U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Triangle Area Water Supply Monitoring Project 

Steering Committee

Quality of Surface-Water Supplies in the 

Triangle Area of North Carolina, 

Water Year 2008

Open-File Report 2012–1013
Falls Lake, 

Wake County,

North Carolina

Cane Creek Reservoir, 

Orange County,

North Carolina

Lake Butner, 

Granville County,

North Carolina
Quality of Surface-Water Supplies in 

the Triangle Area of North Carolina, 

Water Year 2008

By M.J. Giorgino, R.B. Rasmussen, and C.A. Pfeifle 

Prepared in cooperation with the Triangle Area Water Supply Monitoring Project

Steering Committee 

Open-File Report 2012–1013

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Giorgino, M.J., Rasmussen, R.A., and Pfeifle, C.A., 2012, Quality of surface-water supplies in the Triangle area of 

North Carolina, water year 2008: U.S. Geological Survey Open-File Report 2012–1013, 12 p.
iii

Contents

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

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

Monitoring Network ......................................................................................................................................2

Streamgaging Stations ........................................................................................................................2

Water-Quality Sites...............................................................................................................................2

Quality Assurance.................................................................................................................................5

Precipitation and Streamflow......................................................................................................................5

Water Quality..................................................................................................................................................6

Dissolved Oxygen, pH, and Conductance.........................................................................................7

Metals and Trace Elements.................................................................................................................8

Chlorophyll a........................................................................................................................................10

Nitrogen and Phosphorus .................................................................................................................10

What’s Next?.................................................................................................................................................11

References....................................................................................................................................................12

Figures

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

data-collection sites in the upper Cape Fear and Neuse River Basins, 

North Carolina ...............................................................................................................................4 

2–5. Graphs showing: 

2. Monthly precipitation at the Raleigh-Durham International Airport, North

 Carolina (NOAA National Weather Service station KRDU), October 2007 

 through September 2008........................................................................................................5

3. Palmer Hydrological Drought Index values for the Northern Piedmont 

 and Central Piedmont climate divisions of North Carolina, 

 October 2007 through September 2008..............................................................................6

4. Annual runoff measured from October 2007 through September 2008 and 

 normal runoff for the period of record at 10 U.S. Geological Survey 

 streamgaging stations in the Triangle area of North Carolina......................................6

5. Seven-day average streamflow, at U.S. Geological Survey site 1, Eno River at

 Hillsborough in the Neuse River Basin; and U.S. Geological Survey site 19,

 Haw River near Bynum in the Cape Fear River Basin, for the period

 October 2007 through September 2008..............................................................................6

6–8. Maps showing: 

6. Sites in the Triangle area of North Carolina with one or more measurement of

 dissolved-oxygen concentration less than 5 milligrams per liter and

 dissolved-oxygen percent saturation value greater than 110 percent, October 

 2007 through September 2008..............................................................................................7

7. Sites in the Triangle area of North Carolina with one or more concentration 

 of unfiltered iron greater than 1,000 micrograms per liter, unfiltered 

 manganese greater than 200 micrograms per liter, unfiltered mercury 

 greater than 0.012 microgram per liter, and unfiltered copper greater than 

 7 micrograms per liter, October 2007 through September 2008...................................9
iv

8. Sites in the Triangle area of North Carolina with one or more chlorophyll a

 concentration greater than 40 micrograms per liter, October 2007 through

 September 2008......................................................................................................................10

9. Graphs showing dot plots of concentrations of total ammonia plus organic

nitrogen and total phosphorus at sites in the Triangle Area Water Supply 

Monitoring Project study area, October 2007 through September 2008 ...........................11

Tables

1. Water-quality and streamflow monitoring sites for the Triangle Area Water Supply 

Monitoring Project, October 2007 through September 2008 .................................................3

2. Summary of water-quality results for sampling sites in the Triangle Area 

Water Supply Monitoring Project, October 2007 through September 2008..........................

Separate Excel file

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 millimeter (mm)

mile (mi) 1.609 kilometer (km)

Flow rate

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

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 either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). 

Selected acronyms and abbreviations used in this report:

DOC dissolved organic carbon

MCL maximum contaminant level

NCDWQ North Carolina Division of Water Quality

SDWR secondary drinking-water regulation

TAWSMP Triangle Area Water Supply Monitoring Project

USGS U.S. Geological Survey
Abstract

Surface-water supplies are important sources of drinking 

water for residents in the Triangle area of North Carolina, which 

is located within the upper Cape Fear and Neuse River Basins. 

Since 1988, the U.S. Geological Survey and a consortium of 

governments have tracked water-quality conditions and trends 

in several of the area’s water-supply lakes and streams. This 

report summarizes data collected through this cooperative effort, 

known as the Triangle Area Water Supply Monitoring Project, 

during October 2007 through September 2008. Major findings 

for this period include:

• Antecedent drought conditions during 2007 contributed to 

below-average flows at streams throughout the study area 

during 2008. Continuous records from 9 of the 10 project 

stream gages documented below-average streamflow during most of the year.

• More than 8,000 individual measurements of water quality 

were made at a total of 27 sites—15 in the Neuse River 

Basin and 12 in the Cape Fear River Basin.

• North Carolina water-quality standards were exceeded 

one or more times for nine constituents, including dissolved oxygen, dissolved oxygen percent saturation, pH, 

chlorophyll a, mercury, copper, iron, manganese, and 

zinc. Exceedances occurred at 26 sites, 14 of which were 

in the Neuse River Basin, and 12 of which were in the 

Cape Fear River Basin.

