Model setup

Model setup for fracture flow and transport in limestone aquifers - steps

This chapter gives an overview of the recommended steps to setup of a model for the simulation of flow and transport in a fractured limestone aquifer. The following list shows the typical steps to setup a model for contaminant transport in a fractured limestone aquifer. Expand an item (button on the right) to get more information about it.

Formulation of the modeling objectives

Before setting up a model, it is important to define the modeling objectives. The choice of the model concept, model extent, modeling scale and included details should be closely linked to these objectives. One should always aim at including the most relevant features in the model, while keeping it as simple as possible.

The following list gives some examples for modeling objectives:

• Analyze the distribution and potential spreading of a contaminant in an aquifer
• Improve the understanding and predictability of contaminant spreading
• Analyze the influence of transient hydraulic conditions (annual variations, pumping in the area) on plume propagation
• Develop and optimize a remediation strategy for the source zone
• Optimize a remediation strategy for the plume
• Improve risk assessment, e.g. for a drinking water well, in order to assess future actions
• Delineate the capture zone of a well

Definition of the model scope

The scale is also depending on the modeling objectives. The model scope will, for example, be different for the prediction of the spreading of an entire plume (large scale) and for the planning of source zone remedial actions (local scale). An important aspect when choosing the model extent is to have the boundaries sufficiently far away from the most influential features in the area, such as pumping wells. The model extent has to be big enough, so that the boundaries do not influence the results in the area of interest. It should be chosen based on physically meaningful boundaries (f.e. known head isolines or no-flow boundaries).

Source zone remediation needs a different scale than the risk assessment of a contaminant plume for water works. The following scales can be considered:

• well scale
• source zone scale
• intermediate scale for e.g. pumping tests
• plume scale

Based on available data and modeling objectives, the model complexity has to be chosen. A very complex and detailed model is not appropriate if only little field data is available. Then, a simple model can be applied, which can be refined as soon as new data is measured.

Modeling was an integral part in the limestone project. Initially, a rough model based on first measurements and available data was setup and used for the planning of further field work and measurements. These measurements were later on used to improve the model quality.

Conceptualization and setup of a conceptual model including geology and hydrogeology

Geologic modeling

Borehole data, outcrops, geophysical measurements etc. can give valuable information about the geology at a site. Bits of geologic knowledge can be connected to establish a geologic model, that shows different geologic layers. Typically, these layers are characterized by different hydrogeologic properties. The geometry can be stored as a CAD interpolation of the surfaces, that delineate the geologic layers. Tools like GeoScene3D can be very useful to store borehole data and to create interpolation surfaces. They can usually be imported into the model later on.

Data acquisition - measurements to obtain relevant model parameters (see list of parameters for each model)

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Implementation of parameters for selected units in the model domain (homogeneous/heterogeneous)

Based on available data, parameter distributions in the model can be defined. For a flow simulation, the hydraulic conductivity (or permeability) and the porosity have to be specified.

Choice of boundary conditions and sources/sinks

Boundary conditions have to be chosen according to known values, like a constant head or a known inflow.

For transient models: definition of initial conditions

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Mesh generation

When working with complex models it is useful to start with a coarse mesh to test the model and the setup with limited time effort. For the final simulations, a finer grid can be employed. Modern grid generators allow a mesh refinement at specific parts of the mesh. Especially at heterogeneities and fractures, at wells and at concentration fronts, the mesh should be sufficiently fine to resolve the local gradients (e.g. of concentration or heads) appropriately. This can be tested by a grid refinement study, where the mesh is refined and the results are compared. When the solution does not change with a further grid refinement, the grid resolution is sufficient.

Simulation

After setting up the geometry, boundary conditions, initial conditions, material parameters and simulation parameters (simulation time, solver settings), the actual simulation can be run. It is always a good idea to start with a test run, f.e. with a coarse mesh and a short duration, to test if everything is set as desired.

Critical evaluation of the modeling results

Modeling results should be always critically evaluated and tested. It is, for example, helpful, to have a check the mass balance of a model. It is also important to visually inspect the results, by . f.e. visualizing the hydraulic heads and concentrations in the domain and in special areas of interest. Then, it can be checked, if the results look as expected, if the boundary conditions are fulfilled and if there are any disturbances like oscillations in the model domain. Oscillations can be an indication for a too coarse mesh.

Model calibration and validation

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Model reporting

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Flow chart giving an overview of the individual steps

ADD FLOW CHART

Example: Setup of models for a contaminated site with a fractured limestone aquifer (Akacievej, Hedehusene)

The setup of a discrete-fracture model in 2D in COMSOL Multiphysics is described in the following document:

• Instructions for setting up a discrete-fracture model in COMSOL Multiphysics (PDF)

The typical workflow for modeling a contaminated site will be demonstrated using an example field site close to Copenhagen.

Example: Setup of models for a field site (Akacievej, Hedehusene)

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