"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Measuring Suspended-Solids Concentration with Optical Backscatterance Sensors

By> David H. Schoellhamer and Paul A. Buchanan

Abstract

Optical backscatterance sensors can be used to measure suspended-solids concentration (SSC) in surface waters. They provide a means to automate the collection of high resolution (up to 4 Hertz) time series of SSC. The variability of SSC can be measured over a wide range of time scales, such as those of tides, runoff events, and seasons. Disadvantages of optical sensors include calibration requirements, susceptibility to fouling, and sensitivity to suspended particle size.

An optical backscatterance sensor is a cylinder approximately 7 inches long and 1 inch in diameter with an optical window at one end, a cable connection at the other end, and an encased circuit board (Downing and others, 1981). An infrared pulse of light is transmitted through the optical window and is scattered, or reflected, by particles in front of the window to a distance of about 4 to 8 inches at angles up to 165. Some of this scattered, or reflected, light is returned to the optical window where a receiver converts the backscattered light to a voltage output. Sensor operation is controlled with a data logger. Vertical profiles of SSC are measured by lowering an optical sensor from a boat. SSC time series are measured by attaching optical sensors to an anchored wire mooring or to an instrument stand that sits on the bottom of the river or bay.

The accuracies of sensors permanently deployed in San Francisco Bay, California, are checked using a 100 Nephelometric turbidity units (NTU) turbidity solution prepared from a 4000 NTU formazin standard. At the field site, clean sensors are immersed in the standard and the voltage output is recorded in the station log. Monitoring sensor accuracy in a known standard over a period of time aids in identifying output drift and sensor malfunction. The voltage output is proportional to the concentration of suspended solids in the water column at the depth of the sensor. Calibration of the sensor voltage output to SSC will vary depending on the size and optical properties of the suspended solids; therefore, the sensors must be calibrated either in the field or in a laboratory using the same suspended material that is in the field. To insure that the suspended material used for calibration is identical to the actual suspended material, we use field samples to calibrate optical sensors deployed in San Francisco Bay. The calibrations usually are nearly linear with significance levels less than 0.001 (Buchanan and Schoellhamer, 1996).

Optical sensors are more responsive to fine sediment than to coarse sediment. Vertical profiles of SSC and sensor output collected in the Colorado River indicate how particle size affects sensor output. The slope of a calibration curve is approximately SSC divided by voltage output. When 10, 50, and 90 percent of the suspended sediment was sand, the slope of the calibration curve was about 1, 2, and 10 milligrams per liter per millivolt. In Tampa Bay, Florida, and San Francisco Bay and where suspended sediments are silt and clays, SSC and sensor output are nearly linear over a wide range of conditions, which indicates that particle size is not changing enough to affect sensor output.

Plankton may have a small effect on optical sensor output. An experiment in Tampa Bay found that phytoplankton larger than 20 micrograms may interfere with optical sensors when the plankton concentration exceeds several thousand cells per milliliter (Schoellhamer, 1993) and SSC is near or below the sensor response threshold. Zooplankton did not affect optical sensor output.

Sensor fouling interferes with the collection of accurate optical backscatterance data. Typically, biological growth on the sensors increases voltage output. The time required for biological fouling varies depending on water temperature, location, and season. Fouling can occur in as short as 24 to 36 hours during summer in Tampa Bay, but only rarely occurs during winter in San Francisco Bay. Regular servicing trips are required to clean sensors and to collect calibration samples. Optical sensors with automated wiper blades were installed at some sites in San Francisco Bay to reduce fouling, but they were prone to leaking around the wiper blade shaft and were undependable. Other fouling can be caused by animals, such as fish and crabs, that are illuminated by the sensor and cause a voltage spike in the time series. Fouling also has been caused by debris that blocks the optical window, such as during the 1996 controlled flood in the Colorado River. Time series data must be edited to remove fouling. Fouling usually appears as a gradual voltage increase that abruptly returns to background values when cleaned. In tidal environments, the voltage increase is usually accompanied by a change in the temporal symmetry of the signal. Comparison of voltage output from multiple sensors deployed at one site or nearby sites are also used to indicate fouling. Fouling at our San Francisco Bay sites is also identified by collecting samples before and after instruments are cleaned during servicing trips. Efforts to correct fouled time series data by applying shifts and numerical filters have not proven successful, so fouled data are removed from the time series.

References

Buchanan, P.A., and Schoellhamer, D.H., 1996, Summary of suspended-solids concentration data, San Francisco Bay, California, water year 1995: U.S. Geological Survey Open-File Report 96-591, 40 p.

Downing, J.P., Sternburg, R.W., and Lister, C.R.B., 1981, New instrumentation for the investigation of sediment suspension processes in the shallow marine environment: Marine Geology, v. 42, p. 19-34.

Schoellhamer, D.H., 1993, Biological interference of optical backscatterance sensors in Tampa Bay, Florida: Marine Geology, v. 110, p. 303-313.


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