Wisconsin Pesticide Applicator Practice Test

Why is pesticide breakdown much slower in groundwater?

1. 1. High oxygen and light conditions

2. 2. High temperature conditions

3. 3. Low oxygen and light conditions

4. 4. High microbial activity

The correct answer is: 3. Low oxygen and light conditions

Pesticide breakdown in groundwater occurs more slowly due to low oxygen and light conditions. In groundwater, the environment tends to be more stable, lacking the necessary exposure to sunlight and having limited oxygen levels that are found in surface water. Sunlight plays a critical role in the photodegradation process, breaking down many pesticides in open environments, while oxygen is vital for aerobic microbial processes which facilitate biodegradation. In a groundwater setting, the absence of these elements means that chemical and biological processes that would typically lead to pesticide degradation are significantly inhibited. Microbial activity can be lower in these anaerobic conditions, and without sufficient light, photolysis—the breakdown due to light—does not occur. Therefore, the unique conditions present in groundwater contribute to a slower rate of pesticide breakdown, raising concerns about contamination lingering in these water sources.