Total and dissolved rare earth element (REEs) and U concentrations were determined for waters from Lake Paranoá (Brasilia, Brazil) sampled in the dry (November 2012) and wet seasons (July 2013). Shale-normalized REE patterns of all samples display the M-type lanthanide tetrad effect and large positive Gd anomalies, but only the total REE pool shows a positive Ce anomaly, possibly reflecting aeolian and/or fluvial input from surrounding laterites. The positive Gd anomaly increased strongly between 2012 and 2013. It reveals an anthropogenic dissolved Gd source that enters the lake with effluents from wastewater treatment plants, originating from the use of Gd-based contrast agents in magnetic resonance imaging. Because anthropogenic Gd is a tracer for other wastewater-derived substances, such as pharmaceuticals, REE geochemistry offers an inexpensive way to monitor the presence of wastewater-derived substances in the lake, which may be utilized as a drinking water reservoir in the near future.