Prepared in cooperation with the City of Durham

Water-Quality Characteristics, Trends, and Nutrient and 

Sediment Loads of Streams in the Treyburn Development 

Area, North Carolina, 1988–2009 79 ̊

78 ̊30'

35 ̊30'

36 ̊

PERSON

GRANVILLE

DURHAM

Smithfield

Clayton

Raleigh

Garner

Cary

Apex

Durham

Hillsborough

N.

S.

Flat

Knap

of Reeds

Neuse

River

Swift

Creek

Cr.

Flat

Little

Eno

Flat

River

River

River

River

River

Lake

Michie

Falls

Lake

Little River

Reservoir

Lake

Benson

Creek

ORANGE

WAKE

JOHNSTON

Scientific Investigations Report 2013–5062

U.S. Department of the Interior

U.S. Geological Survey
Cover: Map showing location of the Treyburn development and study area in the upper Neuse River Basin, 

North Carolina.
Water-Quality Characteristics, Trends, and 

Nutrient and Sediment Loads of Streams in 

the Treyburn Development Area, 

North Carolina, 1988–2009

By Jason M. Fine, Douglas A. Harned, and Carolyn J. Oblinger 

Prepared in cooperation with the City of Durham

Scientific Investigations Report 2013–5062

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: 2013

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Fine, J.M., Harned, D.A., and Oblinger, C.J., 2013, Water-quality characteristics, trends, and nutrient and sediment 

loads of streams in the Treyburn development area, North Carolina, 1988–2009: U.S. Geological Survey Scientific 

Investigations Report 2013–5062, 59 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Study Area..............................................................................................................................................3

Land Use.................................................................................................................................................3

Study Sites .............................................................................................................................................3

Data Collection...............................................................................................................................................4

Streamflow Data ...................................................................................................................................4

Water-Quality Data...............................................................................................................................4

Field and Laboratory Methods...................................................................................................4

Quality-Assurance Samples ......................................................................................................7

Streamflow Conditions................................................................................................................................12

Water-Quality Conditions ...........................................................................................................................16

Specific Conductance........................................................................................................................16

pH...........................................................................................................................................................19

Dissolved Oxygen................................................................................................................................19

Suspended Sediment .........................................................................................................................19

Metals ..................................................................................................................................................19

Nutrients...............................................................................................................................................22

Nitrogen.......................................................................................................................................22

Phosphorus.................................................................................................................................22

Pesticides.............................................................................................................................................25

Water-Quality Seasonality .........................................................................................................................28

Suspended Sediment .........................................................................................................................28

Dissolved Oxygen................................................................................................................................28

pH...........................................................................................................................................................28

Specific Conductance........................................................................................................................30

Metals ..................................................................................................................................................30

Nitrogen................................................................................................................................................30

Phosphorus..........................................................................................................................................36

Estimation of Nutrient and Sediment Loads and Yields ........................................................................38

Suspended-Sediment Loads and Yields .........................................................................................38

Nitrogen Loads and Yields ................................................................................................................42

Phosphorus Loads and Yields...........................................................................................................47

Trend Analysis .............................................................................................................................................51

Methodology........................................................................................................................................51

Trends in Water-Quality Data...........................................................................................................52

Mountain Creek (Site 6T)..........................................................................................................52

Little River Tributary (Site 8T) ..................................................................................................52

Little River Below Little River Tributary (Site 10TA) .............................................................54

Flat River (Site 5T)......................................................................................................................54

Flat River Tributary (Site 1T).....................................................................................................54

Summary........................................................................................................................................................56

References Cited..........................................................................................................................................57
iv

Figures

1. Map showing location of the Treyburn development and study area in the upper 

Neuse River Basin, North Carolina............................................................................................2

2. Map showing location of study sites in the vicinity of the Treyburn development 

area of North Carolina .................................................................................................................5

3. Graphs showing mean monthly streamflow for water years 1989–93; 1994–98; 

1999–2003; 2004–09, and long-term mean monthly streamflow (1925–2009) at 

Flat River(site 5T), North Carolina ............................................................................................13

4. Graph showing annual rainfall amounts (1988–2009) and long-term mean annual 

rainfall (1980–2009) measured at the National Oceanic and Atmospheric 

Administration site in Durham County, North Carolina ........................................................15

5. Graph showing instantaneous discharge measurements at Flat River tributary 

(site 1T) and Little River tributary (site 8T) in the vicinity of the Treyburn 

development study area, North Carolina, 1994–2009............................................................15

6. Graphs showing discharge at the Little River below Little River tributary (site 10TA), 

Flat River (site 5T), Mountain Creek (site 6T), and Flat River tributary (site 1T) sites 

in the vicinity of the Treyburn development study area, North Carolina...........................16

7. Boxplots showing variability in concentrations of dissolved-oxygen concentrations; 

pH; specific conductance; and suspended-sediment concentrations for selected 

sites in the vicinity of the Treyburn development study area, North Carolina, 

1988–2009 .....................................................................................................................................18

8. Boxplots showing variability in concentrations of dissolved ammonia; dissolved 

nitrite plus nitrate; total ammonia plus organic nitrogen; and total nitrogen for 

selected sites in the vicinity of the Treyburn development study area, 

North Carolina, 1988–2009.........................................................................................................24

9. Boxplots showing variability in concentrations of dissolved orthophosphate, and 

total phosphorus for selected sites in the vicinity of the Treyburn development 

study area, North Carolina, 1988–2009....................................................................................25

10. Boxplot showing monthly suspended-sediment concentrations for Little River 

(site 4T) in the vicinity of the Treyburn development study area, North Carolina, 

1988–2009 .....................................................................................................................................29

11. Boxplot showing monthly dissolved-oxygen concentrations for Flat River tributary 

(site 1T) in the vicinity of the Treyburn development study area, North Carolina, 

1988–2009 .....................................................................................................................................29

12. Boxplot showing monthly pH for Flat River tributary (site 1T) in the vicinity of the 

Treyburn development study area, North Carolina, 1988–2009 ..........................................31

13. Boxplot showing monthly specific conductance for Little River tributary (site 8T), 

North Carolina, 1994–2009.........................................................................................................31

14. Boxplots showing monthly dissolved nitrate nitrogen for Little River below Little River 

tributary (site 10TA), 1994–2009; Flat River (site 5T), 1988–2009; Mountain Creek 

(site 6T), 1988–2009; and Little River tributary (site 8T), 1994–2009, in the Treyburn 

development study area, North Carolina................................................................................32

15. Boxplots showing monthly dissolved ammonia nitrogen for Little River tributary 

(site 8T), 1994–2009, and Little River below Little River tributary (site 10TA), 

1995–2009, in the vicinity of the Treyburn development study area, North Carolina ......33

16. Boxplots showing monthly total ammonia plus organic nitrogen for Little River 

(site 4T), 1988–2009; Little River tributary (site 8T), 1994–2009; Little River below Little 

River tributary (site 10TA), 1995–2009; and Flat River (site 5T), 1988–2009, in the 

vicinity of the Treyburn development study area, North Carolina......................................34
v

17. Boxplots showing monthly total nitrogen for Little River (site 4T), 1988–2009; 

Mountain Creek (site 6T),1994–2009; Little River below Little River tributary 

(site 10TA), 1995–2009; and Flat River (site 5T), 1988–2009, in the vicinity of the 

Treyburn development study area, North Carolina...............................................................35

18. Boxplots showing monthly dissolved orthophosphate for Little River (site 4T), 

1988–2009, and Little River tributary (site 8T), 1994–2009, in the vicinity of the 

Treyburn development study area, North Carolina...............................................................36

19. Boxplots showing monthly total phosphorus for Little River (site 4T), 1988–2009; 

Little River tributary (site 8T), 1994–2009; Little River below Little River tributary 

(site 10TA), 1995–2009; and Flat River tributary (site 1T), 1988–2009, in the vicinity 

of the Treyburn development study area, North Carolina ...................................................37

20. Graph showing annual suspended-sediment yields for sites in the vicinity of the 

Treyburn development study area, North Carolina...............................................................42

21. Graph showing annual total nitrogen yields for sites in the vicinity of the Treyburn 

development study area, North Carolina................................................................................47

22. Graph showing annual total phosphorus yields for sites in the vicinity of the 

Treyburn development study area, North Carolina...............................................................51

23. Graph showing total nitrogen concentrations with LOESS smooth trend line for 

Mountain Creek (site 6T), 1988–2009, in the vicinity of the Treyburn development 

study area, North Carolina ........................................................................................................52

24. Graph showing suspended-sediment concentrations with LOESS smooth trend 

line for Mountain Creek (site 6T), 1988–2009, in the vicinity of the Treyburn 

development study area, North Carolina................................................................................53

25. Graph showing specific conductance with LOESS smooth trend line for Little River 

tributary (site 8T), 1994–2009, in the vicinity of the Treyburn development study 

area, North Carolina...................................................................................................................53

26. Graph showing dissolved nitrate concentration with LOESS smooth trend line for 

Little River below Little River tributary (site 10TA), 1996–2009, in the vicinity of the 

Treyburn development study area, North Carolina...............................................................54

27. Graph showing dissolved nitrite plus nitrate concentration with LOESS smooth 

trend line for Little River below Little River tributary (site 10TA), 1996–2009, in the 

vicinity of the Treyburn development study area, North Carolina......................................54

28. Graph showing total ammonia plus organic nitrogen concentration with LOESS 

smooth trend line for Little River below Little River tributary (site 10TA), 1996–2009, 

in the vicinity of the Treyburn development study area, North Carolina...........................55

30. Graph showing dissolved nitrite plus nitrate concentration with LOESS smooth 

trend line for Flat River (site 5T), 1994–2009, in the vicinity of the Treyburn 

development study area, North Carolina................................................................................55

29. Graph showing total nitrogen concentration with LOESS smooth trend line for 

Little River below Little River tributary (site 10TA), 1996–2009, in the vicinity of the 

Treyburn development study area, North Carolina...............................................................55

31. Graph showing total ammonia plus organic nitrogen concentration with LOESS 

smooth trend line for Flat River (site 5T), 1988–2009, in the vicinity of the Treyburn 

development study area, North Carolina................................................................................55

32. Graph showing suspended-sediment concentration with LOESS smooth trend line 

for Flat River (site 5T), 1988–2009, in the vicinity of the Treyburn development study 

area, North Carolina...................................................................................................................56

33. Graph showing specific conductance with LOESS smooth trend line for Flat River 

tributary (site 1T), 1988–2009, in the vicinity of the Treyburn development study 

area, North Carolina...................................................................................................................56
vi

Tables

1. Data-collection sites in the vicinity of the Treyburn development study area, 

North Carolina ...............................................................................................................................6

2. Concentrations of nutrients in replicate and blank quality-assurance samples 

collected in the vicinity of the Treyburn development study area, North Carolina...........8

3. Concentrations of metals in replicate and blank quality-assurance samples 

collected in the vicinity of the Treyburn development study area, North Carolina.........11

4. Summary statistics for physical properties and suspended sediment collected at 

six sites in the vicinity of the Treyburn development study area, North Carolina, 

1988–2009 .....................................................................................................................................17

5. Summary statistics for metals and minor elements collected at six sites in the 

vicinity of the Treyburn development study area, North Carolina, 1988–2009 .................20

6. Summary statistics for nutrients detected in stream samples collected at six sites 

in the vicinity of the Treyburn development study area, 1988–2009...................................23

7. Organic compounds analyzed by the U.S. Geological Survey National Water Quality 

Laboratory for the Treyburn development project, North Carolina....................................26

8. Concentrations of pesticides that exceed the laboratory reporting level in samples 

collected at three sites in the vicinity of the Treyburn development study area, 

North Carolina, 1988–2009.........................................................................................................28

9. Summary of significant variables and regression coefficients used to develop the 

predictive equations of loads for sites in the vicinity of the Treyburn development 

study area, North Carolina ........................................................................................................39

10. Estimated annual suspended-sediment loads and yields for water years 1987–2009 

at sites in the vicinity of the Treyburn development study area, North Carolina, 

including mean and median load and yield for period of record........................................41

11. Estimated annual nitrate plus nitrite loads and yields for water years 1987–2009 at 

sites in the vicinity of the Treyburn development study area, North Carolina, 

including mean and median load and yield for period of record........................................43

12. Estimated annual ammonia nitrogen loads and yields for water years 1987–2009 at 

sites in the vicinity of the Treyburn development study area, North Carolina, 

including mean and median load and yield for period of record........................................44

13. Estimated annual total ammonia plus organic nitrogen loads and yields for water 

years 1987–2009 at sites in the vicinity of the Treyburn development study area, 

North Carolina, including mean and median load and yield for period of record............45

14. Estimated annual total nitrogen loads and yields for water years 1987–2009 at sites 

in the vicinity of the Treyburn development study area, North Carolina, including 

mean and median load and yield for period of record .........................................................46

15. Estimated annual dissolved orthophosphorus loads and yields for water years 

1987–2009 at sites in the vicinity of the Treyburn development study area......................48

16. Estimated annual dissolved phosphorus loads and yields for water years 1987–2009 

at sites in the vicinity of the Treyburn development study area, North Carolina, 

including mean and median load and yield for period of record........................................49

17. Estimated annual total phosphorus loads and yields for water years 1987–2009 at 

sites in the vicinity of the Treyburn development study area, North Carolina, 

including mean and median load and yield for period of record........................................50
vii

Conversion Factors

Inch/Pound to SI

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 0.4047 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2)

Flow rate

cubic foot per second per 

square mile [(ft3

/s)/mi2]

0.01093 cubic meter per second per 

square kilometer [(m3/s)/km2]

inch per year (in/yr) 2.54 centimeter per year (cm/yr)

Mass

ton, short (2,000 pounds) 0.9072 megagram (Mg)

ton per square mile (ton/mi2

) 0.3503 megagram per square 

kilometer (Mg/km2

)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8

Abbreviations

DENR Department of Environment and Natural Resources

LOESS locally weighted scatterplot smoothing

NSW nutrient-sensitive waters

USGS U.S. Geological Survey

mg/L milligram per liter

μg/L microgram per liter

μS/cm at 25 °C microseimen per centimeter at 25 degrees Celsius

Water-Quality Characteristics, Trends, and Nutrient and 

Sediment Loads of Streams in the Treyburn Development 

Area, North Carolina, 1988–2009

By Jason M. Fine, Douglas A. Harned, and Carolyn J. Oblinger

Abstract 

Streamflow and water-quality data, including concentrations of nutrients, metals, and pesticides, were collected 

from October 1988 through September 2009 at six sites in the 

Treyburn development study area. A review of water-quality 

data for streams in and near a 5,400-acre planned, mixed-use 

development in the Falls Lake watershed in the upper Neuse 

River Basin of North Carolina indicated only small-scale 

changes in water quality since the previous assessment of data 

collected from 1988 to 1998. Loads and yields were estimated 

for sediment and nutrients, and temporal trends were assessed 

for specific conductance, pH, and concentrations of dissolved 

oxygen, suspended sediment, and nutrients.

Water-quality conditions for the Little River tributary and 

Mountain Creek may reflect development within these basins. 

The nitrogen and phosphorus concentrations at the Treyburn 

sites are low compared to sites nationally. The herbicides 

atrazine, metolachlor, prometon, and simazine were detected 

frequently at Mountain Creek and Little River tributary but 

concentrations are low compared to sites nationally.

Little River tributary had the lowest median suspendedsediment yield over the 1988–2009 study period, whereas 

Flat River tributary had the largest median yield. The yields 

estimated for suspended sediment and nutrients were low compared to yields estimated for other basins in the Southeastern 

United States.

Recent increasing trends were detected in total nitrogen concentration and suspended-sediment concentrations 

for Mountain Creek, and an increasing trend was detected in 

specific conductance for Little River tributary. Decreasing 

trends were detected in dissolved nitrite plus nitrate nitrogen, 

total ammonia plus organic nitrogen, sediment, and specific 

conductance for Flat River tributary. 

Water chemical concentrations, loads, yields, and trends 

for the Treyburn study sites reflect some effects of upstream 

development. These measures of water quality are generally 

low, however, compared to regional and national averages.

Introduction

The Treyburn development is a 5,400-acre planned, 

mixed-use development located in the Falls Lake watershed 

in the upper Neuse River Basin of North Carolina (fig. 1). 

The development began in 1986 and consists of residential, 

industrial, and recreational facilities. Development occupied 

0.71 percent of the land area of the watershed in 1986 and 

increased to 1.12 percent by 2005. The remainder of the land 

in the Treyburn development area is forest and farmland.

Three water-supply reservoirs lie just outside the Treyburn development boundaries—Lake Michie to the north, 

Falls Lake to the southeast, and Little River Reservoir to the 

west (fig. 1). Lake Michie and Little River are water supplies 

for Durham, and Falls Lake supplies water for Raleigh. Most 

of the streams in the Treyburn area are classified as WS-IV, 

which means they are in moderately to highly developed 

water-supply watersheds (North Carolina Department of Environment and Natural Resources, 2009). Much of the Treyburn 

development area also is classified as a water-quality critical 

area. A “critical area” is defined as a 0.5-mile- (mi) wide area 

extending from the normal pool elevation of a water-supply 

reservoir or from a water-supply intake on a river (North 

Carolina Department of Environment, Health, and Natural 

Resources, 1993). The WS-IV stream classification requires 

a 50-foot- (ft) wide buffer area on each side of perennial 

streams (North Carolina Division of Water Quality, 2011. In 

addition, no new landfills are allowed within the critical area, 

no new landfills can be located outside the critical area, no 

new sludge or petroleum-contaminated soils can be applied in 

the critical area, and a hazardous-material containment plan 

and structure(s) are required for new industries in the area. In 

the Treyburn development area, residential lots of 1 acre are 

allowed within the critical area, and 0.5-acre lots are allowed 

outside of the critical area. The North Carolina Division of 

Water Quality of the Department of Environment and Natural 

Resources (DENR) lists the control of urban stormwater and 

protection of reservoirs among the priority issues for this part 
2 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

79 ̊

Upper

Neuse River

Basin

LOCATION OF UPPER NEUSE RIVER BASIN IN NORTH CAROLINA

78 ̊30'

35 ̊30'

36 ̊

Neuse River

Basin

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

PERSON

GRANVILLE

DURHAM

Smithfield

Clayton

Raleigh

Garner

Cary

Apex

Durham

Hillsborough

N.

