A temporal iron curtain: The hydro- and multi-isotope geochemistry of iron-rich ground waters emerging at the southern Baltic Sea coast line

Marko Lipka1, Jürgen Sültenfuss2, Zijun Wu1, Peter Escher1, Iris Schmiedinger1, Ulrich Struck3, Olaf Dellwig1, Vera Winde1, Michael E. Böttcher1
1 Geochemie & Isotopenbiogeochemie, Leibniz Institute für Ostseeforschung (IOW)
2 Umweltphysik, Universität Bremen, FRG
3 Natural History Museum Berlin, FRG

P 9.2 in Groundwater-surface water-interactions - processes and methods

Iron-rich groundwater springs emerging at the shoreline of the southern Baltic Sea were examined on a seasonal base for a period of about five years. Besides major, minor, and trace elements, stable isotopes of water (H, O), dissolved inorganic carbon (DIC), and sulfate, tritium and noble gases were analyzed. The stream bed sediment was extracted for the geochemistry of the newly formed precipitates. Subsequently, the hydrogeochemical results were subjected to a thermodynamic analysis via the PHREEQC speciation model.

   The springs emerge from small pits (about 60 cm diameter; up to 15cm depth). Surrounding sediments are sandy with gravels found at depth and corresponding high permeability. The positions of different springs on the shore zone were stable during the investigation period while the shape of the pits and the stream beds may vary due to wind- and wave-driven forces. Selected measurements of spring yield discharges close to 10 L/min. The H-2 and O-18 contents of the spring waters indicate the ground water to originate from relatively young mixed meteoric waters. The springs are dominated by dissolved Ca, Mg, Na, DIC and sulfate, mainly reflecting the interaction with soils and bedrocks in the recharge are. The oxygen-free ground water is rich in Fe, P, and DIC. Dissolved Fe and SO4 originate from the oxidation of pyrite. The C isotope signature of DIC indicates a mixture of biogenic CO2 from the soil zone with further water-rock interaction with carbonate minerals. The streams flow towards the BS and, in contact with the atmosphere, outgas carbon dioxide and takes up oxygen. Upon CO2-degassing, 12C is preferentially desorbed from the aqueous solution.  The changes in the stream composition lead to the formation of FeOOH precipitates in the stream bed before the waters pass to underground drainage into a subterranean mixing zone with brackish BS waters. These ochrous precipitates act as a sink for dissolved phosphate and minor calcium. P : Fe ratios and Ca : Fe ratios are about 0.08 and 0.2, respectively, which are caused by P adsorption and a mixture with minor CaCO3 and/or Ca-phosphate. dating with the tritium-noble gas method yield average ages of the emerging ground waters of around 25 to 32 years with different mixing proportions of tritium-free waters.

   The investigation reveals that the surface precipitation on the beach leads to the formation of SGD essentially free of dissolved iron and strongly depleted in phosphate.  Similar iron phases are an important part of the subterranean estuary  at that coast line, where retention of nutrients and heavy metals influence the release of nutrients into the coastal ecosystem. Fe- and P-rich surface precipitates, however, are transported in suspension into the Baltic Sea during wind-driven flood events.

   Acknowledgements: This work was partly supported by the BONUS+ project AMBER, the DFG graduate research training group BALTIC TRNASCOAST and the Leibniz IOW.