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. 

 

  1. 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.

 

  1. 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).

 

  1. 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

/admin/memo.

 

(* References are on-line at URL /osw/techniques/sediment.html)

 

 

 

 

Janice R. Ward                                                 Thomas H. Yorke, Jr.

Acting Chief, Office of Water Quality             Chief, Office of Surface Water

 

This memorandum does not supersede any other technical memorandum.

 

Distribution:            All WRD Employees