Implementierung der impliziten geologischen 3D-Modellierung in die hydrogeologische Systemanalyse am Beispiel des Hastenrather Grabens - Aachen

  • Implementation of implicit 3D geological modeling into the conceptual modeling workflow - A hydrogeological study of the Hastenrather Graben near Aachen

Burs, David Noel; Rüde, Thomas R. (Thesis advisor); Clauser, Christoph (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019


The main goal of this work is the implementation of a structural three-dimensional (3D) model into the workflow of hydrogeological modeling. Therefore, the Hastenrather Graben near Aachen, Germany serves as study area. Numerical flow simulations are implemented to yield an improved understanding of the fractured and partly karstified limestone Kohlenkalk aquifer in the study area. The Hastenrather Graben drinking water catchment, 15 km east of Aachen, comprises the southern part of the Hastenrather Graben which is part of a NNW-SSE trending horst and graben system. It is located at the transition of the Rhenish Massif in the south and the Lower Rhine Embayment in the north. This region experienced various folding phases and is characterized by complex geological conditions. Thus, Tertiary and Quaternary sediments unconformably overlie the hard-rocks of the folded Variscan basement of Devonian and Carboniferous rocks in some parts. The Hastenrather Graben is limited by the Sandgewand fault towards SW and by the Omerbach fault in the NE. In the southeastern part of the graben NE-SW striking thrusts exist, revealing relicts of allochthonous nappes. A coring campaign, combined with available maps, additional literature research, and pumping tests served as basis for the setup of the 3D model, as well as for the development of the hydrogeological conceptual model, and the parametrization of the numerical model. Thereby, the driving cores confirm and improve the spatial distribution of the nappes. Hydraulic conductivity of the aquifer system varies between 6.0*10^-7 m/s and 6.4*10^-4 m/s with a decreasing trend from the NE graben shoulder to the graben center. The process of 3D geological modeling comprises integrating available geological information using an implicit modeling approach. In the study area, the subsurface is only slightly known in some parts while other parts are well explored because of mining or water extraction. The implicit approach interpolates the geometry of each geological and tectonic element. Instead of defining surfaces by digitizing their shape explicitly, the potential-field is set up by co-kriging of available data on geological interface and orientation. The 3D model comprises all important structural elements of the Hastenrather Graben, including the fault system, deep folded structures, and thrusts in the graben center. It shows vertical throws of the Kohlenkalk at the graben normal faults up to 160 m. The Kohlenkalk formation makes up 30 % (2.9*10^9 m³) of the model volume. Combined with information on effective porosity, the calculation of the volume allows to estimate available groundwater resources. The 3D model helps to assess and evaluate former geological and tectonic conditions. Areas which are dominated by block folding processes are analyzed as an example.Integration of geological structure and hydraulic studies yielded a conceptual model of the hydrogeo-logy, describing the Kohlenkalk aquifer and neighboring layers. In reference to the conceptual model the 3D implicit geological modeling approach allows for calculating groundwater flow across lateral cross-sections at any location of the model. Data on groundwater recharge as well as anthropogenic influences on the groundwater resources are part of the conceptual model. The Hastenrather Graben catchment area consist of two different aquifers, a shallow sedimentary one and the hard-rock Kohlenkalk aquifer. The Kohlenkalk aquifer is limited by the thick Walhorn and Stolberg layer as well as the Condroz sandstone. These formations show lower hydraulic conductivity but are assumed to contribute to the flow regime by leakage effects especially in fractured parts and zones of rock disintegration. Between the shallow aquifer and the Kohlenkalk aquifer a hydraulic connection is assumed in the northern part of the graben. The reason for this is a varying spatial distribution of separating clay layers. Analysis of piezometric heads and resulting groundwater contour maps indicated four hydrogeological units with regards to the Kohlenkalk aquifer. These units are partly limited by faults.By setting up a numerical model of the Hastenrather Graben it is possible to evaluate the conceptual model by means of different scenarios. The 3D model serves as basis for the spatial discretization of the hydrostratigraphic units. Flow simulations confirm the basic assumptions of the conceptual model with and without water withdrawal on the graben shoulder. The simulated water inflow and outflow at the boundaries of the model vary in the predicted ranges which were estimated during the conceptual modeling. Assumptions of the conceptual model relating to the Sandgewand fault are validated by the model. The Sandgewand fault is, at least partly, hydraulically active. The amount of water which is used for drinking water production and mining activities on the graben shoulders is relatively small in comparison to the entire volume of the Kohlenkalk. In conclusion, the main advantages of implementing the 3D modeling process into conceptual and numerical modeling are the various possibilities of visualizing and analyzing the spatial distribution of hydrostratigraphic units in the subsurface, the calculation of their volume and therefore available groundwater resources, the calculation of groundwater flow across lateral cross-sections at any location of the model, and the simplified discretization and parametrization of the numerical model.