Turbidity
Turbidity is a measure of water clarity. High turbidity makes water appear cloudy or muddy.
Why do we measure turbidity?
Figure 7. Fish in turbid water (left), and fish in clear water (right).
Credit: Photo courtesy of Credit Valley Conservation
Turbidity and total suspended solids (TSS) are different
ways to measure similar water quality characteristics.
TSS is the concentration of suspended particles, which
include soil particles (clay, silt, organic matter), algae, and
microscopic organisms.
An increase in turbidity (Figure 1) or suspended solids
can aiso negatively affect aquatic health by:
Clogging fish gills or the filter-feeding systems of
other aquatic animals.
Hindering visibility, making it difficult for predators to
find prey.
Decreasing light penetration into water and
thereby the ability of submerged aquatic plants to
photosynthesize, reducing biomass and growth rates
of aquatic plants.
Reducing fish resistance to disease.
Altering egg and larval development.
Changes in turbidity can also affect other water
quality parameters; increased turbidity is likely to be
accompanied by the following:
Higher temperature and reduced dissolved oxygen
due to increased heat absorption of the water.
Reduced dissolved oxygen due to decreased light
penetration into the water and an associated
decrease in photosynthesis by aquatic plants.
Increased nutrient concentrations and chlorophyll a if
the turbidity is caused by excess algal growth.
The suspended solids contributing to turbidity can affect
water chemistry and microbiology. The particles can
adsorb (take up on their surfaces) pollutants, including
nutrients, metals, and organic compounds, if the
particles settle on the bottom of the waterbody, then
the pollutants settle with them, if bottom sediments are
subsequently disturbed and resuspended, the aquatic
community can be exposed to any adsorbed toxins or
nutrients.
In drinking water, particles can interfere with disinfection
by physically blocking UV rays from reaching
microorganisms. Some microorganisms can make
people sick if they occur in drinking water.
For factsheets on other water quality parameters, visit:
epa.gov/awma/factsheets-water-aual itv-para meters.
1
For more information about the CWA Section 106 Grants Program, visit:
epa.aov/water-pollution-control-section-106-g rants.

-------
Turbidity
What affects turbidity?
Natural factors that increase turbidity inciude:
Runoff caused by precipitation and/or severe
weather.
Disruption of bottom sediments (resuspension) due
to water turbulence from windstorms or rain events.
Bottom-feeding animals moving sediments around.
Small floating organisms suspended in the water
column (plankton, algae, cyanobacteria).
Dead organic matter in the water column.
Wood ash from wildfires that reaches surface water.
Spring snowmelt and precipitation
Summer algal growth in lakes and slower moving
rivers.
Human-induced factors that increase turbidity include:
Stream bank erosion contributing soil to water
(Figure 2).
Erosion in other areas of the watershed caused by
changes in land use (construction, farming, forestry,
and urban development) that cause soil to be carried
in runoff to surface water.
Urban runoff carrying particles from impervious
surfaces to surface water.
Untreated wastewater discharges.
Disturbance and resuspension of bottom sediments
during dredging or boating activity.
Algal growth due to fertilizer use and resulting
increases in nutrients in the water, especially in lakes
or slower moving rivers.
Figure 2. Example of streambank erosion. Credit: Photo courtesy
of Cuyahoga SWCD
What are EPA's recommended criteria for turbidity?
EPA's Quality Criteria for Water (1986) contains the
following general narrative criterion for turbidity: "Settieable
and suspended solids should not reduce the depth of the
compensation point for photosynthetic activity by more
than 10 percent from the seasonally established norm for
aquatic life."
States and tribes have the discretion to set quantitative or
qualitative water quality criteria for turbidity. For example,
narrative criteria may require no increases above naturally
occurring conditions.

-------
Turbidity
How do we measure turbidity?
Turbidity is measured directly using a turbidity meter or
sensor (nepheiometry). Turbidity can also be measured
indirectly through water clarity which is measured in
deeper rivers or lakes using a Secchi disk.
Turbidity is reported in nephelometric turbidity units (IV' Us)
or Secchi depths (in meters) depending on the method
used for measurement. Figure 3 shows a water sample
that was taken to assess turbidity in murky water.
Turbidity can vary both horizontally and vertically in a
waterbody. Water samples should, therefore, be taken at
regular increments across a waterbody and at various
depths (or depth integrated, which is a sample that
represents the entire water column).
Basic field data collected by a water quality monitoring
program should include turbidity along with other
parameters that may influence turbidity, such as
temperature, streamflow, dissolved oxygen, specific
conductance, and pi I.
Figure 3. Water sample taken to assess turbidity. Credit: Photo
courtesy of US EPA
What are the challenges of using turbidity as a water quality parameter?
Turbidity is an optical property of water rather than a
chemical or biological measurement. Caution should
be exercised when using turbidity as a water quality
parameter because high turbidity levels do not necessarily
indicate poor water quality and low turbidity levels do not
necessarily indicate good water quality. Values should,
therefore, be evaluated alongside other parameters.
Measurements made using a Secchi disk as shown
in Figure 4- are qualitative and subject to the accuracy
of the measurer. A related parameter, total suspended
solids (TSS), is the concentration of particles suspended
in the water column that are larger than two microns in
size. Although turbidity is not a direct measure of TSS,
changes in turbidity often correspond with changes in
TSS. In general, higher turbidity values and greater TSS
concentrations are both observed at higher flows. TSS is
reported in units of mg/L.
Figure 4. Secchi disk measuring the Secchi depth in water with
extensive algal growth. Credit: Photo courtesy of USGS
EPA 841F21007D | July 2021

-------