The gist
Bivariate plots are simple x–y scatters where a reference line encodes a specific process — samples that sit on the line confirm it, and the direction they scatter tells you what else is going on.
The idea
Unlike the classification diagrams, each bivariate plot tests one hypothesis. Hydrochart offers several, all on a meq/L basis (except the molar Gaillardet ratios):
Two examples. Left: Na vs Cl with the 1:1 halite line. Right: the ion-exchange plot, where cation exchange falls on a slope −1 line.
The plots and what they test
| Plot | Reference | Reads as |
|---|---|---|
| Na vs Cl | 1:1 line | On the line = halite dissolution or seawater. Above (Na excess) = silicate weathering or cation exchange releasing Na; below (Cl excess) = reverse exchange or evaporite chloride. |
| Ca vs SO₄ | 1:1 line | Points on the line indicate gypsum dissolution; departure suggests other Ca or sulfate sources/sinks. |
| (Ca+Mg) vs (HCO₃+SO₄) | near 1:1 | Carbonate + gypsum dissolution plots near 1:1; ion exchange shifts samples off it. |
| (Na+K−Cl) vs (Ca+Mg−HCO₃−SO₄) | slope −1 | A trend along the slope −1 line is the classic signature of cation (base) exchange. |
| Gaillardet: Ca/Na vs HCO₃/Na | weathering end-members | Molar ratios place a sample between silicate, carbonate and evaporite weathering domains (Gaillardet et al. 1999). |
How to use them. Start from a hypothesis raised by a Piper or Gibbs plot — say "this looks like ion exchange" — then open the matching bivariate plot to confirm or reject it. Dots on the guide line support the process; systematic scatter off it points to a second one.
Watch for
A single plot rarely proves a process on its own. Read two or three together — for example Na–Cl plus the exchange plot plus Ca–SO₄ — so competing explanations (halite, gypsum, silicate weathering, exchange) can be separated rather than confused.