Water can contain suspended solid matter consisting of particles of different sizes (see TSS). While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of a container if a water sample is left to stand (the settleable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated. These small solids cause the water to be turbid or cloudy, and their measurement is called turbidity. Turbidity usually appears as a cloudiness or haziness in the water because the individual particles are mostly too small to be seen without magnification, much like smoke in air.
There are several practical ways of quantifying cloudiness in water, the most direct being some measure of attenuation (that is, reduction in strength) of light as it passes through a sample column of water. The now little-used Jackson Candle method (units=Jackson Turbidity Unit or JTU) is essentially the inverse measure of the length of a column of water needed to completely obscure a candle viewed through it. The more water needed (the longer the water column), the clearer the water. Of course water alone produces some attentuation, and any substances dissolved in the water that produce color can attenuate some wavelengths. Therefore, a property of the particles -- that they will scatter a light beam focused on them -- was selected as a more meaningful measure of turbidity. Turbidity is measured this way in an instrument called a nephelometer. A light detector is setup to the side of a (source) light beam; more light reaches the detector if their are lots of small particles reflecting the source beam than if there are few. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). To some extent, how much light reflects for a given amount of particulates is dependent upon properties of the particles like their shape, color, and reflectivity. For this reason (and the reason that heavier particles settle quickly and do not contribute to a turbidity reading), a correlation between turbidity and TSS is somewhat unique for each location or situation.
Water Quality Standard
Hawaii (US)
Fresh water stream --
Geometric mean of wet season* values not to exceed 5.0 ntu
Geometric mean of dry season** values not to exceed 2.0 ntu
Wet season* values not to exceed 15.0 ntu ten percent
(10%) of the time
Dry season** values not to exceed 5.5 ntu ten percent
(10 %) of the time
Wet season* values not to exceed 25.0 ntu two percent
(2%) of the time
Dry season** values not to exceed 10.0 ntu two percent
(2%) of the time
Estuaries (note separate criteria applicable to Pearl Harbor estuary)
Geometric mean values not to exceed 1.50 ntu (4.00 ntu in PH)
Values not to exceed 3.00 ntu (8.00 ntu in PH) ten percent
(10%) of the time
Values not to exceed 5.00 ntu (15.00 in PH) two percent
(2%) of the time
Embayments --
Geometric mean of wet embayment* values not to exceed 1.50 ntu
Geometric mean of dry embayment** values not to exceed 0.40 ntu
Wet embayment* values not to exceed 3.00 ntu ten percent
(10%) of the time
Dry embayment** values not to exceed 1.00 ntu ten percent
(10 %) of the time
Wet embayment* values not to exceed 5.00 ntu two percent
(2%) of the time
Dry embayment** values not to exceed 1.50 ntu two percent
(2%) of the time
Open Coastal waters --
Geometric mean of wet coast* values not to exceed 0.50 ntu
Geometric mean of dry coast** values not to exceed 0.20 ntu
Wet coast* values not to exceed 1.25 ntu ten percent
(10%) of the time
Dry coast** values not to exceed 0.50 ntu ten percent
(10 %) of the time
Wet coast* values not to exceed 2.00 ntu two percent
(2%) of the time
Dry coast** values not to exceed 1.00 ntu two percent
(2%) of the time
* Measurements made between November 1 and April 30 are wet season values; Wet
embayments receive an average fresh water inflow from land equal to or greater than
1% of embayment volume per day; Wet coast values apply where more than 3 mgd
of freshwater discharge occurs per shoreline mile.
**Measurements made between May 1 and October 31 are dry season values; Dry
embayments receive an average fresh water inflow from land of less than 1% of
embayment volume per day; Dry coast values apply where coastal waters receive
less than 3 mgd of freshwater discharge per shoreline mile.
Republic of Palau
Class 1 fresh waters --
Turbidity (in ntu) shall not be greater than 5% above natural conditions.
Class 2 fresh waters --
Turbidity (in ntu) shall not be greater than 10% above natural conditions.
Class A & AA marine waters --
Turbidity shall not be greater than 1 ntu.
Class B marine waters --
Turbidity shall not be greater than 2 ntu.
Further Reading:
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