What is the least desirable method to dispose of pesticides?

Pouring pesticides into a river represents the least desirable method of disposal due to its severe environmental impact. This method can lead to contamination of water sources, endangering aquatic life, disrupting ecosystems, and potentially posing health risks to communities that rely on that water. Water bodies act as conduits, carrying harmful chemicals over vast areas, which can have long-lasting effects on both wildlife and human populations.

The other methods, although not ideal, can sometimes be more controlled and managed. For example, mixing pesticides with like substances or burning them can be done in regulated environments where emissions and residues can be monitored. While burying pesticides can lead to potential leaching of chemicals into soil and groundwater, it's generally more acceptable than releasing them directly into water systems. Thus, pouring pesticides into a river is unequivocally the most harmful and least desirable method of disposal.