03Where your tap water comes from

The short version: City water starts as rain on a protected watershed or in a river, passes through a treatment plant, and travels through buried tunnels and mains to the tap, pushed by gravity, pumps or both. Pressure comes from height: every foot of water standing above a faucet adds about 0.433 psi.¹⁰¹ Greater Boston drinks from two forested reservoirs in central Massachusetts and is one of five major US systems with a waiver to skip filtration.¹⁰²¹⁰³ The weak points are the pipes themselves: tunnels with no backup, and lead service lines that a federal rule, now being challenged in court, requires most US utilities to replace by the end of 2037.⁶⁷¹⁰⁴

Where does city water come from?

Every piped water supply is a chain of four links: a source (a reservoir, river, lake or aquifer), treatment, a distribution network of mains and service pipes, and storage that keeps pressure steady when demand swings. A failure in any one link reaches the tap.

The source is usually defined by its watershed: the land that drains rain and snowmelt into a particular river or reservoir. A utility that owns or controls a clean watershed starts with cleaner water, and some of the oldest big-city systems were built around that idea.

Greater Boston's supply is a clear example. The Quabbin Reservoir holds 412 billion gallons (about 1.56 billion cubic meters) and the Wachusett Reservoir holds 65 billion gallons, for 477 billion gallons of combined source storage.¹⁰² More than 85% of the land in the two watersheds is forest and wetland, and the watersheds are tested and patrolled.¹⁰⁵ Two older reservoirs, the Sudbury Reservoir and Framingham Reservoir #3, are kept as backup and hold another 7.7 billion gallons.¹⁰²

New York City works on a similar plan at a larger scale. Its Catskill and Delaware supplies come from six reservoirs west of the Hudson River and serve more than 9.5 million people.¹⁰³

The number that matters most for a reservoir system is safe yield: how much water the supply can deliver every day, reliably, through a long drought. For the Massachusetts Water Resources Authority (MWRA), which runs Greater Boston's wholesale system, safe yield is about 300 million gallons per day (mgd), or about 1.1 million cubic meters.¹⁰² Average demand was 194.3 mgd in 2023 and 201.2 mgd in 2024 (a preliminary figure).¹⁰²

Why do some big cities skip filtration?

Most surface-water systems run their water through a sequence of chemical and physical steps that clump fine particles together, let them settle, and filter what is left.

Concord, New Hampshire, which draws from Penacook Lake, describes a typical sequence. A coagulant is added to make particles stick together into floc; the water is mixed fast, then gently, so the floc grows (flocculation); it slows in a sedimentation basin so the heavy clusters settle and are scraped away; and it then passes down through a 3-foot filter of silica sand and granular activated carbon.¹⁰⁶ Chlorine and then chloramine are added afterward for disinfection.¹⁰⁶

A few cities with exceptionally protected sources are legally allowed to skip the filter. The Natural Resources Defense Council (NRDC) counts five major US systems with these filtration waivers: New York, Seattle, Portland (Oregon), San Francisco and Boston.¹⁰³ New York's current waiver was issued by the New York State Health Department in December 2017 and is scheduled to last until 2027.¹⁰³

In our reading, "filtration avoidance" is a regulatory status, not a loophole. A utility keeps it by keeping its watershed clean, through land purchases, patrols and limits on development, and by proving it with water testing.

Boston's waiver was fought over in court. On February 12, 1998, the US government sued MWRA and the Metropolitan District Commission to require filtration under the Safe Drinking Water Act.¹⁰⁷ On May 5, 2000, the federal district court declined to order filtration and accepted MWRA's ozone-based plan instead; the court put the cost of a filtration plant at about $180 million.¹⁰⁷ The First Circuit Court of Appeals affirmed that ruling on July 16, 2001.¹⁰⁷ The idea was not new: when the Quabbin was built, it was described as a pure upland supply, delivered by gravity, that would "not require filtration."¹⁰⁸

What actually happens at the treatment plant?

Since July 2005, metro Boston's water has been treated at the John J. Carroll Water Treatment Plant in Marlborough.¹⁰⁵ It was designed for an average of 275 mgd and a peak of 405 mgd.¹⁰⁵ The steps, in order:¹⁰⁵

The last step is easy to overlook and arguably matters most to anyone living in an old building. Corrosion control means tuning the water's chemistry so it does not eat the metal it travels through.¹⁰⁵

Fluoridation is a policy decision as much as a treatment step, and it has become contested in parts of the US.

