Modeling of karst aquifer genesis: Influence of exchange flow

Abstract. 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 distributed 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 possible not only to derive a consistent flow and transport model for a 150 km 2 karst area but also to combine the use of groundwater flow and transport parameters thereby greatly reducing 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 .

Section: S Oehlmann Et Almentioning

confidence: 99%

Reducing the ambiguity of karst aquifer models by pattern matching of flow and transport on catchment scale

Oehlmann¹,

Geyer

Licha³

et al. 2015

“…This approach was further developed as Conduit Flow Process Mode 1 (CFPM1) for MODFLOW-2005(Shoemaker et al, 2008. This discretecontinuum model approach was applied in a number of modeling studies presented in scientific literature (e.g., Liedl et al, 2003;Bauer et al, 2003;Hill et al, 2010). However, discrete-continuum models based on the Liedl et al (2003) approach simulate conduit flow as quasi-steady without drainable storage.…”

Section: T Reimann Et Al: Representation Of Water Abstractionmentioning

confidence: 99%

Representation of water abstraction from a karst conduit with numerical discrete-continuum models

Reimann

Giese²,

Geyer³

et al. 2014

Abstract. Karst aquifers are characterized by highly conductive conduit flow paths embedded in a less conductive fissured and fractured matrix, resulting in strong permeability contrasts with structured heterogeneity and anisotropy. Groundwater storage occurs predominantly in the fissured matrix. Hence, most mathematical karst models assume quasi-steady-state flow in conduits neglecting conduitassociated drainable storage (CADS). The concept of CADS considers storage volumes, where karst water is not part of the active flow system but hydraulically connected to conduits (for example karstic voids and large fractures). The disregard of conduit storage can be inappropriate when direct water abstraction from karst conduits occurs, e.g., largescale pumping. In such cases, CADS may be relevant. Furthermore, the typical fixed-head boundary condition at the karst outlet can be inadequate for water abstraction scenarios because unhampered water inflow is possible.The objective of this work is to analyze the significance of CADS and flow-limited boundary conditions on the hydraulic behavior of karst aquifers in water abstraction scenarios. To this end, the numerical discrete-continuum model MODFLOW-2005 Conduit Flow Process Mode 1 (CFPM1) is enhanced to account for CADS. Additionally, a fixed-head limited-flow (FHLQ) boundary condition is added that limits inflow from constant head boundaries to a user-defined threshold. The effects and the proper functioning of these modifications are demonstrated by simplified model studies. Both enhancements, CADS and FHLQ boundary, are shown to be useful for water abstraction scenarios within karst aquifers. An idealized representation of a large-scale pumping test in a karst conduit is used to demonstrate that the enhanced CFPM1 is able to adequately represent water abstraction processes in both the conduits and the matrix of real karst systems, as illustrated by its application to the Cent Fonts karst system.

“…Karst genesis simulation would provide process-based information about conduit widening towards a karst spring. Such simulations were employed for instance by Kaufmann and Braun (1999), Liedl et al (2003), Bauer et al (2003), and Hubinger and Birk (2011). They simulate the temporal evolution of a small fracture or fracture network due to solution with coupled transport and hydraulic models.…”

Section: Discussionmentioning

confidence: 99%

Influence of aquifer heterogeneity on karst hydraulics and catchment delineation employing distributive modeling approaches

Oehlmann¹,

Geyer²,

Licha³

et al. 2013

Abstract. Due to their heterogeneous nature, karst aquifers pose a major challenge for hydrogeological investigations. Important procedures like the delineation of catchment areas for springs are hindered by the unknown locations and hydraulic properties of highly conductive karstic zones.In this work numerical modeling was employed as a tool in delineating catchment areas of several springs within a karst area in southwestern Germany. For this purpose, different distributive modeling approaches were implemented in the finite element simulation software Comsol Multiphysics ® . The investigation focuses on the question to which degree the effect of karstification has to be taken into account for accurately simulating the hydraulic head distribution and the observed spring discharges.The results reveal that the representation of heterogeneities has a large influence on the delineation of the catchment areas. Not only the location of highly conductive elements but also their geometries play a major role for the resulting hydraulic head distribution and thus for catchment area delineation. The size distribution of the karst conduits derived from the numerical models agrees with knowledge from karst genesis. It was thus shown that numerical modeling is a useful tool for catchment delineation in karst aquifers based on results from different field observations.