Drinking-water standards: The typical upper limit for nitrate concentration in domestic water supplies is closest to which value?

Introduction / Context:
Nitrate in drinking water is regulated because high levels can cause health issues such as methemoglobinemia in infants. Standards are usually expressed as mg/L (ppm) as nitrate (NO3−) or as nitrate-nitrogen (N). Understanding the common limit is important for source protection and treatment design.

Given Data / Assumptions:

• Concentration units in options are mg/L (ppm) as NO3−.
• Domestic supply typical guideline values are considered.

Concept / Approach:

Many national standards (e.g., IS 10500) set nitrate (as NO3−) at 45 mg/L as a permissible limit, roughly equivalent to about 10–11 mg/L as nitrate-nitrogen. This aligns with health-based guidance and provides a practical screening target for raw and treated sources.

Step-by-Step Solution:

Verification / Alternative check:

Utility compliance reports often list nitrate near or below 45 mg/L (as NO3−) where agricultural runoff is controlled.

Why Other Options Are Wrong:

• 10–30 ppm: Too low if expressed as NO3−; these may align with “as N” values when converted.

Common Pitfalls:

• Confusing “as N” and “as NO3−” units.
• Ignoring seasonal spikes from fertilizer application.

Final Answer:

45 ppm.