The Hydrochemical Facies Evolution Diagram (HFE-D) plots %Na+K against %Cl to track whether a coastal or saline aquifer is undergoing seawater intrusion or freshening — read from the cation-exchange signature.
What it plots
Giménez-Forcada (2010) maps each sample onto a square by two meq-percentages:
- x-axis: % Na+K among the cations
- y-axis: % Cl among the anions
Fresh recharge sits bottom-left (Ca–Mg–HCO₃) and seawater sits top-right (Na–Cl). The straight line between them is conservative mixing. The diagnostic power is in the two off-diagonal corners, which only appear when cation exchange is active.
The four facies
| Facies | Corner | What it means |
|---|---|---|
| Ca–Mg–HCO₃ | Bottom-left | Fresh recharge water — the starting point of the system. |
| Na–Cl | Top-right | Seawater (or a saline end-member) — the mixing partner. |
| Ca–Mg–Cl | Top-left | Intrusion signature. As seawater invades, the aquifer releases Ca/Mg and takes up Na, pushing waters toward Ca–Cl. |
| Na–HCO₃ | Bottom-right | Freshening signature. As fresh water flushes a formerly saline aquifer, exchange reverses — Na is released and Ca taken up. |
Watch for
HFE-D is built for coastal and salinised systems; away from a saline influence the facies labels lose their intrusion/freshening meaning. It relies on a good chloride analysis, and the exact facies boundaries vary between published versions — treat the corners as directions of evolution rather than rigid bins, and confirm any interpretation with the underlying chemistry and, where possible, a time series.