Application of a solute transport model under variable velocity conditions in a conduit flow aquifer: Olalde karst system, Basque Country, Spain (original) (raw)
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Hydrological Processes, 2012
A structure model was used to analyse solute-transport parameter estimates based on tracer breakthrough curves. In the model system, groundwater flow is envisioned to be organised in a complex conduit network providing a variety of short circuits with relative small carrying capacities along different erosion levels. The discharge through the fully filled conduits is limited owing to void geometries and turbulent flow; thus, a hierarchic overflow system evolves where conduits are (re-)activated or dried up depending on the flow condition. Exemplified on the Lurbach-Tanneben karst aquifer, the applicability of the model approach was tested. Information derived from multi-tracer experiments performed at different volumetric flow rates enabled to develop a structural model of the karst network, under constraint of the geomorphological and hydrological evolution of the site. Depending on the flow rate, groundwater is divided into up to eight flow paths. The spatial hierarchy of flow paths controls the sequence of flow path activation. Conduits of the topmost level are strongly influenced by reversible alteration processes. Sedimentation or blocking causes an overflow of water to the next higher conduit. Flow path specific dissolutional denudation rates were estimated using the temporal development of the partial discharge.
Journal of Hydrology, 2016
This paper presents the modelling results of several tracer-tests performed in the cave system of Han-sur-Lesse (South Belgium). In Han-sur-Lesse, solute flows along accessible underground river stretches and through flooded areas that are rather unknown in terms of geometry. This paper focus on the impact of those flooded areas on solute transport and their dimensioning. The program used (Onedimensional Transport with Inflow and Storage: OTIS) is based on the two-region non equilibrium model that supposes the existence of an immobile water zone along the main flow zone in which solute can be caught. The simulations aim to replicate experimental breakthrough curves (BTCs) by adapting the main transport and geometric parameters that govern solute transport in karst conduits. Furthermore, OTIS allows a discretization of the investigated system, which is particularly interesting in systems presenting heterogeneous geometries. Simulation results show that transient storage is a major process in flooded areas and that the crossing of these has a major effect on the BTCs shape. This influence is however rather complex and very dependent of the flooded areas geometry and transport parameters. Sensibility tests performed in this paper aim to validate the model and show the impact of the parametrization on the BTCs shape. Those tests demonstrate that transient storage is not necessarily transformed in retardation. Indeed, significant tailing effect is only observed in specific conditions (depending on the system geometry and/or the flow) that allow residence time in the storage area to be longer than restitution time. This study ends with a comparison of solute transport in river stretches and in flooded areas.
Hydrogeology Journal
Artificial tracer experiments were conducted in the mature karst system of Jeita (Lebanon) under various flow conditions using surface and subsurface tracer injection points, to determine the variation of transport parameters (attenuation of peak concentration, velocity, transit times, dispersivity, and proportion of immobile and mobile regions) along fast and slow flow pathways. Tracer breakthrough curves (TBCs) observed at the karst spring were interpreted using a two-region nonequilibrium approach (2RNEM) to account for the skewness in the TBCs' long tailings. The conduit test results revealed a discharge threshold in the system dynamics, beyond which the transport parameters vary significantly. The polynomial relationship between transport velocity and discharge can be related to the variation of the conduit's crosssectional area. Longitudinal dispersivity in the conduit system is not a constant value (α = 7-10 m) and decreases linearly with increasing flow rate because of dilution effects. Additionally, the proportion of immobile regions (arising from conduit irregularities) increases with decreasing water level in the conduit system. From tracer tests with injection at the surface, longitudinal dispersivity values are found to be large (8-27 m). The tailing observed in some TBCs is generated in the unsaturated zone before the tracer actually arrives at the major subsurface conduit draining the system. This work allows the estimation and prediction of the key transport parameters in karst aquifers. It shows that these parameters vary with time and flow dynamics, and they reflect the geometry of the flow pathway and the origin of infiltrating (potentially contaminated) recharge.
Hydrology and Earth System Sciences, 2015
Assessing the hydraulic parameters of karst aquifers is a challenge due to their high degree of heterogeneity. The unknown parameter field generally leads to a high ambiguity for flow and transport calibration in numerical models of karst aquifers. In this study, a distributive numerical model was built for the simulation of groundwater flow and solute transport in a highly heterogeneous karst aquifer in south western Germany. Therefore, an interface for the simulation of solute transport in one-dimensional pipes was implemented into the software Comsol Multiphysics ® and coupled to the three-dimensional solute transport interface for continuum domains. For reducing model ambiguity, the simulation was matched for steady-state conditions to the hydraulic head distribution in the model area, the spring discharge of several springs and the transport velocities of two tracer tests. Furthermore, other measured parameters such as the hydraulic conductivity of the fissured matrix and the maximal karst conduit volume were available for model calibration. Parameter studies were performed for several karst conduit geometries to analyse the influence of the respective geometric and hydraulic parameters and develop a calibration approach in a large-scale heterogeneous karst system. Results show that it is not only possible to derive a consistent flow and transport model for a 150 km 2 karst area, but that the combined use of groundwater flow and transport parameters greatly reduces model ambiguity. The approach provides basic information about the conduit network not accessible for direct geometric measurements. The conduit network volume for the main karst spring in the study area could be narrowed down to approximately 100 000 m 3 .
