. 

The lowest 20 percent of daily streamflows, from 80 percent to 

100 percent exceedance in figures 32 and 33, are less accurately simulated; however, at Harriston, they account for just 

4.9 percent of total discharge volume. 

Observed daily-mean and simulated daily flows do not 

match in all cases, as can be seen in the 1:1 plots of observed 

to simulated streamflow, (figs. 34–35). The R2 value for figure 34 is 0.39, and 0.41 for figure 35. The mismatch between 

daily values is, in many cases, caused by an offset in timing 

of a day or two between simulated and observed stormflows. 

The primary reason for differences between simulated and 

observed stormflows in most cases is probably differences 

between modeled and actual precipitation. As described in an 

earlier section, Meteorological and Climatic Data, the hourly 

precipitation data used in the South River watershed model 

were taken from the CBM5 model and were derived by spatial 

averaging of records from nearby meteorological monitoring 

stations, which for water years 1991 through 2000 were all 

outside the watershed. Although there are no hourly precipitation records from stations within the watershed during the 

calibration period, differences between actual precipitation 

Figure 23. Observed mean annual flow, South River at Harriston, Virginia (USGS station number 01627500), 

water years 1926–2007.

0

100

200

300

400

500

600

1926 1936 1946 1956 1966 1976 1986 1996 2006

WATER YEAR

MEAN ANNUAL FLOW, IN CUBIC FEET PER SECOND

hydrology and sediment

model calibration period

Figure 23. Observed mean annual flow, South River at Harriston (USGS station number 01627500), Virginia, water years 1926-2007.
38 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Table 22. Primary model transport parameters changed from Chesapeake Bay 

Model Phase 5 values during calibration of the South River watershed model. 

[HSPF, Hydrological Simulation Program–FORTRAN; ET, actual evapotranspiration; 

PERLND, pervious land area; IMPLND, impervious land area; RCHRES, model river reach]

HSPF 

module Parameter name Description

Hydrologic

PERLND INFILT Controls infiltration capacity

ET coefficients Scale evaporation rate input time series

AGWETP Controls ET rate from base flow

Sediment transport

IMPLND Loading factors Control fraction of sediment runoff from 

PERLND to RCHRES

KEIM Controls solids wash-off rate

PERLND Loading factors Control fraction of sediment runoff from 

PERLND to RCHRES

NVSI Represents external application of detached soil

KRER Controls rate of soil detachment

KSER Controls rate of transport of detached soil

RCHRES W Particle settling velocity

TAUCD Critical bed shear stress for deposition

TAUCS Critical bed shear stress for scour

M Erodibility of bed sediment

Table 23. Simulation results for the calibration period water years 1991 through 2000, calibrated model, 

South River, Virginia. 

[in., inches; %, percent]

Runoff

category

Criterion

(percent)

South River near Waynesboro

01626000

South River at Harriston

01627500

Observed 

(in.)

Simulated 

(in.) % Error Observed (in.)

Simulated 

(in.) % Error

Total annual runoff ±10 186.9 182.2 –2.5 185.1 194.6 5.2

Highest 10-percent flow ±10 84.7 86.5 2.0 81.5 87.8 7.7

Lowest 50-percent flow ±15 28.0 25.6 –8.4 31.5 31.4 –0.2

Winter runoff ±15 64.3 61.6 –4.3 63.0 64.7 2.6

Spring runoff ±15 65.9 63.4 –3.9 63.6 66.5 4.5

Summer runoff ±15 26.1 26.7 2.2 26.2 29.7 13.4

Fall runoff ±15 30.8 30.5 –1.1 32.2 33.7 4.7

Total storm volume ±20 19.6 17.1 –13.0 19.0 17.4 –8.5
Watershed Model Results 39

Table 24. Simulation results for the verification period April 1, 2005, through March 31, 2007, calibrated model, 

South River, Virginia. 

[in., inches; %, percent]

Runoff

category

Criterion

(percent)

South River near Waynesboro

01626000

South River at Harriston

01627500

Observed 

(in.)

Simulated 

(in.) % Error Observed (in.)

Simulated 

(in.) % Error

Total annual runoff ±10 35.5 32.5 –8.6 35.3 34.9 –1.0

Highest 10-percent flow ±10 14.0 13.2 –5.5 13.8 13.4 –2.8

Lowest 50-percent flow ±15 6.4 6.1 –4.4 7.0 7.3 3.5

Winter runoff ±15 11.4 9.8 –13.5 10.8 10.5 –3.3

Spring runoff ±15 7.8 7.2 –7.8 7.6 7.9 3.2

Summer runoff ±15 3.8 5.7 51.9 4.2 6.3 50.5

Fall runoff ±15 12.7 9.8 –23.1 12.7 10.3 –18.7

Total storm volume ±20 9.8 9.6 –2.0 9.9 9.7 –1.7

Figure 24. Simulated and observed daily streamflow for the calibration period water years 1991–2000, calibrated 

model, South River near Waynesboro, Virginia (USGS station number 01626000).

10

100

1,000

10,000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

STREAMFLOW, IN CUBIC FEET PER SECOND

Observed

Simulated

WATER YEAR

Figure 24. Simulated and observed daily streamflow for the calibration period water years 1991-2000, calibrated model, South River

near Waynesboro (USGS station number 01626000), Virginia.
40 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

10

100

1,000

10,000

MAR

2005

JUNE

2005

SEPT

2005

DEC

2005

MAR

2006

JUNE

2006

SEPT

2006

DEC

2006

MAR

2007

STREAMFLOW, IN CUBIC FEET PER SECOND

Observed

Simulated

DATE

Figure 25. Simulated and observed daily streamflow during the verification period April 1, 2005, through March 31, 2007,

calibrated model, South River near Waynesboro (USGS station number. 01626000), Virginia.

Figure 25. Simulated and observed daily streamflow during the verification period April 1, 2005, through 

March 31, 2007, calibrated model, South River near Waynesboro, Virginia (USGS station number 01626000).

Figure 26. Simulated and observed daily streamflow during the calibration period water years 1991–2000, 

calibrated model, South River at Harriston, Virginia (USGS station number 01627500).

10

100

1,000

10,000

OCT

1990

OCT

1991

OCT

1992

OCT

1993

OCT

1994

OCT

1995

OCT

1996

OCT

1997

OCT

1998

OCT

1999

OCT

2000

STREAMFLOW, IN CUBIC FEET PER SECOND

Observed

Simulated

DATE

Figure 26. Simulated and observed daily streamflow during the calibration period water years 1991-2000, calibrated model,

South River at Harriston (USGS station number 01627500), Virginia.
Watershed Model Results 41

10

100

1,000

10,000

MAR

2005

JUNE

2005

SEPT

2005

DEC

2005

MAR

2006

JUNE

2006

SEPT

2006

DEC

2006

MAR

2007

STREAMFLOW, IN CUBIC FEET PER SECOND

Observed

Simulated

DATE

Figure 27. Simulated and observed daily streamflow for the verification period April 1, 2005, through March 31, 2007, calibrated model,

South River at Harriston (USGS station number 01627500), Virginia.

Figure 27. Simulated and observed daily streamflow for the verification period April 1, 2005, through 

March 31, 2007, calibrated model, South River at Harriston, Virginia (USGS station number 01627500).

Figure 28. Simulated and observed annual runoff during the calibration period water years 1991–2000, calibrated 

model, South River near Waynesboro, Virginia (USGS station number 01626000).

5

0

10

15

20

25

30

35

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

STREAM RUNOFF, IN INCHES

Observed 

Simulated 

YEAR

Figure 28. Simulated and observed annual runoff during the calibration period water years 1991-2000, calibrated model,

South River near Waynesboro (USGS station number 01626000), Virginia.
42 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 29. Simulated and observed total runoff during the verification period April 1, 2005, 

through March 31, 2007, calibrated model, South River near Waynesboro, Virginia (USGS 

station number 01626000).

Observed 

Simulated 

2

0

4

6

8

10

12

14

16

18

20

2005 2006 2007

ANNUAL STREAM RUNOFF, IN INCHES

DATE

Figure 29. Simulated and observed total runoff during the verification period April 1, 2005, through

March 31, 2007, calibrated model, South River near Waynesboro (USGS station number 01626000),

Virginia.

Figure 30. Simulated and observed annual runoff during the calibration period water years 1991–2000, calibrated, 

South River at Harriston, Virginia (USGS station number 01627500).

STREAM RUNOFF, IN INCHES

Observed 

Simulated 

DATE

5

0

10

15

20

25

30

35

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Figure 30. Simulated and observed annual runoff during the calibration period water years 1991-2000, calibrated, South River at

Harriston (USGS station number 01627500), Virginia.
Watershed Model Results 43

Observed 

Simulated 

2

0

4

6

8

10

12

14

16

18

20

2005 2006 2007

ANNUAL STREAM RUNOFF, IN INCHES

DATE

Figure 31. Simulated and observed total runoff during the verification period April 1, 2005, through

March 31, 2007, calibrated model, South River at Harriston (USGS station number 01627500),

Virginia.

Figure 31. Simulated and observed total runoff during the verification period April 1, 2005, 

through March 31, 2007, calibrated model, South River at Harriston, Virginia (USGS station 

number 01627500).

10

100

1,000

10,000

0 20 40 60 80 100

PERCENT TIME EXCEEDED

STREAMFLOW, IN CUBIC FEET PER SECOND

Observed–Harriston

Simulated–Harriston

Observed–Waynesboro

Simulated–Waynesboro

Figure 32. Flow duration curves for observed and simulated streamflow, calibration period water years 1991-2000, calibrated model,

for the South River near Waynesboro (USGS station number 01626000) and at Harriston (USGS station number 01627500), Virginia.

Figure 32. Flow duration curves for observed and simulated streamflow, calibration period water 

years 1991–2000, calibrated model, for the South River near Waynesboro, Virginia (USGS station 

number 01626000) and at Harriston, Virginia (USGS station number 01627500).
44 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 33. Flow duration 

curves for observed and 

simulated daily streamflow, 

calibrated model, South 

River near Waynesboro , 

Virginia (USGS station number 

01626000) and at Harriston, 

Virginia (USGS station 

number 01627500), verification 

period April 1, 2005, through 

March 31, 2007.

Figure 34. 1:1 comparison 

of simulated and observed 

daily streamflow, calibrated 

model, South River near 

Waynesboro, Virginia (USGS 

station number 01626000), 

water years 1991–2000 

and April 2005 through 

March 2007.

10

100

1,000

10,000

0 20 40 60 80 100

PERCENT TIME EXCEEDED

STREAMFLOW, IN CUBIC FEET PER SECOND

Observed–Harriston

Simulated–Harriston

Observed–Waynesboro

Simulated–Waynesboro

Figure 33. Flow duration curves for observed and simulated daily streamflow, calibrated model, South River near Waynesboro

(USGS station number 01626000) and at Harriston (USGS station number 01627500), Virginia, verification period April 1, 2005,

through March 31, 2007.

1

10

100

1,000

10,000

1 10 100 1,000 10,000

OBSERVED STREAMFLOW, IN CUBIC FEET PER SECOND

SIMULATED STREAMFLOW, IN CUBIC FEET PER SECOND

Calibration period 

Verification period

1:1 line

Figure 34. 1:1 comparison of simulated and observed daily streamflow, calibrated model, South River near Waynesboro

(USGS station number 01626000), Virginia, water years 1991-2000 and April 2005 through March 2007.
Watershed Model Results 45

on the basin and the precipitation time series in the model can 

be inferred from the fact that some observed storms had total 

storm discharge that exceeded the model precipitation volumes for the same period. Similarly, some simulated storms 

have discharge volumes that exceeded observed precipitation 

volumes for the same period based on daily precipitation data 

collected within the watershed.

Sediment Transport Model Calibration Results 

The sediment transport component of the South River 

watershed model was calibrated using a “weight-of-evidence” 

approach (Donigian and Love, 2003), in which multiple 

numeric and qualitative calibration goals were assessed. 

