You said the dam cores cut off the sand veins that were running downslope.

Did you have to seal the "sides" of your ponds, or was the dip of the soil beds steep enough that the dam core trenches cut off the water flow that was moving through the sand veins? (Such that water could not move laterally from your ponds and then down slope past the outside corners of the core trenches?)

P.S. Were the sand veins in the soil horizons above the blue clay later, or actually running through the clay layer?