1

By John R. Gray, G. Douglas Glysson, Lisa M. Turcios, and Gregory E. Schwarz

Water-Resources Investigations Report 00-4191

Comparability of

Suspended-Sediment

Concentration and

Total Suspended Solids Data

U.S. Department of the Interior

U.S. Geological Survey

WRIR 00-4191

August 2000

science for a changing world
2
3

By John R. Gray, G. Douglas Glysson, Lisa M. Turcios, and Gregory E. Schwarz

U. S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00-4191

Reston, Virginia 2000

COMPARABILITY OF

SUSPENDED-SEDIMENT CONCENTRATION

AND TOTAL SUSPENDED SOLIDS DATA
4

U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Department of the Interior

Charles G. Groat, Director

The use of firm, trade, or brand names in this report is for identification purposes only and

does not constitute endorsement by the U.S. Geological Survey

For additional information write to:

U.S. Geological Survey

Chief, Office of Surface Water

Mail Stop 415

12201 Sunrise Valley Drive

Reston, VA 20192

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286, Mail Stop 417

Denver Federal Center

Denver, CO 80225-0286
5

CONTENTS

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

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

Field Techniques and Laboratory Methods .................................................................................................................................. 2

 Field Techniques................................................................................................................................................................... 2

 Laboratory Methods ............................................................................................................................................................. 3

 Suspended-Sediment Concentration Analytical Method ................................................................................................ 3

 Total Suspended Solids Analytical Method .................................................................................................................... 3

 Differences Between the Suspended-Sediment Concentration and

 Total Suspended Solids Analytical Methods ............................................................................................................. 4

Description of Data Used in the Evaluation ............................................................................................................................... 5

 Arizona ................................................................................................................................................................................ 5

 Hawaii .................................................................................................................................................................................. 5

 Illinois.................................................................................................................................................................................. 5

 Kentucky .............................................................................................................................................................................. 5

 Maryland.............................................................................................................................................................................. 5

 Virginia ................................................................................................................................................................................ 6

 Washington .......................................................................................................................................................................... 6

 Wisconsin ............................................................................................................................................................................ 6

 Quality-Control Data ........................................................................................................................................................... 6

Comparability of Suspended-Sediment Concentration and Total Suspended Solids Data.......................................................... 6

 Natural-Water Data .............................................................................................................................................................. 6

 Quality-Control Data .......................................................................................................................................................... 10

Conclusions ................................................................................................................................................................................ 11

References Cited ......................................................................................................................................................................... 12

List of Tables:

1. State in which natural-water samples were collected, collecting organization, collection methods,

 and devices for obtaining subsamples for suspended-sediment concentration and total suspended

 solids analyses ......................................................................................................................................................................... 2

2. Statistical characteristics of paired suspended-sediment concentrations (SSC) and total suspended

 solids (TSS) data for each of eight States, and for the combined data from all States........................................................... 7

List of Figures:

1. Bar graph showing number of paired suspended-sediment concentration values and total suspended

 solids values of the 3,235 data pairs for selected suspended-sediment concentration ranges................................................ 6

2. Scatter plot showing relation between untransformed values of suspended-sediment concentration

 and total suspended solids for 3,235 data points ................................................................................................................... 7

3. Scatter plot showing relation between the base-10 logarithms of suspended-sediment concentration

 and total suspended solids for 3,235 data pairs in the scattergrams plotted .......................................................................... 8

4. Scatter plots showing relation between the base-10 logarithms of suspended-sediment concentration

 and total suspended solids for the data pairs from each State used in the analysis ............................................................... 9

5. Scatter plot showing relation between percent sand-size material in the sample analyzed for

 suspended-sediment concentration and the remainder of suspended-sediment concentration minus

 total suspended solids ............................................................................................................................................................ 10

6. Scatter plot showing relation between total suspended solids and the concentration of suspended

 sediments finer than 0.062 mm in paired suspended-sediment concentration samples ........................................................ 10

7. Graph showing instantaneous water discharges and sediment discharges computed from total

 suspended solids and suspended-sediment concentration data for a stream in the northeastern

 United States, 1998 ................................................................................................................................................................ 11

8. Boxplot showing variability in results of suspended-sediment concentrations and total suspended

 solids analytical methods in quality-control water samples analyzed by a cooperator laboratory........................................ 11
vi6

1

This conversion is true for concentration values <8,000 mg/L. The equivalent value in mg/L for 

concentrations ≥8,000 ppm can be calculated from table 1, American Society of Testing Material (2000), or by 

using the following equation:

where:

Cmg/L= sediment concentration, mg/L, and

Cppm= sediment concentration, ppm

Multiply SI units By

Length

Volume

Temperature

Flow

Concentration (Mass/Volume)

liter (L)

liter (L)

liter (L)

liter (L)

33.82

 2.113

 1.057

 0.2642

ounce fluid (fl. oz)

pint (pt)

quart (qt)

gallon (gal)

degree Celsius (oC) F = 1.8 x°C + 32 degree Fahrenheit (oF)

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

0.03937 inch (in)

1.0

0.0000334

millimeter (mm)

parts per million (ppm1)

ounces per quart (oz/qt)

milligrams per liter (mg/L)

milligrams per liter (mg/L)

Mass

0.03527

0.002205

1.102

ounce, avoirdupois (oz)

ounce, avoirdupois (oz)

ton, short

gram (g)

gram (g)

megagram (Mg)

To obtain inch-pound units

Cmg/L= Cppm/(1-Cppm(6.22 x 10-7)

CONVERSION FACTORS
1

Comparability of Suspended-Sediment

Concentration and Total Suspended Solids Data

By John R. Gray, G. Douglas Glysson, Lisa M. Turcios, and Gregory E. Schwarz

ABSTRACT

 Two laboratory analytical methods — suspended-sediment concentration (SSC) and total suspended solids (TSS)

— are predominantly used to quantify concentrations of

suspended solid-phase material in surface waters of the

United States. The analytical methods differ. SSC data are

produced by measuring the dry weight of all the sediment

from a known volume of a water-sediment mixture. TSS

data are produced by several methods, most of which entail

measuring the dry weight of sediment from a known volume of a subsample of the original. An evaluation of 3,235

paired SSC and TSS data, of which 860 SSC values include

percentages of sand-size material, shows bias in the relation

between SSC and TSS —SSC values tend to increase at a

greater rate than their corresponding paired TSS values. As

sand-size material in samples exceeds about a quarter of the

sediment dry weight, SSC values tend to exceed their corresponding paired TSS values. TSS analyses of three sets of

quality-control samples (35 samples) showed unexpectedly

small sediment recoveries and relatively large variances in

the TSS data. Two quality-control data sets (18 samples)

that were analyzed for SSC showed both slightly deficient

sediment recoveries, and variances that are characteristic of

most other quality-control data compiled as part of the U.S.