Comparisons were made between simulated and observed 

suspended sediment concentration, suspended sediment loads, 

and depth of bed sediment. Comparisons were made using the 

same calibration period (water year 1991 through 2000), the 

same verification period (April 1, 2005, through March 31, 

2007), and the same two calibration sites (Waynesboro 

01626000 and Harriston 01627500) as were used for the calibration of streamflow. Only USGS suspended sediment concentration values were used for quantitative calibration of the 

sediment transport model. Suspended sediment values from 

the VDEQ (reported as total suspended solids as discussed 

earlier) were used only for qualitative and supplemental 

checks during the sediment model calibration.

Time series of regressed daily suspended sediment concentration values were used as the primary “observed” values 

during calibration because the regressed time series have 

values for every time step and therefore, their statistics are 

unbiased towards either stormflow or base flow. The regressed 

suspended sediment concentration time-series data were 

derived from USGS grab samples and multiple linear regression, as described earlier. So that a consistent regression model 

was used throughout the entire simulation period, regressed 

suspended sediment concentration values were used from estimation model number 7 in table 15, which derived suspended 

sediment concentration from normalized streamflow and 

streamflow increase. Observed suspended sediment loads were 

calculated by multiplying observed daily flow values with 

regressed suspended sediment concentration values, whereas 

simulated loads were calculated by HSPF using its mass balance formulas. 

The South River watershed model has coarse spatial discretization relative to channel morphology and sediment distribution patterns. Riffle spacing in the South River is typically 

less than 0.2 mi, for example (Pizzuto and others, 2006, p. 24), 

whereas river reaches in the model are about 8.0 mi long. Due 

to the coarse discretization and the limited sediment transport 

algorithms available in HSPF, the accurate simulation of any 

single observed suspended sediment concentration value was 

Calibration period 

Verification period

1:1 line

1

10

100

1,000

10,000

1 10 100 1,000 10,000

OBSERVED STREAMFLOW, IN CUBIC FEET PER SECOND

SIMULATED STREAMFLOW, IN CUBIC FEET PER SECOND

Figure 35. 1:1 comparison of simulated and observed daily streamflow, calibrated model, South River at Harriston (USGS station

number 01627500), Virginia, water years 1991-2000 and April 2005 through March 2007.

Figure 35. 1:1 comparison 

of simulated and observed 

daily streamflow, calibrated 

model, South River at 

Harriston, Virginia (USGS 

station number 01627500), 

water years 1991–2000 

and April 2005 through 

March 2007.
46 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

not the primary focus of the calibration. Rather, the goal was 

to match simulated and observed suspended sediment concentration statistics and suspended sediment loads. 

The multiple calibration goals were defined as follows:

1. Simulated suspended sediment concentration ranges 

match observed suspended sediment concentration ranges, 

and simulated 50-percent and 10-percent duration suspended sediment concentration values are within 30 percent of observed values.

2. Simulated and observed suspended sediment concentration duration plots show no major differences.

3. Total simulated suspended sediment loads are within 

10 percent of observed loads for the calibration period.

4. The total of the bottom 50 percent of daily simulated 

sediment loads and the top 10 percent of daily simulated 

sediment loads are within 30 percent of observed values.

5. Simulated annual sediment loads show patterns similar to 

observed annual loads.

6. Simulated depth of bed sediment does not show a longterm increasing trend. 

Calibration goal 6 is based on the results of a study by Pizzuto and others (2006), who found that very little fine-grained 

sediment is stored on the channel bed of the South River, an 

amount that is “3 orders of magnitude less than the annual suspended sediment load of the South River, and is volumetrically 

insignificant.” They found that average depth of sediment in the 

channel was less than 0.04 inch downstream from the plant site. 

On the basis of this observation, the model was calibrated so 

that depth of deposited sediment in each model river reach did 

not show long-term increasing trends. This goal was met in the 

calibrated model as shown in figure 36.

Selected sediment transport parameters for each impervious land area, pervious land area, and river reach were 

adjusted during the calibration process to obtain a better fit to 

observations (table 22). Changes to initial CBM5 parameter 

values were made to reduce the amount of sediment entering 

the river, to redistribute sediment loads across flow regimes, 

and to prevent sediment building up on the river bed. For 

impervious land areas, sediment transport coefficients KEIM 

were reduced by 50 percent. For pervious land areas, the coefficients NVSI, KRER, and KSER were reduced by 80 percent. 

For each impervious and pervious land area, coefficients used 

to adjust sediment loads reaching the river (sediment load 

factors) were reduced from CBM5 values (Chesapeake Bay 

0

0.05

0.10

0.15

0.20

0.25

MAY

1990

MAY

1992

MAY

1994

MAY

1996

MAY

1998

MAY

2000

MAY

2002

MAY

2004

MAY

2006

DEPTH OF CHANNEL BED SEDIMENT, IN INCHES

Waynesboro (USGS station number 01626000)

Harriston (USGS station number 01627500)

DATE

Figure 36. Simulated depth of sediment in model river reaches 1 and 4, calibrated South River model, water

years 1991-2000.

Figure 36. Simulated depth of sediment in model river reaches 1 and 4, calibrated South River 

model, water years 1991–2000.
Watershed Model Results 47

Program, 2002, Appendix F; Gary Shenk, Chesapeake Bay 

Program, written commun., 2007) to achieve better sediment mass balance. Sediment load factors are multiplied into 

overland runoff sediment loads before they enter the river to 

improve mass balances. The calibrated South River watershed model uses the same sediment load factor value for all 

pervious and impervious land areas contributing to a reach. 

The load factor for reach 1 is 15.6 percent, reduced from the 

equivalent CBM5 value of 21 percent. For reaches 2–5, the 

load factor is 21 percent, reduced from the equivalent CBM5 

value of 23 percent.

As can be seen in figures 37–44 and tables 25–26, the 

watershed model does a reasonable job of recreating observed 

grab samples, suspended sediment concentration ranges, and 

regressed concentrations and annual loads. The simulated 

annual sediment load at Harriston for the calibration period is 

22,400 tons, of which 98 percent comes from pervious land 

area surfaces, 2 percent from impervious land area surfaces, 

and 0.1 percent from point-source discharges. Total simulated 

loads during the calibration period have an error of +2.1 percent at Harriston and –3.0 percent at Waynesboro, when 

compared to loads calculated from the regressed suspended 

sediment concentration time series. Yearly loads for Waynesboro and Harriston are shown in figures 41–44.

Time series of simulated and observed (regressed) suspended sediment concentration values are shown in figures 37 

and 38 for the Waynesboro station and in figures 39 and 40 for 

the Harriston station. Simulated suspended sediment concentration values have an overall good match to observed values. 

All but one of the six numeric calibration goals was met 

(tables 25 and 26). The goal not met was the lowest 50 percent 

of suspended sediment concentration at the Waynesboro station, which had –46.5 percent error.

Simulated annual sediment loads (figs. 41–44) have 

the same temporal pattern of change as observed (regressed) 

annual loads, but show less range of variation. The difference 

between simulated and observed (regressed) loads in 1996 

is due primarily to a storm on January 19 that had a daily 

Table 25. Comparison of simulated and observed (regressed) daily sediment concentrations and loads, calibration period water years 

1991 through 2000, calibrated model, South River, Virginia. 

[mg/L, milligrams per liter; %, percent]

Criterion

(percent)

South River near Waynesboro

(01626000)

South River at Harriston 

(01627500)

Observed 

(Regressed) Simulated % Error Observed (Regressed) Simulated % Error

Suspended sediment 

concentration (mg/L)

90th percentile ±30 29.4 26.1 –11.4 30.3 33.2 9.5

50th percentile ±30 6.6 6.5 –1.2 6.8 7.5 9.7

Sediment load (tons) Total ±10 125,500 125,000 –0.2 231,700 224,300 –3.2

Top 10% ±30 113,600 116,100 2.2 212,600 204,500 –3.8

Lowest 50% ±30 1,000 1,200 15.7 2,100 2,600 26.6

Table 26. Comparison of simulated and observed (regressed) daily sediment concentrations and loads, verification period April 1, 2005, 

through March 31, 2007, calibrated model, South River, Virginia. 

[mg/L, milligrams per liter; %, percent]

Criterion

(percent )

South River near Waynesboro

(01626000)

South River at Harriston 

(01627500)

Observed 

(Regressed) Simulated % Error Observed (Regressed) Simulated % Error

Suspended sediment 

concentration (mg/L)

90th percentile ±30 23.7 23.9 0.8 24.5 25.7 4.8

50th percentile ±30 7.8 4.1 –46.5 7.6 5.5 –28.5

Sediment load (tons) Total ±10 19,100 18,100 –5.5 29,900 32,800 9.8

Top 10% ±30 16,800 16.600 –1.1 26,300 29,800 13.4

Lowest 50% ±30 300 200 –16.9 500 500 –13.5
48 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 37. Simulated 

daily, observed 

(regressed), 

and sampled 

suspended sediment 

concentrations 

for the calibration 

period, water 

years 1991–2000, 

South River near 

Waynesboro, Virginia 

(USGS station number 

1 01626000).

10

100

1,000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

Observed (regressed)

Virginia Department of Environmental

Quality observed

Simulated

YEAR

Figure 37. Simulated daily, observed (regressed), and sampled suspended sediment concentrations for the calibration period,

water years 1991-2000, South River near Waynesboro (USGS station number 01626000), Virginia.

Figure 38. Simulated 

daily, observed 

(regressed), 

and sampled 

suspended sediment 

concentrations for 

the verification period 

April 1, 2005, through 

March 31, 2007, 

calibrated model, 

South River near 

Waynesboro, Virginia 

(USGS station number 

01626000). 0

1

10

100

1,000

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER 

Observed (regressed)

Virginia Department of Environmental

Quality sampled

U.S. Geological Survey sampled

Simulated

DATE

Figure 38. Simulated daily, observed (regressed), and sampled suspended sediment concentrations for the verification period

April 1, 2005, through March 31, 2007, calibrated model, South River near Waynesboro (USGS station number 01626000), Virginia.
Watershed Model Results 49

Figure 39. Simulated 

and observed 

(regressed) 

suspended sediment 

concentrations 

for the calibration 

period water years 

1991–2000, calibrated 

model, South River 

at Harriston, Virginia 

(USGS station number 

01627500).

1

10

100

1,000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

YEAR

Observed (regressed)

Virginia Department of Environmental Quality observed

Simulated

Figure 39. Simulated and observed (regressed) suspended sediment concentrations for the calibration period water years 1991-2000,

calibrated model, South River at Harriston (USGS station number 01627500), Virginia.

Figure 40. Simulated 

and observed 

(regressed) 

suspended sediment 

concentrations 

for the verification 

period April 1, 2005, 

through March 31, 

2007, calibrated 

model, South River 

at Harriston, Virginia 

(USGS station number 

01627500).

Observed (regressed)

Virginia Department of Environmental

Quality sampled

U.S. Geological Survey sampled

Simulated

0

1

10

100

1,000

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER 

DATE

Figure 40. Simulated and observed (regressed) suspended sediment concentrations for the verification period April 1, 2005,

through March 31, 2007, calibrated model, South River at Harriston (USGS station number 01627500), Virginia.
50 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 41. Annual simulated and observed (regressed) suspended sediment loads for the calibration period 

water years 1991–2000, calibrated model, South River near Waynesboro, Virginia (USGS station number 01626000).

0

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Observed (regressed)

Simulated 

DATE

20,000

40,000

60,000

ANNUAL SUSPENDED SEDIMENT LOAD, IN TONS

Figure 41. Annual simulated and observed (regressed) suspended sediment loads for the calibration period water years 1991-2000,

calibrated model, South River near Waynesboro (USGS station number 01626000), Virginia.

Figure 42. Annual simulated and observed (regressed) suspended sediment loads for 

the verification period April 1, 2005, through March 31, 2007, calibrated model, South 

River near Waynesboro, Virginia (USGS station number 01626000).

5,000

0

10,000

15,000

20,000

25,000

ANNUAL SUSPENDED SEDIMENT LOAD, IN TONS

Observed (regressed)

Simulated 

2005 2006 2007

DATE

Figure 42. Annual simulated and observed (regressed) suspended sediment loads for the verification

period April 1, 2005, through March 31, 2007, calibrated model, South River near Waynesboro (USGS

station number 01626000), Virginia.
Watershed Model Results 51

Figure 43. Annual simulated and observed (regressed) suspended sediment loads for the calibration period, 

water years 1991–2000, calibrated model, South River at Harriston, Virginia (USGS station number 01627500).

