A wastewater discharge containing large amounts of biodegradable organic matter enters a flowing stream. Downstream, bacterial decomposition increases sharply. Dissolved oxygen begins to fall, sensitive fish disappear, and organisms tolerant of low-oxygen conditions become more common. Farther downstream, dissolved oxygen gradually rises and normal aquatic communities begin returning. Which interpretation best explains this sequence?

Question

Accepted Answer

Decomposition raises BOD and consumes dissolved oxygen, creating an oxygen sag, followed by recovery as the waste is diluted and processed.

Answer

Biological decomposition immediately removes all pollutants without affecting dissolved oxygen.

Answer

The decline in oxygen is caused primarily by groundwater recharge rather than decomposition.

Answer

Organic matter directly produces oxygen, followed by temporary oxygen oversaturation.

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A wastewater discharge containing large amounts of biodegradable organic matter ...

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