Every glass of drinking water starts somewhere.
It may begin as rainwater slowly infiltrating through soil and rock into an underground aquifer. It may be taken from a river, lake or reservoir. Or it may emerge naturally from the ground as spring water.
These sources can look very different, and their chemistry can be different too.
The reason is simple:
Water carries information about the environment it has travelled through.
Geology can influence its mineral composition. Rainfall determines how water enters and moves through a catchment. Agricultural land can contribute nutrients and pesticides. Surface contact can expose water more directly to changing weather, runoff and biological activity.
That is why understanding raw water starts with understanding its source.
First: What Is Raw Water?
Raw water is water taken from a natural source before it has been treated for use as drinking water.
The German Environment Agency defines raw water as water abstracted from groundwater or surface-water sources for drinking-water production. If the raw water does not already have the necessary quality, it must be treated before being supplied as drinking water.
This distinction matters.
Raw-water quality is not the same thing as tap-water quality.
A source can contain substances that require treatment while the final drinking water still fully complies with legal requirements.
In Germany, data for larger water-supply zones for 2020–2022 showed that approximately 67.6% of raw water came from groundwater, 15.9% from surface water and 16.5% from other sources such as bank filtrate or artificially recharged groundwater. At the drinking-water stage, more than 99% of samples met the requirements for most microbiological and chemical parameters.
The source affects the starting conditions.
The waterworks determine how those conditions are managed.
Groundwater: Water Shaped Underground
Groundwater begins largely with precipitation.
Rain and melting snow infiltrate the ground, move through soil and geological layers and eventually collect in pores, cracks and cavities underground.
These water-bearing formations are known as aquifers.
The German Environment Agency explains that groundwater is mainly replenished by rainwater infiltrating through soil and the subsurface. It can later emerge naturally through springs and also contributes water to rivers and streams.
Underground does not mean chemically unchanged.
Quite the opposite.
Geology Leaves a Chemical Fingerprint
As groundwater travels through soil and rock, it remains in contact with minerals.
Some of those minerals dissolve.
The final composition therefore depends partly on:
- the rock and sediment through which the water moves
- how long the water remains underground
- pH and other chemical conditions
- the composition of the recharge water
The U.S. Geological Survey identifies geological composition, hydrological properties, recharge-water chemistry and residence time as major natural controls on groundwater quality.
This is how geology can influence parameters such as:
calcium
magnesium
bicarbonate
sulfate
chloride
iron
and other naturally occurring substances.
Germany’s Federal Institute for Geosciences and Natural Resources accordingly maps distinct groundwater types based on their mineralisation and chemical composition in different hydrogeological regions.
The water beneath two regions can therefore have very different chemistry even before human influence is considered.
This Is Also Why Water Hardness Varies
Calcium and magnesium are two of the best-known examples.
Where groundwater interacts with mineral-rich geological formations, higher concentrations of these minerals can dissolve into the water.
That can result in harder water.
Elsewhere, groundwater may contain substantially less calcium and magnesium and therefore be softer.
So when drinking water tastes different between regions or creates different amounts of limescale, geology may be part of the explanation.
The underground journey changes the water.
Does Soil Naturally Filter Groundwater?
To some extent, yes.
When rainwater infiltrates through soil and geological layers, several physical, chemical and biological processes can influence substances carried with it.
Soil passage can retain particles, promote degradation of some substances and change water chemistry.
That natural barrier is one reason groundwater is such an important drinking-water resource.
The German Environment Agency notes that bank filtration and artificial groundwater recharge deliberately make use of the natural filtering effect of soil.
But “naturally filtered” should not be confused with:
“guaranteed free from contaminants.”
Some substances are mobile enough to move through soil and reach groundwater.
And once groundwater becomes contaminated, recovery can be slow.
Groundwater Changes Slowly — Which Is Both an Advantage and a Problem
Groundwater can remain underground for very different lengths of time.
Depending on the aquifer, groundwater residence can range from comparatively short periods to many years, centuries or even longer.
Longer underground residence often makes groundwater less immediately responsive to day-to-day weather than a river or lake.
That relative stability can be advantageous for water supply.
But there is another side.
If persistent pollutants reach groundwater, they may also remain there for a long time.
The European Environment Agency highlights that groundwater pollution is difficult to reverse because contaminants can accumulate and recovery may take decades.
Slow change protects groundwater from some short-term fluctuations.
