Our drinking water comes from surface water (lakes, rivers) and groundwater (aquifers), both replenished by the hydrologic cycle. Learn how water moves through the earth, picks up contaminants, and gets treated before reaching your tap.
Where does our water come from? Most drinking water comes from two sources: surface water (lakes, rivers, and streams) and groundwater (underground aquifers). Both are replenished by the hydrologic cycle — the continuous process of evaporation, condensation, precipitation, and collection that moves water through the earth's system.
With 68% of the earth’s clean water stored in glaciers and ice caps, the majority of what the world uses for drinking comes from surface water sources such as lakes and rivers and groundwater sources in the form of aquifers.
But while these bodies of water appear to be geographically static, the water itself is always flowing from one location to another—continuously going through changes as it comes into contact with various elements along the way.
Because of these changes, understanding the earth’s natural water cycle is critical to identifying the most efficient methods of sourcing and filtering water.
This article will illustrate the journey our drinking water goes through to get to us as it travels throughout the earth’s storehouses, the chemical changes it goes through, the potential contaminants it picks up along the way, and why it’s all important from a purification perspective.
Natural Sources of Water
Surface Water and Groundwater
Most of the water the entire human population uses for drinking comes from the earth's surface or from underground. Surface water makes up about 70 percent of the freshwater used in the United States and about 80 percent worldwide. While there's about three times as much groundwater available, it makes up about 30 percent of the freshwater used in the US.
Regions will use different water sources to meet various needs depending on their geographical makeup. The Eastern part of the US has far more surface water sources than the West, while underground aquifers are scattered fairly evenly throughout.
Of the 322 billion gallons of water withdrawn daily in the US, only 13% is used for public supply. The majority of the water is used for agriculture and thermoelectric power.
Surface Water
Surface water refers to creeks, streams, rivers, lakes, and oceans—anything that gathers on the earth's surface.
It begins with the ice caps at the top of snowy mountains, melting as it trickles downward in what is known as runoff. Moving downhill, small creeks converge into larger streams, which later converge into rivers. Rivers eventually empty out into lakes and oceans.
Colorado River Dam / jonathan varghese / Unsplash
Major water sources, such as the Colorado River, supply over 40 million people with their water needs. Under the Colorado River Compact, this massive water source is irrigated to seven states, providing them with water for agricultural, thermoelectric, and domestic needs.
Groundwater
The term 'groundwater' refers to freshwater that exists beneath the earth's surface in porous chambers of rock known as aquifers. Aquifers vary in size, depth and water volume, and exists in varying locations under the earth's crust.
Locating and estimating the size and volume of aquifers can be challenging and require precise instrumentation and years of research. Agencies such as the United States Geological Survey (USGS) are in charge of monitoring groundwater levels and providing data.
Ogallala aquifer wikimedia commons
The Ogallala Aquifer, a massive basin under the Great Plains of the United States, supplies water to over 2.3 million people in the region, meeting their agricultural and domestic needs.
While groundwater is a major source for the public water sector, about 43 million people (15% of the U.S. population)—rely on private wells as their source of drinking water. Private wells aren't monitored by agencies such as the EPA and the Safe Drinking Water Act, and therefore, aren't required to meet certain water quality standards. This means private well owners are responsible for purifying water themselves.
Stages of the Hydrologic Cycle
The Hydrologic Cycle
The hydrologic cycle is the earth’s process of moving water within its closed-loop system so that different sections of the planet can benefit. It also acts as a natural filtering system so that water is continuously renewed and made clean. The stages in the hydrologic cycle are evaporation, condensation, precipitation, and collection.
Evaporation
Evaporation is the process through which water is transformed into gas and transported into the Earth's atmosphere via heat, typically due to the sun's warmth. As the surface of water bodies heats, moisture from lakes, rivers, and oceans turns water into steam, rising into the air.
While most of the evaporation comes from surface water, other sources, such as soil, plants, and even human perspiration, can also contribute to the overall evaporation process.
Condensation
As evaporated water accumulates in the atmosphere, it condenses and transitions from the gaseous phase back to its liquid form. Condensation leads to the creation of clouds and the eventual generation of rain.
Precipitation
Precipitation is the process by which the accumulated water in the atmosphere, in the form of dense clouds, becomes too heavy to stay afloat and descends back to the Earth's surface in the form of rain, snow, or hail.
