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Turbidity, Total Suspended Solids & Water Clarity
Fondriest
Environmental, Inc.
What are Total Suspended Solids?
Total suspended solids (TSS) are particles
that are larger than 2 microns found in the water column.
Anything smaller than 2 microns (average
filter size) is considered a dissolved solid.
Most suspended solids are made up of
inorganic materials, though bacteria and algae can also contribute to the total
solids concentration.
These solids include anything drifting or
floating in the water, from sediment, silt, and sand to plankton and algae.
Organic particles from decomposing materials
can also contribute to the TSS concentration.
As algae, plants and animals decay, the
decomposition process allows small organic particles to break away and enter
the water column as suspended solids.
Even chemical precipitates are considered a
form of suspended solids.
Total suspended solids are a significant
factor in observing water clarity. The more solids present in the water, the
less clear the water will be.
Some suspended solids can settle out into
sediment at the bottom of a body of water over a period of time.
Heavier particles, such as gravel and sand,
often settle out when they enter an area of low or no water flow.
Although this settling improves water
clarity, the increased silt can smother benthic organisms and eggs.
The remaining particles that do not settle
out are called colloidal or nonsettleable solids. These suspended solids are
either too small or too light to settle to the bottom.
Settleable solids are also known as bedded
sediments, or bedload. These sediments can vary from larger sand and gravel to
fine silt and clay, depending on the flow rate of water.
Sometimes these sediments can move downstream
even without rejoining the suspended solids concentration.
When settleable solids are moved along the
bottom of a body of water by a strong flow, it is called bedload transport.
What is Turbidity?
Turbidity is an optical determination of
water clarity. Turbid water will appear cloudy, murky, or otherwise colored,
affecting the physical look of the water.
Suspended solids and dissolved colored
material reduce water clarity by creating an opaque, hazy or muddy appearance.
Turbidity measurements are often used as an
indicator of water quality based on clarity and estimated total suspended
solids in water.
The turbidity of water is based on the amount
of light scattered by particles in the water column. The more particles that
are present, the more light that will be scattered.
As such, turbidity and total suspended solids
are related. However, turbidity is not a direct measurement of the total
suspended materials in water.
Instead, as a measure of relative clarity,
turbidity is often used to indicate changes in the total suspended solids
concentration in water without providing an exact measurement of solids.
Tannins from decomposing vegetation have
colored this river red.
Turbidity can come from suspended sediment
such as silt or clay, inorganic materials, or organic matter such as algae,
plankton and decaying material.
In addition to these suspended solids,
turbidity can also include colored dissolved organic matter (CDOM), fluorescent
dissolved organic matter (FDOM) and other dyes.
CDOM is also known as humic stain. Humic
stain refers to the tea color produced from decaying plants and leaves
underwater due to the release of tannins and other molecules.
This discoloration is often found in bogs,
wetlands or other water bodies with high amounts of decaying vegetation in the
water.
CDOM can cause water to appear red or brown,
depending on the type of plants or leaves present.
These dissolved substances may be too small
to be counted in a suspended solids concentration, but they are still part of a
turbidity measurement as they affect water clarity.
What is Water Clarity?
Water clarity is a physical characteristic
defined by how clear or transparent water is. Clarity is determined by the
depth that sunlight penetrates in water.
The further sunlight can reach, the higher
the water clarity. The depth sunlight reaches is also known as the photic zone.
The clearer the water, the deeper the photic
zone and the greater the potential for photosynthetic production. The photic
zone (and thus water clarity) has a maximum depth of 200 m based on the light
absorption properties of water.
Water clarity is directly related to
turbidity, as turbidity is a measure of water clarity.
The transparency of water is affected by the
amount of sunlight available, suspended particles in the water column and
dissolved solids such as colored dissolved organic material (CDOM) present in
the water.
Salinity also affects water clarity. This is
due to the effect of salt on the aggregation and settling velocity of suspended
particles.
In other words, salt ions collect suspended
particles and bind them together, increasing their weights and thus their
likelihood of settling to the bottom.
Due to this mechanism, oceans and estuaries
tend to have a higher clarity (and lower average turbidity) than lakes and
rivers.
These marine environments also have a higher
rate of sedimentation as solids are pulled out of the water column to the
seafloor.
Turbidity vs Suspended Solids – What is the
difference?
