4.4 Water Pollution

Water quality definition

The measurement of chemical, physical, and biological characteristics of water, often monitored to inform management strategies for reducing pollution.

Abiotic water quality parameters

Key measurable physical and chemical factors in aquatic systems, including dissolved oxygen (DO), pH, temperature, turbidity, nitrates, phosphates, and total suspended solids.

Biochemical Oxygen Demand (BOD)

The amount of dissolved oxygen required by microorganisms to decompose organic material in a water sample (measured in mg/L over 5 days at 20°C).

Indicator species

Organisms whose presence, absence, or abundance reflects environmental conditions; e.g., pollution-intolerant mayfly nymphs indicate clean water, while pollution-tolerant Tubifex worms indicate poor quality.

Biotic Index

An indirect measure of water quality calculated using the pollution tolerance scores, relative abundance, and diversity of species in a community (e.g., Hilsenhoff or Trent Biotic Index).

Water Quality Index (WQI)

A single, weighted numerical average combining multiple water quality parameters (such as turbidity, pH, BOD, and nutrients) to represent overall contamination levels.

Point-source vs. Non-point-source pollution

Point-source pollution comes from a single, identifiable origin (like factory effluent pipes); non-point-source pollution comes from diffuse, widespread inputs (like agricultural run-off).

Endocrine-disrupting chemicals (TBT)

Synthetic chemical pollutants like tributyltin (TBT) used in antifouling paints that disrupt reproductive systems and cause physiological abnormalities in marine species.

Persistent Organic Pollutants (PCBs)

Non-biodegradable, highly toxic industrial chemical compounds that undergo bioaccumulation and biomagnification through aquatic food webs.

Ocean gyres and marine plastics

Large systems of circulating ocean currents that concentrate buoyant plastic debris into calm central convergence zones, such as the Great Pacific Garbage Patch.

Microplastics in food webs

Plastic fragments smaller than 5 mm that absorb surface toxins, bioaccumulate in organisms across all trophic levels, and biomagnify in top predators including humans.

Stages of Eutrophication

Nutrient enrichment (nitrates/phosphates) causes algal blooms; algae die and decompose via aerobic bacteria, consuming dissolved oxygen and resulting in hypoxia or dead zones.

Harmful Algal Blooms (HABs)

Explosive growths of specific phytoplankton (such as freshwater cyanobacteria releasing hepatotoxins or marine dinoflagellates causing red tides) that produce lethal neurotoxins.

Primary sewage treatment

The initial physical phase of wastewater management involving screening to filter large debris and sedimentation tanks to settle out heavy solid particles.

Secondary sewage treatment

The biological phase of wastewater management that uses aeration and microorganisms to break down dissolved organic matter and significantly lower the biochemical oxygen demand (BOD).

Tertiary sewage treatment

The advanced phase of wastewater management utilizing chemical disinfection and filtration to remove remaining pathogens, fine suspended impurities, and excess dissolved nutrients.