Pardon the terminology or lack thereof. Is there not a way to place threads on the inside of a cap and on the PVC tube? This way the threads are facing down and water will need to go back up the threads to get in.. and then to halt that place a bead of silicon around the base of the cap.

This would allow you to keep it buried and still have access for multiple times.

And if you made a 'vacuum' in the cache I would think it would draw water in. Place a bicycle tire valve on it and put pressure into the container. And or you can add ORings.

Pictures make it easier for me to contemplate possibilities, this is what I meant when I stated PVC Caps
https://duckduckgo.com/?q=pvc+cap+im...%2FSDF-Cap.jpg