Fractionation of stable Ca isotopes in a shallow groundwater aquifer

Calcium has six stable isotopes, which fractionate during biological and inorganic processes, and thus may provide an important tracer in hydrological studies. To evaluate the potential use of Ca isotopes in determining sources and biogeochemical processes, groundwater from a shallow aquifer in NW Germany was investigated. 44Ca/40Ca ratios (expressed as Δ44Ca) of dissolved Ca2+ in water were analyzed by thermal ionization mass spectrometry using a ^{42}Ca-^{48}Ca double spike to correct for fractionation during the analytical procedure. 44Ca/40Ca ratios were normalized to seawater (Δ44Ca = 0), which was used as a standard. Groundwater samples were collected from monitoring wells in depths between 1.5 and 50 meters below surface. The aquifer is composed of Quaternary unconsolidated sediments. pH values increase from 4.5 in shallow wells to 7.6 at deeper levels, indicating low buffering capacities due to long-term decalcification processes in near-surface horizons. Near-surface groundwater is strongly undersaturated with respect to calcite whereas deep groundwater is close to saturation with calcite. Deep groundwater and several shallow wells are characterized by anoxic conditions while in parts of the aquifer oxic conditions prevail. Δ44Ca values of groundwater range between 0.2 and -1.4 ‰, indicating changes in the sources that are contributing to the dissolved Ca2+ loads, and biogeochemical fractionation processes. The main sources of calcium in the groundwater include mineral weathering and atmospheric deposition. Ca isotopes may be fractionated by secondary minerals precipitation, ion-exchange processes, and biological processes. In this presentation, possible scenarios will be discussed. Acknowledgements: Thanks to Dr. P. Groth, Dr. A. Mehling and the personnel of the Harzwasserwerke GmbH, Germany.