Influence of Natural and Anthropogenic Ligands on Metal Transport during Infiltration of River Water to Groundwater

The behavior of dissolved metals, the anthropogenic ligands EDTA and NTA, and the natural ligands for Cu and Zn during infiltration of river water to the adjacent aquifer was investigated at the Glatt River field site (Switzerland). The speciation of EDTA was determined by a combination of experimental methods and of equilibrium calculations. The speciation of EDTA that behaves conservatively with respect to its total concentration changes on the infiltration path. The main EDTA species in the river water are Zn−EDTA, Ca−EDTA, and Fe(III)−EDTA and in some of the groundwaters Mn(II)−EDTA. The speciation of EDTA is strongly influenced by the presence of natural organic ligands for Cu and Zn that bind a large fraction of these metals, even in groundwater. Ni−EDTA is a less significant species than expected from equilibrium calculations. Fe(III)−EDTA is not in equilibrium with other metals due to its slow exchange kinetics. The fraction of EDTA that is present as Fe(III)−EDTA decreases from the river with 35% to 11% in groundwater, within a residence time of about 8 days. This decrease indicates that exchange reactions of the type: Fe(III)−EDTA + Me (ads or aq) → MeEDTA + Fe(III) take place in the pH range of 7−8. Remobilization of metals is therefore possible under these conditions.