Which is an example of nonpoint-source contamination of groundwater?

Nonpoint-source contamination of groundwater refers to pollution that does not originate from a single, identifiable source but instead comes from multiple or diffuse sources. This type of contamination is often associated with rainfall or stormwater runoff that can carry pollutants over a widespread area, affecting groundwater quality.

The correct choice describes how pesticides can dissolve in water and leach through the soil during rain events. When rainwater infiltrates the ground, it can pick up chemicals from various surface sources, such as agricultural fields, lawns, or urban areas, and subsequently transport these pollutants into the groundwater. This process exemplifies nonpoint-source contamination because it is not traceable to a single source; rather, it is the cumulative result of widespread pesticide application.

The other options describe forms of contamination that are more localized or directly traceable to specific sources or activities, thus classifying them as point sources. For instance, back-siphoning at a wellhead and leaching from a pesticide mixing area are both associated with specific locations where contamination can be traced directly to certain practices, making them point-source issues. The discharge from a wastewater treatment plant is also a direct source of contaminants, as it originates from a specific facility.