(photo by Peter Wolter, NRRI)
Sediment plumes in Lake Superior. Suspended sediments are carried to the lake from streams and rivers like
Kingsbury Creek, pictured below.
See how a rainstorm affected Tischer Creek.
Turbidity and TSS
Turbidity is a water quality parameter that refers to how clear
the water is. The greater the amount of total suspended solids
(TSS; also called total suspended sediment) in the water, the
murkier it appears and the higher the measured turbidity. Clay,
silt, and sand from soils, phytoplankton (suspended algae),
bits of decaying vegetation, industrial wastes and sewage are
common suspended solids. Turbidity is usually measured using
an optical instrument called a turbidimeter. If the water is
darkly stained from dissolved organic material (usually coming
from bogs and other wetlands), this may also contribute to decreased
Why is it Important?
Measuring turbidity in streams is extremely important as
an indicator of the concentration of suspended sediments in
the water. Sediments are a natural part of streams and other
water bodies and even the most pristine streams in undeveloped
watersheds will run muddy during high flows.
However, excessive sedimentation in streams and rivers is considered
to be the major cause of surface water pollution in the U.S
(38% of stream miles) followed by pathogens at 36%, and
nutrients at 28%. Nutrients are the leading source of impairment
to lakes, ponds, and reservoirs (US EPA 2000).
High turbidity and suspended solids in streams and lakes may be
caused by many factors including:
- soil erosion associated with agricultural practices, construction site runoff
photos from Stream Corridor Restoration: Principles, Processes, and Practices (10/98).
Interagency Stream Restoration Working Group (FISRWG)
- domestic and industrial wastewater discharge
- urban runoff from roads, parking lots and other impervious surfaces
- flooding and chronically increased flow rates (see Flow)
- algae growth from nutrient enrichment (called eutrophication)
- dredging operations in the stream itself or in feeder tributaries or ditches
photo from Visualizing the Great Lakes: Images of a Region
photo from Stream Corridor Restoration: Principles, Processes, and Practices (10/98).
Interagency Stream Restoration Working Group (FISRWG)
- removal of riparian vegetation and other stream bank disturbances
- too many bottom-feeding fish (such as carp) that stir up bottom sediments
- food for invertebrates, amphibians and fish
- habitat (cover)
- substrate for eggs
- anchors for shoreline sediments
- oxygen production
Expected impacts of pollution and effects on organisms
— Light, food, mechanical,
oxygen and temperature —
Increased turbidity affects a stream and the organisms that
live in it in many ways and if the water becomes too turbid,
it loses the ability to support a wide variety of plants and
other aquatic organisms. Suspended solids may cause the water
color to darken thereby reducing the amount of light available
for aquatic vegetation, algae and mosses to grow by photosynthesis.
Reduced plant matter means less food and habitat for herbivorous
organisms such as snails, insects and juvenile fish. As photosynthesis
slows, less oxygen is released into the water during the daytime
and the plants may even die. As they decompose, bacteria will
use up even more oxygen from the water. Reduced clarity also
interferes with the ability of visual insect and fish predators
to find their prey and may also impair reproduction when visual
cues are a part of courtship and mating.
Fine particulate sediment can also have
purely mechanical effects by clogging sensitive fish and insect
gills, abrading soft tissues, and scouring algal and microbial
"mats" growing on rocks. Growth rates and resistance
to disease may be reduced and proper egg and larval development
may be prevented. As particles of silt, clay, and other organic
materials settle to the bottom, they can suffocate newly hatched
larvae and potentially interfere with particle feeding activities.
Settling sediments can fill in spaces between rocks which
could have been used by benthic aquatic organisms for homes.
The organic matter fraction of these settling solids is a
source of food but too much of it can lead to oxygen depletion
(see the biological oxygen demand section for more details and also
the dissolved oxygen pages on Water on the Web --
This is especially important in cold-water systems, such as
the 12 designated trout streams in Duluth, because their
native fish and insect eggs require continuously high levels
of oxygen to develop properly.
Another negative effect of increased TSS is
that turbid waters usually become warmer because suspended
solids darken the water and absorb more heat from sunlight.
