87Sr/86Sr-Verhältnisse als Tracer für geochemische Prozesse in einem Lockergesteinsaquifer (Liebenau, NW-Deutschland)

Groundwater of the Quaternary unconsolidated sediments in the catchment area of the waterworks of Liebenau II (NW-Germany) is characterized by heterogeneous distributions of dissolved constituents. In order to determine sources of solutes and to estimate flowpaths, Sr isotope analyses were carried out on groundwater, precipitation, and soil samples. ⁸⁷Sr/⁸⁶Sr ratios of the groundwater samples vary between 0.708 and 0.719 and allow to discriminate four groundwater levels. The near-surface groundwater level is characterized by low ⁸⁷Sr/⁸⁶Sr ratios in the range of 0.7099 and 0.7130. At greater depths ⁸⁷Sr/⁸⁶Sr ratios increase up to a value of 0.7187 in the intermediate groundwater level. Deep groundwaters which can be divided into two levels exhibit a remarkable decrease of ⁸⁷Sr/⁸⁶Sr ratios. Groundwater of deep level I shows Sr isotope ratios between 0.7118 and 0.7128, whereas deep level II shows ⁸⁷Sr/⁸⁶Sr ratios of 0.7082 to 0.7087.

Differences in the ⁸⁷Sr/⁸⁶Sr ratios are the result of varying sources of dissolved strontium in the groundwater. These encompass mineral weathering, precipitation, and anthropogenic inputs. Low ⁸⁷Sr/⁸⁶Sr ratios of 0.710 to 0.713 indicate the influence of precipitation and probably fertilizers on the near-surface groundwaters. At greater depths, groundwater chemistry is dominated by weathering processes. Due to significant differences in the ⁸⁷Sr/⁸⁶Sr ratios the dissolution of silicate minerals can be distinguished from the dissolution of carbonate minerals. For the intermediate groundwater level a dominant influence of silicate weathering is confirmed by radiogenic ⁸⁷Sr/⁸⁶Sr ratios between 0.713 and 0.719 and saturation indices < –3.5 for calcite. In contrast, low ⁸⁷Sr/⁸⁶Sr ratios of 0.708 to 0.709 indicate dissolution of carbonate minerals in the deep groundwater, where saturation for calcite is observed.