The gist
A Gibbs diagram plots TDS against an ion ratio to show which natural mechanism — precipitation, rock weathering or evaporation — controls a water's chemistry.
The two panels
Gibbs (1970) uses two semi-log scatter plots. Both share a y-axis of TDS in mg/L (log). The x-axis is a simple weight ratio between 0 and 1:
- Cation panel: Na / (Na + Ca)
- Anion panel: Cl / (Cl + HCO₃)
The three Gibbs fields. Where a sample plots names the dominant control on its chemistry.
The three fields
| Field | Position | Meaning |
|---|---|---|
| Rock | Middle, low ratio, medium TDS | Rock–water interaction (weathering of silicates and carbonates) dominates — Ca and HCO₃ from minerals. |
| Evaporation | Top-right, high ratio, high TDS | Evaporation and crystallisation concentrate the water toward Na and Cl. |
| Precipitation | Bottom-right, high ratio, low TDS | Atmospheric precipitation dominates — dilute water carrying marine-aerosol Na and Cl. |
Reading tip. The classic "boomerang" traces an arc: dilute rainfall at the bottom, rising through rock-dominated waters, up to evaporation-concentrated brines at the top. Where your samples sit along that arc is the headline result.
Watch for
Gibbs is a useful first-order screen, but it's been criticised for oversimplifying — mineral dissolution (gypsum, halite) and human inputs can push samples outside the fields. Treat points that fall outside the boomerang as a prompt to look closer (often anthropogenic influence), and confirm a weathering story with the bivariate ratio plots.