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

Non-point source contamination of groundwater refers to pollutants that do not have a single, identifiable source, but rather come from multiple origins across a broad area. This type of contamination often occurs due to runoff from agricultural fields, urban areas, or other land surfaces where chemicals are applied.

In this scenario, the choice about pesticides that dissolve and leach through soil after rain exemplifies non-point source contamination. As rainfall occurs, water can percolate through the soil, allowing soluble pesticides to move through the soil layers and reach the groundwater. This process illustrates how widespread pesticide use can collectively lead to contamination of groundwater without a single source point, demonstrating the diffuse nature of non-point source pollution.

The other options represent different pathways of contamination. For instance, pesticides washed off during heavy rains can indeed contribute to non-point source pollution as well, but this scenario is typically viewed in the context of surface runoff rather than how they directly interact with groundwater. Stored pesticides leaking from containers and pesticides poured into storm drains represent more direct, identifiable, and localized sources of contamination, classifying them as point sources. Therefore, the focus on leaching through the soil offers a clearer and more accurate representation of non-point source groundwater contamination.