Pesticide runoff can cause what issue?

Pesticide runoff primarily presents significant environmental challenges, particularly through its potential to harm non-target organisms. When pesticides are improperly applied or when there's heavy rainfall, chemicals can wash away from the intended treatment area and enter nearby waterways or ecosystems. This unintentional movement can result in detrimental effects on aquatic life, insects, birds, mammals, and plants that were not the intended recipients of the pesticide application. These organisms may experience toxicity, disruption of reproductive systems, or even mortality, depending on their exposure to the runoff contaminants.

While the other choices address various environmental aspects, they do not accurately capture the primary concern associated with pesticide runoff. For instance, pesticide runoff does not typically lead to an increase in soil nutrients or an improvement in groundwater quality, as these substances often degrade ecological health rather than enhance it. Similarly, the effectiveness of pesticides may not directly reduce due to runoff; rather, it is more likely that runoff disperses the active ingredients away from the target area, potentially leading to ineffective pest control but not necessarily reducing the inherent effectiveness of the pesticide itself. Thus, the main issue with pesticide runoff lies in its potential to harm non-target organisms.