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 instead comes from multiple or diffuse sources. An example of this is when pesticides dissolve in water and leach through the soil after rainfall. In this scenario, rainwater carries the pesticide residues through the soil layers and into the groundwater, making it difficult to pinpoint a specific source of contamination. This is characteristic of non-point-source pollution, as it involves the cumulative impact of various agricultural practices over a broad area rather than a specific localized incident.

In contrast, the other choices present scenarios where contamination can be traced back to a distinct and identifiable source. Dumping leftover pesticides down a well represents a direct, intentional action that results in a specific point of contamination. Back-siphoning occurs when pesticides are drawn back into a water supply, also representing a clear, identifiable source of contamination. Finally, leaching from a pesticide mixing area typically involves localized contamination due to spills or improper storage, which can be directly traced back to that area. All these scenarios are classified as point-source contamination, as they originate from a specific location or event.