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Water science

How geology shapes water

Water chemistry is mostly a record of what the water has flowed over. Hardness, pH, alkalinity and mineral content are all set by rock.

intermediateUpdated 24 August 2026

Rain is essentially pure. Everything dissolved in the water coming out of a tap was picked up on the way — from the atmosphere, the soil and above all the rock. Water chemistry is a record of that journey.

Where hardness comes from

Hardness is geology written into water. Rain is soft everywhere; what it flows over decides what it becomes.

Where hardness comes fromRain falls with almost no dissolved minerals. Where it flows over granite, sandstone or peat, little dissolves and the water stays soft. Where it percolates through chalk, limestone or dolomite, carbonic acid dissolves calcium and magnesium carbonate, and the water becomes hard. The result is that hardness maps closely onto underlying geology rather than onto anything about the water supplier.Rainnaturally softGranite / sandstonelittle dissolvesPeat moorlandorganic acidsChalk / limestoneCaCO₃ dissolvesDolomiteadds magnesiumSoft water<50 mg/l as CaCO₃Hard water>200 mg/l as CaCO₃
Scotland and Wales
Predominantly hard igneous and metamorphic rock, so water is generally soft.
South east England
Chalk and limestone, so water is generally very hard.
What it means for you
Hardness has no adverse health effect. It affects scaling, soap use and taste, and it is set by the rock, not by the water company.

What each geology produces

Geology and the water it produces
GeologyResulting waterWhere
ChalkVery hard, alkaline, calcium-bicarbonate. Highly productive aquifer with dual porositySouthern and eastern England, northern France
LimestoneHard, alkaline. Karst systems can transmit water and contamination very rapidlyPennines, Mendips, Dinaric Alps
DolomiteHard, with a higher magnesium share than limestoneMagnesian Limestone belt of north east England
SandstoneModerately hard, good storage and steady yield. Iron and manganese common where reducingSherwood Sandstone of the English Midlands
GraniteVery soft, slightly acidic, low alkalinity. Poor aquifer — water moves only in fracturesScotland, Cornwall, Brittany
Volcanic rockSoft, silica-rich. Can be highly productive where fracturedIceland, Auvergne, Deccan Traps
PeatSoft, acidic, coloured by dissolved organic carbonUK uplands, Ireland
Gypsum-bearing strataHigh sulphate, permanent hardness, bitter tasteParts of the English Midlands and eastern Europe
Coastal sands and gravelsVariable, vulnerable to saline intrusionCoastal aquifers worldwide

Why the mechanism is carbonate weathering

Rainwater dissolves carbon dioxide from the air and, far more importantly, from soil where root and microbial respiration raise CO₂ concentrations well above atmospheric. That forms carbonic acid, which dissolves calcium carbonate. The reaction is reversible, which is why the same chemistry that dissolves a limestone cave deposits a stalactite in it — and why heating hard water precipitates limescale.

The carbonate system

The chemistry linking atmospheric carbon dioxide, rock weathering, water pH and limescale. It is the single most important equilibrium in natural waters.

The carbonate systemCarbon dioxide dissolves in water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions, and bicarbonate further into carbonate. Calcium carbonate rock dissolves in the presence of carbonic acid, releasing calcium and bicarbonate into the water. Heating reverses this: bicarbonate decomposes, carbon dioxide is driven off, and calcium carbonate precipitates as limescale.dissolvesdissociatesdissolves rockreversesCO₂atmospheric and soilH₂CO₃carbonic acidHCO₃⁻bicarbonate — alkalinityCO₃²⁻carbonateH⁺lowers pHCaCO₃ rocklimestone, chalkCa²⁺ + HCO₃⁻temporary hardnessHeatingkettle, boilerLimescaleCaCO₃ precipitatesOcean acidificationmore CO₂, lower pH
Why hard water scales
Heating drives off carbon dioxide, shifting the equilibrium back toward solid calcium carbonate. The scale in a kettle is the limestone the water dissolved on its way to you.
Temporary versus permanent hardness
Hardness paired with bicarbonate precipitates on heating and is temporary. Hardness paired with sulphate or chloride does not, and is permanent.
Alkalinity
Bicarbonate is the buffer that resists pH change. Waters with little of it — soft upland waters — are the ones acid deposition damaged.
Ocean acidification
The same equilibrium at global scale. More atmospheric CO₂ means more carbonic acid, more hydrogen ions and lower ocean pH.

Why some aquifers are dangerous

Geology also determines natural contamination. Arsenic in the Bengal basin comes from arsenic-bearing sediments releasing it under reducing conditions. Fluoride above guideline values across the East African Rift comes from volcanic rock. Neither is pollution — both are natural, and both affect tens of millions of people.

Vulnerability differs too. A karst limestone aquifer can transmit contamination from a surface input to a spring in days, because water moves through open conduits rather than through the rock matrix. A thick clay-covered sandstone aquifer may take decades. Two aquifers with identical water quality today can have completely different exposure to what happens on the land above them.

Sources

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