4.4 Water Pollution

Water quality

Measurement of the chemical, physical and biological characteristics of water

Abiotic factors of water quality

Dissolved oxygen, BOD, temperature, pH, turbidity, nutrients, metals and suspended solids

Dissolved oxygen (DO)

Amount of oxygen available in water to support aquatic life, measured in mg L⁻¹

Hypoxia

Low dissolved oxygen levels that stress or kill aquatic organisms

Anoxia

Complete absence of dissolved oxygen in water

Biochemical oxygen demand (BOD)

Amount of dissolved oxygen required by microorganisms to decompose organic matter

High BOD

Indicates high organic pollution and low water quality

BOD measurement

Difference between initial DO and DO after 5 days incubation in the dark

Temperature (water quality)

Affects oxygen solubility and metabolic rates of aquatic organisms

Warm water effect on DO

Warm water holds less dissolved oxygen

pH

Measure of water acidity or alkalinity affecting species survival

Turbidity

Lack of water clarity caused by suspended particles

Secchi disk

Tool used to measure turbidity based on water transparency

Nitrates and phosphates

Nutrient pollutants that can cause eutrophication

Eutrophication

Excess nutrient input causing rapid algal growth in water bodies

Cause of eutrophication

Agricultural runoff, sewage, detergents and industrial effluent

Algal bloom

Rapid growth of algae due to excess nutrients

Eutrophication oxygen effect

Algal decay increases BOD and reduces dissolved oxygen

Dead zone

Area of water with little or no oxygen where most life cannot survive

Impact of eutrophication on biodiversity

Loss of species and dominance of pollution-tolerant organisms

Provisioning services impact (eutrophication)

Reduced fish stocks and unsafe drinking water

Cultural services impact (eutrophication)

Reduced recreational and aesthetic value of water bodies

Harmful algal blooms (HABs)

Algal blooms that produce toxins harmful to humans and animals

Freshwater algal toxins

Cyanotoxins that damage liver and nervous systems

Marine algal toxins

Neurotoxins from dinoflagellates causing red tides

Red tide

Marine harmful algal bloom often caused by dinoflagellates

Increasing hypoxia causes

Global warming, stratification, sewage disposal and eutrophication

Stratification

Formation of water layers that prevent oxygen mixing

Primary sewage treatment

Physical removal of large solids by screening and sedimentation

Secondary sewage treatment

Biological breakdown of organic matter using microorganisms

Tertiary sewage treatment

Chemical and physical processes to remove remaining pollutants and pathogens

Indicator species

Species whose presence or absence indicates water quality

Pollution-tolerant species

Organisms that survive in polluted, low-oxygen water

Pollution-intolerant species

Organisms that only survive in clean, well-oxygenated water

Tubifex worms

Indicator species for poor water quality

Mayfly nymphs

Indicator species for good water quality

Biotic index

Indirect measure of water quality based on species tolerance and abundance

Hilsenhoff Biotic Index

Biotic index using pollution tolerance scores from 1–10

Low biotic index value

Indicates clean water with many pollution-intolerant species

High biotic index value

Indicates polluted water with many pollution-tolerant species

Water Quality Index (WQI)

Combined weighted score of multiple water quality parameter

Purpose of WQI

Simplifies complex water quality data into a single value

WHO drinking water guidelines

International standards for safe drinking water quality

Point-source pollution

Pollution from a single identifiable source

Non-point-source pollution

Diffuse pollution from many sources

Domestic sewage pollution

Contains pathogens, nutrients and organic matter

Agricultural runoff pollution

Major source of nitrates and phosphates

Industrial effluent pollution

Includes heavy metals, chemicals and thermal pollution

Thermal pollution

Release of warm water that reduces dissolved oxygen levels

Plastic pollution

Persistent solid waste that harms aquatic ecosystems

Macroplastics

Large plastic items such as bottles, bags and fishing nets

Microplastics

Plastic particles smaller than 5 mm formed from breakdown of macroplastics

Bioaccumulation

Increase in pollutant concentration within an organism

Biomagnification

Increase in pollutant concentration at higher trophic levels

Oil spill

Point-source pollution that blocks light and gas exchange

Reducing water pollution

Changing consumption, legislation, protests, research and legal action

Individual actions to reduce pollution

Reduce plastic use, recycle and dispose of waste responsibly

Government actions to reduce pollution

Taxes, legislation, financial incentives and enforcement

Mitigating eutrophication levels

Reduce pollutant production, reduce release and remove pollutants

Buffer zones

Vegetated areas that absorb nutrients before reaching water bodies

Dredging

Removal of nutrient-rich sediments from water bodies

Aeration

Addition of oxygen to improve water quality