The slab edge had to be sealed against radon. I had decided to use roofing tape (Eternabond Webseal) and after returning to see that the Sanitred had pulled away from the edges of the cement, it was an even more satisfying decision.
First I had to break off the loose bits of concrete from the insulation covered with plastic and then vacuum the dust and gravel to clean the area. Then I applied the tape sealing the edge of the concrete to the plastic and the floor. Then I painted the tape with deck paint that is rated to work on canvas to both waterproof and protect the surface. The system seems to have worked fine and will hopefully stand up to the abuse of the rest of the deconstruction until we decide how to cover the perimeter with a decorative edge.