Diel turbidity fluctuations in streams in Gwinnett County, Georgia
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Continuous turbidity data have been collected since 2001 at 12 water-quality monitoring stations in Gwinnett County, Georgia, as part of a cooperative agreement between the U.S. Geological Survey and the Gwinnett County Department of Public Utilities. With one of the largest real-time turbidity monitoring networks in the nation, this program has led in the development of deployment strategies and data management practices. Though the technology used in continuous monitoring of turbidity is relatively new, sinusoidal diel turbidity fluctuations have been observed at 11 of the 12 Gwinnett County monitoring stations, as well as in other geologic and climatic settings in the United States during baseflow conditions. The fluctuations are represented by elevated turbidity values that occur near sunrise, followed by a decrease throughout the day, with lowest values occurring near sunset. Evening values show a gradual increase through the night to sunrise. Turbidity fluctuations do not show seasonality, except that they are not observable during rainy periods, when stormwater runoff dominates flow conditions. Several mechanisms for turbidity variation—including instrumentation effects, sediment transport, and biological activity—are considered. Coincidence of turbidity and dissolved oxygen fluctuations supports biological activity as a cause of diel turbidity fluctuations. Diel fluctuations in turbidity may have implications for studies that use turbidity as a surrogate for other water-quality properties, such as requiring correction factors for studies that report bacteria concentrations during low-flow conditions.