Which is an example of non-point-source contamination of groundwater?

Non-point-source contamination of groundwater refers to pollution that does not originate from a single, identifiable source but rather comes from various sources over a wide area. When it comes to the example provided, pesticides that dissolve and leach through soil after it rains fit this definition perfectly.

Rainfall can transport pesticides that have been applied to agricultural fields or other landscapes into the soil. As the rainwater travels through the soil, it can carry these dissolved pesticides into the groundwater. This process does not happen from a single location or source but from multiple areas where pesticides may be used, making it non-point-source pollution.

In contrast, other options describe scenarios that are more localized or identifiable. For instance, dumping leftover pesticide products down a well, back-siphoning pesticide spills, or leaching from a pesticide mixing area all represent specific, point-source incidents where contamination can be traced back to a particular action or location. Thus, they do not exemplify non-point-source contamination as clearly as the leaching of pesticides through soil does.