What is the maximum permissible concentration of nitrites (as NO2-) in public water supplies?

Introduction:
Nitrite (NO2-) is an intermediate in the nitrogen cycle and a health-significant parameter. Elevated nitrite indicates recent contamination or nitrification within the distribution system and can pose risks such as methemoglobinemia.

Given Data / Assumptions:

• Public supply standard is being referenced.
• Answer choices: 0.5, 1.0, 1.5, and 200 ppm.

Concept / Approach:
Potable water standards limit nitrite stringently—values are typically in sub-ppm range. A value such as 200 ppm is grossly excessive for any health-based standard; choices near or below 1 ppm are plausible. The commonly cited permissible value for nitrite as NO2- in basic MCQ references is 0.5 ppm for supplies.

Step-by-Step Solution:
Recognize that nitrite standards are far more restrictive than nitrate standards. From the given options, 0.5 ppm aligns with conservative potable limits. Select 0.5 ppm as the maximum permissible value among choices.

Verification / Alternative check:
Cross-compare with typical textbook tables where nitrite limits are ≤ 1 ppm; 0.5 ppm is widely referenced in exam-oriented materials.

Why Other Options Are Wrong:

• 1.0 ppm and 1.5 ppm: Less conservative than commonly prescribed limits.
• 200 ppm: Orders of magnitude too high for drinking water; unrealistic.

Common Pitfalls:

• Confusing nitrate limits (~45 ppm as NO3-) with nitrite limits (much lower).
• Ignoring the difference between “as NO2-” versus “as N.”

Final Answer:
0.5 ppm.