It can also make long-term pollution difficult to correct.
Agriculture Can Influence Groundwater Quality
One of the clearest examples is nitrate.
Nitrogen applied to agricultural land can be transformed into nitrate in the soil. Because nitrate is highly soluble, it can move downward with infiltrating water and enter groundwater.
The German Environment Agency identifies agricultural nutrient inputs as the main reason for elevated nitrate concentrations in German groundwater.
Its latest groundwater monitoring data show that in 2024, 15.7% of the monitoring sites in Germany’s EEA groundwater network exceeded 50 mg/L nitrate.
An important distinction:
These are groundwater monitoring results, not drinking-water-at-the-tap results.
Water utilities can protect abstraction areas, choose suitable wells, blend water or apply treatment where necessary before supplying drinking water.
The data nevertheless demonstrate why source protection matters.
Pesticides Can Travel Underground Too
Plant-protection products and their degradation products can also move through soil.
The German Environment Agency explains that rainfall can transport certain pesticide substances or metabolites through agricultural soil into groundwater. Once there, some may degrade only slowly.
This again shows why the idea of groundwater as simply “rain filtered by soil” is incomplete.
The soil is a protective barrier.
But it is not an absolute barrier against every molecule.
Surface Water: Much More Directly Connected to the Catchment
Surface water includes sources such as:
rivers
lakes
reservoirs
and other bodies of water exposed directly to the surrounding environment.
Unlike deep groundwater, surface water is in immediate contact with the atmosphere, surrounding land and biological activity.
That makes its behaviour different.
Rain Can Change Surface Water Quickly
Imagine a river catchment during several dry weeks.
Then heavy rain arrives.
Water begins moving across:
agricultural fields,
roads,
urban surfaces,
forests,
and exposed soil.
Some of that water eventually reaches streams, rivers and lakes.
Along the way, it may carry sediment, nutrients, organic material and other substances from the catchment.
WHO therefore treats the entire surface-water catchment as part of drinking-water risk management rather than assessing only the point where water is abstracted.
This is an important difference from many groundwater systems:
surface-water quality can respond relatively quickly to events occurring above ground.
Surface Water Is Naturally More Dynamic
A river today is not chemically identical to the same river under all conditions.
Changes can occur with:
- rainfall
- snowmelt
- drought
- river discharge
- water temperature
- algae and biological activity
- upstream wastewater discharges
- agricultural runoff
- sediment movement
That does not mean surface water is inherently “bad”.
It means its raw-water composition can be more variable.
This variability is one reason surface-water treatment systems are designed around the characteristics and risks of the individual catchment.
WHO’s current water-safety approach explicitly assesses risks throughout the system from catchment to consumer.
Agriculture Can Reach Surface Water Differently
The pathway is often different from groundwater contamination.
For groundwater, pollutants commonly move down through soil.
For rivers and lakes, substances can also move across the surface through runoff, enter drainage systems or reach water bodies through erosion and other pathways.
The European Environment Agency reports continuing pesticide pressure in both European rivers and groundwater, illustrating that agricultural chemicals can affect both parts of the water cycle.
Nutrients such as nitrogen and phosphorus can also reach surface waters and contribute to ecological effects such as eutrophication.
Again, the key is not:
agriculture = polluted drinking water.
It is:
land use within a catchment influences the risks that water suppliers must understand and manage.
Groundwater and Surface Water Are Not Separate Worlds
The categories are useful.
Nature is less tidy.
Groundwater can flow into rivers and lakes.
Rivers can recharge groundwater.
Water can repeatedly move between the surface and subsurface.
The U.S. Geological Survey describes groundwater and surface water as interconnected components of the hydrological cycle, continuously exchanging both water and dissolved substances.
The German Environment Agency likewise notes that groundwater feeds rivers and streams, particularly during periods with little rainfall.
This connection is particularly important for drinking-water systems using bank filtration.
What Is Bank Filtration?
A water utility can place wells near a river.
When groundwater is pumped from these wells, some river water moves through the riverbed and surrounding geological material toward the well.
During this underground passage, natural soil and sediment processes can change the water before abstraction.
The German Environment Agency describes bank filtration as a method that deliberately uses this natural soil passage for drinking-water production.
So is bank filtrate groundwater or surface water?
In a practical sense, it contains characteristics of both.
This is a good reminder that source categories describe hydrological pathways, not perfectly isolated containers.
