A Piper diagram packs a water's major-ion chemistry into three linked panels — a cation triangle, an anion triangle, and a central diamond that combines them — so you can read its water type at a glance and spot mixing or evolution across many samples.
The three panels
Every sample appears three times. Its relative cation make-up places a point in the lower-left triangle, its anion make-up places a point in the lower-right triangle, and those two points are projected up into the diamond, where they meet at a single point that describes the water overall.
The main thing to hold onto: the triangles show proportions, not amounts. Each ion is plotted as a percentage of total cations (or anions) in meq/L, and the three always sum to 100%. A dilute water and a concentrated water with the same ion ratios land on exactly the same spot — Piper diagrams are about chemical character, not salinity.
Reading a triangle
Take the cation triangle. Its three corners are pure Ca, pure Mg, and pure Na+K. To read a point, drop a line parallel to each side; where it meets the labelled axis is that ion's percentage. The three readings sum to 100%.
Reading the diamond
The diamond is where classification happens. Its four corners correspond to the four end-member water types, and a sample's position tells you which one it resembles. The two dashed diagonals split it into the classic fields:
| Field | Water type | What it usually means |
|---|---|---|
| Top | Ca–Mg–SO₄–Cl | Non-carbonate (permanent) hardness — alkaline earths with strong acids. Gypsum dissolution, mine drainage, some evaporative waters. |
| Left | Ca–Mg–HCO₃ | Carbonate (temporary) hardness. Typical of fresh, recently recharged groundwater in carbonate or silicate aquifers — the "young water" corner. |
| Right | Na–Cl | Saline / alkali–strong-acid waters. Seawater, brines, evaporation, and the end-member for seawater intrusion. |
| Bottom | Na–HCO₃ | Carbonate alkali. Often produced by cation exchange (Ca/Mg swapped for Na) — deep or confined groundwater, or naturally softened water. |
Patterns across many samples
A single point classifies one water; the real value comes from plotting a whole dataset:
- Mixing lines. Samples strung along a straight line between two clusters suggest mixing of two end-members — for example fresh groundwater and seawater.
- Evolution trends. Groundwater commonly evolves from Ca–HCO₃ near recharge toward Na–Cl down-gradient; a trend across the diamond traces that flow-path chemistry.
- Ion exchange. A shift toward the Na–HCO₃ (bottom) corner without added chloride is a classic base-exchange signature.
- Outliers. A point far from the main group is worth checking — either a real anomaly or an analytical error. Hydrochart's charge-balance flag helps you tell them apart.