U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2018–5077

Prepared in cooperation with the Triangle Area Water Supply Monitoring Project 

Steering Committee

Trends in Water Quality of Selected Streams and Reservoirs 

Used for Water Supply in the Triangle Area of North Carolina, 

1989–2013
Cover. Jordan Lake upstream from U.S. Highway 64 near Wilsonville, North Carolina, June 6, 2018 

(photograph by Jen Schmitz, Triangle J Council of Governments).

Inset photographs. From left to right: U.S. Geological Survey (USGS) hydrologic technican retrieving 

high-flow samples at Northeast Creek near Genlee, NC, April 16, 2018 (photograph by Jessica Cain, USGS); 

Cane Creek near Orange Grove, NC, February 3, 2010 (photograph by Ryan Rasmussen, USGS); USGS 

hydrologic technicians collecting water-quality samples at Little River Reservoir near Bahama, NC, 

August 19, 2009 (photograph by Mary Giorgino, USGS); USGS hydrologic technician collecting samples 

at Jordan Lake near Wilsonville, NC, June 6, 2018 (photograph by Jen Schmitz, Triangle J Council of 

Governments).
Trends in Water Quality of Selected Streams 

and Reservoirs Used for Water Supply in the 

Triangle Area of North Carolina, 1989–2013

By Mary J. Giorgino, Thomas F. Cuffney, Stephen L. Harden, and Toby D. Feaster

Prepared in cooperation with the Triangle Area Water Supply Monitoring Project 

Steering Committee

Scientific Investigations Report 2018–5077

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Giorgino, M.J., Cuffney, T.F., Harden, S.L., and Feaster, T.D., 2018, Trends in water quality of selected streams and 

reservoirs used for water supply in the Triangle area of North Carolina, 1989–2013: U.S. Geological Survey Scientific 

Investigations Report 2018–5077, 67 p., https://doi.org/10.3133/sir20185077.

ISSN 2328-0328 (online)
iii

Acknowledgments

The long-term support of the Triangle Area Water Supply Monitoring Project Steering Committee 

is deeply appreciated. Committee members include representatives from Chatham and Orange 

Counties; the Towns of Apex, Cary, Hillsborough, and Morrisville; the City of Durham, and 

Orange Water and Sewer Authority. The Triangle J Council of Governments provides administrative and coordination support. 

The authors wish to thank Aldo Vecchia, U.S. Geological Survey (USGS), for tireless assistance 

with trend analysis. Aldo Vecchia, Heather Welch, and Chad Wagner, USGS, provided insightful 

comments on the draft report. Ashlyann Arana-Morales, Kitty Kolb, and Silvia Terziotti, USGS, 

provided technical assistance with report preparation. Finally, the authors express sincere 

thanks to all field personnel and science-support staff who diligently collected and quality 

assured streamflow and water-quality data over the duration of the project since its inception in 

October 1988.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Previous Studies ...................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Study Area.......................................................................................................................................................2

Approach.........................................................................................................................................................4

Site Selection.........................................................................................................................................4

Population and Land Cover ................................................................................................................4

Precipitation and Streamflow ............................................................................................................4

Water-Quality Data Compilation.........................................................................................................6

Time-Series Analysis for Water-Quality Trends...............................................................................8

Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions ............................10

Population and Land Cover...............................................................................................................10

Precipitation and Streamflow ..........................................................................................................13

Water Quality.......................................................................................................................................18

Specific Conductance and Major Ions ..................................................................................18

Nutrients and Organic Carbon.................................................................................................45

Total Suspended Solids and Suspended Sediment in Streams .........................................54

Secchi Transparency and Chlorophyll a in Lakes.................................................................59

Summary and Conclusions.........................................................................................................................63

References Cited..........................................................................................................................................64

Figures

1. Map showing location of the Triangle Area Water Supply Monitoring Project

trend sites in the upper Cape Fear and upper Neuse River Basins, North Carolina .........3

2–6. Graphs showing—

2. Population density for trend site watersheds in the Triangle area of

North Carolina, 1990 and 2010 .........................................................................................11

3. Percent change for generalized land-cover categories for trend site

watersheds in the Triangle area of North Carolina from 1992 to 2011 ......................13

4. Annual precipitation in the Northern Piedmont and Central Piedmont climate

divisions of North Carolina, 1989–2013, and long-term average precipitation

at the Raleigh-Durham International Airport, 1981–2010 ............................................14

5. Duration of daily mean flow yields for sites S1, S2, S3, and S4 in the

Neuse River Basin for the concurrent period October 1, 1988, to

September 30, 2013 ............................................................................................................16

6. Duration of daily mean flow yields for sites S5, S6, S7, S8, S9, S10Q, and S11

in the Cape Fear River Basin and sites S9 and S10Q in Morgan Creek

for the concurrent period September 15, 1999, to September 30, 2013 ....................17
vi

7–19. Maps and graphs showing trends at stream and lake sites assessed in the 

Triangle area of North Carolina, 1989–2013—

7. Specific conductance .......................................................................................................37

8. Dissolved calcium concentration ...................................................................................39

9. Dissolved magnesium concentration .............................................................................40

10. Dissolved potassium concentration ...............................................................................41

11. Dissolved sodium concentration .....................................................................................42

12. Dissolved chloride concentration ...................................................................................43

13. Dissolved sulfate concentration .....................................................................................44

14. Dissolved nitrate plus nitrite concentration ..................................................................46

15. Dissolved ammonia concentration .................................................................................48

16. Total organic nitrogen concentration .............................................................................49

17. Total nitrogen concentration............................................................................................51

18. Total phosphorus concentration .....................................................................................53

19. Total organic carbon concentration ...............................................................................55

20. Map and graphs showing trends in suspended solids concentration at stream

sites assessed in the Triangle area of North Carolina, 1989–2013 .....................................57

21. Map and graphs showing trends in suspended-sediment concentration at

stream sites assessed in the Triangle area of North Carolina, 1989–2013 ........................58

22. Map and graphs showing trends in Secchi transparency at lake sites assessed

in the Triangle area of North Carolina, 1993–2013 .................................................................61

23. Map and graph showing trends in phytoplankton chlorophyll a concentration

at lake sites assessed in the Triangle area of North Carolina, 1992–2013 ........................62

Tables

1. Stream and lake sites in the Triangle area, North Carolina, selected for trend

analysis and sources of data ......................................................................................................5

2. Population density estimates for trend site watersheds in the Triangle area of

North Carolina, 1990 and 2010, and change from 1990 to 2010 ...........................................11

3. Generalized land-cover distribution for trend site watersheds in the Triangle area

of North Carolina, circa 1992, circa 2011, and change from 1992 to 2011 .........................12

4. Flow-related watershed features and results of Kendall’s tau statistical test for

detection of monotonic trends in the annual minimum 7-day average flow, annual

mean flow, and annual peak flow at streamflow-monitoring sites in the Triangle

area of North Carolina, 1989−2013 ...........................................................................................15

5. Statistical summary of water-quality properties and constituents aggregated for

all stream and lake sites in the Triangle area of North Carolina for which trend

assessments were conducted .................................................................................................19

6. Statistical summary of water-quality properties and constituents compiled for

trend analysis in the Triangle area of North Carolina, by stream site ...............................20

7. Statistical summary of water-quality properties and constituents compiled for

trend analysis in the Triangle area of North Carolina, by lake site ....................................25

8. Summary of trend results based on data collected during 1989–2013 for specific

conductance, major ions, and nutrients at stream and lake sites assessed in the

Triangle area of North Carolina ................................................................................................30
vii

 9. Summary of trend results based on data collected during 1989–2013 for total 

Conversion Factors 

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

square mile (mi2

) 259.0 hectare (ha)

Flow rate

cubic foot per second per 

square mile ([ft3

/s]/mi2)

0.01093 cubic meter per second per 

square kilometer ([m3/s]/km2)

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

°F = (1.8 × °C) + 32.

Datum

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

Supplemental Information

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
viii

Abbreviations

7Q1 annual minimum 7-day average flow

EPA U.S. Environmental Protection Agency

GIS geographic information system

HPLC high-performance liquid chromatography

lidar light detection and ranging

MCFRBA Middle Cape Fear River Basin Association

N nitrogen

NCDEQ North Carolina Department of Environmental Quality

NLCD 1992 National Land Cover Dataset 1992

NLCD 2011 National Land Cover Database 2011

NOAA National Oceanic and Atmospheric Administration

NPDES National Pollutant Discharge Elimination System

NWIS National Water Information System

P phosphorus

QWTREND a parametric time-series model for detecting trends

SEAKEN Seasonal Kendall test

STORET STOrage and RETrieval database

TAWSMP Triangle Area Water Supply Monitoring Project

TKN total Kjeldahl nitrogen

TON total organic nitrogen

UCFRBA Upper Cape Fear River Basin Association

USGS U.S. Geological Survey

WRTDS Weighted Regressions on Time, Discharge, and Season

WWTP wastewater-treatment plant
Trends in Water Quality of Selected Streams and 

Reservoirs Used for Water Supply in the Triangle Area 

of North Carolina, 1989–2013

By Mary J. Giorgino, Thomas F. Cuffney, Stephen L. Harden, and Toby D. Feaster

Abstract

As the population of the Triangle area in central 

North Carolina increases, the demand for good quality 

drinking water from streams and lakes within the upper 

Neuse and upper Cape Fear River Basins also increases. The 

Triangle area includes Raleigh, Cary, Research Triangle Park, 

Durham, Chapel Hill, and the surrounding communities. 

The U.S. Geological Survey examined temporal trends in 

water quality for 13 stream and 8 reservoir sites in the two 

basins on the basis of data collected during 1989–2013. 

Trends were analyzed by using a fitted time-series model 

that accommodated for shifting trends and variations in 

streamflow at multiple time scales. Seventeen water-quality 

properties and constituents were evaluated, including specific 

conductance and major ions, nutrients, and organic carbon. 

Suspended solids and suspended sediment were examined 

at stream sites; chlorophyll a and Secchi transparency were 

examined at lake sites.

The investigation identified considerable changes in 

population, land cover, streamflow, and selected water-quality 

characteristics in the study area over the 25-year period. 

Specific conductance and concentrations of calcium, magnesium, potassium, sodium, and chloride tended to increase 

throughout the study area. Area-wide increases were also 

observed for organic nitrogen. Trends for other water-quality 

constituents varied on a more site-specific basis because of 

local watershed influences such as changes to wastewatertreatment processes and substantial shifts from rural to urban 

land use. Water quality is influenced by multiple, often 

confounding factors, and thus may change in a manner that 

is not uniform over time. Long-term monitoring is critical 

for tracking these trends and ensuring resiliency of water 

supplies for the future. Results from this study may promote 

the understanding of water-quality response to a growing 

population and to land-cover changes and can assist waterresource managers in the Triangle area in tracking progress 

toward water-quality goals. 

Introduction

The Triangle area of North Carolina includes the cities of 

Raleigh, Cary, Durham, and Chapel Hill; Research Triangle 

Park; and the surrounding communities. Ongoing growth in 

the Triangle area continues to increase the demand for local 

drinking-water supplies. At the same time, urbanization brings 

landscape changes that are likely to alter area hydrology and 

inputs of nutrients, sediment, and other water-quality constituents to lakes and rivers used as sources of drinking water. 

Several streams and lakes in the Triangle area are considered 

impaired because of degraded water quality (North Carolina 

Department of Environmental Quality, 2014), and all surface 

waters in the study area are designated “nutrient sensitive” 

(North Carolina Department of Environmental Quality, 2018). 

Raw water quality determines the type and level of water treatment needed to meet drinking-water standards. McDonald and 

others (2016) estimated that watershed degradation has led to 

increased water-treatment costs at 29 percent of cities globally, 

increasing costs, on average, by 53 percent for operation and 

maintenance and by 44 percent for replacement capital costs.

Recognizing the potential for ongoing population 

growth and landscape change to affect water-supply quality 

and quantity, State and local governments have committed 

to long-term monitoring and periodic assessments to protect 

these valuable resources. In 1988, several local governments 

joined to form the Triangle Area Water Supply Monitoring 

Project (TAWSMP) to systematically evaluate the quality of 

water-supply sources in the region. In partnership with the 

TAWSMP, the U.S. Geological Survey (USGS) has collected 

and analyzed water-quality samples from reservoirs and 

streams and has collected continuous records of streamflow in 

the study area for nearly 30 years. Data collected by USGS, 

State, and local monitoring programs form a valuable, longterm database for streamflow and water quality in the Triangle 

area. To date, however, the number of regional assessments 

conducted for long-term water-quality trends for area water 

supplies is limited. 
2 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Understanding temporal changes is critical for documenting progress toward water-quality goals and informing 

water-supply planning and land-management decisions. 

Therefore, the USGS compiled publicly available streamflow 

and water-quality data and analyzed trends during the 25-year 

period of 1989–2013 for streams and reservoirs in the Triangle 

area. Stream sites with long-term increases or decreases in 

flow were identified, and long-term increases or decreases 

in concentrations of selected water-quality constituents were 

identified for 13 stream sites and 8 reservoir sites. 

Previous Studies

This study supplements a previous analysis of waterquality trends in the Triangle area of North Carolina for the 

period 1983–95 (Childress and Bathala, 1997). At a national 

scale, Sprague and others (2009) evaluated trends in nutrients 

in major rivers of the United States during 1993–2003, including two sites that were evaluated in the current study. Oelsner 

and others (2017) investigated trends in nutrients, sediment, 

ions, carbon, and other water-quality constituents in U.S. rivers and streams during 1972–2012, including five stream sites 

in the current study, by using the Weighted Regressions on 

Time, Discharge, and Season (WRTDS) method (Hirsch and 

others, 2010; Hirsch and De Cicco, 2015). Spruill and others 

(2006) studied trends of suspended sediment and nutrients in 

the upper Cape Fear River Basin during 1976–2004, including 

one site in the current study. Water-quality trends in selected 

Triangle area streams have also been evaluated periodically 

by the North Carolina Department of Environmental Quality 

(NCDEQ) as part of their river basin assessment program. 

The methods used for trend analysis in the current study differ 

from the methods used in previous investigations as discussed 

later in the report.

Purpose and Scope

This report presents a regional comparison of trends in 

water quality at selected water-supply streams and reservoirs 

in the Triangle area of North Carolina. Trends in water quality 

were related to trends in population density, land cover, and 

other relevant watershed characteristics. Watershed and 

streamflow characteristics, trend analysis methods, and factors 

potentially affecting trends are discussed.

Twenty-one sites (13 stream sites and 8 lake sites) were 

evaluated for water-quality trends during the 25-year period 

of 1989–2013, hereafter referred to as “the trend period.” The 

period of analysis was less than 25 years for some sites and 

(or) constituents because sufficient data were unavailable. The 

evaluation of trends generally involved a nested comparison 

of a 1-period trend model for the entire period with a 2-period 

trend model for “early” (1989–2001) and “late” (2002–13) 

subperiods to assess whether trend rates and directions shifted 

over time. Step-change functions were included in the trend 

models when known events contributed to abrupt changes 

in water quality. The trend models that best fit the data were 

selected for use in the regional evaluation. The magnitude of 

uptrends and downtrends was expressed as a percent change 

in concentration; the model-fitted median concentrations 

for beginning and ending years of the trend period also 

were reported.

Seventeen water-quality properties and constituents 

were examined for trends, although not all constituents were 

analyzed at all sites. Water-quality characteristics included 

specific conductance, dissolved ions, nutrients, total organic 

carbon, suspended sediment and (or) solids, chlorophyll a, and 

water clarity. Water-quality data were obtained from publicly 

available sources; specifically, the USGS National Water 

Information System (NWIS) and the U.S. Environmental 

Protection Agency (EPA) STOrage and RETrieval (STORET) 

database. Water-quality datasets that were used as input for 

the trend analyses are available from Cain and others (2018). 

Streamflow data from USGS streamgage-monitoring stations 

used in the trend analyses also were obtained from NWIS. 

The trend-analysis approach used in this study allowed 

site-to-site comparisons throughout the study area during 

consistent time periods. A full discussion of site-specific trends 

was beyond the scope of this report; however, site-specific 

trends are presented when especially noteworthy.

Study Area

The study area is in the upper Neuse and upper 

Cape Fear River Basins in the Piedmont Physiographic 

Province of North Carolina (fig. 1). Characterized by low, 

rolling hills, the Piedmont extends westward to the Blue Ridge 

escarpment and eastward to the Coastal Plain. The geology 

of the region is complex, composed mainly of metamorphic 

and igneous rocks—primarily gneiss and schist—with granitic 

intrusions. The western part of the study area is generally 

located in the Carolina Slate Belt, which consists of heated 

and deformed volcanic and sedimentary rocks, including 

slate, gneiss, and metamudstone. The eastern part of the study 

area is in the Sanford-Durham Triassic Subbasin, which is 

dominated by claystones, siltstones, and other fine-grained 

sedimentary rocks (Bain and Brown, 1981; Daniel and 

Dahlen, 2002). 

Sites selected for trend analysis were in Chatham, 

Durham, Orange, and Wake Counties. From 1990 to 2010, 

the population of these four counties increased 85 percent—

from 737,825 to 1,365,886 people (Forstall, 1996; U.S. Census 

Bureau, 2017). The population of the same four counties 

is projected to increase by another 44 percent by the year 

2030 (North Carolina Office of State Budget and Management, 2017).

Streams and reservoirs in the Triangle area are important 

water-supply sources and also support recreation, aquatic 

life, and other uses. Numerous efforts have been made 

through the years to protect these water supplies and ensure 
Study Area 3

Figure 1. Location of the Triangle Area Water Supply Monitoring Project trend sites in the upper Cape Fear 

and upper Neuse River Basins, North Carolina. (Geologic belts shown in inset map were modified from 

Huffman and others, 2006.)

Cape Fear

River Basin

Neuse

River Basin

Piedmont Coastal Plain

Study

area

Carolina

Slate Belt

Sanford-Durham

Subbasin

Blue Ridge

DURHAM

S4

S3 L2

L1

S1 S2

S5

L3

S9

L5

S6

S10Q

S10

S7

S8

L6

L7 L8

S11

L4

S12

S13

Cary

Raleigh-Durham

International

Airport

# #

#

#

#

#

#

#

#

#

#

#

#

#

79°15' 79° 78°45'

36°

35°45'

Northeast Creek

res18-0031_fig01

EXPLANATION

Basin boundary

Water-quality stream site and number

Water-quality lake site and number

Streamflow site and number

Precipitation station

0 5 10 MILES

0 5 10 KILOMETERS

Base map from digital files of

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

U.S. Department of Commerce, Bureau of Census, 2016

TIGER/Line Files- Political Boundaries

North Carolina Department of Transportation Municipal Boundaries

Falls

Lake

Lake

Michie

Little River

Reservoir

Cane Creek

Reservoir

University

Lake

Jordan

Lake

Pittsboro

Brickhaven

Bynum

Raleigh

Durham

Chapel

Hill

Carrboro

Hillsborough

Apex

ALAMANCE

CHATHAM

LEE

HARNETT

ORANGE

Butner

Burlington

Mebane

Flat River

Cane

Creek

Little River

Eno River

New

Hope Creek

Morgan Creek

Haw River

Deep River

Whit e Oak Creek

Cape Fear

River

Cape Fear

River Basin

Neuse River

Basin

GRANVILLE

WAKE

S13 #

S10Q #

L4
4 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

their long-term sustainability and suitability for use. Various 

regulatory and voluntary efforts have been undertaken to manage inputs of pollutants to the reservoirs and their tributary 

streams. For example, a statewide phosphate-detergent ban 

was enacted in January 1988, which resulted in regionwide 

reductions of instream phosphorus concentrations (Childress 

and Bathala, 1997). Local governments have upgraded 

wastewater-treatment facilities to reduce both phosphorus 

and nitrogen levels, implemented land and water conservation practices, managed stormwater, and implemented best 

management practices to reduce runoff from development. 

Agricultural agencies and landowners have implemented best 

management practices to achieve sediment and nutrient-runoff 

reductions. Stream restoration projects have been constructed 

to restore more natural hydrology, and numerous watershed 

restoration plans have been adopted.

Approach

This section provides an overview of the sites selected 

for trend analysis and the methods used to compile watershed 

characteristics (land cover and population), hydrologic 

data (precipitation and streamflow), and water-quality data. 

Statistical methods used to evaluate trends in streamflow and 

water-quality constituents at the study sites are discussed. 

Site Selection

The geographic extent of the TAWSMP network has 

varied over time. As previously stated, sites selected for the 

current trend analysis were in Orange, Durham, Chatham, and 

Wake Counties. The land area that drains to the selected sites 

extends farther upstream to the headwaters of the Neuse and 

Haw Rivers and downstream to the headwaters of Falls Lake 

and the Cape Fear River near Brickhaven. Stream sites, lake 

sites, and water-quality constituents were selected for trend 

analysis on the basis of available data. 

In the Neuse River Basin, the four stream sites selected 

for water-quality trend analysis were located on the Flat, 

Little, and Eno Rivers, and the two lake sites were in Lake 

Michie and Little River Reservoir. In the Cape Fear River 

Basin, the nine stream sites selected for water-quality trend 

analysis were located on the Cane, Morgan, New Hope, 

Northeast, and White Oak Creeks and the Haw and Cape Fear 

Rivers, and the six lake sites were in Cane Creek Reservoir, 

University Lake, and Jordan Lake (fig. 1; table 1). Morgan 

Creek near Farrington (site S10) was a water-quality sampling 

site with no streamflow gage; thus, Morgan Creek near 

Chapel Hill (site S10Q) was used as the source of streamflow 

data for site S10.

Population and Land Cover 

Watershed boundaries and contributing drainage areas 

for the sites were determined with the USGS StreamStats 

application developed for North Carolina (https://streamstats.

usgs.gov/ss/; Weaver and others, 2012). These features were 

calculated by using a 30-foot (ft) by 30-ft light detection and 

ranging (lidar)-derived digital elevation model. Drainage areas 

for the trend sites ranged in size from 7.54 to 3,160 square 

miles (mi2; table 1). Note that drainage area delineation was 

not applicable for Jordan Lake at Bells Landing (site L8), 

because the site is in a cove of Jordan Lake. Jordan Lake was 

impounded in 1982, and water quality at site L8 primarily 

depends on upstream conditions in the New Hope Creek arm.

Population statistics were compiled by using geographic 

information system (GIS) processes. Watershed boundaries 

were intersected with 1990 and 2010 U.S. Census block 

datasets. The proportion of each census block’s area within 

each watershed was calculated. Each watershed’s population 

was computed as the area-weighted sum of the census block 

populations within the watershed boundary. Population 

density was obtained by dividing the total population by the 

drainage area.

Land-cover data for the trend site watersheds were compiled by using the StreamStats application. The 1992 and 2011 

National Land Cover Datasets (NLCD 1992 and NLCD 2011) 

were used to characterize conditions near the beginning and 

end of the 1989–2013 trend period for each site. Mapping 

methods differ for the two datasets; therefore, comparisons 

should be interpreted broadly. The NLCD 1992 was based 

primarily on circa 1992 Landsat Thematic Mapper data 

(Vogelmann and others, 2001) and used a 21-class land-cover 

classification scheme. The NLCD 2011 was based primarily 

on circa 2011 Landsat satellite data and included a 16-class 

land-cover classification scheme (Homer and others, 2015). 

For this report, detailed land-cover classes were aggregated 

into three generalized categories—developed, agricultural, and 

forested/other, which included water and wetlands.

Precipitation and Streamflow 

Although many factors influence streamflow, including 

basin size and slope, interaction with groundwater, land cover, 

geology, dams, and water-supply withdrawals, streamflow 

amounts are determined primarily by the amount of precipitation that falls in a watershed. Regional precipitation data 

for the Northern Piedmont and Central Piedmont climate 

divisions of North Carolina were obtained from the National 

Oceanic and Atmospheric Administration (NOAA, 2018). 

The Northern Piedmont climate division includes Orange 

and Durham Counties; the Central Piedmont climate division 
Approach 5

Table 1. Stream and lake sites in the Triangle area, North Carolina, selected for trend analysis and sources of data.

[no., number; USGS, U.S. Geological Survey; mi2, square mile; NCDEQ, North Carolina Department of Environmental Quality; AMS, Ambient Monitoring 

System; NC, North Carolina; na, not applicable; SR, Secondary Road; UCFRBA, Upper Cape Fear River Basin Association; USACE, U.S. Army Corps of 

Engineers; MCFRBA, Middle Cape Fear River Basin Association. Gray shading indicates sites in the Neuse River Basin; unshaded rows represent sites in the 

Cape Fear River Basin]

Map no. 

(fig. 1)

USGS 

station no. USGS station name

Drainage 

area 

(mi2)

Source of 

streamflow 

data

Source of 

water-quality 

data

NCDEQ AMS 

station no.

