Dylan,

Yes, trees five feet from the clay liner are a risk. However, it is also how you want your pond to look. You are the one that gets to balance the risk versus the reward.

Are you going to be able to irrigate these trees?

If so, then I would give them lots of water AWAY from the pond during your Cali dry spells.

Have you ever fertilized the oaks?

You also might try fertilizing them (very carefully following the instructions) out to the drip line only in the 180 degree section away from the pond.

If your trees are perfectly happy without running roots through your liner, then they should be less likely to make the attempt.

Regardless, it is always nice to irrigate and fertilize your trees after they get a little abuse due to an earthmoving project.

Good luck on your pond!