A major source of nonpoint source pollution is:

Nonpoint source pollution refers to contaminants that accumulate from multiple diffuse sources rather than a single identifiable point, such as a pipe or drain. Runoff from agricultural land is a significant contributor to this type of pollution. This occurs when rainwater washes over fields treated with fertilizers, pesticides, and herbicides, which can then be carried into nearby water bodies. The chemicals from agricultural runoff can lead to nutrient loading, contributing to problems like algal blooms and depletion of oxygen in aquatic environments.

In contrast, the other options represent sources of point source pollution or are more easily identifiable and managed. Industrial discharges and wastewater treatment plants are specific and regulated outflows, while stormwater systems, although they can contribute to pollution, primarily transport existing pollutants rather than being a diffuse source of new pollutants. Thus, agricultural runoff stands out as a primary example of nonpoint source pollution, significantly impacting water quality and ecosystems.