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Observed (regressed)

Simulated 

DATE

20,000

40,000

60,000

80,000

100,000

120,000

ANNUAL SUSPENDED SEDIMENT LOAD, IN TONS

0

Figure 43. Annual simulated and observed (regressed) suspended sediment loads for the calibration period, water years 1991-2000,

calibrated model, South River at Harriston (USGS station number 01627500), Virginia.

Figure 44. Annual simulated and observed (regressed) suspended sediment loads for 

the verification period, April 1, 2005, through March 31, 2007, calibrated model, South 

River at Harriston, Virginia (USGS station number 01627500).

5,000

0

10,000

15,000

20,000

25,000

ANNUAL SUSPENDED SEDIMENT LOAD, IN TONS

Observed (regressed)

Simulated 

2005 2006 2007

DATE

Figure 44. Annual simulated and observed (regressed) suspended sediment loads for the verification

period, April 1, 2005, through March 31, 2007, calibrated model, South River at Harriston (USGS station

number 01627500), Virginia.
52 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

observed (regressed) load of 40,400 tons but a simulated load 

of only 1,600 tons (figs. 41 and 43). Streamflow also was 

undersimulated for this storm in both cases due to differences 

between modeled and actual precipitation. Similarly, the difference between simulated and observed (regressed) loads in 

1998 occurred primarily during two February storms in which 

streamflow, and therefore sediment load, were undersimulated. 

Sediment transport is dominated by infrequent high-flow 

events, and individual storms can have a large effect on annual 

sediment loads.

The mean observed (regressed) annual sediment load 

from 1991 through 2000 at monitoring station 01627500 is 

23,170 tons/yr (tons per year). This compares to an annual 

mean load calculated in the Pizzuto and others (2006) study of 

10,100 tons at the same location for the period 1967 through 

2003 using different estimation methods. Both the Pizzuto 

load estimate and the “observed” load estimate calculated 

for this study are subject to considerable uncertainty. The 

uncertainty in simulated loads results from uncertainty in 

the suspended sediment concentration regression, sampling 

errors propagating through the calibration, uncertainty in 

flow estimates, use of daily average rather than instantaneous 

flow values, and the inherent uncertainty of representing low 

frequency high load events based on periodic data and linear 

estimation methods. 

Sediment results for the calibrated model during the 

calibration and verification periods are shown in tables 25 and 

26. Simulated loads are comparable to those reported in other 

studies of Chesapeake Bay watersheds (Langland and others, 

2003; Phillips, 2007).

Duration plots for simulated and observed (regressed) 

suspended sediment concentrations are shown in figure 45 

for the calibration period and in figure 46 for the verification 

period. Duration plots are comparable to cumulative distribution functions and express the percent of time that a variable 

(suspended sediment concentration in this case) exceeds a 

given value. The simulated and observed (regressed) suspended sediment concentration duration plots are visually similar at both the Waynesboro and Harriston monitoring stations.

Mercury Transport Model Calibration Results

The mercury transport component of the South River 

watershed model was calibrated by adjusting parameters so 

that hourly simulated total mercury concentrations match 

observed concentrations in grab samples from the river. Unlike 

water and sediment, there are no continuous time series of 

observed mercury concentrations to use as a calibration goal. 

Calibration of mercury transport also differed from water 

Figure 45. Duration plots for simulated and observed (regressed) daily suspended sediment 

concentrations, calibration period water years 1991–2000, calibrated model, South River, Virginia.

1

10

100

1,000

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

Observed (regressed)–Harriston

Simulated–Harriston

Observed (regressed)–Waynesboro

Simulated–Waynesboro

0 20 40 60 80 100

PERCENT TIME EXCEEDED

Figure 45. Duration plots for simulated and observed (regressed) daily suspended sediment concentrations, calibration period

water years 1991-2000, calibrated model, South River, Virginia.
Watershed Model Results 53

and sediment in that it used a 1-year calibration period, from 

April 1, 2005, through March 31, 2006, and a 1-year verification period, from April 1, 2006, through March 31, 2007. 

Three monitoring sites were used for calibration—01626000 

(Waynesboro), 01626920 (Dooms), and 01627500 (Harriston). 

As with sediment calibration, USGS mercury concentration data were used for quantitative comparison to simulated 

results, and data from the VDEQ were used for additional 

visual checks.

Although the calibration process compared observed and 

simulated concentrations, these values have somewhat different characters. Hourly simulated concentrations represent 

average conditions in a river reach over a 1-hour period and 

do not capture variability occurring within a reach or at time 

intervals less than 1 hour. Observed THG concentration values 

represent instantaneous vertically averaged conditions at the 

centroid of flow at a single point along the river. In the actual 

river, there likely is substantial variation of THG concentrations within a reach and within an hour time span, therefore 

simulated concentrations are not expected to match observed 

concentrations exactly. Instead, the goal was to recreate 

observed THG statistics and spatial and temporal patterns.

Initial calibration efforts indicated that downstream from 

the plant site, the model had insufficient sources of mercury 

to the river during stormflow recession periods and during 

other times of declining streamflow when there was no surface 

runoff. This was true despite the fact that hydrology and sediment transport were well calibrated, incorporating all explicitly known mercury sources, and using assigned model input 

mercury concentrations that matched observed values. Results 

from an example simulation with sediment runoff mercury 

adjusted to match storm peak values are shown in figure 47. 

When mercury on runoff sediment was assigned unrealistically 

high concentrations, simulated THG values could be raised to 

match observed values at the low end, but then simulated high 

end values were much too high.

To calibrate the mercury model, an additional mercury 

source was added to the model to mimic channel margin inputs 

of mercury to the river, as discussed in the earlier section of 

this report describing the mercury transport model. Channel 

margin mercury loads are linked to groundwater discharge 

(AGWO) and interflow (IFWO), and therefore occur during 

low and moderate flow and during periods of no surface runoff. The coefficients multiplied into IFWO and AGWO time 

series were adjusted during calibration to match simulated to 

observed THG concentrations. No channel margin inputs were 

used for model river reach 1 because they were not needed to 

achieve good calibration, and because model river reach 1 is 

upstream from the historic mercury source.

Figure 46. Duration plots for simulated and observed (regressed) daily suspended sediment concentration 

for the verification period April 1, 2005, through March 31, 2007, calibrated model, South River, Virginia.

Observed (regressed)–Harriston

Simulated–Harriston

Observed (regressed)–Waynesboro

Simulated–Waynesboro

1

10

100

1,000

SUSPENDED SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

0 20 40 60 80 100

PERCENT TIME EXCEEDED

Figure 46. Duration plots for simulated and observed (regressed) daily suspended sediment concentration for the verification period

April 1, 2005, through March 31, 2007, calibrated model, South River, Virginia.
54 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Final calibrated concentrations for mercury sources to 

the river are shown in table 27. These concentrations are 

within the range of observed concentrations. Although there 

are no observations of interflow (THGF), the calibrated value 

of 10.0 ng/L is reasonable because interflow is conceived as 

precipitation traveling through the shallow subsurface for a 

few days or less before discharging to the river. Therefore it 

is expected that interflow would have a THGF concentration 

between that of groundwater and precipitation (0.07 ng/L and 

21.8 ng/L), respectively.

With the channel margin inputs added to the model, it 

was possible to successfully calibrate the model to match 

observed THG concentrations in the river. The time series of 

calibrated THG values are shown in figures 48A–C for the 

Waynesboro, Dooms, and Harriston stations, respectively. For 

all three monitoring stations, the model produces reasonably 

accurate simulations of water column THG concentrations. 

Simulated THG concentrations from the calibrated model have 

a slightly wider range than observed concentrations because 

grab samples are unlikely to capture the very highest and 

lowest concentrations. Simulated high concentrations during 

storm events agree well with observed storm sample concentrations in figures 48A–C. 

Figure 47. Simulated and observed (USGS samples only) total unfiltered mercury concentration 

distributions showing poor calibration obtained with no channel margin inputs, simulation period 

April 1, 2005, through March 31, 2007, South River at Dooms, Virginia (USGS station number 01626920).

1

10

100

1,000

10,000

TOTAL UNFILTERED MERCURY CONCENTRATION, IN NANOGRAMS PER LITER

0 10 20 30 40 50 60 70 80 90 100

PERCENT TIME EXCEEDED

Observed U.S. Geological Survey samples

Simulated, no channel margin mercury inputs

Figure 47. Simulated and observed (USGS samples only) total unfiltered mercury concentration distributions showing poor

calibration obtained with no channel margin inputs, simulation period April 1, 2005, through March 31, 2007, South River at

Dooms (USGS station number 01626920), Virginia.

Table 27. Input mercury concentrations to the calibrated 

watershed model. 

[THGF, aqueous filterable total mercury; THGSS, total mercury on solids 

suspended in water; ng/L, nanograms per liter; μg/g, micrograms per gram]

Mercury source and 

hydrologic response unit type

Calibrated model 

concentration value

Groundwater THGF concentrations (ng/L)

Uncontaminated pervious land areas 0.7

Contaminated flood-plain areas 2.9

Interflow THGF concentrations (ng/L)

Uncontaminated pervious land areas 10.0

Contaminated flood-plain areas 10.0 to 16.7

Precipitation THGF concentrations (ng/L) 21.8

Runoff sediment associated mercury THGSS concentrations (μg/g)

Uncontaminated pervious land areas 0.061

Contaminated flood-plain areas 7.6 to 16.7
Watershed Model Results 55

Figure 48. Simulated total unfiltered 

mercury concentration time series 

and observed concentrations, 

calibrated model, existing conditions, 

April 2005 through March 2007, 

(A) South River near Waynesboro, 

Virginia (USGS station number 

01626000), (B) South River at Dooms, 

Virginia (USGS station number 

01626920), and (C) South River at 

Harriston, Virginia (USGS station 

number 01627500).

APR

2005

JULY

2005

OCT

2005

JAN

2006

MAY

2006

AUG

2006

NOV

2006

MAR

2007

3.8 nanograms per liter

Calibration period Verification period

1

10

100

1,000

10,000

100,000

Simulated 90-day median total mercury

Simulated hourly total mercury

Target total mercury

U.S. Geological Survey samples

Virginia Department of Environmental

Quality samples

(C)

1

10

100

1,000

10,000

100,000

3.8 nanograms per liter

Simulated 90-day median total mercury

Simulated hourly total mercury

Target total mercury

U.S. Geological Survey samples

Calibration period Verification period

Figure 48. Simulated total unfiltered mercury concentration time series and observed

concentrations, calibrated model, existing conditions, April 2005, through March 2007,

(A) South River near Waynesboro (USGS station number 01626000), Virginia, (B) South

River at Dooms, Virginia (USGS station number 01626920), Virginia, and (C) South River

(B)

APR

2005

JULY

2005

OCT

2005

JAN

2006

MAY

2006

AUG

2006

NOV

2006

MAR

2007

0.1

1

10

100

TOTAL UNFILTERED MERCURY CONCENTRATION, IN NANOGRAMS PER LITER 

Simulated 90-day median total mercury

Simulated hourly total mercury

Target total mercury

U.S. Geological Survey samples

Calibration period Verification period

3.8 nanograms per liter

DATE

(A)

APR

2005

JULY

2005

OCT

2005

JAN

2006

MAY

2006

AUG

2006

NOV

2006

MAR

2007
56 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Simulated mercury concentration values do not match 

all observed values, however. These discrepancies may result 

from a variety of factors including errors in the sediment 

or hydrologic components of the model, errors in mercury 

component input or parameterization, or real world variability at scales below model discretization. During periods 

of extended low flow, when THG concentrations are low, the 

simulated hourly THG concentrations for both Waynesboro 

and Harriston (figs. 48A and 48C) exhibit spurious numerical 

oscillations. These oscillations have an hourly period driven 

by a corresponding oscillation in suspended sediment concentration. As a result of numerical dispersion and(or) rounding 

errors, the model at each hourly time step alternately deposits 

or resuspends a large percentage of the suspended sediment 

that is present in the water column. Most mercury in the water 

column is attached to sediment, and this causes the total water 

column mercury concentrations to oscillate as well. This is 

clearly a numerical artifact of the model simulation and was 

not observed in South River sample data. These oscillations do 

not affect the mass balance of total mercury at periods of a day 

or more, and do not significantly affect the TMDL calculations 

because the 1-hour oscillation period is much shorter than the 

90-day averaging period used to determine the TMDL.

