Excessive
suspended sediment is the number-one cause of water quality
violations in the U.S. Metal and insecticide contaminants are
also leading causes of violations. Most of these contaminants
are bound to suspended sediment. Turbidity can indicate the
presence sediment in surface waters and the chemical contaminates
it carries, however, the relationship between NTUs and SSC can
vary widely in utility as shown below.
It is often assumed that turbidity provides a direct measure
of suspended sediment and that there is a formula or set of
conversion factors with which SSC can be calculated from NTUs.
This is simply not the case and no such formulas exist. Some
also believe that turbidity standards behave the way sediments
do in an OBS sensor or turbidity meter. This is also not true
in general because the size, NIR reflectivity, refractive index,
and shape of sediment particles and turbidity standards (formazin
and SVDB) differ from one another.
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The most important of
these factors in particle size, which can vary be a factor
of 1000 in the environment. The graph below shows
the size distributions of turbidity standards and suspended
sediments in rivers and the ocean. When the sizes of sediment
and standards overlap, there is good reason to believe the
standards will mimic the light-scattering effects of sediment.
While in other size ranges the materials will not be optically
similar. The other thing to realize is that while turbidity
standards are fairly uniform in terms of size, refractive
index and SVDB particles have uniform shape, sediment particles
vary widely from place to place. The SEM images of flocs from
the Columbia and Colorado Rivers illustrate the huge diversity
of size and shape encountered in the environment.
When using OBS sensors to measure SSC, it is unnecessary
to first calibrate them with formazin and then establish a
relationship between NTU values and SSC as many OBS users
continue to do. If your primary objective is to measure SSC,
do the calibration with sediment and avoid the errors introduced
by making two numerical conversions, one from OBS signal to
NTU values and a second one from NTU values to SSC. If you
calibrate directly from OBS signal to SSC, you achieve an
improved standard error in SSC conversions. |
There are situations, however,
you will have a record of turbidity in NTUs and will want
to correlate it with concurrent measurements of SSC in water
samples. There are environments where this works well and
there are others where it is not satisfactory. For example
a large number of turbidity and SSC measurements in runoff
from a freeway is shown on the left-hand log-log graph shown
above. While the general relationship is useful for establishing
time trends in SSC of highway runoff in the monitored location,
the NTU-SSC relationship cannot provide an accurate way
to estimate. At a second location in San Francisco Bay,
however, the SSC is highly correlated with NTU values obtained
with an OBS sensor, right-hand graph. At this monitoring
location, the NTU-SSC relation will provide for the accurate
estimation of SSC from OBS turbidity measurements.
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Reference:
Lewis, Jack. 1996. Turbidity -Controlled Suspended
Sediment Sampling for Runoff-Event Load Estimation.
Water Resources Research Volume 32, No. 7, pp. 2299-2310.
John Downing. 2005. Turbidity Monitoring.
Chapter 24 in: Environmental Instrumentation and Analysis
Handbook. John Wiley & Sons, Pages: 511-546. 2005.
Schoellhamer, D., P. Buchanan, & N. Ganju. 2002. Ten
Years of Continuous Suspended-Sediment Concentration Monitoring
in San Francisco Bay & Delta. Turbidity
& Other Sediment Surrogates Workshop, Reno, NV. 3 pages.
U.S. Geological Survey. 2001. Summary of the
U.S. Geological Survey On-line Instantaneous Fluvial Sediment
and Ancillary Data. Proceedings of the 7th Federal
Interagency Sedimentation Conference, Reno, NV.
Brent, G.C., J.R. Gray, K.P. Smith, and G.D. Glysson. 2001.
A Synopsis of Technical Issues for Monitoring
Sediment in Highway and Urban Runoff. U.S. Geological
Survey, Open-File Report 00-497, 51 pages.
Sheldon, R.W., A. Prakask, and W.H. Sutcliffe, Jr. 1972.
The Size Distribution of Particles in the Ocean.
Limnology and Oceanography, Vol. XVII(3), pp. 327-340.
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