What Causes Turbidity in Water?

Turbidity is cloudiness from particles that scatter light. It is measured in NTU, not with a TDS meter.

Direct answer

Turbidity is usually caused by silt, clay, organic debris, algae, or rust suspended in the water. In rivers and lakes, storms and runoff are the main drivers. In a house, it can be a disturbed well, a dirty tank, or old iron pipes. Cloudy water can hide microbes and make disinfection less reliable — but turbidity is not the same as dissolved salts (TDS).

Definition: what turbidity is. TDS: what TDS measures.

Common causes

Surface water: rain, erosion, construction, farm runoff
Algae blooms in warm, nutrient-rich lakes
Wells: pump surge, a cracked casing, or nearby excavation
Home plumbing: rust from iron pipes after the water has been off
Storage tanks: sludge stirred when the tank is almost empty — see tank cleaning

Why it matters for drinking water

Particles can shield bacteria from chlorine and clog filters. Utilities treat turbidity with coagulation and filtration before disinfection. At home, sudden cloudiness after rain on a well is a reason to test, not only to wait for it to “settle.” Procedure: well water testing procedure.

Turbidity vs hardness vs TDS

Hardness is dissolved calcium and magnesium — water can look clear and still be hard. TDS is all dissolved solids. Turbidity is undissolved particles. Clear water can still be unsafe; cloudy water is a visible warning, not a complete test.

Educational explainer. Limits and lab methods differ by country. About.