Water and Water Quality
2. Sources of Water Pollution
Learning outcomes
- I can identify major sources of water pollution.
- I can distinguish between point-source and non-point-source pollution.
- I can explain how pollutants enter water systems.
- I can describe the impacts of water pollution on organisms and ecosystems.
- I can evaluate methods for reducing water contamination.
Sources of Water Pollution
Water pollution occurs when harmful substances, organisms, or forms of energy enter water and reduce its quality or make it harmful to organisms and ecosystems.
Water pollution can affect:
- rivers
- lakes
- wetlands
- groundwater
- estuaries
- oceans
- drinking-water supplies
Pollutants can enter water directly, or they can be transported by runoff, drainage systems, groundwater, rivers, or the atmosphere.
Common water pollutants include:
- sewage
- fertilizers
- pesticides
- oil
- plastics
- sediments
- industrial chemicals
- heavy metals
- disease-causing microorganisms
- excess nutrients
- heated water
Understanding where pollution comes from and how it travels is important because preventing pollution at its source is often easier than removing it after it has spread through an ecosystem.
Pollutants Move Through the Water Cycle
Water pollution is closely connected to the water cycle.
Rainwater can collect pollutants from:
- farms
- roads
- cities
- construction sites
- industrial areas
Surface runoff then carries these substances into streams, rivers, lakes, and oceans.
Some pollutants infiltrate into soil and eventually enter groundwater.
A pollutant released in one location can therefore travel far from its original source.
For example:
fertilizer on field → rainfall → runoff → stream → river → lake
or:
chemical spill → soil → infiltration → groundwater → well
Point-Source Pollution
Point-source pollution comes from a single, identifiable source.
Examples include:
- a discharge pipe from a factory
- a wastewater treatment outlet
- a leaking storage tank
- a mine drainage pipe
- a sewage pipe
- an oil spill from a specific facility or vessel
Because the source can often be identified, point-source pollution may be easier to:
- monitor
- measure
- regulate
- treat
For example, if polluted water is being discharged from a factory pipe, samples can be collected directly from that outlet.
Non-Point-Source Pollution
Non-point-source pollution comes from many scattered sources rather than one easily identifiable location.
Examples include:
- fertilizer washing from many farms
- pesticides carried from fields
- oil and fuel washed from roads
- sediment from construction areas
- pollutants carried by urban stormwater
- animal waste washed from agricultural land
Rainfall and runoff often transport these pollutants into waterways.
Because there is no single discharge point, non-point-source pollution can be much more difficult to control.
Comparing Point and Non-Point Sources
| Point-Source Pollution | Non-Point-Source Pollution |
|---|---|
| Comes from an identifiable location | Comes from many scattered locations |
| Often enters through a pipe or outlet | Often transported by runoff |
| Usually easier to monitor | Often difficult to monitor |
| Source may be easier to regulate | Requires management across a larger area |
| Example: factory discharge pipe | Example: fertilizer runoff from farmland |
A useful question is:
Can you identify one specific location where the pollutant enters the water?
If yes, it is likely a point source.
If pollution comes from many locations across a landscape, it is more likely non-point-source pollution.
Agricultural Pollution
Agriculture can introduce several types of pollutants into water.
These include:
- fertilizers
- pesticides
- herbicides
- animal waste
- sediment
- microorganisms
Rainfall can wash these substances from fields into nearby waterways.
A simplified pathway is:
farm → rainfall → runoff → stream → river
Fertilizer Pollution
Fertilizers contain nutrients that help crops grow.
Two especially important nutrients are:
- nitrogen
- phosphorus
When too much fertilizer is applied, plants may not absorb all of it.
Rainfall can then transport nutrients into:
- streams
- rivers
- lakes
- groundwater
These nutrients can cause excessive growth of algae and aquatic plants.
This process can lead to eutrophication.
Eutrophication
Eutrophication occurs when excessive nutrients enter a body of water and stimulate unusually rapid growth of algae and aquatic plants.
