TRACING PREFERENTIAL FLUID FLOW ALONG FAULT ZONES IN THE UNCONSOLIDATED SEDIMENTS OF THE LOWER RHINE EMBAYMENT

We hypothesize that the normal faults in the unconsolidated sediments of the Lower Rhine Embayment act as conduit-barrier systems. In such systems fluid flow vertically along faults will be enhanced while across-fault fluid flow is impeded. We proposed that this dual behaviour can be understood by assuming the simultaneous shear of sand as well as the smear of clay along the fault plane. The barrier properties of faults in the Lower Rhine Embayment are reflected in strong hydraulic head gradients that are enhanced by groundwater extractions that are impacting regional scale groundwater flow. Preliminary, idealized models show that these head drops can coexist with vertically enhanced fluid migration. However, hydraulic head data alone are not sufficient to demonstrate this behaviour. For this purpose we have collected a suite of groundwater samples for detailed hydrochemical analyses. These samples are analyzed for stable isotope ratios, trace metals and groundwater ages are being determined (14C and T/3He dating). The results of these analyses are integrated using three- and two dimensional hydrogeological models that are used to calculate transient hydraulic head, temperature and groundwater age distributions.