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Turbidity: a measure of the collective optical properties of a water sample that cause light to be scattered and absorbed rather than transmitted in straight lines.

Turbidity measures the scattering effect that suspended solids have on light: the higher the intensity of scattered light, the higher the turbidity.1 Primary contributors to turbidity include clay, silt, finely divided organic and inorganic matter, soluble colored organic compounds, plankton, and microscopic organisms (American Public Health Association and others, 1992). The measurement is qualitative and cannot be correlated directly as micrograms per liter of suspended solids.

Determination of turbidity is a common component of water-quality assessments.

arrow In surface water, the clarity of a natural body of water is used routinely as an indicator of the condition and productivity of the aqueous system.

arrow In ground water, turbidity commonly is measured during well development and well purging to indicate the extent to which particulates occurring as a result of well installation and sampling activities have been removed.

Turbidity measurements reported for regulatory purposes require a true nephelometric measurement using turbidimeter instruments that meet U.S. Environmental Protection Agency (USEPA) specifications (see 6.7.1).

Turbidity is measured in nephelometric turbidity units (NTU) or Formazin turbidity units (FTU), depending on the method and equipment used. Turbidity measured in NTU uses nephelometric methods that depend on passing specific light of a specific wavelength through the sample. FTU is considered comparable in value to NTU and is the unit of measurement when using absorptometric methods (spectrophotometric equipment). Jackson turbidity unit (JTU) values also approximate NTU but the JTU is no longer in common use. Turbidity values are entered into the USGS National Water Information System (NWIS) database only if the measurement is made in NTU and with instruments that are operated using USEPA-approved methods--not all turbidimeters that display NTU values meet these criteria.

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1Turbidity measurements have not been systematically researched, tested, compared, or quality assured within the U.S. Geological Survey (USGS). Recommendations in this section were compiled from the references cited, instrument handbooks, field experience, and a limited series of tests on available instruments conducted by the USGS Hydrologic Instrumentation Facility (HIF).

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