Positive gadolinium anomaly in surface water and ground water of the urban area Berlin, Germany

Rare-earth element (REE) distribution patterns of surface water and commonly also of ground water from industrialised and highly populated areas show anthropogenic gadolinium (Gd) anomalies. They result from the application of organic Gd compounds, such as the derivatives of the gadopentetic acid (Gd-DTPA), as a contrast medium in magnetic resonance imaging. After excretion from the human body, the Gd complex enters the surface water mostly by the effluents of sewage treatment plants. The chemical complex is very stable over at least 6 months under natural conditions. Owing to its anionic state, it is neither adsorbed onto surfaces of abundant clay mineral particles nor by particulate organic matter, both strongly contrasting with the behaviour of the free REE ions. Thus, this complex behaves like a pseudo-natural marker and provides a tool for tracing the mixing of surface and ground water with recycled water, and the infiltration of surface water into aquifers. In urban areas, where water production is commonly based on bank filtration, the anthropogenic Gd of the surface water can be followed to the water production wells. Like boron, δD and δ18O, Gd can be used as a marker in the estimation of the recycled water fraction in ground water systems. Examples of anthropogenic Gd distribution in surface water, sewage and ground water in Berlin, Germany, are discussed. Mixing ratios of bank filtrates and ground water are estimated.