Stiff diagram

A Stiff diagram is a convenient way of displaying water chemistry data. It is widely used by hydrogeologists and geochemists to display the major ion composition of a water. The diagram itself is a symbol or picture where the shape and size represent the relative proportions and concentrations of the different ions in the water.

Stiff diagrams are usually plotted on maps allowing differences or similarities between different waters to be seen at a glance. The Stiff diagrams can then be used:

1) to help visualize ionically related waters from which a flow path can be dertermined, or;

2) if the flow path is known, to show how the ionic composition of a water body changes over space and/or time.

A typical Stiff diagram is shown in the Figure below. By standard convention, Stiff diagrams are created by plotting the equivalent concentration of the cations to the left of the center axis and anions to the right. The points are connected to form the figure. When comparing Stiff diagrams between different waters it is important to prepare each diagram using the same ionic species, in the same order, on the same scale.

Environmental laboratories typically report concentrations for anion and cation parameters using units of mass/volume, usually mg/L. In order to convert the mass concentration to an equivalent concentration the following mathematical relationship is used:

(mass concentration) * (ionic charge) / (molecular weight) = (equivalent concentration)

For example, a water with a calcium concentration of 120 mg/L would have the following calcium equivalent concentration:

(120 mg/L) * (2 meq/mmol) / (40 mg/mmol) = (5 meq/L)----

References

"The Geochemistry of Natural Waters, Surface and Groundwater Environments, Third Edition" by James I. Drever, University of Wyoming; Prentice Hall, Upper Saddle River NJ, 2002