Statistics for simulated THG concentrations from the 

calibrated model and for USGS observed concentrations at 

the three primary South River monitoring stations are listed 

in table 28. Results for the calibration and verification periods have been combined to produce the statistics in table 28. 

Simulated mercury statistics closely approximate observed 

mercury statistics. Undersimulation of the 90th-percentile 

THG concentration value at Waynesboro is at least partially 

the result of multiple storm samples being collected at that 

monitoring station in October and November 2005 (fig. 48A), 

causing bias in the sample population towards the high end. 

Similarly at Dooms, where a smaller percentage of storm 

samples were collected, the 90th-percentile value is oversimulated (fig. 48B). 

Duration plots for simulated and observed (USGS samples only) mercury concentrations are shown in figures 49A–C

for the Waynesboro, Dooms, and Harriston stations. Curves 

for the simulated THG duration show a reasonable match to 

observed duration curves at all three monitoring stations. 

Simulated mercury loads to the river are listed by source 

in table 29. Nonpoint sources account for 99.7 percent of the 

mercury load to the South River. The largest of the nonpoint 

sources are channel margin inputs, accounting for about 

84 percent of all mercury entering the river. Runoff from land 

surfaces, primarily contaminated flood-plain areas, accounts 

for most of the rest. Point sources, groundwater discharge, 

interflow discharge, and precipitation on the river surface collectively account for less than 1 percent of the mercury load to 

the South River.

Table 28. Statistics for hourly simulated and observed total mercury concentrations, calibrated watershed model, South River, Virginia, 

calibration and verification periods combined, April 1, 2005, through March 31, 2007. 

[Observed values from U.S. Geological Survey samples only; %, percent; ng/L, nanograms per liter; THG, total unfiltered mercury]

THG concentration 

(ng/L)

Waynesboro (01626000) Dooms (01626920) Harriston (01627500)

Observed Simulated % Error Observed Simulated % Error Observed Simulated % Error

90th percentile 14.9 7.1 –52 568.3 988.0 74 895.2 898.8 0.4

50th percentile 1.3 1.2 –9 103.6 69.6 –33 115.0 91.4 –21

10th percentile 0.7 0.7 –5 29.5 31.7 8 29.3 26.5 –10

Table 29. Simulated annual total mercury loads to the South River, calibrated model, existing conditions, 

April 1, 2005, through March 31, 2007. 

[%, percent]

Reach

Total mercury (grams/year)

1 2 3 4 5 Total all reaches

Point sources 1 608 0 0 41 650 (0.3%)

Direct precipitation to river 28 7 2 11 8 55 (0.0%)

Interflow discharge 382 46 48 151 41 667 (0.4%)

Groundwater discharge 54 8 7 24 6 99 (0.1%)

Runoff 573 144 3,998 21,205 3,316 29,237 (15.4%)

Channel margin inputs 0 59,179 82,742 14,551 2,241 158,713 (83.8%)

Totals 1,038 

(1%)

59,992 

(32%)

86,797 

(46%)

35,942 

(19%)

5,653 

(3%)

189,421 (100%)
Watershed Model Results 57

Figure 49. Duration curves for simulated 

and observed (USGS samples only) 

total unfiltered mercury concentrations, 

calibrated model, existing conditions, 

April 1, 2005, to March 31, 2007, South 

River (A) near Waynesboro, Virginia (USGS 

station number 01626000), (B) at Dooms, 

Virginia (USGS station number 0162920), 

and (C) at Harriston, Virginia (USGS station 

number 01627500).

0 10 20 30 40 50 60 70 80 90 100

PERCENT

Observed—calibration period

Simulated hourly—calibration period

Observed—verification period

Simulated hourly—verification period

10

100

1,000

10,000

(C)

Figure 49. Duration curves for simulated and observed (USGS samples only) total

unfiltered mercury concentrations, calibrated model, existing conditions, April 1, 2005, to

March 31, 2007, South River (A) near Waynesboro, (USGS station number 01626000),

Virginia, (B) at Dooms (USGS station number 0162920), Virginia, and (C) at Harriston

(USGS station number 01627500), Virginia.

10

100

1,000

10,000

Observed—calibration period

Simulated hourly—calibration period

Observed—verification period

Simulated hourly—verification period

(B)

0

0.1

1

10

100

UNFILTERED TOTAL MERCURY CONCENTRATION, IN NANOGRAMS PER LITER

Observed—calibration period

Simulated hourly—calibration period

Observed—verification period

Simulated hourly—verification period

(A)
58 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Limitations of the South River Watershed Model 

and Suggestions for Future Investigations

A number of factors limit the accuracy of the HSPF 

South River model simulations. A primary factor is the spatial 

and temporal discretization within the model, particularly 

spatial discretization. Parameters such as water velocity, sediment depth, suspended sediment concentration, and mercury 

concentration exhibit a wide range of values over relatively 

small distances (less than 100 m) in the South River. The 

model treats all such variables as homogeneous within each 

river reach, however. Model reach lengths range from 1.5 to 

24.6 mi. Temporal discretization is less of an issue because 

the model progresses in 1-hour time steps. During periods 

of flooding, however, and particularly at the beginning of a 

runoff event, hydrologic variations can be substantial from one 

hour to the next. There were not enough data to justify finer 

spatial discretization of the model. Collecting data at additional monitoring stations could provide the data needed for 

future calibration of the model at finer spatial scales.

A second limitation of the model is the exclusion of 

physical mechanisms that may be major controls on mercury 

transport. Although inputs of mercury from channel margin 

processes, such as bank collapse and erosion, make up more 

than 80 percent of the mercury load to the river in the calibrated model (table 29), these mercury inputs to the river are 

treated in the model as simple lumped parameter processes 

rather than explicit physical processes. Related to this limitation is the lack of data describing the magnitude and timing 

of these processes. Detailed studies of the timing and spatial 

extent of mercury release from channel margin sediments in 

response to changes in temperature, pH, and oxidative states 

could improve understanding of the channel margin mercury 

loading, for example. Ongoing studies are examining the 

effects of hyporheic flow and bank collapse on loading of 

mercury to the river. These and other future research efforts 

could improve the understanding of channel margin inputs of 

mercury and guide changes to the watershed model that could 

improve its accuracy and certainty in model output.

Deficiencies in model input data are an additional potential source of model error. Precipitation data are the primary 

hydrologic input to the model and have partially known errors, 

as previously discussed, that directly affect simulation results. 

Precipitation has spatial variation at scales smaller than the 

land-county segments used to assign precipitation time series. 

Additional precipitation data could reduce this uncertainty 

in the model. Because most observed mercury concentration 

values are high relative to background levels, model results 

are tied to relatively few calibration data at low concentrations. The model was designed to accurately simulate the full 

range of concentrations currently seen in the watershed, most 

of which are much greater than the TMDL target concentration of 3.8 ng/L. None of the samples collected downstream 

from the plant site in this study had THG concentrations 

below 13 ng/L. If more low concentration mercury data were 

available, perhaps after initial cleanup efforts, it could help 

reduce uncertainty associated with simulation of low mercury 

concentrations. This uncertainty is especially applicable to the 

TMDL Scenarios discussed later in this report, which examine 

potential clean-up efforts and their effects on THG concentrations in the river. Groundwater THGF

 data are sparse, and the 

model value for THGF concentrations in groundwater from 

contaminated pervious lands areas is assigned primarily on 

the basis of data from a single contaminated flood-plain area. 

Additional mercury concentration data for groundwater would 

be useful for reducing uncertainty in future modeling efforts.

Limitations that should be noted by both readers and 

model users are that the HSPF model in its current state relies 

heavily on data collected since 2004, and that data related to 

mercury loading to the river prior to 2004 are relatively sparse 

and generally not produced by low-level detection laboratory 

methods for measuring mercury concentrations. Before 2004, 

there were several early reports on fish tissue concentrations 

and sediment and soil concentrations, but there were few reliable and low-detection mercury concentration data for plant 

outfalls or the South River. Mercury sources to the river and 

mercury concentrations within the river may have changed 

significantly over the past 30 years, but the model does not 

reflect those changes due to insufficient data. This limitation 

does not apply to dates from 2004 to 2007, but should be 

considered if the model is used to simulate mercury transport 

during earlier time periods.

Simulation of Mercury Total Maximum 

Daily Load (TMDL)

Subsequent to calibration of the mercury transport 

component of the watershed model, the model was used 

to simulate a TMDL for mercury in the South, South Fork 

Shenandoah, and Shenandoah Rivers. The TMDL value is 

set at a level to ensure that mercury loads from point sources 

and nonpoint sources can be assimilated without exceeding the criterion of 0.3 mg/kg methylmercury in fish tissue. 

Allocations from point sources, nonpoint sources, and natural 

background sources are included in the TMDL. The South 

River TMDL includes an implicit margin of safety to account 

for uncertainties in calculation of the TMDL. The South River 

was analyzed in detail and results were then extrapolated 

downstream to determine a TMDL for both the South Fork 

Shenandoah and Shenandoah Rivers.

Designation of Endpoints

The South River model links the identified sources of 

mercury to water column concentrations of total mercury 

(THG). An empirical bioaccumulation model then relates THG 

concentrations to fish tissue concentrations. The watershed 

model, in conjunction with the bioaccumulation model, provides the basis for estimating the total assimilative capacity of 

the river and any needed load reductions (Virginia Department 

of Environmental Quality, 2008b). The mercury TMDL for 
Simulation of Mercury Total Maximum Daily Load (TMDL) 59

the South River is then determined as a mercury loading rate 

that is consistent with the endpoint fish tissue methylmercury 

concentration of 0.3 mg/kg.

Numeric endpoint water column total mercury concentration values were determined by the VDEQ based on the 

0.3 mg/kg fish tissue methylmercury level and the empirical 

bioaccumulation model (table 30). These target concentrations were used to evaluate attainment of acceptable water 

quality and represent water-quality goals that will be targeted 

through load-reduction scenarios. The target concentrations 

decrease downstream to account for variations in fish size and 

natural variability of bioaccumulation rates in the Shenandoah 

river system. 

For each modeled river reach, simulated total mercury 

concentrations were compared to target concentrations to 

determine whether a violation had occurred. Simulated 90-day 

median THG concentrations were used in the comparison. On 

a daily basis, simulated 90-day median THG concentrations 

were compared to target concentrations for each river. If for 

that day, the median total mercury concentration for the preceding 90-day period was higher than the target concentration, 

then a violation had occurred.

Existing Conditions 

The calibrated South River watershed model was run 

with a simulation period of April 1, 2005, through March 31, 

2007, to simulate existing conditions in the South River. When 

90-day median THG concentrations are below target endpoint concentrations (table 30), fish are protected from tissue 

mercury concentrations above 0.3 mg/kg. If 90-day median 

THG concentrations exceed target concentrations then fish 

are expected to have tissue mercury concentrations in excess 

of 0.3 mg/kg. Table 31 shows median hourly total mercury 

concentrations for the entire period, April 1, 2005, through 

March 31, 2007 (730 days).

Under existing conditions, median mercury concentrations in the South River are below the target concentration of 

3.8 ng/L only at the Waynesboro monitoring station, upstream 

from the plant site. Below the plant site, median THG concentrations exceed target concentrations by a factor of 5 or more 

(table 31). Rolling 90-day median THG simulated concentrations from the calibrated model under existing conditions are 

shown in figures 48A–C. At the Waynesboro monitoring station, 

90-day median THG concentrations are always below the target 

concentration of 3.8 ng/L, whereas, at Dooms and Harriston, 

90-day median THG concentrations are always far above it.

