Using anthropogenic gadolinium as a tracer to reduce the risk for contamination of river bank filtration systems

Drinking water systems providing water to large cities are frequently located in alluvial or fluvio-glacial aquifers because their high permeability allows high extraction rates. Under normal conditions, these systems are recharged by river bank filtration and groundwater is collected at a sufficient distance to ensure that filtration through the aquifer provides water of good quality. However, during flood events, infiltration from the river bank may increase reducing the transit time, which may result in a higher risk of contamination. Identifying the water origin and its path from the river to the water work is key to establish operational strategies that minimize or prevent contamination during flood events. In this study, we investigate the potential of using anthropogenic gadolinium, which is increasingly used as a contrast agent in magnetic resonance imaging (MRI) and finally verted into rivers, as a conservative tracer to identify pathways of river bank filtration and then contribute to minimize the uncertainty of numerical models.

The test site is located in a rural sub-alpine basin. Several horizontal drains extract water from a fluvio-glacial aquifer by gravity. Under normal conditions, good quality water is collected; during flood events, due to the sudden increase in infiltration and decrease in transit times, water quality might deteriorate, as shown by the appearance of E. coli and coliforms. The concentration of anthropogenic gadolinium was measured in the river, in observation wells, and in the extracted drinking water over several years. The results demonstrated the great potential of gadolinium to identify and delineate the infiltration plumes produced by river bank filtration, which is contributing to reduce model uncertainty and evaluate the best pumping strategy for each of the drains in order to prevent water quality degradation.