Geological Survey’s National Sediment Laboratory Quality

Assurance Program. The method for determining TSS,

which was originally designed for analyses of wastewater

samples, is shown to be fundamentally unreliable for the

analysis of natural-water samples. In contrast, the method

for determining SSC produces relatively reliable results for

samples of natural water, regardless of the amount or percentage of sand-size material in the samples. SSC and TSS

data collected from natural water are not comparable and

should not be used interchangeably. The accuracy and

comparability of suspended solid-phase concentrations of

the Nation’s natural waters would be greatly enhanced if all

these data were produced by the SSC analytical method.

INTRODUCTION

The importance of fluvial sediment to the quality of

aquatic and riparian systems is well established. The U.S.

Environmental Protection Agency (1998) identifies sediment as the single most widespread cause of impairment of

the Nation’s rivers and streams, lakes, reservoirs, ponds,

and estuaries.

Reliable, quality-assured sediment and ancillary data

are the underpinnings for assessment and remediation of

sediment-impaired waters. The U.S. Geological Survey

(USGS) has protocols for the collection of sediment data

(Edwards and Glysson, 1999) and for laboratory analysis

of suspended-sediment samples (Guy, 1969; Matthes and

others, 1991; Knott and others, 1992 and 1993; U.S. Geological Survey, 1998 and 1999a). Most of the laboratory

analytical methods were adapted or developed by the

Federal Interagency Sedimentation Project (1941), approved by the Technical Committee (Glysson and Gray,

1997), and used by most Federal agencies that analyze

fluvial-sediment data.

Data collected, processed, and analyzed using consistent protocols are comparable in time and space. Conversely, data obtained using different protocols may not

be comparable. The focus of this study is the comparability of suspended-sediment concentration (SSC) and total suspended solids (TSS) data. The terms SSC and TSS

are often used interchangeably in the literature to describe the concentration of solid-phase material suspended in a water-sediment mixture, usually expressed in

milligrams per liter (mg/L) (Gregory Granato, U.S. Geological Survey, oral commun., 1999; James, 1999). However, given that all other factors are held constant (such as

particle density and shape), the analytical procedures for

SSC and TSS differ and may produce considerably different results, particularly when sand-size material composes a substantial percentage of the sediment in the

sample.

This report compares the SSC and TSS analytical

methods and derivative data, and demonstrates which of

the data types is the more accurate and reliable. The

evaluation is based on historical SSC and TSS data

collected and analyzed by the USGS and selected cooperators.

The authors appreciate the assistance of: Stephen S.

Anthony, Donna L. Belval, James G. Brown, Ronald D.

Evaldi, Herbert S. Garn, John D. Gordon, Stephen D.

Preston, Daniel J. Sullivan, Richard J. Wagner and Henry

Zajd, Jr. for providing the data used in this report. The

formal reviews of Herbert S. Garn, Mary Ellen Ley, and

Henry Zajd, Jr., were most appreciated, as were informal

reviews by Anne Hoos and Harvey Jobson. Kenneth

Pearsall’s insights and research significantly enhanced

the report. Patricia Greene’s and Roger K. Chang’s support for developing the tables and figures was invaluable.
2

FIELD TECHNIQUES AND LABORATORY METHODS

The paired SSC and TSS results used in this evaluation

were derived from analyses of natural-water samples collected by the USGS and selected cooperators (table 1).

Analyses of all SSC data from natural water were made by

USGS sediment laboratories, and analyses of the TSS data

were made by USGS and cooperating laboratories. Additionally, 53 quality-control samples were prepared by the

USGS and analyzed by a laboratory that provides data to

the USGS.

Field Techniques

The large majority of water samples were collected using

either the equal-width-increment or the equal-discharge-increment method to obtain a composite sample that is representative of the discharge-weighted SSC (Edwards and Glysson,

1999). Some samples, including those obtained by at least one

cooperating agency, were collected by dipping an open bottle

to obtain samples for subsequent TSS analysis. Some of the

paired SSC and TSS samples were collected in-stream sequentially and submitted to laboratories for analysis as whole

samples. The remaining samples were split into subsamples

by using a churn splitter or cone splitter (Ward and Haar, 1990;

Capel and Larson, 1996; Capel and others, 1995).

Table 1. State in which natural-water samples were collected, collecting organization, collection methods,

and devices for obtaining subsamples for suspended-sediment concentration (parameter code 80154) and

total suspended solids (parameter code 00530) analyses

[SSC, suspended-sediment concentration; TSS, total suspended solids; USGS, U.S. Geological Survey]

SSC

(80154)

TSS

(00530)

SSC

(80154) 

TSS

(00530)

SSC

(80154)

TSS

(00530)

Arizonaa USGS USGS USGS, 1999i USGS, 1999i Churn Splitter Churn Splitter

Hawaiib USGS USGS Automatic Sampler Automatic Sampler None Churn Splitter

Illinoisc USGS USGS USGS i ,1999 ; USGS, 1999i Churn Splitter Churn Splitter

Kentuckyd USGS USGS

Open Bottle

Open Bottle None None

Marylande USGS USGS Open Bottle 

USGS, 1999i

; 

 Automatic Sampler

USGS, 1999i

; 

Automatic Sampler Churn Splitter Churn Splitter

Virginiaf USGS and 

Cooperator

USGS and 

Cooperator USGS, 1999i USGS, 1999i None Churn Splitter

Washingtong USGS USGS USGS, 1999i USGS, 1999i None Churn Splitter

Wisconsinh USGS Cooperator USGS, 1999i Open Bottle Cone Splitter Cone Splitter

Sample 

State Collecting Organization Sample Collection Method Subsampling Device

James G. Brown, U.S. Geological Survey, written commun. (1999).

Stephen S. Anthony, U.S. Geological Survey, written commun. (1999).

Daniel J. Sullivan, U.S. Geological Survey, written commun. (1999).

Ronald D. Evaldi, U.S. Geological Survey, written commun. (1999).

Stephen D. Preston, U.S. Geological Survey, written commun. (1999).

USGS

a

b

c

d

e

Donna L. Belval, U.S. Geological Survey, written commun. (1999).

Richard J. Wagner, U.S. Geological Survey, written commun. (1999).

Herbert S. Garn, U.S. Geological Survey, written commun. (1999).

See Edwards and Glysson (1999). 

f

g

h

i

Tests performed by the USGS demonstrate that the

churn splitter and cone splitter can provide unbiased and acceptably precise (generally within 10 percent of the known

value) SSC values as large as about 1,000 mg/L when the

mean diameter of sediment particles is less than about 0.25

mm. At SSC values of 10,000 mg/L or more, the bias and

precision of SSC values in churn splitter subsamples are considered unacceptable (U.S. Geological Survey, 1997; Wilde

and others, 1999).

Cone splitters produce subsamples with SSC values that

are adequately representative of the original sample at 10,000

mg/L, but not at 100,000 mg/L. The accuracy of the cone

splitter for SSC values between 10,000 mg/L and 100,000

mg/L is unknown and is considered unacceptable at concentrations larger than 100,000 mg/L (U.S. Geological Survey,

1997; Wilde and others, 1999).