Simulated mercury loads in the South River under existing conditions are summarized in table 32. Mercury loads 

increase dramatically below the plant site (in reach 2) as a 

result of a variety of point and nonpoint source inputs. The 

annual mercury load of 189 kg/yr at Port Republic can be 

compared to the estimated 109 kg/yr of mercury loading due to 

bank retreat estimated by Rhoades and others (2009). Although 

the time period for the simulations (2005 through 2007) is 

shorter than the averaging period (1937 through 2006) used by 

Rhoades and others (2009), it is noteworthy that the two values 

are relatively close and the total simulated load is higher than 

the estimated load resulting solely from bank retreat.

The results indicate that, as expected, mercury loads to 

the river increase dramatically in model river reach 2, which 

contains the plant site. Mercury attributable to releases from 

the plant site, including legacy mercury entering the river 

through channel margin inputs and contaminated runoff sediment, increases the total load of mercury to the river by a 

factor of more than 100 when compared to background conditions. The relative percentage of different mercury loads also 

changes, as shown in figure 50. Upstream from the plant site, 

most mercury is loaded to the river through interflow or runoff 

of sediment at background THGSed concentrations, whereas 

downstream from the plant site, channel margin inputs dominate mercury loads.

Table 30. Target total mercury water column concentrations for 

rivers in the study. 

[g, grams; ng/L, nanograms per liter; THG, total unfiltered mercury]

Water body

Normalized 

fish size 

(g)

Target water column 

THG concentration 

(ng/L)

South River 218 3.8

South Fork Shenandoah River 253 3.2

Shenandoah River 321 2.5

Table 31. Simulated total mercury concentrations for the South 

River, Virginia, existing conditions, median hourly concentrations 

for the period April 1, 2005, through March 31, 2007. 

[– indicates upstream; ng/L, nanograms per liter]

Model 

river 

reach

Reach 

end node

Miles 

downstream 

from 

plant site

THG 

concentration (ng/L) 

simulated median

1 01626000 –2.8 1.2

2 01626850 2.3 21.7

3 01626920 5.3 69.6

4 01627500 16.5 91.4

5 Port Republic 24.0 93.4

Table 32. Simulated annual mercury loads by reach in the South 

River, existing conditions, April 1, 2005, through March 31, 2007. 

[– indicates upstream; USGS, U.S. Geological Survey; kg/yr, kilograms 

per year]

Model 

river 

reach

Endpoint USGS 

station ID

Miles 

downstream 

from 

plant site

Total HG 

load 

(kg/yr)

1 Waynesboro 01626000 –2.8 1.0

2 Hopeman Parkway 01626850 2.3 61.0

3 Dooms 01626920 5.3 147.8

4 Harriston 01627500 16.5 183.8

5 Port Republic None 24 189.4
60 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 50. Percent mercury loading to the South River from sources upstream (model reach 1) and 

downstream (model reaches 2–5) from the plant site, calibrated model, existing conditions.

Mercury load components

Upstream from the plant site (RCHRES 1)

55%

37%

5%

3%

Mercury load components

Downstream from the plant site 

(RCHRES 2–5)

15%

84%

Point sources

Direct precipitation

Interflow discharge Groundwater discharge

Runoff

Channel margin inputs

EXPLANATION

Figure 50. Percent mercury loading to the South River from sources upstream (model reach 1) and downstream

(model reaches 2-5) of the plant site, calibrated model, existing conditions.

Table 33. Mercury loading rates to watersheds and subwatersheds by model reach, calibrated model, existing conditions, South River, 

Virginia, April 1, 2005, through March 31, 2007. 

[g/yr, grams per year; g/acre/yr, grams per acre per year]

Model river reach

1 2 3 4 5

Reach subwatershed

Subwatershed area (acres) 81,468 13,651 10,129 30,704 14,525

Reach-specific mercury loading rate (g/yr) 1,038 59,992 86,797 35,942 5,653

Reach-specific unit mercury loading rate (g/acre/yr) 0.013 4.395 8.569 1.171 0.389 

Total upstream watershed

Total upstream area (acres) 81,468 95,119 105,248 135,952 150,477

Total watershed Hg loading rate (g/yr) 1,038 61,030 147,827 183,768 189,421
Simulation of Mercury Total Maximum Daily Load (TMDL) 61

Scenarios for Mercury Load Reductions 

Simulated loads to the river were modified to determine 

how water column THG concentrations would respond. Mercury loads were increased or decreased to simulate possible 

future scenarios, as shown in table 34. Scenario 1, “existing 

conditions,” is discussed in the previous section. 

Future Conditions

Under Scenario 2, future conditions are simulated by 

increasing permitted point-source flows to limits set by current 

(2007) discharge permits. These “future conditions” are comparable to “build-out” scenarios used in other TMDL studies 

(Interstate Commission on the Potomac River Basin, 2004). 

Flow rates are increased to maximum permitted values, but 

concentrations of mercury are not changed, reflecting that current permits do not specify maximum concentrations. Under 

future conditions, Invista outfall 011 is assumed to discharge 

directly to the South River under monthly average flow rates. 

Since 2002, flow from outfall 011 has actually been routed 

through the Invista wastewater treatment plant, but under the 

current discharge permit, it is allowed to flow directly to the 

river. It is also assumed that THGF concentrations in precipitation will decline by 19 percent (from 21.8 to 17.6 ng/L) as 

a result of USEPA’s Clean Air Interstate Rule and the Clean 

Air Mercury Rule (Alex Barron, Virginia Department of 

Environmental Quality, written commun., 2008), and that 

interflow THGF concentrations will therefore also decrease by 

19 percent. These changes to point-source loads, atmospheric 

mercury deposition rates, and interflow mercury concentrations are included in Scenarios 2 through 4B.

The results of Scenario 2 show that “future conditions” 

cause higher median THG concentrations in all South River 

reaches, with the highest increase of 43 percent just below the 

plant site at Hopeman Parkway (table 35). These increases 

are due to the higher loads from Invista outfall 011 and the 

somewhat higher mercury loads from point sources that come 

with the assumption that maximum permitted flows are in 

effect. Simulated total mercury loads decrease slightly at the 

Table 34. Mercury load-reduction simulation scenarios. 

[THGF, aqueous filterable mercury; ng/L, nanograms per liter]

Type Scenario Changes to mercury loading

Existing conditions 1 All current mercury loads included.

Future conditions 2 Point sources increased to maximum permitted discharge, outfall 011 added, 

precipitation and interflow concentrations reduced.

Precipitation, 

interflow, and 

spring flow THGF

concentrations 

reduced.

Single source 

reductions

3A Point sources reduced to target stream concentrations.

3B Channel margin inputs eliminated, point sources at maximum permitted.

3C Runoff cleaned up to background conditions, point sources at maximum permitted.

Multiple source 

reductions

4A Channel margin loads eliminated and runoff cleaned to background conditions, 

point sources at max permitted.

4B Additionally reduce point sources to 3.8 ng/L.

Table 35. Changes in median simulated total mercury concentrations, relative to existing conditions, April 1, 2005, 

through March 31, 2007, South River, Virginia. 

[%, percent]

Model

scenario

Waynesboro

(01626000)

Hopeman Parkway

(01626850)

Dooms

(01626920)

Harriston

(01627500) Port Republic

1 ----------------------------------------- Existing conditions -------------------------------------------

2 7% 43% 13% 4% 2%

3A 7% –14% –11% –9% –10%

3B 7% –51% –77% –85% –86%

3C 7% 43% 9% 1% 0%

4A 7% –52% –78% –87% –88%

4B 34% –94% –97% –97% –98%
62 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Waynesboro monitoring station and increase slightly at Hopeman Parkway (table 36). Mercury loads can decrease even 

though median mercury concentrations increase because loads 

are dominated by high flow periods whereas median concentrations are dominated by low flow periods. The violation of 

target concentrations in reaches 2–5 does not change under 

“future conditions.” Simulated THG concentrations under 

future conditions at the Harriston monitoring station are shown 

in figure 51.

Single Source Reductions

Sources of mercury to the South River were reduced one 

at a time to determine the resulting changes in river mercury 

concentrations. Under Scenario 3A, point-source loads are lowered by setting point-source THG concentrations to the target 

THG concentration of 3.8 ng/L, while keeping flow rates at the 

maximum permitted rates used in the “future conditions” of 

Scenario 2. All point-source mercury concentrations are set to 

3.8 ng/L in Scenario 3A. The only other change to mercury 

source loads under Scenario 3A, as compared to the existing 

conditions of Scenario 1, is the reduction of atmospheric 

mercury deposition and interflow THGF concentrations as 

previously described. A comparison of the simulated THG 

values at the Harriston station (01627500) from Scenario 3A 

with those of Scenario 2 shows that reducing point source 

inputs lowers THG concentrations during low-flow periods (figs. 51–52). The results in table 36 indicate that 

point sources make a relatively small contribution to total 

loads. THG loads decrease by 6 percent at Waynesboro and 

decrease by less than 1 percent at reaches 2–5 downstream 

from the plant site. Point sources make a relatively larger 

contribution to median THG concentrations because median 

concentrations are more sensitive than loads to low THG 

concentrations (tables 35–36). Upstream from the plant site 

at Waynesboro, median concentrations increase by 7 percent, as they do under Scenario 2, and downstream from the 

plant site in reaches 2–5, median concentrations decrease 

Table 36. Percentage change to in-stream total mercury loads, 

relative to existing conditions, April 1, 2005, through March 31, 2007, 

South River, Virginia. 

[%, percent]

Model

scenario

Waynesboro

(01626000)

Hopeman 

Parkway

(01626850)

Dooms

(01626920)

Harriston

(01627500)

Port 

Republic

1 ---------------------- Existing conditions ----------------------

2 –5.7% 0.1% 0.0% 0.0% 0.0%

3A –5.9% –0.9% –0.4% –0.3% –0.3%

3B –5.7% –96.9% –96.0% –85.1% –83.8%

3C –7.2% –0.1% –2.7% –13.5% –14.8%

4A –7.2% –97.0% –98.7% –98.7% –98.6%

4B –5.6% –99.6% –99.9% –99.7% –99.9%

Figure 51. Simulated total unfiltered mercury values for the South River at Harriston, 

Virginia (USGS station number 01627500), under future conditions, Scenario 2. (Existing 

90-day median simulated mercury concentrations shown for comparison.)

1

10

100

1,000

10,000

JAN

2005

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

Simulated mercury

Target

90-day median mercury

Existing conditions

3.8 nanograms per liter

DATE

Figure 51. Simulated total unfiltered mercury values for the South River at Harriston (USGS station number 01627500),

Virginia, under future conditions, scenario 2. [Existing 90-day median simulated mercury concentrations shown

for comparison.]
Simulation of Mercury Total Maximum Daily Load (TMDL) 63

from 9–14 percent. Simulated THG concentrations for Scenario 3A at Harriston are shown in figure 52.

Under Scenario 3B, channel margin mercury loads are 

eliminated, simulating the potential future effects of remediation 

strategies focused on streambanks. Reduction of atmospheric 

mercury deposition and interflow THGF concentrations to reflect 

expected future changes are also included. Point sources are 

restored to future conditions. The results of Scenario 3B show 

large declines in both mercury loads and concentrations downstream from the plant site (fig. 53 and tables 35–37). THG loads 

decline by 84 percent to 97 percent, and median THG concentrations decline from 51 percent to 86 percent in reaches 2–5 

downstream from the plant site. Despite the large THG 

concentration declines, all downstream monitoring stations are still in violation of target concentrations 100 percent of the time. Simulated THG concentrations for 

Scenario 3B at Harriston are shown in figure 53.