A common sequence is:
excess fertilizer → nutrient runoff → algal growth → algae die → decomposition increases → dissolved oxygen decreases → aquatic organisms suffer
Bacteria and other decomposers use oxygen while breaking down dead organic material.
If dissolved oxygen falls too low, fish and other organisms may die.
Algal Blooms
An algal bloom is a rapid increase in algae or cyanobacteria in water.
Blooms may:
- reduce light penetration
- disrupt aquatic food webs
- reduce oxygen levels when organisms die and decompose
- alter water quality
Some blooms can also produce toxins.
However, not every algal bloom is toxic.
The ecological effects depend on the organisms involved and environmental conditions.
Pesticides
Pesticides are chemicals used to control organisms considered harmful to crops or other human activities.
They can include:
- insecticides
- herbicides
- fungicides
Pesticides may enter waterways through:
- runoff
- spray drift
- spills
- infiltration
Once present, some pesticides can affect organisms that were not the intended targets.
Possible impacts include:
- reduced reproduction
- changes in growth
- poisoning
- disruption of food webs
Sewage and Wastewater
Sewage contains human waste and other materials carried through wastewater systems.
Untreated or poorly treated sewage can contain:
- harmful microorganisms
- organic matter
- nutrients
- household chemicals
- suspended solids
Sewage can enter waterways through:
- leaking systems
- untreated discharge
- overflowing sewers
- damaged infrastructure
This can create both ecological and human-health problems.
Pathogens
A pathogen is a microorganism or other biological agent capable of causing disease.
Contaminated water may contain:
- bacteria
- viruses
- protozoa
- other disease-causing organisms
Sources can include:
- untreated sewage
- animal waste
- poorly maintained sanitation systems
Contaminated drinking water can therefore create serious health risks.
Treating sewage and protecting drinking-water sources are important ways to reduce these risks.
Organic Waste and Oxygen
Organic material in sewage can affect oxygen levels.
Microorganisms decompose organic matter.
During decomposition, many microorganisms consume dissolved oxygen.
Therefore:
more biodegradable organic waste → more decomposition → greater oxygen demand
If oxygen consumption becomes too high:
dissolved oxygen decreases
Aquatic organisms such as fish may then struggle to survive.
Industrial Pollution
Industrial activities can release many types of contaminants.
Depending on the industry, these may include:
- acids
- alkalis
- solvents
- oils
- heavy metals
- other chemicals
- heated water
Industrial pollution may result from:
- normal discharges
- equipment failures
- spills
- improper waste disposal
- leaking storage facilities
Some industrial pollutants can persist in ecosystems for long periods.
Heavy Metals
Certain metals can become harmful pollutants when present at elevated concentrations.
Examples include:
- mercury
- lead
- cadmium
Possible sources include:
- mining
- industrial processes
- contaminated waste
- improper disposal
Heavy metals can be particularly concerning because some can accumulate in organisms and move through food webs.
Bioaccumulation
Bioaccumulation occurs when a substance builds up inside an organism over time because it is absorbed faster than it is removed.
For example, an aquatic organism may repeatedly absorb a persistent pollutant from:
- water
- sediment
- food
Its internal concentration may gradually increase.
Biomagnification
Biomagnification occurs when the concentration of certain persistent pollutants increases at higher levels of a food chain.
A simplified example is:
water → plankton → small fish → large fish → predator
Predators consume many contaminated prey organisms.
As a result, organisms near the top of a food chain can sometimes contain relatively high concentrations of certain pollutants.
Bioaccumulation and biomagnification are related but different:
bioaccumulation = buildup within one organism
biomagnification = increasing concentration through trophic levels
Oil Pollution
Oil can enter aquatic environments through:
- spills
- leaking pipelines
- ships
- industrial facilities
- road runoff
- machinery
Large spills are highly visible, but smaller releases can also contribute to pollution over time.