NCDEQ AMS 

station name

Stream sites

S1 02085000 Eno River at Hillsborough, NC 66.0 USGS gage at site USGS na na

S2 02085070 Eno River near Durham, NC 141 USGS gage at site USGS, NCDEQ J0770000 Eno River at US 501 

near Durham

S3 0208521324 Little River at SR 1461 near Orange 

Factory, NC

78.2 USGS gage at site USGS, NCDEQ J0820000 Little River at SR 1461 

near Orange Factory

S4 02085500 Flat River at Bahama, NC 149 USGS gage at site USGS, NCDEQ J1070000 Flat River at SR 1614 

near Quail Roost

S5 02096846 Cane Creek near Orange Grove, NC 7.54 USGS gage at site USGS na na

S6 02096960 Haw River near Bynum, NC 1,275 USGS gage at site USGS, NCDEQ, 

UCFRBA

B2100000 Haw River at SR 1713 

near Bynum

S7 02097314 New Hope Creek near Blands, NC 75.9 USGS gage at site USGS, NCDEQ, 

UCFRBA

B3040000 New Hope Creek at 

SR 1107 near Blands

S8 0209741955 Northeast Creek at SR 1100 near Genlee, 

NC

21.1 USGS gage at site USGS, NCDEQ B3660000 Northeast Creek at 

SR 1100 near Nelson

S9 02097464 Morgan Creek near White Cross, NC 8.35 USGS gage at site USGS na na

S10 02097521 Morgan Creek near Farrington, NC 46.0 USGS station 

02097517

USGS, NCDEQ, 

UCFRBA

B3900000 Morgan Creek at SR 

1726 near Farrington

S11 0209782609 White Oak Creek at mouth near Green 

Level, NC

11.9 USGS gage at site USGS na na

S12 02098198 Haw River below B. Everett Jordan Dam 

near Moncure, NC

1,689 USACE Jordan 

Lake dam 

releases

USGS, NCDEQ B4050000 Haw River below Jordan 

Dam near Moncure

S13 0210215985 Cape Fear River at State Highway 42 near 

Brickhaven, NC

3,160 USGS stations 

02098206 and 

02100500

USGS, NCDEQ, 

MCFRBA

B6160000 Cape Fear River at 

NC 42 near Corinth

Lake sites

L1 0208524845 Little River Reservoir at Dam near 

Bahama, NC

97.7 USGS station 

0208521324

USGS na na

L2 02086490 Lake Michie at Dam near Bahama, NC 167 USGS station 

02085500

USGS na na

L3 0209684980 Cane Creek Reservoir at Dam near White 

Cross, NC

31.4 USGS station 

02096846

USGS na na

L4 0209699999 Jordan Lake, Haw River arm near Hanks 

Chapel, NC

1,303 USGS station 

02096960

USGS na na

L5 0209749990 University Lake at Intakes near Chapel 

Hill, NC

30.0 USGS station 

02097464

USGS na na

L6 0209768310 Jordan Lake at Buoy 12 at Farrington, NC 231 USGS stations 

02097314 and 

02097517

USGS na na

L7 0209799150 Jordan Lake above U.S. Highway 64 at 

Wilsonville, NC

285 USGS stations 

02097314 and 

02097517

USGS na na

L8 0209801100 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

na USGS stations 

02097314 and 

02097517

USGS na na
6 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

includes Chatham and Wake Counties. For comparison, the 

long-term, annual average precipitation data for the RaleighDurham International Airport (fig. 1) also were obtained from 

NOAA (2017).

The USGS operates a network of streamflow-gaging 

stations throughout the study area. For each stream-monitoring 

site (table 1), daily mean streamflow values were obtained 

from the USGS NWIS database (U.S. Geological Survey, 

2014). Streamflow values for two water-quality sites (S10 

and S13) were computed by using data from nearby upstream 

gages (table 1). In addition, daily records of releases from the 

Jordan Lake dam were obtained from the U.S. Army Corps of 

Engineers (2018) to estimate flows at the Haw River below 

Jordan dam (site S12; table 1).

Flow was not measured at lake sites; however, the waterquality trend analysis technique used in this study required 

the input of daily flow data, or a representative surrogate, 

for all sites. Therefore, flow records from upstream USGS 

streamgages were used to construct surrogate daily-flow 

values for each lake site (table 1). 

Kendall’s tau test (Helsel and Hirsch, 2002) was used 

to assess trends in streamflow at USGS streamflow-gaging 

stations in the upper Neuse and upper Cape Fear River 

Basins. The period of time for which streamflow records 

were available varied considerably among sites, ranging from 

15 to 90 years. To facilitate comparisons, however, trends 

in streamflow were evaluated for the period 1989–2013 at 

all sites except White Oak Creek (S11), where streamflow 

monitoring began in 1999. To assess the full range of flows, 

trend tests were performed on three flow statistics: annual 

minimum 7-day average flow (7Q1); annual mean flow; and 

annual peak flow. Available approved data were used through 

March 31, 2014, which corresponds with the 2013 climatic 

year. Climatic year is the annual period from April 1 through 

March 31 that conventionally is used by the USGS for analysis 

of low-flow statistics. The USGS uses the annual period from 

October 1 through September 30 (water year) for analysis of 

annual mean and peak flow statistics.

To illustrate how streamflow characteristics varied 

among sites, durations of daily flow were computed for the 

four Neuse River Basin stations for the concurrent period 

of record from October 1, 1988, to September 30, 2013. 

Likewise, durations of daily-flow yields were computed for 

seven Cape Fear River Basin stations for the concurrent period 

of record from September 15, 1999, to September 30, 2013 

(the period for which streamflow data were available for all 

seven sites). To eliminate differences due to drainage area size, 

the duration flow for a site was divided by the drainage area 

of the site, resulting in flow yield in cubic feet per second per 

square mile. 

Water-Quality Data Compilation

Water-quality monitoring has been conducted at the 

study sites by multiple agencies, including the USGS, the 

NCDEQ, and two regional monitoring coalitions. Depending 

on the individual study site, data were compiled from one to 

three sources (table 1). USGS data were retrieved from NWIS 

(U.S. Geological Survey, 2014). The NCDEQ monitors nine of 

the same USGS stream sites as part of its Ambient Monitoring 

System. The NCDEQ site identification numbers and names 

that correspond to the USGS sites are listed in table 1. The 

Upper Cape Fear River Basin Association (UCFRBA) and the 

Middle Cape Fear River Basin Association (MCFRBA) monitor water quality at three and one TAWSMP sites, respectively, 

under Memorandums of Agreement with the NCDEQ and 

adhere to the NCDEQ site-numbering scheme (table 1). Data 

from these two coalitions are quality assured and uploaded 

to STORET by the NCDEQ. Water-quality data for the 

NCDEQ, UCFRBA, and MCFRBA monitoring stations were 

retrieved from the STORET database (U.S. Environmental 

Protection Agency, 2014). All water-quality data used for the 

trend analysis were previously quality assured by the source 

agencies (USGS or NCDEQ) prior to being downloaded from 

the NWIS or STORET databases.

In contrast to the stream sites, only USGS water-quality 

data were used for trend analysis at lake sites. The USGS 

sampled lake sites at multiple depths; however, only 

upper/photic zone samples were included in the trend analysis. 

Although the NCDEQ also sampled many of the TAWSMP 

lakes through its Ambient Lakes Monitoring program, 

NCDEQ sampling historically focused on one season of 

the year (summer) and was conducted once every 5 years 

in accordance with a river-basin rotational schedule. The 

NCDEQ sampled Jordan Lake more frequently; however, 

sampling locations generally did not correspond to TAWSMP 

trend sites. 

Properties and constituents selected for trend analysis 

at stream and lake sites included specific conductance, major 

ions (calcium, magnesium, potassium, sodium, chloride, and 

sulfate), nutrients (nitrate plus nitrite [hereafter referred to 

as nitrate], ammonia, total organic nitrogen, total nitrogen, 

and total phosphorus), and total organic carbon. In addition, 

suspended solids and suspended-sediment data were available 

for stream sites. Chlorophyll a and Secchi depth measurements of water transparency were monitored only at lake sites.

Water-quality data for the 13 stream- and 8 lake-monitoring sites were compiled for the period January 1989 through 

December 2013. For stream sites sampled by more than one 

agency, analytical results for multiple samples occasionally 

were available on the same date. In these cases, only the 

sample with the most complete set of analytical results was 

retained. Most sites had water-quality data for the entire 

25-year period of record, but eight sites had shorter periods of 

record. Water-quality data began in 1990 for site S1, in 1991 

for sites L4, L7, and L8, in 1993 for site L6, and in 2000 for 

site S11. Data for site S4 ended in 2011. At site S13, data for 

major ions ended in 2003, and data for all other constituents 

ended in 2012. 

As might be expected, different stream-sampling methods 

were used during the trend period, both among and within 

agencies. The USGS generally collected cross-sectional, 
Approach 7

depth-integrated composite samples from streams, but 

occasionally automated samplers were used to collect highflow samples from a single point in a stream. The NCDEQ, 

UCFRBA, and MCFRBA typically collected samples from 

a single point at mid-channel or at the point of greatest 

flow. Sampling-method differences may have influenced 

water-quality results, particularly for unfiltered constituents 

that include particulate matter, such as total nitrogen and total 

phosphorus. For trend analysis, results were not differentiated 

on the basis of sampling method, and the data were assumed 

to be representative and comparable. Laboratory analytical 

methods and reporting levels evolved during the trend period 

as well. Analytical approaches for data with multiple reporting 

levels are discussed later in this report. 

The compiled data were reviewed to identify questionable 

results and obvious outliers, which were then excluded from 

the final trend-analysis datasets. Reviews included examining nitrogen-fraction ratios, data plots, plots of streamflow 

versus constituent, and residuals plots from exploratory trend 

analysis. An example of questionable data included one 

sample from site S10 and another from site S12 with reported 

ammonia concentrations that were greater than the concentrations reported for ammonia plus organic nitrogen (theoretically 

impossible). In another case, one very high concentration of 

total suspended solids was reported at Eno River near Durham 

(site S2) during extremely low-flow conditions, which 

suggested that the sampling device may have contacted the 

river bottom and thus contaminated the sample. Clear outliers 

that likely represented episodic events also were eliminated 

from the trend datasets. For example, one sample collected 

at Eno River at Hillsborough (site S1) had an unusually high 

sulfate concentration of 170 milligrams per liter (mg/L) that 

also contributed to an unusual specific conductance value of 

304 microsiemens per centimeter (μS/cm). The data were 

valid but highly atypical for this site; therefore, these two 

values were excluded from the trend analysis. 

Retrievals of water-quality data included remark codes 

associated with analytical results, such as “less than” (<) 

censored values. During February through July 2001, the 

NCDEQ laboratory implemented changes to internal qualityassurance practices and analytical methods that resulted in 

substantial, but temporary, increases in analytical reporting 

levels for nutrient constituents (North Carolina Department of 

Environmental Quality, 2017). Retention of these data would 

have artificially inflated levels used to re-censor data during 

the trend analysis; therefore, data for samples collected by 

NCDEQ during this period were excluded. 

The USGS lake-sampling methods for chlorophyll a

changed in 1992. Prior to 1992, samples were collected at 

1 meter (m) below the water surface by using a grab-sampling 

device. Beginning in 1992, chlorophyll a samples were 

collected throughout the euphotic zone, operationally defined 

as the depth equal to twice the Secchi transparency depth, by 

using a depth-integrating sampler. The change to photic-zone 

sampling occurred to align USGS and NCDEQ lakesampling protocols and thereby improve data comparability 

(Oblinger, 2004). Shifts in chlorophyll a concentrations that 

coincided with this method change were evident at several 

lake sites; therefore, pre-1992 data were excluded from the 

trend analysis. 

For nutrients, all monitoring agencies reported concentrations of the various nitrogen fractions in milligrams per liter as 

nitrogen (N) and concentrations of total phosphorus in milligrams per liter as phosphorus (P). Analytical results for USGS 

nutrient samples included concentrations of total ammonia 

plus organic N, filtered ammonia, filtered nitrate plus nitrite, 

total N, and total P. Nutrient results from NCDEQ, UCFRBA, 

and MCFRBA were based on unfiltered samples and therefore 

included concentrations of total ammonia plus organic N (also 

referred to as total Kjeldahl nitrogen [TKN]), total ammonia, 

total nitrate plus nitrite, and total P. Concentrations reported 

for total ammonia and total nitrate plus nitrite were assumed 

to be equivalent to concentrations of dissolved ammonia 

and dissolved nitrate plus nitrite, respectively, because these 

constituents are present in water only in dissolved form. In 

this study, trends were analyzed for the following nitrogen 

fractions: nitrate plus nitrite, hereafter referred to as “nitrate,” 

ammonia, total organic N, and total N.

Values for total organic N and total N were computed by 

using concentrations reported for the aforementioned, directly 

measured N fractions. Total organic N, which includes both 

particulate and dissolved forms, was computed by subtracting 

ammonia from total ammonia plus organic N. When one or 

both of these constituents was left-censored (less than the 

reporting level), a “<” remark code was or was not applied to 

the computed total organic N concentration as follows: 

• If total ammonia plus organic N was left-censored, the 

remark code was carried forward with the computed 

value for total organic N regardless of whether ammonia was censored or uncensored.

• In those cases where total ammonia plus organic N was 

uncensored and ammonia was left-censored, ammonia 

typically represented a small fraction, less than 20 percent, of the total ammonia plus organic N values; thus, 

the remark code was not applied to the computed total 

organic N values.

Values of total N were computed by summing concentrations 

for total ammonia plus organic N and nitrate plus nitrite. When 

one or both of these constituents was left-censored, the “<” 

remark code was or was not applied to the computed total N 

concentration as follows: 

• If both total ammonia plus organic N and nitrate plus 

nitrite were left-censored, then the remark code was 

carried forward with the computed value for total N.

• If only one of the constituents was left-censored and its 

value represented 40 or more percent of the computed 

value, the remark code also was assigned to the computed total N value; otherwise, the remark code was 

not applied. 
8 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

The final datasets used for trend analysis contained more 

than 47,000 water-quality observations and are available from 

Cain and others (2018). Statistical summaries were useful for 

describing and comparing general water-quality characteristics 

among sites and provided information relevant for analyzing 

and interpreting trends. The total number of observations, 

number and percentage of censored observations, mean and 

standard deviation, and minimum, maximum, and median 

values were computed for the aggregated stream data, the 

aggregated lake data, and each site-constituent pairing. 

Summary statistics for constituents with censored data were 

determined by use of the Kaplan-Meier technique (Helsel, 

2005). Although the concentration data were assumed to have 

a lognormal distribution, this technique provided unbiased 

estimates of percentiles, median, and interquartile range 

for a variety of data distributions, even those that were not 

lognormal. The percentage of censored data in a trend dataset 

was particularly important, because the trend analysis could 

fail to run or produce misleading results when used to analyze 

highly censored datasets. For this reason, trend results were 

not presented for station-constituent datasets that contained 

more than 20 percent censored data. Of 232 site-constituent 

pairings, 18 had high censoring that precluded trends analysis. 

This threshold was exceeded for three water-quality constituents: ammonia at five stream sites (S2, S3, S4, S5, and S6) and 

four lake sites (L5, L6, L7, and L8); total suspended solids at 

two stream sites (S3 and S6); and nitrate at seven lake sites 

(L1, L2, L3, L5, L6, L7, and L8). 

Time-Series Analysis for Water-Quality Trends

Trends in 17 water-quality properties and constituents 

were analyzed on a site-by-site basis. Increases and decreases 

over time were expressed as both percentage difference and 

magnitude of change in annual fitted-median concentrations 

from the beginning to the ending year of the trend period. 

Trends were identified as statistically significant or nonsignificant on the basis of a widely used convention of p-value 

less than 0.05. Conventional tests of significance are informative; however, the choice of a significance level is somewhat 

discretionary. Therefore, the direction and magnitude of 

change were provided for all reported trends regardless of 

the p-value.

The QWTREND program, a parametric statistical 

time-series model for detecting trends developed by the USGS 

(Vecchia, 2000, 2003, 2004a, 2004b, 2005), was used to 

determine whether statistically significant trends existed for 

the 17 water-quality properties and constituents examined in 

this study. QWTREND takes into consideration the relation 

between a property or constituent concentration and both 

concurrent (same day as concentration sample) and lagged 

(days leading up to the concentration sample) flow at multiple 

time scales (annual, seasonal, and daily). Characterizing flowrelated variability at multiple time scales is important because 

many water-quality properties and constituent concentrations 

may depend on streamflow in complex ways that cannot be 

adequately accounted for by using a regression model relating 

concentration only with concurrent streamflow. Accounting 

for as much flow-related variability as possible increases the 

ability to detect trends in concentration independent of trends 

in flow arising from long- and short-term climatic variation. 

QWTREND uses a time-series analysis to express 

log-transformed constituent concentration in terms of additive 

components consisting of a trend, flow-related variability, and 

serially correlated errors: 

log(C) = MC + ANNC + SEASC + HFVC + TRENDC

where 

log denotes the base-10 logarithm; 

C is the concentration, in milligrams or 

micrograms per liter; 

MC is the long-term mean of the log10-transformed 

concentration; 

ANNC is the annual concentration variability; 

SEASC is the seasonal concentration variability; 

HFVC is the high-frequency variability of the 

concentration; and 

TRENDC is the concentration trend (dimensionless). 

ANNC , SEASC , and HVFC represent natural variability in 

concentration for different time scales. ANNC is an estimate of 

the interannual variability in concentration that can be attributed to long-term variability in streamflow. For example, the 

proportion of surface runoff to base flow in streams is affected 

by extended dry and wet periods, which result in changes to 

stream water quality. SEASC is an estimate of the variability in 

concentration that can be attributed to seasonal fluctuations in 

streamflow or factors other than streamflow, such as snowmelt, 

water temperatures, fertilizer applications, or road deicing. 

Both ANNC and SEASC depend on concurrent and lagged daily 

streamflow (Vecchia, 2005, appendix 1). HFVC estimates the 

variability in concentration over time intervals shorter than 

a season (for example, several days or weeks), such as daily 

changes in weather that may cause variability in streamflow 

and water quality. HFVC depends on both concurrent and 

lagged daily streamflow and, in addition, accounts for potential serial correlation between concentration samples separated by short time intervals (several days to several months) 

(Vecchia, 2005, appendix 1). TRENDC estimates long-term 

systematic changes in water-quality concentration that are 

unrelated to streamflow, such as a change in human activities. 

Step (abrupt), monotonic (gradual upward or downward), or 

cyclic trends are possible. Trends can persist for a short time 

before ending or reversing direction. In general, TRENDC can 

consist of any linear combination of piecewise monotonic 

trends, step trends, or other more complicated trends that may 

be considered, depending on the objectives of the trend analysis and the adequacy of the data (for example, sampling frequency, record length) for determining the trend coefficients.
Approach 9

The QWTREND program uses Gaussian maximum likelihood estimation to fit the model parameters, choose the best 

trend model, and determine the significance levels (p-values) 

associated with the trends. For this study, QWTREND was 

used to compare among nested models (null [no trend], 

1-period trend, and 2-period trend), and statistically significant 

trends were identified when the probability of a Type I error 

was less than 5 percent (α = 0.05). Generalized likelihood 

ratio tests (Vecchia, 2005, appendix 1) were used to determine 

if the 2-period trend model was a significantly better fit 

than the 1-period trend model. Additional step trends were 

included in certain cases, when appropriate. The magnitude of 

uptrends and downtrends was expressed as a percent change 

in concentration; the annual model-fitted median concentrations for beginning and ending years of the trend period also 

were reported.

QWTREND analysis uses nonlinear optimization, is 

computation-intensive, requires care in fitting the models 

and verifying assumptions about the model and data, and 

cannot assess highly censored datasets. For this study, the 

following guidelines were used for including a constituent in a 

QWTREND analysis:

• at least 15 years of continuous daily streamflow data

• at least 1 year of flow data prior to the collection of the 

first water-quality sample

• at least 10 years of water-quality data, not necessarily 

consecutive

• at least 4 samples per year

• at least 15 samples in each 3-month season 

(January–March, April–June,...)

• less than 20 percent censored data

QWTREND replaced left-censored (“<”) values with half of 

the largest censored value in the dataset for the constituent and 

site. If an uncensored value was less than half of the largest 

censored value, it was also censored at that value. A time step 

of 3 samples per month (36 samples per year) was used in 

the QWTREND analyses. Many of the concentration values 

were missing, which is permissible, but streamflow values 

were available. The three samples per month represented early 

(1st–10th), middle (11th–20th), or late (21st–31st) periods in 

the month. If a water-quality constituent was measured once in 

an interval, the daily mean streamflow value that corresponded 

to the water-quality measurement date was used for that 

period. If a water-quality constituent was measured more than 

once in an interval, the daily mean streamflow value closest to 

the middle of the interval was used. Likewise, if the waterquality constituent was not measured in an interval, streamflow for the middle of the interval was used (that is, the 5th, 

15th, or 25th of the month for the early, middle, or late period 

in the month, respectively).

Selection of trend analysis time periods was important 

because results are dependent on how the time periods 

are structured. Factors considered in the selection of trend 

analysis time periods included the timing of known watershed 

perturbations that might have affected water quality, such as 

wastewater-treatment plant (WWTP) upgrades; examination 

of residual plots from QWTREND analyses with no trend 

(null model); temporal distribution of available data; definition 

of time periods of sufficient length to provide reasonably 

meaningful information on temporal variability; and changes 

in analytical methods, such as the change in chlorophyll a

methods that occurred in October of 2005. 

Multiple QWTREND analyses were run for each 

site-constituent combination. Identification of trends generally 

involved a nested comparison of models in which a 1-period 

trend model was compared with the null (no trend) model, and 

a 2-period trend model was compared with the corresponding 

1-period trend model. The 1-period trend model consisted 

of a single monotonic (increasing or decreasing) trend for 

the entire period of record. This period generally was 1989 

through 2013, but varied among sites and constituents 

depending on the availability of data. For sites with sufficient 

data both before and after the midpoint of the sampling period, 

the 2-period trend model also was applied. January 1, 2002, 

was selected as the beginning of the second period in order to 

divide the 25-year trend period approximately in half and to 

ensure that each of the smaller periods had at least 10 years 

of data. The 2-period trend model consisted of two piecewise 

monotonic trends for “early” and “late” time periods: 1989 

(or beginning of record) through 2001, and 2002 through 

2013 (or end of record). The generalized likelihood ratio test 

statistic was used to determine if the 2-period trend model 

provided a significantly (α = 0.05) better fit with the data than 

the 1-period trend model. The generalized likelihood ratio test 

is analogous to a partial F-test in a multiple linear regression 

framework (Helsel and Hirsch, 2002). Only the “best fit” 

trend model for each site-constituent pairing was selected 

and presented in the report. For site-constituent combinations 

with short periods of record, a single trend during either the 

early or late period may have been the only possibility that 

could be considered, in which case it was not possible to 

determine if there was a similar (or different) trend during the 

missing period. 

In two cases, site- or constituent-specific models were 

examined if known changes in environmental conditions or 

analytical methods warranted an alternative model. First, a 

municipal WWTP that discharges to Northeast Creek (site S8) 

was upgraded to enhance nitrogen removal in July 2005. All 

constituents at this site were analyzed by using three models: 

a step trend beginning in July 2005; a 1-period (1989–2013) 

monotonic trend; and a model that combined the step and 

1-period trends. Interestingly, for each constituent evaluated at 

site S8, either the step trend or the 1-period trend model was 

selected. The simultaneous step-plus-monotonic trend model 

did not provide a significantly better fit. 
10 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Second, laboratory methods used to analyze 

chlorophyll a samples changed radically in October 2005, 

which warranted including a step function in the trend analyses for this constituent. From 1992 through September 2005, 

chlorophyll a was analyzed using a high-performance liquid 

chromatography (HPLC) method that was highly specific for 

chlorophyll a and no other forms of chlorophyll or accessory 

pigments. On October 1, 2005, the USGS National Water 

Quality Laboratory discontinued the HPLC method; thereafter, samples were analyzed using a modified EPA Method 

445.0, which collectively measured chlorophyll a and 

portions of other algal pigments. Evaluation of split-replicate 

samples conducted prior to the method change indicated 

that up to threefold increases in reported chlorophyll a

concentrations would likely occur. Data collected before 

and after the method change are not equivalent and should 

not be combined for trend analysis without including a step 

function that can accommodate effects of the method change. 

Therefore, the QWTREND models for chlorophyll a simultaneously evaluated a step function at the method change and 

a 1-period trend test for the period of record. This approach 

was critical for distinguishing environmental trends from 

shifts that resulted from the method change.

The seasonal Kendall (SEAKEN) test (Hirsch and 

Slack, 1984; Helsel and Hirsch, 2002) is an alternative trend 

assessment approach often used to evaluate trends in constituent concentrations. A primary advantage of SEAKEN over 

a parametric time-series approach such as QWTREND is that 

SEAKEN is a nonparametric procedure and thus requires 

fewer assumptions than QWTREND. Another advantage of 

SEAKEN is that it allows up to 50 percent censored values, 

whereas QWTREND is not recommended for datasets with 

more than 20 percent censored values. 

There are three important advantages of QWTREND 

compared to SEAKEN. First, QWTREND jointly models 

flow-related variability and trends using both concurrent and 

antecedent daily streamflow, whereas SEAKEN is applied to 

the residuals from a regression model relating concentration 

and concurrent daily flow (flow-adjusted concentrations). 

Thus, in many cases QWTREND explains considerably more 

flow-related variability and makes trends easier to detect 

compared to SEAKEN. Second, QWTREND can be used to 

model complex, nonmonotonic trends, whereas SEAKEN 

assumes a single monotonic trend. Third, QWTREND does 

not require a fixed sampling interval for concentration, 

whereas SEAKEN requires a fixed sampling interval (such as 

monthly sampling). The sampling frequency for QWTREND 

can vary from month to month, the number of samples can 

vary from 0 to 3 samples per month, and the timing of the 

samples within each month is accounted for. SEAKEN 

requires one sample per interval, with strict limitations on 

the number and timing of missing values.

Water-Quality Trends Related to 

Watershed Setting and Hydrologic 

Conditions

Water quality is best understood within the context 

of watershed setting and prevailing hydrologic conditions; 

therefore, changes in population, land cover, and hydrologic 

characteristics during the trend period were examined. Statistical summaries of the water-quality properties and constituents 

selected for trend analysis were useful for identifying site-tosite differences within the study area. Results of water-quality 

trend analyses are presented for the period 1989–2013 or for 

shorter periods within that range, depending on the availability 

and characteristics of the data. The observed trends are 

discussed in relation to population and land-cover change in 

the study area as a whole and in relation to site-specific events 

when warranted. 

Population and Land Cover

The Triangle area experienced rapid development and 

population growth during the trend period. Population density 

increased in all of the trend site watersheds on the basis of 

a comparison of 1990 and 2010 U.S. Census information 

(table 2; fig. 2). Among watersheds, population density 

ranged from 59 to 991 people per square mile (people/mi2) in 

1990 to 86 to 1,355 people/mi2 in 2010. Population increases 

expressed as percent change from 1990 to 2010 ranged from 

a minimum of 26 percent for Eno River at Hillsborough 

(site S1) to 919 percent at White Oak Creek (site S11), with 

a median increase of 43 percent among all sites. In both 1990 

and 2010, population densities were lowest in the watersheds 

of Little River Reservoir (L1), Lake Michie (L2), and Cane 

Creek Reservoir (L3) and their major tributaries—Little River 

(S3), Flat River (S4), and Cane Creek (S5), respectively. The 

most densely populated watersheds were those draining to 

the New Hope arm of Jordan Lake (sites L6 and L7), which 

included New Hope (S7) and Northeast (S8) Creeks near 

Durham, Morgan Creek near Farrington (S10), and White Oak 

Creek (S11) west of Cary. In 1990, population density within 

the White Oak Creek watershed was 133 people/mi2

. By 2010, 

population in that watershed had increased by an order of 

magnitude to 1,355 people/mi2. 

