Water quality impacts: The presence of soluble organic matter in polluted water primarily leads to which adverse effect in receiving streams and treatment systems?

Introduction / Context:
Biodegradable organic substances in wastewater impose a biochemical oxygen demand (BOD) on natural waters. When effluents are discharged without adequate treatment, microorganisms consume dissolved oxygen while degrading organics, thereby threatening aquatic life and altering ecosystem balance.

Given Data / Assumptions:

• Organics are soluble and biodegradable to some extent.
• Receiving water has finite dissolved oxygen capacity.
• No reference is made to toxic or explosive organics; the emphasis is typical municipal/industrial biodegradable load.

Concept / Approach:
Microbial oxidation reduces dissolved oxygen according to the stoichiometry of carbon conversion to CO2 and water. If oxygen consumption exceeds reaeration, the stream experiences a sag in DO concentration, harming fish and aerobic organisms. While nutrients can stimulate plant growth (eutrophication), that is linked to nitrogen and phosphorus; soluble organics primarily drive oxygen depletion. Fire or explosion hazards are not the chief concern in dilute aquatic environments.

Step-by-Step Solution:
Associate soluble organics with BOD.Relate BOD to oxygen uptake by microbes.Conclude that oxygen depletion is the central immediate effect.

Verification / Alternative check:
Classic Streeter–Phelps DO sag models quantify the balance between deoxygenation and reaeration, showing how organic loads depress stream DO to critical levels downstream of outfalls.

Why Other Options Are Wrong:
Undesirable plant growth: Primarily tied to nutrient enrichment, not just carbonaceous organics.Fire/explosion hazards: Not typical at environmental concentrations in open waters.

Common Pitfalls:

• Conflating eutrophication (nutrients) with BOD effects (organics).
• Overlooking temperature and flow effects on reaeration capacity.

Final Answer:
depletion of oxygen.