• Stream samples collected during storm events contained 

elevated concentrations of iron, copper, and total phosphorus relative to non-storm samples.

• The first full year of sampling was completed for a new 

project site at Lake Butner in Granville County. Among 

all lakes sampled during 2008, Lake Butner had the lowest concentrations of total ammonia plus organic nitrogen, 

total phosphorus, chlorophyll a, and specific conductance 

and the highest water clarity.

• Concentrations of nitrogen and phosphorus were within 

ranges observed during previous years; however, Falls 

Lake at U.S. Interstate 85 had elevated levels of nitrate 

plus nitrite and total phosphorus relative to other sites.

• Five lakes had chlorophyll a concentrations in excess of 40 

micrograms per liter at least once during 2008, including 

Little River Reservoir, Falls Lake, Lake Benson, University Lake, and Jordan Lake.

Introduction

The Triangle area of North Carolina, located within the 

upper Cape Fear and Neuse River Basins, is one of the most 

rapidly developing areas of the State. Population growth 

continues to increase demands for water from public suppliers, 

the majority of which draw water from streams and lakes in 

the region. Growth also brings the threat of greater loads of 

pollutants and new contaminant sources that, if not properly 

managed, could adversely affect water quality. 

For the past 20 years, the Triangle Area Water Supply 

Monitoring Project (TAWSMP) has tracked water-quality 

conditions and long-term trends in many of the area’s water-supply 

lakes, rivers, and tributaries. The project has progressed in phases, 

allowing for flexibility in the monitoring network and partners 

and for focusing on specific water-quality issues as they arose 

(http://nc.water.usgs.gov/projects/triangle/overview.html). The 

project currently is in its sixth phase, which covers the period 

July 2007 through June 2012. Objectives for this phase are to

• Extend the existing water-quality database for nutrients, 

sediment, major ions, and metals and trace elements to 

track spatial variations in water quality, loads to lakes, and 

long-term water-quality trends. 

• Continue monitoring at tributary sites during high-flow 

events to increase the understanding of constituent 

concentrations and loads during extreme hydrologic 

conditions. 

Quality of Surface-Water Supplies in 

the Triangle Area of North Carolina, 

Water Year 2008

By Mary J. Giorgino, R.B. Rasmussen, and C.A. Pfeifle
Triangle Area Water Supply 

Monitoring Project Partners, 2008 

Chatham County 

Orange County 

Wake County 

Town of Apex 

Town of Cary 

City of Durham 

Town of Hillsborough 

Town of Morrisville 

City of Raleigh 

Orange Water and Sewer Authority 

South Granville Water and Sewer Authority 

Triangle J Council of Governments

U.S. Geological Survey

For more information on the project, go to: 

 http://nc.water.usgs.gov/projects/triangle/ 

2 Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008

• Investigate the occurrence of mercury in water and 

sediment. 

• Maintain a network of 10 continuous streamgaging stations 

in the study area. 

• Publish an analysis of water-quality trends.

Pursuant to an agreement with several local governments, 

the U.S. Geological Survey (USGS) collects water samples up 

to six times a year at 30 sites, depending on the location and 

constituents being monitored. One additional site consists only of 

a streamflow gage. Water temperature, dissolved oxygen, pH, and 

specific conductance are measured, and analyses are performed to 

determine concentrations of nutrients, metals and trace elements, 

major ions, and total organic carbon at all sites. In addition, 

suspended-sediment concentrations are measured at stream sites, 

and chlorophyll a is analyzed at lake sites. Continuous streamflow 

is recorded at almost all of the stream sites and is funded through 

the TAWSMP and other USGS cooperative programs. The 

USGS also is responsible for data quality assurance, analysis, and 

interpretation, providing the data to the public and maintaining 

the data in perpetuity. Funding for the project is provided by local 

government partners (see sidebar) and by the USGS Cooperative 

Water Program (http://water.usgs.gov/coop/). The Triangle J 

Council of Governments provides organizational support services 

for the TAWSMP. 

Monitoring Network

Since the project began in 1988, several adjustments 

have been made to sampling locations, sampling frequency, 

and constituents that are sampled. During 2008, the TAWSMP 

monitoring network was composed of 31 sites, including 

streamgaging stations and stream and lake water-quality sites. 

Project sampling and analytical methods and quality-assurance 

practices are described by Oblinger (2004).

Streamgaging Stations

Streamflow records are useful for managing water supplies 

and are essential for determining in-stream loads of sediment, 

nutrients, and other constituents and for interpreting water-quality 

trends. The USGS operates 10 continuous-record streamgaging 

stations that are funded through the TAWSMP (table 1). These 

gages record water level and discharge at 15-minute intervals 

and display them in near-real time through the National Water 

Information System Web interface commonly known as NWISWeb (http://waterdata.usgs.gov/nc/nwis/current/?type=flow). 

Precipitation and streamflow data for several additional sites in the 

study area are available through other USGS programs. 

Water-Quality Sites

Water-quality data are used to track current conditions and to 

analyze long-term water-quality trends and pollutant loads in the 

Triangle area. The USGS monitored water quality at 27 different 

sites in the TAWSMP study area during water year 2008. More 

than 8,000 individual water-quality measurements were made, 

not including vertical-profile measurements in study area lakes. 

