In-Situ TROLL 9500 Operators Manual - Manual (Page 126)

121
Multi-Parameter
Water Quality TROLL ®
TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
WHAT IS TURBIDITY?
Turbidity is an indirect measure of the clarity or transparency of water,
and thus an important indicator of its condition and productivity.
Created by suspended matter and microscopic organisms, turbidity
causes light to be scattered and absorbed rather than transmitted
directly through water. Turbidity is the physical characteristic of the
solution that causes light scattering. Turbidity is the opposite of clarity.
The APHA reference work Standard Methods (Eaton and others,
2005) defines turbidity as “an expression of the optical property that
causes light to be scattered and absorbed rather than transmitted with
NO
Turbidity is not . . .
s
light-scattering abilities.
4HE
WHY MEASURE TURBIDITY?
Turbidity measurements
s
sediments (TSS) concentration in water.
s
of a natural water body. Clear water
lets light penetrate more deeply into a
lake or stream than does murky water.
This light allows photosynthesis to occur
and oxygen to be produced.
s
s
rates, can provide a good indication of true formation water.
Higher turbidity levels make it more costly to treat surface water for
use as drinking water. Controlling turbidity may be an effective way to
protect against pathogens in drinking water.
Aesthetic considerations also play a role in our desire to quantify tur-
bidity: Most people would rather look at, drink, or swim in clear water
than in water that appears cloudy, and closely associate appearance
with the health of the body of water.
HOW IS TURBIDITY MEASURED?
Historical methods for measuring turbidity relied on subjective esti-
mates that depended largely on the eye of the beholder.
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served through the length of a glass tube into which
A
The Secchi disk method used in limnological studies
involves submerging a weighted, black-and-white
painted metal plate until the pattern can no longer
Turbidity sensor
Optional wiper
18 TURBIDITY
EPA drinking water 0.3 - 0.5 NTU
Treated water 0 - 1 NTU
&RESH < 10 NTU
Fresh water, 2.5” visibility 240 NTU
Short-term stress to aquatic life
Unsafe level for most aquatic life
Typical turbidity values