The Value of Continuous Turbidity Monitoring in TMDL Programs

The Federal Clean Water Act requires States to establish total maximum daily loads (TMDLs) to meet water-quality criteria and to attain designated beneficial uses for each water body. Sediment and sediment-associated constituents, such as total nutrients and bacteria, are among the most common water-quality impairments in the Nation. The load of a contaminant introduced to a water body varies with both concentration and streamflow, requiring reliable information for both when implementing an effective TMDL program. In most TMDL programs, discrete water samples collected at periodic intervals are used to determine whether established criteria are being met and to determine loads from contributing sources. However, discrete samples do not adequately describe daily, monthly, or annual variability in load characteristics because both concentration and streamflow can fluctuate substantially between samples.

The ability to continuously measure water-quality constituents such as sediment, nutrients, and bacteria frequently is limited by technical and financial constraints. However, turbidity, which often is strongly correlated with sediment, nutrients, and bacteria, can be measured in-stream and on a continuous basis. By developing regression models to establish statistical relations between laboratory-analyzed samples and in-stream turbidity measurements, it is possible to provide continuous estimates of concentration and load under changing streamflow and seasonal conditions.

The information is valuable to State water-quality programs for several reasons. Continuous turbidity data lead to an improved understanding of in-stream processes affecting sediment and sediment-associated constituents. Regression models using turbidity to estimate sediment-associated constituents are better than those using streamflow because there is a better correlation between turbidity and sediment than between streamflow and sediment. Load and yield estimates that are based on continuous data may be more accurate than estimates that are based on discrete samples because the continuous data better capture variability. Continuous concentration estimates can be used to construct duration curves to determine the percentage of time that estimated concentrations exceed existing water-quality criteria or established goals. Because the full range of conditions are displayed, duration curves are a convenient tool for evaluating conditions during any selected time period and making comparisons between sites, seasons, years, and even particular periods of rainfall. In addition, when continuous data are transmitted in near real-time, the information can be used to optimize efforts to collect discrete samples. Continuous data collected over the long term may be used to help identify changes in management practices in the watershed. Finally, continuous data lead to more effective strategies for developing and implementing TMDLs to protect water quality.

Rasmussen, T.J., Ziegler, A.C., Rasmussen, P.P., and Stiles, T.C., 2006, The value of continuous turbidity monitoring in TMDL programs [abs.], in Proceedings of Joint Federal Interagency Conference 2006, April 2-6, 2006: Book of Abstracts, Reno, Nevada: Subcomittee on Sedimentation, Book of Abstracts, p. 312.