Turbidity and total suspended solids refer to
particles present in the water column.
Turbidity and water clarity are both visual
properties of water based on light scattering and attenuation.
All three parameters are related to particles
in the water column, whether directly or indirectly.
Turbidity is determined by the amount of
light scattered off of these particles. While this measurement can then be used
to estimate the total dissolved solids concentration, it will not be exact.
Turbidity does not include any settled solids
or bedload (sediment that “rolls” along the riverbed).
In addition, turbidity measurements may be
affected by colored dissolved organic matter.
While this dissolved matter is not included
in TSS measurements, it can cause artificially low turbidity readings as it
absorbs light instead of scattering it.
Total suspended solids, on the other hand,
are a total quantity measurement of solid material per volume of water.
This means that TSS is a specific measurement
of all suspended solids, organic and inorganic, by mass.
TSS includes settleable solids, and is the
direct measurement of the total solids present in a water body.
As such, TSS can be used to calculate
sedimentation rates, while turbidity cannot 1,6.
Water clarity is strictly relative to
sunlight penetration.
While this is usually determined by the
amount of suspended solids in water, it can also be affected by CDOM and other
dissolved solids.
Water clarity is the most subjective
measurement of these three parameters, as it is usually determined by human
observation.
Why are Turbidity and Total Suspended Solids
Important?
Turbidity and TSS are the most visible
indicators of water quality. These suspended particles can come from soil
erosion, runoff, discharges, stirred bottom sediments or algal blooms.
While it is possible for some streams to have
naturally high levels of suspended solids, clear water is usually considered an
indicator of healthy water .
A sudden increase in turbidity in a
previously clear body of water is a cause for concern. Excessive suspended
sediment can impair water quality for aquatic and human life, impede navigation
and increase flooding risks .
Water Chemistry
In terms of water quality, high levels of
total suspended solids will increase water temperatures and decrease dissolved
oxygen (DO) levels.
This is because suspended particles absorb
more heat from solar radiation than water molecules will. This heat is then
transferred to the surrounding water by conduction.
Warmer water cannot hold as much dissolved
oxygen as colder water, so DO levels will drop.
In addition, the increased surface
temperature can cause stratification, or layering, of a body of water. When
water stratifies, the upper and lower layers do not mix.
As decomposition and respiration often occur
in the the lower layers, they can become too hypoxic (low dissolved oxygen
levels) for organisms to survive.
Photosynthesis Production
Turbidity can also inhibit photosynthesis by
blocking sunlight. Halted or reduced photosynthesis means a decrease in plant
survival and decreased dissolved oxygen output.
The higher the turbidity levels, the less
light that can reach the lower levels of water. This reduces plant productivity
at the bottom of an ocean, lake or river.
Without the needed sunlight, seaweed and bay
grasses below the water’s surface will not be able to continue photosynthesis
and may die.
nderwater vegetation die-off has two main
effects. First, as photosynthetic processes decrease, less dissolved oxygen is
produced, thus further reducing DO levels in a body of water.
The subsequent decomposition of the organic
material can drop dissolved oxygen levels even lower.
Second, seaweed and underwater plants are
necessary food sources for many aquatic organisms. s they die off, the amount
of vegetation available for other aquatic life to feed on is reduced. This can
cause population declines up the food chain.
Erosion
An increase in turbidity can also indicate
increased erosion of stream banks, which may have a long-term effect on a body
of water. Erosion reduces habitat quality for fish and other organisms.
In terms of water clarity, reduced light
penetration due to suspended sediment can obscure aquatic organisms’ vision,
reducing their ability to find food.
These suspended particles can also clog fish
gills and affect growth rates.
Erosion can contribute to shallower,
filled-in lakes and streams as some of the suspended particles settle out.
These settleable solids can suffocate benthic
organisms and fish eggs. In addition, the sediment may smother insect larvae
and other fish food sources.
When this occurs in rivers and channels, the
increased sediment loads can reduce navigability for ships and boats.
In cases of excessive sedimentation,
settleable solids from erosion and runoff can even halt freight passage
completely.
Contamination
Pollutants such as dissolved metals and
pathogens can attach to suspended particles and enter the water.
This is why an increase in turbidity can
often indicate potential pollution, not just a decrease in water quality.
