Environmental Chemistry • Topic 2 of 3

Water & Soil Pollution

Clean water and fertile soil are essential resources. Water pollution is the addition of undesirable substances that make water unfit for its intended use. The main causes are listed below.

Causes of water pollution

Pathogens: disease-causing bacteria and other microorganisms entering from domestic sewage and animal waste (e.g. E. coli), causing cholera, typhoid and dysentery.
Organic wastes: biodegradable matter such as leaves, sewage and food waste. Microbes decompose it using up dissolved oxygen.
Inorganic wastes & chemicals: trace metals (lead, mercury, cadmium, arsenic), nitrates and phosphates from fertilisers, detergents, and industrial effluents. Many are toxic even in small amounts.

Dissolved oxygen and BOD

Aquatic life needs dissolved oxygen (DO); clean water holds about 8–10 mg of O₂ per litre. When organic waste is added, bacteria consume oxygen to break it down. The Biochemical Oxygen Demand (BOD) is the amount of oxygen (in mg L⁻¹) required by microorganisms to decompose the organic matter in 1 litre of water, usually measured over 5 days at 20°C.

Low BOD (< 5 ppm) → clean water with little organic load.
High BOD (> 17 ppm) → heavily polluted water; oxygen is depleted and fish die.

So BOD is inversely linked to water quality: the higher the BOD, the more polluted the water.

International standards for drinking water

Limits set by bodies such as the WHO and BIS protect health, for example: fluoride ≤ 1 ppm (excess causes fluorosis), lead ≤ 50 ppb, nitrate ≤ 50 ppm (excess causes “blue baby” syndrome), and sulphate ≤ ~500 ppm.

Eutrophication

When fertiliser run-off adds nitrates and phosphates to a lake, algae grow explosively (an algal bloom). When the algae die, bacterial decay consumes the dissolved oxygen, suffocating fish and aquatic life. This nutrient-driven ageing of a water body is called eutrophication.

Soil pollution

Soil is polluted mainly by pesticides and industrial waste.

Insecticides (e.g. DDT) kill insects but persist and biomagnify up the food chain.
Herbicides (e.g. sodium chlorate, triazines) kill weeds.
Fungicides (e.g. organomercury compounds) kill fungi but can release toxic mercury.
Industrial waste: heavy metals and acids from factories that poison soil and seep into groundwater.

Persistent pesticides such as DDT are now banned in many countries because they accumulate in fatty tissue and harm birds and humans.

Dissolved oxygen and BOD: clean vs polluted water
Water type	Dissolved O2 (mg/L)	BOD (ppm)	Aquatic life
Clean water	8 to 10	less than 5	Healthy, fish survive
Moderately polluted	4 to 6	5 to 17	Stressed
Heavily polluted	below 4	above 17	Fish die (anoxic)

Solved Examples

Worked Example

Two water samples are tested: sample X has a BOD of 3 ppm and sample Y has a BOD of 22 ppm. Which sample is more polluted, and why?

Solution

BOD measures the oxygen microorganisms need to decompose organic matter; more organic load means higher BOD.
Sample X (BOD 3 ppm) is below 5 ppm, indicating little organic waste.
Sample Y (BOD 22 ppm) is well above 17 ppm, indicating a heavy organic load and oxygen depletion.

Answer: Sample Y is more polluted, because a higher BOD means more organic matter and greater oxygen demand.

Worked Example

Explain how the addition of biodegradable organic waste to a river lowers its dissolved oxygen.

Solution

Aerobic bacteria decompose the organic waste, and this decomposition uses dissolved oxygen.
As more waste is added, bacterial activity increases and oxygen is consumed faster than it is replaced.
The dissolved oxygen falls, which can suffocate fish and other aquatic organisms.

Answer: Bacteria use dissolved oxygen to break down the organic waste, so DO drops as the organic (BOD) load rises.

Worked Example

Describe eutrophication and the role of fertilisers in causing it.

Solution

Run-off from farms carries nitrate and phosphate nutrients into lakes.
These nutrients trigger rapid algal growth (an algal bloom) on the surface.
When the algae die, bacterial decay consumes the dissolved oxygen, killing fish; this nutrient-driven decline is eutrophication.

Answer: Eutrophication is nutrient enrichment (mainly nitrates/phosphates from fertilisers) that causes algal blooms whose decay depletes oxygen and kills aquatic life.

Worked Example

A drinking-water sample contains fluoride at 3 ppm and nitrate at 60 ppm. Comment on its safety against standard limits.

Solution

The recommended limit for fluoride in drinking water is about 1 ppm; 3 ppm is three times higher and can cause fluorosis (mottled teeth, bone damage).
The limit for nitrate is about 50 ppm; 60 ppm exceeds it and can cause methaemoglobinaemia (“blue baby” syndrome) in infants.
Both contaminants are above safe limits.

Answer: The water is unsafe: fluoride (3 ppm) and nitrate (60 ppm) both exceed the recommended limits (~1 ppm and ~50 ppm).

Worked Example

Classify the following soil pollutants by their target: DDT, sodium chlorate, organomercury compound.

Solution

DDT kills insects, so it is an insecticide.
Sodium chlorate kills weeds, so it is a herbicide.
Organomercury compounds kill fungi, so they are fungicides.

Answer: DDT → insecticide; sodium chlorate → herbicide; organomercury compound → fungicide.

Worked Example

Why is the persistent pesticide DDT considered an environmental hazard despite controlling insect-borne disease?

Solution

DDT is chemically stable and does not break down easily, so it persists in soil and water for years.
It is fat-soluble and accumulates in the fatty tissue of organisms, becoming more concentrated up the food chain (biomagnification).
High levels harm birds (thinning eggshells) and pose risks to humans, so DDT is now banned in many countries.

Answer: DDT persists and biomagnifies up the food chain, harming wildlife and humans, which is why it is widely banned.

Key Points

Water pollution comes from pathogens (sewage), biodegradable organic wastes, and inorganic/chemical wastes (heavy metals, nitrates, phosphates).
Dissolved oxygen (DO) in clean water is about 8–10 mg/L; BOD is the oxygen microbes need to decompose organic matter and is higher for more polluted water.
Drinking-water standards limit contaminants, e.g. fluoride ≤ 1 ppm, nitrate ≤ 50 ppm, lead ≤ 50 ppb.
Eutrophication is nutrient (nitrate/phosphate) enrichment that triggers algal blooms whose decay depletes oxygen and kills aquatic life.
Soil is polluted by pesticides — insecticides (DDT), herbicides (sodium chlorate), fungicides (organomercury) — and by industrial waste; persistent pesticides biomagnify.

Quick Check — 4 MCQs

Tap an option to check your answer0 / 4

Q1.A high value of BOD in a water sample indicates:

Explanation: BOD is the oxygen demand of microbes decomposing organic matter; a high BOD means a large organic load, i.e. heavy pollution.

Q2.Eutrophication of a lake is mainly caused by excess:

Explanation: Nitrate and phosphate run-off feeds algal blooms; their decay depletes oxygen, causing eutrophication.

Q3.Match the pesticide to its use: DDT is an example of a/an:

Explanation: DDT is a persistent insecticide; herbicides kill weeds and fungicides kill fungi.

Q4.Dissolved oxygen of clean natural water is approximately:

Explanation: Clean water typically holds about 8–10 mg of dissolved O₂ per litre, enough to support fish.

Practice more MCQs → 📝 Assignment · 30 marks