S.

Flat

Knap

of Reeds

Neuse

River

Swift

Creek

Cr.

Flat

Little

Eno

Flat

River

River

River

River

River

Lake

Michie

Falls

Lake

Little River

Reservoir

Lake

Benson

Creek

Base from digital files of:

U.S. Department of Commerce, Bureau of Census,

 1990 Precensus TIGER/Line Files-Political boundaries, 1991

Environmental Protection Agency, River File 3

U.S. Geological Survey, 1:100,000 scale 

ORANGE

WAKE

JOHNSTON

Treyburn study area

Treyburn development

Weather station—Precipitation

EXPLANATION

Figure 1. Location of the Treyburn development and study area in the upper Neuse River Basin, North Carolina.
Introduction 3

of the upper Neuse River Basin (North Carolina Department 

of Environment and Natural Resources, 2009). All streams in 

the upper Neuse watershed are classified as nutrient-sensitive 

waters (NSW) and are subject to special nutrient management 

(North Carolina Department of Environment and Natural 

Resources, 2009). 

The Treyburn development was designed to minimize 

adverse effects on water quality. Because of the size of the 

development and its proximity to the water-supply reservoirs, 

however, local resource managers need to be able to quantify 

the effects of ongoing land-use conversion on water quality. 

In response to this need, the U.S. Geological Survey (USGS), 

in cooperation with the city of Durham, began a study in 

February 1988 to determine water-quality characteristics of 

surface waters in and around the Treyburn area. Assessing 

water quality at a range of watershed scales and assisting local 

governments are among the primary activities that have been 

identified to meet the USGS mission (U.S. Geological 

Survey, 2007). 

Two previously published reports documented and 

analyzed hydrologic data collected in the Treyburn area. Garrett and Bales (1995) described the data collected at 17 sites 

that were monitored for all or part of February 1988 through 

September 1993. Oblinger and others (2002) discussed the 

collection and analysis of hydrologic and macroinvertebrate 

data that were collected by the USGS from July 1994 through 

September 1998 at six sites in the Treyburn area.

Purpose and Scope

The purpose of this report is to summarize water-quality 

concentration, loads, and trends of selected stream sites flowing in or near the Treyburn development. The extent to which 

land-use conversion has affected water quality at these sites 

also is described. Data used to characterize water quality, seasonality, trends, and loads were collected from October 1988 

through September 2009. 

Study Area

The Treyburn development is located 12 mi north of the 

city of Durham in the Falls Lake watershed (fig. 1). Durham 

and Raleigh are the principal municipalities in the area. The 

combined population of these cities increased 33 percent 

between 2000 and 2009 to about 618,000 (North Carolina 

Office of State Budget, Planning and Management, 2009). 

Three water-supply reservoirs lie just outside the Treyburn development boundary (fig. 1). Lake Michie and the 

Little River Reservoir supply water to the city of Durham. 

Falls Lake, the largest of the three reservoirs, supplies water 

to the city of Raleigh and other nearby municipalities. Most 

of the development is drained by the Little River between the 

Little River Reservoir and where the Little River discharges 

into the Eno River (fig. 1). Tributaries of the Flat River 

drain the eastern edge of the development. Runoff from the 

development reaches Falls Lake through the Eno and Flat Rivers. The development accounts for about 1 percent of the total 

drainage area of Falls Lake. Lake Michie receives no drainage 

from the development, and the Little River Reservoir receives 

only minor runoff.

The climate of the study area is characterized by hot, 

humid summers and mild winters. The mean monthly temperature ranges from about 40 degrees Fahrenheit (°F) in January to 79 °F in July. Precipitation averages about 46.5 inches 

per year (National Oceanic and Atmospheric Administration, 

1988–2009). The topography in the study area is gently sloping to moderately steep. The area is underlain primarily by 

slates as part of the Carolina Slate Belt. Granites underlie part 

of the Flat River Basin, and the soils in the area are predominantly well-drained, sandy loam. Part of the study area is 

underlain by sedimentary rocks of the Triassic Basin with 

generally poorly drained soils (U.S. Department of Agriculture, 1976). 

Land Use

Land use prior to the development of Treyburn in 1985 

was a combination of agricultural (15 percent) and mixed hard 

wood and pine forest (85 percent). Treyburn originally was 

planned to include commercial (45 percent) and residential 

(20 percent) development with completion planned for about 

2006 (Treyburn, Durham County, North Carolina, Zoning 

application for a mixed land use project, app. A, written commun., February 1986). 

By 1994, land use in Treyburn included residential, 

industrial, and recreational development (20 percent) with 

the remainder being mixed forest and abandoned agricultural 

lands (80 percent). Between 2006 and 2011, residential and 

industrial development increased to 24 percent (Brian Pointer, 

U.S. Geological Survey, oral commun., October 2011), with 

basinwide land use paralleling the same percentage of development. As of 2011, the 24-percent developed land area 

includes 1,250 acres of residential, industrial, and recreational 

land, a 210-acre golf course, and 4,300 acres of undeveloped 

forest and abandoned agricultural land (Brian Pointer, U.S. 

Geological Survey, oral commun., October 2011). Upstream 

from Treyburn in the Little and Flat River Basins, land use in 

1988 was forested (55 percent) and agricultural and pasture 

(38 percent); the remainder (7 percent) was developed (Childress and Bathala, 1997).

Study Sites

During the initial phase of the study (1988 through 1993; 

Garrett and Bales, 1995), 17 monitoring sites were active in or 

near the study area. As development at Treyburn progressed, 

the focus of the monitoring network was limited to sites in 

areas that were most affected by land-use changes. Six of the 

original 17 sites were selected to characterize the water-quality 

conditions associated with developed and undeveloped land 
4 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

in and near Treyburn (fig. 2; table 1). Flat River tributary 

(site 1T) drains an undeveloped area entirely within Treyburn that was initially planned for commercial land use. Data 

from site 1T provide a baseline for evaluating the effects of 

development. Little River tributary (site 8T) drains a relatively small area that is densely developed with single-family 

residences and a golf course. Data from site 10TA, Little 

River downstream from the Little River tributary, were used 

to characterize the water quality leaving Little River Reservoir 

and the forested and residential areas in the western part of 

Treyburn. Water quality at Little River (site 4T) and Mountain 

Creek (site 6T), both upstream from the Little River Reservoir, 

are influenced by upstream agricultural land uses. 

Site 6T on Mountain Creek was selected to characterize water quality from a moderately developed area where 

agricultural land use is being converted to residential land use. 

Mountain Creek is northwest of Treyburn and unaffected by 

that development. Data from Flat River (site 5T) were used 

to characterize water quality in a less developed area where 

agricultural land is being converted to forest (Childress and 

Bathala, 1997). Site 6T also is unaffected by the Treyburn 

development.

Data Collection

During the October 1998 through September 2009 study 

period, streamflow and water-quality data were collected at six 

sites in or near the Treyburn study area (fig. 2; table 1). Waterquality data included measurements of physical water-quality 

characteristics and analyses of concentrations of nutrients, 

metals, and pesticides.

Streamflow Data

Continuous streamflow records were collected at five 

sites (1T, 4T, 5T, 6T, and 10TA) to facilitate interpretation 

of water-quality data and to allow for calculation of nutrient 

loads. Stage was measured by using a pressure transducer and 

recorded at 15-minute intervals. Periodic measurements of 

stage and instantaneous streamflow were used to develop a 

stage-discharge relation for calculating streamflow from continuous stage record (Rantz and others, 1982). Instantaneous 

streamflow measurements were made following standard 

USGS methods described by Rantz and others (1982), Mueller and Wagner (2009), and Sauer and Turnipseed (2010). A 

gaging station could not be installed at site 8T because of its 

proximity to the Treyburn golf course. Thus, instantaneous 

streamflow measurements were made each time a waterquality sample was collected at this site.

Water-Quality Data

Water-quality samples were collected at six sites (1T, 

4T, 5T, 6T, 8T, and 10TA) at varying intervals, approximately 

eight times a year, and during several storm events during 

water years 1988–2009 (fig. 2; table 1). Water year is defined 

as the period from October 1 to September 30 and is designated by the year in which the period ends. For example, water 

year 1988 is October 1, 1987, to September 30, 1988. Samples 

were analyzed for nutrients and suspended sediment. Samples 

for analysis of pesticides generally were collected once per 

year. Samples for analysis of metals were collected three times 

per year.

Field and Laboratory Methods

Water-quality sample collection, handling, and analytical procedures were performed in accordance with methods 

described for the Triangle Area Water Supply Monitoring 

Project (Oblinger, 2004) and USGS standard procedures 

(U.S. Geological Survey, variously dated). Stream samples 

for inorganic analysis were collected by using the depth-integrated, equal-width increment method; composited in a polycarbonate churn splitter; and processed and preserved according to USGS standard operating procedures (Edwards and 

Glysson, 1988; Ward and Harr, 1990; Wilde and others, 1998). 

Water samples for analysis of dissolved constituents were 

filtered through a 0.45-micron pore-size membrane-capsule 

filter. Samples collected for pesticide analysis were collected 

in glass containers at midstream using a weighted open-mouth 

sampler or by hand as a grab sample. 

Water temperature, pH, specific conductance, and 

dissolved-oxygen concentration were measured in the field at 

the time of sample collection. Field instruments were calibrated, and results were documented on a daily basis as part of 

the USGS quality-assurance program. 

Chemical analyses were performed by the USGS 

National Water Quality Laboratory in Denver, Colorado, by 

using established methods (Wershaw and others, 1987; Britton 

and Greeson, 1989; Fishman and Friedman, 1989; Fishman, 

1993; Zaugg and others, 1995). Suspended-sediment concentrations were determined in the USGS sediment laboratories in 

Raleigh, North Carolina, and in Louisville, Kentucky, according to methods and procedures described in Guy (1969). Analytical procedures and reporting levels for chemical constituents in water analyzed by the USGS National Water Quality 

Laboratory during this study are described by Oblinger (2004).
Data Collection 5

Bahama

Willardville

Orange

Factory

Fairntosh

Mountain

Little

Little River

tributary

Flat 

 R. 

tributary

River

Little

River

Creek

Little River

Reservoir

Falls

Lake

Lake Michie

0 1 2 MILES

0 1 2 KILOMETERS

36 ̊07'30 ̈

36 ̊10'

36 ̊5'

78 ̊55' 78 ̊50'

River

Eno

Flat

River

5T

6T

4T

8T

10TA

1T

11T

Base from digital files of:

Environmental Protection Agency, River File 3

U.S. Geological Survey, 1:100,000 scale 

8T

4T

EXPLANATION

Treyburn development boundary

Continuous-record gaging station and number

Water-quality site and number

Figure 2. Location of study sites in the vicinity of the Treyburn development area of North Carolina.
6 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009 Table 1. Data-collection sites in the vicinity of the Treyburn development study area, North Carolina. [no., number; USGS, U.S. Geological Survey; mi2, square miles; (ft3/s)/mi2, cubic feet per second per square mile; S, streamflow; Q, water quality; SR, secondary road; —, no data] Site no. (fig. 2) USGS station numbera Site name Drainage area (mi2

)

Type of 

data 

collected

Land cover

Period of record

Mean annual runoff 

([ft3/s]/mi2

)

Mean 

annual 

runoff 

(inches)

Continuous 

streamflow 

record

Water 

quality

1T 0208650112 Flat River tributary near 

Willardville

1.14 S, Q Forested 3/1988–9/1990,

10/1994–9/2009

1988–1991,1994–2009

0.81 11.02

4T 0208521324 Little River at SR 1461 near 

Orange Factory

78.2 S, Q Forest and agricultural 1988–2009 1988–2009 0.901 12.25

5T 02085500 Flat River at Bahama 149 S, Q Mixed forest and agricultural 7/1925–9/2009 1988–1993,

1994–2009

0.95 12.91

6T 0208524090 Mountain Creek at SR 1617 

near Bahama

8.00 S, Q Mixed forest and agricultural 10/1994–9/2009 1988–1991,

1994–2009

0.87 11.63

8T 0208524950 Little River tributary at Fairntosh 0.86 Sb, Q Golf course and residential

—b 1994–2009

—b —b

10TA 0208524975 Little River below Little River 

tributary at Fairntosh

99.0 S, Q Mixed forest, agricultural, 

residential/reservoir outfall

10/1995–9/2009 1995–2009 0.65 8.79

aStation number is assigned by the U.S. Geological Survey on the basis of geographic location. The downstream order number system is used for surface-water sites.bStreamflow was measured only when a water-quality sample was collected.
Data Collection 

7

Quality-Assurance Samples

Two types of quality-assurance samples were collected—

replicates and equipment blanks (table 2). Replicate samples 

were collected by dividing the composite sample into two 

separate samples to determine the repeatability of sample 

analyses. Samples containing water certified as inorganic 

blank water were run through the sample equipment used for 

regular sampling onsite to determine if equipment cleaning or 

sample-processing procedures resulted in contamination by 

any of the compounds of interest. 

Nutrient concentrations in replicate samples are given 

in table 2. Replicate sample results were generally within 

0.01 milligram per liter (mg/L) of the associated environ-

mental sample results, indicating good analytical repeatabil-

ity. Metal concentrations in replicate samples are given in 

table 3. The replicate sample results for metals were all within 

5 percent of the environmental sample except for one alumi-

num sample, indicating good analytical repeatability. 

Blank samples were analyzed for nutrients (table 2) and 

metals (table 3). Concentrations in blank samples generally 

were at or below the reporting level, which was about an order 

of magnitude lower than the reporting level for environmental 

samples. Traces of nutrients and some metals (tables 2 and 3) 

were found in a small number of equipment blank samples. 

The levels found were near or below the reporting level for 

environmental samples for most of the samples. Levels that 

were detected would not affect the analysis of environmental 

samples. 
8 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009 Table 2. Concentrations of nutrients in replicate and blank quality-assurance samples collected in the vicinity of the Treyburn development study area, North Carolina.—Continued [N, nitrogen; P, phosphorus; <, less than; E, estimated; —, no data] Station number and site Sample type code Date Ammonia as N Ammonia plus organic nitrogen, as N Nitrite plus nitrate, as N Total phosphorus, as P Orthophosphate, as P 0208524090, 6T Replicate 4/6/1995 < 0.01 0.2 0.25 < 0.01 < 0.01 0208524090, 6T Replicate 4/6/1995 < 0.01 0.2 0.25 < 0.01 < 0.01 0208524090, 6T Replicate 3/21/2000 0.05 0.58 0.24 0.07 0.01 0208524090, 6T Replicate 3/21/2000 0.04 0.66 0.24 0.06 < 0.01 0208524090, 6T Replicate 6/4/2004 < 0.04 0.39 0.49 0.05 < 0.02 0208524090, 6T Replicate 6/4/2004 < 0.04 0.4 0.5 0.05 < 0.02 0208524090, 6T Replicate 12/11/2007 < 0.020 0.21 < 0.016 0.017 0.006 0208524090, 6T Replicate 12/11/2007 < 0.020 0.21 < 0.016 0.017 E 0.004 0208524090, 6T Replicate 2/18/2009 < 0.020 0.28 0.405 0.033 < 0.008 0208524090, 6T Replicate 2/18/2009 < 0.020 0.3 0.438 0.033 E 0.007 0208524090, 6T Replicate 3/21/2001 E 0.02 0.64 0.31 0.11 0.06 0208524090, 6T Replicate 3/21/2001 E 0.02 0.7 0.31 0.11 0.06 0208524090, 6T Replicate 3/4/2002 E 0.03 0.52 0.28 E 0.06 E 0.02 0208524090, 6T Replicate 3/4/2002 E 0.03 0.53 0.29 E 0.06 E 0.02 0208524090, 6T Replicate 9/23/2009 E 0.016 0.88 0.928 0.147 0.078 0208524090, 6T Replicate 9/23/2009 E 0.017 0.88 0.921 0.149 0.078 0208524975, 10Ta Replicate 5/31/2006 0.017 0.42 0.2 E 0.02 E 0.006 0208524975, 10Ta Replicate 5/31/2006 0.017 0.41 0.19 E 0.03 E 0.006 0208524975, 10Ta Replicate 10/2/2007 < 0.020 0.54 < 0.04 0.05 E 0.004 0208524975, 10Ta Replicate 10/2/2007 E 0.013 0.57 < 0.04 0.04 E 0.004

0208524975, 10Ta Replicate 7/6/2009 0.086 0.89 0.438 0.201 0.11

0208524975, 10Ta Replicate 7/6/2009 0.08 0.97 0.44 0.202 0.106

02085500, 5T Replicate 9/25/1990

— 0.4

— < 0.01

—

02085500, 5T Replicate 9/25/1990

— 0.4

— < 0.01

—

02085500, 5T Replicate 12/6/2000 < 0.04 0.23 < 0.05 < 0.06 < 0.02

02085500, 5T Replicate 12/6/2000 < 0.04 0.22 < 0.05 < 0.06 E 0.01

02085500, 5T Replicate 10/16/2002 0.022 0.76 0.51

— 0.02

02085500, 5T Replicate 10/16/2002 < 0.04 0.77 0.48 0.11 0.02

02085500, 5T Replicate 11/22/2004 < 0.04 0.27 0.32 E 0.02 < 0.02

02085500, 5T Replicate 11/22/2004 < 0.04 0.29 0.32 E 0.02 < 0.02
Data Collection 

9

Table 2. Concentrations of nutrients in replicate and blank quality-assurance samples collected in the vicinity of the Treyburn development study area, North Carolina.—Continued [N, nitrogen; P, phosphorus; <, less than; E, estimated; —, no data]