In 2025, Utah (HB 81, enacted in March) and Florida (SB 700, signed by Governor Ron DeSantis in May) passed laws barring local water systems from adding fluoride.¹⁰⁹ In April 2025, Kennedy said he would tell CDC to stop recommending community water fluoridation and would assemble a task force to study the issue; as the Associated Press noted, states and localities make their own fluoridation decisions.¹¹⁰ The same month, EPA said it would review new scientific information on fluoride to inform a possible revision of its drinking water limit, now a maximum contaminant level of 4.0 mg/L.¹¹¹ A Congressional Research Service report dated May 16, 2025 found no official HHS statement on the change yet, and said the Community Preventive Services Task Force's 2013 recommendation remained its most current.¹¹¹ On May 13, 2025, HHS announced that FDA was moving to remove concentrated ingestible fluoride prescription products for children from the market; HHS Secretary Robert F. Kennedy Jr. called ending ingestible fluoride "long overdue."¹¹² CDC's published recommendation, last reviewed in May 2024, remains community water fluoridation at 0.7 mg/L, the level the US Public Health Service set to balance benefits against potential harms.¹¹³

How does water get from the reservoir to your faucet?

Much of the work is done by gravity. MWRA's Quabbin Tunnel runs in both directions: in high-water months it carries surplus Ware River flow west into the Quabbin, and at other times it carries water east.¹⁰⁸ Its eastern section, the Wachusett-Colebrook Tunnel, was begun in 1926.¹⁰⁸

Closer to the city, the MetroWest Water Supply Tunnel runs 17.6 miles (28 km) from Marlborough to Weston, parallel to the Massachusetts Turnpike.¹¹⁴ It was mined 16 feet (4.9 m) across, finished at 14 feet, and lies 200 to 500 feet (60 to 150 m) underground.¹¹⁴ It went into service in November 2003 at an estimated $665 million, under its 1995 budget of $728 million, and it was built to back up a single aqueduct from the 1940s, the Hultman.¹¹⁴ Inside the metro area, the City Tunnel, City Tunnel Extension and Dorchester Tunnel carry about 60% of daily demand.⁶⁷ MWRA also keeps 282.7 million gallons in covered distribution storage.¹⁰²

From the tunnels, water moves into local mains run by each city or town, then through a service line, the pipe that connects a building to the main under the street.

Pressure follows from height. Each foot of water standing above a point adds about 0.433 pounds per square inch (psi) of pressure there, or roughly 9.8 kilopascals per meter.¹⁰¹ Our arithmetic: a water surface 100 feet (30 m) above a faucet delivers about 43 psi before friction losses; 200 feet delivers about 87 psi. That is why reservoirs sit on high ground, why gravity systems were the first choice for 19th- and early 20th-century engineers, and why pumping is needed wherever the source is too low (or the faucet too high; see Why water reaches the 40th floor).

Why are water towers so high?

The same arithmetic explains the water tower. A tower or standpipe is a reservoir lifted into the air: the height of its water surface sets the pressure in the mains around it. Utilities can pump water up when demand is low and let it flow down when everyone showers at once, and the stored water is there for firefighting.

A 2001 technical brief from the National Drinking Water Clearinghouse at West Virginia University lists the same roles: storage lets a utility pump at the average rather than the peak rate, holds a reserve for firefighting and other emergencies, and keeps pressure uniform.¹¹⁵ Tanks are drawn down during peak hours and refilled when demand is lowest, late at night and early in the morning.¹¹⁵ The brief gives minimum acceptable system pressures of 35 to 40 psi, with maximums generally 100 to 120 psi.¹¹⁵

Why do water mains break?

Pipes age, the ground shifts and freezes, and sudden pressure changes stress joints. When a small main breaks, a street floods and a few blocks lose water. When a trunk line fails, a whole region can lose it.

Greater Boston learned this in May 2010, when a water main break made clear that the tunnels carrying 60% of metro demand had no backup.⁶⁷ MWRA's response is the Metropolitan Water Tunnel Program, which adds about 15 miles (24 km) of deep rock tunnel: a 4.8-mile Northern Tunnel and a 10-mile Southern Tunnel, each 10 to 12 feet across and 200 to 450 feet deep.⁶⁷ The estimated cost is $2.1 billion (in MWRA's proposed FY25 capital plan), with construction planned for about 2027 to 2040.⁶⁷

Redundancy is the recurring theme of modern water engineering in old cities. The MetroWest tunnel backed up a single aqueduct; the new tunnels back up the city tunnels. A system with one path is one bad coupling away from a regional emergency.

How often do mains break nationally? A Utah State University study of 802 utilities in the US and Canada, dated December 2023, found 11.1 breaks per 100 miles of pipe per year, down about 20% from 14.0 in its 2018 study; cast iron failed most often, at 28.6 breaks per 100 miles.¹¹⁶ Utilities also hunt for leaks before they become breaks. Leaking water makes the pipe vibrate, and crews listen at valves and hydrants or through the ground with sensors and microphones; a correlator times when the sound reaches two sensors on either side of a suspected leak to estimate where it is.¹¹⁷

Are there still lead pipes, and who has to replace them?

