Turbidity is the reduction of clarity in water due to the presence of suspended or colloidal particles.
Turbidity is measured by the amount of light which is reflected by the
particles. Turbidity is commonly used as an indicator for the general condition of the
drinking water, but is an easy field water quality
parameter to measure. Turbidity in water is caused by suspended
matter such as clay, silt, and organic matter and by plankton and other
microscopic organisms that interfere with the passage of light through the water
(American Public Health Association, 1998). Turbidity is closely related to
total suspended solids (TSS), but also includes plankton and other
organisms. Turbidity of natural waters tends to increase during runoff
events as a result of increased
overland flow, stream flow, and erosion.
The suspended or colloidal particles, commonly referred to as total
suspended solids (TSS), are all the
suspended solids in water which will
not settle out by gravity. TSS is measured
on a sample of water (which has been
settled) and are those particles which will not pass through a very fine
filter (usually 0.45 micron). The filter is pre-weighed
prior to passing of the water, and
post-weighed. The difference in the two weights is the TSS
concentration (in mg/L).
Turbidity is a measure of how much of the light traveling through water is
scattered by suspended particles. The scattering of light increases with
increasing suspended solid and plankton content. Turbidity in slow moving, deep
waters can be measured using a device called a Secchi disk. A Secchi disk is a
black and white, 20-cm diameter disk. The disk is lowered into the water until
it just disappears from sight. The depth at which the disk disappears is called
the Secchi depth, and is recorded in meters.
A Secchi disk does not work in shallow, fast-moving streams. In these
waters, a turbidimeter (sometimes called a nephelometer) is used. A turbidimeter
measures the scattering of light, and provides a relative measure of turbidity
in Nephelometric Turbidity Units (NTUs). A less expensive method of measuring
turbidity is to evaluate the fuzziness of a mark at the bottom of a clear tube
when a water sample is poured in the tube. Units are reported in Jackson
Turbidity Units (JTUs). This method can only be used in highly turbid waters.
Factors Affecting Turbidity
Because one of the primary factors affecting turbidity is toal suspended
solids, the factors affecting TSS will also affect turbidity. In addition,
organic matter contributes to turbidity.
High Flow Rates
The flow rate of a water body is a primary factor influencing turbidity
concentrations. Fast running water can carry more particles and larger-sized
sediment. Heavy rains can pick up sand, silt, clay, and organic particles from
the land and carry it to surface water. A change in flow rate also can affect
turbidity; if the speed or direction of the water current increases, particulate
matter from bottom sediments may be resuspended.
Soil erosion is caused by disturbance of a land surface. Soil erosion can be
caused by Building and Road Construction, Forest Fires, Logging, and
Mining. The eroded soil particles can be carried by stormwater to
surface water. This will increase the turbidity of the water body.
During storm events, soil particles and debris from streets and industrial,
commerical, and residential areas can be washed into streams. Because of the
large amount of pavement in urban areas, natural settling areas have been
removed, and sediment is carried through storm drains to creeks and rivers.
Wastewater and Septic System Effluent
The effluent from Wastewater Treatment Plants (WWTPs) can add suspended
solids and organic material to a stream. The wastewater from our houses contains
food residue, human waste, and other solid material that we put down our drains.
Most of the solids and organic mateial are removed from the water at the WWTP
before being discharged to the stream, but treatment can’t eliminate
Decaying Plants and Animals
As plants and animals present in a water body die and decay, suspended
organic particles are released and can contribute to turbidity.
Bottom-feeding fish (such as carp) can stir up sediments as they remove
vegetation. These sediments can contribute to turbidity.
Algal blooms can contribute to turbidity. Algal production is enhanced when
nutrients are released from bottom sediments during seasonal turnovers and
changes in water current.
As flood waters recede, they will bring along inorganic and organic particles
from the land surface, and contribute this to the stream.
Water Quality Standards Regarding Turbidity
The U.S. Environmental Protection Agency’s (EPA) Surface Water Treatment
Rule requires systems using surface water or ground water under the direct
influence of surface water to (1) disinfect their water, and (2) filter their
water or meet criteria for avoiding filtration so that at no time can turbidity
go above 5 nephelometric turbidity units (NTUs). Systems that filter must ensure
that the turbidity go no higher than 1 NTU (0.5 NTU for conventional or direct
filtration) in at least 95% of the daily samples in any month.
information about the Water Research Center,