Ammonia plus Station number Nitrite plus nitrate, Total phosphorus, Orthophosphate, Sample type code Date Ammonia as N organic nitrogen, and site as N as P as P as N

0208650112, 1T Replicate 11/25/1996 0.02 < 0.20 < 0.05 < 0.01 < 0.01

0208650112, 1T Replicate 11/25/1996 0.02 < 0.20 0.08 < 0.01 0.020208650112, 1T Replicate 7/19/2001 < 0.04 0.16 0.3 < 0.06 < 0.02

0208650112, 1T Replicate 7/19/2001 < 0.04 0.14 0.3 < 0.06 < 0.02

0208650112, 1T Replicate 10/16/2002 < 0.04 0.49 < 0.06 E 0.03 < 0.020208650112, 1T Replicate 10/16/2002 < 0.04 0.49 < 0.06 E 0.03 < 0.020208650112, 1T Replicate 2/16/2006 < 0.04 0.12 < 0.06 < 0.04 < 0.02

0208650112, 1T Replicate 2/16/2006 < 0.04 0.33 < 0.06 < 0.04 < 0.02

0208650112, 1T Replicate 3/28/2008 < 0.020 E 0.12 < 0.016 0.012 E 0.003

0208650112, 1T Replicate 3/28/2008 < 0.020 E 0.12 < 0.016 0.012 E 0.003

0208650112, 1T Replicate 12/29/2008 < 0.020 0.16 < 0.016 0.014 E 0.007

0208650112, 1T Replicate 12/29/2008 < 0.020 0.15 < 0.016 0.015 E 0.008

0208524090, 6T Blank 2/17/2001 0.002 — < 0.005 — < 0.0070208650112, 1T Blank 3/21/2000 < 0.02 < 0.10 < 0.05 < 0.05 < 0.01

0208650112, 1T Blank 5/15/2000 < 0.02 < 0.10 < 0.05 < 0.05 < 0.01

0208650112, 1T Blank 9/11/2000 0.005 — < 0.005 — < 0.001Not applicable Blank 8/13/1996 < 0.002 — < 0.005 — < 0.001

Not applicable Blank 11/24/1996 < 0.002 — < 0.005 — < 0.001

Not applicable Blank 1/13/1997 < 0.002 — < 0.005 — < 0.001

Not applicable Blank 3/10/1997 < 0.002 — < 0.005 — < 0.001

Not applicable Blank 4/28/1997 < 0.002 — 0.005 — 0.003

Not applicable Blank 9/16/1997 < 0.002 — < 0.005 — < 0.001

Not applicable Blank 10/28/1997 < 0.002 — < 0.005 — 0.002

Not applicable Blank 3/9/1998 < 0.002 — < 0.005 — 0.001

Not applicable Blank 10/27/1998 < 0.002 — < 0.005 — 0.002

Not applicable Blank 4/27/1999 0.004 — 0.007 — 0.002

Not applicable Blank 1/3/2000 0.008 — 0.006 — < 0.001

Not applicable Blank 10/17/2000 0.006 — < 0.005 — < 0.007

Not applicable Blank 2/5/2002 < 0.015 — < 0.013 — < 0.007

Not applicable Blank 10/16/2002 < 0.015 — < 0.022 — < 0.007
10 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009 Table 2. Concentrations of nutrients in replicate and blank quality-assurance samples collected in the vicinity of the Treyburn development study area, North Carolina.—Continued [N, nitrogen; P, phosphorus; <, less than; E, estimated; —, no data] Ammonia plus Station number Nitrite plus nitrate, Total phosphorus, Orthophosphate, Sample type code Date Ammonia as N organic nitrogen, and site as N as P as P as N Not applicable Blank 12/18/2002 < 0.015

— < 0.022

— < 0.007

Not applicable Blank 5/16/2003 < 0.015

— < 0.022

— < 0.007

Not applicable Blank 6/26/2003 < 0.015

— < 0.022

— < 0.007

Not applicable Blank 7/30/2003 < 0.015

— < 0.022

— < 0.007

Not applicable Blank 2/20/2004 < 0.010

— < 0.016

— < 0.006

Not applicable Blank 8/30/2004 < 0.010

— < 0.016

— < 0.006

Not applicable Blank 10/6/2004 < 0.010

— < 0.016

— < 0.006

Not applicable Blank 12/22/2004 E 0.006

— E 0.011

— < 0.006

Not applicable Blank 10/1/2005 E 0.006

— < 0.016

— < 0.006

Not applicable Blank 2/16/2006 < 0.010

— < 0.016

— < 0.006

Not applicable Blank 10/18/2006 < 0.020

— < 0.016

— < 0.006

Not applicable Blank 10/3/2007 < 0.020

— < 0.016

— < 0.006

Not applicable Blank 10/2/2008 0.022

— < 0.016

— < 0.008

Not applicable Blank 4/10/2009 < 0.020

— < 0.016

— < 0.008
Data Collection 11

Table 3. Concentrations of metals in replicate and blank quality-assurance samples collected in the vicinity of the Treyburn development study area, North Carolina. [<, less than; E, estimated; —, no data]

Station number Sample type code Date Alumi- num Cad- mium Chromium Cobalt Copper Iron Lead Manga- nese Mercury Molyb- denum Nickel Silver Zinc

0208524090, site 6T Replicate 12/11/2007 15 < 0.01 E 0.26 < 0.04 < 1.2 41 E 0.04 15.6 < 0.010 < 0.1 0.23 < 0.02 < 2.0

0208524090, site 6T Replicate 12/11/2007 16 < 0.01 < 0.40 < 0.04 < 1.2 41 E 0.04 15.6 < 0.010 < 0.1 0.2 < 0.02 < 2.0

02085500, site 5T Replicate 9/25/1990 210 < 1.0 < 1 < 1.0 2 950 1 220 < 0.1 < 1 2 < 1.0 10

02085500, site 5T Replicate 9/25/1990 210 < 1.0 < 1 < 1.0 2 950 1 220 < 0.1 < 1 2 < 1.0 10

0208524090, site 6T Blank 2/17/2001 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 1

0208524950, site 8T Blank 4/2/2002 < 1.0 < 0.04 < 0.8 < 0.020 < 0.2 E 5 < 0.08 0.5 < 0.011 E 0.1 < 0.06 < 1.0 1.2

0208650112, site 1T Blank 5/15/2000 E 20 < 0.1 < 1.0 < 1.8 < 1.2 < 21 < 1.0 E 2.7 < 0.3 < 1 < 1.8 < 1.0 < 31

Not applicable Blank 11/24/1996 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 < 0.5

Not applicable Blank 1/13/1997 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 < 0.5

Not applicable Blank 10/28/1997 0.6 < 0.3 < 0.2 < 0.2 0.9 < 3 < 0.3 0.1 — < 0.2 < 0.5 < 0.2 0.7

Not applicable Blank 3/9/1998 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 0.0 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 0.7

Not applicable Blank 10/27/1998 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 < 0.5

Not applicable Blank 4/27/1999 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 < 0.5

Not applicable Blank 1/3/2000 0.4 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 39.3

Not applicable Blank 10/17/2000 < 0.3 < 0.3 < 0.2 < 0.2 < 0.2 < 3 < 0.3 < 0.1 — < 0.2 < 0.5 < 0.2 0.7

Not applicable Blank 2/5/2002 < 1.0 < 0.04 < 0.8 0.032 < 0.2 < 10 < 0.08 < 0.1 — < 0.2 < 0.06 < 1.0 1.2

Not applicable Blank 12/18/2002 < 1.6 < 0.04 < 0.8 0.02 < 0.2 < 10 < 0.08 < 0.2 — < 0.3 < 0.06 < 0.2 < 1.0

Not applicable Blank 7/30/2003 E 1.1 < 0.04 < 0.8 < 0.015 < 0.2 < 8 < 0.08 < 0.2 < 0.018 < 0.3 < 0.06 < 0.2 < 1.0

Not applicable Blank 2/20/2004 < 1.6 < 0.04 < 0.8 < 0.014 < 0.4 < 6 < 0.08 < 0.2 < 0.018 < 0.4 < 0.06 < 0.2 < 0.6

Not applicable Blank 10/6/2004 < 1.6 < 0.04 < 0.8 < 0.014 < 0.4 < 6 < 0.08 < 0.2 < 0.010 < 0.4 < 0.06 < 0.2 < 0.6

Not applicable Blank 12/22/2004 E 1.1 < 0.04 < 0.8 0.874 E 0.4 < 6 < 0.08 E 0.2 < 0.010 < 0.4 E 0.04 < 0.2 1.4

Not applicable Blank 10/1/2005 E 0.9 < 0.04 < 0.04 < 0.040 < 0.4 < 6 < 0.08 0.3 < 0.010 < 0.4 E 0.04 < 0.2 2.7

Not applicable Blank 10/18/2006 < 1.6 < 0.04 < 0.12 < 0.04 < 0.40 < 6 < 0.12 E 0.1 < 0.010 < 0.1 0.15 < 0.1 0.91

Not applicable Blank 10/3/2007 < 1.6 < 0.04 < 0.12 < 0.02 < 1.0 < 8 < 0.08 < 0.2 — < 0.2 < 0.20 < 0.1 < 1.8

Not applicable Blank 10/2/2008 < 4.0 < 0.02 < 0.12 < 0.02 < 1.0 < 4 < 0.06 < 0.2 < 0.010 < 0.02 < 0.12 < 0.008 < 2.0
12 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Streamflow Conditions

Streamflow conditions for water years 1988–2009 were 

compared to long-term mean discharge (1925–2009) at the 

Flat River stream gage (site 5T, fig. 3) to provide a context for 

assessing constituent loads. Higher than average streamflow 

occurred during 2003 when monthly means were more than 

double the normal monthly mean streamflow during most 

months of the year. Average or lower than average streamflow 

occurred during most of the 1999, 2001, 2002, 2006, 2007, 

and 2008 water years (at least 9 of 12 months; fig. 3). A major 

drought occurred during all or parts of 5 consecutive years 

(1998–2002; Weaver, 2005). Streamflow during 2000, 2004, 

and 2005 was near the long-term mean.

The mean annual runoff at each of the gaged sites ranged 

from 8.79 to 12.91 inches (in.), or 0.65 to 0.95 cubic foot per 

second per square mile ([ft3/s]/mi2; table 1). Estimates of runoff for Little River (site 10TA), Flat River tributary (site 1T), 

and Mountain Creek (site 6T) are based on 13 to 22 years of 

streamflow record. Mean annual runoff for Flat River (site 5T) 

is a long-term mean based on 84 years of record. Yearly 

precipitation data for a National Oceanic and Atmospheric 

Administration site in Durham is presented in figure 4. During 

several years, tropical cyclones (Hurricanes Fran in 1996; 

Hanna in 2008; and Faye in 2008) contributed almost 20 percent of the annual precipitation.

Continuous streamflow records were not collected at the 

Little River tributary (site 8T). Instantaneous measurements 

of discharge at this site were compared to the discharge record 

for the same date and time at Flat River tributary (site 1T, 

fig. 5). Flat River tributary has a similar drainage area (table 1) 

and is within less than 2 mi of the Little River tributary site, 

so rainfall reasonably was assumed to be similar (fig. 2). 

Hydrographs of discharge for each gaged site in the Treyburn 

development study area are displayed in figure 6. 
Streamflow Conditions 13

1,000

800

600

400

200

0

Mean monthly streamflow, in cubic feet per second

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

1,000

800

600

400

200

0

1989

1990

1991

1992

1993

1925–2009

EXPLANATION

1994

1995

1996

1997

1998

1925–2009

EXPLANATION

A. 1989–1993 water years

B. 1994–1998 water years

Figure 3. Mean monthly streamflow for water years A, 1989–93; B, 1994–98; C, 1999–2003; D, 2004–09, and long-term mean monthly 

streamflow (1925–2009) at Flat River (site 5T), North Carolina.
14 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

1,000

800

600

400

200

0

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

1,000

800

600

400

200

0

Mean monthly streamflow, in cubic feet per second

C. 1999–2003 water years

D. 2004–2009 water years

1999

2000

2001

2002

2003

1925–2009

EXPLANATION

2004

2005

2006

2007

2008

2009

1925–2009

EXPLANATION

Figure 3. Mean monthly streamflow for water years A, 1989–93; B, 1994–98; C, 1999–2003; D, 2004–09, and long-term mean monthly 

streamflow (1925–2009) at Flat River (site 5T), North Carolina.—Continued
Streamflow Conditions 15

Long-term

mean

(1980–2009)

65

60

55

50

45

40

35

30

25

20

15

10

5

0

Water Year

Rainfall, in inches

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

Figure 4. Annual rainfall amounts (1988–2009) and long-term mean annual rainfall (1980–2009) measured at the National Oceanic and 

Atmospheric Administration site in Durham County, North Carolina. Discharge, in cubic feet per second

Flat River tributary (site 1T)

Little River

tributary (site 8T)

Oct.

1994

Oct.

1996

Oct.

1998

Oct.

2000

Oct.

2002

Oct.

2004

Oct.

2006

Oct.

2008

100

10

0.01

0.1

1

Figure 5. Instantaneous discharge measurements at Flat River tributary (site 1T) and Little River tributary (site 8T) in the vicinity of the 

Treyburn development study area, North Carolina, 1994–2009.
16 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Flat River (site 5T)

Moutain Creek (site 6T)

Little River (site 10TA)

Flat River tributary (site 1T)

100,000

10,000

1,000

100

10

1

0.1

Mean daily discharge, in cubic feet per second

Oct.

1994

Oct.

1996

Oct.

1998

Oct.

2000

Oct.

2002

Oct.

2004

Oct.

2006

Oct.

2008

Oct.

1992

Oct.

1994

Oct.

1998

Oct.

1990

Oct.

1988

Oct.

1996

Oct.

2000

Oct.

2008

Oct.

2004

Oct.

2002

Oct.

2006

10,000

1,000

100

10

1

0.1

0.01

Oct.

1994

Oct.

1996

Oct.

1998

Oct.

2000

Oct.

2002

Oct.

2004

Oct.

2006

Oct.

2008

Oct.

1992

Oct.

1994

Oct.

1998

Oct.

1990

Oct.

1988

Oct.

1996

Oct.

2000

Oct.

2008

Oct.

2004

Oct.

2002

Oct.

2006

Figure 6. Discharge at the Little River below Little River tributary (site 10TA), Flat River (site 5T), Mountain Creek (site 6T), and Flat River 

tributary (site 1T) sites in the vicinity of the Treyburn development study area, North Carolina.

Water-Quality Conditions

Physical properties, suspended sediment, nutrients, 

metals, and pesticides were measured during the 1988–2009 

period of record in surface water collected at the six study 

sites. A summary of the results of these analyses is presented 

in this report to characterize water quality at each site and 

make comparisons among sites.

The Flat River tributary site (site 1T) reflects waterquality conditions unaffected by development and was used 

as a baseline for comparison with the other Treyburn sites. 

Water-quality data from a study of forested basins, considered 

representative of background conditions, in the North Carolina 

Piedmont by Caldwell (1992) were also used to characterize 

data from the study sites. Caldwell (1992) used two relatively 

undisturbed forested basins in the Piedmont to determine 

background concentrations of selected chemical constituents 

during both high- and low-flow conditions.

Specific Conductance

Specific conductance is a measure of the ability of water 

to conduct an electric current and is a function of the amount 

and type of ionic material dissolved in water. In forested 

basins in the North Carolina Piedmont, specific conductance 

generally ranges from 5 to 56 microsiemens per centimeter 

at 25 degrees Celsius (μS/cm; Simmons and Heath, 1982). 

The lowest median value of specific conductance in this study 

(68 μS/cm) occurred at the mostly forested Flat River tributary 

(site 1T) within a range for all study sites of 25 to 265 μS/cm 

(table 4). The highest median conductance occurred at the 

Little River tributary (site 8T; 158 μS/cm), which was significantly different from the other sites (Kruskal-Wallis and Tukey 

multiple comparison tests with a 95 percent confidence level 

(p=0.05)). The remaining sites had lower or similar specific 

conductance values (fig. 7) to the Flat River tributary (site 1T). 

These results are similar to the 1994–98 study-period analyses 
Water-Quality Conditions 17

Table 4. Summary statistics for physical properties and suspended sediment collected at six sites in the vicinity of the Treyburn 

development study area, North Carolina, 1988–2009.

