What is a Stiff Diagram?
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. As it is used to compare multiple samples a Stiff diagram is usually not viewed individually, but in a group
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.
Multiple Stiff plots created in Grapher comparing water chemistry from locations within the same watershed
How to Create a Stiff Diagram:
- Click the Home | New Graph | Specialty | Stiff Plot () command.
- Select the data file and click Open to create the graph.
The data must have the cations and anions in separate columns, with each value on a separate row. The sample data below is located in the Samples file named stiff data.xls.
The cation and anion data for the lake water concentrations.
In the data above, Columns A and E are the label columns for the cation and anion names. Columns B and F are the data values being plotted. The value determines how far from the center 0 line the point is located. When this data is plotted it looks like the diagram below.
Grapher stiff plot displaying lake water ion concentrations.
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.
Updated May 2023