Gadolinium als Umwelttracer anthropogenen Einflusses auf Grundwasser : Oberflächengewässer-Grundwasser-Interaktion

  • Gadolinium as environmental tracer of anthropogenic influence on groundwater : Surface water-groundwater-interaction

Boester, Uwe; Rüde, Thomas R. (Thesis advisor); Schwarzbauer, Jan (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Gadolinium (Gd) is an element of the rare earth element (REE) group. It has been used as an environmental tracer in surface waters since 1996. The method is based on the determination of the enrichment of Gd in the environment compared to geological background values. Gd represents a group of contrast agents in magnetic resonance imaging (MRI), which have been emitted into surface waters since the 1980s. Consequently, an anthropogenic enrichment relates to the patients excreting these contrast agents shortly after ambulant medication in hospitals or at home. Sewage treatment is currently unable to hold back Gd from this anthropogenic source. Therefore, the Gd concentration in the receiving channel increases significantly, and it does not only influence surface water but propagates into adjacent groundwater bodies due to its conservative behavior in the environment.As this research focuses on surface water and porous aquifers, however, the anthropogenic Gd-anomaly is a very stable environmental tracer. The investigation of three study sites, which represent a high groundwater velocity and high Gd-concentration case in karst "Mittelstreu" (125 to ca. 3000 m/d), a mid-case "Eisch" and a case with lower groundwater velocities and lower Gd-concentrations in a sandstone aquifer "Brombachsee" (50 m/a). These investigations aim to prove that Gd is a suitable environmental tracer for different usages in solid rock aquifers as well. The karst aquifer study site displays a connection between a river/ponor (input) and three springs (output), which show a Gd anomaly. The sampling of springs and brooks in the vicinity of the karst proved the Gd’s anthropogenic origin. Another study site is situated at the river Eisch in Luxembourg. In this location, the anthropogenic Gd-anomaly in the river is used to determine groundwater dilution of the anthropogenic Gd-Signal in the river over some distance and a groundwater influx diluting the river’s Gd-Signal. The third investigation site shows a different situation. In this case, the anthropogenic Gd-anomaly in the sandstone aquifer originates in a freshwater reservoir filled with river water, leading to the infiltration of the river water’s Gd-signature into the aquifer. The karst and the sandstone aquifer investigation sites display surface water-groundwater interaction and highlight the environmental tracer capabilities of Gd.To quantify the anthropogenic part of the Gd-Signal, natural Gd backgrounds were calculated statistically for each investigation site and at the karst site for surface water and groundwater as well as different aquifer lithologies. The differences between the lithologies and the water types regarding natural Gd-concentrations is the result of a water sampling campaign in the catchment of the springs. Furthermore, two time series were taken at the ponor and the springs, one in summer and one in winter. As a result, the weekly anthropogenic Gd-Signal in surface water and spring water produced Gd-peaks that are correlated between the ponor and the springs. The flux and mass evaluations enable to calculate a distance velocity (2,500 m/d) between the ponor and the springs as well as the surface water share in the spring water (winter: 5%; summer: 20 %). Moreover, the anthropogenic emissions stay the same during the year as the mass-sum is the same in both sampling periods (summer and winter). At the "Brombachsee", the anthropogenic Gd input reaches the lake in pulses, as the lake system receives only water if the river "Altmühl" experiences flooding. However, the Gd-Signal reaches the northern lake shore and infiltrates the sandstone aquifer ("Mittlerer Burgsandstein" (kmBm)). This signal is traceable over one kilometer into the aquifer at its widest distance to the lake shore. Furthermore, the surface area and hot spots of the infiltration are mapped. In addition to Gd, both sites experienced concentration measurements of five other anthropogenic, organic environmental tracers. Of these, just acesulfame could be correlated to Gd at the "Mittelstreu" without quantification. In the "Brombachsee" area, acesulfame just proves the input path but does not have the environmental stability to reach out into the sandstone aquifer.Finally, Gd represents a conservative, anthropogenic, environmental tracer, that is not interacting with its surroundings and, therefore, stays stable over long periods (20 years, "Brombachsee"). It is measured directly out of the sample and the Gd-anomaly is much higher in the surface waters (2-25 times) so that it can be measured even with high dilution in groundwater. All of this taken into account, Gd is a powerful environmental tracer for surface water - groundwater interaction in solid rock aquifers, even over decades.

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