Under Scenario 3C, mercury contaminated sediment 

runoff is cleaned to background conditions to simulate 

a hypothetical future remediation of South River flood 

plains. THGSed concentrations on sediment from contaminated flood-plain areas are reduced to THGSed concentrations of uncontaminated land areas. Reductions of 

atmospheric mercury deposition and interflow THGF concentrations are again included. Point sources and channel 

margin inputs are restored to future conditions. Simulated 

THG concentrations from Scenario 3C at the Harriston 

monitoring station are shown in figure 54. With mercury 

contaminated runoff cleaned up, THG loads decrease 

from 0.1 percent to 14.8 percent at monitoring stations downstream from the plant site. Median THG concentrations under 

this scenario increase relative to existing conditions because 

point sources are assumed to have maximum permitted flows, 

which particularly increases mercury loads to the river during 

base-flow periods. Median THG concentrations increase by 

43 percent at Hopeman Parkway, 9 percent at Dooms, and 

1 percent at Harriston. All monitoring stations downstream 

from the plant site remain in violation of target THG values 

under this scenario. 

Figure 52. Simulated total unfiltered mercury values for the South River at Harriston, 

Virginia (USGS station number 01627500), under future conditions with point sources 

cleaned up, Scenario 3A. (Existing 90-day median simulated mercury concentrations 

shown for comparison.)

1

10

100

1,000

10,000

JAN

2005

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

Simulated mercury

Target

90-day median mercury

Existing conditions

3.8 nanograms per liter

DATE

Figure 52. Simulated total unfiltered mercury values for the South River at Harriston (USGS station number 01627500),

Virginia, under future conditions with point sources cleaned up, scenario 3A. [Existing 90-day median simulated

mercury concentrations shown for comparison.]

Table 37. Percentage of time that simulated 90-day median 

total mercury concentrations exceed the 3.8 nanogram per liter 

target concentration, April 1, 2005, through March 31, 2007, South 

River, Virginia. 

[%, percent]

Model

scenario

Waynesboro

(01626000)

Hopeman 

Parkway

(01626850)

Dooms

(01626920)

Harriston

(01627500)

Port 

Republic

1 0% 100% 100% 100% 100%

2 0% 100% 100% 100% 100%

3A 0% 100% 100% 100% 100%

3B 0% 100% 100% 100% 100%

3C 0% 100% 100% 100% 100%

4A 0% 100% 100% 100% 100%

4B 0% 0% 0% 0% 0%
64 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Figure 53. Simulated total unfiltered mercury values for the South River at Harriston, 

Virginia (USGS station number 01627500), under future conditions with channel margin 

mercury sources cleaned up, Scenario 3B. (Existing 90-day median simulated mercury 

concentrations shown for comparison.)

1

10

100

1,000

10,000

JAN

2005

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

Simulated mercury

Target

90-day median mercury

Existing conditions

3.8 nanograms per liter

DATE

Figure 53. Simulated total unfiltered mercury values for the South River at Harriston (USGS station number 01627500),

Virginia, under future conditions with channel margin mercury sources cleaned up, Scenario 3B. [Existing 90-day

median simulated mercury concentrations shown for comparison.]

Figure 54. Simulated total unfiltered mercury values for the South River at Harriston, Virginia 

(USGS station number 01627500), under future conditions with runoff cleaned up, Scenario 3C. 

(Existing 90-day median simulated mercury concentrations shown for comparison.)

1

10

100

1,000

10,000

JAN

2005

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

Simulated mercury

Target

90-day median mercury

Existing conditions

3.8 nanograms per liter

DATE

Figure 54. Simulated total unfiltered mercury values for the South River at Harriston (USGS station number 01627500),

Virginia, under future conditions with runoff cleaned up, scenario 3C. [Existing 90-day median simulated mercury

concentrations shown for comparison.]
Simulation of Mercury Total Maximum Daily Load (TMDL) 65

Multiple Source Reductions and Total Maximum 

Daily Load (TMDL) Scenario

The results of Scenarios 3A–3C demonstrate that THG 

concentrations cannot be brought below target concentrations 

unless channel margin mercury loads are drastically reduced 

and other loads are also reduced. Under Scenario 4A, multiple 

loads are reduced; channel margin loads are totally eliminated 

and sediment runoff concentrations, THGSed, are reduced to 

background levels. Together, these two sources account for 

99.2 percent of total mercury loads to the river (table 29). 

Precipitation and interflow are again reduced to reflect future 

reductions in atmospheric mercury deposition. The remaining mercury loads are from point sources, groundwater, and 

background mercury levels in precipitation, interflow, runoff, 

and precipitation. Results from Scenario 4A indicate that in 

spite of greatly reduced mercury loads, violations of THG target concentrations still occur at all of the monitoring stations 

downstream from the plant site, as shown in figure 55 for the 

Harriston monitoring station.

Under Scenario 4B, mercury loads to the river are further 

reduced, resulting in mercury concentrations that meet TMDL 

requirements. Mercury loads from channel margin inputs and 

surface runoff are again reduced and a further load reduction is 

made by lowering all point sources to the target THG concentration of 3.8 ng/L. Because this scenario will be used for load 

allocation under TMDL regulations by the VDEQ, all point 

sources are assigned a fixed concentration of 3.8 ng/L, including the Stuart’s Draft wastewater plant and Genicom, which 

under existing conditions have THG concentrations below 

3.8 ng/L. Results show that 90-day median THG concentrations stay below 3.8 ng/L in all river reaches, as shown in 

figure 56 for the Harriston monitoring station. 

TMDL requirements are satisfied under Scenario 4B, and 

the TMDL for mercury to the South River above its confluence 

with the South Fork Shenandoah at Port Republic is 2.0 kg/yr 

(table 38). This value is reasonable from a mass balance 

point of view, considering the low target concentrations. As 

an example, if the target concentration of 3.8 ng/L is multiplied by the simulated annual volume of water passing Port 

Republic, 2.7×1011 L, the result is 1.0 kg of mercury, which 

is less than the TMDL value of 2.0 kg/yr. The TMDL value 

of 2.0 kg/yr is higher because the 90-day median statistic 

reduces the importance of stormflows, which carry most of the 

mercury load.

There are several reasons for the large percentage reduction in mercury loads required to achieve target in-stream 

THG concentrations. The first is that THG concentrations of 

inputs to the river are very high relative to the target concentration of 3.8 ng/L. Precipitation THGF

 is about 5 times 

higher than target THG concentrations, interflow THGF from 

uncontaminated land occupying most of the watershed is 

about 3 times higher than target THG concentrations, and 

some point-source THG concentrations are two to three 

Figure 55. Simulated total unfiltered mercury concentrations for Scenario 4A, reduced channel 

margin and runoff mercury loads, South River at Harriston, Virginia (USGS station number 

01627500). (Existing 90-day median simulated mercury concentrations shown for comparison.)

1

10

100

1,000

JAN

2005

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

Simulated mercury

Target

90-day median mercury

Existing conditions

3.8 nanograms per liter

DATE

Figure 55. Simulated total unfiltered mercury concentrations for Scenario 4A, reduced channel margin and runoff

mercury loads, South River at Harriston (USGS station number 01627500), Virginia. [Existing 90-day median simulated

mercury concentrations shown for comparison.]
66 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

orders of magnitude higher than target THG concentrations. A smallmouth bass at the background reference station 

upstream from the plant site had average mercury concentrations of 0.24 mg/kg from 1999–2007, which indicates that the 

mercury concentrations must be reduced to near background 

levels to achieve the fish tissue mercury concentration goal of 

0.3 mg/kg.

Results of the simulations indicate that large percentage 

reductions in multiple mercury loads to the river would be 

required to lower fish tissue methylmercury concentrations to 

below the 0.3 mg/kg criterion. Although the largest contributing load in the simulations is from channel margin inputs, 

other loads that would need to be addressed include runoff 

from contaminated flood plains and point-source discharges. 

Due to the large reductions needed (over 99 percent of the 

total mercury load), achieving the goal of maintaining fish 

tissue methylmercury concentrations below 0.3 mg/kg in the 

South River will be challenging.

Figure 56. Simulated total unfiltered mercury concentrations for the Total Maximum Daily 

Load Scenario (Scenario 4B), reduced channel margin, runoff, and point-source mercury 

loads, South River at Harriston, Virginia (USGS station number 01627500). (Existing 90-day 

median simulated mercury concentrations shown for comparison.)

1

10

100

1,000

JAN

2005

APR

2005

JULY

2005

OCT

2005

JAN

2006

APR

2006

JULY

2006

OCT

2006

JAN

2007

APR

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

Simulated mercury

Target

90-day median mercury

Existing conditions

3.8

nanograms

per liter

DATE

Figure 56. Simulated total unfiltered mercury concentrations for the Total Maximum Daily Load Scenario

(Scenario 4B), reduced channel margin, runoff, and point-source mercury loads, South River at Harriston

(USGS station number 01627500), Virginia. [Existing 90-day median simulated mercury concentrations

shown for comparison.]

Table 38. Annual mercury loads under Total Maximum Daily Load (TMDL) conditions (Scenario 4B), South River, Virginia.

Total mercury load (grams)

1 2 3 4 5 Total all reaches

Point sources 21.0 72.9 1.1 0.0 16.8 112

Direct precipitation to river 22.6 5.7 1.3 8.9 6.4 45

Interflow discharge 309.2 50.6 39.2 124.8 33.8 558

Groundwater discharge 53.9 7.8 7.5 23.9 5.7 99

Runoff 572.8 96.7 87.0 354.1 105.0 1,216

Channel margin inputs 0.0 0.0 0.0 0.0 0.0 0

Reach loads 980 234 136 512 168 2,029

Watershed loads 980 1,213 1,349 1,861 2,029 2,029
Simulation of Mercury Total Maximum Daily Load (TMDL) 67

Sensitivity of Total Maximum Daily Load (TMDL) 

Results to Model Parameter Values

The sensitivity of model results to input parameter values 

was assessed by altering parameter values and measuring the 

resulting changes to simulated mercury concentrations. Model 

sensitivity was assessed both under existing conditions and 

TMDL conditions (Scenarios 1 and 4B). Input parameters 

selected for the sensitivity analysis were those expected to 

have the greatest control over simulated THG concentrations 

(table 39). Most of the parameters analyzed have multiple values in the calibrated model because of variation by HRU or by 

time period. For the sensitivity analysis, changes were made to 

all values of a parameter on a percentage basis of ±50 percent. 

When precipitation was decreased by 50 percent, for example, 

all daily precipitation values for all HRUs were decreased by 

50 percent. Instream mercury loads are reported to the tenth 

of a kilogram. Because thousands of input values go into the 

simulation of annual mercury loads, the appropriate number 

of significant figures for reporting model results cannot be formally computed. For the purposes of this report, the numbers 

have been rounded as much as possible, while still showing 

the differences among locations and runs.

Under the existing conditions scenario (Scenario 1), 

sensitivity was measured as the change in simulated median 

THG concentration at the Harriston monitoring station for the 

period April 1, 2005, through March 31, 2007. Results of the 

sensitivity analysis under existing conditions are shown in 

table 40. Under the TMDL Scenario, sensitivity was measured 

as the change in the percent of time that 90-day rolling median 

THG concentrations at Harriston exceeded the 3.8 ng/L target 

concentration (table 41).

Results of the sensitivity analysis under existing conditions (table 41) show that median THG concentrations 

are most sensitive to parameters affecting suspension and 

deposition of sediment in the river during low-flow periods 

(TAUCD, TAUCS, and W). Under the TMDL Scenario, 

violation of the 3.8 ng/L target THG concentration is affected 

most by KRER, the sediment detachment rate, and TAUCD, 

the critical shear stress for deposition of suspended sediment. 

THG concentrations are most sensitive, under both existing 

conditions and TMDL conditions, to parameters controlling 

suspended sediment in the river because most mercury in 

the river is attached to suspended sediment. Sensitivities do 

not appear particularly high in this analysis, as ±50 percent 

changes to parameter values cause at most a ±24 percent 

change in THG concentrations and ±4 percent change in time 

that 90-day median concentrations exceed 3.8 ng/L. However, 

because the model treats channel margin loads very simply, 

this sensitivity analysis does not express the full uncertainty 

associated with the channel margin mercury loads. Future 

investigations that improve conceptual understanding and 

computer simulation of processes controlling the channel 

margin inputs have the potential to provide that needed aspect 

of the sensitivity analysis. 

Table 39. Model input parameters changed during sensitivity analysis. 