Increases in population density were reflected in landcover changes in the watersheds. On the basis of comparisons 

of 1992 and 2011 land cover, all trend site watersheds 

experienced increases in developed land and decreases in 

forested land. Only minor changes were observed for agricultural land (table 3; fig. 3). Circa 1992, 7 of the 13 stream site 

watersheds were less than 10 percent developed. By 2011, 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 11

Table 2. Population density estimates for trend site watersheds in the Triangle area of North Carolina, 1990 and 2010, and change from 

1990 to 2010.

[mi2, square mile; USGS, U.S. Geological Survey. Gray shading represents sites in the Neuse River Basin; unshaded rows represent sites in the Cape Fear River 

Basin. Site L8 is not included because watershed characteristics were not applicable for site L8; see “Population and Land Cover” for explanation]

Map no. 

(fig. 1)

USGS 

station no. USGS station name

Population density

1990 

(people/mi2

)

2010 

(people/mi2)

Change from 

1990 to 2010 

(people/mi2

)

Change from 

1990 to 2010 

(percent)

Stream sites

S1 02085000 Eno River at Hillsborough, NC 144 182 38 26

S2 02085070 Eno River near Durham, NC 276 372 96 35

S3 0208521324 Little River at SR 1461 near Orange Factory, NC 74 103 29 39

S4 02085500 Flat River at Bahama, NC 77 118 41 53

S5 02096846 Cane Creek near Orange Grove, NC 59 87 28 47

S6 02096960 Haw River near Bynum, NC 306 429 123 40

S7 02097314 New Hope Creek near Blands, NC 991 1,316 325 33

S8 0209741955 Northeast Creek at SR 1100 near Genlee, NC 522 985 463 89

S9 02097464 Morgan Creek near White Cross, NC 136 183 47 35

S10 02097521 Morgan Creek near Farrington, NC 595 844 249 42

S11 0209782609 White Oak Creek at mouth near Green Level, NC 133 1,355 1,222 919

S12 02098198 Haw River Below B. Everett Jordan Dam near Moncure, NC 330 506 176 53

S13 0210215985 Cape Fear River at State Highway 42 near Brickhaven, NC 243 363 120 49

Lake sites

L1 0208524845 Little River Reservoir at Dam near Bahama, NC 89 120 31 35

L2 02086490 Lake Michie at Dam near Bahama, NC 74 115 41 55

L3 0209684980 Cane Creek Reservoir at Dam near White Cross, NC 61 86 25 41

L4 0209699999 Jordan Lake, Haw River arm near Hanks Chapel, NC 302 423 121 40

L5 0209749990 University Lake at Intakes near Chapel Hill, NC 209 302 93 44

L6 0209768310 Jordan Lake at Buoy 12 at Farrington, NC 661 1,070 409 62

L7 0209799150 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 553 952 399 72

1,400

1,200

1,000

800

600

400

200

0

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7

Site number

1990

EXPLANATION

2010

Population per square mile of drainage area

in 1990 and 2010

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Figure 2. Population density for trend site watersheds in the Triangle area of North Carolina, 1990 and 2010. Site L8 is not 

included because watershed characteristics were not applicable for site L8; see “Population and Land Cover” for explanation.
12 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 3. Generalized land-cover distribution for trend site watersheds in the Triangle area of North Carolina, circa 1992, circa 2011, 

and change from 1992 to 2011.

[mi2, square mile; USGS, U.S. Geological Survey. Gray shading represents sites in the Neuse River Basin; unshaded rows represent sites in the Cape Fear River 

Basin. Site L8 is not included because watershed characteristics were not applicable for site L8; see “Population and Land Cover” for explanation]

Map 

no. 

(fig. 1)

USGS 

station no. USGS station name

Drainage area 

(mi2)

Land-cover distribution, in percent

circa 1992a circa 2011b Change from 1992 to 2011

DevelopedAgriculturalForestedDevelopedAgriculturalForestedDevelopedAgriculturalForested

Stream sites

S1 02085000 Eno River at Hillsborough, NC 66.0 6.4 23.3 70.3 12.5 24.3 63.2 6.1 1.0 –7.1

S2 02085070 Eno River near Durham, NC 141 11.0 15.8 73.2 18.1 17.1 64.8 7.1 1.3 –8.4

S3 0208521324 Little River at SR 1461 near Orange 

Factory, NC

78.2 0.7 24.4 74.8 5.9 27.5 66.7 5.1 3.0 –8.2

S4 02085500 Flat River at Bahama, NC 149 2.5 26.8 70.7 7.1 28.0 65.0 4.5 1.2 –5.7

S5 02096846 Cane Creek near Orange Grove, NC 7.54 0.0 15.8 84.2 4.4 17.0 78.5 4.4 1.2 –5.7

S6 02096960 Haw River near Bynum, NC 1,275 11.4 27.6 61.0 19.7 27.6 52.7 8.2 –0.0 –8.2

S7 02097314 New Hope Creek near Blands, NC 75.9 26.8 5.3 67.9 41.6 5.3 53.1 14.8 –0.0 –14.8

S8 0209741955 Northeast Creek at SR 1100 near 

Genlee, NC

21.1 27.4 6.9 65.7 59.8 1.8 38.3 32.4 –5.0 –27.4

S9 02097464 Morgan Creek near White Cross, NC 8.35 0.7 17.7 81.6 5.3 19.3 75.4 4.6 1.6 –6.2

S10 02097521 Morgan Creek near Farrington, NC 46.0 11.9 9.2 78.9 20.0 10.7 69.3 8.1 1.5 –9.7

S11 0209782609 White Oak Creek at mouth near Green 

Level, NC

11.9 0.7 11.6 87.6 38.7 9.3 52.0 37.9 –2.3 –35.6

S12 02098198 Haw River below B. Everett Jordan 

Dam near Moncure, NC

1,689 11.3 22.2 66.5 21.0 22.5 56.5 9.7 0.3 –10.0

S13 0210215985 Cape Fear River at State Highway 42 

near Brickhaven, NC

3,160 8.5 20.0 71.6 16.8 21.7 61.5 8.3 1.8 –10.1

Lake sites

L1 0208524845 Little River Reservoir at Dam near 

Bahama, NC

97.7 1.7 24.5 73.8 6.9 27.1 66.0 5.2 2.6 –7.8

L2 02086490 Lake Michie at Dam near Bahama, NC 167 2.3 26.1 71.5 6.8 27.2 65.9 4.5 1.1 –5.6

L3 0209684980 Cane Creek Reservoir at Dam near 

White Cross, NC

31.4 0.2 26.9 72.9 5.5 25.1 69.5 5.2 –1.8 –3.4

L4 0209699999 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

1,303 11.2 27.2 61.6 19.4 27.2 53.4 8.2 0.0 –8.2

L5 0209749990 University Lake at Intakes near Chapel 

Hill, NC

30.0 2.4 11.6 86.0 7.4 14.2 78.4 5.0 2.6 –7.7

L6 0209768310 Jordan Lake at Buoy 12 at Farrington, 

NC

231 18.4 5.6 76.0 35.7 6.1 58.2 17.3 0.5 –17.8

L7 0209799150 Jordan Lake above U.S. Highway 64 at 

Wilsonville, NC

285 15.1 5.6 79.3 31.8 6.4 61.9 16.7 0.8 –17.5

a

1992 National Land Cover Database (NLCD) classes were aggregated as follows: Developed = classes 21, 22, and 23; Agricultural = classes 81 and 82; 

Forested = classes 11, 31, 32, 33, 41, 42, 43, and 92. NLCD 1992 class definitions are from Vogelmann and others (2001).

b

2011 NLCD classes were aggregated as follows: Developed = classes 21, 22, 23, and 24; Agricultural = classes 81 and 82; Forested = classes 11, 31, 41, 42, 

43, 52, 71, 90, and 95. NLCD 2011 class definitions are from Homer and others (2015).
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 13

40

30

20

10

0

–10

–20

–30

–40

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7

Site number

Developed

Agricultural

Forested/Other

EXPLANATION

Change in land cover from 1992 to 2011,

in percent

res18-0031_fig03

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Figure 3. Percent change for generalized land-cover categories for trend site watersheds in the Triangle 

area of North Carolina from 1992 to 2011. Site L8 is not included because watershed characteristics were not 

applicable for the site; see “Population and Land Cover” for explanation

only four watersheds remained less than 10 percent developed, 

including Little River (S3), Flat River (S4), Cane Creek (S5), 

and Morgan Creek near White Cross (S9) (table 3). Among all 

study sites, the watershed for White Oak Creek (S11) had the 

greatest overall land-cover change. Developed lands increased 

from less than 1 percent to 39 percent of the watershed, and 

forested lands dropped from 88 to 52 percent. Northeast Creek 

(S8) watershed experienced a 32-percent increase in developed 

land cover and a 27-percent decrease in forested/other lands. 

The lowest amounts of land-cover change were observed in 

the watersheds of Lake Michie (L2) and Cane Creek Reservoir 

(L3) as well as their major tributaries Flat River (S4) and Cane 

Creek (S5), respectively (table 3; fig. 3).

Precipitation and Streamflow 

In a largely unaltered basin, streamflow is strongly 

influenced by physiographic characteristics and climatic conditions. Likewise, trends in streamflow in such a basin often 

are associated with similar trends in climatic conditions. When 

evaluating trends in hydroclimatic records, it is important 

to consider the potential influence of long-term persistence, 

which is the natural pattern of wet years tending to follow wet 

years and dry years tending to follow dry years (Hirsch, 2011). 

In substantially long hydroclimatic records, multidecadal 

variations may lead to multiyear periods that exhibit upward 

and (or) downward fluctuations that are not necessarily 

indicative of long-term changes in the hydroclimatic system 

(Lins and Cohn, 2011). 

Interpretation of streamflow trends can be complicated 

further in basins that are influenced by regulation (for 

example, dams), water withdrawals, and (or) inputs from point 

sources such as wastewater-treatment facilities. The streamflow in such basins may also be influenced by short-term 

hydroclimatic conditions, but the anthropogenic influences 

may mitigate, enhance, or even offset those short-term 

changes in certain parts of the flow regime. Nonetheless, trend 

assessments still may prove useful in the broader context of 

assessing water resources in a basin.

Streamflow sites assessed in this report are located 

within the Northern Piedmont and Central Piedmont climate 

divisions (NOAA, 2016). Total annual rainfall for the trend 

period varied from year to year, and no consistent trend was 

indicated (fig. 4). In both the Northern and Central Piedmont, 

2007 had the lowest annual precipitation, and 2003 had the 

highest. The study area experienced drought conditions in 

2001 and 2007, and precipitation was well below the “climate 

normal” (that is, the annual average for the period 1981–2010) 

of 43.34 inches at the Raleigh-Durham International Airport 

(NOAA, 2017). In addition to the high annual precipitation in 

2003, precipitation was well above normal during 1989, 1995, 

1996, and 2013. 

Many water-quality constituents vary with streamflow; 

therefore, an assessment of streamflow conditions during 

the trend period provides a context for understanding waterquality trends. Streamflow is influenced by a wide range of 

factors that function across multiple time scales. Although the 

magnitude of change can be assessed with confidence for a 

given time period, tests of statistical significance for streamflow trends should be interpreted with caution (Cohn and Lins, 

2005). Certain watershed features influenced flow patterns 

observed at the study sites. At some sites, flows from upstream 

impoundments were regulated; other sites had no upstream 

flow regulation. Sites ranged from headwater streams with 

small, undeveloped watersheds to mid- and large-sized rivers 

that received substantial inflows of treated wastewater and 

urban runoff and (or) were affected by upstream water-supply 

withdrawals (table 4).
14 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

70

60

50

40

30

20

10

0

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Year

Northern Piedmont

Central Piedmont

EXPLANATION

Total precipitation, in inches

Long-term

average

res18-0031_fig04

Figure 4. Annual precipitation in the Northern Piedmont and Central Piedmont climate divisions of North Carolina, 

1989–2013, and long-term average precipitation at the Raleigh-Durham International Airport, 1981–2010.

Among the four streamflow sites in the Neuse River 

Basin, no statistically significant trends were indicated for 

annual minimum 7-day average flows (7Q1) or annual peak 

flows from 1989 through 2013, with the exception of Eno 

River at Hillsborough (site S1) where annual peak flows 

indicated a downward trend. Annual mean flows trended 

downward at all four sites, including the two Eno River sites 

(S1 and S2) (table 4). In the late 1980s, concerns regarding 

competing and unrestricted water-supply withdrawals and 

decreasing flow in the Eno River led local governments 

to voluntarily adopt a “Capacity Use Agreement” (North 

Carolina Department of Natural Resources and Community 

Development, 1987). At the time, substantial water users 

included three public water-supply systems and a mining 

operation. The agreement mandated a minimum instream flow 

at site S1 and capped withdrawals by each user over a series of 

stages of water availability. 

Of the seven streamflow sites in the Cape Fear River 

Basin, five indicated downward trends in the 7Q1 flows during 

the 1989–2013 study period: Cane Creek near Orange Grove 

(S5), Haw River near Bynum (S6), Morgan Creek near White 

Cross (S9), Morgan Creek near Chapel Hill (S10Q, which is the 

source of streamflow data for site S10), and White Oak Creek 

(S11) (table 4). It should be noted that three of the five sites with 

downward trends (S5, S9, and S11) had a number of zero flows 

in the record being analyzed. The Kendall’s tau test is based 

on enumerating concordant (when both the x and y variables 

increase or decrease) and discordant (when x increases and y 

decreases or x decreases and y increases) pairs of x and y data 

(Kendall, 1938). Consequently, when stations have a substantial 

number of zero flows, resulting in numerous pairs of x and y 

data with tied ranks, interpretation of the trend test can become 

tenuous at best. These three sites also have the smallest drainage 

areas (7.54 to 11.9 mi2

) and may respond more rapidly to 

extremes such as droughts. Continued monitoring of streamflow 

at these sites can provide the data needed to assess trends with 

greater confidence. New Hope Creek near Blands (S7) indicated 

no significant trend in 7Q1 flows, and Northeast Creek near 

Genlee (S8) indicated an upward trend. Both sites S7 and 

S8 receive effluent from municipal wastewater-reclamation 

facilities, and neither site is affected by upstream withdrawals 

or substantial flow regulation. The additional and consistent 

input of flow from effluent likely lessened the time periods that 

these sites experienced low-flow conditions. Annual mean flows 

trended downward at five sites in the Cape Fear River Basin 

(S5, S6, S7, S9, and S10Q) during the study period, and no 

significant trend was noted at the remaining two sites (S8 and 

S11). No trends were indicated in the annual peak flows at any 

of the sites. 

Durations of daily flow yields for the four Neuse River 

Basin stations indicated that Flat River (site S4) had a lower 

yield throughout the full range of flows, although there are no 

known water diversions upstream from the site (fig. 5). For the 

other three stations, flow yields were similar at flow durations 

from one to about the 40th percentile. Under low-flow conditions (highest percentiles), however, yields were relatively 

higher at the two Eno River sites (S1 and S2) than in the Little 

River (S3) or Flat River (S4). The higher low-flow yield at 

site S1 likely reflected the upstream minimum-release requirements implemented in the late 1980s. The higher low-flow 

yields at site S2 also reflected the regulated upstream release, 

as well as anthropogenic inputs from a municipal wastewatertreatment facility.

Flow yields at stations located in the Cape Fear River 

Basin differed on the basis of contribution of flow from other 

sources (fig. 6A). The three stations with small drainage areas 

and no known withdrawals or point-source inputs (sites S5, 

S9, and S11) had flow yield curves with a relatively similar 

shape, although the yield at site S11 substantially dropped 

for the lowest flows (percentiles greater than 80). Site S11 

is located in the Triassic basins, a sedimentary geological 

formation with low porosity and permeability, where low 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 15

Table 4. Flow-related watershed features and results of Kendall’s tau statistical test for detection of monotonic trends in the annual minimum 7-day average flow, annual mean flow, and annual peak flow at streamflow-monitoring sites in the Triangle area of North Carolina, 1989−2013.[Flow regulation indicates whether flows are unregulated, regulated, or affected by other factors, including upstream diversions. Major National Pollutant Discharge Elimination System (NPDES) discharges are defined as wastewater-treatment facilities that are permitted through the NPDES and that discharge more than 100,000 gallons per day. Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend results. Abbreviations: USGS, U.S. Geological Survey; mi2, square mile; NC, North Carolina; —, not applicable; ns, not significant]

Map no. 

(fig. 1)

USGS 

station no. USGS station name

Watershed features Annual minimum 7-day average flow Annual mean flow Annual peak flow

Drainage 

area 

(mi2

)

Flow 

regulation

Watersupply 

withdrawals

Major 

NPDES 

discharges 

Period of 

analysis

Kendall’s 

tau p-value Trend direction Period of analysis Kendall’s tau p-value Trend direction Period of analysis Kendall’s tau p-value Trend direction

Neuse River Basin

S1 02085000 Eno River at 

Hillsborough, NC

66.0 Minor1 Yes

— Apr. 1, 1989, to 

Mar. 31, 2014

−0.05 0.7437 ns Oct. 1, 1988, to 

Sept. 30, 2013

−0.47 0.0011 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.33 0.0195 Down

S2 02085070 Eno River near 

Durham, NC

141 Minor1 Yes Yes Apr. 1, 1989, to 

Mar. 31, 2014

−0.08 0.5751 ns Oct. 1, 1988, to 

Sept. 30, 2013

−0.32 0.0250 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.21 0.1350 ns

S3 0208521324 Little River at SR 

1461 near Orange Factory, NC

78.2 Unregulated

—

— Apr. 1, 1989, to 

Mar. 31, 2014

−0.16 0.2722 ns Oct. 1, 1988, to 

Sept. 30, 2013

−0.28 0.0498 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.19 0.1909 ns

S4 02085500 Flat River at 

Bahama, NC

149 Unregulated

—

— Apr. 1, 1989, to 

Mar. 31, 2014

−0.23 0.1123 ns Oct. 1, 1988, to 

Sept. 30, 2013

−0.29 0.0399 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.23 0.1123 ns

Cape Fear River Basin

S5 02096846 Cane Creek near 

Orange Grove, NC

7.54 Unregulated

—

— Apr. 1, 1989, to 

Mar. 31, 2014

−0.30 0.0466 Down2 Oct. 1, 1989, to 

Sept. 30, 2013

−0.38 0.0099 Down Oct. 1, 1989, to 

Sept. 30, 2013

−0.20 0.1804 ns

S6 02096960 Haw River near 

Bynum, NC

1,275 Regulated Yes Yes Apr. 1, 1989, to 

Mar. 31, 2014

−0.37 0.0102 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.30 0.0356 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.26 0.0741 ns

S7 02097314 New Hope Creek 

near Blands, NC

75.9 Minor1 — Yes Apr. 1, 1989, to 

Mar. 31, 2014

−0.01 0.9627 ns Oct. 1, 1988, to 

Sept. 30, 2013

−0.28 0.0498 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.05 0.7312 ns

S8 0209741955 Northeast Creek 

at SR 1100 near Genlee, NC

21.1 Minor1 — Yes Apr. 1, 1989, to 

Mar. 31, 1993; Apr. 1, 1996, to Mar. 31, 2014

0.56 0.0003 Up Oct. 1, 1988, to 

Sept. 30, 1993; Oct. 1, 1995, to Sept. 30, 2013

−0.19 0.2145 ns Oct. 1, 1988, to 

Sept. 30, 2013; Oct. 1, 1995, to Sept. 30, 2013

−0.05 0.7396 ns

S9 02097464 Morgan Creek 

near White Cross, NC

8.35 Unregulated

—

— Apr. 1, 1989, to 

Mar. 31, 2014

−0.44 0.0029 Down2 Oct. 1, 1989, to 

Sept. 30, 2013

−0.42 0.0043 Down Oct. 1, 1989, to 

Sept. 30, 2013

−0.12 0.3990 ns

S10Q 020975173 Morgan Creek 

near Chapel Hill, NC

41.0 Regulated Yes Yes Apr. 1, 1989, to 

Mar. 31, 2014

−0.46 0.0013 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.36 0.0124 Down Oct. 1, 1988, to 

Sept. 30, 2013

−0.06 0.6913 ns

S11 0209782609 White Oak Creek 

at mouth near Green Level, NC

11.9 Unregulated

—

— Apr. 1, 2000, to 

Mar. 31, 2013

−0.49 0.0311 Down2 Oct. 1, 1999, to 

Sept. 30, 2013

−0.21 0.2983 ns Oct. 1, 1999, to 

Sept. 30, 2013

0.20 0.3237 ns

1Flow characteristics reflect effects of minor regulation and (or) diurnal fluctuation caused by permitted discharges, water-supply diversions, and (or) small impoundments upstream from the station. From Weaver (2016).2Trends should be interpreted with caution due to zero-flow periods.3Streamflow data for station 02097517 (site S10Q) were used in the water-quality trend analysis for station 02097521 (site S10).
16 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

0.001

0.01

0.1

1

10

100

0 10 20 30 40 50 60 70 80 90 100

Percentage of time indicated mean flow yield was equaled or exceeded

S1 (66.0)

S2 (141)

S3 (78.2)

S4 (149)

Site number and drainage area,

in square miles

EXPLANATION

Daily mean flow yield, in cubic feet per second per square mile

res18-0031_fig05

Figure 5. Duration of daily mean flow yields, in cubic feet per second per square mile, for sites S1, 

S2, S3, and S4 in the Neuse River Basin for the concurrent period October 1, 1988, to September 30, 

2013. 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 17

0.001

0.0001

0.01

0.1

1

10

100

0 10 20 30 40 50 60 70 80 90 100

Percentage of time indicated mean flow yield was equaled or exceeded

S5 (7.5)

S6 (1,275)

S7 (75.9)

S8 (21.1)

S9 (8.4)

S10Q (41.0)

S11 (11.9)

EXPLANATION

Daily mean flow yield, in cubic feet per second per square mile

A

B

S9 (8.4)

S10Q (41.0)

res18-0031_fig06

0.01

0.001

0.1

1

10

Site number and drainage area,

in square miles

EXPLANATION

Site number and drainage area,

in square miles

Figure 6. Duration of daily mean flow yields, in cubic feet per second per square mile, for 

(A) sites S5, S6, S7, S8, S9, S10Q, and S11 in the Cape Fear River Basin and (B) sites S9 and 

S10Q in Morgan Creek for the concurrent period September 15, 1999, to September 30, 2013.
18 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

yields are typical (Bain and Brown, 1981). The watersheds 

contributing flow to sites S6 and S10Q have upstream watersupply withdrawals. Sites S6, S7, S8, and S10Q receive inputs 

from major wastewater-treatment facilities, and the influence 

of anthropogenic discharges to these streams is evident in the 

higher yields for the lower flows (higher percentiles).

Additional details are evident when durations of daily 

flow yields are compared for the two Morgan Creek stations 

for the same period of record (September 15, 1999, to 

September 30, 2013). Sites S9 and S10Q have very similar 

yields at higher flows but begin to diverge above the 20th 

flow percentile. The influence of anthropogenic sources at site 

S10Q can clearly be seen in the larger yields under low-flow 

conditions (higher percentiles) (fig. 6B).

Water Quality

Trends in concentrations of 17 water-quality properties 

and constituents were analyzed for selected streams and lakes 

used for water supply in the Triangle area of North Carolina. 

Trends are reported in terms of both percent change and 

magnitude of change in fitted annual-median concentrations 

from the first year to the last year of the trend period.

Specific Conductance and Major Ions

Specific conductance is a measure of the ability of water 

to conduct an electrical current at a specified temperature 

(typically standardized to 25 degrees Celsius). Specific 

conductance ranges widely in natural waters, depending 

primarily on underlying geology. In practical terms, specific 

conductance indicates the overall concentration of charged 

ions that are present in a water sample; thus, specific 

conductance generally is closely related to the concentrations 

of individual ions. Individual ions in this report include the 

cations calcium, magnesium, potassium, and sodium, and the 

anions chloride and sulfate. All of these ions occur naturally 

in water; however, concentrations that are elevated above 

naturally occurring levels may indicate anthropogenic inputs.

At stream sites in the study area, specific conductance 

and nutrients were measured by the USGS, NCDEQ, 

UCFRBA, and MCFRBA; thus, there were substantially more 

observations for these constituents than for other properties 

and constituents in the trend datasets (table 5). In contrast, 

major ions and suspended-sediment data were collected only 

by the USGS and therefore had fewer observations. 

Specific conductance varied considerably among study 

sites and with streamflow. Concentrations ranged from 

21 to 996 μS/cm among stream samples and from 42 to 

453 μS/cm among lake samples (table 5). Concentrations at all 

stream sites tended to be highest during low-flow conditions 

and lowest when streamflow was high. This likely could 

be attributed to dilution by incoming stormwater. Streams 

with the highest median specific conductance were those 

located in highly developed watersheds and in proximity to 

National Pollutant Discharge Elimination System (NPDES) 

discharges (tables 4, 6), including New Hope Creek (site S7), 

Northeast Creek (site S8), and Morgan Creek near Farrington 

(site S10). Low specific conductance values were measured at 

sites with relatively undeveloped watersheds with no NPDES 

inputs. Among lake sites, the four sites in Jordan Lake had the 

highest specific conductance (table 7), reflecting the higher 

concentrations observed in their upstream tributaries. Jordan 

Lake sites have large drainage areas and diverse anthropogenic 

inputs, such as treated wastewater and stormwater runoff from 

developed and (or) agricultural lands. 

Trends in specific conductance were analyzed for all 

21 sites in the study area (table 8; fig. 7). The time-series 

model did not produce a trend result for Cane Creek Reservoir 

(site L3). Of the 20 remaining sites, all but one site (S11) 

had sufficient data to evaluate both the 1- and 2-period trend 

models. For White Oak Creek (site S11), only a single trend 

for the later period (2002–13) could be evaluated. 

During 1989–2013, specific conductance significantly 

increased at 13 sites and significantly decreased at 2 sites. 

