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Collection and Use of
Total Suspended Solids Data
by John R. Gray and G. Doug Glysson
An important measure of water quality
is the amount of material suspended in the water. The U.S Geological
Survey (USGS) traditionally has used measurements of suspended-sediment
concentration (SSC) (USGS parameter code 80154) 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 Total Suspended Solids
(TSS) analytical method. The TSS method originally was 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 a relatively inexpensive
laboratory procedure. The TSS analytical method (USGS parameter
code 00530) to determine concentrations of suspended material in
open-channel flow is fundamentally unreliable and can result in
unacceptably large errors.
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. Data from measurements such as TSS, turbidity, and
optical backstatterance are being used with increasing frequency
as surrogates for suspended sediment. Collection methods for these
data are typically less-expensive than those for traditional data-collection
techniques. Additionally, some measurement techniques enable acquisition
of suspended-sediment data on a more frequent basis, such as every
15 minutes. However, proper use of these surrogate measurements
of suspended material requires that a relation between SSC and the
surrogate be defined and documented for each site at which the data
are collected.
Differences between the TSS and SSC
analysis were investigated using 3,235 paired TSS and SSC samples
provided by eight USGS Districts throughout the U.S. (Gray and others
2000), and with 14,446 data pairs from the USGS’s National Water
Information system (NWIS) data base (Glysson and others 2000).
The findings of these studies can be summarized as follows.
1. The TSS analysis normally
is performed on an aliquot of the original sample. The difficulty
in withdrawing an aliquot from a sample that accurately represents
suspended material concentration leads to inherent variability in
the measurement. By contrast, the SSC analysis is performed on
the entire sediment mass of the sample. 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 rarely will 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 from 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 (Figure 1).
The biased TSS data can result in errors in load computations of
several orders of magnitude.
Figure 1. Relation between the base-10 logarithms of suspended-sediment
concentration (SSC) and total suspended solids (TSS) for 3,235 data
pairs in the scattergram. 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.
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 relation. No reliable, straightforward
way presently is available 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 various States’ water-quality criteria
standards for sediment, the USGS wishes to share this information
with its cooperators. The USGS is passing this information on to
the U.S. Environmental Protection Agency’s Office of Water, other
Federal agencies, and State and local agencies that are involved
in collection or use of sediment data.
Summary
It is not appropriate to use TSS data 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. Collection
of samples to determine TSS requires concurrent collection of samples
for suspended-sediment concentration 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 entire range of expected
flows.
Selected 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. *
Glysson, G.D., J.R. Gray, and G.E.
Schwarz, in press, "A Comparison of Load Estimates Using Total
Suspended Solids and Suspended-Sediment Concentration Data,"
in Proceedings of ASCE World Water and Environmental Resources Congress,
May 20-24, 2001, Orlando, FL.
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.
* The references (Gray et al. 2000 and Glysson et al.
2000) discussing comparability and adjustment of TSS and TSS data
are available on-line for downloading at URL http://water.usgs.gov/osw/techniques/sediment.html.
Direct questions or additional information
requests about the USGS policy on the collection and use of total
suspended solids data to:
John Gray, hydrologist, U.S. Geological Survey, Office of Surface
Water, Reston, VA; (703) 648-5318; jrgray@usgs.gov.
Doug Glysson, hydrologist, U.S. Geological Survey, Office of Water
Quality, Reston, VA; (703) 648-5019; gglysson@usgs.gov.
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