GEOLOGICAL SURVEY CIRCULAR 764

Program for Evaluating 

Stream Quality in 

North Carolina

Prepared in cooperation with the 

North Carolina Department of 

Natural and Economic Resources
Program for Evaluating 

Stream Quality in 

North Carolina

By H. B. Wilder and C. E. Simmons

GEOLOGICAL SURVEY CIRCULAR 764

Prepared in cooperation with the 

North Carolina Department of 

Natural and Economic Resources

1978
United States Department of the Interior

CECIL D. ANDRUS, Secretary

Geological Survey

V. E. McKelvey, Director

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South fads Street, Arlington, VA 22202
CONTENTS

Page Page

Abstract .......................................... 1 Natural loads of dissolved and suspended constituents 8

Introduction ...................................... 1 Amount of pollution ............................... 12

Variations in water quality ......................... 2 Trends ............................................ 13

Pollution .......................................... 6 Conclusions and future plans ....................... 15

Natural water quality .............................. 8 References ........................................ 15

ILLUSTRATIONS

Page

FIGURE 1. Map showing locations of sampling stations in North Carolina's water-quality program ................... 2

2. Hydrograph showing flood peaks on Neuse River near Clayton showing water-quality samples collected,

types of analyses, and values of specific conductance and dissolved solids .......................... 3

3. Graph showing variations in specific conductance over two closely-spaced floods, Neuse River near Clayton 4

4. Graph showing relation of specific conductance to discharge for rising, falling, and stable stages of Neuse

River near Clayton, 1974 ....................................................................... 5

5. Hydrographs showing suspended sediment, and total and dissolved lead, Neuse River near Clayton ...... 6

6. Graph showing regressions of lead and manganese with suspended sediment, Neuse River near Clayton .. 7

7. Graph showing current sources of water and transported materials for a flood in an idealized stream .... 8

8. Map showing regionalized dissolved-solids concentrations of natural surface waters during periods of overland runoff and baseflow ....................................................................... 9

9. Diagram showing various proposals for sketching ground-water contribution during a flood .............. 9

10. Graph showing procedure for calculating ground-water contribution from a single flood event, Neuse River

near Clayton .................................................................................. 11

11. Graph showing method of computing ground-water contribution from more than one flood, Neuse River near

Clayton ....................................................................................... 12

12. Graph showing relation between water discharge and specific conductance, Neuse River near Clayton ... 14

13. Graph showing relation between water discharge and dissolved-solids load, Neuse River near Clayton .... 15

TABLES

Pag«

TABLE 1. Statistical summary of selected constituents and the results of regression analyses relating the concentration of constituents to corresponding values of specific conductance, Neuse River near Clayton (1974- 

75 water years)................................................................................ 6

2. Comparison of selected constituents for stations in the Natural-Quality Network with stations in the Primary Network, October 1974-September 1975 .................................................. 13

3. Transport of dissolved matter in Neuse River near Clayton during 1956 and 1974 water years ........... 15

in
ENGLISH-METRIC CONVERSION

[The following table gives the factors used to convert English units to metric or International System (SI).]

Multiply English units by to obtain SI units

feet (ft)

miles (mi)

square feet (ft2 )

square miles (mi2)

feet per mile (ft/mi)

feet per day (ft/d)

cubic feet per second (ft3/s)

cubic feet per second per

square mile (ftVs"1 mi~2) 

cubic feet per day (ft3 /d) 

short ton

tons per day (ton/d) 

tons per cubic feet

(tons/ft3)

0.3048 meters (m)

1.609 kilometers (km)

.0929 square meters (m2)

2.590 square kilometers (km2)

.1894 meters per kilometer (m/km)

.3048 meters per day (m/d)

.02832 cubic meters per second (rn3/s)

.01093 cubic meters per second per

	square kilometer (n^s^km 

.02832 cubic meters per day (m3/d) 

.9072 metric ton (t) 

.9072 metric ton per day (t/d) 

32.03 metric tons per cubic meter 

	(t/m3)

IV
PROGRAM FOR EVALUATING STREAM QUALITY

IN NORTH CAROLINA

By H. B. Wilder and C. E. Simmons

ABSTRACT

The design and objectives of the program for evaluating 

stream quality in North Carolina are described. Using waterquality and streamflow data collected since the 1940's, a study 

is underway to define certain variations in water quality, to 

quantify the effects of man's activities on water quality, and to 

determine long-term trends at key locations on the State's major rivers. Data collected from 47 unpolluted stream sites were 

used to estimate average concentrations for naturally occurring constituents during periods of high and low flow. Methods 

are described for estimating constituent loads derived from 

both natural sources and sources of pollution.

INTRODUCTION

In 1972, regional officials of the U.S. Environmental Protection Agency (then, U.S. Federal 

Water Quality Administration) issued a provisional set of water-quality monitoring guidelines 

to State pollution control officials in the 

Southeastern Region. The North Carolina Department of Natural and Economic Resources invited 

the U.S. Geological Survey to work with them in 

designing a program that would meet the requirements of these Federal guidelines as well as meet 

the combined requirements of the two agencies. A 

joint agency task group extracted the following six 

major objectives from the EPA guidelines and informally designated them as goals for the 

statewide water-quality monitoring program:

1. Determine the suitability of waters of the State 

for specific use.

