Anthropogenic gadolinium as a transient tracer for investigating river bank filtration

The growing use of gadolinium-based contrast agents in magnetic resonance imaging (MRI) leads to an increasing input of anthropogenic gadolinium (Gd _anth) into natural environments. Conventional sewage treatment is unable to remove Gd _anth and since MRI facilities are mainly used on weekdays the Gd _anth inputs to sewage treatment plants are generally higher between Monday and Friday. This transient signal has been traced in both surface water and groundwater through 12-h composite samples collected at high spatial resolutions using depth-discrete rhizon samplers. Propagation of the Gd _anth signal from surface water to groundwater was used to calibrate lumped parameter models. Transit time distributions derived for each sampling site revealed mean transit times of between 0.5 and 10 days. Other metrics, such as peak transit time, were shown to correlate better with observed time lags between peak Gd _anth concentrations in stream water and groundwater. The relatively stable artificial sweetener acesulfame was investigated as a possible additional sewage indicator, but decreasing concentrations along the flow path indicated its attenuation. We have demonstrated that the ideal tracer Gd _anth occurs transiently and can be used to derive groundwater transit times.