In a series of bore holes in an acid-intermediate volcanite rock type measurements of pH and pe (in situ), main ionic constituents, silicic acid and aluminium were made. The bore holes covered the pH interval 6.5-8.6, each with a time invariant solution composition. Field data were used as an experimental test of computerized simulation of weathering reactions by means of equilibrium models. The results suggest that a criterion for an undisturbed groundwater system is a high pH and a low pe where the dissolved main components reach a saturation concentration close to that experimentally found. For lower pH values, however, only the subsystem H+ - Si(OH)4 - Al3+ corresponded to an equilibrium behaviour.

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