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
NOCHANGEINDIRECTIONORmUXLEVELTHROUGHTHESAMPLEv
Turbidity is not . . .
s ADIRECTMEASUREOFCLARITY
s AMEASUREOFCOLOR
s AMEASUREOFSUSPENDEDSOLIDSITISAMEASUREOFTHEIR
light-scattering abilities.
4HE)N3ITU4URBIDITYSENSORISINCONFORMANCEWITHTHE)3/
TURBIDITYSTANDARDWHICHSPECIlES½SCATTEREDLIGHT
WHY MEASURE TURBIDITY?
Turbidity measurements
s CANPROVIDEAREASONABLEESTIMATEOFTHE
total suspended solids
or
sediments (TSS) concentration in water.
s CANTELLUSSOMETHINGABOUTTHEHEALTH
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 CANBEAUSEFULINDICATOROFRUNOFFINTOSURFACEWATERSYSTEMS
s INmOWCELLORINLINEAPPLICATIONSWHENPUMPINGWATERATVERYLOW
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.
)NTHE*ACKSON#ANDLEMETHODFOREXAMPLEACANDLEmAMEISOB
-
served through the length of a glass tube into which
AmUIDSAMPLEISPOUREDUNTILTHERAYSOFTRANSMIT
-
TEDANDSCATTEREDLIGHTAPPEAREQUALANDTHEmAME
essentially disappears. Among several drawbacks to
this method, the reproducibility of standards formu-
lated from natural sediments was difficult to control.
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
&RESHWATERvVISIBILITY
< 10 NTU
Fresh water, 2.5” visibility
240 NTU
Short-term stress to aquatic life
.45
Unsafe level for most aquatic life
.45
Typical turbidity values