USGS water-quality data are available to project partners and the 

public through the NWIS-Web (http://nwis.waterdata.usgs.gov/

nc/nwis/qwdata) or by request from the USGS North Carolina 

Water Science Center (http://nc.water.usgs.gov/index.html).

Nine Triangle area public water-supply lakes are sampled as 

part of the TAWSMP. Little River Reservoir, Lake Michie, Lake 

Butner, Falls Lake, Lake Wheeler, and Lake Benson are located 

in the Neuse River Basin, and Cane Creek Reservoir, University 

Lake, and Jordan Lake are located in the Cape Fear River Basin 

(fig. 1). Falls and Jordan Lakes are large, multipurpose reservoirs 

managed by the U.S. Army Corps of Engineers. The seven smaller 

lakes are used primarily for water supply; however, most of the 

lakes also provide recreational access for area residents. 

The USGS routinely samples four sites at Falls Lake and 

four sites at Jordan Lake six times per year. One site in each of 

the seven smaller lakes is sampled four times per year. At the 15 

lake sites, vertical profiles of field properties (water temperature, 

dissolved oxygen, specific conductance, and pH) are measured, 

along with secchi disk transparency. Water samples are collected 

for analysis of alkalinity, nutrients, major ions, iron, manganese, 

total organic carbon, and chlorophyll a. Additional metals and 

trace elements are sampled twice each year. 

This report summarizes monitoring activities and data 

collected by the USGS for the TAWSMP during October 2007 

through September 2008, also referred to as water year 2008. 

Hydrologic conditions in the Triangle area also are described. 

Ranges of concentrations for water-quality field properties, major 

ions, nutrients, metals and trace elements, chlorophyll a, organic 

carbon, and suspended sediment are presented for each site 

sampled during this period. 
Monitoring Network 3

Table 1. Water-quality and streamflow monitoring sites for the Triangle Area Water Supply Monitoring Project, October 2007 through 

September 2008. 

[USGS, U.S. Geological Survey; runoff, sampled only during runoff events; n/a, not applicable for lake sites; --, data were not collected (refer to footnote); 

USACE, U.S. Army Corps of Engineers; italics indicate site is located in waters listed as impaired by the North Carolina Department of Environment and Natural Resources, Division of Water Quality (2007a) at the time of sampling]

Map 

no. 

(fig. 1)

USGS 

station no. Station location in North Carolina Site type

Data collected during 2008?

Water 

quality

Continuous 

streamflow 

[period of record]

Neuse River Basin

1 02085000 Eno River at Hillsborough Stream Yes Yes [1927–present]

2 02085070 Eno River near Durham Stream (runoff) No Yes [1963–present]

3 0208521324 Little River at SR 1461 near Orange Factory Stream (runoff) Yes Yes [1987–present]

4 0208524845 Little River Reservoir at dam near Bahama Lake Yes n/a

5 02085500 Flat River at Bahama Stream (runoff) Yes Yes [1925–present]

6 02086490 Lake Michie at dam near Bahama Lake Yes n/a

7 02086569 Lake Butner at dam near Butner Lake Yes n/a

8 02086624 Knap of Reeds Creek near Butner Stream (runoff) Yes (Raleigh)1

 [1982–1995; 2006–present]

9 02086849 Ellerbe Creek near Gorman Stream (runoff) Yes (Raleigh)1

 [1982–1995; 2006–present]

10 02086920 Falls Lake at U.S. Interstate 85 near Redwood Lake Yes n/a

11 0208700780 Little Lick Creek above SR 1814 near Oak Grove Stream (runoff) Yes –2

12 0208703650 Falls Lake at NC Highway 50 near Sandy Plain Lake Yes n/a

13 0208708905 Falls Lake at NC Highway 98 near Bayleaf Lake Yes n/a

14 0208718195 Falls Lake above dam at Falls Lake Yes n/a

15 02087588 Lake Wheeler on Swift Creek near Raleigh Lake Yes n/a

16 02087701 Lake Benson at dam near Garner Lake Yes n/a

Cape Fear River Basin

17 02096846 Cane Creek near Orange Grove Stream Yes Yes [1988–present]

18 0209684980 Cane Creek Reservoir at dam near White Cross Lake Yes n/a

19 02096960 Haw River near Bynum Stream (runoff) No (USACE)1

 [1973–present]

20 0209699999 B. Everett Jordan Lake, Haw River arm near Hanks Chapel Lake Yes n/a

21 02097314 New Hope Creek near Blands Stream (runoff) Yes Yes [1982–present]

22 0209741955 Northeast Creek at SR 1100 near Genlee Stream (runoff) Yes Yes [1982–present]

23 02097464 Morgan Creek near White Cross Stream Yes Yes [1988–present]

24 0209749990 University Lake at intakes near Chapel Hill Lake Yes n/a

25 02097517 Morgan Creek near Chapel Hill Streamflow 

only

n/a Yes [1982–present]

26 02097521 Morgan Creek near Farrington Stream (runoff) No –3

27 0209768310 B. Everett Jordan Lake at buoy 12 at Farrington Lake Yes n/a

28 0209782609 White Oak Creek at mouth near Green Level Stream Yes Yes [1999–present]

29 0209799150 B. Everett Jordan Lake above U.S. Highway 64 at Wilsonville Lake Yes n/a

30 0209801100 B. Everett Jordan Lake at Bells Landing near Griffins 

Crossroads

Lake Yes n/a

31 02098198 Haw River below B. Everett Jordan Lake dam near Moncure Stream (runoff) Yes –4

1

 Gage funded through separate agreement with agency shown in parentheses. 