The largest percent increases during the trend period were 

observed at Northeast Creek (site S8, 63.3 percent) and 

New Hope Creek (site S7, 43.2 percent). These sites receive 

municipal wastewater effluent and also have high percentages 

of developed land. In contrast, specific conductance decreased 

25.5 percent at Eno River near Durham (site S2) during 

1989–2013 and 7.2 percent at Jordan Lake near Hanks Chapel 

(site L4) during 1991–2013. Interestingly, major NPDES 

dischargers are located upstream from these sites. As part of 

an assessment of water-quality trends in the Nation’s rivers, 

Oelsner and others (2017) reported similar trends in specific 

conductance during 1992–2002 at several Triangle area sites—

specifically, increasing trends at sites S1, S3, S4, and S7 and a 

decreasing trend at site S2.

Specific conductance at Morgan Creek near Farrington 

(site S10) and Jordan Lake at Buoy 12 (site L6) increased 

significantly (84.5 percent) during the early part of the 

trend period (1989–2001), but did not change significantly 

(5.5 percent) during the later part (2002–13). At White Oak 

Creek (site S11), specific conductance increased significantly 

during 2002–13.

Sufficient data to assess trends for individual ions were 

available at 13 sites, including 5 stream and 8 lake sites 

(table 8). Data for 11 of the 13 sites were sufficient to evaluate 

both the 1- and 2-period trend models for the entire trend 

period. Data for White Oak Creek (site S11) were sufficient 

to evaluate trends for only the late time period (2002–13), 

and data for Cape Fear River near Brickhaven (site S13) were 

sufficient to evaluate trends for only the early time period 

(1989–2001).

As expected, trend patterns for most of the individual 

ions were similar to those observed for specific conductance. 

Concentrations of calcium, magnesium, potassium, sodium, 

and chloride increased significantly at most sites during 

the various time periods that were analyzed. Overall, trend 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 19

Table 5. Statistical summary of water-quality properties and constituents aggregated for all stream and lake sites in the Triangle area 

of North Carolina for which trend assessments were conducted. 

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius; m, meter; diss., dissolved; mg/L, milligram per liter; N, nitrogen; P, phosphorus; μg/L, microgram per liter; USGS, 

U.S. Geological Survey; NCDEQ, North Carolina Department of Environmental Quality; UCFRBA, Upper Cape Fear River Basin Association; MCFRBA, 

Middle Cape Fear River Basin Association; <, less than]

Chemical constituent or property 

(unit)

Number of 

observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum

Stream sites (Data sources: USGS, NCDEQ, UCFRBA, MCFRBA)

Specific conductance 

(μS/cm at 25 °C)

4,748 0 0 209 155 21 147 996

Calcium, diss. (mg/L) 822 0 0 7.41 2.42 1.45 7.03 26.2

Magnesium, diss. (mg/L) 822 0 0 2.79 0.77 0.59 2.72 7.29

Potassium, diss. (mg/L) 822 1 0.1 2.61 1.73 <0.10 2.16 18.0

Sodium, diss. (mg/L) 822 0 0 7.28 5.61 1.17 5.70 47.4

Chloride, diss. (mg/L) 806 0 0 7.90 4.61 1.33 6.67 48.7

Sulfate, diss. (mg/L) 803 1 0.1 6.78 5.39 <0.10 5.20 37.0

Nitrate + nitrite, diss. (mg/L as N) 4,221 129 3.1 1.87 2.94 <0.005 0.636 31.0

Ammonia, diss. (mg/L as N) 4,211 824 19.6 0.075 0.182 <0.002 0.040 8.40

Total organic N (mg/L as N) 4,201 55 1.3 0.58 0.33 <0.01 0.53 4.4

Total N (mg/L as N) 4,212 6 0.1 2.5 3.1 0.11 1.3 31.7

Total P (mg/L as P) 4,173 81 1.9 0.185 0.234 <0.010 0.120 4.10

Total organic carbon (mg/L) 800 0 0 7.8 5.2 0.8 6.5 58.0

Suspended solids (mg/L) 2,779 452 16.3 17 33 <1.0 8.8 570

Suspended sediment (mg/L) 928 0 0 36 100 1 9 1,330

Lake sites (Data source: USGS)

Specific conductance 

(μS/cm at 25 °C)

844 0 0 125 58 42 107 453

Calcium, diss. (mg/L) 822 0 0 6.97 1.55 2.86 7.02 12.0

Magnesium, diss. (mg/L) 822 0 0 2.64 0.48 1.24 2.60 4.25

Potassium, diss. (mg/L) 822 0 0 2.80 0.99 1.20 2.61 8.07

Sodium, diss. (mg/L) 822 0 0 11.5 9.2 1.76 6.90 64.1

Chloride, diss. (mg/L) 812 0 0 10.7 7.2 1.98 7.17 47.0

Sulfate, diss. (mg/L) 810 0 0 10.0 7.4 0.20 7.38 55.2

Nitrate + nitrite, diss. (mg/L as N) 846 382 45.2 0.111 0.193 <0.005 0.017 1.70

Ammonia, diss. (mg/L as N) 846 210 24.8 0.044 0.097 <0.002 0.020 2.30

Total organic N (mg/L as N) 846 2 0.2 0.68 0.26 <0.18 0.64 3.4

Total N (mg/L as N) 846 1 0.1 0.85 0.33 <0.25 0.80 3.4

Total P (mg/L as P) 828 61 7.4 0.048 0.032 0.008 0.040 0.300

Total organic carbon (mg/L) 810 0 0 8.7 1.9 2.1 8.5 17.0

Transparency, Secchi (m) 743 0 0 0.84 0.38 0.10 0.75 3.10

Chlorophyll a, phytoplankton 

(μg/L)

722 1 0.1 18.0 15.8 <0.1 13.2 165.0
20 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 6. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by stream site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius;diss., dissolved; mg/L, milligram per liter; N, nitrogen; P, phosphorus; <, less than]

Map no. 

(fig. 1) USGS station name Number of observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Specific conductance (μS/cm at 25 °C)

S1 Eno River at Hillsborough, NC 193 0 0 91 18 33 90 145 1990 2013

S2 Eno River near Durham, NC 294 0 0 143 83 33 121 615 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

332 0 0 85 26 25 85 315 1989 2013

S4 Flat River at Bahama, NC 401 0 0 77 17 24 79 132 1989 2011

S5 Cane Creek near Orange Grove, NC 223 0 0 86 25 26 82 198 1989 2013

S6 Haw River near Bynum, NC 597 0 0 218 134 33 179 793 1989 2013

S7 New Hope Creek near Blands, NC 613 0 0 285 129 37 275 609 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

425 0 0 407 184 50 433 996 1989 2013

S9 Morgan Creek near White Cross, NC 201 0 0 109 30 36 108 249 1989 2013

S10 Morgan Creek near Farrington, NC 607 0 0 364 152 40 367 750 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 94 29 21 92 204 2000 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

242 0 0 160 54 45 155 311 1989 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

527 0 0 158 53 35 154 359 1989 2012

Calcium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 191 0 0 7.05 1.58 2.90 7.00 11.3 1990 2013

S5 Cane Creek near Orange Grove, NC 215 0 0 6.99 2.34 1.94 6.70 16.4 1989 2013

S9 Morgan Creek near White Cross, NC 195 0 0 9.22 3.04 2.36 8.71 26.2 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 5.92 1.63 1.45 5.92 10.5 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

128 0 0 6.97 1.25 2.50 6.91 10.0 1989 2003

Magnesium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 191 0 0 2.84 0.54 1.10 2.87 4.05 1990 2013

S5 Cane Creek near Orange Grove, NC 215 0 0 2.61 0.80 0.80 2.50 5.65 1989 2013

S9 Morgan Creek near White Cross, NC 195 0 0 3.25 0.91 1.04 3.18 7.29 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 2.22 0.57 0.59 2.19 3.60 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

128 0 0 2.75 0.46 1.30 2.80 4.15 1989 2003

Potassium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 191 0 0 1.73 0.44 0.97 1.70 3.53 1990 2013

S5 Cane Creek near Orange Grove, NC 215 0 0 2.24 1.98 0.40 1.62 15.2 1989 2013

S9 Morgan Creek near White Cross, NC 195 1 0.5 3.17 2.30 <0.10 2.45 18.0 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 3.05 0.77 1.93 2.94 6.28 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

128 0 0 3.42 1.11 1.50 3.31 7.72 1989 2003
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 21

Table 6. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by stream site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius;diss., dissolved; mg/L, milligram per liter; N, nitrogen; P, phosphorus; <, less than]

Map no. 

(fig. 1) USGS station name Number of observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Sodium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 191 0 0 5.50 1.48 1.40 5.50 16.0 1990 2013

S5 Cane Creek near Orange Grove, NC 215 0 0 4.86 1.08 1.17 5.02 8.56 1989 2013

S9 Morgan Creek near White Cross, NC 195 0 0 5.48 1.23 1.41 5.50 9.84 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 7.45 3.35 1.68 6.88 25.6 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

128 0 0 16.6 9.0 2.70 14.0 47.4 1989 2003

Chloride, diss. (mg/L)

S1 Eno River at Hillsborough, NC 187 0 0 6.00 1.94 1.60 5.57 15.0 1990 2013

S5 Cane Creek near Orange Grove, NC 209 0 0 6.18 1.82 1.33 6.00 19.1 1989 2013

S9 Morgan Creek near White Cross, NC 194 0 0 7.06 1.89 2.81 6.70 17.0 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 8.93 5.41 1.60 7.98 48.7 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

123 0 0 14.2 6.9 4.80 12.0 39.3 1989 2003

Sulfate, diss. (mg/L)

S1 Eno River at Hillsborough, NC 186 0 0 5.56 1.74 2.40 5.30 14.0 1990 2013

S5 Cane Creek near Orange Grove, NC 208 0 0 3.93 1.83 0.43 3.70 14.7 1989 2013

S9 Morgan Creek near White Cross, NC 193 1 0.5 4.81 2.45 <0.10 4.30 22.0 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 7.60 4.72 1.37 6.98 32.0 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

123 0 0 15.9 6.9 4.00 14.0 37.0 1989 2003

Nitrate + nitrite, diss. (mg/L as N)

S1 Eno River at Hillsborough, NC 192 0 0.0 0.264 0.111 0.036 0.270 0.600 1990 2013

S2 Eno River near Durham, NC 251 12 4.8 0.317 0.230 <0.01 0.300 1.60 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

295 29 9.8 0.260 0.191 <0.01 0.250 1.32 1989 2013

S4 Flat River at Bahama, NC 309 28 9.1 0.243 0.162 <0.008 0.240 0.800 1989 2011

S5 Cane Creek near Orange Grove, NC 217 8 3.7 0.536 0.395 <0.005 0.530 4.16 1989 2013

S6 Haw River near Bynum, NC 582 1 0.2 1.08 0.718 <0.02 0.870 4.40 1989 2013

S7 New Hope Creek near Blands, NC 545 1 0.2 3.74 3.13 <0.02 3.00 31.0 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

421 3 0.7 4.42 5.19 <0.01 2.30 29.0 1989 2013

S9 Morgan Creek near White Cross, NC 199 9 4.5 0.556 0.361 <0.005 0.540 1.70 1989 2013

S10 Morgan Creek near Farrington, NC 549 3 0.5 4.67 3.10 <0.01 4.00 18.2 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 9 9.7 0.055 0.064 <0.005 0.035 0.430 2000 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

134 1 0.7 0.507 0.265 <0.01 0.480 1.30 1989 2000

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

434 25 5.8 0.510 0.287 0.007 0.520 1.79 1989 2012
22 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 6. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by stream site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius;diss., dissolved; mg/L, milligram per liter; N, nitrogen; P, phosphorus; <, less than]

Map no. 

(fig. 1) USGS station name Number of observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Ammonia, diss. (mg/L as N)

S1 Eno River at Hillsborough, NC 192 20 10.4 0.042 0.077 <0.002 0.030 1.02 1990 2013

S2 Eno River near Durham, NC 251 75 29.9 0.041 0.048 <0.01 0.030 0.460 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

295 136 46.1 0.027 0.045 0.005 0.012 0.460 1989 2013

S4 Flat River at Bahama, NC 309 95 30.7 0.039 0.040 <0.002 0.028 0.320 1989 2011

S5 Cane Creek near Orange Grove, NC 217 57 26.3 0.034 0.049 <0.002 0.018 0.360 1989 2013

S6 Haw River near Bynum, NC 578 177 30.6 0.052 0.075 0.005 0.030 0.820 1989 2013

S7 New Hope Creek near Blands, NC 543 56 10.3 0.085 0.158 0.009 0.050 2.30 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

421 36 8.6 0.150 0.470 <0.01 0.050 8.40 1989 2013

S9 Morgan Creek near White Cross, NC 199 38 19.1 0.058 0.142 <0.002 0.020 1.50 1989 2013

S10 Morgan Creek near Farrington, NC 546 40 7.3 0.128 0.143 <0.01 0.080 1.20 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 16 17.2 0.025 0.020 0.002 0.018 0.104 2000 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

133 2 1.5 0.147 0.137 <0.01 0.110 0.700 1989 2000

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

434 76 17.5 0.070 0.068 0.002 0.060 0.580 1989 2012

Total organic N (mg/L as N)

S1 Eno River at Hillsborough, NC 186 3 1.6 0.39 0.26 0.13 0.33 2.8 1990 2013

S2 Eno River near Durham, NC 250 1 0.4 0.33 0.16 0.01 0.32 1.6 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

294 2 0.7 0.39 0.28 0.05 0.32 2.0 1989 2013

S4 Flat River at Bahama, NC 309 4 1.3 0.38 0.27 <0.01 0.29 2.0 1989 2011

S5 Cane Creek near Orange Grove, NC 216 11 5.1 0.57 0.62 <0.08 0.35 4.4 1989 2013

S6 Haw River near Bynum, NC 578 5 0.9 0.58 0.23 <0.01 0.56 2.0 1989 2013

S7 New Hope Creek near Blands, NC 542 6 1.1 0.76 0.27 <0.07 0.77 2.6 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

421 2 0.5 0.82 0.31 0.06 0.81 3.1 1989 2013

S9 Morgan Creek near White Cross, NC 199 7 3.5 0.47 0.40 0.11 0.37 3.1 1989 2013

S10 Morgan Creek near Farrington, NC 546 11 2.0 0.66 0.25 0.01 0.69 1.9 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0.0 0.59 0.17 0.32 0.57 1.2 2000 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

133 0 0.0 0.42 0.17 0.10 0.39 1.2 1989 2000

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

434 3 0.7 0.64 0.26 0.01 0.61 2.3 1989 2012
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 23

Table 6. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by stream site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius;diss., dissolved; mg/L, milligram per liter; N, nitrogen; P, phosphorus; <, less than]

Map no. 

(fig. 1) USGS station name Number of observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Total N (mg/L as N)

S1 Eno River at Hillsborough, NC 186 0 0 0.70 0.32 0.26 0.63 3.4 1990 2013

S2 Eno River near Durham, NC 251 0 0 0.69 0.28 0.23 0.63 1.8 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

295 1 0.3 0.68 0.36 0.13 0.62 2.5 1989 2013

S4 Flat River at Bahama, NC 309 3 1.0 0.66 0.34 0.11 0.60 2.5 1989 2011

S5 Cane Creek near Orange Grove, NC 217 0 0 1.1 0.8 0.24 0.93 5.5 1989 2013

S6 Haw River near Bynum, NC 582 0 0 1.7 0.8 0.51 1.5 5.3 1989 2013

S7 New Hope Creek near Blands, NC 545 0 0 4.6 3.2 0.53 3.8 31.7 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

421 0 0 5.4 5.3 0.34 3.3 29.2 1989 2013

S9 Morgan Creek near White Cross, NC 199 0 0 1.1 0.6 0.19 0.96 4.4 1989 2013

S10 Morgan Creek near Farrington, NC 547 0 0 5.5 3.2 0.39 4.8 18.6 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 0.67 0.21 0.39 0.63 1.4 2000 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

133 0 0 1.1 0.3 0.45 1.0 2.0 1989 2000

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

434 2 0.5 1.2 0.4 <0.13 1.2 3.2 1989 2012

Total P (mg/L as P)

S1 Eno River at Hillsborough, NC 182 17 9.3 0.044 0.045 <0.010 0.030 0.330 1990 2013

S2 Eno River near Durham, NC 252 4 1.6 0.055 0.049 <0.010 0.040 0.458 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

293 10 3.4 0.062 0.079 <0.010 0.040 0.646 1989 2013

S4 Flat River at Bahama, NC 309 22 7.1 0.062 0.065 <0.010 0.040 0.490 1989 2011

S5 Cane Creek near Orange Grove, NC 214 11 5.1 0.123 0.191 <0.010 0.050 0.997 1989 2013

S6 Haw River near Bynum, NC 563 2 0.4 0.167 0.106 <0.020 0.140 0.820 1989 2013

S7 New Hope Creek near Blands, NC 545 1 0.2 0.354 0.353 <0.010 0.240 4.10 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

420 0 0 0.392 0.341 0.040 0.280 1.82 1989 2013

S9 Morgan Creek near White Cross, NC 198 1 0.5 0.181 0.258 0.013 0.107 2.80 1989 2013

S10 Morgan Creek near Farrington, NC 548 2 0.4 0.216 0.193 <0.010 0.160 1.60 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

89 9 10.1 0.065 0.045 0.026 0.051 0.294 2000 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

134 0 0 0.109 0.047 0.030 0.100 0.270 1989 2000

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

426 2 0.5 0.138 0.106 <0.010 0.120 1.40 1989 2012

Total organic carbon (mg/L)

S1 Eno River at Hillsborough, NC 186 0 0 6.4 3.1 2.7 5.6 26.6 1990 2013

S5 Cane Creek near Orange Grove, NC 200 0 0 7.8 6.6 2.1 5.6 49.5 1989 2013

S9 Morgan Creek near White Cross, NC 194 0 0 6.8 6.4 2.1 5.2 58.0 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

93 0 0 11.5 2.6 6.6 11.1 17.6 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

127 0 0 8.9 2.8 0.8 8.3 22.0 1989 2003
24 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 6. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by stream site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius;diss., dissolved; mg/L, milligram per liter; N, nitrogen; P, phosphorus; <, less than]

Map no. 

(fig. 1) USGS station name Number of observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Suspended solids (mg/L)

S2 Eno River near Durham, NC 195 38 19.5 9.6 21.6 <1.0 5.0 275 1989 2013

S3 Little River at SR 1461 near Orange 

Factory, NC

253 88 34.8 14 49 <1.0 3.0 510 1989 2013

S4 Flat River at Bahama, NC 138 23 16.7 15 53 <1.0 5.0 460 1989 2011

S6 Haw River near Bynum, NC 455 106 23.3 16 41 <1.0 6.0 570 1989 2013

S7 New Hope Creek near Blands, NC 485 39 8.0 25 27 <1.0 18 280 1989 2013

S8 Northeast Creek at SR 1100 near 

Genlee, NC

338 62 18.3 22 38 <1.0 12 558 1989 2013

S10 Morgan Creek near Farrington, NC 487 84 17.2 16 20 <1.0 9.0 243 1989 2013

S12 Haw River below B. Everett Jordan 

Dam near Moncure, NC

150 10 6.7 13 11 1.0 9.0 64 1989 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

278 2 0.7 16 24 <1.0 10 225 1992 2012

Suspended sediment (mg/L)

S1 Eno River at Hillsborough, NC 180 0 0 26 63 1 8 546 1990 2013

S4 Flat River at Bahama, NC 164 0 0 48 84 1 11 581 1989 2011

S5 Cane Creek near Orange Grove, NC 192 0 0 47 169 1 7 1,330 1989 2013

S9 Morgan Creek near White Cross, NC 187 0 0 26 85 1 7 640 1989 2013

S11 White Oak Creek at mouth near Green 

Level, NC

91 0 0 35 59 3 12 351 2000 2013

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

114 0 0 30 46 4 16 290 1989 2003
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 25

Table 7. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by lake site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius; m, meter; diss., dissolved; 

mg/L, milligram per liter; N, nitrogen; P, phosphorus; μg/L, microgram per liter; <, less than]

Map no. 

(fig. 1) USGS station name

Number 

of 

observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Specific conductance (μS/cm at 25 °C)

L1 Little River Reservoir at Dam near 

Bahama, NC

106 0 0 74 10 45 73 99 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 70 11 44 70 100 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 76 10 42 76 111 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 0 0 196 70 85 192 453 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 0 0 95 16 66 93 186 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

97 0 0 172 41 74 166 268 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

132 0 0 158 32 80 157 218 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

89 0 0 158 29 83 159 214 1991 2013

Calcium, diss. (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

106 0 0 5.70 0.77 3.41 5.76 8.20 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 4.97 0.66 2.86 5.00 6.46 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 5.87 0.84 3.56 5.81 9.60 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

104 0 0 8.08 1.17 5.57 7.98 10.8 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 0 0 7.62 1.19 5.38 7.55 12.0 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 8.54 1.16 5.20 8.49 11.7 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

106 0 0 7.66 0.96 5.10 7.58 9.59 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 7.51 0.90 5.02 7.48 9.49 1991 2013

Magnesium, diss. (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

106 0 0 2.28 0.28 1.46 2.31 2.90 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 2.17 0.30 1.24 2.20 2.82 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 2.39 0.29 1.46 2.40 3.80 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

104 0 0 3.21 0.47 2.24 3.15 4.25 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 0 0 2.63 0.34 1.90 2.63 3.73 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 2.90 0.42 1.75 2.87 3.95 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

106 0 0 2.78 0.36 1.80 2.80 3.53 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 2.83 0.33 1.88 2.81 3.52 1991 2013
26 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 7. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by lake site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius; m, meter; diss., dissolved; 

mg/L, milligram per liter; N, nitrogen; P, phosphorus; μg/L, microgram per liter; <, less than]

Map no. 

(fig. 1) USGS station name

Number 

of 

observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Potassium, diss. (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

106 0 0 1.87 0.33 1.20 1.80 2.83 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 1.96 0.40 1.20 1.90 2.97 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 2.26 0.45 1.42 2.15 3.68 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

104 0 0 3.87 1.10 2.11 3.68 8.07 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 0 0 2.30 0.50 1.34 2.22 3.52 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 3.66 0.85 1.70 3.64 5.75 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

106 0 0 3.31 0.62 1.90 3.26 4.49 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 3.36 0.56 1.90 3.40 4.32 1991 2013

Sodium, diss. (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

106 0 0 4.20 0.82 2.21 4.20 5.89 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 4.41 0.88 1.76 4.43 6.55 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 4.04 0.62 2.38 4.03 5.80 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

104 0 0 23.0 12.0 5.84 21.8 64.1 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 0 0 5.78 1.14 3.20 5.69 8.16 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 18.9 6.9 5.07 18.2 45.1 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

106 0 0 16.4 4.9 6.90 16.0 26.8 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 17.1 4.6 7.00 17.0 27.0 1991 2013

Chloride, diss. (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

105 0 0 4.94 1.09 1.98 4.90 7.45 1989 2013

L2 Lake Michie at Dam near Bahama, NC 103 0 0 4.87 0.93 2.29 4.72 7.19 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

105 0 0 5.28 0.74 3.10 5.30 7.03 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 0 0 18.4 8.9 5.34 16.9 47.0 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

105 0 0 5.99 1.38 3.68 5.60 9.71 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

96 0 0 17.3 5.9 4.50 16.8 35.1 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

105 0 0 14.9 4.3 6.41 14.3 25.1 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 15.2 4.0 6.40 14.9 23.9 1991 2013
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 27

Table 7. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by lake site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius; m, meter; diss., dissolved; 

mg/L, milligram per liter; N, nitrogen; P, phosphorus; μg/L, microgram per liter; <, less than]

Map no. 

(fig. 1) USGS station name

Number 

of 

observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Sulfate, diss. (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

105 0 0 4.13 1.22 2.11 3.90 8.27 1989 2013

L2 Lake Michie at Dam near Bahama, NC 103 0 0 3.97 1.17 2.19 3.71 7.86 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

105 0 0 3.90 1.09 1.20 3.90 7.30 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 0 0 19.7 9.4 6.78 17.7 55.2 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

104 0 0 5.45 1.95 0.20 5.20 12.3 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

96 0 0 15.8 5.0 6.92 15.4 30.4 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

104 0 0 14.0 3.4 6.93 13.9 21.8 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 14.3 3.3 6.77 14.4 21.3 1991 2013

Nitrate + nitrite, diss. (mg/L as N)

L1 Little River Reservoir at Dam near 

Bahama, NC

105 36 34.3 0.091 0.101 <0.005 0.073 0.470 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 51 48.1 0.061 0.086 <0.005 0.012 0.360 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 52 49.1 0.102 0.205 <0.005 0.012 1.70 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 12 11.7 0.388 0.327 <0.005 0.360 1.50 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 74 69.8 0.027 0.067 <0.005 0.005 0.470 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 50 51.0 0.071 0.124 <0.005 0.011 0.660 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

132 64 48.5 0.063 0.099 <0.005 0.010 0.630 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 43 47.8 0.102 0.140 <0.005 0.020 0.640 1991 2013

Ammonia, diss. (mg/L as N)

L1 Little River Reservoir at Dam near 

Bahama, NC

105 17 16.2 0.058 0.066 <0.002 0.034 0.350 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 21 19.8 0.055 0.065 <0.002 0.030 0.358 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 21 19.8 0.048 0.093 0.002 0.020 0.650 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 19 18.4 0.030 0.030 <0.002 0.020 0.194 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 35 33.0 0.057 0.228 <0.002 0.012 2.30 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 31 31.6 0.031 0.041 <0.002 0.017 0.220 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

132 40 30.3 0.036 0.042 <0.002 0.020 0.213 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 26 28.9 0.033 0.044 <0.002 0.013 0.223 1991 2013
28 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 7. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by lake site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius; m, meter; diss., dissolved; 

mg/L, milligram per liter; N, nitrogen; P, phosphorus; μg/L, microgram per liter; <, less than]

Map no. 

