Gadolinium as the tracer in urban drainage system and the water supply: A case study of the city of Prague

Gadolinium is present in extremely low concentrations in natural water. It is derived from rock. Since 1988, its concentration has been increased in cities with modern hospitals. Gd-DTP complex is administered to patients as a blood pool contrast agent in magnetic resonance imagining. Very soon after the administration (within 4 to 8 hours) the complex is fully removed from body by urination. In this way, Gd is introduced to urban sewage systems . The complex is very stable and can be traced by very careful chemical analyses of the content of Rare Earth elements in surface, sewage and ground water. The anthropogenic Gd contents range from zero to 44 pmol/kg in surface waters of Prague, the capital of the Czech Republic. Vltava and Berounka rivers, several creeks within Prague and the creeks to which the effluents of sewage treatment plants are discharged show high Gdant concentrations. This may indicates that the sewage system leaks or sewage is directly discharged into surface streams. The Vltava river gains about 0.10 mol-Gd/d when passing Prague. The Gdant behaves conservative in surface waters at time span of water flow through Prague which is about 2 days. These observations indicate that Gd complexes are neither absorbed nor coprecipitated nor undergo ion exchange with organic or inorganic particulate matter. This supports the idea that the Gd complex is stable for a long time in the hydrosphere. Compared with the 10 days average of administered Gd-DTPA in Prague's hospitals (0.14 mol-Gd/d), the randomly measured increase of Gdant in the Vltava river is 40% too low. At the same time the feed of the sewage plant is 11% too low. These deviations largely record either the time-dependent variations of the applications of Gd-DTPA in the hospitals or a partly decomposition of the complex during the biological treatment of the sewage and fixation of free Gd cations on abundant clay minerals by ion exchange. The drinking water of Prague contains some anthropogenic Gd at level below 2 pmol/kg due to the production of drinking water from the Vltava river and Jizera river water. Only the drinking water from Zelivka shows negligible Gdant abundances.