Subsampling will typically increase the variance and (or)

create bias in the concentration and size distribution of solidphase material in a subsample. Significant differences in the

amount of solid-phase material in some paired samples may

have occurred as a result of non-representative splitting of

the original samples, or by collecting consecutive in-stream

samples under conditions of rapidly varying SSC. Similarly,

because the data were obtained by field personnel in eight

States as part of unrelated studies, significant differences
3

may have resulted because of differences in data-collection

techniques. However, the probability of significant bias resulting from consistently selecting samples with larger concentrations of sediment for analyses by one of the methods

would be small based on the large number of paired data

used in the analysis. There is no evidence indicating that

methods used for collecting, processing, or selecting

subsamples for subsequent analysis introduced bias in the relations between SSC and TSS identified in this evaluation.

Laboratory Methods

Two standard methods are widely cited in the United

States for determining the total amount of suspended material in a water sample. These are:

1. Method D 3977-97, “Standard Test Method for Determining Sediment Concentration in Water Samples” of the

American Society for Testing and Materials (American

Society for Testing and Materials, 2000), and

2. Method 2540 D, “Total Suspended Solids Dried at 103°–

105° C” (American Public Health Association, American

Water Works Association, and Water Pollution Control

Federation, 1995).

The differences in these analytical methods, and some

variations used to produce TSS data are described below.

Suspended-Sediment Concentration Analytical

Method. ASTM Standard Test Method D 3977-97 lists three

methods that result in a determination of SSC values in water

and wastewater samples:

1. Test Method A – Evaporation: The evaporation method

may only be used on sediment that settles within the allotted storage time, which can range from a few days to several months. If the dissolved-solids concentration exceeds

about 10 percent of the SSC value, an appropriate correction factor must be applied to the SSC value. The precision and bias of Method A are shown as follows:

2. Test Method B- Filtration: The filtration method is used

only on samples with concentrations of sand-size material

(diameters greater than 0.062 mm) less than about

10,000 mg/L and concentrations of clay-size material of

about 200 mg/L. No dissolved-solids correction is needed.

The precision and bias of Method B are shown as follows:

 3. Test Method C - Wet-sieving filtration: The wet-sievefiltration method also yields a SSC value, but the method

is not as direct as Methods A and B. Method C is used if

the percentage of material larger than sand-size particles is

desired. The method yields a concentration for the total

sample, a concentration of the sand-size particles, and a

concentration for the silt- and clay-size particles. A dissolved-solids correction may be needed, depending on the

type of analysis done on the fine fraction of the samples

and the dissolved-solids concentration of the sample. The

precision and bias of Method C are shown as follows:

These three methods are virtually the same as those used

by USGS sediment laboratories and described by Guy

(1969). Only the Whatman grade 934AH, 24-mm-diameter

filter is used for purposes of standardization. Each method

includes retaining, drying at 103°C ±2°C, and weighing all of

the sediment in a known mass of a water-sediment mixture

(U.S. Geological Survey, 1999a).

Total Suspended Solids Analytical Method. According

to the American Public Health Association, American Water

Works Association, and Water Pollution Control Federation

(1995), the TSS analytical method uses a predetermined

volume from the original water sample obtained while the

sample is being mixed with a magnetic stirrer. An aliquot of

the sample — usually 0.1 L, but a smaller volume if more

than 200 mg of residue may collect on the filter — is withdrawn by pipette. The aliquot is passed through a filter, the

diameter of which usually ranges from 22 to 125 mm. The

filter may be a Whatman grade 934AH, Gelman type A/E,

Millipore type AP40; E-D Scientific Specialties grade 161, or

another product that gives demonstrably equivalent results.

After filtering, the filter and contents are removed and dried

at 103° to 105° C, and weighed. No dissolved-solids

correction is required. The percentages of sand-size and finer

material cannot be determined using the TSS method.

The American Public Health Association, American

Water Works Association, and Water Pollution Control Federation (1995) describe the precision for this method as follows: “The standard deviation was 5.2 mg/L (coefficient of

variation 33 percent) at 15 mg/L, 24 mg/L (10 percent) at 242

mg/L, and 13 mg/L (0.76 percent) at 1,707 mg/L in studies

by two analysts of four sets of 10 determinations each.

Single-laboratory analyses of 50 samples of water and wastewater were made with a standard deviation of differences of

Concentration

Added,

(mg/L)

Concentration

Recovered,

(mg/L)

Standard

Deviation of

Test Method

(mg/L)

Standard

Deviation of

Single Operator

(mg/L)

Bias, 

percent

10

1,000

100,000

 9.4

976 

100,294 

 2.5

36.8

532 

 2.3

15.9

360 

-6 

-2.4

0.3

Concentration

Added,

(mg/L)

Concentration

Recovered,

(mg/L)

Standard

Deviation of

Test Method

(mg/L)

Standard

Deviation of

Single Operator

(mg/L)

Bias, 

percent

10

100

1,000

8

91

961

 2.6

5.3

20.4

 2 

5.1

14.1

-20 

-9 

-3.9

Mixture

Number

Sieve

Diameter

(mm)

Concentration

Added

(mg/L)

Concentration

Recovered

(mg/L)

Bias, 

percent

1

1

2

2

3

3

>0.062

<0.062

>0.062

<0.062

>0.062

<0.062

 1

10

9

91

91

909

 3.4

8.7

5 

79 

107 

832 

240

-13

-44

-13

18

-8

Standard

Deviation

of Test

Method

(mg/L)

Standard

Deviation

of Single 

Operator

(mg/L)

 2.8

4.3

5.9

15.2

12.3

87.2

 2.4

2.9

1.9

11 

5.9

61 

[ mg/L, milligrams per liter]

[ mg/L, milligrams per liter]

[mm, millimeters; mg/L, milligrams per liter]
4

2.8 mg/L.” The standard provides no indication of the size of

particles used in the testing for the method.

In practice, TSS data are produced by a number of variations to the processing methods described in the American

Public Health Association, American Water Works Association, and Water Pollution Control Federation (1995). For example:

• For the collection of TSS samples as part of the Chesapeake Bay Program, field staff pump water from a specified depth into a plastic gallon container. The container is

vigorously shaken, and 0.2 – 1.0 L of the water-sediment

mixture is poured for field filtering and subsequent analysis. (Mary Ley, Interstate Commission on the Potomac

River Basin, the State of Maryland and the Commonwealth

of Virginia, written commun., 2000).

• One State government laboratory produces TSS data by

vigorously shaking the sample and pouring it into a crucible for subsequent analysis. All of the sample is poured

into the crucible unless “there is a lot of suspended material,” in which case only part of the sample is poured (Lori

Sprague, U.S. Geological Survey, written commun., 1999).

