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

Nonpoint-source contamination of groundwater refers to pollutants that do not come from a single, identifiable source but rather from widespread areas. This type of contamination occurs when contaminants are dispersed over a large region, making it difficult to pinpoint exact sources of pollution.

In this case, pesticides that dissolve and leach through soil after it rains exemplify nonpoint-source contamination because rainfall can mobilize the pesticides from various sources, such as agricultural fields or lawns, and then cause them to percolate through the soil into the groundwater. The process is distributed across broad areas, making it a classic example of nonpoint-source pollution.

The other options represent different forms of contamination. Runoff during heavy rain typically concentrates contaminants in surface water first, and pesticides that are spilled during transport are easily traceable to a specific incident or location, categorizing these as point-source pollution. Urban runoff can also contain nonpoint-source pollution, but it represents a specific urban context rather than the broader implication of leaching through soil that characterizes the correct response.