In Reply Refer To:                                         September 24, 2004
Mail Stop 412

Office of Water Quality Technical Memorandum 2004.03

Subject: Revision of NFM Chapter 6, Section 6.7—Turbidity

Purpose of Memorandum

This memorandum announces a rewrite of “Chapter 6, Section 6.7—Turbidity” of the U.S. Geological Survey (USGS) National Field Manual for the Collection of Water-Quality Data (NFM), and the implementation of new reporting units and parameter codes for storing and reporting turbidity data in USGS databases and products (Attachments 1 and 2). Information sources supporting these changes are included. This method will be effective October 1, 2004. The revised NFM Section 6.7 is available online at


The NFM documents USGS protocols for the collection, quality assurance, storage, and publication of water-quality data. NFM Section 6.7—“Turbidity” documents methods for equipment selection, calibration, measurement, and data storage. Use of the revised method will help improve the quality and comparability of reported data, and will reduce the variability associated with such data.

Turbidity is a useful measurement that is growing in popularity and importance in scientific and resource monitoring programs, both within and outside of the USGS. Uses of turbidity data include measuring water clarity for drinking water and ecological applications, indicating visual impairment in water, real-time monitoring of conditions in watersheds, use as a surrogate for suspended-sediment concentration (SSC) and other constituents in water, and estimating transport of contaminants associated with suspended materials (Gray and Glysson, 2003).

However, turbidity itself is not an inherent physical property of water (as is, for example, temperature), but rather is a measure of light scattering through a liquid as measured by detectors with known geometry. The configuration of detectors and the source of light are important factors in the response of the turbidity instrument. Although comparisons among instruments with differing designs are often robust, they can also vary according to the character of the sample’s matrix and particulates. Results from an interagency workshop held in 2002 demonstrated that turbidity data from different sources and instrumentation can be highly variable and are often in disagreement with each other, even when calibration methods are similar (Gray and Glysson, 2003). In effect, instruments with different detector geometries and light sources often do not make equivalent measurements.

The U.S. Environmental Protection Agency (USEPA) method 180.1 established specific requirements for instrument configuration for the measurement of turbidity in drinking water within the United States (U.S. Environmental Protection Agency, 1993). The International Organization for Standardization (ISO) method 7027 established instrument requirements for measurement of drinking water in Europe (International Organization for Standardization, 1999), but uses a different light source than USEPA Method 180.1. Measurements complying with USEPA Method 180.1 are reported in Nephelometric Turbidity Units (NTU), whereas those complying with ISO 7027 are reported in Formazin Nephelometric Units (FNU) in Europe. In practice, during the past two decades most turbidity data in the U.S. have been reported in NTUs, regardless of their light source or detector geometry. Data stored in the USGS’s National Water Information System (NWIS) and in USEPA’s STORET database have been stored under parameter code 00076, and have been reported in NTUs; however, use of that parameter code has not been restricted to instruments that comply with USEPA Method 180.1.


In order to reduce the variability associated with turbidity data, the USGS has created new reporting units for turbidity that are based on the instrument design (Attachment 1). These reporting units correspond to new parameter and method codes for storing turbidity data in the National Water Information System (NWIS). The use of NTU and FNU will be retained, but will be restricted to data from instruments that comply with the specific designs defined in USEPA Method 180.1 and ISO 7027, respectively. Eight new reporting units have been promulgated that correspond to existing and anticipated instrument designs that do not strictly conform to USEPA Method 180.1 or ISO 7027. Finally, the protocol discontinues the use within USGS of Formazin Turbidity Units (FTU), a reporting unit that has been commonly used, but which lacks specificity to any particular measurement technology.

Along with the new reporting units as specified in Attachment 1, users are now required to record the specific instrument make and model using the method code field in NWIS. Table A-1 in NFM Section 6.7 (Attachment 2) provides a current (August 2004) list of each instrument make and model according to its detector configuration and light source as specified for each new reporting unit and shows the appropriate parameter and method codes for each. Method codes are required when storing data in NWIS in order to provide traceability and allow for more informed comparisons of data in the future.

As the USGS is changing its reporting units for turbidity, other organizations are making the same changes. The American Society for Testing and Materials (ASTM) is in the process of making these changes and will use the same new reporting units. The USGS and ASTM have been working together in order to ensure traceability and improve comparability among turbidimeters.