Collection
When droplets of rain, flakes of snow, or pellets of hail reach the Earth, they follow a well-charted course, with some of the water collecting at the top of mountains, rejuvenating rivers, streams, and lakes as it melts downward.
Some of it seeps into the ground, replenishing aquifers and contributing to the groundwater system, where it can be pumped via a well and used for various human purposes.
Surface water will eventually heat up when it makes contact with the sun, evaporating and starting the cycle over again.
How Water Gets Contaminated
Rainwater and Contamination
It is important to understand that while water is still in the form of rain, it is completely free of contaminants—only containing the H₂O molecule. This is because when water evaporates into the atmosphere, it is only the H₂O molecule that converts to gas. All other contaminants are left behind. So, when rain precipitates back down to the earth, it is essentially pure H₂O.
But as it makes contact with the surface of the earth and flows through streams and rivers, it collects deposits of beneficial minerals such as magnesium and calcium, as well as a variety of harmful chemicals and microorganisms.
Healthy minerals, such as magnesium, calcium, iron, and salts, are abundant in mountaintop streams and aquifers, improving the taste and quality of the water. But as water reaches other areas of the earth, the potential for coming into contact with harmful contaminants increases.
Naturally occurring harmful chemicals such as arsenic and nitrates are common in surface and groundwater sources, requiring treatment before drinking. Harmful microorganisms such as bacteria and Giardia are common in streams and rivers where animals often bathe.
Human activity can contribute to the increased presence of contaminants in water sources, primarily through industrial and agricultural processes.
Water also has another opportunity for filtration during the collection phase when it seeps into the ground. The layers of rock in the earth’s crust act as a natural filter, binding to harmful contaminants as water eventually collects in underground aquifers. Thus, groundwater is generally less contaminated than most lakes and larger surface water sources.
How Water Gets to Your Tap
For most households, tap water travels through a six-step municipal system before reaching your faucet.
1. Source water intake. A utility draws water from its source — a river intake, reservoir, or well field. The source determines the treatment challenge: surface water from rivers carries more sediment, organic matter, and biological contamination than deep aquifers, which are naturally filtered but may contain higher concentrations of dissolved minerals like iron or arsenic.
2. Screening and settling. Large debris is removed through coarse screens at the intake. The water then moves to a settling basin where suspended solids sink to the bottom over several hours before treatment begins.
3. Coagulation, flocculation, and sedimentation. Utilities add a coagulant (typically aluminum sulfate) that causes fine particles to clump together into larger masses called floc. Floc settles out in a sedimentation tank, removing turbidity, some bacteria, and organic matter.
4. Filtration. Water passes through layered media — sand, gravel, and activated carbon — which captures remaining particles and reduces taste and odor compounds. Some facilities use membrane filtration for additional pathogen removal.
5. Disinfection. Chlorine, chloramines, UV light, or ozone kill bacteria, viruses, and protozoa. Utilities maintain a residual chlorine concentration throughout the distribution system to prevent recontamination in transit.
6. Storage and distribution. Treated water enters a storage reservoir or elevated tank, then flows through pressurized distribution mains to your home. The journey can span dozens of miles of pipe — during which lead or copper can leach from older service lines or home plumbing, regardless of what left the treatment plant.
For a detailed breakdown of each step, see the 7 steps of the water purification process.
Testing and Filtration
With all water sources, testing and proper purification methods should be applied to guarantee drinkability. Local governments utilize water treatment facilities to test and purify water from contaminants before distributing it to the public. So, water from the tap has been treated for harmful contaminants and microorganisms that can make humans sick.
However, under the Safe Drinking Water Act, some contaminants can come through in trace amounts. For this reason, many homeowners install complete home filtration units at the point of entry to ensure completely safe water for their families.
Private wells aren't monitored by governing agencies—owners are responsible for testing and purifying water themselves.
Gathering water from a stream for drinking may pose health risks due to animals potentially contaminating the water with bacteria and viruses. Water purification tools such as water purification tablets and portable water filters are excellent means of protecting oneself when backpacking or hiking.
Main takeaways
- The two main clean water sources that all humans on the planet draw from are surface water and groundwater.
- The hydrologic cycle is the earth’s natural means of purifying water and distributing it to all regions.
- Rainwater is essentially pure but becomes contaminated as it comes into contact with natural and man-made chemicals and microorganisms along the way.
- Public treatment centers collect, purify, and distribute water for public consumption, and also adhere to water quality standards as set by the EPA.
- Private wells aren't monitored by the EPA and require personal testing and treatment.
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