2. Determine variations in water quality.

3. Determine pollution loads in streams.

4. Determine trends in water quality.

5. Detect contraventions of water-quality standards.

6. Collect data needed for water-quality modeling.

Guidelines from which these goals were extracted have since fallen into disuse, and most of 

these objectives are no longer obvious in the standards. They still, however, represent a worthy set 

of water-quality monitoring goals, and are retained as guiding principles for the cooperative 

program between the U.S. Geological Survey and 

the North Carolina Department of Natural and 

Economic Resources to investigate stream water 

quality. The purpose of this report is to describe 

the philosophy and methodology that are being 

used to approach the goals of the program.

None of the goals are mutually exclusive, and 

data collected primarily for any one of them are 

applicable to most of the others. It is economically 

impossible to collect enough data to accomplish 

every goal at every site at which water-quality 

data must be collected. Therefore, a division of 

responsibility was arranged under which the U.S. 

Geological Survey would make a detailed accounting of water quality at key locations; and the 

North Carolina Department of Natural and 

Economic Resources, with a larger more widely 

distributed staff, would sample a larger statewide 

network designed to furnish information on 

specific sources of pollution. Briefly, pollution is 

defined herein as those changes, both physical 

and chemical, in water quality caused by man's 

activities. Ultimately all data are melded in 

STORET (the U.S. Environmental Protection 

Agency data system), but arrangements were also 

made for prompt local interchange of data when 

desirable.

The final plan called for the U.S. Geological 

Survey to concentrate on studying water-quality 

variability, pollution loads, and trends at key locations on North Carolina's major rivers. These loca-
tions would be selected to provide a general picture of the present status of water quality across 

the State, and to allow the earliest possible prognosis of how water quality might be changing. To 

accomplish these objectives two networks of sampling stations were established:

1. A Primary Network within which presently existing water quality would be studied in 

detail; and

2. A Natural Quality Network intended to deduce 

what water quality would be if man had not 

significantly affected it.

Location of the stations in the two networks, as 

well as the federally-funded National Stream 

Quality Accounting Network (NASQAN) stations 

that make up the rest of the district program, are 

shown in figure 1. Data collection at the NASQAN 

station (Ficke and Hawkinson, 1975) is very similar to that of stations in the Primary Network and, 

for purposes of this discussion, are considered to 

be a part of that network. There are, however, 

some major differences in operational concepts 

between the primary stations and the five NASQAN stations operated in the North Carolina District. Perhaps the major difference is that the 

North Carolina program emphasizes collecting 

samples when conditions warrant rather than 

sampling on some specified time schedule a 

concept familiar in suspended-sediment studies

but little used in chemical-quality investigations. 

An example of the type of coverage considered 

ideal is shown in figure 2. In actual practice, once 

a few floods are defined for a station it is often 

possible to discontinue rigorous coverage of floods, 

and adopt a more convenient sampling procedure 

based on continual evaluations of what kinds of 

data are needed to develop a comprehensive picture of water quality at a particular site. Daily or 

continuous measurements of specific conductance 

are made at all Primary and NASQAN sites. Our 

second departure from the NASQAN concept is 

that most stations in the Primary Network will be 

operated for one or two years, and then rotated on 

a 5 - 10-year schedule. This will allow us to obtain 

coverage at a much greater number of locations.

VARIATIONS IN WATER QUALITY

A major initial objective of the program was to 

gain a better understanding of how water quality 

at each station varies. Since an ultimate objective 

is to periodically account for the transport of as 

many materials as possible in the major rivers of 

the State, it is important to know how materials 

are being transported. Knowing that in freshwater streams most of the varions in materials 

transported are related to variations in 

streamflow, a rather detailed study of flow-related 

changes in quality is a first step in the operations

N

 Natural-quality network sita

 Primary natwork sita

A NASQAN sita

( ) Ntusa River near Clay ton, N.C

0 50 

I

100 

I

150

I

0 50 100 MILES 

FIGURE 1. Sampling stations in North Carolina's water-quality program.

200 KILOMETERS

_J
PER SECOND CUBIC DISCHARGE, IN FEET

X

M

N

T 

(47

1 

\

U

1

\ M !

N

T

(108)

1 

&

M 

N 

Ti

1 D

'/' f,

x/'(8

\ 

\

M

N

T

EXPLANATIC 

imple collectic

ajor inorganic 

utrients 

race metals

ssolved solids 

milligrams per

t-l

50)\ 

\ M

X N (47) 

7) V

>N 

>n

5

r in

liter 

^65) M

A

\

M(55)

N x^^

M X(47) 

N / 

T/

^N

 ""*

X (44)

\

V

\CK

\

\

^

M 

^<^

g 8 * 3§S3 

CONDUCTANCE, MICROMHOS 1C IN SPECIF 

10

AUGUST 1974

FIGURE 2. Flood peaks on Neuse River near Qayton showing water-quality samples collected, types of analyses, and values of

specific conductance and dissolved solids.

of all of our primary stations. Figure 2 shows an 

example of the kind of sampling and analytical 

coverage needed early in the operation of each station. Also shown in figure 2 is a graph of specific 

conductance and values of dissolved-solids for 

each sample. This illustration is based on data 

from Neuse River near Clayton, which will be used 

as an example station throughout this report. It 

may also be good to note here that most of the data 

interpretations thus far has been based on dissolved solids, chiefly because most often a sufficient and reliable historical data base is available 

for this constituent.

Samples from the natural quality stations have 

been analyzed, for a number of inorganic constituents, and nutrients were also determined on 

some of the samples. Although previous experience has indicated little need to sample 

pristine streams such as these for trace metals or 

pesticides, some samples will be analyzed for these 

constituents to provide background information.

