THERMAL ANOMALIES ALONG FAULT ZONES IN THE LOWER RHINE EMBAYMENT AS AN INDICATOR OF PREFERENTIAL FLOW PATHS

Hydraulic head data suggest that most fault zones in the Lower Rhine Embayment, Germany act as significant barriers to lateral groundwater movement. This is suggested by considerable hydraulic head drops (upto ~100 meters) that have developed over several major fault zones as a result of dewatering of open pit mines for lignite. However, groundwater temperature data in the area shows that anomalously warm and cold water is present in several aquifers near fault zones. Temperature anomalies can be up to ~6 oC as compared to an undisturbed geothermal gradient governed by heat conduction only. We use numerical models to show that these patterns can be explained by assuming flow of groundwater with an anomalous temperature from the fault zone into the flanking aquifers. However, this implies that the faults are conduits to vertical fluid flow transporting groundwater up- and downward along the fault plane. Consequently, this analysis supports a model in which faults act as combined conduit-barrier systems in which faults act as vertical conduits but are laterally sealing. Numerical modelling of heat transport by fluid flow allows to evaluate the range of fluid fluxes and fault hydraulic properties needed to produce the observed anomalies.