• Another laboratory analyzed quality-control samples by

using Method 2540D of the American Public Health Association, American Water Works Association, and Water

Pollution Control Federation (1995), with the following

variation: The sample is shaken vigorously and a third of

the desired subsample volume is decanted to a secondary

vessel. This process is repeated twice to obtain a single

subsample for subsequent filtration, drying and weighing.

The reduction in TSS data comparability is not limited to

lack of consistency in processing and analytical methods.

According to James (1999), there is generally no agreed

upon definition of TSS in regard to storm-water runoff, in

part because the settleable part of TSS is not reported in

most storm-water studies.

The problem extends to nomenclature. The terms “SSC”

and “TSS”, or variations thereof, are sometimes attributed to

an incorrect data type. For example, a proposed Total Maximum Daily Load for sediment in Stekoa Creek, Georgia

(U.S. Environmental Protection Agency, Region 4, written

commun., 2000) is based on regional TSS data, which are

compiled from U.S. Geological Survey records; the TSS data

referred to are actually SSC data. Buchanan and Schoellhamer (1998) refer to “suspended-solids concentration data”

for San Francisco Bay. Those data would more appropriately

be referred to as SSC, because the total water-sediment mass

and all sediment were measured in the analysis (Alan

Mlodnosky, USGS, oral commun., 1999).

Part of the problem may be attributable to the origin of

the TSS method and subsequent changes in the types of water for which it is recommended for use. Information available from the American Public Health Association and

American Water Works Association (1946) makes it clear

that the Suspended Solids Method was intended for use for

wastewater effluents (Kenneth Pearsall, U.S. Geological Survey, written commun., 2000). This is more or less consistent

with the Total Suspended Matter Method, which was “intended for use with wastewaters, effluents, and polluted

waters,” as listed in the American Public Health Association,

American Water Works Association, and Water Pollution

Control Federation (1971). A fundamental change took place

in 1976, when the Total Suspended Matter Method was

deemed suitable for “residue in potable, surface, and saline

waters, as well as domestic and industrial wastewaters in the

range up to 20,000 mg/L” by the American Public Health Association, American Water Works Association, and Water Pollution Control Federation (1976). The Suspended Solids and

Total Suspended Matter Methods described above are predecessors of the “Total Suspended Solids Dried at 103°-105°C”

Method, which first appeared in 1985 by that title in the

American Public Health Association, American Water Works

Association, and Water Pollution Control Federation (1985).

In summary, the evidence indicates that the TSS method

was originally designed for wastewater analyses, presumably on

samples collected after a settling step at a wastewater treatment

facility (hence the term “suspended” in TSS). The American

Public Health Association, American Water Works Association,

and Water Pollution Control Federation (1976) expanded the

TSS Method’s applicability in 1976 to include natural water.

Differences Between the SSC and TSS Analytical

Methods. The fundamental difference between the SSC and

TSS analytical methods stems from preparation of the

sample for subsequent filtering, drying, and weighing. A

TSS analysis normally entails withdrawal of an aliquot of the

original sample for subsequent analysis, although as previously

noted, there is evidence of inconsistencies in methods used

in the sample preparation phase of the TSS analyses. The

SSC analytical method measures all sediment and the mass

of the entire water-sediment mixture. Additionally, the percentage of sand-size and finer material can be determined as

part of the SSC method, but not as part of the TSS method.

If a sample contains a substantial percentage of sandsize material, then stirring, shaking, or otherwise agitating

the sample before obtaining a subsample will rarely produce

an aliquot representative of the SSC and particle-size distribution of the original sample. This is a by-product of the

rapid settling properties of sand-size material, compared to

those for silt- and clay-size material, given virtually uniform

densities and shapes as described by Stokes’ Law. Aliquots

obtained by pipette might be withdrawn from the lower part

of the sample where the sand concentration tends to be enriched immediately after agitation, or from a higher part of

the sample where the sand concentration is rapidly depleted.

The physical characteristics of a pipette used to withdraw an aliquot, or subsample, can introduce additional errors in subsequent analytical results. The American Public

Health Association, American Water Works Association, and

Water Pollution Control Federation (1995) specifies use of

“wide-bore pipettes” to withdraw aliquots. The tip opening

of those recommended for use is about 3 mm in diameter

(Kimble-Contes Inc., accessed May 1, 2000). By definition,

the upper limit of sand-size material, which is expressed as

the median diameter, is 2 mm (Folk, 1980). A natural sediment particle’s long axis is almost always larger than its me-
5

dian axis and can be substantially larger. Hence, a single

coarse-grained sand particle or multiple sand-size particles,

particularly when present in large concentrations, may clog

a 3-mm tip pipette under suction.

If the aforementioned lack of consistency in the TSS

analytical procedure extends to variability in diameters of

pipette tips used to withdraw TSS aliquots, the size of particles being excluded from the subsample could vary with

the type of pipette used. Hence, use of a pipette may cause

concentration bias when subsampling if sand-size material

is present in the sample.

Based on Stokes’ Law, subsamples obtained by pouring

sand-rich water-sediment mixtures should be deficient in

sand-size material. Because the fine material concentration

will not normally be altered by the removal of an aliquot,

the differences between the two methods will tend to be

more pronounced as the percentage of sand-size material

 in the sample increases.

Samples collected sequentially in-stream may have different concentrations and size characteristics of solid-phase

material. This may be due to natural variations in the

amounts and composition of solid-phase material in transport, and to variance and (or) bias that is introduced by

sampling procedures. Likewise, a subsample may contain

an amount and size distribution of sediment atypical to that

of the original. However, any differences in SSC and sizedistribution data from paired samples resulting from instream variations or sampling procedures would likely occur

randomly among the 3,235 paired analyses used in this

evaluation.

DESCRIPTION OF DATA USED IN THE EVALUATION

Results of analyses of natural-water samples and of

quality-control samples prepared by the USGS were used

for this evaluation. Natural-water samples for determination

of SSC (parameter code 80154) were collected and analyzed by the USGS (table 1). Natural-water samples for determination of TSS, (parameter code 00530) were collected

by the USGS and cooperating agencies, and analyzed by the

USGS and cooperating laboratories. A total of 3,235 pairs

of SSC and TSS data for natural water were obtained from

the files of USGS District offices.

The paired SSC and TSS data were collected at 65 sampling sites in Arizona, Hawaii, Illinois, Kentucky, Maryland,

Virginia, Washington, and Wisconsin. All but the 12 sampling sites in Kentucky were at USGS streamflow-gaging

stations. The percentage of sand-size material was available

for 860, or about 27 percent, of the SSC samples. The SSC

and TSS natural-water data used in this evaluation were

augmented by analytical results of 53 quality-control

samples prepared by the USGS National Sediment Laboratory Quality Assurance Program (Gordon and others, 2000,

U.S. Geological Survey, 1998; 1999a; 1999b; 2000b).

