The changes in water sample quality are jointly driven by physical, chemical and biological factors and regulated by external conditions. The dominant causes vary in different scenarios, as follows:
I. Physical factors: Altering physical states and sensory characteristics
It does not change the chemical properties of substances, but mainly affects transparency, temperature, turbidity, etc., creating conditions for subsequent reactions.
Temperature change: Heating reduces the solubility of gases (such as a decrease in dissolved oxygen), accelerates water stratification (oxygen deficiency at the bottom), and increases the metabolic rate of microorganisms. If industrial cooling water causes a sudden rise in local water temperature, it can lead to oxygen deficiency in fish.
Hydraulic disturbance: Changes in flow velocity carry sediment and increase turbidity (river flow becomes turbid after heavy rain), severe disturbance raises dissolved oxygen, and too slow flow velocity easily leads to the accumulation of pollutants. For instance, when a reservoir releases water, it stirs up the sediment at the bottom, causing secondary pollution.
Lighting conditions: Sufficient light promotes photosynthesis in algae (increases dissolved oxygen), and strong light accelerates photochemical reactions. Suspended matter blocking light inhibits photosynthesis, leading to a decrease in dissolved oxygen (such as oxygen deficiency at the bottom of deep water bodies).
Particulate matter migration: After suspended particulate matter adsorbs heavy metals and organic matter, it migrates or settles, and the sediment is suspended again (due to the activity of benthic organisms), releasing pollutants. For instance, in the rivers of mining areas, particulate matter adsorbs heavy metals, causing the water quality to exceed the standard.
Ii. Chemical Factors: Altering chemical composition and properties
Through dissolution, oxidation-reduction, acid-base reactions, etc., the form, concentration or toxicity of pollutants can be directly changed.
(1) Input of exogenous chemical substances (core human factor)
Point source pollution: Centralized discharge from fixed outlets, such as organic matter/heavy metals discharged from chemical plants, detergents/disinfectants discharged from domestic sewage, and high ammonia nitrogen wastewater discharged from livestock farms.
Non-point source pollution: Dispersed emissions, such as heavy rain washing farmland and bringing in fertilizers/pesticides, rainwater washing road surfaces and bringing in oil stains/heavy metals, acid rain/industrial waste gas sedimentation and changing the pH of water bodies.
(2) Chemical reactions within water bodies
REDOX: Low-valent heavy metals (Fe²⁺) oxidize and precipitate, and organic matter undergoes anoxic reduction to produce hydrogen sulfide (causing the water body to stink).
Acid-base changes: Acidic wastewater (from mines) lowers pH (dissolving heavy metals), while alkaline wastewater (from papermaking) raises pH (forming scale).
Complexation and precipitation: Organic substances form complexes with heavy metals (increasing solubility), and calcium and carbonate ions form calcium carbonate (with excessive hardness).
Iii. Biological Factors: Altering water quality through metabolism/activity
Biological activities directly consume or produce substances, affecting or deteriorating the self-purification of water quality.
Microbial action: Aerobic microorganisms degrade organic matter/ammonia nitrogen (natural self-purification); Anaerobic microorganisms produce malodorous gases (causing water to stink), and pathogenic microorganisms (Escherichia coli) reduce biological safety.
Aquatic plants and algae: Reeds, etc. absorb nitrogen and phosphorus (purify water quality), and excessive growth leads to shading and oxygen consumption. Algae reproduce normally to increase oxygen. When nitrogen and phosphorus are excessive, algal blooms/red tides occur (consuming oxygen and producing toxins).
Aquatic animals: Fish consume algae (to control algae) and excrete to increase ammonia nitrogen/phosphorus (to promote eutrophication). Benthic animals stir up sediments (releasing pollutants).
Iv. Key Influencing Conditions
Water body fluidity: Flowing water bodies (rivers) have strong self-purification capabilities and slow changes in water quality. Still water bodies (lakes) are prone to oxygen deficiency and rapid accumulation of pollutants.
Water body capacity: Small water bodies (ponds) have weak buffering capabilities, and a small amount of pollution can cause significant changes. Large water bodies (oceans) have strong dilution and slow changes.
Human intervention: Coagulation filtration, constructed wetlands, etc. to improve water quality; Overfishing and land reclamation from lakes are accelerating the deterioration.
Examples of scene-driven reasons
Urban drinking water (pipelines) : pipeline corrosion, disinfectant residue, secondary pollution from pipeline leakage.
Rivers near farmlands: agricultural non-point sources (fertilizers/pesticides), sediment from heavy rain.
Industrial park lakes: industrial wastewater, algal blooms, bottom anaerobic conditions.
Family fish tanks: fish excretion, rotten leftover feed, excessive light causing algae growth, and lack of oxygen due to untimely water change.