A Stiff diagram, or plot, is a graphical representation used by hydrologists and geochemists to compare the ionic composition of water samples between different locations, depths, or aquifers.

Stiff Diagram of a monitoring well water sample created in Grapher

How to read a Stiff diagram:

The values of the cations and anions for a water sample are recorded and plotted in milliequivalents per liter. The left side of the diagram shows the cation concentrations and the right side shows the anion concentrations.

The farther a point is from the center, along the X axis, the larger the ionic concentration. To create the polygonal shape the Stiff diagram is known for the individual ionic points are connected. The relative size of the plot is an indication of the total dissolved solids concentration.

Normally, a Stiff diagram is not viewed individually, but in a group. When reading the group of plots below one could determine that the water sample from Monitoring Well LT15 has the highest concentration of cations and total dissolved solids of the 3 samples. 

Multiple Stiff diagram from locations within the same watershed created in Grapher

Why is a Stiff diagram used?

The purpose of a Stiff diagram is to draw an overall opinion about an aquifer or location by comparing the relationships among multiple Stiff plots. 

By grouping the plots together hydrogeologists can quickly identify the dominant ions present in the sampling group. Once the dominant ions are identified scientists can track changes in the water chemistry at an aquifer over time. Changes in the Stiff diagram over time could indicate changes in the source of the water, changes in the geochemical processes occurring within the aquifer, or changes in the amount or type of contaminants present.

To learn how to create your own Stiff diagram click here: Create a Stiff Diagram in Grapher