What About Spring Water?
A spring appears at the surface when groundwater naturally emerges from the ground.
That means spring water is fundamentally groundwater.
It has already travelled underground before appearing at the surface.
This often creates an appealing image:
water moving through natural rock before emerging as a clear spring.
But appearance alone cannot establish water quality.
A Spring Reflects Its Aquifer
Spring-water chemistry depends on the same kinds of factors that influence other groundwater:
geology,
soil,
groundwater flow,
residence time,
and surrounding land use.
A spring flowing through limestone can have a different mineral composition from one emerging from another type of geological formation.
And the vulnerability of springs can differ dramatically.
Not Every Spring Is Naturally Protected
Some spring systems are relatively well protected from immediate surface influence.
Others respond quickly to rainfall or contamination in the surrounding area.
This is particularly important in karst geology, where water can move rapidly through large fractures, caves and channels in soluble rock such as limestone.
Older WHO guidance on spring protection specifically notes that contaminants can travel through large openings in limestone formations and that springs can become contaminated when pollution sources occur on higher surrounding ground.
So:
clear spring water is not automatically proof of microbiological or chemical purity.
A natural source still has a catchment.
And what happens in that catchment matters.
Rainfall Connects All Three Sources
Rainfall is one of the strongest links between groundwater, surface water and springs.
After precipitation reaches the ground, several things can happen.
Part may:
evaporate
be taken up by vegetation
flow across the surface
enter rivers and lakes
or
infiltrate into soil and recharge groundwater
Water that infiltrates today may not emerge from a spring or abstraction well immediately.
Water flowing across a field into a stream may arrive far more quickly.
That difference in travel time explains a great deal about source-water behaviour.
Heavy Rainfall Can Change the Route Pollutants Take
Rain is essential for replenishing water resources.
But rainfall can also mobilise substances already present in a catchment.
On agricultural land, water can transport nutrients or pesticides.
On bare soil, it can move sediment.
In urban environments, water can wash material from impermeable surfaces into drainage systems and waterways.
The effect depends heavily on rainfall intensity, soil type, land management, geology and the design of local drainage systems.
This is why a single raw-water measurement represents conditions at a particular moment.
Water quality is not independent of weather.
For some sources, weather is one of its strongest short-term drivers.
Drought Changes Water Too
Rainfall is not the only relevant condition.
A lack of rainfall can also change water systems.
During prolonged dry periods:
river flow can decrease,
reservoir levels can fall,
groundwater recharge can decline,
and the relative concentration or importance of different water sources can change.
The European Environment Agency identifies climate change and abstraction as growing pressures on Europe’s groundwater resources.
Climate therefore affects not only how much water is available.
It can also affect the conditions under which raw water is collected and treated.
Geology Can Affect More Than Hardness
Calcium and magnesium are familiar because consumers can often see their effect as limescale.
But geology can influence many other naturally occurring constituents.
WHO notes that naturally occurring chemicals in groundwater can sometimes be relevant to drinking-water safety, citing substances such as arsenic and fluoride in some regions of the world.
The important point is not that groundwater generally contains dangerous concentrations of these substances.
It is that natural does not mean chemically identical everywhere.
Some water-quality differences arise without any human pollution at all.
Water-rock interactions alone can create substantial regional variation.
Source Type Does Not Tell You Everything
It would be convenient if the three categories worked like this:
groundwater = clean
surface water = contaminated
spring water = pure
They do not.
A well-protected groundwater aquifer can provide excellent raw water.
Another groundwater body may experience nitrate or pesticide pressure.
A surface-water reservoir in a carefully protected catchment can provide excellent source water.
A river downstream of intensive land use may present a different challenge.
One spring may be well protected.
Another may respond rapidly to surface contamination.
Source type matters.
Source-specific conditions matter more.
Protected Catchments Are Part of Water Treatment
The cheapest contaminant to remove is often the one that never reaches the water source.
That is why drinking-water management begins before the water enters a treatment plant.
In Germany, groundwater protection areas and other preventive measures help protect drinking-water sources from contamination. The German Environment Agency describes source protection as the first part of a multi-barrier approach, followed where necessary by raw-water treatment, secure distribution and properly designed building installations.
WHO follows the same basic principle through water safety planning:
manage risk from catchment to consumer.
Modern drinking-water safety therefore involves both:
protecting the source
and
treating the water where necessary.