The revised protocol also clarifies the use of appropriate calibration standards, in accordance with ASTM provisions. Although “scratch” formazin is recognized as the primary calibration standard, commercially-prepared solutions of stabilized formazin and styrene divinylbenzene polymer beads are also accepted as calibrants. However, consistent use of one type of calibrant is required to minimize errors introduced during calibration.

The revised turbidity protocol in Section 6.7 provides additional tools for USGS users. A decision tree will help the user determine the most appropriate instrument type for a given application. An appendix lists specific turbidity instruments, including manufacturer and model, so that users can look up the appropriate NWIS parameter code and method code for data entry.

Implementation and Data Handling

The revised turbidity protocol, including the use of new parameter codes and method codes, will be implemented on October 1, 2004. Concurrently with implementation of the new parameter codes, parameter code 00076 is no longer to be used for storing new data. This code will eventually be locked so that it will not accept new data. Historical data, stored previously under NWIS parameter code 00076, may be migrated to the new parameter codes on a case by case basis, only where knowledge of specific instruments and their instrument configuration is sufficient to allow selection of an appropriate reporting unit, parameter code, and method code according to Table A-1. These changes should be done with the guidance of the District Water-Quality Specialist. Attachment 3 provides information on a program that is available to aid in migrating historical data to the new parameter and method codes.

Training on the new data reporting and storage requirements will take place through cyber seminars initially to be held on September 22 and 28, 2004. Information will also be provided in the Field Water-Quality Methods for Ground Water and Surface Water training course (QW1028TC) and the NFM.

NWISWeb (real-time)

Currently, NWISWeb is incapable of displaying or retrieving method codes with real-time continuous data. Efforts are underway through the USGS Surface Water Users Group (SWUG) to add database functionality so that method codes would be displayed along with real-time data. As an interim measure, station descriptions for sites that serve continuous turbidity data to the web must be updated to include a description of the turbidity instrumentation used at the site, along with the appropriate method code.


Districts may need documentation or references in report products to cooperators to support the use of the new reporting units. Some appropriate documents are listed in the ‘Reference’ section of this memo and in NFM Section 6.7. All products should use the new reporting limits and list the instruments used for the study. Guidelines for rounding data for reports are provided in NFM Section 6.7 (Table 6.7-6).

Future modifications and maintenance

New parameter and method codes will be added as the technology for measuring turbidity evolves. These will be included in, and announced with, the quarterly NWIS updates and in the “Comments and Errata” page of the online NFM.

Sources of information

Approved Methods

NFM Section 6.7 (Anderson, Chauncey W., 2004)

ASTM – Low-Level Static Method (ASTM International, 2003)

ISO 7027 (International Organization for Standardization, 1999)

Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Site Selection, Field Operation, Calibration, Record Computation, and Reporting, Water-Resources Investigations Report 00-4252 (Wagner and others, 2000)

USEPA 180.1 (U.S. Environmental Protection Agency 1993)


Anderson, C.W., 2004, Turbidity, (version 2): U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A6, section 6.7, accessed September 24, 2004 from

ASTM International, 2003, D6855-03 Standard test method for determination of turbidity below 5 NTU in static mode: ASTM International, Annual Book of Standards, Water and Environmental Technology, 2003, vol. 11.01, West Conshohocken, Pennsylvania, 13 p. []

Davies-Colley, R.J., and Smith, D.G., 2001, Turbidity, suspended sediment, and water clarity— A review: Journal of the American Water Resources Association, v. 37, no. 5, p. 1085–1101.

Gray, J.R., and Glysson, G.D., 2003, Proceedings of the Federal Interagency Workshop on turbidity and other sediment surrogates, April 30-May 2, 2002, Reno, Nevada: U.S. Geological Survey, Circular 1250, 56 p. []

International Organization for Standardization, 1999, Water quality — Determination of turbidity: Geneva, Switzerland, International Organization for Standardization, ISO 7027, 10 p.

U.S. Environmental Protection Agency 1993, Methods for the determination of inorganic substances in environmental samples: Cincinnati, OH, U.S. Environmental Protection Agency EPA/600/R-93/100, 178 p.

Wagner, R.J, Mattraw, H.C., Ritz, G.F., and Smith, B.A., 2000, Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Site Selection, Field Operation, Calibration, Record Computation, and Reporting, U.S. Geological Survey Water-Resources Investigations Report 00-4252, 53 p. []

Timothy L. Miller /s/
Chief, Office of Water Quality

This memorandum does not supersede any other Office of Water Quality Technical Memorandum.

Distribution: All WRD Employees

Attachments (in attached file)