2

 Instantaneous streamflow measured during sampling.

3

 Streamflow from nearby gage, USGS station 02097517.

4

 Streamflow computed from USACE releases from the Jordan Lake dam.
4 Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008

Figure 1. Location of Triangle Area Water Supply Monitoring Project data-collection sites in the upper Cape Fear and 

Neuse River Basins, North Carolina.

GRANVILLE

DURHAM

WAKE

Neuse

Pittsboro

Bynum

Haw

Jordan

Lake

Shearon

Harris

Res.

Falls

Lake

Lake

Michie

Lake

Orange

Basin boundary

Streamgage and number

Lake site and number

Stream site and number

Runoff-only stream site and number

National Weather Service precipitation

 station

EXPLANATION

78 79 30' 

3545'

36

Apex

Cary Raleigh

Lake

Wheeler

Lake

Benson

Swift

Cr.

River

Durham

Little River

Res.

Little

River

River

Hillsborough

Chapel

Hill

Eno River

Flat

Carrboro

University

Lake

Cane Cr.

Res.

CASWELL

ALAMANCE

CHATHAM

LEE HARNETT

JOHNSTON

ORANGE

PERSON

River

River

Deep

Brickhaven

Neuse

River

Cape Fear Basin

River

Basin

1

14

13

12

11

10

9

2

4

3

8

6

7

5

15

16

17

18

23

20

19

30

28

29

31

31

28

30

25

27

26

22

21 24 25

z

z

Cape Fear

River

Base from digital files of:

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

 1990 Precensus TIGER/Line Files-Political boundaries, 1991

U.S. Environmental Protection Agency, River File 3

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

0 5 10 KILOMETERS

0 5 10 MILES

LOCATION OF STUDY AREA IN THE NEUSE AND CAPE FEAR RIVER

BASINS IN NORTH CAROLINA

Neuse Cape

Fear
Precipitation and Streamflow 5

Four stream sites are routinely sampled by the USGS on a 

bimonthly basis for field properties, nutrients, major ions, total 

organic carbon, and suspended sediment, and twice per year for 

metals and trace elements (table 1). Eleven additional stream 

sites are sampled by the USGS only during selected storm-runoff 

events to supplement data that are collected monthly by the 

North Carolina Division of Water Quality (NCDWQ) Ambient 

Monitoring Network. These runoff-only stream sites are not all 

sampled by the USGS during every year. A project goal is to 

collect 16 runoff-event samples each year; the actual number 

varies from year to year depending on whether targeted flow 

conditions occur. 

Alkalinity is determined in the field at the time of sampling. 

Other chemical analyses are performed at the USGS National 

Water Quality Laboratory in Denver, Colorado. Suspendedsediment samples are analyzed at the USGS Eastern Region 

Sediment Laboratory in Louisville, Kentucky.

During water year 2008, routine sampling of the 15 lake sites 

and 4 stream sites was conducted on schedule with the following 

exceptions. Low water prevented access to Falls Lake at U.S. 

Interstate 85 during December 2007. Cane Creek and Morgan 

Creek near White Cross were not sampled during October and 

December 2007 because both streams were dry. White Oak Creek 

was stagnant with no observable flow during December 2007; 

therefore, samples were not collected. 

Eight runoff sites were sampled once or twice, resulting 

in a total of 12 samples (table 1). It is important to note that 

results for the eight streams sampled only during runoff events 

likely do not represent typical water-quality conditions for these 

streams. In addition, one runoff-event sample was collected at 

Cane Creek (site 17), which is a bimonthly stream site, during 

September 2008.

Quality Assurance

Quality-control samples, including deionized water 

blanks, sampling equipment blanks, field blanks, and replicate 

environmental samples, were collected and reviewed throughout 

the year to ensure that project data-quality objectives were 

met (Oblinger, 2004). Approximately 15 percent of the total 

number of samples collected were quality-control samples. A 

review of the quality-control and environmental data revealed 

that capsule filters had contaminated two samples for dissolved 

organic carbon (DOC) during April 2008. Consequently, a Teflon 

filtration apparatus and glass fiber filters were subsequently used 

for processing DOC samples, and the contaminated results were 

rejected. No other quality-assurance problems were observed.

Precipitation and Streamflow

Antecedent drought conditions during water year 2007 

contributed to below-average flows at streams throughout the study 

area during water year 2008. Flows were below average despite 

the fact that precipitation measured at the National Weather Service 

station at the Raleigh-Durham International Airport (fig. 1) from 

Figure 2. Monthly precipitation at the Raleigh-Durham 

International Airport, North Carolina (NOAA National Weather 

Service station KRDU), October 2007 through September 2008 

(Location shown in fig. 1. Source: State Climate Office of North 

Carolina (2008b)).

October 2007 through September 2008 totaled 51.6 inches, which 

exceeded the long-term average annual precipitation of 43.1 inches 

(fig. 2). The yearly total included more than 9 inches of rain that 

fell during September 2008; rainfall for prior months in water year 

2008 was more typical of long-term average amounts (fig. 2). 

10

9

8

7

6

5

4

3

2

1

0

Oct. Nov.

