[1] Hydraulic tomography is a method to identify the three-dimensional spatial distribution of hydraulic properties in the subsurface. We propose a tomographic approach providing the inversion of travel times of hydraulic or pneumatic tests conducted in a tomographic array. The inversion is based on the relation between the peak time of a recorded transient pressure curve and the diffusivity of the investigated system. The development of a transformation factor enables the inversion of further travel times besides the peak time of a transient curve. As the early travel times of the curve are mainly related to preferential flow features while the inversion based on late travel times are reflecting an integral behavior, it can be assumed that the different inversion results reflect the properties of the overall system. By comparing the different reconstructions the system interpretation therefore becomes more comprehensive and reliable. Furthermore, the similarity of the proposed hydraulic tomographic approach to seismic travel time tomography allows us to apply the inversion algorithms which are used for seismic tomography. We therefore apply the method of staggered grids, which enables to refine the grid resulting in a higher nominal resolution, to data from a set of interference tests arranged in a tomographic array. The tests were conducted in an unsaturated fractured sandstone cylinder in the laboratory. The three-dimensional reconstructions of the diffusivity distribution are found to be highly reliable and robust. In particular, the mapped fracture of the sandstone cylinder coincides with our reconstructed diffusivity distribution.

[1] This paper is intended to provide insight into the controlling mechanisms of karst genesis based on an advanced modeling approach covering the characteristic hydraulics in karst systems, the dissolution kinetics, and the associated temporal decrease in flow resistance. Karst water hydraulics is strongly governed by the interaction between a highly conductive low storage conduit network and a low-conductive high-storage rock matrix under variable boundary conditions. Only if this coupling of flow mechanisms is considered can an appropriate representation of other relevant processes be achieved, e.g., carbonate dissolution, transport of dissolved solids, and limited groundwater recharge. Here a parameter study performed with the numerical model Carbonate Aquifer Void Evolution (CAVE) is presented, which allows the simulation of the genesis of karst aquifers during geologic time periods. CAVE integrates several important features relevant for different scenarios of karst evolution: (1) the complex hydraulic interplay between flow in the karst conduits and in the small fissures of the rock matrix, (2) laminar as well as turbulent flow conditions, (3) time-dependent and nonuniform recharge to both flow systems, (4) the widening of the conduits accounting for appropriate physicochemical relationships governing calcite dissolution kinetics. This is achieved by predefining an initial network of karst conduits (''protoconduits'') which are allowed to grow according to the amount of aggressive water available due to hydraulic boundary conditions. The increase in conduit transmissivity is associated with an increase in conduit diameters while the conductivity of the fissured system is assumed to be constant in time. The importance of various parameters controlling karst genesis is demonstrated in a parameter study covering the recharge distribution, the upgradient boundary conditions for the conduit system, and the hydraulic coupling between the conduit network and the rock matrix. In particular, it is shown that conduit diameters increase in downgradient or upgradient direction depending on the spatial distribution (local versus uniform) of the recharge component which directly enters the conduit system.INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 1894 Hydrology: Instruments and techniques; KEYWORDS: karst hydrology, aquifer evolution, groundwater flow, pipe flow, calcite dissolution kinetics, numerical modeling Citation: Liedl, R., M. Sauter, D. Hückinghaus, T. Clemens, and G. Teutsch, Simulation of the development of karst aquifers using a coupled continuum pipe flow model, Water Resour.

A tracer test in a carbonate aquifer is analyzed using the method of moments and two analytical advection-dispersion models (ADMs) as well as a numerical model. The numerical model is a coupled continuum-pipe flow and transport model that accounts for two different flow components in karstified carbonate aquifers, i.e., rapid and often turbulent conduit flow and Darcian flow in the fissured porous rock. All techniques employed provide reasonable fits to the tracer breakthrough curve (TBC) measured at a spring. The resulting parameter estimates are compared to investigate how each conceptual model of flow and transport processes that forms the basis of the analyses affects the interpretation of the tracer test. Numerical modeling results suggest that the method of moments and the analytical ADMs tend to overestimate the conduit volume because part of the water discharged at the spring is wrongly attributed to the conduit system if flow in the fissured porous rock is ignored. In addition, numerical modeling suggests that mixing of the two flow components accounts for part of the dispersion apparent in the measured TBC, while the remaining part can be attributed to Taylor dispersion. These processes, however, cannot reasonably explain the tail of the TBC. Instead, retention in immobile-fluid regions as included in a nonequilibrium ADM provides a possible explanation.

[1] This paper presents a numerical model study simulating the early karstification of a single conduit embedded in a fissured system. A hybrid continuum-discrete pipe flow model (CAVE) is used for the modeling. The effects of coupling of the two flow systems on type and duration of early karstification are studied for different boundary conditions. Assuming fixed head boundaries at both ends of the conduit, coupling of the two flow systems via exchange flow between the conduit and the fissured system leads to an enhanced evolution of the conduit. This effect is valid over a wide range of initial conduit diameters, and karstification is accelerated by a factor of about 100 as compared to the case of no exchange flow. Parameter studies reveal the influence of the exchange coefficient and of the hydraulic conductivity of the fissured system on the development time for the conduit. In a second scenario the upstream fixed head boundary is switched to a fixed flow boundary at a specified flow rate during the evolution, limiting the amount of water draining toward the evolving conduit. Depending on the flow rate specified, conduit evolution may be slowed down or greatly impaired if exchange flow is considered.INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 1829 Hydrology: Groundwater hydrology; 1899 Hydrology: General or miscellaneous; KEYWORDS: karst hydrology, numerical modeling, aquifer evolution, conduit system, fissured system Citation: Bauer, S., R. Liedl, and M. Sauter, Modeling of karst aquifer genesis: Influence of exchange flow, Water Resour.

[1] The finite maximum length of a steady state contaminant plume is determined by developing and employing a new analytical solution which overcomes two drawbacks associated with existing approaches. First, we account for a sharp front caused by the complete consumption of the pollutant (''electron donor'') and some electron acceptor in an instantaneous binary reaction occurring at the front. This approach is not based on purely conservative or first-order degradation models which lead to theoretically infinite plumes and, in addition, depend on a concentration threshold. Second, a vertical aquifer cross section with finite thickness is selected as a model in order to better represent the supply of electron acceptors mostly entering the aquifer from the top. This type of setting allows investigation of the impact of aquifer thickness on plume length. An implicit representation of the donor-acceptor front in a finite vertical domain previously required numerical solutions of the underlying advection-dispersion-reaction equation; we provide for the first time an analytical solution of this two-dimensional transport problem. The length of the plume is found to be given by the point of intersection of the donor-acceptor front and the aquifer bottom. Furthermore, a rather simple and highly accurate approximation is derived to compute the steady state plume length. A comprehensive sensitivity analysis reveals that results are most strongly influenced by aquifer thickness, followed by vertical transverse dispersivity and, to a somewhat lesser extent, by chemical reaction parameters. Longitudinal dispersivity has practically no effect on plume length, and furthermore, there is zero impact of linear velocity. With regard to groundwater risk assessment at the field scale it is also important to note that the present approach is meant to provide an upper bound on the actual plume length. Further research activities may be directed to refine the transport model by considering, for instance, degradation inside the plume and the limited vertical extent of the contaminant source.