What is biological oxygen demand (BOD), and how does it relate to water quality?

• A. BOD measures the amount of oxygen microbes use to decompose organic matter.
• B. BOD measures the rate of water evaporation.
• C. BOD indicates the number of bacteria in water.
• D. BOD measures the amount of oxygen microbes use to decompose organic matter; higher BOD means poorer water quality.

Answer: (D)

BOD is a measure of how much dissolved oxygen microorganisms need to break down organic material in water. This is a direct gauge of organic pollution: when there’s more organic matter, microbes consume more oxygen, reducing the amount available for other aquatic life. So, higher BOD means less oxygen in the water and poorer water quality. The statement that includes both the definition and the link to quality—that BOD measures the oxygen used by microbes to decompose organic matter and that higher BOD corresponds to poorer water quality—is the strongest answer. The other ideas either miss the connection to water quality or describe something unrelated (evaporation, or just the number of bacteria).