Does Groundwater Require Less Treatment Than Surface Water?
Sometimes.
But there is no universal rule.
Well-protected groundwater may require comparatively limited treatment if its natural composition is favourable.
Other groundwater can require treatment for substances such as iron, manganese, hardness-related constituents or anthropogenic contaminants.
Surface water often presents a broader range of changing physical, chemical and microbiological conditions because it is directly exposed to the catchment.
Its treatment may therefore involve multiple stages.
But treatment design is always source-specific.
The relevant question is not:
“Which source is best?”
It is:
“What is present in this particular raw water, and how should it be managed?”
Why Two Groundwater Sources Can Still Be Completely Different
Even two wells labelled simply “groundwater” can produce different water.
Imagine:
Well A
A deep aquifer beneath mineral-rich rock with a long residence time.
Well B
A shallow aquifer beneath intensively farmed land.
Both are groundwater.
But the first may be influenced strongly by long-term mineral-water interactions.
The second may respond more strongly to recent recharge and land use.
Hydrogeology determines how fast water moves, where it came from and what it encountered on the way.
The source label therefore tells only the beginning of the story.
Why Source Information Is Useful for Consumers
Knowing whether local drinking water originates primarily from groundwater or surface water can help explain:
regional mineral profiles,
differences in hardness,
why specific treatment processes are used,
and why water utilities monitor particular substances.
But source information cannot replace an actual water analysis.
Knowing that water originates from groundwater does not reveal its exact calcium concentration.
Knowing it originates from a reservoir does not tell you its final tap-water composition.
And knowing a spring looks clear does not establish that it is safe to drink untreated.
Source tells you where to start asking questions.
Analysis tells you what is actually in the water.
From Source Water to Tap Water
The most important distinction in this entire discussion is between source-water challenges and finished drinking-water quality.
Germany’s drinking-water data illustrate this clearly.
Groundwater can face significant environmental pressures from nitrate and pesticides. Surface waters can receive nutrients and other pollutants from their catchments.
Yet Germany’s public drinking-water supply continues to show very high compliance with regulatory requirements, with more than 99% compliance for most monitored microbiological and chemical parameters in the most recently published national report covering 2020–2022.
This is possible because the drinking-water system does more than simply collect water.
It combines:
source protection
raw-water monitoring
appropriate treatment
distribution management
and
regulatory surveillance.
There Is No Universally “Best” Water Source
Groundwater offers natural underground storage and is often comparatively stable.
Surface water can provide very large volumes and is an essential source in many regions.
Springs can provide naturally emerging groundwater where hydrogeological conditions allow it.
Bank filtration can intentionally combine surface-water availability with underground passage.
Each has advantages.
Each has vulnerabilities.
Each needs to be understood in its specific environmental context.
The right source for a drinking-water system depends on far more than whether the water comes from above or below ground.
The Source Is the First Chapter of Water Quality
Water does not begin at the tap.
Its chemistry has already been shaped by everything that came before:
the rain that replenished it
the soil it passed through
the rock it contacted
the fields and cities in its catchment
the amount of time it spent underground
and
the treatment processes applied before distribution.
At Klar2O, that distinction is fundamental to understanding water treatment.
Before asking which treatment technology makes sense, first understand the water itself:
Where did it come from?
What influenced it along the way?
And what is actually present at the point where the water will be used?
Because groundwater, surface water and spring water are not simply three different names for water.
They are three different journeys.
Sources
German Environment Agency, FAQs on Nitrate in Groundwater and Drinking Water, including definitions of raw water, groundwater, spring water, bank filtration and artificial groundwater recharge.
German Environment Agency, Groundwater Quality, updated February 2026.
German Environment Agency, Nutrients and Pollutants, updated February 2026.
German Environment Agency, Effects of Plant Protection Products on Groundwater and Drinking Water, updated February 2026.
German Environment Agency and Federal Ministry of Health, Report on Drinking-Water Quality in Germany 2020–2022, published 2025.
German Federal Institute for Geosciences and Natural Resources (BGR), Geogenic Groundwater Quality of Germany.
European Environment Agency, Europe’s Groundwater — A Key Resource Under Pressure.
European Environment Agency, Pesticides in Rivers, Lakes and Groundwater in Europe.
World Health Organization, Protecting Surface Water for Health.
World Health Organization, Water Safety Planning.
U.S. Geological Survey, Ground-Water Quality and Water Chemistry.