Comparison of sulphate concentrations in groundwaters and atmospheric precipitation suggests that about 30 % of groundwater sulphate may be of atmospheric origin. Another source of sulphate ion is the oxidation of sulphur compounds by microorganisms in the soil or in the aquifer. The sulphates of atmospheric and biogenic origin are mixed in the soil and subsequently the mixture gets into the groundwater. The ð34S data suggest, that ð34S of biogenic sulphates is the same as that of atmospheric sulphates. Processes such as: the assimilation of sulphates by plants, their decay and oxidation of organic compounds do not change the ð34S significantly. Significant differences between ð18O values of sulphates of atmospheric and biogenic origin, which are created by microbiological sulphur-oxidizing processes, may be useful for interpretation of the processes in the groundwater system.

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