[Median the same as 50th percentile; <, less than]

Statistic North Carolina ambient 

water-quality criteria 

Little River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

tributary 

(site 8T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Specific conductance (microsiemens per centimeter)

Minimum 35 37 41 44 29 25

25th percentile 62 84 114 70 65 55

Median Not applicable 77 98 158 80 76 68

75th percentile 91 112 197 94 87 84

Maximum 115 201 265 143 106 116

Number of analyses 78 164 142 135 190 169

pH (standard units)

Minimum 6 to 9 (except where lower pH 

occurs under natural conditions, 

such as in swamp waters

5.8 5.4 5.8 5.5 5.5 5.3

25th percentile 6.6 6.6 6.5 6.5 6.6 6.2

Median 6.9 6.9 6.8 6.8 6.9 6.6

75th percentile 7.2 7.2 7.1 7 7.1 6.9

Maximum 8.1 7.7 7.4 8 7.9 7.5

Number of analyses 77 164 141 134 186 164

Dissolved oxygen (milligrams per liter)

Minimum 4.6 5.8 5 4 3.3 2.2

25th percentile 4.0 (minimum instantaneous value) 8 8 7.3 7.2 7.3 7.9

Median 9.7 9 8.5 8.5 8.6 9.4

75th percentile 10.8 10.6 10.4 10.6 10.7 11

Maximum 16.4 14.1 14.6 15 17 14.8

Number of analyses 65 155 133 126 177 153

Suspended sediment (milligrams per liter)

Minimum 2 <1 1 3 1 1

25th percentile 6.5 5 7 4 7 5

Median Not applicable 28 10 13 10 13 12

75th percentile 127 33 41 20 68 42

Maximum 476 542 438 267 885 2,100

Number of analyses 53 159 142 135 163 158

a

North Carolina Department of Environment and Natural Resources, 2009.
18 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

A B

C D

Interquartile

range

20

15

10

5

0

Dissolved-oxygen concentration, in

milligrams per liter

4T 6T 8T

Station name

10TA 5T 1T 4T 6T 8T 10TA 5T 1T

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

pH, in standard units

Station name

4T 6T 8T

Station name

10TA 5T 1T 4T 6T 8T 10TA 5T 1T

Station name

300

250

200

150

100

50

0

Specific conductance, in microsiemens per 

centimeter at 25 degrees Celsius

Suspended-sediment concentration, in

milligrams per liter

EXPLANATION

95th percentile

75th percentile

Mean

50th percentile

25th percentile

5th percentile

10,000

1,000

100

10

1

Figure 7. Variability in concentrations of A, dissolved-oxygen concentrations; B, pH; C, specific conductance; and D, suspendedsediment concentrations for selected sites in the vicinity of the Treyburn development study area, North Carolina, 1988–2009.
Water-Quality Conditions 19

noted by Oblinger and others (2002). The higher specific 

conductance noted for the Little River tributary and Mountain 

Creek may be due to development or differences in geology 

within these basins. 

pH

The pH of water is fundamental to the nature of chemical reactions that occur in water. A lower limit of 6.0 pH units 

has been established by the North Carolina Department of 

Environment and Natural Resources (2007) as the criterion for 

waters used for fishing and recreation. Little variation in pH 

occurred among the study sites; however, median pH at Flat 

River tributary (site 1T) is significantly lower than the other 

sites. Median values for all the study sites ranged from 6.6 to 

6.9, and the range of minimum values was 5.3 to 5.8 (table 4; 

fig. 7). The Caldwell (1992) study of background conditions in 

the Piedmont reported pH values ranging from 5.0 to 7.5.

Dissolved Oxygen

Dissolved-oxygen concentrations ranged from 2.2 to 

17.0 mg/L (table 4; fig. 7). No significant difference in median 

dissolved-oxygen concentration was noted among the sites. 

A lower instantaneous limit of 4.0 mg/L dissolved oxygen 

is the criterion for protection of aquatic life (North Carolina 

Department of Environment and Natural Resources, 2007). 

The minimum instantaneous dissolved-oxygen concentrations 

at Flat River (site 5T) and Flat River tributary (site 1T) were 

below this criterion. Low dissolved-oxygen concentrations in 

the study basins generally are associated with extreme lowflow conditions.

Suspended Sediment

Excessive sedimentation is frequently identified as 

the cause for stream impairment and habitat degradation in 

North Carolina (North Carolina Division of Water Quality, 

2000). Suspended-sediment concentrations at the study sites 

(table 4; fig. 7) ranged from less than 1 mg/L at Mountain 

Creek (site 6T) to 2,100 mg/L at Flat River tributary (site 1T). 

Median concentrations ranged from 10 mg/L at Mountain 

Creek (site 6T) and at Little River (site 10TA) to 28 mg/L at 

Little River near Orange Factory (site 4T). The concentration and median ranges increased from those observed in the 

study by Oblinger and others (2002). No significant differences in median suspended-sediment concentrations were 

noted among the sites. The Little River site (site 10TA) was 

previously reported by Oblinger and others (2002) to have the 

smallest range in suspended-sediment concentrations during 

the 1994–98 study period, probably because the site is located 

downstream from a reservoir that traps some of the sediment. 

Although the range and the frequency of higher concentrations 

during the study period increased at the Little River site from 

those listed in the Oblinger and others study (2002), the range 

is the smallest among the study sites.

Metals 

Samples were analyzed for total concentrations of aluminum, arsenic, cadmium, cobalt, copper, iron, lead, manganese, 

mercury, molybdenum, nickel, selenium, silver, and zinc 

(table 5). Water-quality criteria for the protection of freshwater 

aquatic life for arsenic, cadmium, copper, iron, lead, manganese, mercury, nickel, selenium, silver, and zinc have been 

established by the North Carolina Department of Environment 

and Natural Resources (2007; table 5).

Total iron and manganese concentrations frequently 

exceeded North Carolina water-quality criteria. Oblinger and 

others (2002) and Garrett and Bales (1995) reported similar findings in earlier reviews. Median iron concentrations 

exceeded the 1,000-microgram per liter (μg/L) criterion at all 

sites except the Flat River tributary (site 1T) and the Little 

River tributary (site 8T). The 75th percentile for manganese 

concentrations exceeded the 200 μg/L criterion for all sites 

except the Flat River tributary (site 1T). Iron and manganese 

are common components of area geologic materials and soils.

The frequency at which concentrations of arsenic, cadmium, cobalt, copper, lead, mercury, molybdenum, nickel, 

selenium, silver, and zinc in water samples from the study area 

exceeded analytical reporting limits was generally low, typically not exceeding laboratory reporting levels. Concentrations 

of these metals typically were less than water-quality criteria 

(North Carolina Department of Environment and Natural 

Resources, 2007). The highest concentrations of arsenic and 

lead were in samples from Flat River (site 5T), Mountain 

Creek (site 6T), and Little River (site 4T); however, the maximum concentrations of arsenic and lead did not exceed criterion levels at any site. Arsenic and lead are naturally present in 

area soils. Mapped stream data from the Department of Energy 

National Uranium Resource Evaluation Program indicate high 

concentrations of arsenic and lead in stream sediment in the 

study area (Reid, 1993). 
20 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 5. Summary statistics for metals and minor elements collected at six sites in the vicinity of the Treyburn development study 

area, North Carolina, 1988–2009.—Continued 

[All units in micrograms per liter; median the same as 50th percentile; —, no data; <, less than, which varied throughout the study period]

Statistic North Carolina ambient 

water-quality criteriaa

Little River

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

tributary 

(site 8T)

Little River below 

Little River tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Aluminum

Minimum 50 10 30 — 100 20

25th percentile 100 90 200 — 300 100

Median 600 300 600 — 700 300

75th percentile Not applicable 1,400 900 900 — 1,300 1,000

Maximum 4,000 4,600 5,100 — 4,700 8,000

Number of analyses 40 60 50 — 20 60

Arsenic

Minimum < 1 < 0.6 < 1 — < 1 < 1

25th percentile < 1 < 1 < 1.9 — < 1 < 1

Median < 5 < 1.9 1 — < 2 < 1.9

75th percentile 50 0.75 < 5 2 — < 5 < 5

Maximum 11 14 5 — 15 11

Number of analyses 55 69 52 — 39 81

Cadmium 

Minimum < 0.04 < 0.014 < 0.04 — < 0.1 < 0.014

25th percentile < 0.11 < 0.22 < 0.11 — < 1 < 0.22

Median < 1 < 1 < 1 — < 1 < 1

75th percentile 2 < 5 < 1 < 1 — < 5 < 5

Maximum 1 0.61 0.066 — 1 3

Number of analyses 58 69 52 — 42 81

Chromium

Minimum < 0.4 < 0.4 < 0.8 — < 0.8 < 0.8

25th percentile < 1 < 1 < 1 — < 1 < 1

Median 0.35 < 1 0.28 — < 5 < 5

75th percentile 50 2.4 0.57 0.91 — 2 1.2

Maximum 130 18 3.9 — 30 16

Number of analyses 58 69 52 — 42 81

Cobalt

Minimum < 1 <0 .04 < 1 — < 1 < 1

25th percentile < 2.4 < 1 < 1 — < 1 < 1

Median 1 < 3.4 < 3.4 — 1 < 3.6

75th percentile Not applicable 1.7 1.5 0.72 — 1.5 1

Maximum 7.1 6.2 3 — 3 5.5

Number of analyses 25 62 52 — 24 63

Copper

Minimum < 1 < 1 1 — < 1 < 1

25th percentile < 50 < 50 2.2 — < 50 < 50

Median 1.2 1.4 4 — 2 1.6

75th percentile 7 3.8 3 5.3 — 4 3

Maximum 39 76 10 — 81 29

Number of analyses 58 69 52 — 42 81

Iron

Minimum 360 40 270 — 480 210

25th percentile 750 770 680 — 930 420

Median 1,200 1,100 860 — 1,300 690

75th percentile 1,000 2,200 2,200 1,400 — 2,100 1,500

Maximum 8,000 8,000 4,700 — 8,300 13,000

Number of analyses 58 68 52 — 42 81
Water-Quality Conditions 21

Table 5. Summary statistics for metals and minor elements collected at six sites in the vicinity of the Treyburn development study 

area, North Carolina, 1988–2009.—Continued 

[All units in micrograms per liter; median the same as 50th percentile; —, no data; <, less than, which varied throughout the study period]

Statistic North Carolina ambient 

water-quality criteriaa

Little River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

tributary 

(site 8T)

Little River below 

Little River tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Lead

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

25

< 1 

< 5 

0.51

2.2

14

58

< 1 

< 5 

1

2.8

16

69

< 1

< 1

1

1.5

5

52

—

—

—

—

—

—

< 1 

< 5 

1

3

21

42

< 1

< 1

0.11

1.1

9

81

Manganese

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

200

10

49

94

350

1,400

58

10

80

200

460

3,500

69

60

110

150

260

610

52

—

—

—

—

—

—

20

75

170

270

860

42

10

30

50

100

570

79

Mercury

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

0.012

< 0.01

< 0.1

< 0.1

< 0.2

0.1

62

< 0.01

< 0.1

< 0.1

< 0.2

1.9

70

< 0.01

< 0.1

< 0.1

0.01

0.52

52

—

—

—

—

—

—

< 0.01

< 0.1

< 0.2

< 0.2

0.2

43

< 0.01

< 0.1

< 0.1

< 0.2

0.1

80

Nickel

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

88

< 1 

< 10 

0.7

1.8

80

56

< 1

< 1

< 2

1

20

68

< 1

< 1.8

0.41

1

4.5

51

—

—

—

—

—

—

< 1

< 5

1

3

50

40

< 1

< 1.8

0.36

1.2

30

76

Selenium

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

5

< 1

< 1

0.08

0.15

1.4

40

< 0.4

< 1

< 1

< 2.6

0.37

62

< 0.4

< 1

< 2

0.1

0.64

52

—

—

—

—

—

—

 < 1

< 1

< 1

< 2

0.24

24

< 0.12

< 1

< 1

< 2.6

1.1

63

Silver

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

0.06

< 1

< 1 

< 1

< 5

0.14

41

< 0.016

< 0.30

< 1

< 1

5

67

< 0.016

< 0.16

< 0.43

< 1

2

52

—

—

—

—

—

—

< 0.16

< 1 

< 1 

< 5

0.02

40

< 0.016

< 0.43

< 1

< 1

0.033

77

Zinc

Minimum

25th percentile

Median

75th percentile

Maximum

Number of analyses

50

< 2

< 10

< 50

10

340

58

< 2

< 10

< 31

6.8

44

68

< 2

< 10

1

10

36

51

—

—

—

—

—

—

< 10

< 10

< 50

11

37

42

< 2

< 10

 < 31

2.3

40

80 a

North Carolina Department of Environment and Natural Resources, 2009.
22 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Nutrients

Samples from the study sites were analyzed for total 

ammonia plus organic nitrogen, dissolved nitrite plus nitrate, 

dissolved nitrate, dissolved ammonia, total phosphorus, and 

dissolved orthophosphate. Samples from the relatively undisturbed, forested Flat River tributary (site 1T) had the smallest 

median concentrations and ranges for these nutrients (table 6). 

Nitrogen

Median total ammonia plus organic nitrogen concentrations for the Treyburn study sites ranged from 0.21 mg/L at the 

Flat River tributary (site 1T) to 0.52 mg/L at the Little River 

below Little River tributary (site 10TA; table 6; fig. 8). Median 

concentrations of nitrite plus nitrate ranged from 0.06 mg/L at 

the Flat River tributary (site 1T) to 0.31 mg/L at the Mountain 

Creek site (site 6T; table 6; fig. 8). Caldwell (1992) reported 

that background Piedmont concentrations of nitrite plus nitrate 

measured in forested or undeveloped basins ranged from less 

than 0.10 to 0.44 mg/L. Median ammonia concentrations for 

the treyburn study sites ranged from less than 0.04 mg/L at the 

Flat River Tributary site (site 1T) to 0.04 mg/L at Little River 

below Little River tributary (site 10TA; table 6; fig. 8). The 

Caldwell (1992) study reported that ammonia concentrations 

in forested or undeveloped basins ranged from 0.01 to 0.08 

mg/L. Median total nitrogen concentrations for the Treyburn 

sites ranged from 0.39 mg/L at Flat River tributary (site 1T) 

to 0.88 mg/L at Little River (site 4T; table 6; fig. 8). Background total nitrogen concentrations (Caldwell, 1992) ranged 

from less than 0.02 to 1.5 mg/L. The median values for total 

ammonia plus organic nitrogen, dissolved ammonia, dissolved 

nitrite plus nitrate, and total nitrogen concentration for the 

least developed site, Flat River tributary (site 1T), were lower 

than the median concentrations for the other sites.

Phosphorus

Median total phosphorus concentrations ranged from 

0.02 mg/L at the Flat River tributary (site 1T) to 0.06 mg/L at 

the Little River (site 4T; table 6; fig. 9). Background Piedmont 

total phosphorus concentrations reported by Caldwell (1992) 

ranged from less than 0.01 to 0.11 mg/L. The U.S. Environmental Protection Agency recommended limit of 0.1 mg/L 

for total phosphorus concentrations in streams was exceeded 

at Little River tributary (site 8T) at the 75-percent frequency 

interval and at all sites at the 95-percent frequency interval. 

However, phosphorus concentrations for the Treyburn sites 

generally were low compared to sites nationally (Harned and 

others, 1995; Mueller and others, 1995; Oblinger and others, 2002). Median orthophosphate concentrations ranged 

from less than 0.018 mg/L at Flat River tributary (site 1T) 

to 0.02 mg/L at the Little River site (site 4T; table 6; fig. 9). 

Orthophosphate generally is present in small concentrations 

because it is readily bioavailable.
Water-Quality Conditions 23

Table 6. Summary statistics for nutrients detected in stream samples collected at six sites in the vicinity of the Treyburn development 

study area, 1988–2009. 

[All units in milligrams per liter; median the same as 50th percentile; <, less than which, varied throughout the study period]

Statistic North Carolina ambient 

water-quality criteriaa

Little River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

tributary 

(site 8T)

Little River below 

Little River tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

 Total ammonia plus organic nitrogen

Minimum < 0.18 < 0.20 < 0.20 0.28 0.13 < 0.10

25th percentile 0.38 0.29 0.28 0.44 0.30 0.12

Median 0.50 0.34 0.42 0.52 0.40 0.21

75th percentile Not applicable 0.85 0.60 0.78 0.62 0.60 0.40

Maximum 1.97 1.97 4.54 1.44 2.10 1.41

Number of samples 60 164 143 135 169 158

Nitrite plus nitrate

Minimum 0.04 0.02 < 0.04 0.03 < 0 .016 < 0.016

25th percentile 0.17 0.20 0.07 0.14 0.13 < 0 .06

Median 10 (nitrate) 0.26 0.31 0.13 0.23 0.23 0.06

75th percentile 0.43 0.42 0.31 0.32 0.32 0.10

Maximum 0.70 0.76 1.15 1.10 0.80 0.51

Number of samples 34 145 143 135 143 134

Ammonia

Minimum < 0.020 < 0.015 < 0.015 < 0 .015 < 0.002 < 0.015

25th percentile < 0 .02 < 0 .02 < 0 .04 0.02 < 0.04 < 0.02

Median 0.01 0.01 0.03 0.04 0.02 < 0.04

75th percentile Not applicable 0.02 0.04 0.04 0.06 0.04 0.01

Maximum 0.13 0.32 1.95 0.27 0.14 0.12

Number of samples 34 145 143 135 143 134

Total nitrogen

Minimum 0.36 0.31 0.19 0.38 0.26 0.12

25th percentile 0.67 0.61 0.41 0.61 0.52 0.26

Median 0.88 0.77 0.75 0.76 0.67 0.39

75th percentile Not applicable 1.22 0.97 1.22 0.92 0.98 0.59

Maximum 2.32 2.34 5.27 2.10 2.48 1.44

Number of samples 56 148 121 129 145 80

Total phosphorus

Minimum 0.01 < 0 .01 < 0.10 < 0.01 < 0.01 < 0.01

25th percentile 0.03 0.02 0.01 0.03 0.02 < 0.05

Median 0.06 0.04 0.04 0.04 0.04 0.02

75th percentile Not applicable 0.18 0.08 0.15 0.07 0.07 0.04

Maximum 0.59 0.67 0.87 0.45 0.43 0.23

Number of samples 58 164 143 135 168 159

Orthophosphate

Minimum 0.01 < 0.006 < 0 .01 < 0.006 < 0.006 <0 .001

25th percentile 0.01 < 0 .018 < 0.018 < 0.018 < 0.018 < 0 .01

Median 0.02 0.01 0.01 0.01 0.01 < 0 .018

75th percentile Not applicable 0.03 0.02 0.07 0.02 0.02 0.01

Maximum 0.08 0.18 0.76 0.32 0.18 0.12

Number of samples 31 145 143 135 140 134

a

North Carolina Department of Environment and Natural Resources, 2009.
24 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

2.0

1.5

1.0

0.5

0

Dissolved ammonia, in milligrams per liter as nitrogen

1.25

1.00

0.75

0.50

0.25

0

Dissolved nitrite plus nitrate concentration,

in milligrams per liter

5

4

3

2

1

0

Total ammonia plus organic nitrogen concentration,

in milligrams per liter

4T 6T 8T

Station name

10TA 5T 1T 4T 6T 8T

Station name

10TA 5T 1T

4T 6T 8T

Station name

4T 6T 8T 10TA 5T 1T

Station name

10TA 5T 1T

A B

C

6

5

4

3

2

1

0

Total nitrogen concentration, in

milligrams per liter

D

EXPLANATION

95th percentile

75th percentile

Mean

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 8. Variability in concentrations of A, dissolved ammonia; B, dissolved nitrite plus nitrate; C, total ammonia plus organic nitrogen; 

and D, total nitrogen for selected sites in the vicinity of the Treyburn development study area, North Carolina, 1988–2009.
Water-Quality Conditions 25

A

0.8

0.6

0.4

0.2

0

4T 6T 8T 10TA 5T 1T

Dissolved orthophosphate, in milligrams per liter

Station name

B

1.0

0.8

0.6

0.4

0.2

0

4T 6T 8T 10TA 5T 1T

Total phosphorus, in milligrams per liter

Station name

EXPLANATION

95th percentile

75th percentile

Mean

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 9. Variability in concentrations of A, dissolved orthophosphate, and B, total phosphorus for selected sites in the vicinity of the 

Treyburn development study area, North Carolina, 1988–2009.

