Following preconcentration steps, analysis of water
samples by ICPMS can provide data for the complete set of rare-earth elements (REEs) including the monoisotopic REEs (Pr, Tb, Ho, and Tm). This is a major improvement for studies of redox-independent anomalies of La and Gd in shale-normalized (suffix 'SN') REE patterns. The presence of positive LaSN and GdSN anomalies in seawater is well established. The anomalies result from the higher marine particle reactivity of these elements compared to their neighbours in the REE series, which is due to the former's
tendency to form more stable surface complexes on inorganic and organic (-ally coated) particles. However, it is not known yet, whether these anomalies are entirely generated within seawater itself or whether they are already typical of continental run-off.
During a project devoted to the study of the behaviour of Y and REEs in river and lake waters, it was found that rivers draining non-industrialized and thinly populated regions in Sweden (Vermland) and Japan (Hokkaido), do not show
positive GdSN anomalies. In contrast, the river Rhine and its tributaries from the Rhenish Massiv, which drain densely populated areas in western Germany, all show positive GdSN anomalies (GdSN/Gd*SN: 1.6 - 2.5) that are significantly stronger than those of seawater (GdSN/Gd*SN: ×1.2). The most pronounced positive GdSN anomalies ever recorded occur in the river Wupper (GdSN/Gd*SN: 8.5) which enters the river Rhine north of Cologne in western Germany, and in the Spree and Havel river system (max. GdSN/Gd*SN: 140 !) in the Berlin/Potsdam area in eastern Germany. The fact that anomalous concentrations exist exclusively for Gd and the large size of these positive GdSN anomalies strongly suggest that this additional Gd is of anthropogenic origin.
In case of the river Wupper, the most significant Gd input occurs in the vicinity of the city of Wuppertal. In the Berlin/Potsdam area, the anomaly predominantly results from input of "cleaned" waters from the waste water treatment plant 'Ruhleben' into surface waters. This input yielded Gd concentrations as high as 9,000 pMol/kg. Sampling of this input on 10 consecutive days in July '95 (Sa. 22nd to Mo. 31st) showed maxima during weekends and a minimum in the middle of the week. Comparison of REE data determined for a sample aliquot that had been acidified prior to <0.2 µm filtration (containing dissolved + particulate REEs) and REE data determined for an aliquot acidified after <0.2 µm filtration (containing only dissolved REEs) showed similar abundances of Gd in both sub-samples, in contrast to higher abundances of all other REEs in the former. This indicates that in these waters Gd is considerably less particle-reactive than the other REEs, suggesting that the speciation of (the anthropogenic) Gd is dominated by an extremely stable solution-complex. This may explain why Gd is only incompletely removed during waste water treatment. Since aquifers fed by surface waters contribute to the water supply of the cities of Berlin and Potsdam, samples of tap (drinking) water from several parts of these cities were analyzed for REEs. It turned out that most of these samples display positive GdSN anomalies (GdSN/Gd*SN: up to 12 !).
So far, extremely high Gd input appears to be confined to the cities of Berlin and Wuppertal, whereas in the other areas Gd contamination is less pronounced albeit clearly recognizable. Provided the reason for the Gd anomaly is the same in all regions, this suggests that Berlin and Wuppertal are the major centres of production, application, and/or disposal of Gd compounds, although some Gd emission occurs almost everywhere. One may speculate that the ultimate origin of the anthropogenic Gd may be gadopentetic acid used as
contrast agent in magnetic resonance tomography (MRT). The application of gadopentetic acid during medical treatment may explain why in densely populated areas some contamination of surface waters exists everywhere, and why the Berlin/Potsdam area with its almost 5 million inhabitants shows the most pronounced anthropogenic Gd input. Note that even a conservative estimate of the annual consumption of gadopentetic acid for MRT suggests that in the Berlin area between 10 and 100 kg gadolinium per year are disposed with the waste waters.
The positive GdSN anomalies shown by many German rivers indicate that even the distribution of rather "exotic" trace elements such as the REEs may be strongly affected by anthropogenic contamination. Considering that most data sets on REE concentrations in river waters have been determined for rivers draining industrialized and/or densely
populated areas, future studies should focus on more remote river systems. Moreover, it should be noted that the REE
distribution in seas, such as the Baltic Sea, that have only restricted exchange with open ocean waters but receive freshwater from heavily industrialized regions may have been contaminated by anthropogenic REE input.