Where Does Your Tap Water Come From?

Two sources, different characteristics

Every drop of tap water in the United States starts as either surface water or groundwater. Where your water comes from determines its natural mineral content, what contaminants are most likely, how much treatment it receives, and what it tastes like. Understanding your source helps you make better decisions about home filtration and interpret your city\'s water quality data.

Surface water

Surface water comes from rivers, lakes, and reservoirs. According to the EPA, about 68% of Americans get their drinking water from surface sources. Major examples include the Great Lakes system (serving Chicago, Detroit, Cleveland, and other cities), the Colorado River (serving Las Vegas, Phoenix, Los Angeles), and the Catskill/Delaware watersheds (serving New York City).

Surface water characteristics:

• More treatment required. Because surface water is exposed to the environment, it picks up sediment, bacteria, parasites (like Giardia and Cryptosporidium), agricultural runoff, and organic matter. Treatment plants use coagulation, sedimentation, filtration, and disinfection to make it safe.
• Higher disinfection byproducts. The organic matter in surface water reacts with chlorine during treatment, producing trihalomethanes (THMs) and haloacetic acids (HAAs). The EPA limits THMs to 80 ppb and HAAs to 60 ppb, but surface water systems tend to run closer to those limits than groundwater systems.
• More chlorine taste. Surface water systems typically maintain higher chlorine residuals (up to 4 mg/L allowed by EPA) because the water travels through longer distribution systems and needs ongoing disinfection protection.
• Variable quality. Surface water quality changes seasonally. Spring snowmelt and heavy rains increase turbidity and runoff. Summer heat promotes algal blooms that can produce taste and odor compounds or toxins like microcystin. Fall leaf decomposition adds organic matter.
• Variable hardness. Depends on watershed geology. New York City surface water is very soft (around 20 ppm). Colorado River water is very hard (over 300 ppm).

Groundwater

Groundwater comes from wells drilled into aquifers, which are underground layers of rock or sediment that hold water. About 32% of Americans rely on groundwater for their public water supply, and nearly all rural households on private wells use groundwater exclusively.

Groundwater characteristics:

• Natural filtration. Water percolating through rock and soil is naturally filtered of particles and most bacteria. This means less treatment is needed at the surface. Many groundwater systems require only disinfection, not full filtration.
• Higher mineral content. Groundwater dissolves minerals from the rock it passes through. Limestone aquifers produce hard water high in calcium and magnesium. Sandstone aquifers tend to produce softer water.
• Naturally occurring contaminants. Depending on geology, groundwater can contain arsenic (common in western US; EPA MCL is 10 ppb), radium (common in parts of the Midwest; EPA MCL is 5 pCi/L), uranium, fluoride, and iron. These are not signs of pollution; they come from the rock itself.
• More consistent quality. Groundwater quality does not change with seasons or weather events the way surface water does. Aquifers are buffered from surface conditions. However, contamination from spills or industrial activity can persist for decades once it reaches an aquifer.
• Lower disinfection byproducts. Less organic matter means less reaction with chlorine, so DBP levels are typically lower.

How water gets from source to your tap

The journey from source to tap involves several steps, each of which can affect quality:

1. Collection. Surface water is collected in reservoirs or intake structures on rivers. Groundwater is pumped from wells.
2. Treatment. Water treatment plants remove particles, kill pathogens, and adjust chemistry. Common treatment steps include coagulation (adding chemicals to clump particles), sedimentation (letting clumps settle), filtration (passing water through sand, gravel, or membranes), and disinfection (chlorine, chloramine, UV, or ozone).
3. Distribution. Treated water enters a network of pipes, storage tanks, and pump stations. The US has an estimated 2.2 million miles of water distribution pipes. Many are decades old. The American Society of Civil Engineers grades US drinking water infrastructure at a C-minus.
4. Service lines. The pipe connecting the water main to your home. An estimated 6 to 10 million homes still have lead service lines, according to the EPA. The 2021 Bipartisan Infrastructure Law allocated $15 billion to replace them.
5. Home plumbing. Your home\'s own pipes, fixtures, and solder are the final segment. Lead solder was used until 1986. Brass fixtures can contain up to 8% lead (reduced to 0.25% by the 2014 Reduction of Lead in Drinking Water Act).

Why your water source matters for filtration choices

If your source is...	Common concerns	Recommended filtration
Surface water (river/lake)	Chlorine taste, disinfection byproducts, seasonal quality changes	Carbon filter for taste; RO if DBPs are high
Groundwater (well, public)	Hardness, iron staining, arsenic, radium	Softener for hardness; RO for arsenic/radium
Private well	Bacteria, nitrate, arsenic, no regulatory oversight	UV for bacteria; RO for chemicals; annual testing essential
Mixed/blended source	Varies seasonally as the utility blends sources	Depends on specific data; check your CCR

How to find your water source

Several ways to find out where your tap water comes from:

• CheckMyTap: Your city page on CheckMyTap lists the water source and key quality data.
• Consumer Confidence Report (CCR): Your utility is required to publish this annually. It lists the water source, treatment methods, and test results. Search "[your city] water quality report" or check your utility\'s website.
• EPA Safe Drinking Water Information System: The federal database of all public water systems, including their source water type.

Related resources

• What is hard water?
• PFAS (forever chemicals) explained
• Lead in drinking water
• Water quality when moving to a new city