2007 2008

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

Precipitation, in inches

Monthly total

1971–2000 normal

EXPLANATION

A better indication of hydrologic conditions is provided by 

the monthly Palmer Hydrological Drought Index scores for water 

year 2008 (fig. 3). This index reflects the long-term, cumulative 

effects of drought on hydrologic characteristics, such as lake levels 

and streamflow. Negative values indicate dry periods, and positive 

values indicate wet periods. The Palmer Hydrological Drought 

Index indicates that dry conditions prevailed during October 2007 

through August 2008 in the Northern Piedmont (including Orange, 

Durham, and Granville Counties) and during October 2007 

through September 2008 in the Central Piedmont (including 

Chatham and Wake Counties) of North Carolina (fig. 3).

Streamflow data collected at the 10 gaging stations 

funded by the TAWSMP were reviewed, quality-assured, 

and published in the 2008 USGS Annual Water Data Report 

(http://nc.water.usgs.gov/reports/WDR/). The data also are 

available online through the USGS NWIS-Web (http://waterdata.

usgs.gov/nc/nwis/). Streamflow during water year 2008 was 

lower than normal at all sites in the study area, with the exception 

of Northeast Creek near Genlee (site 22; fig. 4). At this site and 

New Hope Creek near Blands (site 21), a high percentage of 

the in-stream flow consisted of treated effluent from municipal 

water reclamation facilities. These continuous inputs mitigated 

the effects of drought on streamflow. Drought conditions were 

especially pronounced at Cane Creek near Orange Grove 

(site 17) and Morgan Creek near White Cross (site 23), which 

are headwater streams with small drainage areas, and at the Eno 

River at Hillsborough (site 1), which is located downstream from a 

municipal water-supply diversion. 
6 Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008

Figure 4. Annual runoff measured from October 2007 through 

September 2008 and normal runoff for the period of record at 10 

U.S. Geological Survey streamgaging stations in the Triangle area 

of North Carolina. 

Below-average flow conditions that were prevalent during 

the spring and summer of 2007 persisted through much of water 

year 2008 as illustrated by hydrographs for the Eno River at 

Hillsborough in the Neuse River Basin and the Haw River near 

Bynum in the Cape Fear River Basin (fig. 5). Only in late August 

through September 2008 did flows approximate normal levels, 

except for brief pulses in response to storm events. Periods of 

zero flow were recorded at five streams in the study area during 

October 2007, including Little River near Orange Factory 

(site 3), Flat River at Bahama (site 5), Cane Creek near Orange 

Grove (site 17), Morgan Creek near White Cross (site 23), and 

White Oak Creek near Green Level (site 28).

10,000

1,000

100

10

1

0.1

100,000

10,000

1,000

100

10

Oct. Nov.

2007 2008

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

7-day average discharge, in cubic feet per second

90th percentile

76–90

25–75

10–24

10th percentile

Flow

EXPLANATION

Percentile classes

A. Eno River at Hillsborough

B. Haw River near Bynum

Figure 5. Seven-day average streamflow, in cubic feet per 

second, at (A) site 1, Eno River at Hillsborough in the Neuse River 

Basin; and (B) site 19, Haw River near Bynum in the Cape Fear 

River Basin, for the period October 2007 through September 2008 

(locations in fig. 1 and table 1).

Figure 3. Palmer Hydrological Drought Index for the Northern 

Piedmont and Central Piedmont climate divisions of North 

Carolina, October 2007 through September 2008 (Source: State 

Climate Office of North Carolina (2008a)).

2

1

0

–1

–2

–3

–4

–5

Drought severity index

Northern Piedmont

Central Piedmont

EXPLANATION

Northern Piedmont

Central Piedmont

Oct. Nov.

2007 2008

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

25

20

15

10

5

0

1 2

Site

3 5 17 21 22 23 25 28

Annual runoff, in inches

Water year 2008

Normal for period of record

EXPLANATION

Water Quality 

Water-quality data were reviewed, quality-assured, 

and published in the USGS 2008 Annual Water Data 

Report (http://nc.water.usgs.gov/reports/WDR/). The 

data also are available online through the NWIS-Web 

(http://waterdata.usgs.gov/nc/nwis/). Ranges of 

concentrations observed at each site for 46 properties or 

constituents are presented in table 2 (Excel file). Although 

lake sites were sampled at multiple depths, only data 

collected from near-surface waters are summarized in this 
Water Quality 7

report. The USGS also collected mid-depth and near-bottom 

samples for nutrients, iron, and manganese at all lake sites. 

These data may be obtained from NWIS-Web or by request 

from the USGS North Carolina Water Science Center.

In-stream water-quality standards have been adopted by 

the North Carolina Division of Water Quality for 23 

constituents monitored by the TAWSMP (table 2). All project 

sampling sites are in waters classified for water-supply use; 

therefore, applicable standards are the most stringent values 

established to protect freshwater aquatic life, water supply, 

or human health (North Carolina Department of Natural 

Resources, Division of Water Quality, 2007b). Concentration 

ranges that are shown in table 2 in bold font indicate that at least 

one sample for the constituent exceeded a North Carolina waterquality criterion at that location. Federal maximum contaminant 

levels (MCLs) and secondary drinking-water regulations 

(SDWRs) have been established by the U.S. Environmental 

Protection Agency for 20 of the monitored constituents 

(http://www.epa.gov/safewater/contaminants/index.html). These 

criteria are applicable only to treated potable water—not to raw 

water supplies; however, they are provided here for reference. 

