Groundwater flow and contaminant transport modelling

GSL has access to a range of software - such as FEFLOW® and MODFLOW - for the solution of groundwater flow and contaminant transport problems. An example is our recent use of FEFLOW® in local-scale groundwater flow modelling to improve the understanding of the near-surface hydrogeology in the area in which it is proposed to construct new facilities for low-level radioactive waste (LLW) disposal at Dounreay. This project is being led by Dr. Dev Reedha.

The surface topography of the Dounreay area (Figure 1) was derived from detailed survey data, Ordnance Survey (OS) 10m Digital Terrain Model data and digitised elevation contours from the 1:10,000 OS survey data.

Figure 1: Surface topography derived for the local-scale hydrogeological model

The thicknesses of the materials making up the near-surface zone were derived from an interpretation of geophysical survey data, and, together with pre-existing data from boreholes and trenches, were used to generate surfaces for the boundaries between the different geophysical units. These maps were then used to generate a 3D finite-element grid in FEFLOW® (Figure 2).

Figure 2: 3D projection of the finite-element mesh used for the local-scale hydrogeological model

Following the specification of boundary conditions, initial conditions and hydrogeological parameters, an initial Base Case model was calibrated using measured groundwater heads and observations of water flows in the study area. An uncertainty analysis was carried out to further constrain / inform some of the uncertainties in the broad conceptual model underpinning the initial Base Case, and to explore the response of the model to changing one or more of the input parameters.

A number of figures were generated, such as plots of groundwater head distributions, recharge and runoff/interflow/discharge locations, and depths to water table across the model domain to help assess and interpret the modelling results. In addition, groundwater heads were calculated and compared against measurements, and mass balances were computed at reference locations in the model domain. Figure 3 shows an example of the groundwater head distribution computed by the 3D local-scale hydrogeological model, shown for a cross-section of the model for illustration.

Figure 3: Groundwater head distribution computed by the 3D local-scale hydrogeological model, shown for a cross-section of the model for illustration

FEFLOW® is a state-of-the-art finite-element package for simulating steady-state and transient groundwater flow and transport processes in both partially-saturated and fully-saturated systems. Models can be developed in 2D or 3D using the finite-element method, which is more appropriate than the finite-difference method for modelling flow and mass transport in complicated geological and hydrogeological environments. FEFLOW® provides robust numerical algorithms, interactive graphics and data visualisation tools. A major feature is its powerful display and analysis ability, including 3D visualisation and animation of models and modelling results.