How the U.S. Water Supply Works — Infrastructure

The System

From watershed to tap: a chain with many links.

The United States has approximately 148,000 public water systems serving roughly 300 million people. These range from large metropolitan utilities serving millions of customers to small community systems serving a few hundred. About 15 million additional households rely on private wells not covered by federal drinking water regulation.

The Safe Drinking Water Act of 1974, administered by the EPA, establishes national standards for contaminants in public water systems. The law requires systems to monitor and treat water to meet maximum contaminant levels, report results to customers annually, and notify the public immediately of violations that pose immediate health risks. Boil water advisories, the most visible public notification, are issued when microbial contamination is confirmed or suspected.

Water infrastructure is among the most capital-intensive in the country and among the most deferred. The American Society of Civil Engineers has graded U.S. drinking water infrastructure a C-minus, citing an estimated $625 billion infrastructure funding gap over the next 20 years. A water main breaks somewhere in the United States approximately every two minutes. Lead service lines, which can contaminate water as it travels from the main to the tap, still serve an estimated 9 million homes.

How It Works: End to End

1

Source water

Surface water (rivers, lakes, reservoirs) or groundwater (wells, aquifers) is the raw material. Source water quality determines treatment complexity.

2

Intake and screening

Raw water enters the treatment plant through intakes. Screens remove large debris.

3

Coagulation and flocculation

Chemicals cause fine particles to clump together into larger masses called floc, which can be removed.

4

Sedimentation and filtration

Floc settles out and remaining particles are removed by sand, gravel, or membrane filters.

5

Disinfection

Chlorine, chloramine, ozone, or UV light kills pathogens. Residual disinfectant protects water during distribution.

6

Distribution

Treated water enters pressurized pipes that carry it to homes and businesses. Pumping stations maintain pressure throughout the network.

Vulnerabilities

Age, pressure, and contaminants the system wasn't designed for.

The average water pipe in the United States is 45 years old. Many older systems use pipes made of cast iron, asbestos cement, or lead-soldered copper, all of which degrade over time. Water main breaks allow both water loss (estimated at 14 to 18 percent of treated water in many systems) and the potential for contamination to enter the distribution system when pressure drops.

Drought and source water contamination are growing concerns. Agricultural runoff containing nitrates, pharmaceuticals, and emerging contaminants such as PFAS (per- and polyfluoroalkyl substances) are present in source water at levels that older treatment systems were not designed to address. The EPA has been in the process of setting enforceable limits on PFAS compounds, which are now detectable in water supplies serving hundreds of millions of Americans.

Aging pipes

Average pipe age is 45 years. A main breaks every 2 minutes somewhere in the U.S. Lead service lines still serve 9 million homes.

Drought and source depletion

Western aquifers and reservoirs face long-term depletion pressure. Colorado River allocations exceed reliable flows.

Contamination

PFAS, nitrates, and other emerging contaminants are present in many source water supplies at levels older treatment systems cannot address.

Power dependence

Water systems require continuous electricity for pumping and treatment. Grid outages disable water pressure within hours.

148K

Public water systems in the United States

EPA SDWA data

2min

Frequency of water main breaks across the U.S.

ASCE Infrastructure Report Card

9M

Homes still served by lead service lines

EPA estimate

$625B

Estimated water infrastructure funding gap over 20 years

ASCE 2021 Report Card

What This Means for You

Understanding the system is the first step.

Water system vulnerabilities, from main breaks to treatment failures, are the reason household water storage matters. Knowing how your local system works, and where to find boil water advisory notifications, is the starting point. The preparedness guide covers storage quantities, treatment methods, and what to do during a water outage.

See the preparedness guide