Modeling of karst aquifer genesis: Influence of exchange flow
Water Resources Research, 2003
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.
Journal of Hydrology, 2010
Karst aquifers are heterogeneous media where conduits usually drain water from lower permeability volumes (matrix and fractures). For more than a century, various approaches have used flood recession curves, which integrate all hydrodynamic processes in a karst aquifer, to infer physical properties of the movement and storage of groundwater. These investigations typically only consider flow to the conduits and thus have lacked quantitative observations of how pressure transfer and water exchange between matrix and conduit during flooding could influence recession curves.
Dual-Flow Phenomena and Longitudinal Dispersivity Interpretation in a Karstic Aquifer
Sinkholes and the Engineering and Environmental Impacts of Karst, 2003
In situ tracer tests constitute one of the best means to study flow dynamics and transport properties of karst aquifers. In this paper, we attempted to interpret karst-conduit flow and transport parameters by using two different models: the linear graphical method and the Qtracer2 program. The latter program permits us to quantify the karst-conduit parameters (morphological type, flow patterns). The linear graphical method allows us to account for two elementary curves (two-layers flow model) that fit the entire breakthrough curve, notably when strong tailing effects occur. The flow parameters previously calculated by Qtracer2 show that flow in the conduit is dual, with a turbulent-core flow and a pseudo-laminar flow against the wall of the conduit within the so-called viscous sublayer. The lower the flow-rate, the higher the viscous sublayer. This latter result is in agreement with the decrease of the second elementary curve contribution to the total area of the breakthrough curve with increasing flow-rate. We agree that partitioning of flow would also induce a partitioning of solute transport (via the turbulent flow and the viscous sublayer). From a geomorphological view, the karst hydrosystem studied appears to be well established around a main conduit that carries the most important part of the flow.
Prediction of Flow in a Karstic Aquifer, Case Yumagual, Cajamarca, Peru
38th IAHR World Congress - "Water: Connecting the World", 2019
The study describes the variables to establish the conceptual hydrogeological model and define the hydrogeological units, the recharge zones, the discharge zones, the direction of flow, the piezometric behavior, among others. The hydrogeological numerical model was developed in steady state (natural conditions) and transitory regime. The error percentage in the modeling is 6%, which is considered acceptable when dealing with such heterogeneous media. The permeability data were obtained from the permeability and pumping tests carried out in the area, obtaining the following ranges:
Solute transport in solution conduits exhibiting multi-peaked breakthrough curves
Journal of Hydrology, 2012
Solute transport in karst aquifers is primarily constrained to solution conduits where transport is rapid, turbulent, and relatively unrestrictive. Breakthrough curves generated from tracer tests are typically positively-skewed and may exhibit multiple peaks. In order to understand the circumstances under which multi-peaked positively skewed breakthrough curves occur, physical experiments utilizing singleand multiple-flow channels were conducted. Experiments also included waterfalls, short-term solute detention in pools, and flow obstructions. Results demonstrated that breakthrough curve skewness nearly always occurs to some degree but is magnified as immobile-flow regions are encountered. Multi-peaked breakthrough curves occurred when flow in the main channel became partially occluded from blockage in the main channel that forced divergence of solute into auxiliary channels and when waterfalls and detention in pools occurred. Currently, multi-peaked breakthrough curves are fitted by a multi-dispersion model in which a series of curves generated by the advection-dispersion equation are fitted to each measured peak by superimposing the measured breakthrough curve to obtain a combined model fit with a consequent set of estimated velocities and dispersions. In this paper, a dual-advection dispersion equation with first-order mass transfer between conduits was derived. The dual-advection dispersion equation was then applied to the multi-peaked breakthrough curves obtained from the physical experiments in order to obtain some insight into the operative solute-transport processes through the acquisition of a consequent set of velocities, dispersions, and related parameters. Successful application of the dual-advection, dispersion equation to a tracer test that exhibited dual peaks for a karst aquifer known to consist of two connected but mostly separate conduits confirmed the appropriateness of using a multi-dispersion type model when conditions warrant.