Collection and Use of Total Suspended Solids Data

In Reply Refer To:
Mail Stop 412 or                                   November 27, 2000
Mail Stop 415

OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM NO. 2001.03
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 2001.03

Subject:  Collection and Use of Total Suspended Solids Data

USGS Policy on Collection and Use of Total Suspended Solids Data:

1.   The use of Total Suspended Solids data (TSS, parameter code 00530)
resulting from the analysis of water samples to determine the concentration
of suspended material in water samples collected from open channel flow and
calculations of fluxes based on these data is not appropriate. Collection
of samples to determine TSS requires concurrent collection of samples for
suspended sediment concentration (SSC) analysis.  Concurrent SSC analysis
can only be discontinued after it is conclusively documented in a published
report that the TSS data, on a site-by-site basis, can adequately represent
SSC data over the whole range of flows that can be expected.

2.   The SSC analytical method, ASTM D 3977-97, Standard Test Method for
Determining Sediment Concentration in Water Samples (ASTM, 1999), is the
USGS standard for determining concentrations of suspended material in
surface water samples.  This method is used by all USGS sediment
laboratories, and by cooperating laboratories certified to provide
suspended-sediment data to the USGS.

Background:

An important measure of water quality is the amount of material suspended
in the water.  The USGS has traditionally used measurements of
suspended-sediment concentration as the most accurate way to measure the
total amount of suspended material in a water sample collected from the
flow in open channels.  Another commonly used measurement of suspended
material is the TSS analytical method.  This method was originally
developed for use on wastewater samples, but has been widely used as a
measure of suspended material in stream samples because it is mandated or
acceptable for regulatory purposes and is an inexpensive laboratory
procedure.  Using the TSS analytical method (parameter code 00530) to
determine concentrations of suspended material in open channel-flow can
result in unacceptably large errors and is fundamentally unreliable.

Summary of Recent Studies:

Studies on the accuracy of the SSC analytical method by ASTM (1999) and the
USGS Branch of Quality Systems (Gordon and others, 2000) have shown that
the SSC analysis represents an accurate measure of the concentration of the
suspended sediment in a sample.  Other measurements such as TSS, turbidity,
and data obtained from optical backscatter instruments are often used as
surrogates for suspended sediment and are often less expensive to collect
and (or) analyze and some may be collected on a near-continuous basis.
However, proper use of these surrogate measurements of suspended material
requires that a relationship between SSC and the surrogate be defined and
documented for each site at which the data are collected.

Differences between the TSS and SSC analyses were investigated using 3,235
paired TSS and SSC samples provided by eight USGS Districts (Gray and
others, 2000), and with 14,466 data pairs from the USGS's NWIS data base
(Glysson and others, 2000).  The findings of these studies can be
summarized as follows:

1.   The TSS analysis is normally performed on an aliquot of the original
sample.  The difficulty in withdrawing an aliquot from a sample that truly
represents suspended material concentration leads to inherent variability
in the measurement.  By contrast, SSC analysis is performed on the entire
sample, thus measuring the entire sediment mass. If a sample contains a
substantial percentage of sand-size material - more than about 25 percent -
then stirring, shaking, or otherwise agitating the sample before obtaining
a subsample will rarely produce an aliquot representative of the suspended
material and particle-size distribution of the original sample.

2.   TSS methods and equipment differ among laboratories, whereas SSC
methods and equipment used by USGS sediment laboratories are consistent,
and are quality assured by the National Sediment Laboratory Quality
Assurance Program (OSW Technical Memorandum 98.05; Gordon and others,
2000).

3.   Results of the TSS analytical method tend to produce data that are
negatively biased by 25 to 34 percent with respect to SCC analyses
collected at the same time and can vary widely at different flows at a
given site.  The biased TSS data can result in errors in load computations
of several orders of magnitude.

Analysis of paired data for TSS and SSC (Glysson and others, 2000)
indicates that in some cases, it might be possible to develop a relation
between SSC and TSS at a given site.   At least 30 paired sample points,
evenly distributed over the range of concentrations and flows encountered
at the site, would be needed to define such a relationship.  There is no
reliable, straightforward way to adjust TSS data to estimate suspended
sediment without corresponding SSC data.

Because the TSS analytical method is widely used outside of the USGS for
the determination of suspended-material concentrations in water samples for
open channel flow, and because the TSS analysis is specified in several
States' water-quality criteria standards for sediment, it would be
appropriate for USGS District offices to share this information with their
cooperators.  The Offices of Water Quality and Surface Water are passing
this information on to the U.S. Environmental Protection Agency's Office of
Water and to other Federal agencies that are involved in using sediment
data.  For questions or additional information, contact Doug Glysson (OWQ,
gglysson@usgs.gov) or John Gray (OSW, jrgray@usgs.gov).

References:

ASTM, 1999, D 3977-97, Standard Test Method for Determining Sediment
Concentration in Water Samples, Annual Book of Standards, Water and
Environmental Technology, 1999, Volume 11.02, p 389-394.

Glysson, G. D., J. R. Gray, and L. M. Conge, 2000, "Adjustment of Total
Suspended Solids Data for Use in Sediment Studies," in the Proceeding of
the ASCE's 2000 Joint Conference on Water Resources Engineering and Water
Resources Planning and Management, July 30 ? August 2, 2000, Minneapolis,
MN, 10 p. *

Gordon, J. D., Newland, C. A., and Gagliardi, S. T., 2000, Laboratory
Performance in the Sediment Laboratory Quality-Assurance Project, 1996-98:
U. S. Geological Survey Water-Resources Investigations Report 99-4184, 69
p.

Gray, J. R., G. D. Glysson, L. M. Turcios, and G. E. Schwarz, 2000,
Comparability of Suspended-Sediment Concentration and Total Suspended
Solids Data, U.S. Geological Survey Water-Resources Investigations Report
00-4191, 14 p. *

U.S. Geological Survey, 1998, A National Quality Assurance Program for
Sediment Laboratories Operated or Used by the Water Resources Division:
Office of Surface Water Technical Memorandum No. 98.05, accessed November
13, 2000 from URL
http://water.usgs.gov/admin/memo/SW/sw98.05.html.

(* References are on-line at URL
http://water.usgs.gov/osw/techniques/sediment.html)


Janice R. Ward /s/                       Thomas H. Yorke, Jr. /s/
Acting Chief, Office of Water Quality    Chief, Office of Surface Water

This memorandum does not supersede any other technical memorandum.

Distribution:  All WRD Employees

(See attached file: TSS.0103.doc)
********************************************
Janice Ward
Acting Chief, Office of Water Quality
US Geological Survey
Reston, VA
703-648-6871
jward@usgs.go