I think that is what I have, groundwater flowing INTO the impoundment with (hopefully) nowhere else to go. Everything slopes (alibet VERY slightly) towards my "valley". The hills on both sides are full of clay and they should keep the water channeled into my pond.
I'm sure the core will go deeper than the bottom of the pond, hopefully to the clay that's just about everywhere around here.
Bad news...we got a BUNCH of rain today. What has been dug is full of water...well not FULL, about a foot deep. I pushed a stick into the bank where the water line is to see how it does. I guess it will be a while before I see any more action. It's a slick mess out there. I have a few "grand canyons" where the water went flowing into the pond, literally waterfalls. We got 1.75 inches in about an hour or two.
I'm planting rye grass for the winter to get some roots going. In spring I'll go for Bahaia. I want to stop any erosion and don't mind cutting the grass. It gives me a chance to listen to audiobooks. I've got 7.5 acres to cut, it takes about 6 hours.