Contaminants include bacteria, protozoa,
nutrients (e.g. nitrates and phosphorus), pesticides, mercury, lead and other
metals.
Several of these pollutants, especially heavy
metals, can be detrimental and often toxic to aquatic life.
The addition of nutrients can encourage the
development of harmful algal blooms.
When the suspended solids concentration is
due to organic materials, particularly sewage effluent and decaying organic
matter, the presence of bacteria, protozoa and viruses are more likely.
These organic suspended solids are also more
likely to decrease dissolved oxygen levels as they are decomposed.
Human Concerns
These microbes and heavy metals can impact
not only aquatic organisms, but drinking water as well.
Organic suspended solids, such as decomposing
matter or sewage effluent often naturally include high levels of microorganisms
such as protozoa, bacteria and viruses.
Such pathogens contribute to waterborne
diseases like cryptosporidiosis, cholera and giardiasis.
Turbid water, whether due to organic or
inorganic material, cannot be easily disinfected, as the suspended particles
will “hide” these microorganisms.
In a lake or river, turbidity may also reduce
visibility of underwater structures such as logs or large boulders, negatively
affecting a water body’s recreational use.
In industrial processes, turbidity can
contribute to clogged tanks and pipes. The particles can also scour machines,
potentially damaging them.
What Contributes to Suspended Solids?
Suspended solids in a body of water are often
due to natural causes.
These natural solids include organic
materials such as algae, and inorganic materials such as silt and sediment.
Some algae, such as phytoplankton, are
regular occurrences, especially in the ocean. Inorganic materials can easily
become suspended due to runoff, erosion and resuspension from seasonal water
flow.
However, when suspended solids exceed
expected concentrations, they can negatively impact a body of water.
Excess over background amounts are often
attributed to human influence, whether directly or indirectly.
Pollution may contribute to either organic or
inorganic suspended solids, depending on the source.
Algae, sediment and pollution will affect
water quality in different ways depending on the quantity present.
Algae
Algae are plantlike, photosynthesizing
organisms that can thrive in both freshwater and saltwater.
These organisms come in different sizes, from
microscopic phytoplankton to giant sea kelp forests.
Both the phytoplankton and seaweed forms of
algae will consume nutrients in the water and can increase dissolved oxygen
levels through photosynthesis.
When they die, however, the organic material
is decomposed by microbes in the water column. This decomposition process can
decrease dissolved oxygen levels to below normal levels.
Seaweed and kelp are found rooted to the
seafloor, but phytoplankton and other micro-algae can be found at the water’s
surface or throughout the water column.
In particular, cyanobacteria, or blue-green
algae, have floating mechanisms that keep them at the surface, blocking
sunlight from the water.
These phytoplankton contribute to the total
suspended solids concentration, while rooted vegetation or attached
streambed-mat forms of algae do not.
However, if these rooted algae become
detached (usually when the algae dies or if it is forcefully removed), then
their mass becomes part of the suspended solids measurement.
Algal blooms can coat the surface of the
water and prevent light from penetrating.
The most obvious examples of algae’s
contribution to turbidity are found in algal blooms.
An algal bloom occurs when an excessive
amount of algae grows quickly across the surface of a body of water.
These blooms usually occur due to an influx
of nutrients such as nitrogen and phosphorus due to agricultural runoff or
decomposition, though warmer water temperatures and longer daylight hours also
contribute to their growth.
Floating algal blooms can block sunlight,
release toxins, and deplete oxygen levels in a body of water.
While some algae growth occurs naturally
(often seasonally), excessive growth is often attributed nutrient pollution.
Turbidity monitoring can be used to determine
if an increase in suspended solids is natural or due to agricultural runoff.
Sediment: Runoff and Erosion
Sediment is comprised of any solid material
that can be transported by water, wind or ice.
It is usually defined as the soil particles
(including silt, clay and sand) that are deposited on the bottom of a body of
water.
These particles are usually classified by
size from smallest (clay is less than 0.00195 mm in diameter) to largest
(coarse sand can be up to 1.5 mm) 19. Silt falls in between, ranging from
0.0049 to 0.047 mm.
In areas of high flow, even rocks can be
considered sediment as they are deposited in water.
However, not all sediment is suspended. The
amount and size of suspended sediment is dependent on water flow.
