Groundwater in shallow unconsolidated sedimen- tary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ 2 Ha nd δ 18 Ov alues in comparison to regional modern groundwater recharge, and 4 He concentrations up to 1.7×10 −4 cm 3 (STP) g -1 ±2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ∼10 7 years. Although fluid exchangebetweenthedeepbasalaquifersystemandtheupper aquifer layers is generally impeded by confining clay layers and lignite, this study's geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, thegeochemicaldatareportedherealsosubstantiateaconduit- barrier model of fault-zone hydrogeology in unconsolidated sedimentarydeposits,aswell ascorroboratingtheconceptthat faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary basins worldwide are far reaching and of particular concern for carboncapture and storage (CCS) programmes, impacts of deepshale gasrecoveryforshallowgroundwateraquifers,and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids.