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Why does surface water usually need more treatment than groundwater?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

Surface water typically requires more extensive treatment than groundwater because it is more susceptible to contamination and contains higher levels of suspended impurities.1

Most surface water sources, such as rivers and lakes, are exposed to the environment and are often polluted to some degree. These sources frequently contain suspended silt, organic material, decaying vegetation, and various microbes from animal wastes. Additionally, surface water often exhibits high turbidity—cloudiness caused by small suspended particles—which is aesthetically undesirable and can interfere with disinfection by shielding pathogenic organisms from treatment.12

In contrast, groundwater is generally free of microbes and suspended solids due to the natural filtration that occurs as water percolates through soil and rock. While groundwater may contain higher concentrations of dissolved minerals from its contact with the earth, it typically requires less processing than surface water, often needing only disinfection as a precautionary measure to protect public health.12