Arizona. A total of 122 SSC and TSS sample pairs

were collected at a USGS streamflow-gaging station on

Pinal Creek at Inspiration Dam near Globe (station number

09498400) in central Arizona from 1982-98. The samples

were collected about monthly or bimonthly using techniques

described by Edwards and Glysson (1999). A churn splitter

was used to obtain subsamples of the water-sediment mixture. The USGS sediment laboratory in Iowa City, Iowa,

analyzed the subsamples for SSC and TSS (James G.

Brown, U.S. Geological Survey, written commun., 1999).

Hawaii. According to Hill (1996), 13 SSC and TSS

sample pairs were collected at three streamflow-gaging stations in the Kamooalii drainage basin, Oahu, Hawaii, from

1985-89, as a component of a large-scale highway-construction study. The SSC samples were collected by a US PS-69

automatic pumping sampler. The TSS samples were collected by a Manning automatic pumping sampler. A churn

splitter was used to obtain subsamples for TSS analyses.

The SSC samples were analyzed by the USGS sediment

laboratory in Oahu. The TSS samples were analyzed by the

USGS National Water Quality Laboratory in Denver, Colorado (Stephen S. Anthony, U.S. Geological Survey, written

commun., 1999).

Illinois. A total of 223 SSC and TSS sample pairs were

collected at 8 USGS streamflow-gaging stations in the upper

Illinois River Basin from 1988-90 (Sullivan and Blanchard,

1994). Samples were collected according to techniques described by Edwards and Glysson (1999). A churn splitter

was used to obtain subsamples for SSC and TSS analyses.

SSC samples were analyzed at the USGS sediment laboratory in Iowa City, Iowa, using the evaporation method. TSS

samples were analyzed by an Illinois State laboratory using

the nonfilterable residue, gravimetric method (Daniel

Sullivan, U.S. Geological Survey, written commun., 1999).

Kentucky. A total of 95 SSC and TSS sample pairs were

collected at 12 sampling locations in the Ohio River Basin in

May 1999. SSC and TSS samples were collected at each site

for one day over several hours at about 1-hour intervals.

Samples were collected using an open-bottle sampler because of the low stream velocities. No splitting devices were

used to obtain subsamples. The USGS sediment laboratory

in Louisville, Kentucky, analyzed the SSC samples. A contract laboratory performed the TSS analyses (Ronald Evaldi,

U.S. Geological Survey, written commun., 1999).

Maryland. A total of 1,561 SSC and TSS sample pairs

were collected at 6 streamflow-gaging stations in the

Patuxent River Basin, Maryland, as part of the USGS

Patuxent Nonpoint Source study during the years 1985-98

(Preston and Summers, 1997). The sampling frequency was

monthly, with additional samples collected during periods of

storm runoff. The monthly base-flow samples were collected using the equal-width-increment method (Edwards

and Glysson, 1999), and the storm-runoff samples were collected using an automatic sampler. A churn splitter was

used for both monthly and storm samples of both SSC and

TSS. The SSC samples were analyzed at USGS sediment

laboratories in Lemoyne, Pennsylvania, and Louisville,

Kentucky. The TSS samples were analyzed using a pipette

and filtration method by a Maryland State laboratory

(Stephen D. Preston, U.S. Geological Survey, written

commun., 1999).
6

0

200

400

600

800

1000

1200

1400

Number of Paired Suspended-Sediment Concentrations

 and Total Suspended Solids Values 

Number of SSC Values 

with Percent-Sand

SSC≤1

1<SSC≤10

10<SSC≤100

100<SSC≤1,000

1,000<SSC≤10,000

SSC>10,000

1 value = 

25,600 mg/L 87

742

814

622

109

19

309

431

96

n = 106 5

n = 1,051

n = 1,245

n = 718

n = 114

Number of SSC Values 

without Percent-Sand 

Virginia. A total of 188 SSC and TSS sample pairs were

collected at 7 streamflow-gaging stations in Virginia during

the years 1975-95. Paired SSC and TSS samples were collected every other month by the USGS except during some

low-flow periods as part of the River Input Monitoring Program (U.S. Geological Survey, 2000a). Techniques described

by Edwards and Glysson (1999) were used to collect all

samples. A churn splitter was used to obtain subsamples for

TSS analyses. The USGS collected most of the samples, except during some low-flow periods when the Virginia Department of Environmental Quality collected the samples. SSC

analyses were performed by USGS sediment laboratories. A

Virginia State laboratory performed the TSS analyses (Donna

L. Belval, U.S. Geological Survey, written commun., 1999).

Washington. A total of 817 SSC and TSS sample pairs

were collected at 25 streamflow-gaging stations in Washington during the years 1973-98, as part of various projects.

Techniques described by Edwards and Glysson (1999) were

used to collect all SSC and TSS samples. A churn splitter

was used to obtain subsamples for TSS analyses. The SSC

and TSS samples were analyzed at a USGS sediment laboratory in Tacoma, Washington, through September 1982.

Thereafter, samples were analyzed at the USGS Cascades

Volcano Observatory Sediment Laboratory (Richard J.

Wagner, U.S. Geological Survey, written commun., 1999).

Wisconsin. A total of 216 SSC and TSS sample pairs

were collected at 3 streamflow-gaging stations on streams in

the Lake Michigan watershed, Wisconsin, as part of an evaluation of the differences in results of

water-quality monitoring caused by

differences in sample-collection

methods (Kammerer and others,

1998). Low-flow samples were

collected in August and October

1993, and high-flow samples were

collected in April-July 1994. The

SSC samples were collected using

techniques described by Edwards

and Glysson (1999). The TSS

samples were collected concurrently with the SSC samples by the

Wisconsin Department of Natural

Resources using an open bottle.

Subsamples for SSC and TSS

analyses were obtained using a

cone splitter. SSC samples were

analyzed by the USGS sediment

laboratory in Iowa City, Iowa. TSS

samples were analyzed by a Wisconsin State laboratory (Herbert S.

Garn, U.S. Geological Survey,

written commun., 1999).

Quality-Control Data. The SSC

and TSS natural-water data used in

this evaluation were augmented by

analytical results of quality-control

samples from a cooperating laboratory. Known amounts of water and sediment were used to

constitute quality-control samples as part of the USGS National Sediment Laboratory Quality Assurance Program. The

National Sediment Laboratory Quality Assurance Program is

designed as an interlaboratory-comparison evaluation to provide a measure of bias and variance of suspended-sediment

data analyzed by laboratories operated or used by the USGS.

The quality-control samples received by the participating

laboratories were identified as such.

The quality-control samples were submitted in five

batches to a cooperating laboratory during 1997-99. Of the

quality-control samples, the first 35 were shipped as batch

numbers 1997-1, 1997-2, and 1998-1 and were analyzed for

TSS. Eighteen quality-control samples were shipped as batch

numbers 1998-2 and 1999-1 and analyzed for SSC using the

evaporation method (Kenneth Pearsall, U.S. Geological Survey, 1999, oral commun.).

