What type of water source usually requires more treatment before distribution?

Surface water typically requires more treatment before it can be distributed for public consumption compared to other sources like groundwater. This is primarily due to the higher susceptibility of surface water to contamination from various sources such as agricultural runoff, industrial discharge, and pollution from urban areas. Surface water bodies, including rivers, lakes, and reservoirs, can carry a wide array of pathogens, nutrients, and chemicals that necessitate extensive filtration and disinfection processes to ensure the water is safe to drink.

In contrast, groundwater usually has a lower level of contaminants due to natural filtration through soil and rock layers, enabling it to often require less treatment. Rainwater, while it can be clean, may still need to be filtered and treated depending on collection methods and potential contaminants in the environment. Desalinated water, derived from seawater, while it requires specific processes to convert saltwater to freshwater, generally involves different treatment challenges rather than the widespread contaminants typical of surface water sources.