Help showing groundwater overlay on minimap?

[Boolet]

Hello! I have been looking for help in adding layers to the minimap and I've seen similar questions asked before, but none of them seem to have been answered.

I'm creating a mod for finite water, but I want to feature ground water prominently - water that can be found anywhere on the map and which will flow towards the lowest point. The plan is to let offshore pumps pull from lakes as usual, but the pumps will not be able to pull water at their full rate when the lake is too low, and the lake will only be replenished by rainfall and by the flow of ground water back into it. In future versions I want to add well pumps and rainfall collectors too.

So the question is: How do I create an overlay for the minimap to show the ground water density? The effect I want is essentially covering high-water areas with deep blue and showing low-water areas in light blue, just like what pollution does. I already have the ground water and its behaviors stored in data, i just need a way to visualize it!

Thanks in advance!