COMPARABILITY OF SUSPENDED-SEDIMENT CONCENTRATION AND TOTAL SUSPENDED SOLIDS DATA

Natural-Water Data

The relation between SSC and TSS data was evaluated by

comparing all available paired SSC and TSS natural-water data,

and subsets of those data for each State. The number of paired

SSC and TSS values for selected SSC concentration ranges

with and without particle-size data are shown in figure 1.

 Of the 3,235 natural-water SSC samples used in this study,

Figure 1. Number of paired suspended-sediment concentration (SSC) values

and total suspended solids (TSS) values of the 3,235 data pairs for selected

suspended-sediment concentration ranges, in milligrams per liter.
7

74 percent had values less than or equal to 100 mg/L; only one

value (25,600 mg/L) exceeded 10,000 mg/L (figure 1).

Statistical characteristics of SSC and TSS paired data for

each State and for all paired data are given in table 2. Sixtysix percent of all TSS values are smaller than their

corresponding paired SSC values. Eighty-three percent of all TSS values are smaller than their paired

SSC value when SSC values exceed the 3rd quartile

value. For each State except Kentucky (38 percent

for 24 paired samples), 61 to 100 percent of the TSS

values are smaller than their paired SSC value when

SSC values exceed the 3rd quartile value. To summarize, SSC values tend to exceed their corresponding

paired TSS values. This tendency becomes stronger

at larger values of SSC.

Relations between all 3,235 paired TSS and SSC

measurements are shown in figures 2 and 3. According to Glysson and others (2000), there is no simple,

straightforward way to adjust TSS data to estimate

SSC if paired samples are not available. Relations

identified herein are not recommended for use in adjusting TSS data unless supported by additional research.

The data shown in figure 2 are plotted without

transformation and include the two ordinary least

squares regression lines obtained by regressing TSS

on SSC (the lower line) and SSC on TSS (the upper line).

Because of measurement errors associated with the collection processing, and analysis of the data, neither line can be

interpreted as an unbiased estimate of the true relation

Source of SSC

and TSS Paired

Data 

3rd

Quartile

mg/L 

Number

of values 

Number

of values

>0 mg/L 

Number of

values when

SSC value is

> 3rd Quartile

value 

Number of

values 

>0 mg/L when

SSC value is

> 3rd Quartile

value

Percentage of

values 

>0 mg/L when

SSC value is

> 3rd Quartile

value

Arizona

Hawaii

Illinois

Kentucky

Maryland

Virginia

Washington

Wisconsin

153.25

353.0 

48.5 

10.2 

324.0 

16.0 

30.0 

80.25

122

13

223

95

1,561

188

817

216

93

13

111

28

1,071

105

518

184

31

3

56

24

390

44

203

54

30

3

34

9

328

40

179

54

97%

100%

61%

38%

84%

91%

88%

100%

All Paired Data1 108.03,235 2,123

Percentage

of values

>0 mg/L

for all

paired data 

76%

100%

50%

29%

69%

56%

63%

85%

66% 809 672 83%

SSC Values SSC Minus TSS

1 Based on statistics using all 3,235 paired data; some values vary slightly from those calculated using summary 

 statistics from the eight States.

Table 2. Statistical characteristics of paired suspended-sediment concentration (SSC) and total suspended

solids (TSS) data for each of eight States, and for the combined data from all States

[mg/L, milligrams per liter; >, greater than]

0 5,000 10,000 15,000 20,000 25,000

0

5,000

10,000

15,000

20,000

25,000

2,500

7,500

12,500

17,500

22,500

27,500

Suspended-Sediment Concentrations, in mg/L 

Total Suspended Solids, in mg/L 

Line of equal value

Line resulting from regressing

SSC on TSS (Upper Bound)

Line resulting from regressing

TSS on SSC (Lower Bound)

Figure 2. Relation between untransformed values of

suspended-sediment concentration and total suspended solids

for 3,235 data points.
8

10,000

1,000

100

10

1

0.1

Total Suspended Solids, in mg/L

Suspended-Sediment Concentrations, in mg/L

0.1 1 10 100 1,000 10,000

Not Plotted: SSC=25,600 mg/L Line of Equal Value

 TSS=19,100 mg/L

between the two measurement methods. In fact, the existence

of measurement error implies the system of equations

describing the two measurements is insufficiently identified,

making estimation of an unbiased relation impossible

without additional information on the variance of the

measurement error for at least one of the measurements

(Klepper and Leamer, 1984). However, the two least squares

regression lines can be used to bound the true slope and

intercept coefficients (Frisch, 1934). In the case of TSS and

SSC, the least squares intercepts are very small relative to the

range of the data. Consequently, the two regression lines

effectively form consistent upper and lower bounds on the

true relation between TSS and SSC. These bounds imply that

TSS is biased downward relative to SSC by a proportionate

amount of 25 to 34 percent. Given the large skew apparent in

the data, this finding is tentative and requires confirmation

using a statistical or functional transformation yielding

homoscedastic residuals.

The relation between SSC and TSS for all 3,235 pairs of

transformed data using the base-10 logarithm and the line of

equal value are shown in figure 3; the relations for each State

and lines of equal value are shown in figure 4. Trends in the

scattergrams plotted for all data compared to those with data

that were segregated by State show some similarities,

including a tendency for the data to plot to the right of the

line of equal value, particularly at larger values of SSC.

As described previously, at least two factors associated

with the TSS analysis can result in subsamples obtained by

pipette or by pouring that are deficient in sand-size material.

Rapidly falling sand-size material can be difficult to withdraw representatively, particularly if pipette subsamples are

obtained from near the surface and (or) if the subsample is

not withdrawn immediately after mixing. Also, coarser sand

particles may plug the pipette intake, precluding withdrawal

of a representative mixture. Subsamples obtained by

pouring are also unlikely to contain representative amounts

of sand-size material. In contrast, the amount or percentage

of sand-size material in a SSC sample has no effect in bias

because all sediment in the original sample is used in the

SSC analysis.

The relation between sand-size material and TSS bias

was examined using the 860 paired SSC and TSS values for

which the amounts of material coarser and finer than 0.062

mm in the SSC sample are known. Percent sand-size material, percent finer material, and the total mass of sand-size

material were included in the analysis. All but one of the

paired data associated with particle sizes are for streams in

Illinois, Virginia, and Washington.

The relation between percent sand-size material associated with the SSC sample, and the SSC minus TSS remainder is shown in figure 5. No bias is apparent when sand-size

material composes less than about a quarter of the sample’s

sediment mass. Above about a third sand-size material, the

large majority of the SSC values exceed their paired TSS values. The increase in bias at larger SSC values as percent

sand-size values increase is consistent with the observation

that splitting original samples that contain a substantial percentage of sand-size material will rarely produce subsamples

with a SSC or particle-size distribution similar to those of the

original.

