Antti Pasanen & Kimmo Hentinen,Geological Survey of Finland, P.O. Box 1237, FI-70211 KUOPIO, FINLAND, antti.pasanen[at]gtk.fi, kimmo.hentinen[at]gtk.fi
FEFLOW is an integrated software package for modelling fluid flow, groundwater age and transport of dissolved material in groundwater. It uses Finite Element Method (FEM) technique in discretization of the continuous domain modelling area into set of discrete sub-domains or elements. The division to smaller elements allows accurate representation of complex geometry, inclusion of dissimilar material properties, easy representation of the total solution and capture of local effects.
In FEFLOW, the complex geological model of the hydrogeological characteristics is preferably imported to produce the geometry of the modelling area. A simple geometry can also be produced in FEFLOW. FEFLOW is able to model two-dimensional and three-dimensional geometries with one to several layers. The geometry is then meshed using the provided meshing tools to create the element network used in calculations. The hydrological and hydrogeological parameters, such as hydraulic conductivity, groundwater recharge, hydraulic head etc., are given to appropriate elements or nodes in the mesh. Also, the boundary conditions are set. When the parametrization is satisfactory the model can be solved as steady-state or transient for saturated or partially saturated ground. The solution should be checked for convergence error and statistical reliability.
Description of the method
FEFLOW is a groundwater flow modelling software package that can be used to model the flow and transport of groundwater and dissolved material in subsurface. FEFLOW can use shapefiles exported from geological information systems (GIS) to create the modelling domain. Vertical configuration of the modelled domain may also be modified using data from external file. Model domain and its vertical configuration may also be created and modified only using FEFLOW without any imported data. In addition to the model domain generation FEFLOW has good built-in mesh generators.
In groundwater flow modelling, FEFLOW has the ability to model confined, partially confined or unconfined conditions, it can handle the phreatic surfaces in several different ways, and it allows the use of 1st, 2nd and 3rd kind of boundary conditions and a specific well boundary condition with physical constraints. The material properties, boundary conditions and constraints can be time-dependent, parameters can be defined applying a user-defined equations and the coupling with surface water flow simulation with Mike software (DHI-Wasy, 2014a) .
In solute transport FEFLOW can simulate single-species and reactive multi-species transport in groundwater and unsaturated zone, including sorption and chemical reactions. Wells, fractures and e.g. mine voids can be modelled as so-called Discrete Feature Elements (DHI-Wasy, 2014c). These features are one dimensional points and lines or two dimensional faces. Discrete feature dimension has to be always smaller than the model dimension. Fractures and mine voids may create preferential flow paths which can be potential routes for pollution and affecting the water balance of the mine. FEFLOW is also capable of density-dependent modelling. Density or viscosity can have a significant effect on the groundwater flow e.g. in coastal region or in very deep mines.
The modelling software and methodology is widely in use and is being developed continuously. The evaluation is based on FEFLOW version 6.2.
An example of a case study using FEFLOW modelling from the Luikonlahti mine area in Eastern Finland can be found in http://arkisto.gtk.fi/2013/125_2013.pdf (Pasanen & Backnäs (Eds) 2013).
Groundwater flow modelling gives an essential tool for estimating the effects of mining in groundwater and in reducing risks involved with water in underground workings and in mine water balance and management. In closed mines the modelling can predict groundwater flows and transport of pollutants. Results of the modelling are only as good as the model is. If the research data for the model is limited from the area of interest (e.g. closed mine), the results of the modelling don’t have much value.
FEFLOW includes a programming interface which can be used e.g. for coupling groundwater model with surface water model or any other model. This is an important feature as modelling is used more and more nowadays for multiple purposes. FEFLOW toolbox also includes PEST-parameter estimation and uncertainty analysis tool integrated with FEFLOW. Especially the parameter estimation feature is important for model calibration in groundwater modelling.
One disadvantage of FEFLOW is its ability to handle phreatic surface crossing layer interfaces with layers having significant difference in the hydraulic conductivities. For example in two layer models where soil and bedrock are present, model becomes computationally challenging if the phreatic surface crosses the interface between soil and bedrock. This can happen especially in steep terrains. One way to handle this kind of problem is to use confined setup (Wels et al. 2012). The situation isn’t confined in reality so this modification produces error into the model. Computationally confined problem is linear which makes it much easier to solve than non-linear unconfined case. Increasing residual water depth for unconfined layers may lead to better convergence. Table1 shows some advantages and disadvantages of FEFLOW.
Table 1. Advantages and disadvantages of FEFLOW. Modified from Wels et al. 2012.
DHI-Wasy 2014a. FeFlow, Groundwater Flow. http://www.feflow.com/solutetransport0.html, 20.1.2014
DHI-Wasy 2014b. FeFlow, Solute Transport.http://www.feflow.com/solutetransport.html, 20.1.2014
DHI-Wasy 2014c. FeFlow, Fracture Modeling.http://www.feflow.com/fracture.html?&tx_ttnews[month]=05&tx_ttnews[tt_news]=52, 20.1.2014
Pasanen, A. & Backnäs, S. (Eds); Backnäs, S., Forsman, P., Karlsson, T., Kauppila, P., Kauppila, T., Koikkalainen, K., Kollanus, V., Komulainen, H., Kousa, A., Makkonen, S., Mäkinen, J., Nikkarinen, M., Pasanen, A., Solismaa, L.,Tornivaara, A., Tuomisto, J. & Waissi-Leinonen, G. 2013. MINERA-hankkeen tapaustutkimus: Riskinarviointimenetelmientestaaminen Luikonlahden ja Kylylahden kaivosalueella. Geological Survey of Finland, Archive report 125/2013. 243 p. (in Finnish) http://arkisto.gtk.fi/2013/125_2013.pdf
Wels, C., Mackie D. & Scibek, J. 2012. Guidelines for Groundwater Modelling to Assess Impacts of Proposed Natural Resource Development Activities, Report no. 194001 for the British Columbia Ministry of Environment Water Protection & Sustainability Branch, Robertson GeoConsultants Inc. & SRK Consulting Inc. Canada. 385 pages. http://www.env.gov.bc.ca/wsd/plan_protect_sustain/groundwater/groundwater_modelling_guidelines_final-2012.pdf, 11.2.2015