We argue that the observed conduit‐barrier behavior of fault zones in siliciclastic sedimentary aquifer systems can be understood by considering a strongly anisotropic hydraulic structure in the fault. Hydraulic anisotropy in the fault is expected from a variety of mechanisms including clay‐smearing, drag of sand, grain re‐orientation and vertical segmentation of the fault plane. In this paper, we present an algorithm to predict fault zone width, lithological heterogeneity and hydraulic anisotropy. Estimation of these parameters is based upon the amount of fault throw and the clay‐content of the lithologies flanking the fault zone. A suite of steady‐state flow models are presented using an idealized stratigraphy consisting of alternating clay and sand‐rich layers that are offset by a fault zone. These conceptual simulations show the impact of a fault zone on shallow (<500 m) fluid flow patterns and solute transport for different scenarios of fault throw. Fault width varies along the fault zone and increases from an average width of ~2 m for a throw of 50 m to ~8 m for a throw of 200 m. Hydraulic anisotropy in the fault zone in these scenarios is predicted to range between two to three orders of magnitude. Our results show that faults can form a preferential path way between aquifers at different depths over vertical distances of several hundreds of meters (that are otherwise separated by confining units) when fault permeability is strongly anisotropic. However, in the same scenario anomalously high hydraulic head gradients across the fault would still suggest that they act as an effective barrier to lateral groundwater flow. This has important implications for the assessment of the risk of a spread of contaminated groundwater or the reconstruction of hydrocarbon migration within sedimentary basins.

The recent increase in fresh‐water discharge during low‐flow conditions as observed in many (sub‐)Arctic Rivers has been attributed to a reactivation of groundwater flow systems caused by permafrost degradation. Hydrogeological simulations show how groundwater flow conditions in an idealized aquifer system evolve on timescales of decades to centuries in response to climate warming scenarios as progressive lowering of the permafrost table establishes a growing shallow groundwater flow system. Ultimately, disappearance of residual permafrost at depth causes a sudden establishment of deep groundwater flow paths. The projected shifts in groundwater flow conditions drive characteristic non‐linear trends in the evolution of increasing groundwater discharge to streams. Although the subsurface distribution of ice will markedly influence the system response, current modeling results suggest that late‐stage accelerations in base flow increase of streams and rivers, are to be expected, even if surface air temperatures stabilize at the current levels in the near future.

Using numerical models, we evaluate hydrogeological regime changes in high‐latitude river basins under conditions of ground surface warming. These models describe transient heat‐ and fluid flow coupled to the hydrogeological impacts of phase‐changes from ice to liquid water. We consider an idealized unconsolidated sedimentary aquifer system in which groundwater flow is driven by topography, representing a series of small drainage basins in riverine terrain of relatively subdued topography. Various temporal and spatial surface temperature conditions are considered to control the initial permafrost distributions for the simulations. The simulated rates of increase in groundwater contribution to streamflow during and after permafrost thaw, are in the order of magnitude comparable to hydrogeological regime changes over the past decades as reported for several (sub‐)Arctic rivers. The simulations further show that two distinct features of the subsurface response control the temporal evolution of base flow increase: (1) shifts in aquifer permeability architecture during permafrost degradation and (2) uptake of water into aquifer storage when sub‐permafrost hydraulic heads rise. Model analysis shows that the latter process delays base flow increase by several decades to centuries. In order to evaluate the relative importance of both processes in natural systems, the current hydraulic regime of sub‐permafrost aquifer systems as well as patterns of permafrost heterogeneity, taliks and their hydraulic connectivity are insufficiently known.