Pesticides

Water samples for analysis of selected pesticides were 

collected twice a year until 1998 and, thereafter, approximately once a year at Mountain Creek (site 6T), Little River 

tributary (site 8T), and Flat River tributary (site 1T). The 12 

most commonly detected pesticides of the 120 compounds 

tested (table 7) are listed in table 8. 

Little River tributary (site 8T) had the greatest number 

of pesticides detected and generally the greatest frequency of 

detections and highest concentrations (table 8). As noted in the 

previous study of Treyburn (Oblinger and others, 2002), the 

most commonly detected pesticides were herbicides typically 

used to control weeds in turf for homes and golf courses.

Four herbicides (atrazine, metolachlor, prometon, 

and simazine) were frequently detected at Mountain Creek 

(site 6T) and Little River tributary (site 8T). The herbicide 

pendimethalin was frequently detected at the Little River 

tributary (site 8T). Three herbicides (atrazine, metolachlor, and 

alachlor) were detected at the Flat River tributary (site 1T). 

Herbicide concentrations at these sites were low compared to 

sites nationally (Oblinger and others, 2002; Gilliom and 

others, 2007).
26 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 7. Organic compounds analyzed by the U.S. Geological Survey National Water Quality Laboratory for the Treyburn development 

project, North Carolina.—Continued

[Reporting level, in micrograms per liter, may have changed throughout the duration of the project. —, not applicable]

Compound name CAS numbera Reporting level

2,4-D 94-75-7 0.06

2,4-D methyl ester 1928-38-7 0.2

2,4-D plus 2,4-D methyl ester — 0.06

2,4-DB 94-82-6 0.02

2,6-Diethylaniline 579-66-8 0.0060

2-Chloro-4-isopropylamino-6-amino-s-triazine (CIAT) 6190-65-4 0.06

2-Chloro-6-ethylamino-4-amino-s-triazine (CEAT) 1007-28-9 0.08

2-Hydroxy-4-isopropylamino-6-ethylamino-s-triazine (OIET) 2163-68-0 0.06

3(4-Chlorophenyl)-1-methyl urea 5352-88-5 0.10

3-Hydroxycarbofuran 16655-82-6 0.06

Acetochlor 34256-82-1 0.010

Acifluorfen 50594-66-6 0.08

Alachlor 15972-60-8 0.008

Aldicarb 116-06-3 0.12

Aldicarb sulfone 1646-88-4 0.08

Aldicarb sulfoxide 1646-87-3 0.08

Aldrin 309-00-2 0.013

alpha-Endosulfan 959-98-8 0.012

alpha-HCH 319-84-6 0.0040

Atrazine 1912-24-9 0.008

Azinphos-methyl 86-50-0 0.12

Bendiocarb 22781-23-3 0.04

Benfluralin 1861-40-1 0.014

Benomyl 17804-35-2 0.06

Bensulfuron-methyl 83055-99-6 0.06

Bentazon 25057-89-0 0.06

Bromacil 314-40-9 0.06

Bromoxynil 1689-84-5 0.12

Butylate 2008-41-5 0.0040

Caffeine 58-08-2 0.08

Carbaryl 63-25-2 0.04

Carbofuran 1563-66-2 0.04

Carbophenothion 786-19-6 0.02

Chloramben, methyl ester 7286-84-2 0.20

Chlordane, technical mix 57-74-9 0.1

Chlorimuron-ethyl 90982-32-4 0.08

Chlorpyrifos 2921-88-2 0.02

cis-Permethrin 61949-76-6 0.010

Clopyralid 1702-17-6 0.14

Cyanazine 21725-46-2 0.022

Cycloate 1134-23-2 0.04

Dacthal 1861-32-1 0.0076

Dacthal monoacid 887-54-7 0.04

Desulfinylfipronil — 0.012

Desulfinylfipronil amide — 0.029

Diazinon 333-41-5 0.0060

Dicamba 1918-00-9 0.06

Dichlorprop 120-36-5 0.04

Dieldrin 60-57-1 0.008

Dinoseb 88-85-7 0.04

Diphenamid 957-51-7 0.04

Disulfoton 298-04-4 0.02

Diuron 330-54-1 0.04

Endrin 72-20-8 0.012

EPTC 759-94-4 0.0056

Ethalfluralin 55283-68-6 0.006

Ethion 563-12-2 0.018

Ethoprophos 13194-48-4 0.016

Fenuron 101-42-8 0.06

Fipronil 120068-37-3 0.018

Fipronil sulfide 120067-83-6 0.012
Water-Quality Conditions 27

Table 7. Organic compounds analyzed by the U.S. Geological Survey National Water Quality Laboratory for the Treyburn development 

project, North Carolina.—Continued

[Reporting level, in micrograms per liter, may have changed throughout the duration of the project. —, not applicable]

Compound name CAS numbera Reporting level

Fipronil sulfone

Flumetsulam

Fluometuron

Fonofos

Heptachlor

Heptachlor epoxide

Imazaquin

Imazethapyr

Imidacloprid

Lindane

Linuron

Malathion

MCPA

MCPB

Metalaxyl

Methiocarb

Methomyl

Metolachlor

Metribuzin

Metsulfuron methyl

Mirex

Molinate

Napropamide

Neburon

Nicosulfuron

Norflurazon

Oryzalin

Oxamyl

p, p’-DDD

p, p’-DDE

p, p’-DDT

p, p’-Methoxychlor

Parathion

Parathion-methyl

Pebulate

Pendimethalin

Phorate

Picloram

Polychlorinated biphenyls, total

Prometon

Propachlor

Propanil

Propargite

Propham

Propiconazole

Propoxur

Propyzamide

Siduron

Simazine

Sulfometuron-methyl

Tebuthiuron

Terbacil

Terbufos

Thiobencarb

Toxaphene

Tri-allate

Tribufos

Triclopyr

Trifluralin

120068-36-2

98967-40-9

2164-17-2

944-22-9

76-44-8

1024-57-3

81335-37-7

81335-77-5

138261-41-3

58-89-9

330-55-2

121-75-5

94-74-6

94-81-5

57837-19-1

2032-65-7

16752-77-5

51218-45-2

21087-64-9

74223-64-6

2385-85-5

2212-67-1

15299-99-7

555-37-3

111991-09-4

27314-13-2

19044-88-3

23135-22-0

72-54-8

72-55-9

50-29-3

72-43-5

56-38-2

298-00-0

1114-71-2

40487-42-1

298-02-2

1918-02-1

1336-36-3

1610-18-0

1918-16-7

709-98-8

2312-35-8

122-42-9

60207-90-1

114-26-1

23950-58-5

1982-49-6

122-34-9

74222-97-2

34014-18-1

5902-51-2

13071-79-9

28249-77-6

8001-35-2

2303-17-5

78-48-8

55335-06-3

1582-09-8

0.024

0.08

0.04

0.0048

0.008

0.009

0.10

0.08

0.08

0.0040

0.04

0.1

0.04

0.2

0.04

0.04

0.12

0.020

0.012

0.14

0.006

0.0040

0.008

0.02

0.32

0.04

0.04

0.12

0.016

0.002

0.01

0.002

0.02

0.008

0.016

0.012

0.019

0.10

0.1

0.012

0.006

0.010

0.020

0.040

0.038

0.06

0.0036

0.04

0.006

0.06

0.028

0.024

0.018

0.016

1

0.0046

0.02

0.08

0.018 a

CAS Registry Number® is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the CASRNs through CAS 

Client Services.
28 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 8. Concentrations of pesticides that exceed the laboratory reporting level in samples collected at three sites in the vicinity of 

the Treyburn development study area, North Carolina, 1988–2009. 

[Units are in micrograms per liter; <, less than]

Constituent Reporting limit 

concentration

Mountain Creek 

(site 6T)

Little River tributary 

(site 8T)

Flat River tributary 

(site 1T)

Maximum 

concentration

Number of 

detections/

number of 

samples

Maximum 

concentration

Number of 

detections/

number of 

samples

Maximum 

concentration

Number of 

detections/

number of 

samples

Herbicides

2, 4-D <0.01 <0.01 0 / 16 <0.01 0 / 17 <0.01 0 / 16

Alachlor <0.1 0.005 1 / 16 0.008 1 / 17 0.01 3 / 16

Atrazine <0.002 0.12 15 / 16 1.37 14 / 17 0.011 10 / 16

Benfluralin <0.002 <0.01 0 / 14 0.014 4 / 15 <0.01 0 / 15

Metolachlor <0.002 0.022 9 / 14 0.023 7 / 15 0.02 6 / 15

Metribuzin <0.004 <0.028 0 / 14 0.011 1 / 15 <0.028 0 / 15

Pendimethalin <0.004 0.045 2 / 14 1.37 10 / 15 <0.022 0 / 15

Promaton <0.01 0.017 10 / 14 0.014 7 / 15 <0.018 0 / 15

Simazine <0.005 0.101 8 / 14 6.7 15 / 15 <0.011 1 / 15

Trifluralin <0.002 <0.009 0 / 14 0.011 5 / 15 <0.009 0 / 15

Insecticides

Chlorpyrifos <0.01 <0.14 0 / 18 0.05 2 / 16 <0.014 0 / 15

Dissolved chlorpyrifos <0.004 <0.005 0 / 14 0.023 4 / 15 <0.006 0 / 15

Water-Quality Seasonality

Water chemistry varies with season. Many factors influence this variation, including seasonal differences in streamflow, temperature, basin land use, and biological activity. 

Seasonal variation in concentration of chemical constituents 

is an important factor to account for in trend analysis and in 

monitoring for resource management.

Suspended Sediment

Monthly distributions of suspended-sediment concentrations at several Treyburn sites showed seasonality. Little River 

(site 4T), Mountain Creek (site 6T), Flat River (site 5T), and 

Eno River (site 11T) showed particularly low suspendedsediment concentrations in June and July, with higher concentrations during the spring and fall. Eno River (site 11T, fig. 1) 

is included for comparison because it includes data from an 

earlier (1983–99) pre-study time period. December and January suspended-sediment concentrations were low compared to 

spring and fall concentrations at the Little River (site 4T), Flat 

River (site 5T), and Flat River tributary (site 1T) sites. These 

seasonal patterns are most evident for Little River (site 4T,

 fig. 10). Possible causes for these seasonal differences may 

include influences on sediment transport due to decreased 

erosion caused by leaf cover in the summer and land-surface 

disturbance due to agricultural or construction activities in the 

spring and increased source contribution of organic sediment 

from leaf litter in the fall. Frozen ground in winter may reduce 

sediment transport.

Dissolved Oxygen

Dissolved-oxygen concentrations showed a strong seasonal pattern at all the Treyburn study sites. Dissolved-oxygen 

concentrations generally were highest in February, ranging 

from 10 to 15 mg/L, and were lowest in July and August, typically ranging from 5 to 9 mg/L. An example of this seasonal 

pattern is shown in figure 11 for Flat River (site 5T). Oxygen 

solubility in water varies with temperature. As water temperature decreases, the concentration of oxygen that can dissolve 

in water increases.

pH

Measurements of pH made at the time of sampling 

showed a pattern of slightly higher summertime levels compared to other seasons for all the sites. The higher measurements may be due to increased biological activity in the summer. Increased pH means lower hydrogen ion concentration 
Water-Quality Seasonality 29

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

300

250

200

150

100

50

0

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

20

15

10

5

0

Dissolved-oxygen concentration, in milligrams per liter Suspended-sediment concentration, in milligrams per liter

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Interquartile

range

Figure 10. Monthly suspended-sediment concentrations for Little River (site 4T) in the vicinity of 

the Treyburn development study area, North Carolina, 1988–2009.

Figure 11. Monthly dissolved-oxygen concentrations for Flat River tributary (site 1T) in the vicinity of 

the Treyburn development study area, North Carolina, 1988–2009.
30 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

in the water. Biological respiration produces carbon dioxide, 

which combines with hydrogen ions to form bicarbonate, and 

a decrease in pH. An example of this pattern is shown by pH 

variability for the Flat River (site 5T, fig. 12). 

Specific Conductance

Specific conductance showed a pattern of slightly 

elevated levels in the summer and fall compared to other 

seasons for all the sites. An example of this pattern is 

shown in figure 13 for Little River tributary (site 8T).

Metals

No distinct seasonal patterns were observed for metals. 

The occurrences of periods of high or low concentrations 

appear to be unrelated to season.

Nitrogen

Seasonal patterns in monthly frequency distributions of 

nitrogen constituent concentrations were observed for several 

of the study sites. Nitrate concentrations at Little River below 

Little River tributary (site 10TA, fig. 14A) peaked in the winter 

and were lowest in the fall, a pattern also observed for the 

Flat River, but with low concentrations in late summer as well 

(site 5T, fig. 14B), and Mountain Creek (site 6T, fig. 14C). 

This pattern of nitrate concentration, however, is reversed for 

the Little River tributary (site 8T, fig. 14D), where some of the 

high nitrate values occurred during the fall. The Little River 

tributary (site 8T) watershed includes golf course and residential land uses. The higher fall nitrate concentrations may be 

associated with turf fertilization. 

Monthly frequency distributions of ammonia concentrations showed a slight seasonal increase during April through 

September at Little River tributary (site 8T, fig. 15A) and 

peaked during June and July at Little River below Little River 

tributary (site 10TA, fig. 15B). Total ammonia plus organic 

nitrogen concentrations were highest in the spring at Little 

River (site 4T, fig. 16A), in the summer at Little River tributary (site 8T, fig. 16B), and from July to November at Little 

River below Little River tributary (site 10TA, fig. 16C). Total 

ammonia plus organic nitrogen peaked in the winter and in the 

fall at Flat River (site 5T, fig. 16D). The summer and spring 

nutrient increases may reflect the release of nutrients due to 

decomposition of leaf litter and the increased use of fertilizers.

Total nitrogen concentrations at Little River (site 4T, 

fig. 17A) and Flat River (site 5T, fig. 17D) generally were 

lowest during June through August. Concentrations were 

higher from December through March at Mountain Creek 

(site 6T, fig. 17B) and Little River below Little River tributary (site 10TA, fig. 17C). Causes of seasonal variation in 

nitrogen concentrations include variation in nutrient uptake 

by vegetation in the summer (decreasing nitrogen influx to 

the streams), land-surface disturbance due to agricultural or 

construction activities (increasing nitrogen inputs), fertilizer 

application in the spring and fall (increasing concentrations), 

and predominance of groundwater inflow (generally decreasing concentrations).
Water-Quality Seasonality 31

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

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

7.75

7.50

7.25

7.00

6.75

6.50

6.25

6.00

5.75

5.50

5.25

5.00

275

250

225

200

175

150

125

100

75

50

25

0

Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

pH, in standard units

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 12. Monthly pH for Flat River tributary (site 1T) in the vicinity of the Treyburn development 

study area, North Carolina, 1988–2009.

Figure 13. Monthly specific conductance for Little River tributary (site 8T), North Carolina, 1994–2009.
32 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

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

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

1.0

0.8

0.6

0.4

0.2

0

Dissolved nitrate nitrogen, in milligrams per liter

A. Little River below Little River tributary (site 10TA) 1.0

0.8

0.6

0.4

0.2

0

B. Flat River (site 5T)

1.0

0.8

0.6

0.4

0.2

0

C. Mountain Creek (site 6T) 1.0

0.8

0.6

0.4

0.2

0

D. Little River tributary (site 8T)

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 14. Monthly dissolved nitrate nitrogen for A, Little River below Little River tributary (site 10TA), 1994–2009; B, Flat River (site 5T), 

1988–2009; C, Mountain Creek (site 6T), 1988–2009; and D, Little River tributary (site 8T), 1994–2009, in the Treyburn development study 

area, North Carolina.
Water-Quality Seasonality 33

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

0.2

0.1

0

0.2

0.1

0

Dissolved ammonia, in milligrams per liter as nitrogen

A. Little River tributary (site 8T) B. Little River below Little River tributary (site 10TA)

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 15. Monthly dissolved ammonia nitrogen for A, Little River tributary (site 8T), 1994–2009, and B, Little River below Little River 

tributary (site 10TA), 1995–2009, in the vicinity of the Treyburn development study area, North Carolina.
34 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

2.0

1.5

1.0

0.5

0

Total ammonia plus organic nitrogen, in milligrams per liter as nitrogen

A. Little River (site 4T) B. Little River tributary (site 8T)

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

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

2.0

1.5

1.0

0.5

0

C. Little River below Little River tributary (site 10TA) D. Flat River (site 5T)

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 16. Monthly total ammonia plus organic nitrogen for A, Little River (site 4T), 1988–2009; B, Little River tributary (site 8T), 

1994–2009; C, Little River below Little River tributary (site 10TA), 1995–2009; and D, Flat River (site 5T), 1988–2009, in the vicinity of the 

Treyburn development study area, North Carolina.
Water-Quality Seasonality 35

3.0

2.5

2.0

1.5

1.0

0.5

0

3.0

2.5

2.0

1.5

1.0

0.5

0

3.0

2.5

2.0

1.5

1.0

0.5

0

Total nitrogen, in milligrams per liter as nitrogen

A. Little River (site 4T) B. Mountain Creek (site 6T)

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

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

C. Little River below Little River tributary (site 10TA) D. Flat River (site 5T)

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 17. Monthly total nitrogen for A, Little River (site 4T), 1988–2009; B, Mountain Creek (site 6T),1994–2009; C, Little River below 

Little River tributary (site 10TA), 1995–2009; and D, Flat River (site 5T), 1988–2009, in the vicinity of the Treyburn development study area, 

North Carolina.
36 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Phosphorus

Few distinct seasonal patterns were apparent in monthly 

distributions of total and dissolved phosphorus concentrations 

at the study sites. No seasonal effect was evident in dissolved 

phosphorus concentrations. Orthophosphate concentrations 

were higher from August to November at Little River (site 4T, 

fig. 18A) and July through September at Little River tributary (site 8T, fig. 18B). Total phosphorus concentrations were 

generally higher in March, April, July, and August at Little 

River (site 4T fig. 19A), July through September at Little 

River tributary (site 8T, fig. 19B) and Flat River tributary 

(site 1T, fig. 19D), and July through November at Little River 

below Little River tributary (site 10TA, fig. 19C). Higher total 

phosphorus concentrations may be associated with the higher 

sediment concentrations that tended to occur during the spring 

and fall. Generally lower overall concentrations at Little River 

below Little River tributary (site 10TA) may be a result of 

phosphorus retention in the reservoir upstream from that site. 