To see how gross concentrations of ionized constituents varied throughout the two flood peaks

depicted in figure 2, a plot of the relation between 

specific conductance and stream discharge (fig. 3) 

was made. This plot agrees with the findings of 

Hendrickson and Krieger (1964), as well as 

various other investigators, in that distinctive 

differences often are noticed between the quality 

of water for equivalent discharges on the rising 

and falling stages of floods. These differences 

were not significant, however, for the second of 

the two closely spaced floods, which demonstrates 

the effect antecendent conditions can have on the 

transport of materials by streams. These 

phenomena were investigated further by plotting 

daily values of specific conductance for rising, falling, and stable streamflows for a full year, as 

shown in figure 4. This disclosed that, while higher specific conductances (and therefore dissolved 

solids) do tend to occur on rising stages, there is 

enough intermixing of the points that, should it 

become desirable to estimate a dissolved solids 

value using discharge, there would be little advantage in differentiating between rising and falling 

stages. The goodness of fit of the points about a
10,000

8-7

Q

I

UJ 

CO

cc. Ill

Q.

tL

o

00

D

o

UJ

o 

cc.

1000

o

UJ

I

I

100

EXPLANATION

-->--- Aug 4-6, 1974

 » Aug 6-14, 1974

Average regression line for 1974 water year

50 100 150 200 250 300 

SPECIFIC CONDUCTANCE, IN MICROMHOS PER CENTIMETER AT 25° C

FIGURE 3. Variations in specific conductance over two closely-spaced floods, Neuse River near Clayton.
I Wf WW

Q

z

o

o

LU

CO

<r

LU

Q.

H

LU

LU

LL

O

CO

D

0 1000

z

LU

tr

I

o

CO

Q

DC

LU

|

mn

i

___

"*"" 

_

T 

T

T

A

A

^ T?T

T TA

T A

VT

A T

w ^VA

.J7 A

T YV T

E XPLANATION

A Rising discharge

v Falling discharge

T Peak discharge

J.B

A

V

£* ^ I ^^

TTv

v ^ T

AVixr"^ i

j7 -r 1

^ T^£k A£

r^yC7 A

^Tw" A

%*vr TT

\ A

A V V^A7

w v^ 2^

VwA. v

V

*g A^T ^ y

A ^^L

AVVW

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^ IV V\7

viv T X

^W A

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v v^y A ry A V ' vv

\7^ V

A

A

aseflow

v

v

i 

t;

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A

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t V

v T

^ A

7? T

^7

TT ^ v

»w

V ^

__

__

-

i /i'lH&i

V iii -L . " 

J 1* J

0 50 100 150 200 250

SPECIFIC CONDUCTANCE, IN MICROMHOS PER CENTIMETER AT 25° C

300

FIGURE 4. Relations of specific conductance to discharge for rising, falling, and stable stages of Neuse River near

Clayton, 1974.
line also indicates that it might be possible to 

make a reasonable estimate of annual dissolved 

solids loads using streamflow data alone. This is 

probably true for unpolluted streams, but now, in 

most of the larger streams in North Carolina, the 

relation between conductance and streamflow is 

slowly changing.

Because specific conductance is the only continuous or daily value relative to chemical quality 

that is measured at most of this district's water - 

quality stations, the relationship between it and 

other dissolved constituents is of critical importance if calculations of loads are to be made. Steele 

(1970) has documented computer methodology for 

making such comparisons, and several of the 

same programs were used to compute statistical 

analyses and the correlation coefficients shown in 

table 1. Where significant, these correlations will 

eventually be used to calculate loads of individual 

constituents and investigate their relation to 

economic and population growth.

TABLE 1. Statistical summary of selected constituents and 

the results of regression analyses relating the concentration of 

constituents to corresponding values of specific conductance, 

Neuse River near Clayton (1974-75 water years)

200

Standard 

No. error of 

Constituent samples Range estimate

Water discharge, ftVs 36

Dissolved silica, mg/L 32

Dissolved calcium, mg/L 32

Dissolved magnesium, mg/L 32

Dissolved sodium, mg/L 32

Dissolved potassium, mg/L 32

Bicarbonate, mg/L 32

Dissolved Sulfate, mg/L 32

Dissolved chloride, mg/L 32

Dissolved solids (sum), mg/L32

209- 16,200

4.9-19

3.3-9.6

.9-2.8

2.6-37

1.5-3.4 9-50

6.3-15

2.7-41

34- 163

1.8

.8

.3

2.1

.3

3.6

1.9

2.8

9.2

Correlation 

coefficient

0.64

.86

.89

.76

.96

.83

.96

.49

.94

.98

A subject of increasing interest recently has 

been the transport of trace metals by rivers. 

Wilder (1972) found that arsenic in Sugar Creek 

near Charlotte, N. C., was being transported primarily in conjunction with suspended sediment. 

Similarly, as shown in figure 5, the Neuse River 

also appears to transport lead in conjunction with 

the suspended material. The relations of total 

(suspended plus dissolved) lead and manganese to 

suspended sediment are shown in figure 6. 

Although far from perfect, these relationships 

might be useful in making rough estimates of the 

amount of lead and manganese being transported 

by the stream.

EXPLANATION 

 W»ttr discharge

 o Suspended sediment

 - x- Total lead (dissolved and 

suspended)

 » Dissolved lead

oci2? 

j*

18

,|120 

w z

25

Q W

So 

w 5

08

So 40 

z <

LU UJ 

0. -I

AUGUST 1974

FIGURE 5. Suspended sediment, and total and dissolved 

lead, Neuse River near Clayton.

POLLUTION

Perhaps the most immediate goal of the primary network is to determine the extent to which 

surface waters of the State are already polluted. 

