Recall
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How can we monitor air, water and soil pollution?
Write down what BOD stands for and the definition.
How can BOD be used to identify pollution?
Learning outcomes
You should all be able to:
 Describe an indirect method of measuring pollution
levels.
Most of you should be able to
 Explain an indirect method of measuring pollution
levels – biotic index
 Explain how BOD can be used to assess pollution
levels in water.
Biochemical Oxygen Demand
A measure of the amount of dissolved
oxygen required to break down the
organic material in a given volume of
water through aerobic biological
activity.
Biochemical Oxygen Demand
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Amount of oxygen in the water required to
decompose organic wastes (dead plants, leaves,
grass clippings, manure, sewerage, food waste, etc.)
Indirect measure of pollution
Starter
Analyse the diagram and explain what is happening.
How can BOD be used to assess
pollution levels?
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High BOD means low Dissolved Oxygen for plants /
fish / invertebrates.
High BOD = a lot of organic pollution
Determined by number of organisms that are
present and the rate or respiration.
BOD indicates organic matter pollution
How to measure BOD
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Take a sample of water
Measure the oxygen level
Place it in a dark place at 20°C for 5 days.
After 5 days measure the oxygen again
BOD = difference between 2 measurements.
Source of pollutant
BOD (mg dissolved oxygen needed l-1)
Unpolluted river
<5
Treated sewage
20-60
Raw domestic sewage
350
Cattle slurry
10,000
Paper pulp mill
25,000
Test yourselves
Four factories discharge effluent containing organic matter into rivers. The table shows
the volume of discharge into the river and the resulting biological oxygen demand.
Factory
Volume of Effluent / 1000 l day-1
BOD / mg l -1
A
14.0
27
B
1.0
53
C
3.0
124
D
0.8
33
1 Explain whether these pollution data are for point source or non-point sources.
2 Which pollution source, point source or non-point source is easier to regulate?
Explain your choice.
3 Which factory is adding most to the BOD of the river into which it discharges?
Indirect methods of measuring pollution
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BOD
Abiotic factors
Biotic factors
Pollution Indicators
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What are pollution indicators?
The organisms in polluted water tell you that there is
something wrong.
Examples of pollution indicators

Lichens are plants that grow in exposed places such as rocks or tree
bark. Air pollutants dissolved in rainwater, especially sulfur dioxide, can
damage lichens, and prevent them from growing. This makes lichens
natural indicators of air pollution. For example:
bushy lichens need leafy lichens can
really clean air
survive a small
amount of air
pollution
crusty lichens can
survive in more
polluted air
no lichens air is
heavily polluted
with sulfur dioxide.
Examples of pollution indicators

Some invertebrates like
bloodworms and rat-tailed
maggots, are able to survive
in polluted waters
Examples of pollution indicators
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While others like mayfly nymphs
and stonefly nymphs can only be
found in very clean water.
This is what happens when a stream or river
becomes polluted
1.
2.
3.
4.
5.
Pollutant is added, which kills some living things.
Micro-organisms decompose them, they respire and
use up the oxygen
Lack of oxygen kills other organisms, but a few
species stay alive
As the water flows, oxygen levels gradually
increase
Other living things recover too.
Sewage pollution and invertebrates
Distance
downstream
from where
sewage enters
the river (m)
What
Invertebrates found (not drawn to scale)
the
water is
like
Oxygen
levels
Sewage
enters here 010
Dark and
cloudy v.
smelly
Chironomous larva
rat-tailed maggot
Falling
quickly
10-100
Cloudy
bad
smell
Tubiflex worm
mosquito larva
Very low
Sewage pollution and invertebrates
100-200
Slight smell
beginning to
clear
Flatworm
caddis fly larva
Gradually
rising
200+
Clear
Stonefly larva
mayfly larva
Back to
normal
Freshwater shrimp
Stonefly nymph
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Stonefly nymphs prefer rocky,
stony, or gravel bottoms in
cool, well-oxygenated, swiftmoving streams or small
rivers. Any effluent that
reduces the oxygen content of
the stream quickly kills the
nymphs.
Biological Monitoring
Working Party (BMWP) = 10
Flattened Mayfly Nymph
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These are only found in
very clean water
containing lots of oxygen.
They absorb oxygen from
the water through their
gills.
They are primary
consumers that eat plants
and algae.
BMWP = 10
Cased caddis Fly Larvae
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Caddisflies use silk (like butterflies) to build cases from
gravel, twigs, needles, or sand. Different species build
distinct cases, but they often lose them when removed from a
stream.
BMWP = 8 -10
Cranefly Larvae
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Up to four inches long with a
Fleshy, plump, rounded
segmented body
Its digestive track (internal
organs) can be seen moving
back and forth as it crawls.
No legs and the back end
usually has several extensions
or finger-like lobes.
BMWP = 5
Flatworms
22
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Flatworms are unsegmented
worms with flattened bodies.
Their flatness allows them to
shelter beneath stones.
Being flat increases the area
of skin exposed to the water,
which improves the flatworm's
oxygen uptake.
BMWP = 5
Freshwater Mite
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Tiny animals related to spiders.

Up to 8mm.
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They have an oval body and 8
legs.

Many are brightly coloured.

BMWP = 4
Leeches
 Flat, Worm-like, segmented
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body.
Many are scavengers or feed
on other invertebrates.
Suckers at both ends are
used for attachment, feeding,
and locomotion.
They swim gracefully and
quickly in an up-and-down
motion.
BMWP = 3
Red Midge Larvae (Bloodworms)
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These are sometimes called
'bloodworms' because of
their bright red colour, but
they are not worms at all.
They are midge larvae.
They eat dead organic
matter and can tolerate very
low oxygen levels.
BMWP = 2
Tubifex Worms
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These are thread-like worms
that live in the mud on stream
bottoms.
They are filterfeeders
They can tolerate extremely
low levels of oxygen and are
therefore found in polluted
water.
BMWP = 1
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This shows
freshwater
fauna as
indicators of
river
pollution.
Let’s do some monitoring of our own…
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Read the student instructions and divide the activities
between the group – who will take what role?
Make sure that you have all the necessary
equipment.
Visit areas A-D and decide which you think is the
most polluted.
Complete your results table and method as follows:
Monitoring Pollution

Roles: Julio = scribe, Emilia = rain analyst, Rodrigo = particulate analyst, Melanie = photographic
investigator, Megan = Freshwater species investigator.
pH testing of rain samples using UI paper
Analysis of particulate levels using dust collector cards & hand lenses – given % cover score
Research into lichens & analysis of photographic evidence – given score of
present/absent/abundant
Research into preferred habitats of freshwater species, and analysis of communities present
1.
2.
3.
4.
Area
pH of Rain
Particulate %
Lichen score
Freshwater species present
Rank 1-4
A
B
C
D
- Find source of pollution – traffic survey & check local industry
- Lobby local council to introduce traffic calming measures
- send leaflets to local industry to inform them of measures to reduce the pollution produced
bushy lichens need leafy lichens can
really clean air
survive a small
amount of air
pollution
crusty lichens can
survive in more
polluted air
no lichens air is
heavily polluted
with sulfur dioxide.