Lower general phosphorus concentrations for the Flat River 

tributary (site 1T) may be due to the basin being mostly forest.

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

0.4

0.3

0.2

0.1

0

Dissolved orthophosphate, in milligrams per liter

A. Little River (site 4T) B. Little River tributary (site 8T)

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 18. Monthly dissolved orthophosphate for A, Little River (site 4T), 1988–2009, and B, Little River tributary (site 8T), 1994–2009, in 

the vicinity of the Treyburn development study area, North Carolina.
Water-Quality Seasonality 37

0.4

0.3

0.2

0.1

0

0.4

0.3

0.2

0.1

0

0.4

0.3

0.2

0.1

0

0.4

0.3

0.2

0.1

0

Total phosphorus, in milligrams per liter

A. Little River (site 4T) B. Little River tributary (site 8T)

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

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

C. Little River below Little River tributary (site 10TA) D. Flat River tributary (site 1T)

EXPLANATION

95th percentile

75th percentile

50th percentile

25th percentile

5th percentile

Interquartile

range

Figure 19. Monthly total phosphorus for A, Little River (site 4T), 1988–2009; B, Little River tributary (site 8T), 1994–2009; C, Little 

River below Little River tributary (site 10TA), 1995–2009; and D, Flat River tributary (site 1T), 1988–2009, in the vicinity of the Treyburn 

development study area, North Carolina.
38 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Estimation of Nutrient and Sediment 

Loads and Yields

Annual instream load estimates from 1988 to 2009 of 

total nitrogen, dissolved ammonia, total ammonia plus organic 

nitrogen, dissolved nitrite plus nitrate, total phosphorus, 

dissolved phosphorus, dissolved orthophosphate, and suspended sediment were calculated using the statistical program 

S-LOADEST (Runkel and others, 2004). Documentation is 

contained in the publicly available USGS library for S-PLUS 

for Windows, release 2.1 (Slack and others, 2003; http://water.

usgs.gov/software/library.html). The load estimates were 

obtained by using the best combination of seven variables in a 

log-linear regression model based on rankings by the Akaike 

information criterion (Cohn and others, 1989, 1992; Gilroy 

and others, 1990) and a review of residuals. The full sevenvariable model is

( )

() () π π

= + + + +

+ +

lnL a ln Q a lnQ a

a t sin 2 a t cos 2 e

0 1 2

2

3

2

5 6 , 4 +

a t

a t

(1)

where 

L is Load (Q * c); 

ln is natural logarithm function; 

c is concentration, in milligrams per liter; 

Q is instantaneous discharge at time of 

concentration sampling, in cubic feet per 

second; 

t is time, in decimal years; 

sin is sine function; 

cos is cosine function; 

π is 3.14169; 

a0, a1, a2, a3,

a4, a5, a6 are coefficients of the regression model 

(a0 is intercept); and 

e is model error term.

The discharge terms (a1ln Q and a2 (ln Q)2) in the model 

address variability in concentration resulting from discharge 

variability. The time terms (a3 t and a4 t

2

) adjust for variability 

resulting from a linear time trend in concentration, and the 

sine and cosine terms adjust for seasonal variability in concentration. Bias generated in the estimated load when the load 

is transformed from log to linear units was corrected using 

the minimum variance unbiased estimator correction (Bradu 

and Mundlak, 1970). Censored data were statistically adjusted 

using the adjusted maximum likelihood estimator 

(Cohn, 1988). 

The models selected by the Akaike information criterion 

ranking used for load estimation for total nitrogen, dissolved 

ammonia nitrogen, dissolved nitrite plus nitrate, total ammonia 

plus organic nitrogen, total phosphorus, dissolved phosphorus, dissolved orthophosphate, and suspended sediment are 

listed in table 9. Load (tons) was divided by the drainage area 

(square miles) to estimate yield to allow comparison between 

basins.

Suspended-Sediment Loads and Yields

Suspended-sediment load, or the total mass of suspended 

sediment transported by a stream, is a function of the interaction of physical characteristics of the upstream stream basin, 

including topographic and physiographic factors, geology, 

soil characteristics, precipitation, land use, land cover, landmanagement practices, flow, and antecedent conditions of the 

watershed. Estimation of suspended-sediment yield allows 

for basin comparisons of sediment delivery and can be used 

to assess the effectiveness of management actions. Sediment 

concentrations, loads, and yields are strongly correlated with 

streamflow so that the greatest loads (and therefore yields) 

for any period generally are associated with the highest flows. 

Results from this study are compared to results from analysis 

by Staub and others (2010) of suspended-sediment yields from 

48 basins in the Southeastern United States.

Model statistics and review of residuals indicated a reasonable fit of the models to the data (table 9). The time terms 

produced by the model give an indication of whether the load 

has changed with time. One or both of the seasonal terms in 

the regression model were significant for each site, suggesting 

strong seasonal variation in suspended-sediment load for all 

sites. 

The annual suspended-sediment loads and yields for the 

study sites (table 10) show considerable basin-to-basin and 

year-to-year variation. Up to 23 complete years (1987–2009) 

of flow data were used in model development. The estimated 

loads and yields reflect the varying total streamflow from each 

basin for the years indicated. The highest loads and yields for 

all sites occurred during water years 1996 and 2003, largely 

due to high streamflow that occurred during those years 

(fig. 20). 
Estimation of Nutrient and Sediment Loads and Yields 39

Table 9. Summary of significant variables and regression coefficients used to develop the predictive equations of loads for sites in the vicinity of the Treyburn development study area, North Carolina.—Continued [ln, natural logarithm function; p, probability, 0.000 indicates p <0.0001; Q, discharge; ID, Insufficient data to run model; Blanks indicate variables not used in the prediction model]

Intercept p lnQ p (lnQ)2 p Time p Time2 p sin(time) p cos(time) p

Little River (site 4T)

Total nitrogen –1.294 0.000 1.156 0.000 –0.021 0.005 0.011 0.037 –0.120 0.012 –0.125 0.044

Dissolved ammonia nitrogen ID ID ID ID ID ID ID ID ID ID ID ID ID ID

Dissolved nitrite plus nitrate ID ID ID ID ID ID ID ID ID ID ID ID ID ID

Total ammonia plus organic nitrogen –2.513 0.000 1.197 0.000 –0.188 0.003 –0.125 0.121

Total phosphorus –4.637 0.000 1.290 0.000 0.016 0.107 –0.112 0.237 –0.216 0.083

Dissolved phosphorus ID ID ID ID ID ID ID ID ID ID ID ID ID ID

Total orthophosphate ID ID ID ID ID ID ID ID ID ID ID ID ID ID

Total suspended sediment 1.482 0.000 1.647 0.000 –0.484 0.009 –0.313 0.184

Mountain Creek (site 6T)

Total nitrogen –4.561 0.000 1.139 0.000 0.007 0.140 –0.007 0.122 0.003 0.000 –0.024 0.485 –0.123 0.001

Dissolved ammonia nitrogen –8.420 0.000 1.166 0.000 0.031 0.019 0.152 0.191 –0.169 0.110

Dissolved nitrite plus nitrate –5.601 0.000 1.115 0.000 –0.042 0.000 0.134 0.074 –0.301 0.000

Total ammonia plus organic nitrogen –5.650 0.000 1.199 0.000 0.023 0.000 –0.115 0.011 –0.206 0.000

Total phosphorus –8.038 0.000 1.356 0.000 0.029 0.005 0.014 0.202 0.003 0.086 –0.319 0.000 –0.265 0.002

Dissolved phosphorus –8.624 0.000 1.329 0.000 –0.299 0.028 –0.185 0.153

Total orthophosphate –9.136 0.000 1.256 0.000 0.037 0.002 –0.021 0.175 0.010 0.012 –0.296 0.006 –0.310 0.002

Total suspended sediment –1.854 0.000 1.588 0.000 0.035 0.022 0.015 0.319 –0.005 0.058 –0.185 0.136 –0.397 0.001

Little River below Little River tributary (site 10TA)

Total nitrogen –1.989 0.000 1.021 0.000 –0.009 0.064 0.023 0.000Dissolved ammonia nitrogen –5.026 0.000 1.039 0.000 –0.021 0.055 –0.024 0.103 –0.275 0.003 –0.133 0.125

Dissolved nitrite plus nitrate –3.504 0.000 0.979 0.000 –0.023 0.017 0.019 0.128 0.005 0.126 0.367 0.000 0.435 0.000

Total ammonia plus organic nitrogen –2.419 0.000 1.034 0.000 –0.007 0.125 0.025 0.000 –0.114 0.001 –0.084 0.013

Total phosphorus –4.944 0.000 1.018 0.000 0.054 0.002

Dissolved phosphorus –5.690 0.000 1.048 0.000 0.080 0.012 –0.355 0.026 0.198 0.189

Total orthophosphate –6.482 0.000 0.975 0.000 –0.445 0.026 0.172 0.377

Total suspended sediment 0.669 0.000 1.081 0.000 –0.161 0.092 –0.187 0.051
40 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009 Table 9. Summary of significant variables and regression coefficients used to develop the predictive equations of loads for sites in the vicinity of the Treyburn development study area, North Carolina.—Continued [ln, natural logarithm function; p, probability, 0.000 indicates p <0.0001; Q, discharge; ID, Insufficient data to run model; Blanks indicate variables not used in the prediction model] Intercept p lnQ p (lnQ)2 p Time p Time2 p sin(time) p cos(time) p Flat River (site 5T) Total nitrogen –1.869 0.000 1.168 0.000 0.003 0.257 –0.004 0.325 0.002 0.044 –0.063 0.087 –0.063 0.100 Dissolved ammonia nitrogen –5.794 0.000 1.082 0.000 0.016 0.038 –0.043 0.019 0.021 0.854 –0.529 0.000 Dissolved nitrite plus nitrate –3.568 0.000 1.235 0.000 –0.028 0.000 –0.033 0.043 Total ammonia plus organic nitrogen –2.988 0.000 1.170 0.000 0.021 0.000 –0.001 0.820 0.003 0.003 –0.150 0.001 –0.176 0.000 Total phosphorus –5.240 0.000 1.300 0.000 0.022 0.000 0.003 0.760 0.003 0.038 –0.252 0.000 –0.248 0.000 Dissolved phosphorus –6.307 0.000 0.000 0.000 0.042 0.038 –0.253 0.022 –0.232 0.029 Total orthophosphate –6.493 0.000 1.166 0.000 0.013 0.030 –0.055 0.000 –0.254 0.000 –0.234 0.000 Total suspended sediment 0.877 0.000 1.564 0.000 0.036 0.000 –0.052 0.000 –0.499 0.000 –0.409 0.000 Flat River tributary (site 1T) Total nitrogen –7.839 0.000 1.156 0.000 0.024 0.020 –0.033 0.000 0.003 0.043 –0.128 0.071 –0.217 0.002 Dissolved ammonia nitrogen –10.824 0.000 0.921 0.000 –0.059 0.011 0.190 0.185 –0.505 0.000 Dissolved nitrite plus nitrate –8.898 0.000 0.879 0.000 –0.019 0.206 –0.072 0.000 –0.012 0.016 0.175 0.167 –0.314 0.010 Total ammonia plus organic nitrogen –7.773 0.000 1.205 0.000 0.024 0.002 –0.012 0.096 0.005 0.000 –0.247 0.000 –0.211 0.001 Total phosphorus –9.864 0.000 1.302 0.000 0.024 0.030 –0.323 0.000 –0.338 0.000 Dissolved phosphorus –10.903 0.000 1.181 0.000 0.062 0.096 –0.262 0.129 –0.330 0.047 Total orthophosphate –11.357 0.000 1.086 0.000 –0.049 0.019 0.030 0.795 –0.283 0.033 Total suspended sediment –3.376 0.000 1.559 0.000 0.042 0.013 –0.025 0.104 –0.388 0.002 –0.548 0.000
Estimation of Nutrient and Sediment Loads and Yields 41

Table 10. Estimated annual suspended-sediment loads and yields for water years 1987–2009 at sites in the vicinity of the Treyburn 

development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual suspended-sediment load, in tons Annual suspended-sediment yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 32,346 217

1988 1,005 2,629 13 18

1989 12,060 34,142 172 154 229 151

1990 5,673 12,204 121 73 82 106

1991 6,032 13,320 77 89

1992 4,736 12,061 61 81

1993 10,633 32,727 136 220

1994 3,512 11,926 45 80

1995 7,789 1,604 17,758 474 100 200 119 416

1996 27,232 7,176 241,509 2,501 348 897 1,621 2,194

1997 5,258 439 1,344 10,654 72 67 55 14 72 63

1998 14,397 2,327 2,553 42,252 417 184 291 26 284 365

1999 11,795 2,151 1,410 38,907 502 151 269 14 261 440

2000 3,653 756 1,034 6,878 27 47 94 10 46 24

2001 3,161 462 553 5,709 44 40 58 6 38 39

2002 556 154 43 509 5 7 19 0.4 3 5

2003 29,638 4,606 4,393 47,536 466 379 576 44 319 409

2004 3,583 315 631 5,675 15 46 39 6 38 13

2005 3,313 144 704 6,608 20 42 18 7 44 18

2006 2,423 614 393 2,241 30 31 77 4 15 26

2007 4,874 494 936 6,744 103 62 62 9 45 91

2008 11,763 979 524 17,656 267 150 122 5 118 234

2009 3,479 201 681 3,567 34 44 25 7 24 30

Mean 8,026 1,495 1,169 26,329 310 103 187 12 177 272

Median 5,066 614 704 12,061 103 65 77 7 81 91
42 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

10

100

1,000

10,000

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Mean Median

Annual suspended-sediment yield, in tons per square mile

Mountain Creek (site 6T)

Little River below Little River

 tributary (site 10TA)

Little River (site 4T)

Flat River (site 5T)

Flat River tributary (site 1T)

EXPLANATION

Figure 20. Annual suspended-sediment yields for sites in the vicinity of the Treyburn development study area, North Carolina.

Comparison of sediment yields for the basins indicated 

that Little River below Little River tributary (site 10TA) had 

the lowest median yield over the period of record (7 tons per 

square mile (tons/mi2 )), and Flat River tributary (site 1T) 

had the largest median yield (91 tons/mi2 ) during the period 

of record. The yields estimated for the study sites are low 

compared to suspended-sediment yields estimated for other 

basins in the Southeast (1973–2005; Staub and others, 2010). 

Median suspended-sediment yields for 20 basins in the Southeast ranged from 5.59 to 1,106 tons/mi2 (Staub and others, 

2010). Median yields for the Treyburn sites ranged from 7 to 

91 tons/mi2, which are within the 10th to 65th percentiles of 

Southeast median suspended-sediment yields (Staub and others, 2010). 

Nitrogen Loads and Yields

Loads and yields were estimated for dissolved nitrite plus 

nitrate (table 11), dissolved ammonia nitrogen (table 12), total 

organic plus ammonia nitrogen (Kjeldahl nitrogen, table 13), 

and total nitrogen (table 14; fig. 21). The multiple-regression 

models used to estimate nitrogen loads are listed in table 9. 

Median 1987–2009 period of record nitrogen yields were 

lowest for Little River below Little River tributary (site 10TA) 

for dissolved nitrite plus nitrate (0.02 ton/mi2), total organic 

plus ammonia nitrogen (0.28 ton/mi2), and total nitrogen 

(0.42 ton/mi2). Median period of record dissolved ammonia 

nitrogen yields were lowest for Flat River tributary (site 1T; 

0.008 ton/mi2). The Flat River had the highest period of record 

median dissolved nitrate plus nitrite nitrogen yields (site 5T; 

0.30 ton/mi2

). Flat River (site 5T) and Mountain Creek 

(site 6T) had the highest median dissolved ammonia nitrogen 

yields (0.031 ton/mi2 for both sites). Little River (site 4T) and 

Mountain Creek (6T) had the highest median total ammonia 

plus organic nitrogen period of record yields (0.49 ton/mi2 

for both sites) and the highest median total nitrogen yields 

(0.81 and 0.78 tons/mi2

, respectively). 

