Pond segregation

Giant zip-lock bags.

Hermetic means airtight. I thought watertight was going to be rough, but airtight is harder.

Unless you explain the project better, I would think the only way to accomplish this would be to take a large airtight pipe or tube, put the isotope in and then fill with pond water. Hermetically seal and place just in the pond water, right at the edge. Obviously, neither air nor pond water will be exchanged between the container and the pond, but that seems to be what you want for this project. Then, as the pond dries up, you push the airtight container every few days to keep it in the pond water, right at the edge.

Thanks Blickers.
I'm not sure I fully understand your suggestion. Allow me to explain myself better (hopefully...).
We have a seasonal pond/puddle where our organism of interest lives along the edges. By adding stable isotope to the water we can later follow our organisms as they disperse from the pond. If we simple drop our isotope into the pond, most of it will mix into the full body of the water and our labeling will be inefficient. I would like to form some kind of barrier along the edge to enclose the 'shore-water'. Gradually, as the pond dries out, I will need to move the barrier farther into the pond...
Hope this helps!
Thanks!

I take it this barrier will be temporary? That is you plan to build the barrier, add the isotope to the water, then remove the barrier so the fish will disperse? The barrier is not hermetic-the word you want is "watertight".

One question-will the barrier separate the shore water from the pond's interior water around the whole circumference of the pond, like in Figure A? Or just a small section of the pond, with most of the pond's shorewater unseparated from the pond's interior water, like in Figure B?