4.4 Water pollution

A pollution explanation names source, pollutant, pathway, receptor and response.

Sewage, agriculture, industry, urban runoff, solid waste and oil use different pathways. Point sources are easier to locate; diffuse runoff needs catchment controls.

In November 2015, Montreal released an estimated 5–8 billion litres of untreated sewage into the St Lawrence River during sewer work. Local evidence reports pollution returning to normal after 4–10 days. Upstream/downstream DO, BOD, coliform, turbidity and nutrient monitoring can track the impact; sewage treatment and interception target the source and pathway.

A complete pollution case links a named location and source to a pollutant pathway, measured environmental impact and management response.

A visible pipe is not the only source; diffuse agricultural and urban runoff can dominate.

Plastic control works best upstream: prevent input first, then intercept or recover material already in rivers and seas.

Currents concentrate floating debris in oceanic gyres; UV and abrasion fragment many polymers rather than biologically recycling them. Large pieces entangle or are ingested. Microplastics enter food chains, can accumulate and magnify through trophic transfer, and can transport additives or toxins adsorbed to their surfaces.

A river boom can catch bottles, but reuse, collection and producer responsibility prevent the next load and lost fishing gear.

It removes escaped material but leaves upstream production, litter and gear-loss pathways unchanged.

Fragmentation is not disappearance; smaller particles can be harder to remove and easier to ingest.

Choose a measurement from the suspected mechanism, then compare matched sites or times with replication.

Organic waste suggests BOD and dissolved oxygen; fertilizer suggests nitrate/phosphate tests; heated effluent suggests temperature; sediment suggests turbidity and total suspended solids; industry may require metal-specific tests. Use oxygen and pH probes, a thermometer, a Secchi disc and nitrate/phosphate tests as appropriate.

Upstream DO is 9 mg/L and downstream is 4 after a sewage outlet; pair the result with BOD and flow data.

Matched upstream/downstream sites, repeated at comparable times, help separate the suspected input from weather, flow and seasonal variation.

A WQI can summarize, but one average cannot replace mechanism-specific measurements.

BOD is dissolved oxygen consumed by microbes decomposing biodegradable organic matter under a specified test condition.

In the five-day, 20°C test, BOD = initial DO − final DO. More biodegradable material means more microbial respiration and less oxygen left for aquatic life.

If DO falls from 8 to 2 mg/L, BOD is 6 mg O₂/L.

High BOD does not mean the water contains abundant oxygen: it means microbes used oxygen rapidly, leaving aquatic organisms under greater oxygen stress.

BOD is oxygen consumed, not oxygen concentration left in the river.

A nutrient input triggers a bloom only when that nutrient was limiting producer growth under the stated conditions.

Nitrate and phosphate can enter from fertilizer, sewage and products. Eutrophication names enrichment and producer response; oxygen depletion is a later consequence of death and decomposition.

If phosphate is limiting in a lake, a phosphate pulse can increase algae; adding nitrate alone may not.

Limitation data plus source and timing, not nutrient presence alone.

More nutrient is not automatically more bloom; identify the limiting resource.

Excess limiting nutrients can trigger bloom → shading → death → decomposition → high BOD → hypoxia or anoxia.

Aerobic decomposers consume oxygen as dead biomass accumulates. Sediment and decomposition can recycle nutrients, feeding another bloom and reinforcing the decline.

A summer algal bloom dies; bacteria use oxygen overnight, fish surface for air, and bottom water becomes hypoxic.

Death adds organic matter; decomposition raises oxygen demand and lowers DO.

Nutrient enrichment and hypoxia are stages in a chain, not synonyms.

Pollution becomes a social cost when an ecological change removes food, recreation, health protection or cultural services.

Low oxygen lowers fish survival; turbidity reduces recreation; toxic blooms threaten drinking water and livestock; treatment and monitoring costs rise. Identify who experiences each loss.

A fishery closure protects health but removes income from fishers, while residents may gain safer water.

Services distribute costs and benefits unevenly; one measure cannot represent all impacts.

An ecosystem service is not only a market price; health, culture and recreation also count.

Pollution control can prevent the activity, intercept the release, or repair legacy damage; the best package uses the level that matches the pathway.

Level 1 reduces pollutant-producing activity, such as changing fertilizer or detergent use. Level 2 limits release through buffer strips, sewer repair or nutrient-removing wastewater treatment. Level 3 removes legacy pollution and restores the ecosystem, for example by removing nutrient-rich mud and reintroducing plant or fish species.

Reducing fertilizer at the field is prevention; a buffer strip intercepts runoff; dredging nutrient-rich sediment treats the legacy.

Continuing upstream input can refill the system; pair release control with source reduction.

A visible cleanup is not always the highest-leverage intervention; diagnose the source first.