TRACING FLUID FLOW ALONG FAULTS IN UNCONSOLIDATED SEDIMENTARY AQUIFERS IN THE LOWER RHINE EMBAYMENT

Ground water flow in siliciclastic sedimentary aquifer systems can be influenced by the hydraulic properties of faults. The entrainment of both sand and clay into the fault zone during fault slip can potentially enhance flow along the fault plane as well as impede transport across the fault. Recently, Bense and Person (Water Resources Research, 42, W05421, doi:10.1029/2005WR004480,2006) have proposed a novel approach for representing fault zone permeability that incorporates this conduit/barrier behavior. In this model, fault zone hydraulic anisotropy develops as vertical slivers of sand and clay are added to the fault zone during fault growth/throw. In order to test this approach, we plan to collect a suite of environmental isotope and noble gas samples from groundwaters hosted within unconsolidated sediments of the Lower Rhine Embayment, Germany. This field site was chosen because long term pumping associated with open pit lignite mine dewatering operations have created a regionally extensive cone of depression that reveals the influence of faults on groundwater flow patterns. Using an available dense (thousands of wells) network of piezometers, we plan to map geochemical, thermal, and isotopic patterns around fault zones in unprecedented detail. Specifically, we will apply a suite of age-dating techniques (4He, Tritium, C-14) to infer preferential flow along fault planes which we expect will lead to anomalous groundwater age distributions around fault zones. Subsequently, we will use these data to constrain high-resolution, three dimensional finite element models constructed to test our new and existing (e.g. Shale Gouge Ratio) fault permeability algorithms. Our study has implications both for water resource managers (e.g. Rio Grande Rift) and petroleum engineers charged with assessing pressure compartment development in petroleum reservoirs.