No exceedances of State water-quality criteria were 

observed for 13 of the 23 constituents for which standards 

exist, including hardness, chloride, fluoride, sulfate, nitrate, 

arsenic, cadmium, chromium, lead, molybdenum, nickel, 

selenium, and silver. Exceedances were observed for nine 

water-quality constituents, including dissolved oxygen, 

dissolved oxygen percent saturation, pH, chlorophyll a, filterable and total recoverable mercury, copper, iron, manganese, 

and zinc (table 2). Knap of Reeds Creek (site 8) was the only 

site sampled during water year 2008 that had no exceedances 

of water-quality standards for the constituents that were 

measured.

Dissolved Oxygen, pH, and Conductance

Dissolved-oxygen concentrations less than 5 milligrams 

per liter (mg/L) were observed occasionally at seven lake 

and three stream sites in the study area (table 2; fig. 6A). 

Low concentrations were reflective of the prevailing drought 

and stagnant conditions in selected streams and lakes. At the 

other extreme, lake sites occasionally showed supersaturated 

dissolved-oxygen concentrations and elevated pH during 

periods of active algal photosynthesis. Dissolved-oxygen 

saturation values greater than 110 percent can be interpreted 

as exceeding the dissolved gases standard and were recorded 

at seven lake sites (table 2; fig. 6B) typically during summer 

months when the lakes were thermally stratified. One 

dissolved-oxygen saturation value greater than 110 percent 

also was measured at White Oak Creek at mouth near Green 

Level (site 28) on August 4, 2008, when the stream had no 

measurable flow and excessive algal growths were observed 

at the site. A single pH value that exceeded the North Carolina 

water-quality standard of 9 was observed at Jordan Lake at 

Bells Landing (site 30) on October 17, 2007.

Figure 6. Sites in the Triangle area of North Carolina with one 

or more measurement of (A) dissolved-oxygen concentration 

less than 5 milligrams per liter and (B) dissolved-oxygen percent 

saturation value greater than 110 percent, October 2007 through 

September 2008 (locations in fig. 1 and table 1).

Eno

Eno

1 Ri ver 10

4

6

15 16

23

28

27

24

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

Neuse

River

Cape Fear Basin

River

Haw

Basin 

River

Flat River

Little River

Neuse

River

Ri ver

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

Neuse

River

Cape Fear Basin

River

Haw

Basin 

River

Flat River

Little River

Neuse

River

79° 78°30'

36°

35°52'30"

36°07'30"

79° 78°30'

36°

35°52'30"

36°07'30"

15 16

28

30

20

29

24

18

Base from digital files of:

U.S. Environmental Protection Agency, River File 3

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

A

B

0 5 10 MILES

0 5 10 KILOMETERS

28

27

Basin boundary

Lake site and number

Stream site and number

EXPLANATION

28

24

Basin boundary

Lake site and number

Stream site and number

EXPLANATION
8 Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008

With the exception of aluminum, iron, and manganese, 

all metal and trace element concentrations were less than 

Federal drinking-water MCLs or SDWRs (table 2) during 

water year 2008. As noted previously, MCLs and SDWRs are 

applicable to treated drinking water rather than untreated source 

water; however, they are included in this report for reference. 

Aluminum, iron, and manganese concentrations occasionally 

exceeded SDWRs (table 2); however, concentrations were 

similar to those observed in the study area during previous years 

(http://wdr.water.usgs.gov/). 

Although no standard has been adopted for specific 

conductance, interesting variations among sites have been 

shown for this constituent. The highest conductance values 

were recorded at Falls Lake at U.S. Interstate 85 (site 10) and 

Jordan Lake, Haw River arm (site 20). Lake Butner (site 7) 

had the lowest conductance values and greatest water 

transparency among the study lakes (table 2). 

Metals and Trace Elements

Metals and trace element samples were collected twice per 

year at all stream and lake sites. In addition, iron and manganese 

were monitored at multiple depths during each lake-sampling 

event. 

The State aquatic-life standard for total recoverable iron is 

1,000 μg/L; however, the NCDWQ is proposing to eliminate this 

standard in 2010 (http://h2o.enr.state.nc.us/csu/trirev_SW.html). 

Iron concentrations exceeded 1,000 μg/L at 10 streams and 1 lake 

site in the study area during water year 2008 (table 2; fig. 7A). 

Storm-related (runoff) stream samples consistently exceeded 

the standard. Iron concentrations in lake-surface samples were 

all less than 1,000 μg/L, except for samples collected at Falls 

Lake at U.S. Interstate 85 (site 10; table 2), where four of the five 

samples had concentrations above the standard. Total recoverable 

manganese samples exceeded the 200 μg/L water-supply standard 

at nine lake sites and five streams sites during water year 2008 

(table 2; fig. 7B). Iron and manganese tend to be substantially 

higher in lake-bottom waters than near-surface samples during 

summer stratification.

Mercury samples were collected at all sites during water year 

2008 as part of a parallel investigation of mercury in the study 

area. Higher concentrations of mercury in water generally were 

observed in lakes compared to streams; however, some streams 

exceeded the 0.012-μg/L State standard during runoff events. 

