What is the primary source of surface water contamination?

Runoff and erosion serve as the primary source of surface water contamination due to their widespread impact across various landscapes. When rain falls, water flows over land surfaces, picking up contaminants such as fertilizers, pesticides, sediment, and other pollutants from agricultural fields, urban areas, and construction sites. This process can introduce a significant load of harmful substances into rivers, lakes, and streams.

In addition to the direct transfer of these contaminants, erosion can exacerbate the problem by removing topsoil, which often contains additional pollutants. This combination leads to increased levels of turbidity and toxicity in surface water, impacting not only water quality but also the health of aquatic ecosystems and potentially affecting human health as well.

Other options may certainly contribute to surface water contamination, but they usually represent more localized or specific circumstances compared to the pervasive nature of runoff and erosion, which can occur practically anywhere there are disturbances in the landscape.