Groundwater flow is usually the main mechanism for the spreading of contaminants and other solutes. While the groundwater flow is a function of a number of regional factors such as topography, location of main watercourses and controlled water levels, the dispersion of contaminants is mainly a function of local factors, such as soil characteristics and the properties of the dissolved constituent.

In most cases changes in the solute concentration field will not measurably affect the groundwater flow field. This enables the flow model to be independently constructed and calibrated. After which the calculated groundwater flow is used for the actual solute transport modelling.

Solute transport in coupled unsaturated and saturated zone

MD-SAT is a GIS application specifially designed to calculate the quality of shallow groundwater based on landuse and load (for instance nitrogen). De application combines standard and widely used models for quality predictions.

The application uses the module FLUZO for flow in the unsaturated zone. The results can directly be used to calculate groundwater quality in the saturated zone (TRIWACO-FLAIRS, MODFLOW, MICROFEM, FEFLOW, etc.). For instance in combination with the modules TRACE and WELCON+.

Solute transport in saturated zone along pathlines

WELCON+ is used to simulate the change in concentration as a function of time and space, accounting for advection, dispersion, diffusion, retardation and first-order decay.

The module SORWACO calculates nonequilibrium transport and sorption of organic chemicals during aquifer remediation. Processes for which equilibrium is reached quickly as well as processes for which equilibrium is reached only slowly are taken into account.

Solute transport in saturated zone with R/MT3D-MS

Simulation of solute transport with these modules is based on a groundwater flow field of a finite difference model (i.e. MODFLOW). In order to guarantee a sufficient level of detail and not letting the boundary conditions influence the solute transport simulation, the following methodology is usually chosen: Regional flow model, local flow model and localised solute transport.

RT3D and MT3D with triwaco uses a methology which is much more efficient. The user can define an area (telescoping) within the regional groundwater model which the interface translates into a localised groundwater flow field. This groundwater flow field is then directly combined with the localised transport model making the intermediate stage obsolete, making simulation runs taking much less time.

The input for the solute transport model can be created by making maps with initial concentrations in graphical input module DIGEDIT. The output of the model is standard R/MT3D and automatically converted to the Triwaco type parameters set, so presentation and visualisation of results can easily be made in the graphical output module TRIPLOT from which te results directly can be exported to a GIS.