Yes. EPA estimates that "up to 9 million homes" in the US are still served by them.¹¹⁸ Treatment plants adjust water chemistry to reduce how much lead and copper leaches out of old plumbing, which is the corrosion control step described above.¹⁰⁵

Flint, Michigan, shows what happens when that step is skipped. On April 25, 2014, the city switched from the Detroit system to the Flint River to save money, and it did not add the anti-corrosion treatment that builds a protective scale inside old pipes, so lead leached into the water.¹¹⁹ Flint was switched back to Detroit water in October 2015.¹¹⁹

On October 8, 2024, EPA announced the Lead and Copper Rule Improvements (LCRI).¹¹⁸ Its main terms, as of October 2026:

• Most US water systems must replace lead service lines, and lines of unknown material, within 10 years; systems with a high share of lead lines may get more time.¹¹⁸
• The action level, the lead concentration that triggers required steps by a utility, drops from 15 to 10 micrograms per liter (parts per billion).¹¹⁸
• The baseline inventory and replacement plan are due November 1, 2027, when mandatory replacement begins.¹⁰⁴
• The required average replacement rate of 10% a year is measured from December 31, 2030, and replacement must be complete by December 31, 2037.¹⁰⁴
• Partial replacements, where only the utility's section of a line is swapped, are banned from November 1, 2027, except during emergency repairs.¹⁰⁴
• The Bipartisan Infrastructure Law provided $15 billion over five years for lead line work.¹¹⁸

The rule is being challenged in court; see "What's changing now" below.

In Boston

Greater Boston's water system is a two-level arrangement. MWRA is the wholesaler: it owns the reservoirs, tunnels and treatment plant and sells water to cities and towns, which run their own local pipes. MWRA describes its reach two ways on the same web page, as "2.5 million people in 53 communities" and as "over 3.1 million customers in 61 communities."⁶⁷

The system is notable for using less water than it did a generation ago. Withdrawals ran above safe yield in the 1980s, and after MWRA started a conservation program in 1986, they dropped below safe yield by 1989.¹⁰² Demand was about 200 mgd in 2024, well under the Carroll plant's 275 mgd design average.¹⁰⁵

Dry years still register. MWRA has listed the Quabbin in "Below Normal" drought status since February 2026, and it remained there as of June 1, 2026.¹⁰² With 477 billion gallons in storage and demand around 200 mgd, our arithmetic says the reservoirs hold several years of supply, which is why a "Below Normal" listing is a watch status rather than an emergency.

On lead, MWRA's corrosion control is the regional defense.¹⁰⁵ Replacing the pipes themselves is up to the local water departments.

In Boston, the Boston Water and Sewer Commission (BWSC) replaces the private section of a lead service line, from the property line into the building, "at no cost to the owner" for eligible properties.¹²⁰ It began covering that cost in August 2023, using federal Bipartisan Infrastructure Law money.¹²¹ As of March 2024, BWSC had identified roughly 3,000 lead lines, said there might be thousands more, and only about 400 owners had used the free program.¹²² We did not find a BWSC target date for finishing.

How it changed

US city water started with gravity. New York's Croton Aqueduct, opened in 1842, fed reservoirs that sat about 60 feet above the city.⁴⁹

Boston built its supply westward in stages. Work on the Wachusett-Colebrook Tunnel began in 1926.¹⁰⁸ Construction of the Quabbin began in 1936, by damming the Swift River.¹⁰⁸ To make room for it, the towns of Dana, Enfield, Greenwich and Prescott were disincorporated at midnight on April 28, 1938, under legislation passed in 1927.¹²³ About 2,500 residents were displaced, and about 7,600 graves were moved to Quabbin Park Cemetery in Ware.¹²³

Filling began on August 14, 1939, by MWRA's account (Mass Humanities says July 1939³⁰), and finished in 1946, when water first flowed over the spillway; at completion it was described as "the largest man-made reservoir in the world devoted solely to water supply."¹⁰⁸ (The longer story of how cities came to build systems like this is in How cities came to need infrastructure.)

The late 20th century brought regulation and conservation. MWRA took over Greater Boston's water and sewer systems in 1985.¹²⁴ Its conservation program began in 1986.¹⁰² The filtration lawsuit ran from 1998 to 2001, and the Carroll plant opened in 2005, with UV added in 2014.¹⁰⁷¹⁰⁵ The MetroWest tunnel followed in 2003, and the 2010 break started the next round of tunnel building.¹¹⁴⁶⁷

What's changing now (2026)

• The lead rule is in court. Water utilities led by the American Water Works Association (AWWA) have challenged the LCRI in the D.C. Circuit.¹²⁵ NRDC intervened on December 20, 2024; final briefs, including EPA's defense of the rule, were filed April 17, 2026; and oral argument was held September 30, 2026.¹²⁵ A decision is expected about 6 to 12 months after argument.¹²⁵ A Tennessee state guide to the rule, published in May 2026, notes that its guidance "may change pending EPA rule updates."¹⁰⁴
• New York's waiver runs to 2027. The current filtration waiver for the Catskill and Delaware supplies is scheduled to last until 2027, so a renewal decision is approaching.¹⁰³
• Boston's backup tunnels are about to start. Construction of MWRA's Northern and Southern tunnels is planned to begin around 2027.⁶⁷
• Drought watch. The Quabbin has been in "Below Normal" drought status since February 2026.¹⁰²

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