Which of the following is a secondary indicator?

• A. Total coliform
• B. E. coli
• C. pH
• D. Odor

Answer: (D)

In water quality, indicators are split into those that signal potential health risk and those that reflect overall quality or consumer acceptability. Primary indicators include organisms like total coliforms and E. coli because their presence points to fecal contamination and possible pathogens. Odor is a secondary indicator; it signals that water quality may be deteriorating in a perceptible way—often hinting at organic decay, treatment byproducts, or chemical changes—without proving the presence of harmful organisms. pH is a chemical parameter that affects water chemistry and treatment decisions but isn’t used as a health-related indicator in the same sense as the primary ones. So, odor best fits the concept of a secondary indicator.