Most drinking water comes from one of two places: surface water (rivers, lakes, and reservoirs) or groundwater (underground aquifers tapped by wells). Globally, groundwater accounts for about half of all freshwater withdrawn for domestic use. In the United States, large cities and towns typically draw from surface water or a blend of surface and groundwater, while rural communities and roughly 10% of Americans rely on private wells.
Surface Water: Rivers, Lakes, and Reservoirs
Surface water is the most visible source of drinking water and the one that supplies most large population centers. It collects in rivers, streams, lakes, and reservoirs, and public utilities pump it to treatment plants before sending it through distribution pipes to homes and businesses. In the U.S., public water systems withdraw roughly 23,800 million gallons per day from surface sources for public supply alone.
Major cities often depend on surface water from surprisingly distant locations. New York City gets its water primarily from the Catskill and Delaware watersheds, a network of reservoirs more than 100 miles north of Manhattan that serves about 9.5 million people. Buffalo draws entirely from the Great Lakes. Chicago pulls from Lake Michigan, and Los Angeles imports water through aqueducts stretching hundreds of miles into the Sierra Nevada and Colorado River systems. These large-scale supply networks are engineered to collect, store, and transport water from regions with reliable rainfall or snowmelt to population centers that couldn’t sustain themselves on local sources alone.
Because surface water sits exposed to the environment, it picks up sediment, microorganisms, and pollutants more readily than groundwater does. That’s why protecting the watersheds surrounding these sources is a priority for utilities: cleaner source water means lower treatment costs and fewer risks of contamination from sewage, agricultural runoff, or industrial discharge.
Groundwater and Aquifers
Groundwater fills the spaces between sand, gravel, and rock formations underground. These water-bearing layers, called aquifers, can sit just below the surface or hundreds of feet deep. Unlike surface water, groundwater is isolated from the atmosphere and often under natural pressure, which is part of what allows it to be pumped upward through wells.
Extracting groundwater is straightforward in concept: a well is drilled into an aquifer, and a pump draws water up. Pumping pulls water toward the well from all directions, creating a funnel-shaped dip in the water table called a cone of depression. In some areas, natural springs form where the underground water level meets the land surface, and these can also be developed for municipal supply.
Groundwater tends to be naturally cleaner than surface water because soil and rock act as filters as rain and snowmelt seep downward over years or decades. But aquifers aren’t limitless. The High Plains Aquifer, one of the largest in the U.S., formed from sediment eroded off the Rocky Mountains over millions of years. When communities pump water out faster than nature replenishes it, the water table drops. If the drawdown is severe enough, the aquifer can physically compact and lose storage capacity permanently.
Private Wells vs. Public Systems
About 90% of Americans get their water from one of the more than 148,000 public water systems regulated by the EPA. The remaining population, concentrated in rural areas, relies on private wells that tap into local groundwater. The difference matters because federal and state drinking water rules do not apply to private wells. If you own a well, you’re responsible for testing and maintaining it yourself.
State-certified laboratories can test well water for common contaminants like nitrates, coliform bacteria, volatile organic compounds, and pH imbalances. Local or state health departments often offer this testing, but there’s no requirement that well owners do it on any particular schedule. That makes private wells both more independent and more vulnerable to undetected contamination from nearby agriculture, septic systems, or naturally occurring minerals in the rock.
How Raw Water Becomes Drinking Water
Whether it starts in a reservoir or an aquifer, water goes through a treatment process before reaching your tap. The standard sequence used by most municipal plants involves five steps.
- Coagulation: Chemicals (typically aluminum or iron-based salts) are added to bind dirt and small particles together.
- Flocculation: Gentle mixing encourages those bound particles to clump into larger, heavier masses called flocs.
- Sedimentation: The heavy flocs sink to the bottom of the water, separating out most of the solid material.
- Filtration: The clearer water on top passes through several filters to catch whatever remains.
- Disinfection: A chemical disinfectant, usually chlorine or a related compound, kills any remaining bacteria, viruses, or parasites.
After disinfection, plants typically adjust the water’s pH to prevent pipe corrosion and add fluoride. Groundwater sources sometimes need less intensive treatment because the natural filtration through rock has already removed much of the sediment and many microorganisms, but disinfection is still standard.
Recycled Water as a Growing Source
A small but expanding share of drinking water now comes from treated wastewater. More than 70 water reuse projects across the U.S. serve over 8 million people daily. Orange County, California operates one of the largest facilities, producing up to 100 million gallons of recycled drinking water per day.
The process involves treating municipal wastewater to standards that meet or exceed those for conventional drinking water, then often blending it with groundwater or passing it through an aquifer before it re-enters the supply. Because the advanced treatment strips out nearly everything, minerals and salts are added back in afterward to improve taste, and the pH is adjusted to protect pipes. Water reuse is particularly important in drought-prone regions where surface water and groundwater reserves are under increasing strain.