The gist
A Chadha diagram is a Piper diamond straightened into a simple rectangle: two difference axes place each sample in one of four quadrants that name both the water type and the likely process.
The two axes
Chadha (1999) replaces the two triangles with two differences, each in meq-percent:
- X-axis: (Ca+Mg) − (Na+K). Positive means alkaline earths dominate; negative means alkali metals dominate.
- Y-axis: (CO₃+HCO₃) − (SO₄+Cl). Positive means weak acids (carbonate) dominate; negative means strong acids dominate.
The four Chadha quadrants. Each names a water type and the process it typically indicates.
The four quadrants
| Quadrant | Type | Process |
|---|---|---|
| Top-right (+,+) | Ca–Mg–HCO₃ | Recharging / fresh groundwater |
| Top-left (−,+) | Na–HCO₃ | Base ion exchange (Ca/Mg → Na); deeper groundwater |
| Bottom-left (−,−) | Na–Cl | Seawater / saline water |
| Bottom-right (+,−) | Ca–Mg–Cl | Reverse ion exchange |
Why use it over Piper. The quadrants map cleanly onto hydrochemical processes, and because it's a single rectangular plot it reproduces well in reports and handles large datasets without the triangles getting crowded. It's a natural companion to the Piper diamond, not a replacement.
Watch for
Like all classification plots, Chadha describes proportions, not concentration — check TDS or EC separately for salinity. Samples near the axes sit between processes and shouldn't be over-interpreted.