Why is pesticide breakdown slower in groundwater?

• A. High microbial activity
• B. Dry soil conditions
• C. Low oxygen and light conditions
• D. High oxygen and bright light

Answer: (C)

Groundwater breaks down pesticides more slowly because the environment is dark and low in oxygen. Photodegradation from sunlight can’t happen underground, so that pathway doesn’t contribute to cleaning the pesticide. At the same time, the limited oxygen in groundwater means aerobic microbes—the ones that use oxygen to biodegrade many pesticides—work much more slowly. Some anaerobic microbial activity can occur, but it typically degrades pesticides much less efficiently. The combination of no light and little oxygen keeps the overall breakdown rate low, so pesticides persist longer in groundwater. If there were plenty of oxygen and sunlight, degradation would occur more quickly.