(fig. 1) USGS station name

Number 

of 

observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Total organic N (mg/L as N)

L1 Little River Reservoir at Dam near 

Bahama, NC

105 0 0 0.48 0.13 0.18 0.47 0.86 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 0.52 0.13 0.28 0.51 0.96 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 0.62 0.23 0.29 0.58 1.6 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 1 1.0 0.79 0.29 <0.18 0.75 2.0 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 1 0.9 0.72 0.29 <0.18 0.65 2.2 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 0.95 0.36 0.31 0.90 3.4 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

132 0 0 0.71 0.14 0.32 0.71 1.2 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 0.67 0.12 0.38 0.66 1.2 1991 2013

Total N (mg/L as N)

L1 Little River Reservoir at Dam near 

Bahama, NC

105 0 0 0.64 0.16 0.35 0.61 1.3 1989 2013

L2 Lake Michie at Dam near Bahama, NC 106 0 0 0.65 0.15 0.35 0.64 1.2 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

106 0 0 0.78 0.29 0.35 0.73 2.1 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

103 0 0 1.2 0.37 0.47 1.1 2.6 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 1 0.9 0.82 0.39 <0.25 0.75 3.2 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 1.1 0.35 0.42 1.0 3.4 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

132 0 0 0.82 0.16 0.45 0.81 1.3 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

90 0 0 0.82 0.19 0.45 0.80 1.6 1991 2013

Total P (mg/L as P)

L1 Little River Reservoir at Dam near 

Bahama, NC

103 11 10.7 0.029 0.014 <0.010 0.030 0.082 1989 2013

L2 Lake Michie at Dam near Bahama, NC 104 15 14.4 0.034 0.015 <0.010 0.030 0.080 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

103 18 17.5 0.030 0.025 0.008 0.029 0.230 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

101 2 2.0 0.091 0.044 <0.010 0.080 0.250 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

103 6 5.8 0.046 0.017 <0.010 0.045 0.093 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

96 1 1.0 0.076 0.034 0.010 0.074 0.300 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

130 4 3.1 0.040 0.011 <0.010 0.040 0.072 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

88 4 4.5 0.037 0.012 <0.010 0.037 0.092 1991 2013
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 29

Table 7. Statistical summary of water-quality properties and constituents compiled for trend analysis in the Triangle area of 

North Carolina, by lake site.—Continued

[Summary statistics for constituents with censored data were determined by use of the Kaplan-Meier technique (Helsel, 2005). Gray shading indicates that more 

than 20 percent of the data for a constituent were censored. Abbreviations: μS/cm, microsiemens per centimeter; °C, degree Celsius; m, meter; diss., dissolved; 

mg/L, milligram per liter; N, nitrogen; P, phosphorus; μg/L, microgram per liter; <, less than]

Map no. 

(fig. 1) USGS station name

Number 

of 

observations

Number 

censored

Percent 

censored Mean Standard deviation Minimum

50th 

percentile 

(median)

Maximum Start year End year

Total organic carbon (mg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

106 0 0 7.7 1.8 4.7 7.5 12.4 1989 2013

L2 Lake Michie at Dam near Bahama, NC 103 0 0 8.6 2.1 2.1 8.3 16.5 1989 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

105 0 0 7.9 2.0 2.9 7.7 16.0 1989 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

101 0 0 8.6 1.8 4.7 8.4 14.7 1991 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

106 0 0 8.5 2.0 4.8 8.6 15.0 1989 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

96 0 0 10.7 1.4 8.0 10.6 15.5 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

105 0 0 9.0 1.5 2.3 8.9 17.0 1991 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

88 0 0 8.4 1.0 5.8 8.3 10.9 1991 2013

Transparency, Secchi (m)

L1 Little River Reservoir at Dam near 

Bahama, NC

86 0 0 1.09 0.36 0.50 1.00 2.50 1993 2013

L2 Lake Michie at Dam near Bahama, NC 86 0 0 0.95 0.36 0.30 0.88 2.50 1993 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

85 0 0 1.30 0.54 0.35 1.20 3.10 1993 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

98 0 0 0.66 0.21 0.20 0.65 1.40 1993 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

85 0 0 0.72 0.23 0.25 0.70 1.45 1993 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

98 0 0 0.50 0.16 0.10 0.50 1.40 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

124 0 0 0.78 0.21 0.40 0.73 1.60 1993 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

81 0 0 0.82 0.21 0.45 0.80 1.40 1993 2013

Chlorophyll a, phytoplankton (μg/L)

L1 Little River Reservoir at Dam near 

Bahama, NC

85 0 0 10.0 10.5 1.0 7.5 75.5 1992 2013

L2 Lake Michie at Dam near Bahama, NC 85 1 1.2 10.0 7.7 <0.1 8.3 37.6 1992 2013

L3 Cane Creek Reservoir at Dam near 

White Cross, NC

85 0 0 13.4 14.1 0.2 8.9 87.8 1992 2013

L4 Jordan Lake, Haw River arm near 

Hanks Chapel, NC

94 0 0 19.1 15.5 0.8 14.8 64.3 1992 2013

L5 University Lake at Intakes near 

Chapel Hill, NC

84 0 0 24.0 23.9 1.2 17.6 165.0 1992 2013

L6 Jordan Lake at Buoy 12 at Farrington, 

NC

90 0 0 30.2 17.7 0.9 27.8 74.5 1993 2013

L7 Jordan Lake above U.S. Highway 64 

at Wilsonville, NC

120 0 0 19.4 10.9 1.2 18.1 52.3 1992 2013

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

79 0 0 16.1 9.4 2.4 14.8 50.3 1992 2013
30 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Specific conductance (μS/cm at 25 °C)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 6.2 0.0309 88 94

S2 Eno River near Durham, NC 1-period 1989–2013 –25.5 0.0019 156 118

S3 Little River at SR 1461 near Orange Factory, NC 1-period 1989–2013 16.3 0.0002 79 92

S4 Flat River at Bahama, NC 1-period 1989–2011 8.6 0.0838 76 82

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 –4.0 0.2318 92 88

S6 Haw River near Bynum, NC 1-period 1989–2013 14.7 0.0489 170 194

S7 New Hope Creek near Blands, NC 1-period 1989–2013 43.2 <0.0001 196 279

S8 Northeast Creek at SR 1100 near Genlee, NC 1-period 1989-2013 63.3 <0.0001 275 446

S9 Morgan Creek near White Cross, NC 1-period 1989–2013 7.6 0.0483 107 115

S10 Morgan Creek near Farrington, NC 2-period 1989–2001 84.5 <0.0001 180 324

2002–2013 5.5 0.2666 328 346

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 25.1 0.0021 78 97

S12 Haw River below B. Everett Jordan Dam near Moncure, NC 1-period 1989–2013 19.1 0.0006 140 166

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

1-period 1989–2012 9.0 0.0143 138 150

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 15.6 <0.0001 65 76

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 12.0 0.0006 63 71

L3 Cane Creek Reservoir at Dam near White Cross, NC none 1989–2013 model error

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 –7.2 0.0010 171 160

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 23.8 <0.0001 90 111

L6 Jordan Lake at Buoy 12 at Farrington, NC 2-period 1993–2001 25.5 <0.0001 128 158

2002–2013 5.8 0.0909 160 168

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 25.4 <0.0001 131 163

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 8.3 0.0177 144 156

Calcium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 8.2 0.0170 6.84 7.40

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 6.6 0.1371 7.03 7.48

S9 Morgan Creek near White Cross, NC 1-period 1989–2013 20.0 <0.0001 8.77 10.50

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 48.9 <0.0001 4.60 6.75

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 5.4 0.0943 6.76 7.11

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 13.4 <0.0001 5.30 6.00

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 4.5 0.0880 4.94 5.16

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –12.9 0.0001 6.84 5.98

2002–2013 12.7 0.0024 5.98 6.71

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 14.6 0.0012 7.46 8.53

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 26.5 <0.0001 6.84 8.61

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 20.6 <0.0001 7.64 9.16

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 19.9 <0.0001 6.82 8.15

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 16.4 <0.0001 6.68 7.76

Magnesium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 8.7 0.0019 2.78 3.01

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 7.8 0.0714 2.61 2.82

S9 Morgan Creek near White Cross, NC 1-period 1989–2013 10.1 0.0191 3.18 3.49

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 26.2 <0.0001 1.95 2.44

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 9.5 0.1976 2.64 2.87
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 31

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Magnesium, diss. (mg/L)—Continued

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 14.2 <0.0001 2.12 2.42

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 13.8 <0.0001 2.05 2.33

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –5.0 0.0246 2.39 2.28

2002–2013 2.8 0.2203 2.27 2.33

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 17.5 0.0001 2.89 3.39

L5 University Lake at Intakes near Chapel Hill, NC 2-period 1989–2001 8.1 0.0063 2.54 2.74

2002–2013 18.0 <0.0001 2.75 3.21

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 25.9 <0.0001 2.52 3.16

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 18.0 <0.0001 2.48 2.91

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 13.1 <0.0001 2.55 2.88

Potassium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 13.5 0.0005 1.58 1.79

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 –1.5 na 1.77 1.75

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –20.7 0.0017 3.00 2.39

2002–2013 –6.9 0.2853 2.38 2.22

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 22.9 <0.0001 2.89 3.52

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 18.0 <0.0001 2.92 3.43

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 3.9 0.2506 1.88 1.95

L2 Lake Michie at Dam near Bahama, NC 2-period 1989–2001 –6.3 0.0596 2.05 1.92

2002–2013 16.8 0.0001 1.93 2.24

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –22.6 <0.0001 2.84 2.23

2002–2013 10.2 <0.0001 2.22 2.44

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 2.7 na 3.42 3.51

L5 University Lake at Intakes near Chapel Hill, NC 2-period 1989–2001 –9.9 0.0055 2.68 2.42

2002–2013 1.4 0.3681 2.42 2.45

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 28.5 <0.0001 3.18 4.06

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 22.5 <0.0001 2.95 3.60

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 13.0 na 3.04 3.43

Sodium, diss. (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 10.5 0.0173 5.13 5.66

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 5.1 0.1426 5.14 5.40

S9 Morgan Creek near White Cross, NC 1-period 1989–2013 30.0 <0.0001 5.12 6.62

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 55.2 <0.0001 5.21 7.93

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 –0.4 na 14.09 14.06

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 27.9 <0.0001 3.61 4.60

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 8.4 0.0773 4.11 4.45

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –5.1 0.1257 4.08 3.88

2002–2013 16.3 0.0003 3.89 4.50

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 2-period 1991–2001 –23.4 0.0325 21.48 16.67

2002–2013 –4.5 0.3615 16.56 15.85

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 38.4 <0.0001 5.19 7.13

L6 Jordan Lake at Buoy 12 at Farrington, NC 2-period 1993–2001 45.2 <0.0001 11.35 16.14

2002–2013 13.6 0.0246 16.37 18.49

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 33.0 <0.0001 13.15 17.40

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 8.1 0.3055 15.24 16.44
32 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Chloride, diss. (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 37.1 <0.0001 5.05 6.89

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 5.5 0.1023 6.31 6.65

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –2.1 0.3376 6.62 6.49

2002–2013 47.2 <0.0001 6.54 9.47

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 48.9 <0.0001 6.30 9.22

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 15.4 0.0723 11.59 13.30

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 32.4 <0.0001 4.07 5.36

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 6.0 0.1542 4.50 4.76

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –5.0 0.1518 5.71 5.43

2002–2013 15.0 0.0012 5.45 6.22

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 1.1 na 14.35 14.49

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 42.2 <0.0001 5.19 7.34

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 46.9 <0.0001 13.27 19.28

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 41.6 <0.0001 11.68 16.41

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

2-period 1991–2001 23.6 <0.0001 11.86 14.52

2002–2013 6.6 0.1942 14.60 15.52

Sulfate, diss. (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 –3.8 0.3771 5.32 5.12

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 2.8 0.8875 3.45 3.55

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –19.5 0.0116 4.82 3.91

2002–2013 21.0 0.0529 3.91 4.70

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 –13.1 0.3078 6.37 5.57

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 –14.8 0.0010 14.72 12.65

L1 Little River Reservoir at Dam near Bahama, NC 2-period 1989–2001 –4.2 0.2701 4.74 4.55

2002–2013 –21.8 <0.0001 4.53 3.57

L2 Lake Michie at Dam near Bahama, NC 2-period 1989–2001 –3.3 0.3105 4.62 4.49

2002–2013 –27.7 <0.0001 4.45 3.26

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –12.0 0.0055 3.75 3.32

2002–2013 –20.7 <0.0001 3.29 2.64

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 –42.9 <0.0001 21.38 12.37

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 27.4 0.0003 4.73 6.00

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 9.7 0.0094 13.49 14.76

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 2-period 1991–2001 –15.6 0.0001 15.28 13.00

2002–2013 5.1 0.1804 12.97 13.61

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 –26.4 <0.0001 16.79 12.45

Nitrate + nitrite, diss. (mg/L as N)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 –12.3 0.0561 0.255 0.224

S2 Eno River near Durham, NC 1-period 1989–2013 –47.3 0.0001 0.324 0.178

S3 Little River at SR 1461 near Orange Factory, NC 1-period 1989–2013 –18.2 0.0381 0.198 0.163

S4 Flat River at Bahama, NC 1-period 1989–2011 –24.5 0.2921 0.204 0.155

S5 Cane Creek near Orange Grove, NC 2-period 1989–2001 –29.0 0.0079 0.553 0.398

2002–2013 34.9 0.0194 0.398 0.530

S6 Haw River near Bynum, NC 1-period 1989–2013 –24.3 0.0001 1.096 0.834

S7 New Hope Creek near Blands, NC 2-period 1989–2001 –54.2 <0.0001 4.350 2.056

2002–2013 30.9 0.0005 2.035 2.630

S8 Northeast Creek at SR 1100 near Genlee, NC Step, July 2005 1989–2013 –75.5 <0.0001 5.682 1.365
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 33

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Nitrate + nitrite, diss. (mg/L as N)—Continued

S9 Morgan Creek near White Cross, NC 1-period 1989–2013 –48.4 0.0116 0.500 0.262

S10 Morgan Creek near Farrington, NC 2-period 1989–2001 41.3 0.0050 2.773 3.864

2002–2013 –31.3 0.0056 3.855 2.692

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 86.2 0.0001 0.034 0.062

S12 Haw River below B. Everett Jordan Dam near Moncure, NC Early period 1989–2000 –37.2 <0.0001 0.547 0.350

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

1-period 1989–2012 –6.2 0.5967 0.431 0.405

L1 Little River Reservoir at Dam near Bahama, NC none 1989–2013 highly censored

L2 Lake Michie at Dam near Bahama, NC none 1989–2013 highly censored

L3 Cane Creek Reservoir at Dam nr White Cross, NC none 1989–2013 highly censored

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 2-period 1991–2001 3.3 0.3951 0.306 0.316

2002–2013 –23.3 <0.0001 0.314 0.244

L5 University Lake at Intakes near Chapel Hill, NC none 1989–2013 highly censored

L6 Jordan Lake at Buoy 12 at Farrington, NC none 1993–2013 highly censored

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC none 1991–2013 highly censored

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

none 1991–2013 highly censored

Ammonia, diss. (mg/L as N)

S1 Eno River at Hillsborough, NC 2-period 1990–2001 –34.8 <0.0001 0.042 0.028

2002–2013 –0.2 0.3909 0.028 0.028

S2 Eno River near Durham, NC none 1989–2013 highly censored

S3 Little River at SR 1461 near Orange Factory, NC none 1989–2013 highly censored

S4 Flat River at Bahama, NC none 1989–2011 highly censored

S5 Cane Creek near Orange Grove, NC none 1989–2013 highly censored

S6 Haw River near Bynum, NC none 1989–2013 highly censored

S7 New Hope Creek near Blands, NC 1-period 1989–2013 –39.0 0.0043 0.063 0.039

S8 Northeast Creek at SR 1100 near Genlee, NC Step, July 2005 1989–2013 –57.3 <0.0001 0.121 0.043

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –49.9 0.0001 0.056 0.029

2002–2013 –20.1 0.2083 0.028 0.023

S10 Morgan Creek near Farrington, NC 2-period 1989–2001 9.3 0.3490 0.088 0.096

2002–2013 –50.0 0.0004 0.095 0.049

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 27.6 na 0.019 0.024

S12 Haw River below B. Everett Jordan Dam near Moncure, NC Early period 1989–2000 –54.4 0.0001 0.139 0.066

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

2-period 1989–2001 –29.2 0.0001 0.091 0.065

2002–2012 –2.6 0.3840 0.065 0.063

L1 Little River Reservoir at Dam near Bahama, NC 2-period 1989–2001 –73.4 <0.0001 0.117 0.033

2002–2013 –25.2 0.1647 0.032 0.024

L2 Lake Michie at Dam near Bahama, NC 2-period 1989–2001 –80.9 <0.0001 0.117 0.024

2002–2013 125.4 0.0001 0.023 0.051

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –69.0 <0.0001 0.129 0.042

2002–2013 87.5 0.0277 0.041 0.076

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 –17.7 0.2059 0.040 0.033

L5 University Lake at Intakes near Chapel Hill, NC none 1989–2013 highly censored

L6 Jordan Lake at Buoy 12 at Farrington, NC none 1993–2013 highly censored

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC none 1991–2013 highly censored

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

none 1991–2013 highly censored
34 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Total organic N (mg/L as N)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 29.1 0.0005 0.29 0.37

S2 Eno River near Durham, NC 1-period 1989–2013 20.1 0.0099 0.26 0.31

S3 Little River at SR 1461 near Orange Factory, NC 1-period 1989–2013 48.3 <0.0001 0.25 0.36

S4 Flat River at Bahama, NC 1-period 1989–2011 29.4 0.0015 0.26 0.34

S5 Cane Creek near Orange Grove, NC 2-period 1989–2001 –19.3 0.0748 0.37 0.30

2002–2013 60.7 0.0001 0.30 0.48

S6 Haw River near Bynum, NC 1-period 1989–2013 44.9 <0.0001 0.39 0.56

S7 New Hope Creek near Blands, NC 1-period 1989–2013 75.0 <0.0001 0.48 0.83

S8 Northeast Creek at SR 1100 near Genlee, NC 1-period 1989–2013 43.9 na 0.61 0.87

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –27.2 0.0065 0.43 0.32

2002–2013 –13.4 0.2131 0.31 0.27

S10 Morgan Creek near Farrington, NC 1-period 1989–2013 46.9 <0.0001 0.48 0.70

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 6.1 0.2987 0.56 0.60

S12 Haw River below B. Everett Jordan Dam near Moncure, NC Early period 1989–2000 –11.0 0.2401 0.45 0.40

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

1-period 1989–2012 30.6 0.0001 0.51 0.66

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 17.7 <0.0001 0.40 0.46

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 26.5 <0.0001 0.53 0.67

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –10.6 0.0878 0.42 0.37

2002–2013 31.5 <0.0001 0.38 0.49

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 8.2 0.5541 0.65 0.71

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 18.4 <0.0001 0.53 0.63

L6 Jordan Lake at Buoy 12 at Farrington, NC 2-period 1993–2001 50.3 <0.0001 0.68 0.99

2002–2013 –17.6 0.0147 1.00 0.83

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 2-period 1991–2001 34.9 <0.0001 0.55 0.74

2002–2013 –8.8 0.0355 0.74 0.68

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 –3.5 0.5376 0.67 0.64

Total N (mg/L as N)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 8.8 0.1229 0.60 0.65

S2 Eno River near Durham, NC 1-period 1989–2013 –17.6 0.0062 0.71 0.59

S3 Little River at SR 1461 near Orange Factory, NC 1-period 1989–2013 10.7 0.1750 0.52 0.58

S4 Flat River at Bahama, NC 1-period 1989–2011 –0.7 0.8065 0.55 0.55

S5 Cane Creek near Orange Grove, NC 2-period 1989–2001 –33.0 <0.0001 1.12 0.76

2002–2013 62.6 <0.0001 0.77 1.22

S6 Haw River near Bynum, NC 1-period 1989–2013 –6.5 0.1692 1.62 1.51

S7 New Hope Creek near Blands, NC 2-period 1989–2001 –43.9 <0.0001 5.12 2.94

2002–2013 35.5 <0.0001 2.92 3.91

S8 Northeast Creek at SR 1100 near Genlee, NC Step, July 2005 1989–2013 –61.0 <0.0001 6.73 2.54

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –37.0 <0.0001 1.27 0.81

2002–2013 –32.5 0.0001 0.80 0.55

S10 Morgan Creek near Farrington, NC 2-period 1989–2001 41.6 0.0003 3.45 4.82

2002–2013 –26.0 0.0053 4.82 3.61

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 12.0 0.0093 0.62 0.69

S12 Haw River below B. Everett Jordan Dam near Moncure, NC Early period 1989–2000 –29.9 <0.0001 1.27 0.91

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

2-period 1989–2001 –13.8 0.0071 1.24 1.07

2002–2012 24.9 0.0003 1.08 1.33

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 –13.5 0.0009 0.72 0.63
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 35

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Total N (mg/L as N)—Continued

L2 Lake Michie at Dam near Bahama, NC 2-period 1989–2001 –13.2 0.0063 0.84 0.73

2002–2013 16.9 0.0086 0.73 0.85

L3 Cane Creek Reservoir at Dam nr White Cross, NC 2-period 1989–2001 –33.8 <0.0001 1.09 0.73

2002–2013 31.2 0.0010 0.73 0.95

L4 Jordan Lake, Haw River Arm near Hanks Chapel, NC 1-period 1991–2013 2.5 0.8415 1.14 1.17

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 2.9 <0.0001 0.70 0.71

L6 Jordan Lake at Buoy 12 at Farrington, NC 2-period 1993–2001 21.4 0.0075 0.91 1.09

2002–2013 –8.5 0.1804 1.09 1.00

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 2-period 1991–2001 25.9 <0.0001 0.71 0.89

2002–2013 –13.0 0.0046 0.89 0.78

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 –3.3 0.5716 0.84 0.81

Total P (mg/L as P)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 –11.0 0.4624 0.039 0.035

S2 Eno River near Durham, NC 1-period 1989–2013 –25.9 0.0043 0.048 0.036

S3 Little River at SR 1461 near Orange Factory, NC 1-period 1989–2013 15.1 0.1371 0.033 0.038

S4 Flat River at Bahama, NC 1-period 1989–2011 –21.1 <0.0001 0.048 0.038

S5 Cane Creek near Orange Grove, NC 2-period 1989–2001 –34.4 0.0053 0.078 0.052

2002–2013 42.2 0.0224 0.052 0.073

S6 Haw River near Bynum, NC 2-period 1989–2001 –20.5 0.0116 0.217 0.174

2002–2013 –44.4 <0.0001 0.171 0.097

S7 New Hope Creek near Blands, NC 2-period 1989–2001 –44.8 <0.0001 0.397 0.225

2002–2013 2.2 0.3930 0.222 0.227

S8 Northeast Creek at SR 1100 near Genlee, NC 1-period 1989–2013 –41.0 0.0007 0.388 0.232

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 –63.6 <0.0001 0.234 0.089

2002–2013 –56.7 <0.0001 0.085 0.038

S10 Morgan Creek near Farrington, NC 1-period 1989–2013 –6.9 0.3865 0.164 0.153

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 53.5 <0.0001 0.045 0.068

S12 Haw River below B. Everett Jordan Dam near Moncure, NC Early period 1989–2000 –41.8 <0.0001 0.126 0.075

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

2-period 1989–2001 20.3 0.0431 0.108 0.129

2002–2012 –20.5 0.0347 0.129 0.104

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 –2.2 0.4028 0.037 0.036

L2 Lake Michie at Dam near Bahama, NC 2-period 1989–2001 –17.6 0.0007 0.041 0.034

2002–2013 25.4 0.0006 0.034 0.043

L3 Cane Creek Reservoir at Dam nr White Cross, NC 1-period 1989–2013 –8.1 na 0.033 0.030

L4 Jordan Lake, Haw River arm near Hanks Chapel, NC 1-period 1991–2013 48.6 0.0008 0.064 0.094

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 10.6 0.3833 0.035 0.039

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 57.8 na 0.055 0.086

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 2-period 1991–2001 36.1 <0.0001 0.033 0.044

2002–2013 –10.2 0.0404 0.044 0.040

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

1-period 1991–2013 15.3 0.0039 0.035 0.040
36 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 8. Summary of trend results based on data collected during 1989–2013 for specific conductance, major ions, and nutrients at 

stream and lake sites assessed in the Triangle area of North Carolina.—Continued

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward trend 

results. Gray shading indicates no trend results are presented due to model error or because more than 20 percent of the data were censored. Abbreviations: USGS, 

U.S. Geological Survey; NC, North Carolina; μS/cm, microsiemens per centimeter; °C, degree Celsius; <, less than; diss., dissolved; mg/L, milligram per liter; 

na, not applicable; N, nitrogen; P, phosphorus]

Map no. 

(fig. 1) USGS station name Best-fit trend model Trend period

Percent 

change p-value

Fitted annual 

median 

concentration for 

first year 

in period

Fitted annual 

median 

concentration for 

last year 

in period

Total organic carbon (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 19.8 0.0015 5.2 6.2

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 16.6 0.1846 5.5 6.3

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 0.5 0.3980 5.5 5.5

2002–2013 –21.1 0.0147 5.5 4.4

S11 White Oak Creek at mouth near Green Level, NC Late period 2002–2013 –2.6 0.4386 11.4 11.1

S13 Cape Fear River at State Highway 42 near 

Brickhaven, NC

Early period 1989–2001 6.1 0.6315 8.2 8.6

L1 Little River Reservoir at Dam near Bahama, NC 1-period 1989–2013 10.8 na 7.1 7.8

L2 Lake Michie at Dam near Bahama, NC 1-period 1989–2013 14.4 0.0183 8.4 9.6

L3 Cane Creek Reservoir at Dam nr White Cross, NC 1-period 1989–2013 14.4 0.1670 7.1 8.1

L4 Jordan Lake, Haw River arm near Hanks Chapel, NC 2-period 1991–2001 6.0 0.2227 8.4 8.9

2002–2013 –12.3 0.0151 8.9 7.8

L5 University Lake at Intakes near Chapel Hill, NC 1-period 1989–2013 18.2 0.0149 7.4 8.7

L6 Jordan Lake at Buoy 12 at Farrington, NC 2-period 1993–2001 12.6 0.0086 10.0 11.2

2002–2013 –8.2 0.1145 11.2 10.4

L7 Jordan Lake above U.S. Highway 64 at Wilsonville, NC 1-period 1991–2013 3.7 0.3833 8.6 9.0

L8 Jordan Lake at Bells Landing near Griffins 

Crossroads, NC

2-period 1991–2001 5.7 0.1394 8.3 8.7

2002–2013 –9.5 0.0264 8.7 7.9
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 37

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

e, cnat cudnoc cfii ceps Change in 

ecnat cudnoc cfii ceps Percent change in in microsiemens per centimeter 1208040 –40 –80

–12

00

500400300200

100

0

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

×

S6

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

×L3

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig07

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberNo trend result and site number

S13

L6

S2S5

Figure 7. Trends in specific conductance at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
38 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

patterns observed for these major ions were remarkably 

similar throughout the study area. 