The faster the flow, the larger the particle
that can be suspended. Higher flow rates can also support a higher
concentration of suspended solids.
Particles larger than 0.5 mm usually settle
out as water flow decreases 19. Most of the suspended sediment that remains
(colloidal solids) consists of fine sand, silt, and clay.
The majority of suspended sediment present in
water bodies comes from runoff and erosion.
If the land surrounding a body of water has
only sparse vegetation, the topsoil can easily be washed away into the water.
Highly vegetated areas will absorb most of
the runoff, keeping the body of water clearer.
In addition to collecting suspended particles
from runoff, rivers and streams can slowly erode soft riverbanks due to the
constant water flow.
An increase in river volume and flow (due to
rain or other causes) can increase the rate of erosion.
On the other side of the spectrum,
bedrock-based streams may not have much sediment available to suspend.
The local geology will determine natural
turbidity levels based on normal flow rates, soil type, land structure and
vegetation.
If the surrounding land is altered by
agriculture, construction or other soil-disturbing use, it can accelerate
erosion and runoff, increasing turbidity.
Pollution
Any potentially harmful substance that is
added to the environment by humans, whether directly or indirectly, is
considered pollution.
This can vary from bacteria riding along on a
sewage plant discharge, to coal and iron ore particulates that float in from a
mining zone.
If these pollutants are larger than 2
microns, they will contribute to the total suspended solids concentration.
Some of the more common suspended solid
pollutants are pathogens (bacteria, protozoa, helminths), microbeads (from
exfoliating soaps), wastewater effluent, sewage, airborne particulates, and
road particles (e.g. asphalt and tire flecks).
Colored wastewater discharge and dyes are
pollutants that will affect turbidity, but not suspended solids.
Nutrients like nitrate and phosphorus are
often considered pollutants, but as they are a dissolved substance, they do not
contribute directly to the suspended solids concentration.
Instead, they are an indirect contributor as
they fuel algal blooms, which do affect TSS and turbidity.
These dissolved nutrients, along with
dissolved metals, chemicals, and refractory organics, will impact the quality
of a body of water.
Nitrate and phosphorus can cause
eutrophication (excessive plant and algae growth) which in turn causes low
dissolved oxygen levels due to plant respiration and microbial decomposition.
Refractory organics are often carcinogenic,
while heavy metals and other chemicals can be toxic to aquatic organisms.
While these contaminants can enter the water
as a dissolved substance, many of them ride along on grains of soil or other
larger pieces of pollution (e.g. paint flecks or asphalt particles).
When this is the case, they can be picked up
in suspended sediment samples.
Chemical dyes will affect turbidity readings
as the colored molecules will affect light absorption, but they will not be
included in a suspended solids measurement.
Factors that Influence Turbidity
Suspended solids can be comprised of organic
and inorganic materials such as sediment, algae, and other contaminants.
However, there are specific factors that can
affect turbidity levels in a body of water. These are water flow, point source
pollution, land use and resuspension.
Water Flow and Weather
Turbidity and water flow are causally related
18. High flow rates keep particles suspended instead of letting them settle to the
bottom.
Thus, in rivers and other naturally-occurring
high flow environments, turbidity can be a constant presence.
In these areas, it is important to monitor
for changes in turbidity at the same point each time to ensure that the data is
not affected by a lower or higher water velocity.
Weather, particularly heavy rainfall, also
affects water flow, which in turn affects turbidity. Rainfall can increase
stream volume and thus stream flow, which can resuspend settled sediments and
erode riverbanks.
Rain can also directly increase the level of
total suspended solids through runoff. As water flows over a surface, it can
pick up particles and deposit them in a body of water.
Runoff can also wash away topsoil, and
contribute to riverbank erosion. If the flow rate increases enough, it can
resuspend bottom sediments, further raising TSS concentrations.
In areas of dry, loose soil or
earth-disturbed sites (e.g. mining or construction areas), wind can blow dust,
sediment and other particles into the water. The addition of new particles will
increase the suspended solids concentration.
However, wind will generally not increase
turbidity levels in the water alone. In wave-dominated estuaries and coastal
areas, turbidity is naturally low.
In comparison, tidal areas, where the water
flow is strong enough to resuspended bottom sediments, have high natural turbidity
levels.