[HSPF, Hydrological Simulation Program–FORTRAN]

Type HSPF module Name Description Original values

Hydrology EXT SOURCES Precipitation Precipitation rates (inches per year) 39–43

Sediment PERLND>SEDMNT>DETACH KRER Sediment detachment rate coefficient 0.02–4.0

PERLND>SEDMNT>SOSED1 KSER Sediment runoff transport coefficient 0.1–20.0

RCHRES>SEDTRN 

(Silt and clay only)

W Settling velocity 0.0035–0.03

TAUCD Critical shear stress—Deposition 0.6

TAUCS Critical shear stress—Suspension 0.02–0.15

M Erodibility of bed sediment 0.03–0.07

Mercury sorption 

in river

RCHRES>GQUAL>ADSEDS KD Adsorption coefficient 1

ADRATE Mercury phase transfer rate coefficient 25
68 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Table 40. Results of sensitivity analysis for existing conditions, changes to median concentrations for the period April 1, 

2005, through March 31, 2007, resulting from + or –50 percent in model parameter values, South River at Harriston, Virginia 

(U.S. Geological Survey station number 01627500). 

[HSPF, Hydrological Simulation Program–FORTRAN; THG, total unfiltered mercury; ng/L, nanograms per liter; %, percent; in/yr, inches per year]

Parameter

Expected 

range

Median THG (ng/L)

Type HSPF module Name Description Original 

values –50% Existing conditions +50%

Hydrology EXT SOURCES Precipitation Precipitation rates 

(in/yr)

39–43 Observeda

27.1–66.5

92.3 91.4 91.1

Sediment PERLND>SEDMNT>

DETACH

KRER Sediment detachment 

rate coefficient

0.02–4.0 0.05–0.75b 79.0 91.4 96.1

PERLND>SEDMNT>

SOSED1

KSER Sediment runoff 

transport coefficient

0.1–20.0 0.1–10.0b 80.5 91.4 96.9

RCHRES>SEDTRN 

(Silt and Clay only)

W Settling velocity 

(in/yr)

0.0035–0.03 0–0.10c 78.8 91.4 108.7

TAUCD Critical shear stress—

Deposition

 0.6 0.001–1.0b 76.0 91.4 98.3

TAUCS Critical shear stress—

Suspension

0.02–0.15 0.01–3.0b 95.6 91.4 71.1

M Erodibility of bed 

sediment

0.03–0.07 0.001–5.0b 101.1 91.4 79.7

Mercury 

sorption 

in river

RCHRES>GQUAL>

ADSEDS

KD Adsorption 

coefficient

1 0.003–10.0d 89.7 91.4 92.1

ADRATE Mercury phase 

transfer rate 

coefficient

25 10–5–anyc 89.5 91.4 92.3

a

 See report section Meteorological and Climatic Data 

b

 U.S. Environmental Protection Agency (2006) 

c

 Bicknell and others (2001); 

d

 Horvat and others (2004), Lyons and others (1997), Gbondo-Tugbawa and Driscoll (1998)

Table 41. Results of sensitivity analysis under the Total Maximum Daily Load (TMDL) Scenario, changes 

to percent of time that 90-day median concentrations exceed the 3.8 ng/L target concentration, resulting 

from + or –50 percent in model parameter values, South River at Harriston, Virginia (U.S. Geological Survey 

station number 01627500), April 1, 2005, through March 31, 2005. 

[%, percent]

Parameter Time in violation, in percent

Type Name Description –50% Scenario 4B +50%

Hydrology Precipitation Precipitation rates (inches per year) 0 0 0

Sediment KRER Sediment detachment rate coefficient 0 0 3.9

KSER Sediment runoff transport coefficient 0 0 0

W Settling velocity (inches per year) 0 0 0

TAUCD Critical shear stress—Deposition 0 0 2.5

TAUCS Critical shear stress—Suspension 0 0 0

M Erodibility of bed sediment 0 0 0

Mercury 

sorption 

in river

KD Adsorption coefficient 0 0 0

ADRATE Hg phase transfer rate coefficient 0 0 0
Simulation of Mercury Total Maximum Daily Load (TMDL) 69

Implications for the Downstream South Fork 

Shenandoah River and Shenandoah River

The South Fork Shenandoah River and Shenandoah River 

are also part of the mercury TMDL, and are discussed here 

in light of the South River modeling results. Monitoring data 

were not collected in the South Fork Shenandoah and Shenandoah Rivers specifically for this study, but the VDEQ regularly 

collects surface-water samples for THG analysis at sites along 

the South Fork Shenandoah River. The most downstream site 

with sufficient mercury data for comparison to the South River 

is the South Fork Shenandoah River near Luray, Virginia 

(01629500) (fig. 57). 

Twenty-seven data pairs of THG concentration values 

from the VDEQ were used to compare THG concentrations in 

the South Fork Shenandoah River to those in the South River. 

The analysis compared THG concentrations from the South 

River at Harriston (01627500) to those from the South Fork 

Shenandoah near Luray, Virginia (01629500), which were collected within 6 hours of each other between August 2001 and 

January 2007. A linear fit to the plot of South Fork Shenadaoah River vs. South River THG concentrations (fig. 57) has 

an r2 value of 0.81, indicating a strong positive correlation. 

Instantaneous THG loads at sample times were estimated by 

multiplying THG concentrations by streamflow. THG loads 

were usually smaller at Luray than upstream in the South 

River at Harriston, with 22 of 27 sampling events showing 

a downstream decrease in load and an average decrease of 

25 percent. This is in spite of the fact that the South Fork 

Shenandoah River near Luray receives drainage from the 

South River and from other tributary streams, most notably the 

Middle and North Rivers.

A possible explanation for the decrease in mercury load 

at the Luray monitoring station is that mercury from the South 

River may be sorbing to suspended solids in the South Fork 

Shenandoah and being deposited on the channel bed. Although 

sediment-deposition characteristics of the South Fork Shenandoah are not well known, long-term accumulation of large 

volumes of sediment seems unlikely for that reach.

For this TMDL, mercury concentrations in the South 

Fork Shenandoah near Luray were estimated from a mixing model of South River water and water at background 

THG concentrations from other tributaries to the South Fork 

Shenandoah. It is assumed that all mercury exiting the South 

River remains in the water column and moves downstream 

to the South Fork Shenandoah. Based on the ratio of annual 

streamflow for the South Fork Shenandoah River near Luray 

to annual streamflow for the South River at Port Republic 

from the HSPF model (4.96), it is estimated that 79.9 percent 

of the flow at Luray originates from outside the South River 

Figure 57. Comparison of total unfiltered mercury concentrations (Virginia Department of 

Environmental Quality samples only) from the South River at Harriston, Virginia (USGS station number 

01627500) to same-day total unfiltered mercury concentrations from the South Fork Shenandoah 

River near Luray, Virginia (USGS station number 01629500).

R2=0.81

0

0 50 100 150 200 250 300 350 400 450

TOTAL UNFILTERED MERCURY CONCENTRATION, IN NANOGRAMS PER LITER,

SOUTH RIVER AT HARRISTON, VIRGINIA 

TOTAL UNFILTERED MERCURY CONCENTRATION, IN NANOGRAMS

PER LITER, SOUTH FORK SHENANDOAH RIVER

NEAR LURAY, VIRGINIA 

10

20

30

40

50

60

Figure 57. 1:1 Comparison of total unfiltered mercury concentrations (Virginia Department of Environmental Quality samples

only) from the South River at Harriston (USGS station number 01627500) to same-day total unfiltered mercury concentrations from

the South Fork Shenandoah River near Luray (USGS station number 01629500), Virginia.
70 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

watershed. All water entering the South Fork Shenandoah 

River above Luray from uncontaminated subwatersheds is 

assumed to have background THG concentrations equal to 

1.81 ng/L, the mean for observed THG concentrations (VDEQ 

samples only) in the North River. THG concentrations for the 

South Fork Shenandoah near Luray were then estimated as a 

mix of South River mercury and background aqueous mercury. 

This estimation was made for existing conditions (Scenario 1) 

and for the TMDL Scenario (Scenario 4B). Resulting time 

series are shown in figure 58 relative to the 3.2 ng/L target 

concentration for the South Fork Shenandoah River.

The results shown in figure 58 indicate that 90-day 

median THG concentrations in the South Fork Shenandoah 

River under existing conditions exceed the 3.2 ng/L target 

concentration. Under the TMDL Scenario (4B), in which runoff, channel margin inputs, and point sources are cleaned up, 

90-day median concentrations stay below the target concentration of 3.2 ng/L; therefore, no violations occur. The mercury 

load originating from areas other than the South River is 

estimated at 1.8 kg/yr, based on Luray flow volume minus 

South River Flow volume times the background mean THG 

concentration of 1.8 ng/L in the North River. The mercury 

TMDL for the South Fork Shenandoah River near Luray is 

therefore 3.8 kg/yr, obtained by adding 1.8 kg/yr to the South 

River mercury TMDL. By use of the same methods, a mercury 

TMDL of 4.1 kg/yr is estimated for the South Fork Shenandoah at Front Royal, Virginia (01631000), just upstream from 

the confluence with the North Fork Shenandoah River.

The Shenandoah River, formed at the confluence of 

the North Fork and South Fork Shenandoah Rivers in Front 

Royal, Virginia, also requires a mercury TMDL. Therefore, 

a TMDL was estimated using the same methods as those 

used for the South Fork Shenandoah River. However, there 

are few mercury data available for the Shenandoah River 

and there is no streamflow-gaging station in the affected 

part of the river, from Front Royal downstream to the Craig 

Run confluence. Annual average flow for the Shenandoah 

River at Craig Run is 2,791 ft3/s, based on the USGS EDNA 

calculator (http://edna.usgs.gov/), which calculates drainage 

areas from digital elevation models and flow accumulation 

regression methods (Vogel and others, 1999; http://edna.usgs.

gov/). The South River occupies 9.8 percent of the drainage 

area and is assumed to provide 9.8 percent of flow in the 

Shenandoah River at the confluence with Craig Run. The 

remaining 90.2 percent of flow is assumed to originate from 

uncontaminated parts of the watershed that contribute water 

at background mercury concentrations. By again assuming 

that water from uncontaminated subwatersheds has a background THG concentration of 1.81 ng/L, THG concentrations 

were estimated on a daily basis for the Shenandoah River at 

the confluence with Craig Run (fig. 59). The TMDL for mercury in the Shenandoah River at the Craig Run confluence, 

calculated as the sum of mercury from the South River plus 

mercury from uncontaminated subwatersheds, is 6.1 kg/yr 

(table 42).

Daily estimated mercury—Total Maximum Daily Load Scenario

Target

90-day median—Total Maximum Daily Load Scenario

Existing conditions 90-day median

3.2 nanograms per liter

0.1

10

100

1,000

MAR

2005

JULY

2005

OCT

2005

JAN

2006

MAY

2006

AUG

2006

NOV

2006

FEB

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

DATE

Figure 58. Simulated total unfiltered mercury concentrations for the Total Maximum Daily Load Scenario

(Scenario 4B), reduced channel margin, runoff, and point-source mercury loads, South Fork Shenandoah

River near Luray (USGS station number 01629500), Virginia. [Existing 90-day median simulated mercury

concentrations shown for comparison.]

1

Figure 58. Simulated total unfiltered mercury concentrations for the Total Maximum Daily 

Load Scenario (Scenario 4B), reduced channel margin, runoff, and point-source mercury 

loads, South Fork Shenandoah River near Luray, Virginia (USGS station number 01629500). 

(Existing 90-day median simulated mercury concentrations shown for comparison.)
Simulation of Mercury Total Maximum Daily Load (TMDL) 71

Figure 59. Estimated total unfiltered mercury concentrations for the Total Maximum Daily 

Load Scenario (Scenario 4B), reduced channel margin, runoff, and point-source mercury loads, 

Shenandoah River at the confluence with Craig Run, Clarke County, Virginia. (Existing 90-day 

median simulated mercury concentrations shown for comparison.)

