__________ is a major contributor to nutrient enrichment, or "nutrient loading" of surface waters.

Runoff is the primary contributor to nutrient enrichment or "nutrient loading" of surface waters because it involves the movement of water over land surfaces into water bodies. When precipitation occurs, whether in the form of rain or melted snow, it can dissolve various nutrients and pollutants from the soil, agricultural fields, roads, and urban areas as it flows. This nutrient-laden runoff ultimately enters rivers, lakes, and streams, significantly impacting water quality and ecosystem health.

The other processes, although they play roles in the hydrological cycle, do not contribute to nutrient loading in the same direct manner. Rainfall itself can introduce some nutrients, but it is mainly the runoff capturing and transporting those nutrients that lead to enrichment. Evaporation is focused on the process of water transitioning from liquid to vapor and does not involve nutrient transport. Sedimentation refers to the settling of particles out of the water column, which can affect nutrients already present but does not directly contribute new nutrients to surface waters.