Calcium trends ranged from a 12.9 percent decrease 

to a 48.9 percent increase during the various trend periods 

(table 8; fig. 8). A 1-period trend model provided the best fit 

for calcium data at 10 sites. During the 1-period trend period 

(1989–2013), calcium significantly increased at eight sites 

and showed smaller, nonsignificant uptrends at two additional 

sites. Calcium decreased significantly at Cane Creek Reservoir 

(site L3) during 1989–2001, but this downtrend was offset by 

a significant uptrend during 2002–13. At White Oak Creek 

(site S11), calcium increased significantly during 2002–13 and 

increased by a smaller, nonsignificant amount at Cape Fear 

River near Brickhaven (site S13) during 1989–2001.

Magnesium trends ranged from –5.0 to 26.2 percent 

(table 8; fig. 9). Similar to calcium, magnesium increased 

significantly during 1989–2013 at nine sites and to a lesser, 

nonsignificant degree at one site. Magnesium decreased 

significantly at Cane Creek Reservoir (site L3) during 

1989–2001, but this decrease was more or less offset by a 

nonsignificant uptrend during 2002–13. Magnesium increased 

significantly at White Oak Creek (site S11) during 2002–13 

and by a smaller, nonsignificant amount at Cape Fear River 

near Brickhaven (site S13) during 1989–2001.

Potassium trends were generally similar to calcium 

and magnesium trends, with notable exceptions at sites S9 

and L5. Overall, trends ranged from –22.6 to 28.5 percent 

(table 8; fig. 10). During 1989–2013, potassium increased 

significantly at three sites and showed nonsignificant uptrends 

at three sites and a nonsignificant downtrend at one site. A 

2-period trend model best fit the potassium concentrations at 

four sites. Significant potassium downtrends were noted for 

Morgan Creek near White Cross (site S9) and University Lake 

(site L5) during 1989–2001 followed by small, nonsignificant 

trends during 2002–13 compared to consistent uptrends for 

calcium and magnesium for those sites. Potassium decreased 

slightly at Lake Michie (site L2) during 1989–2001, then 

increased significantly during 2002–13. Potassium also 

increased significantly at White Oak Creek (site S11) during 

2002–13 and increased at Cape Fear River near Brickhaven 

(site S13) during 1989–2001.

Sodium trends also were generally similar to calcium and 

magnesium trends at all sites except site L4. Trends ranged 

from –23.4 to 55.2 percent (table 8; fig. 11), with uptrends at 

most sites. Sodium increased significantly during 1989–2013 

at five sites and to a lesser, nonsignificant degree at three 

sites. Significant uptrends also were observed at White Oak 

Creek (site S11) and Cane Creek Reservoir (site L3) during 

2002–13. Sodium increased significantly at Jordan Lake at 

Buoy 12 (site L6) during both the early and late periods, 

although the rate of increase was substantially higher during 

the early period. For Jordan Lake near Hanks Chapel (site L4), 

a significant downtrend (23.4 percent) in sodium was observed 

during 1991–2001 and a small, nonsignificant downtrend was 

observed during 2002–13. 

Chloride trends ranged from –5.0 to 48.9 percent 

and trended upward at most sites, similar to calcium and 

magnesium (table 8; fig. 12). Chloride increased significantly 

during 1989–2013 at five sites and by a lesser, nonsignificant 

amount at three sites. Significant uptrends also were observed 

during 2002–13 at Morgan Creek near White Cross (site S9), 

White Oak Creek (site S11), and Cane Creek Reservoir, and 

during 1991–2001 at Jordan Lake at Bells Landing (site L8). 

For the Cape Fear River near Brickhaven (site S13), sodium 

also trended upward during 1989–2001, but the trend was not 

statistically significant.

Evidence is increasing that urbanization raises specific 

conductance and concentrations of major ions in downstream 

waters through nonpoint-source inputs. Increases in calcium 

and magnesium in streams have been attributed to weathering 

of concrete and other carbonate materials, and increases in 

sodium and chloride have been related to road-salt runoff 

(Kaushal and others, 2017; Moore and others, 2017). Likewise, increasing chloride concentrations in North American 

lakes have been attributed to road-salt application (Dugan 

and others, 2017). In the current study, some of the largest 

percent increases in major ion concentrations were observed 

at White Oak Creek (site S11), where calcium, sodium, and 

chloride increased by more than 40 percent during the 12-year 

period 2002–13. The White Oak Creek watershed (site S11) 

had the greatest increase in developed land cover as well as 

pronounced increases in major ion concentrations and specific 

conductance during the entire study period. For other sites 

with significant increases in specific conductance, such as 

sites S7, S8, and S10, data were insufficient to assess trends 

in individual ions. Because increasing concentrations of ions 

have implications for water treatment, agricultural use, and 

aquatic health, it could be beneficial to obtain sufficient data to 

track future trends at additional locations in the study area.

Although urbanization also has been associated with 

increasing sulfate concentrations in other areas, sulfate in the 

Triangle area trended downward at most sites (table 8; fig. 13). 

Sulfate trends ranged from – 42.9 to 27.4 percent. Concentrations increased significantly during 1989–2013 at only two 

sites—University Lake (L5) and Jordan Lake at Buoy 12 

(L6); these uptrends were similar to uptrends for the previous 

ions. Sulfate concentrations for the remaining sites trended 

downward or remained relatively unchanged, in sharp contrast 

to uptrends for most of the other major ions. Sulfate concentrations in atmospheric deposition have declined during the 

last several decades due to advances in air-pollution controls, 

which may have contributed to the observed downward trends.

The previous discussion focused on percent changes 

(either increasing for decreasing) in major ion concentrations. 

The effects of the trends on water quality, however, need to be 

assessed both in terms of the relative (percent) changes and 

with respect to actual concentration changes (table 8). A large 

percent change at one site may not have as much effect (in 

terms of concentrations) as a smaller percent change at another 

site. For example, some of the largest percent increases in 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 39 Figure 8. Trends in dissolved calcium concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

WAKE

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S12

L6

L5

L4

L3

L7 L8

S13

S1

S2

S3

S4

S5

L2

L1

Annual median dissolved calcium

concentration, in milligrams per liter

Percent change in dissolved calcium

concentration

604020

–20–40–60

0

1210

86420

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

res18-0031_fig08

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberInsufficient data and site number

S11

S5

L3

S6
40 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S12

L6

L5

L4

L3

L7

L8

S13

S1

S2

S3

S4

S5

L2

L1

Annual median dissolved magnesium

concentration, in milligrams per liter

Percent change in dissolved magnesium

concentration

302010

–10–20–30

043210

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

res18-0031_fig09

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberInsufficient data and site number

S1

S5

L3

S6

Figure 9. Trends in dissolved magnesium concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 41

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7

L8

S13

S1 S2

S3

S4

S5

L2

L1

S13

L4

S9

L2

S6

Annual median dissolved potassium

concentration, in milligrams per liter

Percent change in dissolved potassium

concentration

40302010

–10–20–30–40

0543210

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

Trends_Figure_10

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

Figure 10. Trends in dissolved potassium concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989-2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
42 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

res18-0031_fig11

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S6

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1

S2

S3

S4

S5

L2

L1

Annual median dissolved sodium

concentration, in milligrams per liter

Percent change in dissolved sodium

concentration

604020

–20–40–60

0

25201510

50

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

S11

L2

S6

S13

L4

Figure 11. Trends in dissolved sodium concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 43

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

GRANVILLE

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

S7

S9

S11

S10

S12

L6

L5

L4

L3

L7

L8

S13

S1 S2

S3

S4

S5

L2

L1

Annual median dissolved chloride

concentration, in milligrams per liter

Percent change in dissolved chloride

concentration

604020

–20–40–60

0

25201510

50

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig12

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

S1

L2L3

S6

Figure 12. Trends in dissolved chloride concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
44 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S12

L6

L5

L4

L3

L7

L8

S13

S1

S2

S3

S4

S5

L2

L1

Annual median dissolved sulfate

concentration, in milligrams per liter

Percent change in dissolved sulfate

concentration

604020

–20–40–60

0

25201510

50

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig13

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

L5

S5S6

S11

S9

Figure 13. Trends in dissolved sulfate concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 45

major ions were observed at White Oak Creek (site S11). 

However, concentrations of these ions for site S11 at the end 

of the trend assessment period (2013) were still low compared 

to many other sites (figs. 8, 11, 12). 

Among the lake sites, University Lake (L5) had some of 

the largest percent increases in sodium, chloride, and sulfate 

concentrations. However, concentrations in 2013 for site L5 

were still well below concentrations for several other lake sites 

(figs. 11, 12, 13). Two lake sites, Jordan Lake at Buoy 12 (L6) 

and Jordan Lake above U.S. Highway 64 (L7) had large 

percent increases in sodium and chloride concentrations 

during 1993–2013 and 1991–2013, respectively, and also had 

the highest concentrations of those ions in 2013 compared to 

all of the remaining sites (figs. 11, 12). For site L6, the annual 

median sodium concentration increased from about 11.3 to 

18.5 mg/L from 1993 to 2013, and the annual median chloride 

concentration increased from about 13.3 to 19.3 mg/L during 

the same period.

It is interesting to note that specific conductance for three 

stream sites upstream from the New Hope arm of Jordan Lake 

(S7, S8, and S10) had the largest increases in specific conductance during the study period and by far the highest specific 

conductance in 2013 (fig. 7). Although major ion trends were 

not available for these sites, the specific conductance trends 

indicate that the source of the increasing major ion concentrations in Jordan Lake is probably the upstream tributaries.

Trend results for sulfate (fig. 13) provide an interesting 

contrast to the trends observed for other major ions. Like 

sodium and chloride, significant uptrends in sulfate concentrations were observed for lake sites at University Lake (L5) and 

Jordan Lake at Buoy 12 (L6). However, sulfate concentrations 

for lake sites at Cane Creek Reservoir (L3), Haw River arm 

of Jordan Lake (L4), and Jordan Lake at Bells Landing (L8) 

trended strongly downward. Similarly, for several sites in 

the upper Neuse River Basin portion of the study area (S1, 

L1, and L2), sulfate concentrations decreased in contrast to 

increases in specific conductance and concentrations of all the 

other ions (figs. 7–13). 

Nutrients and Organic Carbon

Nitrogen and phosphorus are the primary elements 

essential for plant and animal nutrition. Nutrient dynamics in 

streams and lakes differ. Excessive concentrations of nutrients 

accelerate the eutrophication of lakes, which can lead to 

water-quality problems like algal blooms, nuisance macrophyte growth, fish kills, taste-and-odor problems, and loss of 

biodiversity and recreational enjoyment. Nutrient enrichment 

primarily affects lakes; however, similar water-quality 

problems may be observed in slow-moving streams. 

In this study, data were sufficient to assess trends for total 

organic nitrogen, total nitrogen, and total phosphorus at all 

of the stream and lake sites (table 8). Data for 19 sites were 

sufficient to evaluate both 1- and 2-period trend models for the 

entire trend period. Data for White Oak Creek (site S11) were 

sufficient to evaluate trends only for the late time period 

(2002–13), and data for Haw River below Jordan Dam 

(site S12) were sufficient to evaluate trends for only the early 

time period (1989–2000). As described below, trends for nitrate 

or ammonia were not reported at several sites where more than 

20 percent of the observations were below reporting levels.

Nitrogen occurs in water in several chemical forms and 

is cycled through the atmosphere, hydrosphere, biosphere, 

and lithosphere through a complex set of chemical and 

biological processes. The major forms of nitrogen that occur in 

water include nitrate plus nitrite, ammonia, and total organic 

nitrogen. Nitrate is the predominant form of inorganic nitrogen 

in surface waters, typically occurring with lesser amounts of 

nitrite and ammonia. For this study, nitrate and nitrite were 

analyzed as a combined fraction; these results hereafter are 

referred to as “nitrate.” Nitrate concentrations in excess of 

10 mg/L as N may cause methemoglobinemia (“blue baby 

syndrome”) in young children (North Carolina Department of 

Health and Human Services, 2018); therefore, concentrations 

of nitrate in water supplies have been studied extensively. In 

the Triangle area, concentrations of nitrate above 10 mg/L 

were occasionally observed during the trend period at three 

stream sites, including New Hope Creek (S7), Northeast Creek 

(S8), and Morgan Creek near Farrington (S10); these sites also 

had the highest median concentrations of nitrate during the 

trend period (table 6). In contrast, the maximum concentration 

of nitrate observed at lake sites was 1.70 mg/L—well below 

10 mg/L. Overall, 45.2 percent of lake-sample values were 

censored (table 5). Nitrate typically was below detection at 

these lake sites during summer months, likely due to assimilation by phytoplankton, and was more likely to be detected 

during fall turnover, winter, and early spring. At seven lake 

sites, more than 30 percent of samples had concentrations 

below reporting levels. At the remaining lake site, Jordan Lake 

near Hanks Chapel (L4), the median nitrate concentration 

during the trend period was an order of magnitude higher than 

at other lake sites (table 7). 

Nitrate trends were assessed at 14 sites, including 13 

stream sites and 1 lake site (table 8). Statistically significant, 

downward trends in nitrate concentrations were observed 

at five sites for the 1989–2013 period; downward but 

nonsignificant trends were observed at another three sites 

(table 8; fig. 14). It should be noted that many of these 

downtrends represented little change in terms of actual nitrate 

concentrations. However, a pronounced change occurred 

at Northeast Creek (site S8), where nitrate concentrations 

declined by 75.5 percent after July 2005, when an upstream 

WWTP implemented nitrogen-removal practices. This step 

trend corresponded to a substantial (4.317 mg/L) decrease 

in annual fitted median concentrations. During 1989–2000, 

nitrate decreased significantly at Haw River below Jordan 

dam (site S12). Nitrate concentrations at White Oak Creek 

(site S11) increased 86.2 percent during 2002–13, which was 

the maximum percent increase observed during the study; 

however, the magnitude of change was very small, with annual 

median concentration increasing only 0.028 mg/L (from 

0.034 to 0.062 mg/L) during that period. 
46 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

GRANVILLE

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberSignificant downward step trend and site numberGreater than 20 percent censored and site number

S11

L4

S9S4

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

S7

S9

S11

S10

S12

L6

L5

L4

L3

L7 L8

S13

S1 S

S

2

3

S4

S5

L2

L1

S8

L6

Annual median dissolved nitrate plus nitrite

concentration, in milligrams per liter

as nitrogen

Percent change in dissolved nitrate plus

nitrite concentration

6040

100

8020

–20–40–60–80

–100

06543210

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig14 Figure 14. Trends in dissolved nitrate plus nitrite concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 47

Two-period trend analysis for nitrate showed mixed 

results at the remaining four sites. At New Hope Creek 

(site S7), annual median concentrations decreased 1.720 mg/L 

overall from 1989 to 2013, with a significant downtrend 

during 1989–2001 followed by a significant, but smaller, 

uptrend during 2002–13. In 1994, a major municipal WWTP 

upstream from site S7 implemented biological nutrient 

removal, which may have contributed to decreasing nitrate 

concentrations during the early trend period (Sydney Miller, 

City of Durham, Department of Water Management, written 

commun., March 2, 2018). The maximum-magnitude increase 

in nitrate concentrations was observed at Morgan Creek 

near Farrington (site S10), where annual fitted medians rose 

1.090 mg/L from 1989 to 2001; however, this uptrend was 

offset by a 1.163 mg/L decrease from 2002 to 2013 (table 8; 

fig. 14). In 2007, a major municipal WWTP upstream from 

site S10 installed deep-bed denitrification filters, which may 

have contributed to decreasing nitrate concentrations during 

the late trend period (Sandra Bradshaw, Orange Water and 

Sewer Authority, written commun., February 28, 2018). It 

is interesting that the largest reductions in nitrate occurred 

at sites S7, S8, and S10 during the late trend period, despite 

considerable development and population growth in these 

watersheds; however, annual median nitrate concentrations 

in 2013 remained higher than at other sites in the study area 

(fig. 14). In an analysis of nutrient yields among 48 streams in 

central and eastern North Carolina, Harden and others (2013) 

noted that high point-source contributions (greater than 10 percent of total streamflow) had a major influence on nitrate 

and total nitrogen yields. Nitrate trends observed at sites S5 

and L4, while statistically significant, accounted for small 

changes in actual concentrations. In other water-quality trend 

studies, decreases in stream nitrate concentrations have been 

associated with decreases in agricultural land use. However, in 

the Triangle area, agricultural land uses were generally stable 

during the trend period (table 3).

Ammonia is produced when microorganisms decompose 

or convert organic matter that contains nitrogen. Ammonia is 

rapidly transformed to other forms of nitrogen when oxygen 

is present and is readily utilized by phytoplankton. Not 

surprisingly, therefore, ammonia concentrations in Triangle 

area streams and lake (near-surface) samples were generally 

low. The median ammonia concentration was 0.040 mg/L 

as N among all stream samples and 0.020 mg/L among all 

lake samples. For stream samples, 19.6 percent of values 

were censored (below reporting limits), and 24.8 percent of 

lake-sample values were censored (table 5). In lakes, ammonia 

typically was below detection during summer months when 

phytoplankton were relatively more abundant and upper 

waters were well oxygenated. 

Ammonia trends were not reported for the five stream 

sites (table 6) and four lake sites (table 7) where more than 

20 percent of the data were censored. Sufficient uncensored 

data were available to analyze trends in ammonia at eight 

stream sites and four lake sites (table 8; fig. 15). From 1989 

to 2013, ammonia significantly decreased at New Hope Creek 

(site S7) and showed a smaller, nonsignificant downtrend at 

Jordan Lake near Hanks Chapel (site L4). At Northeast Creek 

(site S8), a downward step trend (57.3 percent; 0.078 mg/L) 

corresponded to nitrogen-removal implementation at the 

upstream WWTP in July 2005. During the earlier period of 

1989–2001, statistically significant, downward trends were 

noted at seven additional sites (table 8; fig. 15). At Lake 

Michie (site L2) and Cane Creek Reservoir (site L3), ammonia 

concentrations decreased from 1989 to 2001, then increased 

from 2002 to 2013. However, despite the recent uptrends, 

annual median ammonia concentrations for those sites in 2013 

remained well below the values in 1989. The reasons for the 

prevailing downtrends in the study area are unclear other than 

for the previously described step trend at site S8. 

Total organic nitrogen (TON) in water consists of 

living organisms, such as phytoplankton and zooplankton, 

suspended and dissolved materials transported from upstream 

sources, and various intermediate products of organic matter 

decomposition. At stream sites, the maximum TON value 

observed during the trend period of 1989–2013 was 4.4 mg/L 

as N at Cane Creek (site S5); Northeast Creek (site S8) had the 

highest median concentration (0.81 mg/L; table 6). In general, 

concentrations were lower at stream sites in the Neuse River 

Basin than in the Cape Fear River Basin (table 6). Among lake 

sites, Jordan Lake at Farrington (site L6) had the maximum 

TON concentration (3.4 mg/L) as well as the highest median 

concentration (0.90 mg/L). 

In contrast to nitrate and ammonia, concentrations of 

TON generally increased throughout the study area during 

1989–2013 (table 8; fig. 16). Statistically significant, upward 

trends in TON concentrations were noted at 11 sites for 

the 1989–2013 period; upward but nonsignificant trends 

were noted at an additional 2 sites; and a downward but 

nonsignificant trend was noted at 1 site (L8). The largest 

increase of 75.0 percent (0.35 mg/L) was noted at New Hope 

Creek (site S7). In contrast to the substantial reductions in 

nitrate and ammonia that were observed at Northeast Creek 

(site S8), TON trended upward by 43.9 percent (0.26 mg/L) at 

this site, although the trend was not statistically significant. At 

sites with uptrends, increases in annual median concentration 

generally ranged from 0.05 to 0.20 mg/L.

At five sites (S5, S9, L3, L6, and L7), 2-period trend 

models best fit the TON data. At sites S5 and L3, nonsignificant downtrends during the early period were followed by 

significant uptrends during the late period, resulting in higher 

annual median concentrations in 2013. At two sites in Jordan 

Lake (L6, L7), TON increased significantly through 2001, 

then decreased slightly from 2002 to 2013. However, annual 

median concentrations at the end of the trend-analysis period 

remained higher than concentrations at the beginning of the 

record for both sites. In contrast to the generally increasing 

trends observed at most sites in the study area, TON decreased 

significantly at Morgan Creek near White Cross (site S9), 

where annual fitted-median concentrations decreased by 

0.16 mg/L from 1989 to 2013. A nonsignificant uptrend in 

TON concentrations was noted at White Oak Creek (site S11) 
48 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

Annual median dissolved ammonia

concentration, in milligrams per liter

as nitrogen

Percent change in dissolved ammonia

concentration

150100

50

–50

–100–150

0

0.160.140.120.100.080.060.040.02

0

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig15

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberSignificant downward step trend and site numberGreater than 20 percent censored and site number

L2

S11

S7

L4

S8

L6

Figure 15. Trends in dissolved ammonia concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989-2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 49

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

GRANVILLE

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

Percent change in total organic nitrogen

concentration

80604020

–20–40–60–80

0

1.21.00.80.60.40.2

0

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median total organic nitrogen

concentration, in milligrams per liter

as nitrogen

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig16

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site number

S3

L4

S9

L8

Figure 16. Trends in total organic nitrogen concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989-2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
50 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

for the late period, and a nonsignificant downtrend was noted 

at Haw River below Jordan dam (site S12) for the early period. 

Childress and Bathala (1997) previously reported 

downward trends in TON concentrations during 1989–95 at 

sites S5, S9, S12, S13, L1, and L3, which agreed with the 

downward trends noted at sites S5, S9, S12, and L3 during the 

early period (1989–2001) of the current study. Interestingly, 

TON tended to decrease during the first several years of the 

trend period at several additional sites in the current study, 

then shifted to an increasing pattern around 1995 to 2000. This 

pattern was observed even at sites where a significant upward 

trend was ultimately selected for the entire trend period of 

1989–2013. The regional consistency suggested these trends 

were influenced by factors that affected the entire study area. 

Atmospheric deposition is one factor that may have 

contributed to the observed trends in TON. Globally, emissions of anthropogenic nitrogen increased following the 

mid-1900s as fossil-fuel combustion and intensive agriculture 

accelerated (Galloway and Cowling, 2002; Liu and others, 

2013). Oxidized forms of nitrogen (nitrous oxide and nitrite), 

which are attributed primarily to combustion of fossil fuels, 

formerly dominated emissions in the United States. As a 

result of successful regulatory controls, emissions of oxidized 

nitrogen fractions have declined during the last 20 years; however, emissions of reduced forms of nitrogen (ammonia and 

ammonium) have increased and now dominate wet deposition. 

Increases in atmospheric ammonia have been attributed to 

increases in regional livestock production, nitrogen fertilizer 

application, and use of emission-control devices that convert 

oxidized forms of nitrogen to reduced forms (Li and others, 

2016). Nitrogen deposition has also increased outside of the 

United States—in Western Europe and rapidly industrializing nations such as China (Liu and others, 2013). Organic 

forms of nitrogen are also present in the atmosphere, often 

bound with fine particulate matter (aerosols), but data for 

atmospheric organic nitrogen are limited (Holland and others, 

2005; Cornell, 2011). Nutrient enrichment from deposition of 

organic nitrogen is not well understood, and more information 

is needed to support management decisions (Jickells and 

others, 2013; Samy and others, 2013).

Links between shifting nitrogen deposition and instream 

nitrogen concentrations have not been well documented. In 

the Triangle, concentrations of nitrate and ammonia generally 

trended down, while TON trended up. If increased ammonia 

is being deposited, it would likely be quickly converted to 

oxidized forms and (or) assimilated into the aquatic food 

web. Further research is clearly needed to better understand 

the mechanisms responsible for observed increases in 

TON concentrations. 

Total nitrogen concentrations in the Triangle study area 

during 1989–2013 ranged from 0.11 to 31.7 mg/L as N among 

stream sites and from <0.25 to 3.4 mg/L among lake sites 

(table 5). Total nitrogen concentrations varied widely among 

stream sites and were higher in New Hope Creek (site S7), 

Northeast Creek (site S8), and Morgan Creek near Farrington 

(site S10) than at other stream sites (table 6). In contrast, 

concentrations were relatively similar among lake sites, with 

medians ranging from 0.61 to 1.1 mg/L (table 7).

Because trends among inorganic and organic nitrogen 

fractions often diverged at sites, trends in total nitrogen were 

complex (table 8; fig. 17). During 1989–2013, total nitrogen 

did not change significantly at six sites. A statistically significant, but small, upward trend was observed at University 

Lake (site L5). Significant, downward trends were noted at 

Eno River near Durham (site S2) and Little River Reservoir 

(site L1). The most pronounced decrease in total nitrogen was 

a step trend at Northeast Creek (site S8), where total nitrogen 

decreased by 61.0 percent (4.19 mg/L) after July 2005. This 

decrease was largely driven by the reduction in nitrate that 

occurred when nitrogen removal was implemented at the 

upstream WWTP. Total nitrogen concentrations decreased 

significantly at Haw River below Jordan dam (site S12) during 

the early period (1989–2000), but increased significantly at 

White Oak Creek (site S11) during the late period (2002 to 

2013). Both of these trends, although statistically significant, 

accounted for relatively small changes in concentration. 

