What environmental factor commonly causes non-point pollution?

Non-point source pollution is characterized by its diffuse and widespread nature, as opposed to the specific, identifiable sources of point source pollution. Rainfall or snowmelt plays a significant role in this process because as water moves across the land surface, it collects various pollutants from a range of sources. These can include fertilizers, pesticides, oil, heavy metals, and sediments that are present in soil and on impervious surfaces.

When precipitation occurs, it can wash these contaminants into nearby streams, rivers, lakes, and oceans. This dispersed pattern of pollution is why rainfall or snowmelt is a prominent environmental factor contributing to non-point source pollution. In contrast, the other options represent specific or localized sources of pollution – such as industrial wastewater, landfill leachate, and household sewage – which can easily be traced back to their origin, thus classifying them as point source pollution. This distinction highlights the nature of non-point pollution and explains why rainfall or snowmelt is the correct answer.