What is the difference between point-source and non-point-source pollution?

Point sources discharge from a discrete, identifiable outlet such as a pipe, whereas non-point sources, such as agricultural runoff or urban stormwater, enter water diffusely across the landscape and are harder to monitor and control.

Why does organic pollution lower dissolved oxygen in a river?

Microorganisms consume oxygen as they break down biodegradable organic matter; when the oxygen demand exceeds the rate of reaeration, dissolved oxygen falls, stressing or killing oxygen-dependent aquatic life.

Water Pollution

Water pollution is the degradation of water quality by chemical, physical, or biological agents that impair its use and harm aquatic life.

Definition

The introduction into water bodies of substances or energy that alter water quality so as to impair beneficial uses, harm aquatic organisms, or threaten human health.

Scope

This area covers the contamination of surface waters, including rivers, lakes, estuaries, and oceans, by point and non-point sources. It addresses oxygen-demanding wastes, nutrients and eutrophication, pathogens, toxic chemicals, sediments, thermal loading, oil, and plastics, together with the indicators used to measure water quality. Treatment and remediation are referenced where they bear on restoring quality, while groundwater contamination is treated under soil and land contamination and detailed hydrogeochemistry under soil and water science.

Sub-topics

Core questions

What distinguishes point-source from non-point-source water pollution?
How do oxygen-demanding wastes and nutrients degrade aquatic ecosystems?
Which parameters are used to characterize water quality?
How do oil, plastics, and thermal discharges affect aquatic systems?

Key theories

Dissolved oxygen sag and self-purification: Biodegradable organic loading depletes dissolved oxygen downstream of a discharge before reaeration restores it, producing the characteristic oxygen-sag curve described by Streeter and Phelps that governs a stream's assimilative capacity.
Nutrient limitation and eutrophication: Excess inputs of limiting nutrients, chiefly phosphorus in fresh water and nitrogen in many marine settings, stimulate algal production whose decay depletes oxygen, driving the progressive enrichment known as eutrophication.

Clinical relevance

Water pollution affects drinking-water safety, fisheries, recreation, and ecosystem health; quantifying contaminant loads and water-quality indicators underpins discharge regulation, treatment design, and watershed management.

Evidence & guidelines

Water-quality assessment commonly references indicator frameworks such as those compiled by UNESCO, WHO, and UNEP, used here descriptively to explain how pollution is measured rather than as regulatory thresholds.

History

Concern over waterborne disease and organic pollution grew through the nineteenth and twentieth centuries; the Streeter-Phelps oxygen-sag model of 1925 gave stream pollution a quantitative basis, and later decades broadened attention to nutrients, toxics, oil, and plastics.

Seminal works

davis2008
metcalf2014
chapman1996

Frequently asked questions

What is the difference between point-source and non-point-source pollution?: Point sources discharge from a discrete, identifiable outlet such as a pipe, whereas non-point sources, such as agricultural runoff or urban stormwater, enter water diffusely across the landscape and are harder to monitor and control.
Why does organic pollution lower dissolved oxygen in a river?: Microorganisms consume oxygen as they break down biodegradable organic matter; when the oxygen demand exceeds the rate of reaeration, dissolved oxygen falls, stressing or killing oxygen-dependent aquatic life.