Clarifying turbidity — the potential and limitations of turbidity as a surrogate for water-quality monitoring
The U.S. Geological Survey has several ongoing projects throughout the Nation that use con-tinuous monitoring of instream turbidity to develop es-timates of water quality. The recent development of several in situ sensors has made it possible to continu-ously monitor turbidity in “real time.” The statistical relations between turbidity and certain sediment or sediment-bound, water-quality constituents have made it possible to provide a “real-time” estimate of concen-trations of certain water-quality constituents such as total suspended solids, suspended sediment concentra-tion, various nutrients, and bacteria. Continuously monitored instream turbidity may provide more accurate concentration predictions than traditional surrogates such as discharge; however, there are many issues and limitations regarding turbidity. These include: (1) different methods and technologies used to measure turbidity, (2) effects that physical properties of the solids and streamwater have on the measurement of turbidity, and (3) the best deployment strategy for measuring instream turbidity. This paper summarizes the potential and limitations of turbidity as a water-quality surrogate including selected methods and technologies for measuring turbidity, factors that affect turbidity readings, and some of the issues encountered when monitoring instream turbidity. The paper also presents some of the preliminary relations between turbidity and water-quality constituent concentrations for data collected in Gwinnett County, Georgia.