Anthropogenic gadolinium in estuaries and tropical Atlantic coastal waters from Fortaleza, Northeast Brazil

Highlights: • Anthropogenic gadolinium (Gd_anth) was detected in coastal waters from Fortaleza. • Conservative Gd_anth behavior allows to track its levels at the three city sources. • About 25 kg of Gd_anth are released annually by the Fortaleza outfall into the ocean. • This emerging contaminant is a suitable tracer for water management purposes. Gadolinium-based contrast agents are worldly used for medical magnetic resonance imaging and are an emerging contaminant in natural waters. We investigated the dissolved fraction of Gd in coastal waters from Fortaleza city and observe positive Gd anomalies in the wastewater outfall area as well as in two local river estuaries indicating that the city is a significant source of anthropogenic Gd (Gd_anth) to the ocean. Based on this synoptic study and on the conservative behavior of Gd_anth we trace a highly concentrated sewage-based source which accounts for 2200 pmol kg⁻¹ and to an annual discharge of 25 kg of Gd to the ocean. We also trace minor sources from the two rivers and estimate that the levels of wastewater dilution within freshwater prior to mixing with seawater accounted for 4.8%–14% of the Cocó River discharge and 1.4%–3.9% of the Ceará River discharge at the time of the sampling. Gd is consequently a suitable and promising tracer for water management and forensic purposes in Fortaleza. In order to guide the application of this method to other coastal waters impacted by metropolitan areas in the world, we propose a conceptual model for Gd_anth behavior within salinity gradients and apply it to revisit previous studies.