Wind-driven turbidity increases only occur in
shallow zones where waves are tall enough to resuspend sediment.
Tides, wind, and rain can influence turbidity
levels due to their effect on water flow and introduced sediment loads. Tributaries
can also alter turbidity.
When a freshwater stream or river enters a
saltwater estuary, the change in water flow can cause turbidity levels to
increase. This mixing area is often called a turbidity maximum zone.
These zones tend to have little aquatic
vegetation due to the high suspended solids concentrations.
Estuaries are often subject to tidal
influences as well, which can pull in sand and sediment from the shoreline and
resuspended bottom sediments.
turbidity_runoff_suspended_sediment_ocean
If pollution can be tracked to a single,
identifiable source, it is considered point-source pollution.
Point-source pollution can increase turbidity
through the addition of suspended solids and colored effluent (wastewater) to a
body of water.
For water quality, common examples include
discharge pipes from factories and wastewater treatment plants.
In addition, farms can also fall under the
category of point-source pollution. These sources can release harmful pathogens
(bacteria) and chemicals into the water, in addition to suspended solids.
Many factories, wastewater treatment plants,
and sewage treatment plants discharge effluent into local water bodies or sewer
systems.
Sometimes this water is treated or filtered
before it is discharged, but sometimes it is not.
The EPA has created several guidelines for
effluent discharge, but they are all based on the technology used, and not the
final impact on the local water body.
While most wastewater treatment plants
include a settling period in the treatment process, this does not affect
colloidal (nonsettleable) solids.
When this wastewater is discharged, these
suspended solids may still be present unless treated with additional filters.
In addition, colored effluent cannot be trapped by a filter.
While dyes and colored dissolved organic
material (CDOM) are not included in a suspended solids measurement, they will
contribute to turbidity readings due to their effects on light absorption.
Farms that are identified as point sources
often allow fertilizer and animal waste to enter local bodies of water.
Most agricultural pollution is due to runoff,
and not a specific discharge. While this runoff is not intentional, it can be
detrimental to water quality as these pollutants are untreated.
Animal wastes can increase pathogen
concentrations in the water, while the fertilizer can contribute to
eutrophication and excessive algal growth.
Land Use
A major factor in increased turbidity and
total suspended solids concentrations is due to land use.
Construction, logging, mining and other
disturbed sites have an increased level of exposed soil and decreased
vegetation.
Agricultural areas are also considered
disturbed areas after they are tilled.
Land development, whether it is agricultural or
construction, disturbs and loosens soil, increasing the opportunities for
runoff and erosion.
The loosened soils caused by these sites can
then be carried away by wind and rain to a nearby body of water.
This leads to an increase in runoff rates, causing
erosion and increased turbidity in local streams and lakes.
Settleable solids in the runoff can be
deposited on the bottom of a lake, river or ocean, damaging benthic habitats.
Erosion due to land use is considered a
non-point source of turbidity. The use of silt fences and sedimentation basins
at construction sites can prevent soils from reaching nearby water sources.
In addition to increasing turbidity levels
through suspended sediment, agricultural runoff often includes nutrients as
well.
Due to the presence of these nutrients, this
runoff can fuel the growth of algal blooms. These effects can be seen in local
streams, lakes, and even estuaries like the Chesapeake Bay.
Water quality can be affected anywhere that
these nutrients and sediments are carried. No-till farming practices can reduce
the potential for erosion and help maintain nearby water quality.
Sediment- and pollutant-filled runoff can
also occur in urban areas. When it rains, soil, tire particles, debris and
other solids can get washed into a water system.
This often occurs at a high flow rate due to
the amount of impervious surface areas (e.g. roads and parking lots). Water
cannot penetrate these surfaces, so sediment cannot settle out.
Instead, the stormwater runoff flows right
over the pavement, carrying the suspended solids with it.
Even in areas with storm drains, these drains
usually lead directly to a local water source without filtration.
To minimize the pollution and turbidity
caused by urban runoff, stormwater retention ponds can be constructed. These
basins allow suspended particles to settle before water drains downstream.
Resuspension
Even carp and other bottom-feeding fish can
contribute to increased turbidity levels. As they remove vegetation, sediment
can become resuspended in the water.
Sediment at the bottom of a body of water can
be stirred up by shifting water flow, bottom-feeding fish, and anthropogenic
causes such as dredging.