The yields estimated for the study sites are low compared 

to nitrogen yields estimated for other basins in the Southeastern United States (1973–2005). Median dissolved nitrite plus 

nitrate yields for 48 basins in the Southeast ranged from 0.01 

to 16.0 tons/mi2 (Staub and others, 2010). Median 1987–2009 

period of record yields for the study sites ranged from 0.02 to 

0.30 ton/mi2, which are within the 13th to 35th percentiles of 

yields for nitrite plus nitrate in the Southeast. Median dissolved ammonia yields for 22 basins across the Southeast 

ranged from 0.01 to 3.01 tons/mi2 (Staub and others, 2010). 

Median dissolved ammonia yields for the Treyburn study sites 

ranged from 0.008 to 0.031 ton/mi2, which are within the 10th 

to 40th percentiles of yields for dissolved ammonia in the 

Southeast. 

Median total organic plus ammonia nitrogen yields 

for 47 basins across the Southeast ranged from 0.13 to 

34.6 tons/mi2 (Staub and others, 2010). Median period of 

record yields for the Treyburn study sites ranged from 0.28 

to 0.49 ton/mi2, which are within the range of the 9th to 38th 

percentiles of total organic plus ammonia nitrogen for sites 

in the Southeast. Median total nitrogen yields for 47 basins 

across the Southeast ranged from 0.12 to 53.2 tons/mi2 (Staub 

and others, 2010). Median total nitrogen yields for the Treyburn study sites ranged from 0.42 to 0.81 ton/mi2

, which are 

within the range of 7th to 35th percentiles of the yields at the 

Southeastern sites.
Estimation of Nutrient and Sediment Loads and Yields 43

Table 11. Estimated annual nitrate plus nitrite loads and yields for water years 1987–2009 at sites in the vicinity of the Treyburn 

development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Year

Annual nitrate plus nitrite load, in tons Annual nitrate plus nitrite yield, in tons per square mile

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 84.62 0.57

1988 25.46 0.17

1989 101.76 0.030 0.68 0.026

1990 67.83 0.041 0.46 0.036

1991 50.20 0.34

1992 33.94 0.23

1993 89.89 0.60

1994 49.40 0.33

1995 2.22 41.17 0.058 0.28 0.28 0.051

1996 4.22 92.34 0.091 0.53 0.62 0.080

1997 2.62 3.94 54.21 0.080 0.33 0.04 0.36 0.070

1998 3.61 5.49 96.59 0.095 0.45 0.06 0.65 0.083

1999 2.26 3.49 45.56 0.072 0.28 0.04 0.31 0.063

2000 3.13 2.79 44.76 0.065 0.39 0.03 0.30 0.057

2001 1.41 1.29 24.79 0.036 0.18 0.01 0.17 0.031

2002 0.59 0.13 6.02 0.013 0.07 0.001 0.04 0.012

2003 6.11 10.06 112.72 0.105 0.76 0.10 0.76 0.092

2004 1.67 1.78 28.06 0.029 0.21 0.02 0.19 0.026

2005 1.35 1.82 31.99 0.030 0.17 0.02 0.21 0.026

2006 1.40 0.94 12.69 0.012 0.17 0.01 0.09 0.010

2007 2.05 2.20 30.12 0.022 0.26 0.02 0.20 0.019

2008 1.53 1.06 21.48 0.012 0.19 0.01 0.14 0.011

2009 1.90 1.50 26.17 0.015 0.24 0.02 0.18 0.013

Mean 2.40 2.81 50.95 0.047 0.30 0.03 0.34 0.042

Median 2.05 1.82 44.76 0.036 0.26 0.02 0.30 0.031
44 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 12. Estimated annual ammonia nitrogen loads and yields for water years 1987–2009 at sites in the vicinity of the Treyburn 

development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual ammonia nitrogen load, in tons Annual ammonia nitrogen yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 9.26 0.062

1988 2.44 0.016

1989 12.96 0.032 0.087 0.028

1990 7.01 0.028 0.047 0.025

1991 4.66 0.031

1992 3.63 0.024

1993 9.63 0.065

1994 5.18 0.035

1995 0.35 5.73 0.016 0.044 0.038 0.014

1996 0.84 13.23 0.021 0.105 0.089 0.018

1997 0.26 3.94 5.44 0.014 0.032 0.040 0.037 0.012

1998 0.71 5.49 9.92 0.018 0.088 0.056 0.067 0.016

1999 0.38 3.49 5.28 0.012 0.047 0.035 0.035 0.011

2000 0.34 2.79 4.62 0.009 0.043 0.028 0.031 0.008

2001 0.18 1.29 2.82 0.006 0.022 0.013 0.019 0.005

2002 0.06 0.13 0.59 0.002 0.007 0.001 0.004 0.002

2003 0.97 10.06 13.17 0.019 0.121 0.102 0.088 0.016

2004 0.15 1.78 2.67 0.005 0.019 0.018 0.018 0.004

2005 0.12 1.82 2.57 0.005 0.015 0.018 0.017 0.005

2006 0.17 0.94 1.37 0.003 0.022 0.009 0.009 0.002

2007 0.25 2.20 2.36 0.006 0.031 0.022 0.016 0.005

2008 0.23 1.06 2.57 0.004 0.028 0.011 0.017 0.004

2009 0.18 1.50 2.39 0.005 0.022 0.015 0.016 0.005

Mean 0.34 2.81 5.63 0.012 0.043 0.028 0.038 0.011

Median 0.25 1.82 4.66 0.009 0.031 0.018 0.031 0.008
Estimation of Nutrient and Sediment Loads and Yields 45

Table 13. Estimated annual total ammonia plus organic nitrogen loads and yields for water years 1987–2009 at sites in the vicinity of 

the Treyburn development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual total Kjeldahl nitrogen load, in tons Annual total Kjeldahl nitrogen yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 93.98 0.63

1988 15.50 26.73 0.20 0.18

1989 75.95 126.51 0.72 0.97 0.85 0.63

1990 54.72 79.45 0.67 0.70 0.53 0.59

1991 46.40 62.10 0.59 0.42

1992 30.98 45.51 0.40 0.31

1993 69.92 121.94 0.89 0.82

1994 32.32 68.30 0.41 0.46

1995 42.75 5.75 67.27 0.55 0.55 0.72 0.45 0.48

1996 91.49 16.83 197.25 1.65 1.17 2.10 1.32 1.45

1997 49.15 3.93 39.46 81.59 0.30 0.63 0.49 0.40 0.55 0.26

1998 82.58 9.66 69.44 159.66 0.78 1.06 1.21 0.70 1.07 0.69

1999 46.14 7.28 37.29 95.66 0.75 0.59 0.91 0.38 0.64 0.66

2000 39.50 5.05 33.11 75.33 0.18 0.51 0.63 0.33 0.51 0.16

2001 27.49 2.55 16.62 44.82 0.15 0.35 0.32 0.17 0.30 0.14

2002 7.21 1.03 1.69 10.60 0.04 0.09 0.13 0.02 0.07 0.03

2003 154.08 16.06 135.71 242.47 1.01 1.97 2.01 1.37 1.63 0.89

2004 35.16 2.64 22.90 58.54 0.12 0.45 0.33 0.23 0.39 0.10

2005 34.34 1.77 26.60 63.51 0.16 0.44 0.22 0.27 0.43 0.14

2006 20.74 2.98 13.38 28.91 0.11 0.27 0.37 0.14 0.19 0.10

2007 36.64 3.77 35.49 65.83 0.39 0.47 0.47 0.36 0.44 0.35

2008 36.15 4.29 17.80 62.40 0.59 0.46 0.54 0.18 0.42 0.52

2009 34.37 2.60 27.46 60.02 0.32 0.44 0.32 0.28 0.40 0.28

Mean 48.35 5.75 36.69 84.28 0.50 0.62 0.72 0.37 0.57 0.44

Median 38.07 3.93 27.46 67.27 0.39 0.49 0.49 0.28 0.45 0.35
46 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 14. Estimated annual total nitrogen loads and yields for water years 1987–2009 at sites in the vicinity of the Treyburn 

development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual total nitrogen load, in tons Annual total nitrogen yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 197.35 1.32

1988 24.14 51.97 0.31 0.35

1989 105.97 233.55 1.35 1.36 1.57 1.19

1990 83.00 142.59 1.22 1.06 0.96 1.07

1991 67.89 110.29 0.87 0.74

1992 44.21 80.30 0.57 0.54

1993 100.48 214.39 1.28 1.44

1994 50.38 113.20 0.64 0.76

1995 63.22 7.02 102.18 0.95 0.81 0.88 0.69 0.84

1996 124.91 16.67 290.41 2.39 1.60 2.08 1.95 2.09

1997 79.44 6.27 59.50 128.41 0.54 1.02 0.78 0.60 0.86 0.47

1998 119.65 12.24 105.76 266.92 1.38 1.53 1.53 1.07 1.79 1.21

1999 65.55 8.00 48.84 137.94 1.10 0.84 1.00 0.49 0.93 0.97

2000 67.52 7.41 50.66 114.67 0.31 0.86 0.93 0.51 0.77 0.28

2001 46.04 3.54 24.17 70.38 0.26 0.59 0.44 0.24 0.47 0.23

2002 12.87 1.47 2.57 16.36 0.06 0.16 0.18 0.03 0.11 0.05

2003 240.14 19.33 191.39 367.26 1.45 3.07 2.42 1.94 2.46 1.27

2004 61.82 4.22 33.18 86.84 0.18 0.79 0.53 0.34 0.58 0.15

2005 61.18 3.21 40.49 103.59 0.24 0.78 0.40 0.41 0.70 0.21

2006 36.70 4.15 17.79 44.32 0.15 0.47 0.52 0.18 0.30 0.13

2007 63.02 6.37 52.43 109.16 0.49 0.81 0.80 0.53 0.73 0.43

2008 54.22 5.97 23.57 99.19 0.59 0.69 0.75 0.24 0.67 0.52

2009 63.70 5.39 41.25 104.31 0.37 0.81 0.67 0.42 0.70 0.33

Mean 74.37 7.42 53.20 138.50 0.77 0.95 0.93 0.54 0.93 0.67

Median 63.46 6.27 41.25 110.29 0.54 0.81 0.78 0.42 0.74 0.47
Estimation of Nutrient and Sediment Loads and Yields 47

0.01

0.10

1.0

10

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Mean Median

Annual total nitrogen yield, in tons per square mile

Mountain Creek (site 6T)

Little River below Little River

 tributary (site 10TA)

Little River (site 4T)

Flat River (site 5T)

Flat River tributary (site 1T)

EXPLANATION

Figure 21. Annual total nitrogen yields for sites in the vicinity of the Treyburn development study area, North Carolina.

Phosphorus Loads and Yields

Loads and yields were estimated for dissolved orthophosphorus (table 15), dissolved phosphorus, (table 16), and 

total phosphorus (table 17; fig. 22). The multiple-regression 

models used to estimate phosphorus loads are provided in 

table 9. Median period of record yields were lowest for Flat 

River tributary (site 1T) dissolved phosphorus (0.010 ton/mi2) 

and dissolved orthophosphorus (0.005 ton/mi2) and for Little 

River (site 10TA) total phosphorus (0.04 ton/mi2). Mountain Creek (site 6T) had the highest median period of record 

total phosphorus yields (0.10 ton/mi2), dissolved phosphorus 

yields (0.03 ton/mi2), and dissolved orthophosphorus yields 

(0.02 ton/mi2

).

The 1987–2009 period of record annual median phosphorus yields estimated for the study sites are low compared to 

phosphorus yields estimated for other basins in the Southeast 

(1973–2005). Median dissolved orthophosphorus yields for 

the study sites ranged from 0.005 to 0.02 ton/mi2. Dissolved 

orthophosphate yields for 22 basins across the Southeast 

ranged from medians of 0.004 to 1.37 tons/m2 (Staub and 

others, 2010). The study sites fall in the range of the 7th to 

27th percentiles of yields for dissolved orthophosphate in 

the Southeast. Total phosphorus yields for 47 basins across 

the Southeast ranged from medians of 0.005 to 5.64 tons/mi2 

(Staub and others, 2010). Median annual total phosphorus 

yields for the study sites ranged from 0.04 to 0.10 ton/mi2, 

which fall in the range of the 18th to 40th percentiles of 

Southeastern yields.
48 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 15. Estimated annual dissolved orthophosphorus loads and yields for water years 1987–2009 at sites in the vicinity of the 

Treyburn development study area.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual orthophosphorus load, in tons Annual orthophosphorus yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 5.42 0.036

1988 1.34 0.009

1989 6.61 0.020 0.044 0.017

1990 3.75 0.019 0.025 0.016

1991 2.87 0.019

1992 2.02 0.014

1993 5.12 0.034

1994 2.65 0.018

1995 0.41 2.64 0.012 0.052 0.018 0.010

1996 1.33 8.85 0.019 0.167 0.059 0.017

1997 0.16 1.79 2.66 0.009 0.020 0.018 0.018 0.008

1998 0.41 2.55 5.23 0.016 0.051 0.026 0.035 0.014

1999 0.37 1.72 3.27 0.011 0.047 0.017 0.022 0.010

2000 0.17 1.29 2.06 0.006 0.021 0.013 0.014 0.005

2001 0.09 0.58 1.18 0.004 0.011 0.006 0.008 0.004

2002 0.04 0.09 0.25 0.001 0.005 0.001 0.002 0.001

2003 0.69 4.80 6.20 0.016 0.086 0.049 0.042 0.014

2004 0.09 1.00 1.33 0.003 0.012 0.010 0.009 0.003

2005 0.05 1.03 1.34 0.004 0.007 0.010 0.009 0.003

2006 0.14 0.44 0.58 0.002 0.018 0.004 0.004 0.002

2007 0.15 1.36 1.26 0.005 0.019 0.014 0.008 0.004

2008 0.29 0.57 1.29 0.004 0.037 0.006 0.009 0.004

2009 0.12 0.82 0.98 0.004 0.015 0.008 0.007 0.003

Mean 0.30 1.39 3.00 0.009 0.038 0.014 0.020 0.008

Median 0.16 1.03 2.64 0.006 0.020 0.010 0.018 0.005
Estimation of Nutrient and Sediment Loads and Yields 49

Table 16. Estimated annual dissolved phosphorus loads and yields for water years 1987–2009 at sites in the vicinity of the Treyburn 

development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual dissolved phosphorus load, in tons Annual dissolved phosphorus yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 2.47 0.02

1988 1.69 0.65 0.02 0.00

1989 10.52 3.72 0.01 0.13 0.02 0.01

1990 6.90 2.30 0.01 0.09 0.02 0.01

1991 5.87 1.92 0.08 0.01

1992 4.26 1.48 0.05 0.01

1993 10.26 4.23 0.13 0.03

1994 4.42 2.43 0.06 0.02

1995 6.52 0.33 2.68 0.01 0.08 0.04 0.02 0.01

1996 13.86 1.09 9.40 0.04 0.18 0.14 0.06 0.03

1997 6.93 0.23 2.15 3.23 0.01 0.09 0.03 0.02 0.02 0.01

1998 13.64 0.53 4.01 6.97 0.02 0.17 0.07 0.04 0.05 0.02

1999 7.21 0.50 2.62 4.90 0.03 0.09 0.06 0.03 0.03 0.02

2000 5.84 0.29 2.00 3.33 0.01 0.07 0.04 0.02 0.02 0.01

2001 4.46 0.14 1.00 2.11 0.01 0.06 0.02 0.01 0.01 0.01

2002 0.99 0.07 0.12 0.46 0.00 0.01 0.01 0.00 0.00 0.00

2003 26.69 0.99 9.87 13.56 0.05 0.34 0.12 0.10 0.09 0.04

2004 5.11 0.17 1.95 3.16 0.01 0.07 0.02 0.02 0.02 0.01

2005 5.29 0.10 2.28 3.44 0.01 0.07 0.01 0.02 0.02 0.01

2006 3.35 0.18 0.97 1.64 0.01 0.04 0.02 0.01 0.01 0.01

2007 6.01 0.23 3.57 3.93 0.02 0.08 0.03 0.04 0.03 0.02

2008 6.92 0.28 1.65 4.49 0.03 0.09 0.04 0.02 0.03 0.02

2009 5.97 0.15 2.50 3.73 0.02 0.08 0.02 0.03 0.03 0.02

Mean 7.40 0.35 2.67 3.75 0.02 0.09 0.04 0.03 0.03 0.02

Median 5.99 0.23 2.15 3.23 0.01 0.08 0.03 0.02 0.02 0.01
50 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Table 17. Estimated annual total phosphorus loads and yields for water years 1987–2009 at sites in the vicinity of the Treyburn 

development study area, North Carolina, including mean and median load and yield for period of record.

