I'd dig by hand in the very shallow area of the settlement pond to see if it has a liner, whether a PVC or clay liner. If it's a PVC liner, then I'd just bite the bullet, dig it out with an excavator and put in another geotex liner by BTL. For the help it does with the 1/2 ac pond, it's cheap insurance.
If it's a clay liner, you can dig the settlement pond out with an excavator (do it with no water in it, even if you have to pump it dry). Watch the color of the sediment that comes out, don't dig deep with each scoop. Scrape to make a pile, scoop that pile out of the pond. When the color of the dirt coming out changes, that's most likely the clay liner that you have reached.