Dredging projects, which remove built-up
sediment in navigation channels, are a major source of resuspended sediments in
the surrounding water.
Dredging can cause high turbidity levels as
it disturbs large amounts of settled sediment in a relatively short period of
time.
These stirred-up particles are mostly silt
and sand. When they resettle, they can alter habitats, smother fish eggs and
suffocate bottom-dwelling organisms.
TSS and Turbidity Units
Total suspended solids, as a measurement of
mass are reported in milligrams of solids per liter of water (mg/L) 18. Suspended
sediment is also measured in mg/L 36.
The most accurate method of determining TSS
is by filtering and weighing a water sample.
This is often time consuming and difficult to
measure accurately due to the precision required and the potential for error
due to the fiber filter.
Turbidity, on the other hand, is most often
measured with a turbidity meter.
Turbidity is reported in units called a
Nephelometric Turbidity Unit (NTU), or a Jackson Turbidity Unit (JTU).
The JTU was the original turbidity unit based
on the visibility of candlelight in a tube (Jackson Candle Turbidimeter).
However, this method is considered out of date and inaccurate in comparison to
newer methods.
While some organizations consider the two
units to be approximately equal, there are some specific differences 20.
In particular, NTU is more precise and has a
wider range (JTU cannot measure above 25 JTU/NTU) 43.
In addition, NTU is the standard unit of many
broadband output (400-680 nm wavelength) turbidity meters.
Nephelometric refers to the measurement
technology used.
This technology method requires the
photodetector in the meter to be placed at a 90-degree angle from the
illumination source.
As light bounces off the suspended particles,
the photodetector can measure the scattered light.
The USGS also suggests the use of the
Formazin Nephelometric Unit (FNU) if a turbidity meter only has a
monochrome/infrared output, as opposed to the white/broadband output.
This applies to instruments that are in
compliance with the European drinking-water protocol, including most
submersible turbidity meters.
Both NTU and FNU will show equal measurements
when calibrating as they both use nephelometric technology, but may operate differently
in the field due to the different light source.
Turbidity meters that use FNU units are able
to compensate for dissolved colored materials (such as humic stain), while NTU
turbidity meters cannot.
Water clarity, when not measured in terms of
turbidity, is measured by Secchi depth. This measurement is based on the depth
that a black and white Secchi disc can be lowered into a body of water.
At the point visibility is lost, the depth of
the disc is recorded, and is known as the Secchi depth.
High Secchi depths correspond with low
turbidity levels, while low Secchi depths are associated with high levels of
suspended solids. This method is generally only useful in oceans, lakes and
deep, low-flow rivers.
In marine environments, a larger solid white
disc is often used, while some shallower lakes use a black disc and take a
horizontal measurement.
Due to the effects of salt on suspended
sediment, ocean clarity is often much higher than lake or river clarity. Most
Secchi disc records reach around 65-80 m 39.
Water clarity has a theoretical limit of 200
m, based on light penetration and calculations with distilled and ultrapure
water. However, most Secchi discs are not large enough to be seen at that
depth.
In shallower streams, a Secchi tube can be
used. A Secchi tube is usually one meter long and is filled with collected
water.
A small Secchi disc is then lowered into the
tube and read at the point of disappearance, just as it is in a larger body of
water.
Turbidity Meters and Measurements
Regardless of whether readings are in NTU,
FNU or other less common units, it is important to note that a turbidimeter’s
optical design will affect turbidity readings.
As turbidity is a measurement of light
scatter, the placement and designs of the detectors with the meter can
influence the readings.
This simply means that raw data from two
different turbidity meters cannot be directly compared without an established
relationship between them.
Turbidity readings can vary based on
wavelengths emitted, light source instability, high particle density or due to
the presence of colored dissolved or suspended material.
The more detectors present in a turbidimeter,
the less variability there will be in measurements.
Typical Levels
In most situations, a total suspended solids
concentration below 20 mg/L appears clear, while levels over 40 mg/L may begin
to appear cloudy.
In comparison, a turbidity reading below 5
NTU appears clear, while a reading of 55 NTU will start to look cloudy and a
reading over 500 NTU will appear completely opaque.
It is important to note that this is
dependent on the size and nature of the suspended solids.