Although few streams are so contaminated that 

they are not suitable, with reasonable treatment, 

for almost any use, preliminary analyses of 

historical data (Wilder and Slack, 1971) indicate 

that man's activities have significantly changed 

their chemical characteristics. The U.S. Geological Survey, in cooperation with various State 

agencies, the most recent of which is the North 

Carolina Department of Natural and Economic 

Resources, began systematic investigations of the 

quality of streams in North Carolina in 1943. 

Other U.S. Geological Survey analyses have been 

available (Ray and Randolph, 1927) as early as 

1906. There are not, however, enough historical 

data to determine directly the chemical character 

of most streams before man became a dominant 

influence on their quality. As an alternative to 

having adequate data for making "before and
10,000 1000 I I I I I I I I

EXPLANATION 

O Manganese

XLead

r Correlation coefficient

100 1000 

SUSPENDED SEDIMENT, IN MILLIGRAMS PER LITER

FIGURE 6. Regressions of lead and manganese with suspended sediment, Neuse River near Clayton.

after" comparisons, some aspects of the degree of 

pollution in streams in North Carolina are being 

estimated using modern data. Current sources of 

water (and thus the sources of dissolved and suspended matter) over an idealized flood on a typical 

stream can be generalized as shown in figure 7.

The placements of the division lines in figure 7 

are, to a degree, arbitrary; and the chief purpose 

of the illustration is to delineate the components 

that control the quality of most streams.

For periods when water quality is constant the 

total dissolved load of any constituent being
EXPLANATION 

GW - Ground water 

OR - Overland runoff 

P - Pollution

TIME

FIGURE 7. Current sources of water and transported 

materials for a flood in an idealized stream.

transported by a stream can be calculated as 

follows:

Lt =kQtCt ,

where Lt = total dissolved load for period, 

k = constant depending on units of

measurement used. 

Qt = total streamflow for period, and 

Ct = concentration of the constituent.

(1)

From figure 7 it follows that:

total 

load

_ overland 

~ runoff 

load (L«r)

ground-water 

' load (L )

" "

and, combining equations 1 and 2,

Lp Lt Lor Lgy,

 k(QtCt QorCor Qgufieu)'

pollution 

+ load (LP)

QpCP), (2)

(3)

Only Q,and Ct are subject to direct measurement. 

If methods can be found to make usefully accurate 

estimates of the components needed to compute

natural loads (Co Qor and pollution

could be separated from the total load being 

transported by a stream by use of equation 3.

NATURAL WATER QUALITY

To determine natural water quality of surface 

waters, streams were sought across the State that 

appeared not to be significantly affected by man's 

activities. More than 200 sites were investigated 

as possible candidates for a natural-water-quality 

network, and 47 were selected as being reasonably

representative of pristine quality from the major 

hydrologic environments of the State. These sites 

have been sampled four times thus far twice at 

baseflow and twice during floods. The baseflow 

samples are assumed to represent the quality of 

ground-water inflow to the streams, and the floodflow samples are assumed to represent the quality 

of overland runoff. These analyses have been 

used, in conjunction with geologic maps, to 

regionalize some aspects of natural quality. As an 

example, a regional map of natural dissolved 

solids is shown in figure 8.

To avoid significant amounts of pollution it was 

necessary to establish most of the natural-quality 

sites on streams with very small drainage areas 

ranging from 0.3 to 17 mi2 (0.8 to 44 km2). A basic 

assumption of the program is that, under similar 

natural conditions, there is no significant 

difference between the quality of water in small 

streams and that in large streams. It is recognized 

that larger streams tend to be more deeply 

entrenched than smaller ones, but this would not 

seem to invalidate this assumption unless ground 

water from an aquifer containing more soluble 

materials is intercepted. The only area in North 

Carolina where this would appear to be a potential 

problem is in part of the Coastal Plain, where 

deeper entrenchment might introduce inflow from 

underlying formations. Admittedly, more study of 

the quality of very shallow ground water and its 

relation to stream quality is needed.

When information from the natural-quality network is transferred to larger drainage areas, 

multiple types of ground waters are often included. For these sites a set of values are calculated by summing the products of the individual 

values and their portion of the drainage area. For 

example, dissolved solids constants for Neuse 

River near Clayton (figure 8) are computed as 

follows:

a)

Rock type Part of basin 

(fig. 8) in rock type

(II) 

Overland

runoff 

constant

Adjusted 

overland

runoff 

constant 

(I) x (II)

(III)

Base

runoff

constant

Adjusted

base

runoff

constant

<I) x (III)

Group B . 

Group C .

0.49

0.51

20mg/L lOmg/L 30mg/L 15mg/L 

20mg/L lOmg/L 50mg/L 26mg/L

New constants 20 mg/L ........ 41 mg/L

NATURAL LOADS OF DISSOLVED AND 

SUSPENDED CONSTITUENTS

To calculate natural loads for streams in which 

the quality of overland runoff and ground-water

8
18

Total dissolved solids, 

in milligrams per liter

Overland runoff 

Baseftow

Mixed metamorphic and igneous rocks 

Granite and gneiss 

Volcanic rocks 

Sandhills 

Sedimentary rocks 

 Drainage area of Neuse River near Clayton

FIGURE 8. Regionalized dissolved-solids concentrations of natural surface waters during periods of overland runoff and

baseflow.

50

I

100

i

150 200 KILOMETERS

I____I

100 150 MILES

inflow are different, as determined by the naturalquality network, the amounts of overland runoff 

(Qor) and ground water runoff (Q^,) must be estimated. Although the influence of pollution on 

water quality is often dominant, the actual volume 

of polluted water entering fairly large streams is 

usually relatively insignificant compared to the 

total flow; and the influent water most often 

comes from the same stream in the first place. 