Warm water holds less oxygen than cold water, so O2 levels
will decrease in addition to the direct effects associated
with increased on cold- and cool-water adapted native species.
This adds to the other increasing effects of urbanization
such as heating by parking lots and roads and by the removal
of shading by riparian vegetation. (see temperature)
— Toxic compound and nutrient associations —
Suspended solids also provide adsorption surfaces and a route
of transmission for many organic contaminants, heavy metals,
and some nutrients. Many of the most toxic industrial compounds
such as dioxins and furans, PCB's (polychlorinated biphenyls),
PAH's (polycyclic aromatic hydrocarbons), many pesticides
and heavy metals such as mercury, cadmium, lead, zinc, and
chromium are "sticky" molecules that adhere to both
fine organic and clay particles. The particles may provide
a route of accumulation into the food web via ingestion but
they may also act to bind the pollutants in areas of deep
water where particles can settle out. In deep lakes this may
be essentially permanent , but in streams it is likely to
be only temporary until the next strong flushing event from
Sediments can also be a major source of the
plant nutrients phosphorus, nitrogen (in its ammonium form)
and iron. In sensitive receiving waters, particularly northern
Minnesota lakes, excess nutrients can over stimulate algal
and higher plant growth leading to a host of water quality
problems. See the WOW,
websites for further information.
— Drinking water, water quality standards and aesthetic effects —
Suspended sediments also interfere with the recreational
use and aesthetic enjoyment of water but there are because
of the wide variation in TSS and turbidity there are generally
no numerical criteria for TSS or turbidity that apply to all
streams and lakes. The
Water Quality Rules for the Waters
of Minnesota are developed by the Minnesota Pollution Control Agency.
General provisions are included to prevent the degradation
of any water, irrespective of a specific standard and a general
anti-degradation statement. The GENERAL STANDARDS FOR DISCHARGERS
TO WATERS OF THE STATE (Chapter 7050.0210 Subparagraph 2)
further states that Nuisance conditions are prohibited and
"No sewage, industrial waste,
or other wastes shall be discharged from either point or nonpoint
sources into any waters of the state so as to cause any nuisance
conditions, such as the presence of significant amounts of
floating solids, scum, visible oil film, excessive suspended
solids, material discoloration, obnoxious odors, gas ebullition,
deleterious sludge deposits, undesirable slimes or fungus
growths, aquatic habitat degradation, excessive growths of
aquatic plants, or other offensive or harmful effects."
In Class 2A (coldwater fishery) and Class 2B
(Cool or warmwater fishery) Waters the State has set chronic
turbidity standards of 10 NTU and 25 NTU, respectively where
"chronic standard" means the highest water concentration
of a toxicant to which organisms can be exposed indefinitely
without causing chronic toxicity (7050.0220 SPECIFIC STANDARDS
OF QUALITY AND PURITY BY ASSOCIATED USE CLASSES). This definition
allows for periodically high fluctuations that are associated
with storm events. State Water Quality Rules also set specific
limits for TSS for the discharges from industrial and municipal
wastewater treatment plants, usually in the range of 30-60
Turbidity also adds real costs to the treatment
of surface water supplies used for drinking water since the
particulate material causing it must be virtually eliminated
for effective disinfection to occur. Adding chlorine in a
variety of forms is the typical process used to disinfect
domestic water, but the source water must be clarified by
filtration to an extremely high degree prior to chlorination
(see LINK to Duluth Water Treatment Plant, MN Dept of Health
and EPA DRINKING WATER sites for further information). This
is because many disease-causing microorganisms adhere to particlulates
and as a result receive less exposure to disinfection processes.
Lakes and Ponds
The major source of turbidity in the open water zone of most lakes
in northern Minnesota is usually phytoplankton, but closer
to shore, particulates may also be clays and silts from shoreline
erosion or resuspended bottom sediments. Both of these turn
the western arm of Lake Superior near Duluth brown on a windy
day as in the image at the top of this section. Organic detritus
from stream and/or wastewater discharges may also contribute
turbidity to lakes as can soil erosion from streams draining
agricultural land (see image to right).
Dose effects - How much and for how long?