Total recoverable mercury concentrations above the standard were 

measured at 17 stream and lake sites (table 2; fig. 7C); at 9 of 

those sites, filterable mercury also exceeded the standard at least 

once during water year 2008 (table 2). A maximum concentration 

of 0.235 μg/L total recoverable mercury was recorded at 

Falls Lake at NC Highway 50 (site 12) on October 3, 2007. All 

mercury concentrations were less than the Federal drinking-water 

MCL of 2 μg/L. 

Copper concentrations greater than the State water-quality 

standard of 7 μg/L were measured at four stream sites: Little River 

(site 3), Ellerbe Creek (site 9), New Hope Creek (site 21), and 

Northeast Creek (site 22). All samples with total recoverable 

copper concentrations greater than 7 μg/L were collected from 

streams in association with runoff events (table 2; fig. 7D). 

A maximum concentration of 17.4 μg/L was recorded at 

Little River on September 6, 2008. The only concentration of zinc 

that was greater than the State water-quality standard of 50 μg/L 

occurred at Ellerbe Creek on September 6, 2008 (52.8 μg/L). 

Overall, zinc concentrations were lower at lake sites than at stream 

sites (table 2).

Dry streambed at site 17, Cane Creek near Orange Grove, 

North Carolina, October 2, 2007. (Photograph by R.B. Rasmussen, 

U.S. Geological Survey)
Water Quality 9

Figure 7. Sites in the Triangle area of North Carolina with one or more concentration of (A) unfiltered iron greater than 

1,000 micrograms per liter, (B) unfiltered manganese greater than 200 micrograms per liter, (C) unfiltered mercury greater than 

0.012 microgram per liter, and (D) unfiltered copper greater than 7 micrograms per liter, October 2007 through September 2008 

(locations in fig. 1 and table 1).

River

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

Neuse

River

Basin

Cape Fear

River

Basin 

1 10

23

28

Haw

River

9

3

22 21

Flat River

River

Little River

Eno Rive r

Rive r

NeuseNeuse

River

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

Neuse

River

Basin

Cape Fear

River

Basin Haw

River

Flat

Little River

Eno

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

Neuse

River

Basin

Cape Fear

River

Basin Haw

River

Flat River

River

Little River

Eno

Rive rRive r

NeuseNeuse

RiverRiver

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

Neuse

River

Basin

Cape Fear

River

Basin Haw

River

Flat

Little River

Eno

5

5

1

10

17

10

6

12

23

3

5

4

17 11

79° 78°30' 79° 78°30'

15 16

29

27

24

Basin boundary

Lake site and number

Stream site and number

Runoff-only stream site

 and number

A EXPLANATION B

36°

31

10

7

9

21

5

9

21

17

3

78 79 30'  78 79 30' 

6

12 13

14

4

16

29

20

30

24

18

C D

Base from digital files of:

U.S. Environmental Protection Agency, River File 3

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

22

0 5 10 MILES

0 5 10 KILOMETERS

5

4

10 Basin boundary

Lake site and number

Stream site and number

Runoff-only stream site

 and number

EXPLANATION

5

4

10 Basin boundary

Lake site and number

Stream site and number

Runoff-only stream site

 and number

EXPLANATION

3

Basin boundary

Runoff-only stream site

 and number

EXPLANATION

35°52'30"

36°07'30"

36°

35°52'30"

36°07'30"
10 Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008

Nitrogen and Phosphorus

The USGS collected nutrient samples six times per year 

at 4 routine stream sites, four to six times per year at 15 lake 

sites, and during every sampling of 8 runoff-only stream sites. 

Nutrient fractions that were analyzed included total ammonia 

plus organic nitrogen (also known as total kjeldahl nitrogen), 

ammonia, nitrite, nitrate plus nitrite, orthophosphorus, and 

total phosphorus. Nutrient concentrations were within ranges 

observed during previous years (http://wdr.water.usgs.gov/). A 

State water-quality standard exists only for nitrate (10 mg/L), 

and no samples exceeded that concentration. 

Total ammonia plus organic nitrogen concentrations ranged 

from 0.26 mg/L at Morgan Creek near White Cross (site 23) 

to 1.7 mg/L at Little River near Orange Factory (site 3) and 

varied widely among sites and sampling dates (table 2; fig. 9A). 

Total phosphorus concentrations ranged from 0.01 mg/L 

at Lake Butner (site 7) to 0.62 mg/L at Falls Lake at U.S. 

Interstate 85 (site 10; table 2; fig. 9B). Except for the maximum 

concentration, all other samples had concentrations less than 

Chlorophyll a

Concentrations of chlorophyll a were measured only at 

lake sites. During water year 2008, concentrations greater than 

the North Carolina water-quality standard of 40 μg/L were 

observed at least once at 10 of the 15 lake sites, including Little 

River Reservoir, 3 sites in Falls Lake, Lake Benson, University 

Lake, and the 4 Jordan Lake sites (table 2; fig. 8). Seasonal 

patterns differed among lakes. A maximum chlorophyll a

concentration of 96 μg/L was recorded at University Lake on 

October 5, 2007. Hot, dry weather during October 2007 also 

provided an opportunity for algae to thrive at the four Jordan 

Lake sites and the two upstream sites in Falls Lake. During 

April 2008, concentrations greater than 40 μg/L were observed 

in Little River Reservoir and the near-dam site in Falls Lake. 

A chlorophyll a concentration of 69.4 μg/L was measured at 

Lake Benson during August 2008. Lake Butner consistently 

had the lowest levels of chlorophyll a, with concentrations 

ranging from 1.9 to 6.3 μg/L. 