Oil can:
- coat feathers and fur
- damage habitats
- expose organisms to harmful chemicals
- contaminate shorelines
- interfere with feeding and reproduction
Plastics
Plastic pollution is now found in many aquatic environments.
Sources include:
- litter
- poorly managed waste
- lost fishing equipment
- industrial plastic materials
- stormwater
- wastewater
Large plastic items can injure animals through:
- entanglement
- ingestion
Plastic also gradually breaks into smaller pieces.
Microplastics
Microplastics are very small pieces of plastic.
They can originate from the breakdown of larger plastic objects or enter the environment already small.
Sources can include:
- fragmented packaging
- synthetic fibres
- tyre wear particles
- industrial materials
Microplastics can enter:
- rivers
- lakes
- oceans
- sediments
- food webs
Scientists continue to investigate their environmental and biological effects.
Sediment Pollution
Soil particles naturally enter rivers, but human activities can greatly increase erosion.
Sources include:
- construction
- agriculture
- deforestation
- mining
- damaged riverbanks
Excess sediment can make water cloudy or turbid.
High turbidity can:
- reduce light penetration
- reduce photosynthesis
- cover fish spawning areas
- bury bottom-dwelling organisms
- transport other pollutants attached to sediment particles
Urban Runoff
Cities contain many impermeable surfaces such as:
- roads
- parking lots
- roofs
- sidewalks
Rainwater flows rapidly across these surfaces.
Along the way, it can collect:
- oil
- fuel
- metals
- litter
- sediment
- animal waste
- chemicals
Stormwater systems may transport this mixture into nearby waterways.
A simplified pathway is:
road pollutants → rainfall → storm drain → stream
Household Pollution
Everyday activities can also contribute to water pollution.
Potential contaminants include:
- cleaning products
- paints
- solvents
- oils
- medicines
- garden chemicals
Improper disposal can allow these substances to enter sewage systems, storm drains, soil, or groundwater.
Correct disposal reduces the chance of contamination.
Mining
Mining can expose rocks and minerals that were previously underground.
Water moving through mining areas can sometimes carry:
- metals
- acidic drainage
- sediment
- processing chemicals
Pollution may then enter surface water or groundwater.
Careful management of mine waste and drainage is therefore important.
Thermal Pollution
Not all water pollution involves adding a chemical.
Thermal pollution occurs when human activities significantly change water temperature.
For example, some industrial facilities use water for cooling.
If warmer water is returned to a river or lake, it can alter aquatic conditions.
Temperature affects:
- dissolved oxygen
- metabolism
- reproduction
- species distributions
Generally, warmer water can hold less dissolved oxygen than colder water.
Groundwater Pollution
Groundwater can become contaminated when pollutants infiltrate through soil.
Possible sources include:
- leaking fuel tanks
- landfills
- fertilizers
- pesticides
- industrial spills
- poorly managed waste
- leaking sewage systems
Groundwater pollution can be particularly difficult to manage because underground contamination may:
- move slowly
- spread through aquifers
- remain unnoticed for long periods
- be difficult and expensive to remove
Effects on Aquatic Ecosystems
Water pollution can affect ecosystems in many ways.
Possible consequences include:
- reduced dissolved oxygen
- fish deaths
- reduced biodiversity
- reproductive problems
- habitat degradation
- excessive algal growth
- altered food webs
- toxic effects
- reduced photosynthesis
- changes in species populations
Different pollutants produce different effects.
The concentration, duration of exposure, and sensitivity of the organisms also matter.
Dissolved Oxygen
Aquatic animals require dissolved oxygen.
Low dissolved oxygen can occur when large quantities of organic material are decomposed.
A simplified chain is:
organic pollution increases
↓
decomposer activity increases
↓
oxygen consumption increases
↓
dissolved oxygen decreases
↓
aquatic organisms experience stress
Severe oxygen depletion can produce areas where relatively few oxygen-dependent organisms survive.
Water Pollution and Food Webs
Pollution can affect organisms indirectly as well as directly.
Suppose a pollutant reduces a population of aquatic insects.
