Water quality index — what does turbidity indicate? Turbidity in a water sample is primarily an indication of the presence of:

Introduction / Context:
Turbidity is a routine water-quality parameter measured to assess the clarity of water. Elevated turbidity can impact filtration, disinfection efficiency, and aesthetic quality. Interpreting turbidity correctly is essential for treatment design and monitoring.

Given Data / Assumptions:

• Turbidity measured by nephelometric methods correlates with light scattering.
• Typical contributors include clay, silt, organic colloids, and microorganisms.
• Dissolved substances do not generally scatter light strongly enough to cause turbidity.

Concept / Approach:
Turbidity arises from suspended and colloidal particles that scatter light. Inorganic fines (clays, silts) and organic colloids increase scattering, resulting in higher NTU values. Dissolved solids pass through filters and usually do not cause turbidity, while floating solids (large debris) are often removed by screening and are not the main driver of turbidity readings.

Step-by-Step Solution:
Identify what scatters light: suspended/colloidal particles.Link elevated NTU to fine solids rather than purely dissolved species.Conclude that turbidity primarily indicates suspended inorganic matter (and colloids).

Verification / Alternative check:
Jar tests and filtration reduce turbidity by removing particulate matter; conductivity or TDS measurements, not turbidity, track dissolved ions.

Why Other Options Are Wrong:

• Dissolved solids: can make water hard or saline but do not significantly increase turbidity.
• Floating solids: too large; turbidity meters are sensitive to fine particles.
• Dissolved gases: have negligible effect on turbidity.

Common Pitfalls:

• Confusing color (from dissolved organics) with turbidity; color is different from light-scattering turbidity.

Final Answer:
Suspended inorganic matter and colloids