Therefore, it is usually satisfactory to assume that 

total flow is equal to overland runoff plus groundwater inflow (Q, = Qor+QsJ- To estimate the 

amounts of each, it is necessary to separate the 

streamflow hydrograph into its component parts, 

namely Qor and Q^,.

There are a number of methods proposed in the 

literature for separating hydrographs. Most of 

these involve sketching a somewhat arbitrary 

"ground-water inflow" hydrograph on the total 

streamflow hydrographs and estimating Qor as 

the difference between Qt and Q^ on a daily, or 

shorter period, basis. There are, however, pitfalls 

in these methods. The most obvious pitfall is that 

one never knows exactly where to sketch the 

ground-water inflow line. It is not the purpose of

this report to discuss in detail the various proposals for hydrograph separation; however, a few 

representative suggestions for sketching the Qgw 

hydrograph for a single idealized flood are shown 

in figure 9. It is doubtful if any of these except that 

of Daniel and others (1970) is experimentally 

defensible, but hydrologists can obtain useful 

answers with any of the methods. The answers are

EXPLANATION

vSculz,1974

 Bleasdaleand others, 1963 

° Cable and others, 1971 

o Daniel and others, 1970 O ^ ' 

CC

X

o to

a

0 TIME

FIGURE 9. Diagram of various proposals for sketching 

ground-water contribution during a flood.
not, however, generally closely reproducible from 

hydrologist to hydrologist; thus line-sketching 

methods of hydrograph separation are more of an 

art than a science.

Another pitfall of importance in water -quality 

related separations is that there is no way of accounting for channel storage, which can be significant during the early rising and falling stages of a 

flood, thus making daily or other short-term 

calculations of loads subject to considerable error 

especially during recession.

After evaluating several techniques for separating hydrographs the method adopted is based on 

one developed by Rorabaugh (1964) and Daniel 

(1976), which deals with "events" rather than 

time periods. Rorabaugh, using a heat-flow 

analogy, derived the following equation for 

ground-water discharge into one side of a unit 

length of stream, following a uniform impulse of 

recharge to the entire drainage area:

q = 2T(h0/a)

(4)

where: q = ground- water discharge per unit

length of stream, 

T = transmissivity of aquifer, 

hQ height of recharge impulse, 

a = distance to ground-water divide, 

t = time since recharge impulse

occurred, and 

S = storage coefficient of the aquifer.

By evaluating equation 4 Rorabaugh found that 

when -^- becomes about 0.2, a plot of the 

logarithm of discharge versus time becomes a 

straight line. Glover (1964) showed that this corresponds to the point at which one -half of the 

water from a recharge pulse will have entered a 

stream as ground-water inflow. At this point all of 

the terms in equation 4 except the first one 

become insignificant and the rate of ground-water 

inflow at any time thereafter can be calculated 

with the simpler equation:

(5)

Rorabaugh integrated equation 5 with respect 

to time to obtain the volume (V) of a recharge impulse remaining in storage on one side of a stream

at any time, t, from t ^ -: = to T = oo:

Tt which when applied at a2S = 0.2 is doubled and

converted to base 10 logarithms to compute total 

recharge by the equation:

.933 (6)

2.3

The quantity 0.933 ^ip is the slope of the recession 

described by equation 5. In practice this slope is 

determined empirically by seeking periods on the 

discharge hydrograph when the recession is 

allowed to go undisturbed (by precipitation, 

regulation, etc.) far enough beyond the point at 

which q becomes a straight line on a semilog plot, 

to make a visual determination of its trace. As illustrated in figure 10, this trace can then be extended across one log cycle to determine the slope 

of the recession. (Generally the slope determination is made during the non-growing season to 

eliminate the effects of evapotranspiration). Once 

the slope is known it can be used to calculate both 

.fl£3- and the critical time (t^ at which the reces0 2<z2S sion becomes a straight line (tc = -= fK ), if the

time of the recharge impulse is known. Once both 

the slope of the recession and ^^are known they 

can be substituted into equation 6 along with the 

instantaneous discharge at tc to calculate the 

undrained volume of water from a given recharge 

impulse, which will be one-half of the total volume 

derived from the recharge impulse.

Throughout this discussion it has been assumed 

that the initial head difference, from the aquifer 

to the stream is caused by a recharge impulse that 

instantaneously raises the water table by an 

amount h0 everywhere within the aquifer. Actually, the head at any time is a cumulative result 

of a number of factors that interact to produce an 

effective h0 at the boundary of the stream channel. Among these factors are the amount and 

duration of rainfall, the initial depth to the water 

table, the dimensions of the aquifer, the amount of 

water stored in it; and, perhaps most important, 

the "backwater" effects created when rapidly rising stream levels create a reverse gradient from 

the stream to the aquifer. An additional consequence is that ground water entering the stream 

during any recession is a mixture of water from 

the most recent precipitation with that from an

10
2000

Q

o 1000

o

UJ 

CO

OC

Ul

Q.

