June 23, 2010
The ability to represent wells in a regional three-dimensional numerical groundwater model is crucial for many modeling exercises. Such wells can either be measurement wells where groundwater levels are measured as a function of time, or wells can be pumping wells in which water is removed from the well and a drawdown cone is created in the water table surrounding the well.
There are numerical difficulties surrounding the representation of such wells. For example, one particular site may contain hundreds of wells. One methodology is to represent each well as a cylindrical object in a 3D model. This approach is typically problematic when there are many wells as the nodal requirements for representing such geometric objects become excessive. Therefore the industry has typically resorted to representing pumping wells as single nodes or a line of vertical nodes in a numerical model. Pumping gradients are then applied to these single or linear nodes as head or flux conditions. Although this methodology greatly reduces the nodal requirements of regional models to the point that they can now be solved in reasonable time, there is a downside. The downside is that extreme gradients may be created next to wells in which pumping occurs. These gradients can easily create numerical instabilities in a model and can potentially invalidate the results. One way to handle these potential numerical instabilities is through mesh refinement next to pumping wells. However, SVFlux™ remains the only commercial package which offers fully automatic mesh refinement.
In order to overcome the numerical difficulties surrounding well objects, SoilVision Systems Ltd. has recently added a wells object which may be added to any 2D or 3D SVFlux™ numerical groundwater model. The wells object applies an exponential decay function to the zone surrounding the well screens in the numerical model. The well screen itself is represented simply by a line of nodes. This approach numerically smooths the sharp gradients normally encountered with internal pumping wells and greatly stabilizes the use of wells on complex 3D numerical models.
In benchmarking and trials of the new system, it was found that it is possible that high gradients are created in the zones surrounding a well object. The user can somewhat control how steep these gradients are by defining a zone of influence in the numerical model. Despite this, it was noted during trials of the new system that mesh refinement was triggered in the numerical model due to the high gradients. Therefore it can be seen that the mesh refinement in SVFlux™ still plays an important role in the successful solution of 3D regional models where pumping wells are utilized.
Well objects can be applied to any 2D or 3D groundwater model under the Boundaries > Wells Manager menu option, shown here:
Wells Manager menu
The new well object was extensively benchmarked against classic examples. Benchmarking examples may be found in the SVFlux™ verification manual.
Examples of the use of the new well object may be seen under the RegionalFlow and WellPumping projects located in the distribution models. Our distributed models are included with our free download of SVOffice™ 2009. Users are welcome and encouraged to open and examine these models to see the potential applications to their situation. The figures below illustrates some of the numerical models in which the well object was applied.
RegionalFlow > Borehole_3D
RegionalFlow > Boreholes_3D
RegionalFlow > AquiferExtractionNearConstantSource
Results from the aquifer model shown above
Results from an aquifer extraction model