Splitting samples that contain small percentages of sandsize material are more likely to produce subsamples with

concentrations and particle-size distributions similar to the

original. The relation between TSS and the concentration of

material finer than 0.062 mm for 860 of the paired SSC and

TSS data with associated particle-size distribution data is

shown in figure 6. The concentration of fine material was

calculated as follows:

At TSS values that exceed about 5 mg/L of fine material, the SSC and TSS data are more or less evenly

distributed around the line of equal value (figure 6).

This suggests that the TSS method can provide relatively unbiased results when the large majority of material in a sample is finer than 0.062 mm.

The importance of bias in the relation between SSC

and TSS characterized in figure 3 can be magnified

when TSS data are used to compute sediment discharges. Sediment discharges increase when the product of water discharge and SSC increases (Porterfield,

1972). Additionally, the mobility of coarse material

tends to increase with larger flow velocities. Because

of the strong tendency for SSC to exceed TSS at larger

values of SSC (see figures 3 and 4), calculating discharges of TSS will usually result in underestimates of

Figure 3. Relation between the base-10 logarithms of

suspended-sediment concentration (SSC) and total suspended solids (TSS) for 3,235 data pairs in the scattergrams

plotted. All SSC and TSS values less than 0.25 mg/L were

set equal to 0.25 mg/L to enable plotting the data on

logarithmic coordinates.

 C<0.062mm is the concentration of material finer than 

0.062 mm in diameter,

is suspended-sediment concentration, and

is percent sand-size material associated 

with the SSC value.

SSC

Percent≥ 0.062mm

C<0.062mm = SSC [1- (Percent≥0.062mm /100)]
9

Figure 4. Relation between the base-10 logarithms of suspended-sediment concentration (SSC) and total

suspended solids (TSS) for the data pairs from each State used in the analysis. All SSC and TSS values less than

0.25 mg/L were set equal to 0.25 mg/L to enable plotting the data on logarithmic coordinates.

Not Plotted: SSC=25,600 mg/L

TSS=19,100 mg/L

Total Suspended Solids, in mg/L Total Suspended Solids, in mg/L Total Suspended Solids, in mg/L Total Suspended Solids, in mg/L

Suspended-Sediment Concentration, in mg/L Suspended-Sediment Concentration, in mg/L

Arizona

Illinois

Maryland

Washington

Hawaii

Kentucky

Virginia

Wisconsin

Line of Equal Value Line of Equal Value

Line of Equal Value Line of Equal Value

Line of Equal Value Line of Equal Value

Line of Equal Value Line of Equal Value

100

1,000

10

1

1 10 100 1,000

10,000

0.10.1 10,000

1,000

10

1

1 10 100 1,000

10,000

0.10.1 10,000

100

1,000

10

1

1 10 100 1,000

10,000

0.10.1 10,000

100

1,000

10

1

1 10 100 1,000

10,000

0.10.1 10,000

100

1,000

10

1

1 10 100 1,000

10,000

0.10.1 10,000

100

1,000

10

100

1

1 10 100 1,000

10,000

0.10.1 10,000

1,000

10

100

1

1 10 100 1,000

10,000

0.10.1 10,000

1,000

10

100

1

1 10 100 1,000

10,000

0.10.1 10,000
10

Line of Equal Value

0.1

1

10

100

1,000

Concentration of Suspended Sediment

Finer than 0.062 mm, in mg/L

0.1 1 10 100 1,000

Total Suspended Solids, in mg/L

10,000

Figure 6. Relationship between total suspended solids and

the concentration of suspended sediments finer than 0.062mm

in paired suspended-sediment concentration samples. All SSC

and TSS values less than 0.25 mg/L were set equal to 0.25 mg/L

to enable plotting the data on logarithmic coordinates.

0

200

-200

400

600

1,000

Suspended-Sediment Concentration Minus 

Total Suspended Solids, in mg/L

0 20

Percent Sand-Size Material in the Suspended-Sediment Concentration Sample

40 60 80 100

Two values not plotted:

43% sand, 2,810 mg/L

54% sand, 2,450 mg/L

Ordinary

Least-Squares

Regression Line

800

the suspended solid-phase discharges compared to those estimates that are computed from SSC data. TSS discharge underestimates may be negligible for streams conveying a predominantly fine material load over the range of discharges.

Substantial underestimates of TSS discharges can be expected for streams conveying sediment loads that exceed

about a third sand-size material in composition, and

with percentages and concentrations of sand-size material that increase with discharge.

Figure 7 shows an example of the influence of bias

resulting from using TSS data to calculate instantaneous sediment discharges for a stream in the northeastern United States. All the TSS and SSC samples

used to compute sediment discharges from October 15

through December 24, 1998 were collected by a cooperating agency using an open bottle and analyzed by

the cooperator’s laboratory. The apparent order-ofmagnitude change in sediment discharges between November and December 1998 was not related to any instream change in solid-phase transport, but to a change

in analytical procedures (Henry Zajd, Jr., U.S. Geological Survey, oral commun., 2000). TSS analyses

were performed on all samples collected in October

and November 1998, and SSC analyses were used to

produce subsequent data. The USGS did not publish

daily sediment discharges for the pre-December period

shown in figure 7 because the TSS data used in the

computations were considered unreliable.

Quality-Control Data

Box plots that show the results of quality-control

samples analyzed for SSC and TSS by a cooperating

laboratory participating in the USGS National Sediment Laboratory Quality Assurance Program are

shown in figure 8. The samples were analyzed in five

sample sets. Box plots for sample sets 1997-1, 1997-2,

and 1998-1 represent TSS analytical results. Box plots

for study sample sets 1998-2 and 1999-1 represent

SSC analytical results. This figure illustrates two important characteristics related to sediment-data quality.

First, both the SSC and TSS data tend to be negatively biased. The combined data for all samples analyzed as part of the Sediment Laboratory Quality Assurance Program from 1996 through September 2000

have a median concentration bias of -1.83 percent; the

25th percentile is -4.39 percent; and the 75th percentile

is 0.00 percent. The bias primarily reflects a loss of

some sediment, such as through a filter, or an inability

to weigh accurately very small amounts of fine material in the SSC analytical procedure. The SSC median

percent bias values for both study sets are about -2 and

-4 percent of the known sediment mass. In contrast,

TSS median percent bias values for the three study sets

range from -6 to -23 percent from the known sediment

mass; the mean difference in TSS median percent bias

from the known sediment mass is -16 percent. Only

for sample set 1997-2 does any quartile include the

TSS value for the known sediment mass. The median percent

bias in TSS sample set 1997-1 and in 1998-1 exceeds three

F-pseudosigmas2

 from the mean value of all measured sediment mass measurements reported in the USGS National

Figure 5. Relation between percent sand-size material in the

sample analyzed for suspended-sediment concentration and the

remainder of suspended-sediment concentration minus total

suspended solids.

2

The F-pseudosigma is a nonparametric statistic analogous to the standard deviation that is calculated by using the 25th and 75th percentiles in a data set. It is resistant to the effect of extreme outliers.
11

Sediment Laboratory Quality

Assurance Program. The

analytical method used by

the laboratory for determination of TSS in natural-water

samples was deemed unacceptable by the U.S. Geological Survey (USGS,

1999b).