500

o

OQ

D 

O

Ul 

CD 

QC 

<

O 

W

Q

100

t2 -ti = 70 days (slope of recession)

0.993a2 S/T = 70 days (equation 6)

therefore, a2 S/T = 75 days

and, tc = 0.2 (75) = 15 days (recession becomes

straight on semilog plot; one-half of

recharge impulse has drained) 

V = 2q (0.933a2 S/T)/2.3 = 2 (340) (70)72.3 

V = 20,700cfs-days

20 25 

FEB

10 15 20 25 

MAR

10 15 20 25 

. APR

10 15 20

MAY

FIGURE 10. Procedure for calculating ground-water contribution from a single flood event, Neuse River near Clayton.

indeterminate number of preceding periods of precipitation. From an operational standpoint this 

creates a problem of when to select time tg, i.e., the 

time at which the recharge impulse necessary for 

the mathematical analyses is assumed to occur. J. 

F. Daniel (oral commun., 1976) suggests that t0 be 

assigned at the point at which the flood hydrograph of the stream being studied reaches a 

peak. For some streams this may be too early and 

for others too late, but the effect on runoff 

volumes should not significantly affect the results 

of this investigation.

Other simplifying assumptions relative to basin 

geometry, aquifer characteristics, and recharge 

distribution that are inherent in the application of 

the theory to real situations are discussed by 

Rorabaugh (1964) and, more extensively, by 

Daniel (1976). In general these assumptions 

become reasonably valid at and beyond the critical 

time, which is where the theory is applied in this 

instance.

A step-by-step example of the use of this

method for estimating the contribution of ground 

water for a period including only a single flood is 

shown in figure 10. The smoothness of the recession curve in this example is marred by upstream 

diversions of water through two municipal water 

systems and its return through two municipal 

waste-treatment plants. The consumptive use of 

these diversions is relatively small and their effect 

on the general trace of the recession is minor.

In working with a single flood, as illustrated in 

figure 10, it is assumed that all ground water 

reaching the stream during the flood, and the 

following flow recession, results from the increase 

in ground-water heads caused by recharge in 

response to the precipitation that produced the 

flood. In humid regions, such as North Carolina, 

the time between recharge periods (and "floods") 

is seldom long enough for the effects of the preceding recharge period to disappear. In fact, most 

recessions do not proceed long enough to assume a 

straight-line shape on hydrographs. Under such 

conditions ground-water discharge resulting from

11
more than one rechrage period must be taken into 

account. When the critical time (tc) is reached, the 

recession becomes linear (on semilog plot) and the 

principle of superposition is valid (Rorabaugh, 

1964, p. 434). For a series of recharge-flood 

periods, the ground-water discharge at any time 

can be calculated as the sum of the incremental 

discharges of previous periods. Similarly, the 

volume of ground-water runoff for several 

recharge periods can be found by summing the 

calculated runoff for each event. Once the slope of 

a "type" recession is determined it is a simple matter to "extend" an abbreviated recession to tc. An 

example of calculating ground-water inflow to 

Neuse River near Clayton for more than one 

recharge pulse is shown in figure 11.

After determining Cgw and Cor from the 

natural-quality network, plus Q and Qor by the

method described above, natural loads (Ln) can be 

calculated as follows:

Ln = 0.0027 (QgwCgw Qor or C) (7)

It should be noted that Qgw and Qor as used in 

equation 7 are totals for an extended period 

(usually one year), and, as such, are actually 

volumes.

AMOUNT OF POLLUTION

Natural loads from equation 7 are subtracted 

from total loads from equation 1 to get an estimate 

of pollution loads. Usually total loads are computed as the sum of daily loads for the period in 

question. In some cases where discharge and concentrations are changing rapidly, however, it may 

be desirable to subdivide a day into shorter

2000

1000

8UJ

oc

UJ 

Q_

O

CO

D 

O

UJ

(D 

OC 

<

I 

O

500

100 vi=2(qt -QA) (0.933a2 S/T)/2.3

= 2(290-155) (70) /2.3

=8220 cfs-days 

V2 = 2(qtc2 -qB ) 70/2.3=2 (340-230) 70/2.3

=6700 cfs-days 

V3 =2(qtc3 -qc) 70/2.3

= 2(350-205) 70/2.3=8830 cfs-days

=8220+6700+8830 

=23,7 50 cfs-days

20 25 5 10 15 20 25 5 10 15 20 25 5 10 15 20

FEB MAR APR MAY

FIGURE 11. Method of computing ground-water contribution from more than one flood, Neuse River near Clayton.

12
periods. Porterfield (1972) discusses how to determine when subdivision is advisable in suspendedsediment studies. His discussion should be helpful 

in studies of materials being transported in solution as well.

Under most conditions, daily concentrations are 

required for adequate computation of total loads. 

The only daily measurements made in this 

program are specific conductance and temperature. Therefore the only constituents for 

which loads can be calculated are those that correlate meaningfully with specific conductance.

The degree to which a stream is polluted may be 

shown in other ways. The simplest is a visual comparison with data from similar environments that 

are not significantly polluted. Table 2 shows such 

comparisons for three streams in the Mountain 

and Piedmont -Coastal Plain provinces of North 

Carolina.

TRENDS

A major emphasis in many modern waterquality programs is the detection of water-quality

trends. Thus far only a few reports have included 

methodology for determining trends on a practical 

level, and these methods have tended to be ambiguous or limited in scope. Statistical methods of 

testing water-quality data for trends have not 

been notably successful for a number of reasons. 

Perhaps the most important reason is that the 

matrix of data in which water-quality trends are 

buried is often so "noisy" that separation of the 

trends themselves is extraordinarily difficult. 

Some progress has been reported by Steele and 

others (1974), but it is still uncertain to what extent trends of one to three percent per year are 

separable from "noise" that can amount to a few 

hundred percent, as seems to be the common case 

in North Carolina.

