What source is known for contributing to surface water contamination by pesticides?

The correct answer highlights that runoff and erosion play a significant role in the contamination of surface water by pesticides. When it rains or when irrigation occurs, water can wash over treated areas, carrying away pesticide residues with it. This runoff can travel along with soil particles and enter nearby streams, lakes, and rivers, thereby contaminating these bodies of water.

Soil erosion often exacerbates this problem, as disturbed soil is more susceptible to being carried away with water flow, increasing the amount of pesticide that enters surface water systems. Additionally, human activities like farming can intensify both runoff and erosion through practices that disturb the soil, further heightening the risk of pesticide contamination in surface water.

In contrast, other options involve processes that are less directly associated with surface water exposure. Groundwater leaching refers to pesticides moving through the soil profile into groundwater supplies, which is a different concern. Indoor application residues typically affect indoor environments and are not a direct source for surface water contamination. Atmospheric deposition involves pesticides being carried by wind and settling in a different area, which, while it can contaminate surface water, is a less direct route than runoff. Thus, runoff and erosion are the primary mechanisms through which pesticides enter surface water systems.