FEFLOW’s boundary constraints are a robust and efficient measure to establish maximum pumping rates in well boundaries and avoid that unrealistic pumping volumes are extracted from the model. These constraints act on the well boundaries by switching the boundary type from prescribed flux to prescribed head, with boundary values equating to the value of the constraint.
In situations where the groundwater level moves below the constraint level due to different mechanisms (for example, open pit and tunnel boundaries), the prescribed head boundary implemented by the boundary constraint will provide water into the aquifer, which may not be desired in the simulation.
A FEFLOW plugin have been developed to improve the simulation of well boundaries in these situations. A maximum pumping rate for each well is calculated prior to the time-step execution, using a Theis-forward solution. This solution utilises the transmissivity and storage terms defined by the user, and the time-step length. If the prescribed pumping rate is higher than the maximum pumping rate estimated by the forward solution, the boundary value is adjusted to the maximum rate.
The plugin has been tested in synthetic and real-world cases in open pit mines. Preliminary results suggest that the plugin works efficiently and helps with the model stability, since it eliminates the need for boundary constraint checks. Nevertheless, it may not be ideal in situations where multiple boreholes are located close to each other, as the Theis-forward solution does not account for borehole interference.