Evaluation of a volume integral over a 2D plane

Dear support,

I need to evaluate the following volume integral over a 2D surface obtained using the threshold filter:

I have already obtained the divergence of f_vw term and the only thing left is to integrate this expression. I have noticed that for another variable like density (rho) (which changes between 0.25 and 0.35), I have an integral value of 0.00707796 which is way lower from what I would expect, you have here the contour with the legend:

I am using the IntegrateVariables filter and I guess that it is that low because the data is 2D (no Y dimension). Is there any better way to obtain 2D integral values?

Thank you in advance!