Fish that depend on those insects may then have less food.
Predators that depend on those fish may also be affected.
Therefore, pollution can spread ecological effects through an entire food web.
Water Pollution and Humans
Humans depend on clean water for:
- drinking
- agriculture
- food production
- recreation
- sanitation
- industry
Polluted water can:
- increase water-treatment costs
- damage fisheries
- affect recreation
- contaminate food
- reduce freshwater availability
- create health risks
Protecting water quality therefore has ecological, social, and economic importance.
Preventing Agricultural Pollution
Agricultural pollution can be reduced through methods such as:
- applying fertilizers at appropriate amounts and times
- reducing unnecessary pesticide use
- preventing animal waste from entering waterways
- reducing soil erosion
- maintaining vegetation near streams
- using cover crops
- managing irrigation carefully
A riparian buffer is an area of vegetation beside a waterway.
Vegetation can help:
- slow runoff
- trap sediment
- absorb some nutrients
- stabilize streambanks
Treating Wastewater
Wastewater treatment can remove or reduce many contaminants before water is released.
Treatment may involve:
- screening large debris
- allowing solids to settle
- biological decomposition of organic material
- nutrient removal
- disinfection
More advanced treatment may be needed for certain contaminants.
The basic principle is:
remove pollutants before the water returns to the environment.
Industrial Pollution Control
Industries can reduce water contamination by:
- treating wastewater
- containing hazardous chemicals
- monitoring discharge
- preventing leaks
- recycling process water
- safely storing waste
- responding rapidly to spills
Preventing pollution is often preferable to trying to remove contamination after it enters a river or aquifer.
Reducing Plastic Pollution
Possible strategies include:
- reducing unnecessary single-use plastics
- improving waste collection
- preventing litter
- improving recycling where appropriate
- capturing waste before it enters waterways
- improving stormwater management
- designing products that create less persistent waste
No single strategy solves the entire problem.
Effective solutions usually involve several approaches.
Cleaning Up Oil Spills
Oil-spill responses may use methods such as:
- floating barriers called booms
- skimmers
- absorbent materials
- controlled recovery techniques
The appropriate response depends on:
- type of oil
- spill size
- weather
- location
- ecosystem sensitivity
Cleanup can reduce damage, but preventing spills remains extremely important.
Reducing Urban Runoff
Cities can reduce polluted runoff using:
- rain gardens
- retention ponds
- wetlands
- permeable pavement
- vegetated areas
- improved stormwater treatment
These approaches can slow runoff and allow some pollutants to be trapped or treated before water reaches natural waterways.
Source Control vs Cleanup
There are two broad approaches to water pollution.
Source control
Prevent pollutants from entering the environment.
Examples:
- reducing fertilizer overuse
- treating industrial wastewater
- preventing oil leaks
- disposing of chemicals correctly
Cleanup
Remove pollution after contamination occurs.
Examples:
- removing spilled oil
- cleaning contaminated sediments
- treating polluted groundwater
In many cases:
prevention is easier and less expensive than large-scale cleanup.
Evaluating Pollution-Control Methods
A good solution should be evaluated using several factors.
Ask:
- How effectively does it remove or prevent pollution?
- How much does it cost?
- Can it operate over a large area?
- Does it create another type of waste?
- Does it require continuous maintenance?
- Does it protect ecosystems?
- Is it practical for the community or industry involved?
There is rarely one perfect solution for every situation.
Example: Fertilizer Runoff
Suppose farmers near a lake use large amounts of fertilizer.
After heavy rain:
fertilizer → runoff → river → lake
Nutrient levels increase.
This may cause:
nutrients → algal bloom → decomposition → oxygen depletion → fish stress or death
Possible solutions include:
- reducing unnecessary fertilizer use
- applying fertilizer at appropriate times
- planting buffer vegetation
- reducing erosion
- monitoring nutrient concentrations
This example shows why understanding the source, pathway, impact, and solution is important.
