Urban aquifers are a potential alternative, with which to cover the rising demand for freshwater. However, they are commonly polluted by a wide range of anthropogenic substances, including rare earth elements (REEs). In this context, it is essential to investigate the presence and behaviour of these pollutants, in order to safely use urban groundwater. An example of these contaminants is Gadolinium, which can reach the environment after it has been used for medical purposes. Gadolinium anomalies were detected in hydraulically connected surface and subsurface water bodies in Barcelona, northeast Spain. A striking factor in this study, is that Gadolinium pollution in groundwater was less frequent and more attenuated than in surface water, suggesting that Gadolinium attenuation may be prevalent in the subsurface. Batch experiments were conducted and modelled to ascertain processes affecting Gadolinium in the sampled aquifers. Experimental results have (i) demonstrated that the concentration of dissolved Gadolinium decreased 90% in 24 days, and (ii) suggested that sorption plays a key role in Gadolinium attenuation. The modelling of the experiments provides essential parameters, with which to predict the behaviour of Gadolinium. Our results have significant implications for understanding the evolution of anthropogenic Gadolinium in aquifers in general, which is of paramount importance for determining where groundwater can be safely used after its natural or artificial infiltration.

Green infrastructure drainage systems are innovative treatment units that capture and treat stormwater. Unfortunately, highly polar contaminants remain challenging to remove in conventional biofilters. To overcome treatment limitations, we assessed the transport and removal of stormwater vehicle-related organic contaminants with persistent, mobile, and toxic (in short: PMTs) properties, such as 1H-benzotriazole, NN'-diphenylguanidine, and hexamethoxymethylmelamine (PMT precursor), using batch experiments and continuous-flow sand columns amended with pyrogenic carbonaceous materials, like granulated activated carbon (GAC) or wheat-straw derived biochar. Our results indicated that all investigated contaminants were subjected to nonequilibrium interactions in sand-only and geomedia-amended columns, with kinetic effects upon transport. Experimental breakthrough curves could be well described by a one-site kinetic transport model assuming saturation of sorption sites, which we inferred could occur due to dissolved organic matter fouling. Furthermore, from both batch and column experiments, we found that GAC could remove contaminants significantly better than biochar with higher sorption capacity and faster sorption kinetics. Hexamethoxymethylmelamine, with the lowest organic carbon-water partition coefficient (K_OC) and largest molecular volume among target chemicals, exhibited the lowest affinity in both carbonaceous adsorbents based on estimated sorption parameters. Results suggest that sorption of investigated PMTs was likely driven by steric and hydrophobic effects, and coulombic and other weak intermolecular forces (e.g., London–van der Waals, H-bonding). Results from extrapolating our data to a 1-m depth geomedia-amended sand filter suggested that GAC and biochar could enhance the removal of organic contaminants in biofilters and last for more than one decade. Overall, our work is the first to study treatment alternatives for NN'-diphenylguanidine and hexamethoxymethylmelamine, and contributes to better PMT contaminant removal strategies in environmental applications.