Environmental Engineering Reference
In-Depth Information
To reduce the variability among instruments meas-
uring identical in-stream turbidity conditions, a
USGS protocol (Anderson 2005) requires that tur-
bidity data be reported based on instrument design
in one of ten units, comprising eight new reporting
units in addition to the two established reporting
units, the nephelometric turbidity unit and the for-
mazin nephelometric unit (USGS 2008d). These ten
reporting units provide a systematic method by
which to characterize the type of turbidimeter used
and are intended to improve the comparability of
turbidity data.
Commercially available optical instruments
operate on one of two bulk-optic principles.
Transmissometers use a light source beamed directly
at the sensor. The instrument measures the fraction
of light from a collimated light source (typically
within the visible range at about 660 nm) that reaches
a light detector. The fraction of light reaching
the detector is converted to a beam attenuation
coeffi cient, which is related to SSC. Few turbidime-
ters operate on the transmissometry principle.
Nephelometers measure visible or infrared (IR) light
scattered by suspended particles (rather than light
transmitted through particles). They measure scat-
tering in a (SSC-dependent) volume less than a few
cubic centimeters. Most turbidimeters measure 90 °
scattering. Optical backscatterance instruments
(OBS) (Downing
et al.
1981; Downing 1983) are a
type of nephelometer designed to measure less than
180 ° backscattered IR light in a volume on the
order of a few cubic centimeters. Figure 1.5 shows
examples of nephelometry and optical-backscatter
sensors.
Two instruments widely used for
in situ
applica-
tions are the YSI Model 6136 turbidimeter (manu-
factured by YSI, Inc.), which measures IR scatter at
90 °, and OBS-3+ (manufactured by Campbell
Scientifi c, Inc.), which measures IR backscattered at
about 140-160 °. Transmittance and scatterance are
functions of the density, size, color, index of refrac-
tion, and shape of suspended particles (Conner & De
Visser 1992; Sutherland
et al.
2000).
In summer 2008, the purchase price of an
in situ
nephelometric turbidimeter with sonde, wiper, and
controller was about US$5000. The cost of an OBS
and cable without a wiper was about equal to the
average cost of a fully equipped
in situ
nephelometric
turbidimeter.
less risk to fi eld personnel, coupled with advanced
technological capabilities, is leading to a new era in
fl uvial-sediment monitoring. The following sections
describe theoretical principles (Gray & Gartner
2004), selected examples of fi eld applications, and
advantages and limitations of fi ve suspended-
sediment-surrogate technologies that cover a range
of transport conditions and are considered to be
acceptable or promising by the USGS.
1.2.1 Turbidity (bulk optics)
Patrick P. Rasmussen, John R. Gray, Andrew C.
Ziegler, G. Douglas Glysson, & Chauncey W.
Anderson
1.2.1.1 Background and theory
Turbidity is an expression of the optical properties
of a sample that cause light rays to be scattered and
absorbed rather than transmitted in straight lines
through the sample (Ziegler 2003; Anderson 2005).
According to the USGS (2004), “Turbidity itself is
not an inherent physical property of water (as is, for
example, temperature), but rather is a measure of
light scattering through a liquid as measured by
detectors with known geometry,” and hence is oper-
ationally defi ned. Measurements of turbidity are the
most common means of determining water clarity
and computing SSC in US rivers (Pruitt 2003). The
instrument-measurement realm of a turbidimeter is
usually a point in a stream (Secchi disk measure-
ments being a notable exception). Both instrument
and cross-section calibrations are normally
performed.
The confi guration of detectors and the source of
light are important factors in the response of the
turbidity instrument. Although comparisons among
instruments with differing designs are often robust,
they can also vary according to the character of the
sample's matrix and particulates. Results from an
interagency workshop held in 2002 demonstrated
that turbidity data from different sources and instru-
mentation can be highly variable and are often in
disagreement with each other, even when instru-
ment-calibration methods are similar (Gray &
Glysson 2003). In effect, instruments with different
detector geometries and light sources often do not
make equivalent measurements.