Typical turbidity and TSS levels are
difficult to quantify due to their natural variation by season, local geology,
water flow and weather events.
During a low-flow period, most rivers and
lakes are fairly clear with a turbidity reading below 10 NTU. These readings
can easily jump into the hundreds due to runoff during a rainstorm, snowmelt or
a dredging project.
In general, marine environments have lower
turbidity levels than freshwater sources. The salinity of the ocean or estuary
will cause the the suspended solids to aggregate, or combine.
As the aggregate weight increases, the solids
begin to sink and will settle on the seafloor.
This effect offers greater water clarity than
is available in most lakes and rivers. The higher the salinity, the greater the
effect.
However, in tidal zones, a turbidity maximum
may occur due to the constant resuspension of these settled solids 16.
Freshwater sources may also carry out additional suspended particles into the
delta.
As the concentrations of total suspended
solids are difficult to measure and predict, most states do not have a set
standard.
Even the National Academy of Sciences only
recommends that “TSS should not reduce light penetration by more than 10%”.
Kentucky does not have a quantitative
standard for acceptable levels of total suspended solids. Instead, they simply
state that there should be no adverse affects to the body of water or its
inhabitants.
Michigan is another example of a state with
only a “narrative standard” for total suspended solids and turbidity.
There is no set level or concentration, only
a recommendation against unnatural physical properties (e.g. turbidity, color,
films, floating or suspended solids) in “injurious” quantities.
Instead, many countries and organizations
have established recommended turbidity levels from a baseline of prior
measurements.
In the case of drinking water, recommended
levels are based on several filtration and disinfection studies.
The Ireland EPA advises treatment plants to
have turbidity levels below 0.2 NTU, with a mandatory maximum of 1 NTU for
drinking water.
According to the World Health Organization,
water for human consumption should have turbidity levels below 1 NTU, though
for some regions, up to 5 NTU is allowed if it can be proven to be disinfected.
The American Water Works Association suggests
that a level of 5 NTU or lower is acceptable for recreation purposes.
As a state example, the North Carolina code
allows up to 10 NTU for trout waters, 25 NTU for non-trout streams and as high
as 50 NTU for non-trout lakes.
Other states have determined allowable
fluctuations from an established baseline.
The state of Washington does not have a
standard for TSS, but it does for turbidity, depending on the body of water.
In some streams, turbidity cannot increase by
more than 5 NTU from the baseline. For others, turbidity may be allowed to
fluctuate by up to 20%.
Consequences of Unusual Levels
In addition to to being a warning sign for
pollution, suspended solids can harbor pathogens such as bacteria and protozoa.
These microorganisms attach to the suspended
particles, aiding in their transportation and hiding them from disinfectants.
These pathogens can infect aquatic or human
life if the sediment is not removed.
Algal blooms, while initially increasing
dissolved oxygen levels, may create hypoxic conditions as they decompose.
When an algal bloom appears, it blocks
sunlight from reaching any submerged vegetation, killing those plants and
decreasing the amount of dissolved oxygen produced.
Then, when the bloom dies off, microbes
consume more oxygen as they decompose the organic material. This causes
dissolved oxygen levels to plummet even lower, creating hypoxic (low DO) or
even anoxic (no DO) conditions.
Furthermore, some blooms produce toxins that
are damaging to aquatic and human life. These harmful algal blooms include
cyanobacteria, red tide (Karenia brevis) and ciguartera (gambierdiscus toxicus).
Settleable Solids
Settleable solids can impair lakes and other
water bodies. If sedimentation rates are high, they can alter and often destroy
fish habitats and spawning beds.
If eggs or benthic organisms are present,
they can become buried by the sediment and die.
Sediment deposition can reduce egg and embryo
survival by reducing oxygen supply and crusting over the egg, preventing the
embryo from escaping.
As sediment build-up increases, the shallower
body of water means an increased risk of flooding and a decrease in
navigability for boats and ships.
Dredging projects attempt to remove excessive
sediment deposits from navigation channels, but this can be just as damaging to
the local fish habitats and spawning beds.
Turbidity
High turbidity levels can diminish visibility
and often feeding behaviors, in addition to physically harming aquatic life.
The suspended solids may disrupt the natural
movements and migrations of aquatic populations.
