Tracer tests to infer the drainage of the multiple porosity aquifer of Luxembourg Sandstone (Grand-Duchy of Luxembourg): implications for drinking water protection

The Luxembourg Sandstone is the main strategic aquifer of the Grand-Duchy of Luxembourg, providing one third of the total drinking water resource in the country. In the unconfined part, the aquifer discharges through more than 400 springs, which are used for supply. The long-lasting diffuse contamination by pesticides (more recently Metolachor ESA) has been previously simulated assuming the aquifer as an equivalent porous medium, and using environmental tracers and baseflow characteristics. A statistical analysis of the whole data set of artificial tracer tests made for this aquifer (112 injections involving 102 springs) emphasized the role of the main fissures (NE–SW) for preferential groundwater flow and the importance of including them in a conceptual model involving a multiple-porosity aquifer. The results show that these fissures are influencing the whole flow system. The average peak velocity is 16 m/h and the average first arrival (maximum) velocity is 52 m/h (32 m/h for injection-well to spring traces alone), with a recorded maximum velocity of 350 m/h. The threats of infiltrating streams within catchments have also been characterized, suggesting the usefulness of tracer tests for optimizing protective measures. The main benefit of the study is the improvement in the ability to delineate safeguard zones accurately, according to more representative velocities and the strong anisotropy.