Daily estimated mercury—Total Maximum Daily Load Scenario

Target

90-day median—Total Maximum Daily Load Scenario

Existing conditions 90-day median

2.5 nanograms per liter

0.1

10

100

1,000

MAR

2005

JULY

2005

OCT

2005

JAN

2006

MAY

2006

AUG

2006

NOV

2006

FEB

2007

TOTAL UNFILTERED MERCURY CONCENTRATION,

IN NANOGRAMS PER LITER 

DATE

Figure 59. Estimated total unfiltered mercury concentrations for the Total Maximum Daily Load Scenario

(Scenario 4B), reduced channel margin, runoff, and point-source mercury loads, Shenandoah River at the

confluence with Craig Run, Clarke County, Virginia. [Existing 90-day median simulated mercury concentrations

shown for comparison.]

1

Table 42. Estimated total maximum daily loads (TMDLs) for mercury for listed waters in 

the Shenandoah Valley, Virginia. 

[THG, total unfiltered mercury; ng/L, nanograms per liter; kg/yr, kilograms per year]

River

Target THG 

concentration 

(ng/L)

Total mercury 

TMDL 

(kg/yr)

South River at Port Republic, Virginia 3.8 2.0

South Fork Shenandoah River near Luray, Virginia 3.2 3.8

South Fork Shenandoah River at Front Royal, Virginia 3.2 4.1

Shenandoah River at Craig Run, Virginia 2.5 6.1
72 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

Summary

The U.S. Geological Survey (USGS), in cooperation 

with the Virginia Department of Environmental Quality and 

the U.S. Environmental Protection Agency, conducted a study 

to develop a total maximum daily load (TMDL) for methylmercury in fish tissue in the South, South Fork Shenandoah, 

and Shenandoah Rivers of Virginia. These rivers have fish 

with methylmercury concentrations above the U.S. Environmental Protection Agency criterion of 0.3 mg/kg (milligrams 

methylmercury per kilogram of fish tissue). A numerical 

watershed model based on Hydrological Simulation Program–FORTRAN (HSPF) software was developed to simulate 

water, sediment, and mercury transport in the South River 

watershed. This model was calibrated with field data collected 

in this study. Data were also compiled from other studies 

to describe other media including fish tissue and to expand 

coverage of downstream rivers and time periods prior to 2005. 

On the basis of results from the calibrated watershed model, 

the mercury load to the South River under existing conditions 

for the period April 2005 through March 2007 is estimated at 

189 kilograms per year. Using a site-specific empirical bioaccumulation model, the Virginia Department of Environmental 

Quality calculated concentrations of methylmercury in fish 

tissue from water column concentrations of total mercury. 

On the basis of the bioaccumulation model, to reduce fish 

tissue methylmercury concentrations below the U.S. Environmental Protection Agency criterion of 0.3 mg/kg, water 

column concentrations of total mercury need to be below 

target concentrations of 3.8 ng/L (nanograms per liter) in the 

South River, 3.2 ng/L in the South Fork Shenandoah River, 

and 2.5 ng/L in the Shenandoah River. Reductions in mercury 

loads to the South River were simulated using the calibrated 

HSPF model to determine the source-load reductions required 

to meet these conditions. Simulation results indicate that the 

TMDL for mercury in the South River that would be protective of methylmercury in fish tissue is 2.0 kilograms of total 

mercury per year. A mixing model and conservative mercury 

transport based on the South River modeling results were used 

to calculate mercury TMDLs for the South Fork Shenandoah 

and Shenandoah Rivers, which were 4.1 and 6.1 kilograms of 

mercury per year, respectively. Under the assumptions used in 

this study, if mercury loads to the South River are reduced to 

TMDL levels, fish tissue methylmercury concentrations in the 

South Fork Shenandoah River and Shenandoah River will also 

be reduced to less than 0.3 mg/kg.

Major findings and conclusions of this study are: 

• The calibrated South River watershed model simulates 

observed characteristics of streamflow, sediment load, 

and mercury transport. 

• Analysis of mercury loads to the South River indicates 

that nonpoint-source loads account for over 99 percent 

of total loads under existing conditions.

• Channel margin mercury load, a nonpoint-source load, 

makes up an estimated 84 percent of total mercury load 

to the South River. The channel margin mercury loads 

originate from contaminated sediment in close proximity to the river, but the pathways and mechanism(s) 

responsible for moving the channel margin mercury to 

the river are not well understood. 

• A 99 percent or greater reduction in the current mercury load delivered to the South River is required to 

meet target water-column mercury concentrations 

that are protective of the U.S. Environmental Protection Agency 0.3 mg/kg criterion for methylmercury in 

fish tissue. 

• The mercury TMDL is estimated to be 2.0 kg/yr for the 

South River at Port Republic, 4.1 kg/yr for the South 

Fork Shenandoah River at Front Royal, and 6.1 kg/yr 

for the Shenandoah River at the Craig Run confluence.

Acknowledgments

The South River Science Team has provided a valuable 

forum for exchanging information, ideas, and expertise about 

mercury in the South River. Many members of the South 

River Science Team provided data and insight that improved 

this study, and the Virginia Department of Environmental 

Quality and DuPont cosponsored the forum. Many thanks go 

to Douglas Moyer of the U.S. Geological Survey for his expert 

guidance in the collection of water-quality data and construction 

of the Hydrological Simulation Program–FORTRAN (HSPF) 

model; to Gary Shenk of the Chesapeake Bay Foundation 

and Phil Zarriello of the U.S. Geological Survey for valuable 

assistance with the model; and to Robert Brent of the Virginia 

Department of Environmental Quality, who provided strong 

scientific collaboration throughout the study.
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Appendix 1. Data Sources Used to Define 

Point-Source Loading in the Model
78 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia Appendix 1. Data sources used to define point-source loading.—Continued [ft3/s, cubic feet per second; mg, milligrams; VAPDES, Virginia Pollution Discharge Elimination System; %, percent; FPBS, Frew Pond Baker Spring; TSS, total suspended solids; SSC, suspended sediment concentration; Hg, mercury; ng/L, nanograms per liter; THG, total unfiltered mercury; mg/L, milligrams per liter; DEQ, Department of Environmental Quality; conc., concentration; GW, groundwater; K, hydraulic conductivity; Disch., discharge] Point sources Discharge (ft3/s) Suspended solids concentration (TSS or SSC), mg/L Total unfiltered Hg concentration (THG), ng/L Name Model ID Data source and type Model value assignment Data source and type Model value assignment Data source and type Model value assignment Periods with data Periods of no data Periods with data Periods of no data Periods with data Periods of no data Stu Draft STP 101-103 VAPDES monthly 2001–2007 Monthly observed values Average monthly VAPDES monthly 2001–2007 Monthly observed values Average monthly Periodic samples, n=1

— THG = 0.7 ng/L

Loth 

Spring

201-203 None

— Estimate = 25% of FPBS None

— Use FPBS average TSS = 9 mg/L None

— Use FPBS avg. THG = 4.1 ng/L

Invista Outfalls Data from Invista, DuPont, and VAPDES Data from Invista, DuPont, and VAPDES Data from DuPont and DEQ

001 211-213 Daily 

1998–2007

Daily 

observed values

Average monthly Periodic 

samples, n=45

— Base flow TSS average = 2.6 mg/L 

Stormflow TSS average = 3.4 mg/L 

Periodic 

samples, n=41

— Regression on Flow

003 231-233 Periodic 

measurements

— Regression on daily 

river flow

Periodic 

samples, n=31

— Base flow TSS average = 4.5 mg/L 

Stormflow TSS average = 8.9 mg/L

Periodic 

samples, n=40

— Base flow THG avg. = 41.2 ng/L

Stormflow THG avg. = 61.6 ng/L 

004 241-243 Periodic 

measurements

— Regression on daily 

river flow

Periodic 

samples, n=13

— Base flow TSS average = 1.1 mg/L

Stormflow TSS average = 25.7 mg/L

Periodic 

samples, n=8

— Base flow THG avg = 15.5 ng/L

Stormflow THG avg = 42.1 ng/L 

006 Ignored based on insignificant contribution Ignored based on insignificant contribution Ignored based on insignificant contribution008 281-283 Periodic 

measurements

— Regression on daily 

river flow

Periodic 

samples, n=26

— Base flow TSS average = 6.3 mg/L

Stormflow TSS average = 32 mg/L

Periodic 

samples, n=31

— Base flow THG avg. = 298.4 ng/L

Stormflow THG avg. = 134.9 ng/L 

009 291-293 Periodic 

measurements

— Base flow = 0 

Stormflow simulated 

by HSPF

Periodic 

samples, n=4

— No base flow 

Stormflow average TSS = 7.2 mg/L

Periodic 

samples, n=4

— No base flow 

Stormflow THG avg. = 81.9 ng/L

010 221-223 Periodic 

measurements

— Base flow = 0 

Stormflow simulated 

by HSPF

Periodic 

samples, n=3

— No base flow 

Stormflow average TSS = 46.8 mg/L

Periodic 

samples, n=4

— No base flow 

Stormflow THG avg. = 228.4 ng/L

011 271-273 Daily 1998–2002 

and overflow event monitoring 2005–2006

Daily 

observed values

Average monthly Periodic 

samples, n=28

— Base flow TSS average = 24.5 mg/L

Stormflow TSS average = 20.3 mg/L

Periodic 

samples, n=37

— Base flow THG avg. = 2,278 ng/L 

Stormflow THG avg. = 393 ng/L 

012 274-276 None

— Base flow = 0 

Stormflow simulated 

by HSPF

None

— Set = 010 conc. at given time 

and flow

None

— Set = 010 conc. at given time 

and flow

013 277-279 Overflow event 

monitoring

Individual 

overflow events

Average annual volume None

— Set = 001 conc. for stormflow events None

— Set = 001 conc. for stormflow events
Appendix 1 79

Appendix 1. Data sources used to define point-source loading.—Continued[ft3/s, cubic feet per second; mg, milligrams; VAPDES, Virginia Pollution Discharge Elimination System; %, percent; FPBS, Frew Pond Baker Spring; TSS, total suspended solids; SSC, suspended sedimentconcentration; Hg, mercury; ng/L, nanograms per liter; THG, total unfiltered mercury; mg/L, milligrams per liter; DEQ, Department of Environmental Quality; conc., concentration; GW, groundwater; K, hydraulic conductivity; Disch., discharge]

Point sources Discharge (ft3/s) Suspended solids concentration (TSS or SSC), mg/L Total unfiltered Hg concentration (THG), ng/L

Name Model ID Data source and type Model value assignment Data source and type Model value assignment Data source and type Model value assignment Periods 

with data Periods of no data Periods with data Periods of no data Periods with data Periods of no data

014 224-226 Overflow event 

monitoring

Individual 

overflow events

Average annual volume None — Set = 001 conc. for 

stormflow events

None — Set = 001 conc. for stormflow events

FPBS 204-206 Periodic 

monitoring

— Average plus seasonal 

variation of ± 29%

Periodic 

samples, n=3

— Average TSS = 9 mg/L Periodic 

samples, n=3

— Average THG = 4.1 ng/L

GW 

Disch.

207-209 Darcian flux 

GW levels (7 wells), slug test K (5 wells)

— Average flux None — Assume TSS = 0 Periodic 

samples, n=32

— Average THG = 6.9 ng/L

Wboro 

STP

261-263 VAPDES monthly 

2001–2007

Monthly 

observed values

Average monthly VAPDES 

monthly 2001–2007

Monthly 

observed values

Average monthly values Periodic 

samples, n=1

— THG = 7.6 ng/L

Genicom 301-303 VAPDES monthly 

2001–2007

Monthly 

observed values

Average monthly None — Assume TSS = 0 Periodic 

samples, n=1

— THG = 0.2 ng/L

Alcoa 501-503 VAPDES monthly 

2001–2007

Monthly 

observed values

Average monthly VAPDES 

monthly 2001–2007

Monthly 

observed values

Average monthly values Periodic 

samples, n=1

— THG = 18.3 ng/L
80 Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia

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phone: 804–261–2600

email: dc_va@usgs.gov

Or visit the Virginia Water Science Center Web site at:

http://va.water.usgs.gov

Eggleston—Mercury Loads in the South River and Simulation of Mercury TMDLs: Shenandoah Valley, Virginia—Scientific Investigations Report 2009–5076

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