Lake

Michie

Little River

Reservoir

Cane Cr.

Reservoir

University

Lake

Lake

Wheeler

Lake

Benson

Lake

Orange

Falls

Lake

Jordan

Lake

10

4

16

27

24

Haw

River

Neuse

River

Basin

Cape Fear

River

Basin 

Flat River

Little River

Eno River

Neuse

River

79° 78°30'

36°

35°45'

Base from digital files of:

U.S. Environmental Protection Agency, River File 3

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

0 5 10 MILES

0 5 10 KILOMETERS

12

14

20

30 29

20

Basin boundary

Lake site and number

EXPLANATION

Figure 8. Sites in the Triangle area of North Carolina with one or 

more chlorophyll a concentration greater than 40 micrograms per liter, 

October 2007 through September 2008 (locations in fig. 1 and table 1).
What’s Next? 11

0.4 mg/L, and 80 percent of the samples had concentrations 

less than 0.1 mg/L. Relatively higher concentrations (greater 

than 0.20 mg/L) were recorded at the Falls Lake Interstate 

85 site and in storm-event samples from Little River, Flat 

River, Ellerbe Creek, Little Lick Creek, and New Hope Creek 

(fig. 9B). Relatively lower concentrations (less than or equal 

to 0.05 mg/L) were recorded at the Eno River at Hillsborough, 

several small lakes (Little River Reservoir, Lake Michie, Lake 

Butner, Lake Wheeler, and Cane Creek Reservoir), the two 

downstream sites in Falls Lake (site 13 and 14), Jordan Lake at 

Bells Landing (site 30), and White Oak Creek (fig. 9B). 

Concentrations of ammonia, nitrite, nitrate plus nitrite, 

and orthophosphorus were less than laboratory reporting levels 

in 64, 49, 42, and 71 percent of samples, respectively. Most 

reportable concentrations were noted in streams throughout 

the year and in lakes during fall through spring. As in 

Figure 9. Dot plots of concentrations of (A) total ammonia plus organic nitrogen and (B) total phosphorus at sites in the Triangle 

Area Water Supply Monitoring Project study area, October 2007 through September 2008 (locations in fig. 1 and table 1).

previous years, lake sites generally had low concentrations 

of these dissolved, inorganic nutrient fractions near the water 

surface during summer months (June and August 2008) 

when nutrients tend to be taken up by phytoplankton. The 

somewhat riverine site 20, Jordan Lake Haw River arm, 

was an exception. Nitrate plus nitrite was above reporting 

levels during all six sampling events, ranging from 0.024 

to 0.976 mg/L (table 2). Bottom-water concentrations of 

ammonia, orthophosphorus, and total phosphorus were 

higher than near-surface concentrations during the summer 

when lakes were thermally stratified, consistent with results 

from previous years (http://wdr.water.usgs.gov/). The highest 

concentrations reported in the study area for nitrate plus 

nitrite (1.53 mg/L), orthophosphorus (0.417 mg/L) and total 

phosphorus (0.62 mg/L) were observed on October 3, 2007, 

at site 10, Falls Lake at U.S. Interstate 85 (table 2).

0 0.5 1.0 1.5

A. Total ammonia plus organic nitrogen (milligrams per liter as nitrogen)

0 0.2 0.4 0.6

B. Total phosphorus (milligrams per liter as phosphorus)

EXPLANATION

Lake site routine sample

Stream site routine sample

Stream site runoff sample

1

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

20

21

22

23

24

27

28

29

30

31

Site

What’s Next?

During 2009, the USGS plans to continue monitoring 

streamflow and water quality at the TAWSMP sites in 

the Upper Neuse and Cape Fear River Basins to include 

upgrading data transmitters at streamgages, which will 

allow hourly updates of critical water-level and streamflow 

data to the Web. The field data-collection phase of a special 

investigation of mercury in water and bed sediment is 

expected to be completed followed by data analysis and 

interpretation. Plans are to continue the analysis of long-term 

trends in selected water-quality constituents.
12 Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008

References

North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, 2007a, North 

Carolina 2006 303(d) list, June 19, 2007: Raleigh, North 

Carolina Division of Water Quality, 125 p., accessed 

May 20, 2009, at: http://h2o.enr.state.nc.us/tmdl/documents/

303d_Report.pdf.

North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, 2007b, Surface 

waters and wetlands standards (May 1, 2007), NC 

Administrative Code 15A NCAC 2B: Raleigh, 

Environmental Management Commission, 134 p., accessed 

May 20, 2009, at: http://h2o.enr.state.nc.us/admin/rules/

codes_statutes.htm. 

Oblinger, C.J., 2004, Triangle Area 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.

State Climate Office of North Carolina, 2008a, Climate 

Division data, accessed July 21, 2009, at 

http://www.nc-climate.ncsu.edu/climate/climdiv.php.

State Climate Office of North Carolina, 2008b, NC CRONOS 

database, accessed November 21, 2008, at 

http://www.nc-climate.ncsu.edu/cronos.
Prepared by

U.S. Geological Survey

Science Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

A PDF version of this publication is available online at

http://pubs.usgs.gov/of/2012/1013/
Giorgino and others—Quality of Surface-Water Supplies in the Triangle Area of North Carolina, Water Year 2008—Open-File Report 2012–1013