A Gaillardet diagram plots molar ion ratios on log axes — Ca/Na against HCO₃/Na — to show whether a water's dissolved load comes mainly from silicate weathering, carbonate weathering or evaporite dissolution.
What it plots
Gaillardet et al. (1999) normalise the major cations to sodium, because Na behaves conservatively and is contributed by every source. Each sample becomes a point on a log–log scatter:
- x-axis: HCO₃ / Na (molar ratio)
- y-axis: Ca / Na (molar ratio)
Because both axes are ratios to Na, the three weathering sources fall in distinct regions and real waters plot on mixing lines between them.
The three end-members
| Source | Position | What it means |
|---|---|---|
| Silicates | Low Ca/Na, low HCO₃/Na | Weathering of silicate minerals (granite, basalt, sandstone). Releases Na and Si with relatively little Ca. |
| Carbonates | High Ca/Na, high HCO₃/Na (upper right) | Dissolution of limestone and dolomite. Delivers abundant Ca (and Mg) with bicarbonate. |
| Evaporites | Low HCO₃/Na, low–moderate Ca/Na | Dissolution of halite and gypsum. Halite drives Na and Cl up, pulling ratios toward the lower-left. |
Watch for
The plot needs Na, Ca and HCO₃ in mg/L; samples missing sodium can't be placed and are skipped. Sodium from sources other than weathering — sea-salt aerosol near the coast, or anthropogenic inputs — will shift ratios and can mimic a silicate signal, so interpret coastal and impacted waters with care. The end-member fields shown here are representative positions, not hard boundaries; use them as a guide, not a classifier.