Oelsner and others (2017) also reported decreasing trends in 

total nitrogen for the Eno River near Durham (site S2) and 

New Hope Creek (site S7) during 1992–2012, in agreement 

with the current study. Although they noted a significant 

increase in total nitrogen for the Eno River at Hillsborough 

(site S1), the current study found a nonsignificant upward 

trend for the same site.

At nine sites, 2-period trend models best fit the total 

nitrogen data, indicating that trends shifted in direction and 

(or) rate of change during the trend period. Total nitrogen 

decreased at Morgan Creek near White Cross (site S9) during 

both the early and late periods, with annual median concentrations decreasing from 1.27 to 0.55 mg/L. Site S9 was the only 

site where all nitrogen fractions decreased significantly during 

the trend period. At five sites (S5, S7, S13, L2, and L3), total 

nitrogen trended downward during 1989–2001, then upward 

from 2002 to 2013. At three sites (S10, L6, and L7), total 

nitrogen trended upward during 1989–2001, then decreased 

from 2002 to 2013. Most of these trends were small in terms 

of actual concentration changes. However, the 2-period 

trends at New Hope Creek (site S7) and Morgan Creek near 

Farrington (site S10) were more substantial and were largely 

attributable to underlying trends in nitrate, similar to the 

pattern observed at site S8. 

At many sites where 2-period trend models were selected 

for total nitrogen, annual median concentrations were similar 

at the beginning (circa 1989) and end (2013) of the trend 

period. A simplistic 1-period trend analysis might have led 

to the misleading conclusion that there was no change over 

time. Trends tended to change in direction and magnitude 

over time; therefore, one should not assume that uniform or 

monotonic trends will adequately describe water quality. In 

fact, such oscillations are evidence of the highly dynamic 

nature of nitrogen cycling. Furthermore, analysis of trends in 

total nitrogen alone would not have reflected the complex and 

underlying trends among the various nitrogen fractions.
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 51

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

GRANVILLE

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

CHATHAM

LEE HARNETT

ORANGE

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S

S

2

3

S4

S5

L2

L1

Percent change in total nitrogen

concentration

80604020

–20–40–60–80

0876543210

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median total nitrogen

concentration, in milligrams per liter

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig17

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberSignificant downward step trend and site number

S11

S3

L8

S12

S8

Figure 17. Trends in total nitrogen concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
52 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Like nitrogen, phosphorus occurs in water in several 

forms. Total phosphorus encompasses both inorganic and 

organic, and dissolved and particulate fractions. Sources of 

phosphorus in watersheds include decomposition of organic 

matter, human and animal waste, fertilizer, and soil runoff. 

In lakes, internal loading from sediment is another source 

of phosphorus. Excessive levels of phosphorus contribute 

to accelerated eutrophication, harmful algal blooms, and 

oxygen depletion. Water-quality standards for phosphorus 

currently have not been established in North Carolina but are 

under development. 

At Triangle area sites, concentrations of total phosphorus 

were generally lower in lakes than in their upstream tributaries. Median concentrations among stream sites ranged from 

0.030 to 0.280 mg/L as P, and a maximum concentration 

of 4.10 mg/L was measured at New Hope Creek (site S7; 

table 6). Among lake sites, median concentrations ranged 

from 0.029 to 0.080 mg/L and were highest at Jordan Lake 

near Hanks Chapel (site L4) and Jordan Lake at Buoy 12 

(site L6). The maximum lake concentration of 0.300 mg/L was 

measured at Jordan Lake at Buoy 12 (site L6; table 7). Stream 

and lake sites in the Neuse River Basin generally had lower 

concentrations of total phosphorus than most sites in the Cape 

Fear River Basin. 

One-period trends in total phosphorus varied among sites 

in the study area (table 8; fig. 18). A statistically significant, 

upward trend in total phosphorus concentrations was noted 

at Jordan Lake near Hanks Chapel (site L4) and Jordan Lake 

at Bells Landing (site L8) during 1991–2013; upward but 

nonsignificant trends were noted at sites S3 and L5 during 

1989–2013 and at site L6 during 1993–2013. The upward 

trend at site L4 represented a 48.6 percent increase, with 

annual median concentrations increasing from 0.064 to 

0.094 mg/L. It is of note that total phosphorus also increased 

by a substantial 57.8 percent at Jordan Lake at Farrington 

(site L6) during 1993–2013, from an annual median of 

0.055 to 0.086 mg/L; however, this trend was not statistically 

significant. Statistically significant downward trends during 

1989–2013 were observed at Eno River near Durham, Flat 

River at Bahama, and Northeast Creek (sites S2, S4, and S8). 

Nonsignificant downward trends were noted at sites S10, L1, 

and L3 during 1989–2013, and at site S1 during 1990–2013. 

Oelsner and others (2017) previously reported downward 

trends in total phosphorus for sites S2 and S7 during 

1992–2012, in agreement with the current study. 

Trends for shorter periods varied considerably from 

site to site. Total phosphorus concentrations decreased 

significantly at Haw River below Jordan dam (site S12) 

during 1989–2000. At Haw River near Bynum (site S6) and 

Morgan Creek near White Cross (site S9), concentrations 

trended downward during both early and late periods but 

at different rates of change (table 8). At New Hope Creek 

(site S7), total phosphorus concentrations decreased from 

0.397 mg/L in 1989 to 0.225 mg/L in 2001 (44.8 percent), 

which was the largest magnitude change observed during this 

study, followed by no significant trend during 2002–13. Total 

phosphorus concentrations increased significantly at White 

Oak Creek (site S11) during 2002–13, with annual medians 

rising from 0.045 to 0.068 mg/L. Significant uptrends for four 

more sites were noted during either the early or late period 

(site S5, 2002–13; site S13, 1989–2001; site L2, 2002–13; 

site L7, 1991–2001) and were offset by similar downtrends 

during the corresponding period; consequently, annual median 

concentrations for those sites in 2013 were similar to values at 

the beginning of the study period. Oelsner and others (2017) 

previously reported significant downtrends in total phosphorus 

during 1992–2012 at sites S2 and S7, which generally agreed 

with results from the current study; however, they reported 

significant uptrends in total phosphorus at sites S1 and S4 

during 2002–12, which were not supported by results from 

the current study. The current study included data from 2013, 

which may help account for the different trend results.

Some of the statistically significant downtrends at stream 

sites involved very small changes in total phosphorus concentration; however, annual median concentrations decreased 

more than 0.100 mg/L at four sites, including Haw River near 

Bynum (S6), New Hope Creek (S7), Northeast Creek (S8), 

and Morgan Creek near White Cross (S9). Three of these sites 

(S6, S7, and S8) have substantial inputs of treated municipal 

wastewater. Childress and Bathala (1997) previously analyzed 

water-quality trends in the Triangle area during 1983–95. They 

concluded that total phosphorus concentrations in all study 

streams showed a step-trend decrease after 1988, coinciding 

with the adoption of a statewide phosphate-detergent ban 

and increased phosphorus-removal practices at municipal 

wastewater-treatment facilities. The current study showed 

that total phosphorus continued to decline at sites S6, S7, 

and S8 through 2013 despite additional development and 

population growth in the watersheds; however, annual median 

concentrations in 2013 remained higher than at other sites. The 

significant decreases in total phosphorus and all nitrogen fractions at Morgan Creek near White Cross (site S9) may have 

resulted from improved land-management practices, including 

the closure of a dairy operation, restoration of riparian buffers, 

and livestock exclusion fencing, in this relatively undeveloped 

watershed (Ruth Rouse, Orange Water and Sewer Authority, 

written commun., February 27, 2018). 

The upward trend in total phosphorus at Jordan Lake 

near Hanks Chapel (site L4) might appear to contradict the 

decreasing trend observed upstream at site S6, although it is 

interesting that the 2013 annual median concentrations at the 

two sites were in close agreement. Spruill and others (2006) 

previously noted that total phosphorus concentrations at Haw 

River near Bynum (site S6) decreased significantly during the 

period 1981–2004 and attributed this trend to loss of cropland, 

improved land-management practices, adoption of stringent 

wastewater-treatment practices in the watershed, and the 

statewide phosphate-detergent ban. 

Total organic carbon provides an estimate of the amount 

of organic matter in water and, thus, is an important indicator 

of water quality for drinking-water suppliers. Elevated levels 

of total organic carbon in source water have been associated 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 53

DURHAM

CHATHAM

LEE HARNETT

ORANGE

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S

S

2

3

S4

S5

L2

L1

Percent change in total phosphorus

concentration

80604020

–20–40–60–80

0

0.50.40.30.20.1

0

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median total phosphorus

concentration, in milligrams per liter

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig18

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site number

S11

L5

S9

L3

Figure 18. Trends in total phosphorus concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
54 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

with increased formation of disinfection by-products during 

water treatment and with bacterial regrowth in waterdistribution systems. Organic carbon in water originates from 

natural organic matter and synthetic sources. 

Total organic carbon data were available for five stream 

sites and eight lake sites in the study area. Concentrations 

ranged from 0.8 to 58.0 mg/L at the stream sites (table 6) 

and from 2.1 to 17.0 mg/L at the lake sites (table 7). White 

Oak Creek (site S11; 11.1 mg/L) and Jordan Lake at Buoy 12 

(site L6; 10.6 mg/L) had the highest median concentrations 

among stream and lake sites, respectively. 

Trends for total organic carbon were analyzed for the 

13 sites for which data were available (table 8; fig. 19). All 

but two sites had sufficient data to evaluate both the 1- and 

2-period trend models. Trends could be evaluated only for 

the late period (2002–13) at White Oak Creek (site S11) and 

only for the early period (1989–2001) at Cape Fear River 

near Brickhaven (site S13). One-trend models indicated that 

total organic carbon increased significantly at three sites from 

1989 to 2013, including Eno River at Hillsborough (S1), Lake 

Michie (L2), and University Lake (L5) and nonsignificantly 

at four additional sites (S5, L1, L3, and L7). The maximum 

percent increase was observed at Eno River at Hillsborough 

(site S1; 19.8 percent), and the maximum concentration 

increase was observed at University Lake (site L5; 1.3 mg/L). 

The uptrend at site S1 was corroborated in a national assessment of stream water-quality trends for the period 1992–2012 

(Oelsner and others, 2017).

A nonsignificant downtrend was observed at White Oak 

Creek (site S11) during the late period, and a nonsignificant 

uptrend was noted at the Cape Fear River (site S13) during 

the early period. Trends at the four remaining sites were 

best described by 2-period trend models that included both 

uptrends and downtrends. The downward trend at Morgan 

Creek near White Cross (site S9) during 2002–13 represented 

the greatest percent decrease (21.1 percent). Annual median 

concentrations decreased 1.1 mg/L from 2002 to 2013 at both 

site S9 and Jordan Lake near Hanks Chapel (site L4). 

Total Suspended Solids and Suspended 

Sediment in Streams

Total suspended solids and suspended sediment were 

measured in study area streams. Both constituents provide 

information on the amount of solid-phase material suspended 

in the water column, but the results are not interchangeable 

because of differences in sampling technique and laboratory 

analytical methods (Gray and others, 2000). Total suspended 

solids data generally demonstrate a negative or low bias but 

may still be useful for trend comparisons. 

Total suspended solids were measured by the NCDEQ, 

UCFRBA, and MCFRBA at nine sites, but trends were not 

analyzed at two of these sites (S3 and S6) because more than 

20 percent of the values were censored (tables 6, 9). Sufficient 

data were available to evaluate both the 1- and 2-period trend 

models at the remaining seven sites (table 9; fig. 20). Total 

suspended solids decreased significantly from 1989 to 2013 

at three sites, including Flat River at Bahama (S4), New 

Hope Creek (S7), and Northeast Creek (S8). A nonsignificant 

downtrend was observed at the Eno River near Durham 

(site S2). The most pronounced downtrend (42.3 percent, with 

annual median concentrations declining from 23 to 10 mg/L) 

was associated with a step trend at Northeast Creek (site S8) 

that began July 2005. It is not known whether the decrease 

in total suspended solids was directly related to upgrades at 

the upstream wastewater-treatment facility or if other factors 

were involved. Downtrends occurred at sites S7 and S8 

despite substantial increases in population density, loss of 

forested land, and gains in developed area in the watersheds 

(tables 2 and 3). 

Two-period trend tests revealed shifts in trend direction 

and (or) magnitude at the three remaining sites (S10, S12, 

and S13). The overall magnitude of change at these sites was 

relatively small—annual median concentrations at the beginning and end of the trend period changed by only 3 or 4 mg/L. 

Suspended-sediment samples were collected and 

analyzed by the USGS at most stream sites in the study area; 

however, only six sites had sufficient data for analyzing 

trends (table 9; fig. 21). Suspended-sediment concentrations 

decreased significantly during the entire trend period at Eno 

River at Hillsborough (site S1; 44.7 percent) and Flat River 

(site S4; 56.7 percent), equivalent to reductions in annual 

median concentrations of 5 mg/L and 9 mg/L, respectively. 

Reasons for the downtrends at these sites are uncertain but 

could relate to improved land-management practices. In 

addition, suspended-sediment concentrations at site S1 may 

have been influenced by the construction of a new reservoir 

upstream (West Fork Eno Reservoir) in 2000. Although there 

was little overall change in percent agricultural land cover in 

the Flat River watershed during the trend period (table 3), this 

watershed has experienced a loss of cropland and an increase 

in lands used for pasture and hay, which typically have lower 

rates of soil loss. Suspended-sediment concentrations also 

decreased significantly (45.4 percent; 10 mg/L) at Cape 

Fear River near Brickhaven (site S13) during 1989–2001. A 

downward trend noted at Morgan Creek near White Cross 

(site S9) during 2002–13 was statistically significant, but 

was associated with a negligible change in concentration 

(2 mg/L). The only stream site with a significant, increasing 

trend was White Oak Creek (S11) for the late period 2002–13 

(72.6 percent; 8 mg/L). The watershed for this site had the 

highest increases in population density (table 2) and amount of 

developed land (table 3) of all the watersheds in the study.

Comparison of trends for total suspended solids and 

suspended sediment was possible for two stream sites. At Flat 

River (site S4), trends for both constituents were computed for 

the 1989–2011 period. Significant downtrends were observed 

for both total suspended solids and suspended sediment. 

At Cape Fear River near Brickhaven (site S13), significant 

downward trends were observed for total suspended solids 

trends during 1992–2001 and for suspended sediment during 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 55 CHATHAM LEE HARNETT

ORANGE

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

Percent change in total organic carbon

concentration

302010

–10–20–30

0

1210

86420

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median total organic carbon

concentration, in milligrams per liter

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig19

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

S1

S5S6

S11

L4

Figure 19. Trends in total organic carbon concentration at stream and lake sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
56 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 9. Summary of trend results based on data collected during 1989–2013 for total suspended solids and suspended sediment at 

stream sites assessed in the Triangle area of North Carolina. 

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward 

trend results. Gray shading indicates no trend results are presented because more than 20 percent of the data were censored. Abbreviations: USGS, U.S. Geological Survey; NC, North Carolina; mg/L, milligram per liter; <, less than]

Map no. 

(fig. 1) USGS station name Best-fit trend model

Trend 

period

Percent 

change p-value

Fitted annual 

median 

concentration 

for first year 

in period

Fitted annual 

median 

concentration 

for last year 

in period

Suspended solids (mg/L)

S2 Eno River near Durham, NC 1-period 1989–2013 –10.0 0.3055 12 7

S3 Little River at SR 1461 near Orange Factory, NCnone 1989–2013 highly censored

S4 Flat River at Bahama, NC 1-period 1989–2011 –29.2 0.0045 9 7

S6 Haw River near Bynum, NC none 1989–2013 highly censored

S7 New Hope Creek near Blands, NC 1-period 1989–2013 –37.2 <0.0001 25 16

S8 Northeast Creek at SR 1100 near Genlee, 

NC

Step, July 2005 1992–2012 –42.3 <0.0001 23 10

S10 Morgan Creek near Farrington, NC 2-period 1989–2001 –7.4 0.3107 12 11

2002–2013 38.0 0.0020 11 15

S12 Haw River below B. Everett Jordan Dam 

near Moncure, NC

2-period 1989–2001 –8.8 0.2899 11 10

2002–2013 –30.5 0.0119 10 7

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

2-period 1992–2001 –30.5 0.0147 15 11

2002–2012 4.8 0.3625 11 11

Suspended sediment (mg/L)

S1 Eno River at Hillsborough, NC 1-period 1990–2013 –44.7 0.0003 12 7

S4 Flat River at Bahama, NC 1-period 1989–2011 –56.7 <0.0001 16 7

S5 Cane Creek near Orange Grove, NC 1-period 1989–2013 3.9 0.8875 7 8

S9 Morgan Creek near White Cross, NC 2-period 1989–2001 27.1 0.1582 6 7

2002–2013 –37.6 0.0270 7 5

S11 White Oak Creek at mouth near Green 

Level, NC

Late period 2002–2013 72.6 0.0002 11 19

S13 Cape Fear River at State Highway 42 

near Brickhaven, NC

Early period 1989–2001 –45.4 0.0005 22 12
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 57 CHATHAM LEE HARNETT

ORANGE

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S

S

2

3

S4

S5

L2

L1

Percent change in total suspended solids

concentration

604020

–20–40–60

0

3025201510

50

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median total suspended solids

concentration, in milligrams per liter

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig20

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberSignificant downward step trend, and site numberGreater than 20 percent censored and site numberInsufficient data and site number

S10

S13

S4S2

S8S6

L4

Figure 20. Trends in suspended solids concentration at stream sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
58 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 CHATHAM LEE HARNETT ORANGE DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S12

L6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

Percent change in suspended sediment

concentration

80604020

–20–40–60–80

0

25201510

50

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median suspended sediment

concentration, in milligrams per liter

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig21

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberSignificant downward trend and site numberInsufficient data and site number

S11

S5S4S6

Figure 21. Trends in suspended-sediment concentration at stream sites assessed in the Triangle area of North Carolina, 1989–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.] 
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 59

the early period 1989–2001. Therefore, trend directions 

for these two constituents were in agreement at both sites, 

although the trend magnitudes were higher for suspended 

sediment (table 9).

Secchi Transparency and Chlorophyll a in Lakes

Secchi transparency and chlorophyll a were measured 

only at lake sites. All data included in the trend analysis were 

obtained from samples collected by the USGS. The USGS 

began measuring Secchi transparency in the study area in 

1993; therefore, the trend-analysis period for this water-quality 

property is 1993 to 2013. Secchi transparency is a measure 

of water clarity and can be used to estimate the depth of the 

water column where sufficient light is available to support 

the growth of phytoplankton and (or) rooted aquatic plants. 

Secchi-depth measurements provide a useful indicator of lake 

productivity. Highly productive, or eutrophic, lakes tend to 

have shallower Secchi depths. In this study, median Secchi 

depths measured at lake sites ranged from 0.50 m at Jordan 

Lake at Buoy 12 (site L6) to 1.20 m at Cane Creek Reservoir 

(site L3; table 7). 

Water clarity did not change significantly at University 

Lake (site L5) or Jordan Lake near Hanks Chapel (site L4) 

during the study period. Water clarity increased significantly at 

six lake sites during 1993–2013 (table 10; fig. 22), including 

Little River Reservoir (L1), Lake Michie (L2), Cane Creek 

Reservoir (L3), and three sites in the New Hope Creek arm of 

Jordan Lake (L6, L7, L8). Significant increases ranged from 

24.8 to 65.6 percent, which corresponded to magnitudes of 

change ranging from 0.12 to 0.46 m. The greatest increases in 

clarity were seen at Cane Creek Reservoir (site L3; 0.46 m) 

and Little River Reservoir (site L1; 0.41 m). Both of these 

reservoirs were constructed in 1988. Newly impounded reservoirs typically undergo a sequence of water-quality changes 

characterized by an initial surge of eutrophication followed 

by recovery and stabilization (Thornton and others, 1996). 

The initial surge of productivity is fueled by decaying organic 

matter from flooded vegetation and soils. The duration of this 

period depends on the amount of organic matter that was left 

in the flooded area, but typically lasts several years. It is likely 

that water clarity improved as these two reservoirs stabilized. 

Chlorophyll a, the primary plant pigment active in 

photosynthesis, is useful as an indicator of algal biomass 

and, thus, lake productivity. Step-trend analysis compared 

concentrations for the periods before and after the change in 

chlorophyll a laboratory methods that occurred on October 1, 

2005. As anticipated, this change resulted in apparent 

increases in chlorophyll a concentrations at all lake sites, 

ranging from 37.1 to 426 percent (table 10). The 1-period 

trend results (table 10; fig. 23) indicated that chlorophyll a

decreased significantly from 1992 to 2013 at three lake sites, 

Lake Michie (L2), University Lake (L5), and Jordan Lake at 

Buoy 12 (L6). Nonsignificant downward trends were noted 

at Little River Reservoir (site L1), Cane Creek Reservoir 

(site L3), and Jordan Lake near Hanks Chapel (site L4). 

Chlorophyll a concentrations increased at Jordan Lake at 

Wilsonville (site L7) and Jordan Lake at Bells Landing (site L8), but these upward trends were not 

statistically significant. 

It should be noted that chlorophyll a concentrations 

vary seasonally, and annual median concentrations are not 

especially meaningful from an environmental standpoint. The 

computed annual median concentrations for 2013 incorporate 

effects of both the step and 1-period trends and are presented 

solely for the purpose of summarizing model-fitted concentrations at the beginning and end of the trend-analysis period. 

Given the highly variable nature of chlorophyll a concentrations in lakes, trend results computed for these data should 

be interpreted broadly and with caution. It would be expected 

that decreasing trends in chlorophyll a would be associated 

with increases in Secchi depth, and this inverse relation was 

observed at all lake sites except two Jordan Lake sites—L7 

and L8 (figs. 22 and 23).
60 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

Table 10. Summary of trend results based on data collected during 1992–2013 for Secchi transparency and chlorophyll a at lake sites 

assessed in the Triangle area of North Carolina. 

[Bold red font indicates statistically significant (at p-value less than 0.05) upward trend results, and bold blue font indicates statistically significant downward 

trend results. Abbreviations: USGS, U.S. Geological Survey; NC, North Carolina; m, meter; <, less than; μg/L, microgram per liter]

Map no. 

(fig. 1) USGS station name Best-fit trend model

Trend 

period

Percent 

change p-value

Fitted annual 

median 

concentration 

for first year 

in period

Fitted annual 

median 

concentration 

for last year 

in period

Transparency, Secchi (m)

L1 Little River Reservoir at Dam near Bahama, NC1-period 1993–2013 54.5 <0.0001 0.89 1.30

L2 Lake Michie at Dam near Bahama, NC 1-period 1993–2013 24.8 0.0393 0.55 0.67

L3 Cane Creek Reservoir at Dam near White 

Cross, NC

1-period 1993–2013 65.6 0.0005 0.82 1.28

L4 Jordan Lake, Haw River arm near Hanks 

Chapel, NC

2-period 1993–2001 –1.1 0.3965 0.57 0.56

2002–2013 18.1 0.0540 0.57 0.67

L5 University Lake at Intakes near Chapel 

Hill, NC

1-period 1993–2013 7.0 0.6242 0.53 0.56

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 40.0 0.0033 0.44 0.61

L7 Jordan Lake above U.S. Highway 64 at 

Wilsonville, NC

1-period 1993–2013 39.6 <0.0001 0.63 0.86

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

1-period 1993–2013 35.8 0.0011 0.67 0.89

Chlorophyll a, phytoplankton (μg/L) 

[Linear trend from 1992 to 2013 and step trend associated with a laboratory method change on October 1, 2005]

L1 Little River Reservoir at Dam near 

Bahama, NC

1-period 1992–2013 –43.1 0.1626 3.6 6.4

 Step, 

Oct. 2005

206.2 0.0008

L2 Lake Michie at Dam near Bahama, NC 1-period 1992–2013 –77.5 0.0003 6.5 8.0

 Step, 

Oct. 2005

426.0 <0.0001

L3 Cane Creek Reservoir at Dam near White 

Cross, NC

1-period 1992–2013 –58.8 0.0818 3.0 4.7

 Step, 

Oct. 2005

269.8 0.0005

L4 Jordan Lake, Haw River arm near Hanks 

Chapel, NC

1-period 1992–2013 –30.7 0.2584 7.9 14.6

 Step, 

Oct. 2005

166.1 0.0012

L5 University Lake at Intakes near Chapel 

Hill, NC

1-period 1992–2013 –73.9 0.0002 14.2 10.2

 Step, 

Oct. 2005

167.9 0.0002

L6 Jordan Lake at Buoy 12 at Farrington, NC 1-period 1993–2013 –39.5 0.0208 18.4 38.3

 Step, 

Oct. 2005

239.6 <0.0001

L7 Jordan Lake above U.S. Highway 64 at 

Wilsonville, NC

1-period 1992–2013 24.2 0.2904 9.6 16.3

 Step, 

Oct. 2005

37.1 0.1238

L8 Jordan Lake at Bells Landing near 

Griffins Crossroads, NC

1-period 1992–2013 6.9 0.3874 11.2 17.8

 Step, 

Oct. 2005

49.6 0.0551
Water-Quality Trends Related to Watershed Setting and Hydrologic Conditions 61 CHATHAM LEE HARNETT ORANGE

DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

Percent change in Secchi transparency

80604020

–20–40–60–80

0

1.41.21.00.80.60.40.2

0

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

Annual median Secchi transparency,

in meters

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig22

EXPLANATION

Significant upward trend and site numberNonsignificant upward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

L1

L4L4

S6

Figure 22. Trends in Secchi transparency at lake sites assessed in the Triangle area of North Carolina, 1993–2013. [Vertical lines in the trend plots indicate the starting and ending points of the trend. Two adjacent symbols at a site represent 2-period trends.]
62 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 CHATHAM LEE HARNETT ORANGE DURHAM

Site number

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 L1 L2 L3 L4 L5 L6 L7 L8

Pittsboro

Bynum

78°45’ 79°

Apex

Cary Raleigh

Durham

Hillsborough

Butner

Chapel

Hill

Carrboro

Brickhaven

0 5 KILOMETERS

0 5 MILES

S8

36°

35°45’

WAKE

Falls

Lake

LakeMichie

Little River

Reservoir

Flat

River

University

Lake

Cane CreekReservoir

Cape Fear

River Basin

Neuse River

Basin

Cape Fear

River

Little River

Eno River

Jordan

Lake

Deep Rive

H

r

a w River

GRANVILLE

S7

S9

S11

S10

S1

L

2

6

L5

L4

L3

L7 L8

S13

S1 S2

S3

S4

S5

L2

L1

a Percent change in phytoplankton chlorophyll 80604020 –20 –40 –60 –80

0

Neuse Cape Fear Neuse Cape Fear

STREAMS LAKES

S6

Base from digital files of U.S. Department of Commerce,Bureau of Census, 2016, TIGER/Line Files-Political boundariesU.S. Geological Survey, 1:100,000-scale hydrographyNorth Carolina Department of TransportationMunicipal Boundaries 

res18-0031_fig23

EXPLANATION

Nonsignificant upward trend and site numberSignificant downward trend and site numberNonsignificant downward trend and site numberInsufficient data and site number

L7L2L1

S6

Figure 23. Trends in phytoplankton chlorophyll a concentration at lake sites assessed in the Triangle area of North Carolina, 1992–2013. [Depicts only environmental trends. Step-trend effects resulting from a laboratory method change in October 2005 are not shown. See text for a detailed explanation. Vertical lines in the trend plots indicate the starting and ending points of the trend.]
Summary and Conclusions 63

Summary and Conclusions

Lakes and their tributary streams are critical sources of 

public drinking water for residents of the Triangle area of 

North Carolina. This report provides an assessment of trends 

in streamflow and water-quality constituent concentrations 

at selected sites in the study area from 1989 through 2013. A 

statewide phosphate-detergent ban was adopted in 1988, and, 

since then, numerous additional nutrient-reduction and watersupply watershed protection measures have been put in place 

in the study area to address point-source and nonpoint sources 

of nutrients and other pollutants. In addition, population, urban 

development, and water-supply demands have continued to 

expand in the study area. Results from the current trend study 

are useful for understanding progress toward water-quality 

goals in the region and for understanding how water quality 

is responding to increased population, land-cover change, and 

hydrologic alteration. 