Example: Urban Pollution
Imagine rain falling on a large parking lot.
The water collects:
- oil
- tyre particles
- litter
- sediment
The runoff enters a storm drain and then flows into a nearby stream.
This is primarily:
non-point-source pollution
because pollutants are being collected across a broad urban area.
Possible controls include:
- permeable surfaces
- stormwater ponds
- filters
- rain gardens
- improved waste management
Example: Factory Discharge
A factory releases contaminated water from one pipe into a river.
This is:
point-source pollution
Because the entry point is known, authorities can potentially:
- sample the discharge
- measure pollutant concentrations
- require treatment
- establish discharge limits
- monitor compliance
This illustrates why point sources are often easier to identify and regulate.
Common Mistakes
Thinking All Water Pollution Is Visible
Polluted water does not always look dirty.
Some pollutants are:
- dissolved
- colorless
- microscopic
Clear-looking water is not necessarily safe.
Confusing Point and Non-Point Sources
Point source = identifiable discharge location
Non-point source = diffuse pollution from many locations
Assuming Fertilizer Is Always Beneficial
Fertilizer supports plant growth on land, but excessive nutrients entering water can disrupt aquatic ecosystems.
Thinking Algae Directly Remove All the Oxygen
A major oxygen decrease often occurs when dead algae and other organic matter are decomposed by microorganisms.
Confusing Bioaccumulation and Biomagnification
Bioaccumulation = buildup within an organism
Biomagnification = increasing concentration through a food chain
Thinking All Plastics Immediately Become Microplastics
Larger plastic materials may gradually fragment into smaller pieces over time. Microplastics can also enter the environment already very small.
Assuming Groundwater Cannot Be Polluted
Pollutants can infiltrate through soil and reach groundwater.
Once groundwater becomes contaminated, cleanup can be difficult.
Thinking Pollution Is Only Chemical
Pollution can also involve:
- microorganisms
- sediments
- heat
- excess nutrients
- solid waste
Assuming Cleanup Is Always the Best Solution
Once pollutants spread through an ecosystem or aquifer, removing them may be difficult and expensive.
Prevention is often more effective.
Key Terms
Water pollution — Contamination or alteration of water that reduces its quality or harms organisms or human uses.
Pollutant — A substance, organism, or form of energy that causes harmful environmental effects.
Point-source pollution — Pollution originating from a specific identifiable location.
Non-point-source pollution — Pollution originating from many diffuse sources across an area.
Runoff — Water flowing across the land surface, potentially carrying pollutants.
Wastewater — Used water containing waste materials from homes, industries, or other activities.
Sewage — Wastewater containing human waste and associated contaminants.
Pathogen — A biological agent capable of causing disease.
Nutrient pollution — Excessive addition of nutrients such as nitrogen and phosphorus to water.
Eutrophication — Nutrient enrichment of water that can cause excessive biological growth and oxygen depletion.
Algal bloom — Rapid growth of algae or cyanobacteria in a body of water.
Dissolved oxygen — Oxygen dissolved in water and available to aquatic organisms.
Turbidity — Cloudiness of water caused by suspended particles.
Bioaccumulation — Buildup of a substance inside an organism over time.
Biomagnification — Increase in concentration of certain persistent substances at higher trophic levels.
Microplastics — Very small pieces of plastic present in the environment.
Thermal pollution — Harmful alteration of water temperature caused by human activity.
Riparian buffer — Vegetated land beside a waterway that can help reduce runoff, erosion, and pollution.
Groundwater contamination — Pollution of underground water stores.
Key Takeaways
- Water pollution occurs when contaminants reduce water quality or harm organisms and ecosystems.
- Pollutants can enter surface water through direct discharge or runoff.
- Pollutants can also infiltrate into groundwater.
- Point-source pollution comes from an identifiable source such as a discharge pipe.
- Non-point-source pollution comes from many diffuse sources such as farms and urban areas.
- Agriculture can introduce fertilizers, pesticides, sediment, and animal waste.