Very high levels of turbidity for a short period of time
may not be significant and may even be less of a problem than
a lower level that persists longer. The figure below shows
how aquatic organisms are generally affected.
|Schematic adapted from "Turbidty:
A Water Quality Measure", Water Action Volunteers,
Monitoring Factsheet Series, UW-Extension, Environmental
Resources Center. It is a generic, un-calibrated impact
assessment model based on Newcombe, C. P., and J. O. T.
Jensen. 1996. Channel suspended sediment and fisheries:
a synthesis for quantitative assessment of risk and impact.
North American Journal of Fisheries Management. 16: 693-727.
Summer Storm at Tischer Creek
Ever noticed the flow of your local stream changing from
a clear trickle to a mud flow after a storm?
See what happened
in Tischer Creek from a couple of inches
of rain the night of July 7, 2003.
Use our Data Visualization Tools
to find your own examples.
Reasons for Natural Variation
Turbidity varies seasonally, and in larger bodies of water
with depth, in response to natural and human-caused physical,
chemical and biological changes in streams and lakes. Mineral
and organic particles washed in from the watershed vary
largely in response to hydrological events such as storms and
snowmelt that are seasonal and vary widely in intensity,
timing and duration from year to year.
In Duluth, stream flow regimes
throughout the year are typically characterized by low,
or base-flow conditions that most commonly occur in summer
and winter, the spring snowmelt runoff (high flow) period,
and sporadic periods of storm runoff (high flow).
Duluthians know better than most people about how variable these
periods are and how different years can be. We must be
careful to interpret water quality data in light of how the streams
are flowing. This is the major reason for the need
for consistent long-term monitoring programs. Without decades-long strings
of data acquired using comparable and quality assured
methodology, it is impossible to tease out the cause
and effect relationships between human activities and suspected environmental
problems (see our QA/QC page for details).
TSS - Methodology
Calculate TSS as:
TSS(mg/L) = ([A-B]*1000)/C
- Final dried weight of the filter
- Initial weight of the filter
- Volume of water filtered
How do we measure TSS directly?
Usually, we measure turbidity to provide a cheap estimate of the
total suspended solids or sediments (TSS) concentration
(in milligrams dry weight/L). Basically we pre-weigh a
disc of filter paper made from tiny glass fibers and then
suck a measured volume of water through it. The filter
is then dried and re-weighed to calculate the weight of
particulate material that was in he water sample. It's
pretty simple but tedious which is why a turbidimeter
is useful. We can also burn the filter at 475 °C in
a lab furnace to vaporize the organic matter, which we
can then calculate as the difference between the total
solids and the mineral residue remaining on the combusted
filter. This is tedious and difficult to do accurately
for low turbidity water - the reason why a turbidimeter
is often used. The transparency tube is even cheaper but
not easily automated and not as sensitive as a turbidimeter.
How do TSS and turbidity compare?
The figure below shows how the two measurements compare for an erodible shoreline
area of Lake Independence (a Water-on-the-Web lake in the Minneapolis-St.
Paul metro area). The relationship can vary considerably between different
aquatic systems and even at different times for the same stream or lake so
there's no simple conversion factor to use.
A general rule of thumb:
1 mgTSS/L ~ 1.0 to 1.5 NTU's of turbidity
BUT - turbidity scattering depends on particle size so this only an approximation.
Allan, J.D. 1995. Stream Ecology:
Structure and Function of Running Waters. Chapman and
Hauer, F.R. and Lamberti, G.A. (ed.'s) 1996. Methods
in Stream Ecology. Academic Press Inc. San Diego, CA.
Merritt. R.W. and Cummins, K.W. (ed.'s) 1984. An Introduction to the Aquatic
Insects (2nd edition). Kendall/Hunt Publishing Company. Dubuque, IA.
Michaud, J.P. 1991. A citizen's guide to understanding and monitoring lakes and streams.
Publ. #94-149. Washington State Dept. of Ecology, Publications Office, Olympia, WA, USA (360) 407-7472.
Tester, J.R. 1995. Minnesota's Natural Heritage: An Ecological Perspective.
University of Minnesota Press. Minneapolis, MN.
US EPA 2000 National Water Quality Inventory: 1998 Report to Congress. www.epa.gov/305b/98report Last updated October 5, 2000.