Which of the following is NOT a way to help prevent pesticide contamination of groundwater?

The choice to dispose of leftovers in unauthorized areas stands out as it directly contradicts best practices for environmental safety and responsible pesticide management. Proper disposal of pesticide leftovers is crucial for preventing environmental contamination, including contamination of groundwater. This practice not only risks polluting the soil and water but also goes against legal and regulatory requirements designed to protect public health and the environment.

On the other hand, delaying application if heavy rain is expected, preventing backsiphoning, and avoiding applying near wells are all proactive measures aimed at reducing the risk of pesticides reaching groundwater. Heavy rain can lead to runoff, which may carry pesticides into water sources; hence delaying applications during such conditions mitigates this risk. Preventing backsiphoning ensures that pesticides do not accidentally get sucked back into the water supply during mixing or application. Applying pesticides away from wells further minimizes the likelihood of contamination by ensuring that any potential drift or runoff does not enter water sources used for drinking or irrigation.

Thus, while the first three approaches serve to protect water resources, disposing of leftovers in unauthorized areas actively contributes to the problem of groundwater contamination, making it the correct answer for the question asked.