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.