USGS OFR 03-346
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Figure 7

Eh-pH diagramm showing measurements of a number of spring waters. Most waters are consistent with equilibrium between dissolved ferrous iron and solid ferric hydroxide (log K=-37.1). Two of the measurements are consistent with equilibrium between dissolved aqueous iron species. It is not known why the electrode responded to the dissolved species in these two cases rather than the dissolved-solid equilibrium that most measurements reflected, since all of the springs looked similar in physical characteristics.