In what type of water are biological contaminants most commonly found?

Biological contaminants, such as bacteria, viruses, and protozoan cysts, are predominantly found in surface water sources. This is largely due to the direct exposure of surface water to the environment, which can include agricultural runoff, animal waste, and sedimentation processes. Rainfall and natural bodies of water, like rivers, lakes, and reservoirs, can accumulate various microorganisms present in the surrounding ecosystems.

Unlike surface water, groundwater is generally less susceptible to these contaminants because it is filtered naturally through soil and rock layers, which provide an additional layer of protection. Tap water may also contain disinfectants that minimize biological contamination, while desalinated water undergoes processes designed to remove such organisms completely. As a result, the presence and abundance of biological contaminants is most often associated with surface water, which is why this choice stands out as the most accurate answer in this context.