A review of the principles of turbidity measurement

Abstract

Turbidity of water due to the presence of suspended sediment is measured and interpreted in a variety of ways, which can lead to the misinterpretation of data. This paper re-examines the physics of light scattering in water, and exposes the extent to which the reporting of turbidity data is inconsistent. It is proposed that the cause of this inconsistency is the fact that the accepted turbidity standards USEPA Method 180.1, ISO 7027 and GLI Method 2 are mutually inconsistent, as these standards give rise to a large number of measurement units that are not based on the optical properties of light absorption and scattering by suspensions in water, but by the arbitrary definition of the degree of turbidity being due to a concentration of formazin or other similar polymer-based calibration standard. It is then proposed that all turbidity-measuring devices should be calibrated with precise optical attenuators such as neutral density filters. Such calibration would allow for the definition of a beam attenuation coefficient for every turbidity-measuring instrument which would be cross-comparable with any other instrument calibrated in the same way. The units for turbidity measurements should be based on attenuation and reported as dB m−1. It is also proposed that a new standard should be drafted according to this attenuation-based method, and this new standard should also define the nomenclature for reporting data collected at any specific scattering angle in terms of an attenuation in dB m−1. The importance of multi-parameter turbidity measurements for the improvement of the quality of turbidity data and the application of parameter-rich data sets to new methods of sediment characterization are discussed. It is suggested that more research into multi-parameter turbidity measurements is needed, as these new methods will facilitate an increase in parity between turbidity and suspended sediment concentration, a relationship that is subjective.