The civil engineers who design these ponds do flow studies of the ponds to try to monitor how much will actually flow out of the ponds, through the bottom, let's say.
This is not my area of expertise, but the numbers I have seen for what they call the water conductivity are very low in these ponds. Because of the clay makeup, the water does not flow readily through these systems, so they have a very low permeability.
This is more the realm of the engineers. When they build these ponds, they determine the water conductivity of these systems. I don't know the units, but they come up with numbers that show a certain flow rate, and then they can say whether there's a leaky pond or not. They try to design these ponds with very low water conductivity.