Theoretical methods are available for compensating for the effects of "noise" produced by 

seasonal changes, serial correlations (not really 

noise), and others. These, however, force assumptions of distribution upon the data that are not 

usually valid for water-quality data other than 

perhaps temperature. Sherwani and Moreau

TABLE 2. Comparison of selected constituents for stations in the Natural-Quality Network with stations in the Primary Network,

October 1974-September 1975

Constituent

Total nitrogen

(mg/L).

Dissolved phosphorous

(mg/L).

Total phytoplankton

(cells/mL).

Fecal coliform

(col./lOOmL).

Biochemical oxygen

demand (mg/L).

Periphyton biomass

ash wt. (g/m2 ).

Periphyton biomass

dry wt. (g/m2 ).

Dissolved organic

carbon (mg/L).

Chromium (/ug/L)

Total .....................

Suspended ................

Dissolved

Bottom ...................

Lead (/u,g/L)

Total .....................

Suspended ................

Dissolved .................

Bottom ...................

Zinc (/ug/L) 

Total .....................

Suspended ................

Dissolved .................

Bottom ...................

Mountain

Cataloochee Creek 

at Cataloochee, N.C. 

(natural-quality)

0.18-1.7

.01 -.02

>260-320

'<10- 2,000

<1-2.0

1.5-3.9

2.3-6.2

3.6-4.9

1- <10

1-<10 0-1

<10

0-58

0-58

0 9

<10-10

5-40

5-40

0

20

province

French Broad River 

at Marshall, N.C. 

(primary)

0.66-4.8

.02 -.09

1700-1900

20-10,000

.9-3.9

3.1-18

4.6-21

3.6-15

<10-90

<10-90

0

<10

4-250

2-250 0-20

<10

30-440

30-430 0-50

40-60

Piedmont -Coasta

Turner Swamp 

near Eureka, N.C. 

(natural-quality)

0.58-1.7

.02 -.05

64-490

>10-3700

.3-3.1

4.6-18

6.2-18

6.0-

<10-10

<10-10

0

<10

4-21

1-21

1-3

<10-10

4-10

0-7

0-10

10-60

1 Plain province

Neuse River near 

Calyton, N.C. 

(primary)

0.33-3.1

.04 -.90

1700- 28000

40- 3000

1.6-6.1

4.6-5.4

6.9-8.5

7.4-12

0-10

0-10

0

10-20

2-130

2-130

0-8

2 <10-250

7-40

0-40

0-20

40

1 Single nontypical value not included. 

' One value in this concentration range.

13
(1975) have suggested that it is possible to avoid 

the effects of serial correlations by increasing the 

time interval between sampling. The main 

problem with the infrequent-sample method, 

however, is that it fails to define the system with 

sufficient accuracy to detect very small systematic 

changes. Sherwani and Moreau also stated (p. 91), 

"If the objective is to detect a certain percentage 

change, say, ten percent, in the mean concentration of a pollutant between one year and the next, 

the error in each of the annual means would have 

to be less than half that magnitude, i.e., 5 percent." An analysis of historical data will quickly 

disclose that this statement imposes severe limitations on the acceptable sampling interval for most 

streams. If it is assumed that an annual mean 

based on daily measurements (of most anything) 

is accurate, which in itself may be open to question, then means based on monthly or even weekly 

samples are not likely to be sufficiently accurate 

to reflect trends of the magnitude mentioned 

earlier (1 to 3 percent). In fact, after experimenting with some of our historical data it appears impractical, if not impossible, to detect trends using 

concentration data alone. Steele and others (1974) 

compensated for the effect of variations in annual 

discharge by using discharge-weighted daily 

values of specific conductance to define trends in a 

number of streams from different parts of the 

United States. His techniques will be evaluated for 

streams in North Carolina.

In North Carolina, and probably most other 

States, it is impossible to detect trends at the end 

of a one- or two-year study unless historical data 

are available for comparison. Therefore, a desirable prerequisite for establishing a primary network station at a site is that a daily sampling station should have been located there in the past. 

Fortunately, during the period 1943 67, daily stations were operated for at least one full year at 82 

nonestuarine sites in North Carolina, which gives 

a large base of background information to compare with present data. Unfortunately, much of 

the information collected in the historical 

program is not the kind that is receiving emphasis 

today. Historical data do, however, provide a basis 

for making some immediate estimates of how fast 

the quality of waters is changing.

Of the several possible methods for determining 

trends in water quality, the most reasonable 

results have been obtained using comparisons of 

water quality with discharge which, to some

degree, compenstate for variations in stream discharge. A quick comparison of whether waterquality changes have taken place over a period of 

time can usually be obtained by plotting specific 

conductance (or the concentration of the desired 

constituent) versus discharge for several widely 

spaced years, as shown in figure 12 for Neuse 

River near Clayton. Statistical tests of significance also need to be conducted in most cases. 

Such a method will also show at what flows the 

greatest concentration changes have occurred.

A second method for detecting changes involves 

plotting dissolved-solids loads versus stream discharge (fig. 13). This method, however, is more 

complex in that it requires the prior computation 

of daily loads. Referring to the two preceding illustrations of the Neuse River, the water quality 

as reflected by dissolved solids has apparently 

deteriorated during low flows since 1956 but has 

improved at extremely high flows. This situation 

might be explained by an increase in pollution 

from point sources, which would greatly affect 

water quality at low flows; whereas, a decrease in 

pollution from non-point sources, such as farming, 

has occurred during high flows and is reflected in 

lower concentrations of constituents in overland 

runoff.

