Minerals and ions
The dissolved constituents that give water its character — where each comes from in the geology, what it does, and how it is removed.
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.
The second contributor to hardness, and the one that gives some mineral waters a distinctly bitter edge.
Present from geology, sea spray, road salt and — importantly — from ion exchange water softening, which removes hardness by substituting sodium for calcium and magnesium.
The dominant anion in most fresh waters and the source of nearly all alkalinity. Bicarbonate is the reason hard water scales when heated.
A conservative ion that moves with water essentially unchanged, which makes it a useful tracer of saline intrusion, road salt and wastewater influence.
One of the most common causes of consumer complaint, producing orange or brown discoloured water and staining laundry, without generally posing a health risk at the concentrations involved.
Contributes permanent hardness and, at higher concentrations, a bitter taste and a laxative effect that is the basis of several historic spa waters.
Behaves similarly to iron but stains black rather than orange, and is harder to oxidise. Increasingly treated as a health parameter as well as an aesthetic one.
Usually a minor constituent of fresh water, since it is readily taken up by plants and clay minerals.
Dissolved silicon, present in almost all natural water and a persistent operational problem for membranes and boilers because it forms a hard, difficult scale.