StiffDiagramm

Stiff Diagramm, commonly known as a Stiff diagram, is a graphical method used in hydrochemistry to visualize the major-ion composition of water samples. Developed in the 1930s, it serves to compare groundwater and surface water, identify characteristic water types, and infer hydrogeochemical processes such as mineral dissolution, ion exchange, mixing, and evaporation.

Construction and interpretation rely on major ions, typically expressed as milliequivalents per liter (meq/L). The diagram is drawn as a diamond or rhombus with two halves: cations plotted on the left and anions on the right. The ions usually considered are Ca2+, Mg2+, Na+, and K+ on the cation side, and Cl−, SO4 2−, HCO3− (and CO3 2− where present) on the anion side. For each ion, the meq/L value is converted to a distance from the center along its respective side, and the points are connected to form a polygon. The resulting shape resembles a diamond, hence the name.

The pattern of the Stiff diagram helps identify water types and geochemical processes. For example, a Ca–HCO3 type indicates carbonate dissolution, while a Na–Cl type may signal seawater intrusion or ion exchange effects. Similar diagrams among samples suggest similar recharge areas or water-rock interaction histories; distinct shapes point to mixing or different sources.

Limitations include reliance on accurate major-ion measurements and the use of only a subset of dissolved constituents. The diagram is qualitative and sensitive to data quality, alkalinity approximations, and unit consistency. Despite these limitations, the Stiff diagram remains a classic tool in hydrogeology for rapid, visual comparison of water chemistries.