I am using a large transient flow and transport model to model remediation scenarios with a set of pumping wells. I have tried to both use a well BC with a minimum head limit and an hydraulic head BC.
The steady state extraction flux I am using was recommended by a client. However it is way too high for the aquifer and the well BC turns to an Hydraulic head BC after the first time step.
So basically, after the first time step (day 0.01 out of 100 year) both boundary conditions are identical: An Hydraulic head BC set at the same value, at the same nodes, with no constraints (I have exported the data to check in excel).
However the results differ for each model. The model which started as a well BC achieve a much better contaminant removal. When looking at the fluid rate budget for each well (or HH BC) I can see that the model which started with Well BC is achieving a slightly higher "pumping rate" (eg for one of the well: 0.0210 l/s VS 0.0249 l/s).
My questions are:
1. Why is there such difference for the exact same model?
2. Is one of these BC setup more reliable than the other?