Population density and percentage of developed lands 

increased in all study-site watersheds during the trend period, 

but intensity of growth varied considerably among watersheds. 

Overall, gains in developed lands were accounted for by 

losses in forested lands, with little change in percentages of 

agricultural land. Streamflow trended downward at several 

streamgaging sites, especially those with upstream withdrawals or small watersheds. Ongoing, continuous monitoring of 

streamflow is critical for documenting whether these changes 

are short-term fluctuations or long-term trends. One should 

note that flow at various Triangle area streams is influenced by 

upstream impoundments, inflows of treated municipal wastewater and urban runoff, and (or) water-supply withdrawals. 

Across the study area, upward trends in specific conductance, calcium, magnesium, sodium, and chloride likely were 

related to regional patterns of population growth, increases in 

developed land, and decreases in forested lands. These upward 

trends are consistent with those observed in other areas in 

the United States that are undergoing urbanization. Sites that 

began the trend period with the highest specific conductance 

values also experienced the greatest increases, specifically, 

three tributaries to the upper New Hope arm of Jordan Lake. 

For several sites with strong trends in specific conductance, 

data were insufficient to assess trends in individual ions. 

Because elevated concentrations of different ions have varying 

implications for water treatment, agricultural use, and aquatic 

health, it could be beneficial to consistently monitor ion 

concentrations at additional locations in the study area.

Concentrations of nitrate and ammonia decreased in 

most stream sites; trends could not be determined at most lake 

sites. At Northeast Creek, nitrate plus nitrite concentrations 

decreased 75.5 percent and ammonia concentrations decreased 

57.3 percent after an upstream wastewater-reclamation 

facility implemented nitrogen-removal practices. In contrast, 

concentrations of total organic nitrogen increased at most 

stream and lake sites throughout the study area, suggesting 

that broad, rather than site-specific, factors were responsible 

for the increase. Potential interactions between atmospheric 

and aquatic nitrogen cycling in the Triangle area are not well 

understood but warrant further research. Because trends for 

inorganic and organic nitrogen fractions often differed, trends 

in total nitrogen were not as informative as those computed for 

the individual nitrogen fractions. At nine sites, total-nitrogen 

trend directions and (or) magnitude changed significantly during the 25-year trend period. Monotonic trend tests would not 

have characterized these changes adequately. Total phosphorus 

concentrations decreased at most stream sites, suggesting that 

stream water quality in the Triangle area continues to benefit 

from the phosphate-detergent ban and wastewater-treatment 

upgrades that were implemented in the late 1980s and early 

1990s. However, total phosphorus increased significantly 

at one formerly rural stream that underwent considerable 

increases in population density and land development. Trends 

in total phosphorus in Triangle lakes were mixed and generally 

not statistically significant; however, upward trends in Jordan 

Lake warrant continued monitoring. 

Total suspended solids and suspended sediment decreased 

significantly at most stream sites in the study area. A comparison of trends for these two constituents was only possible 

for two stream sites. At both sites, trend directions for total 

suspended solids and suspended sediment were in agreement; 

however, trend magnitude was higher for suspended sediment. 

Secchi transparency increased significantly at six lake 

sites, indicating that water clarity improved during 1993–2013. 

After step increases related to a laboratory method change 

were accounted for by the trend time-series analysis, significant downward trends in chlorophyll a concentrations were 

noted for three lake sites, nonsignificant downward trends 

were noted for three additional lake sites, and nonsignificant 

upward trends were noted for the remaining two lake sites. 

In addition to the regional trends described in this study, 

trends observed at individual sites also reflected the influence 

of local factors. For example, nutrient reductions were 

observed at three stream sites downstream from wastewaterreclamation facilities that implemented nutrient-removal 

processes. At another stream site, conservation efforts 

likely reduced nutrient inputs from nonpoint sources in the 

watershed. These improvements in water quality occurred 

at these four sites despite concurrent population growth and 

development. In contrast, nutrients and suspended sediment 

increased at a stream site in a formerly rural watershed that 

experienced tremendous development during the trend period. 

These indicators of deteriorating water quality at the site were 

likely related to hydrologic alteration and increased inputs 

from nonpoint sources.

Several important lessons were learned in the course 

of this study. Changes in sampling and laboratory methods 

may directly affect water-quality data and, thus, trend results. 

Method changes that are well documented by data-collection 

agencies and appropriately incorporated into trend analysis 

and other data-interpretation activities can benefit the resulting 

analyses. Data collected by different agencies can be highly 

complementary; for example, U.S. Geological Survey highflow samples and samples from rural streams supplement 
64 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

fixed-interval sampling conducted by the North Carolina 

Department of Environmental Quality, Upper Cape Fear River 

Basin Association, and Middle Cape Fear River Basin Association. Data from different agencies can be “harmonized” 

for use in a combined dataset provided assumptions about the 

data are verified. For long-term trend analysis, approaches 

that accommodate disparities in sampling frequency and data 

gaps are preferred, because sampling programs typically shift 

over time. Lastly, streamflow data are essential for interpreting 

water-quality data and typically benefit a wide audience of 

users. Streamgages, coupled with historical water-quality data, 

are “magnets” for supplemental monitoring and data-analysis 

efforts. Since the project began in late 1988, Triangle Area 

Water Supply Monitoring Project stream sites have been 

included in several regional and national studies, leveraging 

investments made by local project partners. It is important 

to continue the programs that have been put in place in 

the Triangle area to protect water supplies and to continue 

tracking water quality over time.

Results of this study suggest that the simple question, “Is 

water quality getting better or worse?” is overly simplistic, 

especially in areas that are rapidly changing like the Triangle 

area of North Carolina. Water quality is influenced by 

multiple, often confounding factors. Within similar timeframes, watersheds may experience increases in population, 

developed lands and impervious surfaces, groundwater and 

surface-water withdrawals, wastewater inputs, septic tank 

installations, and urban stormwater runoff. At the same time, 

open spaces and riparian buffers, urban and agricultural best 

management practices, wastewater-treatment improvements, 

and water-conservation measures may be implemented; and 

agricultural land uses may shift—for example, from row crops 

to pasture. Therefore, as long-term data become available for 

analysis, it has become apparent that water-quality changes 

are not uniform over time. Long-term monitoring and flexible 

analytical approaches are critical for determining how ongoing 

growth and development may affect water quality and for 

ensuring the resiliency of water supplies for the future.

References Cited

Bain, G.L., and Brown, C.E., 1981, Evaluation of the Durham 

Triassic basin of North Carolina and technique used to 

characterize its waste-storage potential: U.S. Geological 

Survey Open-File Report 80–1295, 132 p., accessed 

February 15, 2018, at https://pubs.usgs.gov/of/1980/1295/

report.pdf.

Cain, J.L., Harden, S.L., and Giorgino, M.J., 2018, Datasets 

for trends in water quality of selected streams and reservoirs 

used for water supply in the Triangle area of North Carolina, 

1989–2013: U.S. Geological Survey data release, accessed 

May 11, 2018, at https://doi.org/10.5066/F7MS3S17. 

Childress, C.J.O., and Bathala, N., 1997, Water-quality trends 

for streams and reservoirs in the Research Triangle area of 

North Carolina, 1983–95: U.S. Geological Survey WaterResources Investigations Report 97–4061, 18 p.

Cohn, T.A., and Lins, H.F., 2005, Nature’s style—Naturally 

trendy: Geophysical Research Letters, v. 32, L23402, 

accessed June 27, 2017, at https://doi.org/10.1029/

2005GL024476. 

Cornell, S.E., 2011, Atmospheric nitrogen deposition—

Revisiting the question of the importance of the organic 

component: Environmental Pollution, v. 159, no. 10, 

p. 2214–2222. [Also available at https://doi.org/10.1016/

j.envpol.2010.11.014.] 

Daniel, C.C., III, and Dahlen, P.R., 2002, Preliminary 

hydrogeologic assessment and study plan for a regional 

ground-water resource investigation of the Blue Ridge and 

Piedmont Provinces of North Carolina: U.S. Geological 

Survey Water-Resources Investigations Report 2002–4105, 

60 p., accessed April 11, 2018, at https://pubs.er.usgs.gov/

publication/wri024105.

Dugan, H.A., Bartlett, S.L., Burke, S.M., Doubek, J.P., 

Krivak-Tetley, F.E., Skaff, N.K., Summers, J.C., Farrell, 

K.J., McCullough, I.M., Morales-Williams, A.M., Roberts, 

D.C., Ouyang, Z., Scordo, F., Hanson, P.C., and Weathers, 

K.C., 2017, Salting our freshwater lakes: Proceedings of the 

National Academy of Sciences, v. 114, no. 7, p. 4453–4458, 

accessed March 1, 2018, at https://doi.org/10.1073/

pnas.1620211114.

Forstall, R.L., 1996, Population of states and counties of the 

United States—1790 to 1990: U.S. Bureau of the Census, 

225 p. [Also available at https://www.census.gov/

population/www/censusdata/pop1790-1990.html.]

Galloway, J.N., and Cowling, E.B., 2002, Reactive nitrogen 

and the world—200 years of change: AMBIO—A Journal 

of the Human Environment, v. 31, no. 2, p. 64–71.

Gray, J.R., Glysson, G.D., Turcios, L.M., and Schwarz, G.E., 

2000, Comparability of suspended-sediment concentration 

and total suspended solids data: U.S. Geological Survey 

Water-Resources Investigations Report 00–4191, 14 p.

Harden, S.L., Cuffney, T.F., Terziotti, S., and Kolb, K.R., 

2013, Relation of watershed setting and stream nutrient 

yields at selected sites in central and eastern North Carolina, 

1997–2008: U.S. Geological Survey Scientific Investigations Report 2013–5007, 47 p., accessed February 23, 2018, 

at https://pubs.usgs.gov/sir/2013/5007/.

Helsel, D.R., 2005, Nondetects and data analysis—Statistics 

for censored environmental data: Hoboken, N.J., Wiley 

Interscience, John Wiley & Sons, Inc., 250 p.
References Cited 65

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, 510 p.

Hirsch, R.M., 2011, A perspective on nonstationarity and 

water management: Journal of the American Water 

Resources Association, v. 47, no. 3, p. 436–446.

Hirsch, R.M., and De Cicco, L.A., 2015, User guide to 

exploration and graphics for RivEr Trends (EGRET) and 

dataRetrieval—R packages for hydrologic data (version 2.0, 

February 2015): U.S. Geological Survey Techniques and 

Methods, book 4, chap. A10, 93 p., accessed February 16, 

2018, at https://doi.org/10.3133/tm4A10. 

Hirsch, R.M., Moyer, D.L., and Archfield, S.A., 2010, 

Weighted regressions on time, discharge, and season 

(WRTDS), with an application to Chesapeake Bay River 

inputs: Journal of the American Water Resources Association, v. 46, no. 5, p. 857–880, accessed April 19, 2018, at 

https://doi.org/10.1111/j.1752-1688.2010.00482.x. 

Hirsch, R.M., and Slack, J.R., 1984, A nonparametric trend 

test for seasonal data with serial dependence: Water 

Resources Research, v. 20, no. 6, p. 727–732.

Holland, E.A., Braswell, B.H., Sulzman, J., and Lamarque, 

J.-F., 2005, Nitrogen deposition onto the United States and 

Western Europe—Synthesis of observations and models: 

Ecological Applications, v. 15, no. 1, p. 38–57.

Homer, C.G., Dewitz, J.A., Yang, L., Jin, S., Danielson, P., 

Xian, G., Coulston, J., Herold, N.D., Wickham, J.D., and 

Megown, K., 2015, Completion of the 2011 National Land 

Cover Database for the conterminous United States—

Representing a decade of land cover change information: 

Photogrammetric Engineering and Remote Sensing, 

v. 81, no. 5, p. 345–354, accessed June 22, 2017, at 

https://www.mrlc.gov/nlcd2011.php.

Huffman, B.A., Pfeifle, C.A., Chapman, M.J., Bolich, R.E., 

Campbell, T.R., Geddes, D.J., Jr., and Pippin, C.G., 2006, 

Compilation of water-resources data and hydrogeologic 

setting for four research stations in the Piedmont and 

Blue Ridge Physiographic Provinces of North Carolina: 

U.S. Geological Survey Open-File Report 2006–1168, 

102 p., accessed April 20, 2018, at https://pubs.usgs.gov/

of/2006/1168/.

Jickells, T., Baker, A.R., Cape, J.N., Cornell, S.E., and Nemitz, 

E., 2013, The cycling of organic nitrogen through the atmosphere: Philosophical Transactions of the Royal Society B, 

Biological Sciences, v. 368, accessed February 25, 2018, at 

https://doi.org/10.1098/rstb.2013.0115.

Kaushal, S.S., Duan, S., Doody, T.R., Haq, S., Smith, R.M., 

Newcomer Johnson, T.A., Delaney Newcomb, K., Gorman, 

J., Bowman, N., Mayer, P.M., Wood, K.L., Belt, K.T., and 

Stack, W.P., 2017, Human-accelerated weathering increases 

salinization, major ions, and alkalinization in fresh water 

across land use: Applied Geochemistry, v. 83, p. 121–135, 

accessed July 10, 2017, at https://doi.org/10.1016/

j.apgeochem.2017.02.006. 

Kendall, M.G., 1938, A new measure of rank correlation: 

Biometrika, v. 30, no. 1/2, p. 81–93.

Li, Y., Schichtel, B.A., Walker, J.T., Schwede, D.B., Chen, 

X., Lehmann, C.M.B., Puchalski, M.A., Gay, D.A., and 

Collett, J.L., Jr., 2016, Increasing importance of deposition 

of reduced nitrogen in the United States: Proceedings 

of the National Academy of Sciences, v. 113, no. 21, 

p. 5874–5879, accessed February 24, 2018, at 

https://doi.org/10.1073/pnas.1525736113.

Lins, H.F., and Cohn, T.A., 2011, Stationarity—Wanted dead 

or alive?: Journal of the American Water Resources Association, v. 47, no. 3, p. 475–480, accessed April 25, 2018, at 

https://doi.org/10.1111/j.1752-1688.2011.00542.x.

Liu, X., Zhang, Y., Han, W., Tang, A., Shen, J., Cui, Z., 

Vitousek, P., Erisman, J.W., Goulding, K., Christie, P., 

Fangmeier, A., and Zhang, F., 2013, Enhanced nitrogen 

deposition over China: Nature, v. 494, p. 459–462, accessed 

April 25, 2018, at https://doi.org/10.1038/nature11917.

McDonald, R.I., Weber, K.F., Padowski, J., Boucher, T., and 

Shemie, D., 2016, Estimating watershed degradation over 

the last century and its impact on water-treatment costs for 

the world’s large cities: Proceedings of the National Academy of Sciences, v. 113, no. 32, p. 9117–9122, accessed 

April 25, 2018, at https://doi.org/10.1073/pnas.1605354113.

Moore, J., Bird, D.L., Dobbis, S.K., and Woodward, G., 2017, 

Nonpoint source contributions drive elevated major ion 

and dissolved inorganic carbon concentrations in urban 

watersheds: Environmental Science and Technology 

Letters, v. 4, no. 6, p. 198–204, accessed July 10, 2017, 

at https://doi.org/10.1021/acs.estlett.7b00096. 

National Oceanic and Atmospheric Administration [NOAA], 

2016, Climate divisions with counties: National Weather 

Service, Climate Prediction Center website, accessed 

June 28, 2016, at http://www.cpc.noaa.gov/products/

analysis_monitoring/regional_monitoring/CLIM_DIVS/

states_counties_climate-divisions.shtml. 

National Oceanic and Atmospheric Administration [NOAA], 

2017, Data tools—1981–2010 normals: National Centers 

for Environmental Information website, accessed June 29, 

2017, at https://www.ncdc.noaa.gov/cdo-web/datatools/

normals. 
66 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013

National Oceanic and Atmospheric Administration [NOAA], 

2018, Climate at a glance: National Centers for Environmental Information website, accessed April 12, 2018, at 

https://www.ncdc.noaa.gov/cag/divisional/time-series. 

North Carolina Department of Environmental Quality, 2014, 

2014 North Carolina water quality assessment for 305(b): 

North Carolina Department of Environmental Quality, 

Integrated Report Files website, 2014 Integrated Report, 

accessed February 7, 2018, at https://files.nc.gov/ncdeq/

Water%20Quality/Planning/TMDL/303d/2014v2/2014_

NC_WQ_Asmnt2.16.15.pdf. 

North Carolina Department of Environmental Quality, 2017, 

Important information for users of NC DWR ambient water 

quality monitoring data, August 2017, version 2.6: North 

Carolina Department of Environmental Quality website, 

accessed February 12, 2018, at https://files.nc.gov/ncdeq/

Water%20Quality/Environmental%20Sciences/ECO/

AMS_Data_Explanations_v2_6.pdf. 

North Carolina Department of Environmental Quality, 

2018, River basin classification schedule: North Carolina 

Department of Environmental Quality website, accessed 

February 7, 2018, at https://deq.nc.gov/river-basinclassification-schedule. 

North Carolina Department of Health and Human Services, 

2018, Is your well-water safe?: North Carolina Department 

of Health and Human Services, accessed June 7, 2018, at 

http://epi.publichealth.nc.gov/oee/docs/wellwater.pdf.

North Carolina Department of Natural Resources and Community Development, 1987, Eno River area capacity use 

investigation, executive summary: Raleigh, N.C., Division 

of Water Resources, 34 p.

North Carolina Office of State Budget and Management, 2017, 

Population overview, 2010–2036: North Carolina Office of 

State Budget and Management, accessed February 13, 2017, 

at https://ncosbm.s3.amazonaws.com/s3fs-public/demog/

countytotals_populationoverview.html.

Oblinger, C.J., 2004, Triangle Area Water Supply Monitoring 

Project, October 1988 through September 2001, North 

Carolina—Description of the water-quality network, sampling and analysis methods, and quality-assurance practices: 

U.S. Geological Survey Open-File Report 2004–1278, 

56 p., accessed April 25, 2018, at https://pubs.usgs.gov/

of/2004/1278/. 

Oelsner, G.P., Sprague, L.A., Murphy, J.C., Zuellig, R.E., 

Johnson, H.M., Ryberg, K.R., Falcone, J.A., Stets, E.G., 

Vecchia, A.V., Riskin, M.L., De Cicco, L.A., Mills, T.J., 

and Farmer, W.H., 2017, Water-quality trends in the 

Nation’s rivers and streams, 1972–2012—Data preparation, 

statistical methods, and trend results: U.S. Geological 

Survey Scientific Investigations Report 2017–5006, 136 p., 

accessed February 16, 2018, at https://doi.org/10.3133/

sir20175006. 

Samy, S., Robinson, J., Rumsey, I.C., Walker, J.T., and 

Hays, M.D., 2013, Speciation and trends of organic 

nitrogen in Southeastern U.S. fine particulate matter 

(PM2.5): Journal of Geophysical Research—Atmospheres, 

v. 118, p. 1996–2006, accessed February 24, 2018, at 

https://doi.org/10.1029/2012JD017868. 

Sprague, L.A., Mueller, D.K., Schwarz, G.E., and Lorenz, 

D.L., 2009, Nutrient trends in streams and rivers of the 

United States, 1993–2003: U.S. Geological Survey Scientific Investigations Report 2008–5202, 196 p., accessed 

February 26, 2018, at https://pubs.usgs.gov/sir/2008/5202/.

Spruill, T.B., Jen, P.S., and Rasmussen, R.B., 2006, Suspended 

sediment and nutrients in the upper Cape Fear River Basin, 

North Carolina, 2002–04, with an analysis of temporal 

changes, 1976–2004: U.S. Geological Survey Scientific 

Investigations Report 2005–5271, 40 p., accessed June 15, 

2017, at https://pubs.usgs.gov/sir/2005/5271/pdf/report.pdf.

Thornton, J., Steel, A., and Rast, W., 1996, Reservoirs, 

chap. 8 of Chapman, D. (ed.), Water quality assessments—A 

guide to use of biota, sediments and water in environmental 

monitoring: UNESCO/WHO/UNEP, 41 p., accessed 

February 22, 2018, at http://www.who.int/water_

sanitation_health/resourcesquality/wqachapter8.pdf. 

U.S. Army Corps of Engineers, 2018, B. Everett Jordan–Cape 

Fear River Basin, daily elevation, inflow and outflow data: 

Wilmington District Water Management web interface, 

accessed February 14, 2018, at http://epec.saw.usace.army.

mil/jord.htm. 

U.S. Census Bureau, 2017, American FactFinder: 

U.S. Census Bureau website, accessed February 13, 2017, 

at https://factfinder.census.gov/faces/nav/jsf/pages/

index.xhtml.

U.S. Environmental Protection Agency, 2014, STORET 

Central Warehouse: U.S. Environmental Protection 

Agency web interface, accessed August 20, 2014, at 

https://ofmpub.epa.gov/storpubl/dw_pages.querycriteria. 

U.S. Geological Survey, 2014, USGS water data for the 

Nation: U.S. Geological Survey National Water Information 

System database, accessed September 17, 2014, at 

https://doi.org/10.5066/F7P55KJN.
References Cited 67

Vecchia, A.V., 2000, Water-quality trend analysis and sampling 

design for the Souris River, Saskatchewan, North Dakota, 

and Manitoba: U.S. Geological Survey Water-Resources 

Investigations Report 00–4019, 77 p., accessed April 25, 

2018, at http://nd.water.usgs.gov/pubs/wri/wri004019/.

Vecchia, A.V., 2003, Water-quality trend analysis and 

sampling design for streams in North Dakota, 1971–2000: 

U.S. Geological Survey Water-Resources Investigations 

Report 03–4094, 73 p., accessed June 26, 2017, at 

http://nd.water.usgs.gov/pubs/wri/wri034094/.

Vecchia, A.V., 2004a, Using time series analysis to analyze 

trends in concentration: Proceedings of the National 

Monitoring Conference, Chattanooga, Tenn., May 17–20, 

2004, 19 p., accessed April 25, 2018, at https://acwi.gov/

monitoring/conference/2004/.

Vecchia, A.V., 2004b, Statistical techniques for trend and 

load estimation: Proceedings of the National Monitoring 

Conference, Chattanooga, Tenn., May 17–20, 2004, 74 p., 

accessed April 25, 2018, at https://acwi.gov/monitoring/

conference/2004/.

Vecchia, A.V., 2005, Water-quality trend analysis and sampling 

design for streams in the Red River of the North Basin, 

Minnesota, North Dakota, and South Dakota, 1970–2001: 

U.S. Geological Survey Scientific Investigations Report 

2005–5224, 54 p., accessed September 16, 2015, at 

http://pubs.usgs.gov/sir/2005/5224/.

Vogelmann, J.E., Howard, S.M., Yang, L., Larson, C.R., 

Wylie, B.K., and Van Driel, J.N., 2001, Completion of the 

1990’s National Land Cover Data Set for the conterminous 

United States: Photogrammetric Engineering and Remote 

Sensing, v. 67, p. 650–662, accessed June 22, 2017, at 

https://www.mrlc.gov/nlcd1992.php.

Weaver, J.C., 2016, Low-flow characteristics and flowduration statistics for selected USGS continuous-record 

streamgaging stations in North Carolina through 2012 

(ver. 1.1, March 2016): U.S. Geological Survey Scientific 

Investigations Report 2015–5001, 89 p., accessed June 27, 

2017, at https://doi.org/10.3133/sir20155001. 

Weaver, J.C., Terziotti, S., Kolb, K.R., and Wagner, C.R., 

2012, StreamStats in North Carolina—A water-resources 

web application: U.S. Geological Survey Fact Sheet 

2012–3137, 4 p., accessed April 25, 2018, at 

https://pubs.usgs.gov/fs/2012/3137/.
Manuscript was approved on June 5, 2018

Publishing support was provided by the USGS Science Publishing 

Network, Reston Publishing Service Center 

For additional information, contact

Director, South Atlantic Water Science Center

U.S. Geological Survey

720 Gracern Road

Stephenson Center, Suite 129

Columbia, SC 29210

Or visit the South Atlantic Water Science Center website at

https://www.usgs.gov/water/southatlantic/
Giorgino and others—Trends in Water Quality of Streams and Reservoirs in the Triangle Area of North Carolina, 1989–2013—Scientific Investigations Report 2018–5077

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185077