Water Quality – Interpreting Excess Nitrates in River Water What does a high concentration of nitrates (NO3−) in river water most commonly indicate regarding sewage pollution timing and oxidation stage?

Introduction / Context:
Assessing organic pollution in surface waters relies on indicator species and nitrogen species transformation. Sewage introduces reduced nitrogen (ammonia), which is oxidized to nitrites and then nitrates by nitrifying bacteria. Understanding which nitrogen form dominates provides insight into how long ago the pollution occurred and how far oxidation has progressed.

Given Data / Assumptions:

• Observed: elevated nitrate concentration (NO3−) in river water.
• Oxygenated conditions allow nitrification over time.
• No unusual industrial nitrate sources are assumed.

Concept / Approach:

Organic pollution timeline: fresh sewage → ammonia (NH3/NH4+) peak; as microbial oxidation proceeds, nitrites (NO2−) appear; with further oxidation, nitrates (NO3−) accumulate. Therefore, high nitrates typically indicate that pollution is not fresh but has undergone complete nitrification, pointing to past sewage contamination rather than a recent discharge.

Step-by-Step Solution:

Verification / Alternative check:

River self-purification models and standard water-quality texts align nitrate prevalence with downstream recovery zones following an upstream sewage input, after DO has been restored and nitrification proceeds.

Why Other Options Are Wrong:

• Recent/immediate pollution: would show elevated ammonia and possibly nitrite with low DO.
• No pollution: contradicted by elevated nitrate in the absence of non-sewage sources.

Common Pitfalls:

Ignoring agricultural runoff as an alternate nitrate source; not corroborating with BOD, DO, or coliform counts; misreading nitrite as nitrate.

Final Answer:

Past pollution of water with sewage