- Excess nitrogen and phosphorus can contribute to eutrophication.
- Eutrophication can lead to algal growth, decomposition, and reduced dissolved oxygen.
- Sewage can introduce pathogens, nutrients, and organic material.
- Industrial activities can introduce chemicals, oils, and metals.
- Plastic pollution can affect aquatic organisms and produce microplastics.
- Sediment pollution can increase turbidity and damage habitats.
- Pollution can move through food webs.
- Bioaccumulation and biomagnification describe different ways pollutants can become concentrated in organisms.
- Groundwater pollution can be especially difficult to detect and clean up.
- Water pollution can reduce biodiversity and disrupt ecosystem processes.
- Pollution can also threaten drinking water, food supplies, recreation, and human health.
- Wastewater treatment, better agricultural practices, industrial controls, and improved stormwater management can reduce contamination.
- Preventing pollutants from entering water is often more effective than trying to remove them later.
A useful framework for analyzing water pollution is:
SOURCE → POLLUTANT → PATHWAY → IMPACT → SOLUTION
Check Your Understanding
Basic Concepts
1. Define water pollution.
2. What is a pollutant?
3. Explain the difference between point-source and non-point-source pollution.
4. Give three examples of point-source pollution.
5. Give three examples of non-point-source pollution.
Pollution Pathways
6. Explain how rainfall can transport pollutants into a river.
7. Describe how a chemical spilled on land could eventually contaminate groundwater.
8. Explain why pollutants released far upstream can affect ecosystems downstream.
9. How can urban storm drains contribute to water pollution?
10. Explain how pollutants can move through the water cycle.
Agriculture
11. Identify three agricultural pollutants.
12. Explain how fertilizer can reach a lake.
13. What nutrients commonly contribute to eutrophication?
14. Explain how a riparian buffer can reduce agricultural pollution.
15. Why can excessive pesticide use affect organisms other than agricultural pests?
Eutrophication
16. Define eutrophication.
17. Put the following events into a logical sequence:
- oxygen decreases
- nutrients enter water
- decomposition increases
- algae grow rapidly
18. Explain why decomposition can reduce dissolved oxygen.
19. Describe one possible effect of low dissolved oxygen on aquatic organisms.
20. Explain why reducing nutrient runoff can help prevent eutrophication.
Other Pollution Sources
21. Describe two ways sewage can harm water quality.
22. Explain why heavy metals can be particularly harmful in aquatic ecosystems.
23. Distinguish between bioaccumulation and biomagnification.
24. Describe two ecological effects of oil pollution.
25. What are microplastics?
26. Explain how sediment can damage an aquatic ecosystem.
27. Define thermal pollution and describe one possible effect.
Pollution Control
28. Give three ways agricultural runoff can be reduced.
29. Explain how wastewater treatment reduces pollution.
30. Give three methods industries can use to reduce water contamination.
31. Describe two methods for reducing polluted urban runoff.
32. Explain why preventing pollution is often preferable to cleaning it up later.
Application and Analysis
33. Fertilizer from many farms enters a river after heavy rainfall. Is this mainly point-source or non-point-source pollution? Explain.
34. Wastewater containing a pollutant leaves a factory through one identifiable pipe. Classify the pollution source and explain your reasoning.
35. A lake experiences a large algal bloom shortly after heavy rain washes fertilizer from surrounding farmland. Explain the likely connection.
36. Fish begin dying several days after a large algal bloom. Explain one mechanism that could cause the fish deaths.
37. A groundwater aquifer has been contaminated by a leaking underground chemical tank. Explain why this pollution may be difficult to manage.
38. Compare the likely effectiveness of cleaning pollution from a river with preventing the pollutant from entering the river in the first place.
39. A city wants to reduce pollutants entering streams after storms. Propose three strategies and explain how each would help.
40. Choose one example of water pollution and analyze it using:
SOURCE → POLLUTANT → PATHWAY → IMPACT → SOLUTION.