[Blanks indicate insufficient data to estimate load and (or) yield]

Annual total phosphorus load, in tons Annual total phosphorus yield, in tons per square mile

Year

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little 

River below Little 

River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

Little 

River 

(site 4T)

Mountain 

Creek 

(site 6T)

Little River 

below 

Little River 

tributary 

(site 10TA)

Flat River 

(site 5T)

Flat River 

tributary 

(site 1T)

1987 20.57 0.14

1988 1.69 3.50 0.02 0.02

1989 10.52 23.86 0.05 0.13 0.16 0.04

1990 6.90 11.64 0.05 0.09 0.08 0.04

1991 5.87 9.88 0.02 0.08 0.07 0.02

1992 4.26 7.85 0.07 0.05 0.05 0.06

1993 10.26 21.76 0.19 0.13 0.15 0.17

1994 4.42 10.17 0.04 0.06 0.07 0.03

1995 6.52 1.23 11.34 0.09 0.08 0.15 0.08 0.08

1996 13.86 5.02 58.74 0.11 0.18 0.63 0.39 0.10

1997 6.93 0.55 3.90 10.99 0.03 0.09 0.07 0.04 0.07 0.02

1998 13.64 1.75 7.63 29.94 0.02 0.17 0.22 0.08 0.20 0.02

1999 7.21 1.84 3.60 20.16 0.01 0.09 0.23 0.04 0.14 0.00

2000 5.84 0.78 3.65 9.59 0.15 0.07 0.10 0.04 0.06 0.13

2001 4.46 0.44 1.87 6.78 0.02 0.06 0.05 0.02 0.05 0.01

2002 0.99 0.20 0.20 1.10 0.02 0.01 0.03 0.00 0.01 0.02

2003 26.69 4.00 15.55 46.29 0.02 0.34 0.50 0.16 0.31 0.01

2004 5.11 0.50 2.68 8.51 0.05 0.07 0.06 0.03 0.06 0.05

2005 5.29 0.27 3.37 10.13 0.08 0.07 0.03 0.03 0.07 0.07

2006 3.35 0.76 1.56 4.39 0.04 0.10 0.02 0.03

2007 6.01 0.82 4.69 11.76 0.08 0.10 0.05 0.08

2008 6.92 1.59 2.25 17.90 0.09 0.20 0.02 0.12

2009 5.97 0.54 3.89 10.64 0.08 0.07 0.04 0.07

Mean 7.40 1.35 4.22 15.98 0.059 0.09 0.17 0.04 0.11 0.05

Median 5.99 0.78 3.60 10.99 0.050 0.08 0.10 0.04 0.07 0.04
Trend Analysis 51

0.01

0.10

1.0

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Mean Median

Annual total phosphorus yield, in tons per square mile

Mountain Creek (site 6T)

Little River below Little River

 tributary (site 10TA)

Little River (site 4T)

Flat River (site 5T)

Flat River tributary (site 1T)

EXPLANATION

Figure 22. Annual total phosphorus yields for sites in the vicinity of the Treyburn development study area, North Carolina.

Trend Analysis 

Statistical trends in water-quality data collected at the 

stream sites flowing in or near the Treyburn development were 

evaluated to determine the extent to which land-use conversion has affected water quality at these sites. Data used to 

characterize water-quality trends were collected from October 

1988 through September 2009. 

Methodology

The Seasonal Kendall test described by Hirsch and others 

(1982) was used for trend analysis (see also Crawford and 

others 1983; Schertz and Hirsch 1985; and Helsel, 1993). The 

Seasonal Kendall test is a nonparametric or distribution-free 

procedure developed to detect monotonically (one direction) 

increasing or decreasing trends over time in water-quality data 

that show seasonality. The Seasonal Kendall test adjusts for 

seasonal variability by comparing seasonally grouped constituent concentrations and adjusts for the effects of streamflow 

with locally weighted scatterplot smoothing (LOESS) curves. 

The Seasonal Kendall test examines pairs of values over time, 

assigning a plus if an increase occurs from one value to the 

next, or a minus if a decrease occurs. The pairs of values from 

the same seasonal period are compared (Crawford and others, 

1983; Schertz and Hirsch, 1985; Shertz and others, 1991). In 

the case where seasons are months, January median values 

from each year are compared to the previous January median 

values; February median values from each year are compared 

to the previous February median values, and so on. If the total 

number of pluses from the comparison of pairs is greater than 

the number of minuses, an increasing trend is indicated. Conversely, if more minuses occur than pluses, a decreasing trend 

is indicated. Depending on the frequency of data collection, 

the seasonal period can be greater than a month. The use of 

the median by the Seasonal Kendall test reduces the effect of 

outliers and provides some protection against serial correlation 

in the data (Schertz and Hirsch, 1985). A significance level 

(alpha) of 0.05 was considered to show statistical significance 

of the trend test. Multiple time periods were examined for 

trends for each site to identify all possible monotonic trends. 

Statistical tests for trends in water quality over time were 

performed using S-ESTREND version 1.1, which is a “USGS 

plug-in” version of ESTREND in S-PLUS (version 6.1), a 

PC-based statistical software package. Documentation is 

contained in the publicly available USGS library for S-PLUS 

for Windows, release 2.1 (Slack and others, 2003). ESTREND 

software can be downloaded from the USGS Web page http://

water.usgs.gov/software/library.html.

For the trend-test procedure, a minimum of 50 observations over a minimum of 5 years was required. A model using 

flow-adjusted concentrations was selected for constituents 

with less than 5-percent censoring (less-than values), only one 

reporting level, and daily flow data. LOESS flow adjustment 

and the Seasonal Kendall test for uncensored data were used. 

This nonparametric test calculates trends on the flow-adjusted 

concentrations. 

A simple five-parameter multiple-regression load model, 

which includes a time term, with terms for adjustment for 

flow and season ln L = a0

 + a1 ln Q + a2 t +a3 sin (2 π t) + a4cos 

(2 π t) + e, was also run for each constituent to provide a test 
52 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

of time trend over the period of record. A time term significance level of less than or equal to 0.05 was considered to 

show a statistically significant trend for the period of record. 

This alternate approach to testing for trend is included for 

comparison in the description of the Seasonal Kendall trend 

results. 

Trends in Water-Quality Data

Data tested for time trends included specific conductance, dissolved oxygen, dissolved ammonia nitrogen, total 

ammonia and organic nitrogen, dissolved nitrite plus nitrate, 

total nitrogen, dissolved orthophosphate, total phosphorus, 

and suspended-sediment concentrations. Trend analysis was 

conducted on six sites: Little River (site 4T), Mountain Creek 

(site 6T), Little River tributary (site 8T), Little River below 

Little River tributary (site 10TA), Flat River (site 5T), and 

Flat River tributary (site 1T). No significant time trends in any 

constituent were detected for the Little River site (site 4T). No 

significant trends were detected for dissolved oxygen, dissolved ammonia nitrogen, dissolved orthophosphate, or total 

phosphorus at any site. Only results for trend tests that used 

flow adjustment of constituent concentrations are reported. 

The Oblinger and others (2002) study used the Seasonal 

Kendal method to test for trends at Flat River (site 5T), Flat 

River tributary (site 1T), and Mountain Creek (site 6T) for 

the 1988–98 study period. Downward trends in total nitrogen, 

ammonia plus organic nitrogen, and organic nitrogen were 

detected for the Flat River tributary, suggesting a small reduction in organic nitrogen over time at this site.

Mountain Creek (Site 6T)

A statistically significant increasing trend in total nitrogen 

concentration (1993–2009) was detected for Mountain Creek 

(site 6T). A graph of the total nitrogen concentrations (fig. 23) 

over time shows a step decline in total nitrogen prior to 1993, 

with a slight increase in concentration after 1993. The time 

term of the five-parameter multiple regression model of the 

complete 1988–2009 period of record showed no statistical 

significance. 

A statistically significant increasing trend in suspendedsediment concentration was detected for the 1996–2009 period 

for Mountain Creek (fig. 24). The regression model of the 

longer 1988–2009 data showed no statistical significance for 

sediment. The trend may reflect effects of increased residential 

development in the basin during the 1996–2009 part of the 

study period.

Little River Tributary (Site 8T)

A statistically significant increasing trend in specific conductance (1996–2009) was detected for Little River tributary 

(fig. 25). Regression was not run for specific conductance 

because load analysis for specific conductance is not appropriate. No other trends were evident for this site.

1988 1990 19941992 1996 1998 2000 2008200620042002 2010

0.0

0.5

1.0

1.5

2.0

2.5

Total nitrogen, in milligrams per liter as nitrogen

Figure 23. Total nitrogen concentrations with LOESS smooth trend line for Mountain Creek (site 6T), 1988–2009, in the vicinity of the 

Treyburn development study area, North Carolina.
Trend Analysis 53

19901988 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Suspended sediment, in milligrams per liter500

100

1,000

10

5

0

50

Figure 24. Suspended-sediment concentrations with LOESS smooth trend line for Mountain Creek (site 6T), 1988–2009, in the vicinity of 

the Treyburn development study area, North Carolina.

1994 1996 1998 2000 2002 2004 2006 2008 2010

275

250

225

200

175

150

125

100

75

50

25

0

Specific conductance, in microsiemens per centimeter

 at 25 degrees Celsius

Figure 25. Specific conductance with LOESS smooth trend line for Little River tributary (site 8T), 1994–2009, in the vicinity of the 

Treyburn development study area, North Carolina.
54 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009

Little River Below Little River Tributary 

(Site 10TA)

Statistically significant increasing trends in dissolved 

nitrate (1996–2009, fig. 26) and dissolved nitrite plus nitrate 

(1996–2009, fig. 27) were detected for Little River below 

Little River tributary with the Seasonal Kendal test. The fiveparameter regression model of the 1996–2009 data showed no 

statistical significance of the time term for either nutrient. The 

reason for this lack of concurrence may be due to an inadequate regression model fit or incomplete data transformation to 

meet model assumptions. Nonparametric (Seasonal Kendall) 

techniques do not require data transformation. The regression 

model showed significant increasing trends for total ammonia 

plus organic nitrogen (1995–2009, fig. 28) and total nitrogen 

(1995–2009, fig. 29); however, the Seasonal Kendall test did 

not detect these long-term trends. A trend that is detected by 

both methods is more likely to reflect a true trend. Although 

the different trend test approaches do not produce the same 

results, one test or the other indicated increasing trends for all 

the measured nitrogen species at the Little River, suggesting 

that an increase in nitrogen over time has occurred. 

Flat River (Site 5T)

The regression model showed a significant decreasing 

trend for dissolved nitrite plus nitrate nitrogen (1994–2009, 

fig. 30), which was not detected by the seasonal Kendall test. 

The seasonal Kendall test detected a significant decreasing 

trend in total ammonia and organic nitrogen for 1993–2009 

(fig. 31); however, the regression showed no significant trend 

for the long-term period of record (1988–2009). Both tests 

detected a decreasing 1988–2009 sediment trend for the Flat 

River (fig. 32). 

Flat River Tributary (Site 1T)

A statistically significant decreasing trend in specific 

conductance (1988–2009) was detected for Flat River tributary 

(fig. 33). Regression was not run for specific conductance. No 

other trends were evident for this site.

1996 1998 2000 2002 2004 2006 2008 2010

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Dissolved nitrate, in milligrams per liter as nitrogen

Figure 26. Dissolved nitrate concentration with LOESS smooth 

trend line for Little River below Little River tributary (site 10TA), 

1996–2009, in the vicinity of the Treyburn development study area, 

North Carolina.

1995 2000 2005 2010

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Dissolved nitrite plus nitrate, in milligrams per liter as nitrogen

Figure 27. Dissolved nitrite plus nitrate concentration with 

LOESS smooth trend line for Little River below Little River tributary 

(site 10TA), 1996–2009, in the vicinity of the Treyburn development 

study area, North Carolina.
Trend Analysis 55

1995 2000 2005 2010

0.1

0.5

0.3

0.9

0.7

1.3

1.1

1.5

Total ammonia plus organic nitrogen, in 

milligrams per liter as nitrogen

Figure 28. Total ammonia plus organic nitrogen concentration 

with LOESS smooth trend line for Little River below Little River 

tributary (site 10TA), 1996–2009, in the vicinity of the Treyburn 

development study area, North Carolina.

1995 2000 2005 2010

0.50

0

1.00

1.50

2.00

2.25

1.25

0.25

1.75

0.75

Total nitrogen, in milligrams per liter as nitrogen

Figure 29. Total nitrogen concentration with LOESS smooth 

trend line for Little River below Little River tributary (site 10TA), 

1996–2009, in the vicinity of the Treyburn development study area, 

North Carolina.

Dissolved nitrite plus nitrate, in milligrams per liter as nitrogen

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

0.8

0.7

0.9

0.6

0.5

0.4

0.3

0.2

0.1

0

Figure 30. Dissolved nitrite plus nitrate concentration with 

LOESS smooth trend line for Flat River (site 5T), 1994–2009, in the 

vicinity of the Treyburn development study area, North Carolina.

19901988 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

0

0.50

1.00

1.50

2.00

2.25

0.25

1.25

1.75

0.75

Total ammonia plus organic nitrogen, in 

milligrams per liter as nitrogen

Figure 31. Total ammonia plus organic nitrogen concentration 

with LOESS smooth trend line for Flat River (site 5T), 1988–2009, 

in the vicinity of the Treyburn development study area, North 

Carolina.
56 Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009 Suspended sediment, in milligrams per liter

19901988 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

500

5,000

100

1,000

10,000

10

5

0

50

Figure 32. Suspended-sediment concentration with LOESS 

smooth trend line for Flat River (site 5T), 1988–2009, in the vicinity 

of the Treyburn development study area, North Carolina.

19901988 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

10

30

50

70

90

100

110

60

80

20

40

120

 Specific conductance, in microsiemens per centimeter

at 25 degrees Celsius

Figure 33. Specific conductance with LOESS smooth trend line 

for Flat River tributary (site 1T), 1988–2009, in the vicinity of the 

Treyburn development study area, North Carolina.

Summary

Temporal distributions of water-quality characteristics of 

streams in the Treyburn development study area indicated only 

small-scale changes in water quality during the 1988–2009 

period-of-record study that might be related to landscape 

changes. Evaluation of loads, yields, and trends for nutrients 

and suspended sediment indicated variability with respect to 

streamflow conditions during the study period. Streamflow 

and water-quality data for this study were collected from 

October 1988 through September 2009 at five sites in the 

Treyburn study area and water-quality samples were collected 

from an additional site without continuous flow measurement. 

Water-quality data collected included physical water-quality 

characteristics and concentrations of nutrients, metals, and 

pesticides. 

Higher than average streamflow occurred during 2003. 

Streamflow during 2000, 2004, and 2005 was near the 

long-term mean. Average or lower than average streamflow 

occurred during most of the remaining years.

The lowest median value of specific conductance for 

the study sites occurred at the Flat River tributary (site 1T), 

the least developed of the study sites. The highest median 

value occurred at the Little River tributary (site 8T), the most 

developed site. Higher median specific conductance noted for 

the Little River below Little River tributary (site 10TA) and 

Mountain Creek (site 6T) may be due to development within 

these basins. 

Median pH for the Flat River tributary (site 1T) was significantly lower than for the other sites. No significant differences in median suspended-sediment concentration were noted 

among the sites. Iron and manganese were frequently detected 

over the 1987–2009 study period at all sites at concentrations 

greater than water-quality criterion levels. The frequency of 

detection of the other sampled metals at all sites is generally low, not usually exceeding laboratory reporting levels or 

criterion levels.

The smallest median nutrient concentrations occurred 

in the Flat River tributary (site 1T), which represents more 

undeveloped watershed conditions. The median values of the 

different nitrogen species at the Treyburn sites all fell within 

previously published background concentration ranges for the 

Piedmont Province in North Carolina. Phosphorus and nitrogen concentrations for the Treyburn sites are low compared to 

sites nationally.

Little River below Little River tributary (site 10TA) had 

the greatest number and frequency of pesticides detected and 

generally the highest concentrations. Four herbicides (atrazine, metolachlor, prometon, and simazine) were frequently 

detected at Mountain Creek (site 6T) and Little River tributary (site 8T). Three herbicides (atrazine, metolachlor, and 

alachlor) were detected at the Flat River tributary (site 1T). 

Herbicide concentrations at these sites were low compared to 

sites nationally.

Seasonality in constituent concentrations was apparent 

in the study sites. Little River below Little River tributary 
References Cited 57

(site 10TA), Mountain Creek (site 6T), Flat River (site 5T), 

and Eno River (site 11T) showed low suspended-sediment 

concentrations in June and July and January and December, with higher concentrations during the spring and fall. 

Dissolved-oxygen concentrations are highest in February and 

are lowest in July and August. Total nitrogen concentrations 

at Little River (4T) and Flat River (site 5T) are generally 

lowest during June through August. Nitrogen concentrations 

are higher from December through March at Mountain Creek 

(site 6T) and Little River below Little River tributary (site 8T). 

Few distinct seasonal patterns were apparent in phosphorus 

concentrations at the study sites.

Annual suspended-sediment loads and yields for the 

study sites showed considerable basin-to-basin and year-toyear variation. Flow data measured over the 1988–2009 period 

of record were used to generate load and yield estimates. The 

highest loads and yields for all sites occurred during water 

years 1996 or 2003. 

Little River below Little River tributary (site 10TA) 

had the lowest median suspended-sediment yield over the 

study period, and Flat River tributary (site 1T) had the largest 

median yield during the study period. The yields estimated for 

the study sites are low compared to suspended-sediment yields 

estimated for other basins in the Southeastern United States. 

The nitrogen and phosphorus yields estimated for the study 

sites are also low compared to nitrogen yields estimated for 

other basins in the Southeast.

Statistically significant increasing trends in total nitrogen concentration (1993–2009) and suspended-sediment 

concentrations (1996–2009) were detected for Mountain 

Creek (site 6T). A statistically significant increasing trend 

in specific conductance (1996–2009) was detected for Little 

River tributary (site 8T). Decreasing trends for the Flat River 

(site 5T) in dissolved nitrite plus nitrate nitrogen (1994–2009) 

and in total ammonia and organic nitrogen (1993–2009) were 

indicated. A decreasing 1987–2009 sediment trend for the Flat 

River (site 5T) and a statistically significant decreasing trend 

in specific conductance for Flat River tributary (site 1T) were 

detected. Few trends, however, were detected by both regression and the Seasonal Kendall test, suggesting ambiguity in 

these results. 

Water-chemical concentrations, loads, yields, and trends 

for the Treyburn study sites reflect some effects of upstream 

development. These measures of water quality, however, were 

generally within regional and national averages or considered 

low compared to those averages.

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Prepared by:

USGS Science Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

USGS Publishing Service Center staff:

Kay Naugle, Editor

Jeffery L. Corbett, Illustrator

Gregory L. Simpson, layout

For additional information regarding this publication, contact:

Director

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3916 Sunset Ridge Road

Raleigh, NC 27607

email: dc_nc@usgs.gov

Or visit USGS North Carolina Water Science Center at:

http://nc.water.usgs.gov/
Fine and others—Water Quality of Streams in the Treyburn Development Area, North Carolina, 1988–2009—Scientific Investigations Report 2013–5062