Fish that rely on sight and speed to catch
their prey are especially affected by high turbidity levels. These fish often
flee areas of high turbidity for new territories.
For the fish that remain in the turbid
environment, suspended sediment can begin to physically affect the fish.
Fine sediment can clog fish gills and lower
an organism’s resistance to disease and parasites.
Some fish may consume suspended solids,
causing illness and exposing the fish to potential toxins or pathogens on the
sediment.
If the consumed sediment does not kill the
fish, it can alter the organism’s blood chemistry and impair its growth.
Turbidity will also affect submerged plant
growth. Levels over 15 NTU are considered detrimental to bay grass growth in
estuary zones.
As turbidity increases, the amount of light
available to submerged aquatic vegetation (SAV) decreases.
Without enough light, photosynthesis will
stop, and the SAV will no longer produce dissolved oxygen.
In addition to reducing the dissolved oxygen
concentration in the water, the plants will eventually die.
As the aquatic vegetation dies off, the
organisms that feed on it will also decline due to the reduced food sources
available. If turbidity levels remain elevated, the effects can be seen up the
food chain.
Even aquatic life that does not strongly
depend on vegetation for survival will be affected by the low dissolved oxygen
levels. If these fish and invertebrate cannot escape the anoxic area, they will
die.
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Both organic and inorganic particles of all
sizes can contribute to the suspended solids concentration.
Some sediment will settle to the bottom of a
body of water, while others remain suspended.
This river owes its muddy appearance to high
turbidity levels.
Tannins from decomposing vegetation have
colored this river red.
Ocean water is usually clearer than freshwater
due to the effect of salinity on suspended solids.
Salt ions can cause suspended particle to
aggregate and settle at the bottom of a body of water.
While turbidity and total suspended solids
often overlap, there are a few outlying factors that only contribute to one or
the other.
Is this water clear, or murky, or just slightly
opaque? Human observation of clarity allows for personal perception and
judgement.
A sediment-laden river flows into Tuscaloosa
Lake. Photo Credit: City of Tuscaloosa via USGS
Suspended solids can increase the temperature
of water as they absorb additional heat from the sun. This can also cause
dissolved oxygen levels to drop below the thermocline, creating hypoxic
conditions.
Suspended solids, particularly algae, can block
sunlight from reaching submerged plants. This can cause dissolved oxygen levels
to drop, as the plants rely on respiration (consuming oxygen) instead of
photosynthesis.
Bank erosion along a river can be cause by
runoff , flooding or strong water flow.
Wastewater effluent can carry pathogens and
other contaminants into a water body if it is not treated properly.
Turbidity is caused by include organic
materials such as algae, and inorganic materials such as silt and sediment.
Different algae can float in the water or be
found rooted on a riverbed. Some, like kelp and seaweed, look like underwater
plants.
Algal blooms can coat the surface of the water
and prevent light from penetrating.
Sediment particles can be fine silt or clay,
sand and even gravel. Photo Credit: USGS via Massachusetts Bay Program
Runoff causes erosion, washing soil and other
particulates into a body of water.
Pollution ranges from large garbage to
microplastics, flecks of metal or asphalt, and chemical dyes.
Stream flow and turbidity are often directly
related; as water flow increases, so will turbidity levels.
Stream flow and
turbidity are often directly related; as water flow increases, so will
turbidity levels.
Heavy rainfall will cause turbidity to spike,
as this storm event graph shows. This is due to increased water flow and
increased sediment from runoff.
Heavy rainfall will cause turbidity to spike,
as this storm event graph shows. This is due to increased water flow and
increased sediment from runoff.
Turbid rivers can carry their suspended
sediments into the ocean.
Here is an example of point source pollution.
Construction sites loosen soil that could run
off into a body of water.
Urban runoff flushes contaminants such as
sediment, asphalt and tire particles.
Dredging project underway at Kings Lake.
Secchi discs are used to measure water clarity.
At 5 NTU, water still appears clear. It is
cloudy at 55 NTU and opaque at 515 NTU.
Turbidity will often spike annually due to
spring rains and snow melt.
Saltwater is typically clearer than freshwater.
Drinking water should have less than 5 NTU,
preferably less than 1 NTU and ideally below 0.1 NTU.
As this graph shows, the appearance of an algal
bloom results in a dramatic dissolved oxygen decrease shortly thereafter.
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