(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.

Watersheds Water Quality Primer Water Quality Impacts Organisms Water and Wastewater Landscape

WATER QUALITY

PRIMER

Conversion Tables

Temperature

Weight

Length

Area

Volume

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 clarity.

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

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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

http://www.seagrant.umn.edu/pubs/vgl/

channelization

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

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Macrophytes provide:

food for invertebrates, amphibians

and fish

habitat (cover)

substrate for eggs

anchors for shoreline sediments

oxygen production

too many bottom-feeding fish (such as carp) that stir up bottom sediments

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 -- link1

and link2). 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 high flows.

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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, LakeAccess, and Shoreland Management 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 that:

"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 mg TSS/L.

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.

LakeSuperiorStreams - Turbidity & TSS http://www.lakesuperiorstreams.org/understanding/param_turbidity.html

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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.

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.

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).

Some examples from Duluth's streams

Short-term events: Data "snapshots" from short-term high runoff events for the monitored Duluth

streams may be found in the STREAMS section of this website (look in the "Storm Data" column of

the data availability table).

What does the graph mean?

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Look at one of the rain events (blue bars). After each rain event:

Stream depth increases, due to

runoff.

Turbidity increases.

Sediments are washed from the

watershed into the stream.

Loose sediments from

construction projects, roads,

parking lots, and un-vegetated

soils are carried into the

streams.

Conductivity (salt concentration)

decreases due to dilution from

the large volume of water

entering the stream during the

storm

Seasonal and annual variations: This data graph (coming soon!) shows the variability we've seen so

far at Kingsbury Creek.

Turbidity measurement

There are actually several ways to estimate turbidity and these are all being used in

DuluthStreams. Field and lab turbidimeters measures the scattering effect suspended particles have

on light.

Field turbidity measurements at Chester, Kingsbury and Tischer Creeks are made with submersible

turbidity sensors that are described in the Quality Assurance section.

"Bench" turbidimeters are used for manually collected water samples. We

compare stream water to calibration standards in the NRRI-UMD Water

Quality Lab.

Transparency tubes (also called turbidity tubes)

are being used by volunteer efforts at local schools

(see Riverwatch). The method follows that used by

volunteers throughout Minnesota in the Citizen

Stream Monitoring Program (coordinated by the

Minnesota Pollution Control Agency). The "T-Tube"

gives us a measure of clarity similar to what

limnologists and lake volunteers measure when

they use a Secchi Disk in deep water (WOW for

details). It involves looking down a tube at a black

and white disk and recording how much stream

water is needed to make the disk disappear.

Additional details may be found at WaterOn-the-Web.

What in the world are Nephelometric Turbidity Units (NTU's)?

They are the units we use when we measure Turbidity. The term Nephelometric refers to the way

the instrument estimates how light is scattered by suspended particulate material in the water. The

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TSS - Methodology

Calculate TSS as:

TSS(mg/L) = ([A-B]*1000)/C

where:

Final dried weight of the filter

(in milligrams)

A.

Initial weight of the filter

(in milligrams)

B.

Volume of water filtered

(in Liters)

C.

Nephelometer, also called a turbidimeter, has a photocell (similar to the one on your camera or

your bathroom nightlight) set at 90 degrees to the direction of the light beam to estimate scattered

rather than absorbed light. This measurement generally provides a very good correlation with the

concentration of particles in the water that affect clarity.

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.

LakeSuperiorStreams - Turbidity & TSS http://www.lakesuperiorstreams.org/understanding/param_turbidity.html

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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.

REFERENCES:

Allan, J.D. 1995. Stream Ecology: Structure and Function of Running Waters. Chapman and Hall.

London.

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.

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