Second, the variances associated with the TSS quality-control data are large

compared to those for SSC

data (figure 8). The least

variable data – those from

sample set 1997-1 – range

from -18 to -32 percent of

the known value, and the difference between the 1st and

3rd quartile values is 9 percent. In comparison, the

most variable SSC data –

those from sample set 1999-

1 – range from 0 to -5 percent; the difference in the 1st

and 3rd quantile values is 4 percent.

In terms of bias and variance, the TSS results from two

of the first three sample sets – 1997-1 and 1998-1 – were

considered unacceptable by the U.S. Geological Survey (U.S.

Geological Survey, 1998; 1999a). The SSC results from

study sample sets 1998-2 and 1999-1, which were produced

by the same laboratory, are considered among the most accurate of all laboratories that participated in the USGS National

Sediment Laboratory Quality

Assessment Program (John

Gordon, U.S. Geological

Survey, oral commun.,

2000).

CONCLUSIONS

Of the two analytical

methods examined for measuring the mass of solidphase material in natural-water samples — suspendedsediment concentrations

(SSC), and total suspended

solids (TSS), — data produced by the SSC technique

are the more reliable. This is

particularly true when the

amount of sand in a sample

exceeds about a quarter of

the dry sediment mass. This

conclusion is based on the

following observations:

1. The SSC analytical

procedure entails measurement of the entire mass of sediment

and the net weight for the entire sample. In contrast, only a

part of the water-sediment mixture is typically used in the TSS

analysis. Difficulties in, and variations for methods associated

with obtaining TSS subsamples can result in determinations of

solid-phase characteristics that are substantially different from

those of the original sample.

Figure 8. Variability in results of suspended-sediment concentrations and total suspended solids analytical methods in quality-control water samples analyzed by a cooperator laboratory. (John D. Gordon, U.S. Geological Survey, written commun., 2000).

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

15-Oct 22-Oct 29-Oct 5-Nov 12-Nov 19-Nov 26-Nov 3-Dec 10-Dec 17-Dec 24-Dec

Instantaneous Sediment Discharge, in Megagrams per Day

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Instantaneous Water Discharge, in Cubic Meters per Second

Instantaneous Sediment

Discharges from TSS Data Instantaneous

Sediment Discharges

from SSC Data

Instantaneous Water Discharges

Figure 7. Instantaneous water discharges, and sediment discharges computed

from total suspended solids (TSS) and suspended-sediment concentration (SSC)

data for a stream in the northeastern United States, 1998.

Study Number

Total Suspended Solids

Percent Difference From Known Values

Suspended-Sediment

Concentrations

*

*

30

20

10

1997-1 1997-2 1998-1 1998-2 1999-1

0

-10

-20

-30

EXPLANATION

LARGEST VALUE LESS THAN 

OR EQUAL TO THE 75th 

PERCENTILE PLUS 1.5 TIMES 

INTERQUARTILE RANGES

UPPER QUARTILE

MEDIAN

LOWER QUARTILE

SMALLEST VALUE GREATER

THAN OR EQUAL TO THE 

25th PERCENTILE MINUS

1.5 TIMES INTERQUARTILE

RANGES

REMAINING 1 PERCENT OF

THE DATA NOT INCLUSIVE 

IN THE DISTRIBUTION TAILS
12

2. Subsampling by pipette or by pouring from an open

container will generally result in production of a sedimentdeficient subsample. An analysis of 3,235 paired SSC and

TSS natural-water samples from eight States showed that

SSC values tend to exceed their paired TSS values, particularly at larger values of SSC. This is consistent with the assumption that most subsamples used to determine the TSS

data were obtained by pipette or by pouring from an open

container.

3. An analysis of 860 paired SSC and TSS natural-water

samples for which relative amounts of sand-size and finer

material are known for the SSC sample were used to determine the effect of sand-size particles on the TSS analysis.

SSC values tend to be larger than their paired TSS values as

the percentage of sand-size material exceeds about a quarter

of the mass of sediment in the sample. Additionally, a relation between values of TSS and the paired SSC material finer

than 0.062 mm showed that for samples with TSS values exceeding about 5 mg/L, the paired SSC and TSS data are more

or less evenly distributed around the line of equal value.

Sand-size material is more difficult to subsample than finer

material due to the large fall velocity of sand-size material as

described by Stokes’ Law.

The tendency for SSC values to exceed their paired TSS

values has important ramifications for computations of suspended solid-phase discharges; those computed using TSS

data will often underestimate solid-phase discharges. This is

particularly true for sites when the percentages of sand-size

material in the water samples exceed about a third and where

concentrations and percentages of sand-size material in

transport increase with flow.

4. Fifty-three quality-control samples from a

cooperator’s laboratory — three sample sets totaling 35 TSS

analyses of subsamples obtained by pouring from original

samples, and two sample sets totaling 18 SSC analyses —

were used to compare bias and variance introduced by use of

the TSS and SSC analytical methods. Two of the three

sample sets analyzed for TSS had unacceptably large mean

negative bias. Variances associated with all three TSS

sample sets were at least double those associated with the

SSC quality-control results from the same laboratory. The

two SSC sample sets analyzed by the same laboratory had

small variances compared with those for the three TSS

sample sets. The slight negative bias values associated with

the SSC sample sets were consistent with data analyzed by

most laboratories participating in the USGS National Sediment Laboratory Quality Assurance Program.

5. Review of the literature indicates that the TSS

method originated as an analytical method for wastewater,

presumably for samples collected after a settling step at a

wastewater treatment facility. The results of this evaluation

do not support use of the TSS method to produce reliable

concentrations of solid-phase material in natural-water

samples. The TSS method is being misapplied to samples

from natural water.

Some SSC and TSS data may be comparable, particularly when the percentage or amount of sand-size material in

the sample is less than about 25 percent. TSS values from

analyses of samples collected following a settling step for

coarser sediments, such as those obtained for compliance

purposes at sewage treatment plants and water treatment facilities, may be reliable. However, because relatively few

TSS data are associated with the percent sand-size and finer

material from SSC samples, it is usually impossible to identify which if any TSS data may be biased. Some of the TSS

data may reflect the mass of suspended solids in natural-water samples, but there are currently no absolute means to

identify those data, nor a generally reliable procedure to correct biased TSS data.

The TSS method, which was originally designed for

analyses of wastewater samples, is shown to be fundamentally unreliable for the analysis of natural-water samples. In

contrast, the SSC method produces relatively reliable results

for samples of natural water, regardless of the amount or percentage of sand-size material in the samples. SSC and TSS

data collected from natural water are not comparable and

should not be used interchangeably. The accuracy and comparability of suspended solid-phase concentrations of the

Nation’s natural waters would be greatly enhanced if all

these data were produced by the SSC analytical method.

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