A colour-coded Piper is an ordinary Piper plot in which every sample is coloured by its position in the diamond. Chemically similar waters get similar colours — so you can carry that colour straight onto a map, cross-section or time series (Peeters 2014).
It's a Piper, first
Everything about reading a Piper still applies: the cation and anion ternaries project into the central diamond, and where a sample lands names its water type. If that's new, start with the Piper guide — the axes, arrows and percentage scales here are identical.
What the colour adds
Peeters (2014) set out to fix the Piper's main weakness: it is hard to link a sample's position in the plot back to where it sits in the aquifer. The remedy is a continuous background colour field across the diamond. Each sample takes the colour of its diamond position, and you can then colour the same points on a map with the identical colours. A patch of blue on the map instantly reads as fresh Ca–HCO₃ water, a red patch as saline Na–Cl, and the gradients between them as mixing.
The colour key
The four corners of the diamond are the end-member water types, and the colours blend smoothly between them:
| Corner | Colour | Water type |
|---|---|---|
| Bottom-left | Blue | Ca–HCO₃ — fresh recharge |
| Top-right | Red | Na–Cl — saline / seawater |
| Top-left | Amber | Ca–Cl |
| Bottom-right | Purple | Na–HCO₃ |
Watch for
Colour is only as trustworthy as the analysis behind it, so keep an eye on charge balance. Hydrochart fills the diamond with a continuous bilinear blend of the four corner colours — the same idea as Peeters' scheme (turn a diamond position into a reusable colour), though his original uses a perceptually-uniform HSV map, so exact hues differ from his figures. Treat the colour as a spatial-linking aid, and read the water type itself from the diamond position, just as on any Piper.