Mineral / ion
Calcium
The dominant cation in most fresh water and the principal contributor to hardness. Calcium is where limescale comes from, and also the main reason mineral waters differ in taste.
Key facts
- Chemical form
- Ca²⁺
- Typical range
- Soft supplies 5–30 mg/l; chalk groundwater 100–140 mg/l; some mineral waters exceed 400 mg/l.
Where it comes from
Dissolved from limestone, chalk, dolomite and gypsum. Carbon dioxide in rainwater and soil makes weak carbonic acid, which dissolves calcium carbonate — the same process that carves cave systems.
Typical concentrations
Soft supplies 5–30 mg/l; chalk groundwater 100–140 mg/l; some mineral waters exceed 400 mg/l.
What it does
- The main component of limescale, together with bicarbonate
- Contributes a full, slightly sweet taste
- A dietary source of calcium, though usually a minor one relative to food
- Reduces the plumbosolvency of water, so hard-water areas tend to have lower lead at the tap
Removal
Removed by ion exchange softening, reverse osmosis, nanofiltration or lime softening. Not removed by carbon filtration or boiling — boiling precipitates it as scale but the remaining water is not softened in any useful sense.
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- British Geological Survey — Groundwater levels, aquifer mapping and baseline chemistry. Open — attribution required · Open Government Licence v3.0
- National Center for Biotechnology Information — PubChem chemical database. Open · US Government work — public domain