It is also possible to summarize changing conditions at a station in tabular form as illustrated in 

table 3. Notice that gross loads are somewhat

500

LU rr 

O uj

< UJ 

t|

Is

§£

100

10 i i i 1111 i i i 1111

100 1000 10,000 

WATER DISCHARGE, IN CUBIC FEET PER SECOND

FIGURE 12. Relation between water discharge and specific 

conductance, Neuse River near Clayton.

14
1000

100

10

100 1000 10,000 

WATER DISCHARGE, IN CUBIC FEET PER SECOND

FIGURE 13. Relation between water discharge and dissolved-solids load, Neuse River near Clayton.

misleading when making comparisons because, as 

with concentrations, they are dependent upon the 

amount of streamflow, and even over a span of 

several years significant trends can be lost in the 

noise of an unusually wet or dry year. A somewhat 

more realistic means of comparing loads is by converting them to some unit value (for example, 

tons/million ft3), as in table 3, which lessens the 

effects of nonuniform flow conditions because the 

value is determined by dividing the annual load by 

the total annual flow volume.

TABLE 3. Transport of dissolved matter in Neuse River near 

Clayton during 1956 and 1974 water years

Source of 

dissolved 

matter

Average 

dissolved 

solids1 

(mg/L)

Average 

discharge

(ftVs)

Total 

annual 

discharge

(ft3/yr)

Annual 

load2 

(tons/yr)

Unit load 

(tons/ 

million ft3 )

1956

Natural .

Pollution 

Total 75 874 2.76 x 1010

21,100 

28,900 

50,000

0.76 

1.05 

1.81

1974

Natural .

Pollution 

Total 84 1064 3.36 x 1010

25,400 

36,700 

62,100

0.76 

1.09 

1.85

Increase (percent)

Pollution 

Total 12

20 

27 

24

0 

4 

2

1 Computed as unweighted average of measurements.

2 Computed as sum of daily loads.

CONCLUSIONS AND FUTURE PLANS

What is presented here represents our initial 

efforts to design and implement a program that 

will meet both Federal and State needs for a continuing inventory of water-quality conditions in 

North Carolina. No implication is intended that 

this represents the ultimate in the design of 

programs to investigate water quality or that the 

methods of collecting and analyzing the data are a 

model for all to follow. In many respects this article is preliminary because methods of analyzing 

the data are still in a very rudimentary state. For 

example, in North Carolina we have only recently 

begun to investigate chemical constituents other 

than dissolved solids and the major ionic constituents. Socio-economic changes should be evaluated in conjunction with water-quality changes to 

establish a base for making predictions as to when 

certain aspects of water quality might become intolerable if present environmental practices are 

continued. Techniques used for regionalization of 

factors involved in natural quality and for separation of hydrographs must be refined and standardized.

REFERENCES

Bleasdale, A., Boulton, A. G., Ineson, J., and Law, F., 1963, 

Study and assessment of water resources: Conservation of 

Water Resources, Inst. Civil Engineers Proc., p. 121-136.

Cable, L. W., Daniel, J. F., Wolf, R. J., and Tate, C. H., 1971, 

Water resources of the Upper White River basin, eastcentral Indiana: U.S. Geol. Survey Water-Supply Paper 

1999-C, p. G13.

Daniel, J. F., 1976, Estimating evapotranspiration from 

streamflow records: Water Resources Research, June 

1976, p. 360-364.

Daniel, J. F., Cable, L. W., and Wolf, R. J., 1970, Ground

 water-surface water relation during periods of overland

How: U.S. Geol. Survey Prof. Paper 700 -B, p. B219 -B223.

Ficke, J. F., and Hawkinson, R. O., 1975, The National Stream 

Quality Accounting Network (NASQAN) Some questions and answers: U.S. Geol. Survey Circ. 719, 23 p.

Glover, R. E., 1964, Ground-water movement: U.S. Bur. 

Reclamation Eng. Mon. 31.

Hendrickson, G. E., and Krieger, R. A., 1964, Geochemistry of 

natural waters in the Blue Grass region, Kentucky: U.S. 

Geol. Survey Water-Supply Paper 1700, 132 p.

Porterfield, George, 1972, Computations of fluvial sediment 

discharge: U.S. Geol. Survey Techniques Water-Resources 

Inv., book 3, chap. C3, 66 p.

Ray, C. E., Jr., and Randolph, E. E., 1927, Preliminary report 

on the chemical quality of surface waters of North 

Carolina relative to industrial use: North Carolina Dept. 

of Conserv. and Devel., Econ. Paper 61.

15
Rorabaugh, M. I., 1964, Estimating changes in bank storage Steele, T. D., Gilroy, E. J., and Hawkinson, R. O., 1974, An

and ground-water contribution to streamflow: Internat. assessment of areal and temporal variations in

Assoc. Sci. Hydrology Pub. 63, p. 432-441. streamflow quality using selected data from the National

Sculz, E. F., 1974, Problems in applied hydrology: Water Stream Quality Accounting Network: U.S. Geol. Survey

Resources Pub., Fort Collins, Colo., p. 315. open-file report 74-217.

Sherwani, I. K., and Moreau, T. H., 1975, Strategies for water Wilder, H. B., 1972, Investigation of the occurrence and

quality monitoring: Water Resources Research Inst. of the transport of arsenic in the upper Sugar Creek Watershed,

Univ. of North Carolina, Kept. No. 107, 137 p. Charlotte, North Carolina: U.S. Geol. Survey Prof. Paper

Steele, T. D., 1970, Beneficial uses and pitfalls of historical 800 -D, p. D205-D210.

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16

*U.S. Government Printing Office: 1978-777-102/19