Professional engineers who specialize in the matter. My son is a civil
engineer with water specialty. His consulting company deals with this
sort of things all the time. They check water tables, flood mitigation
strategy, drainage, recycling, etc. all the time.

How do I find such engineers in my area (seattle)?

Is there some sort of professional associations I can request referrals
from?

In answer to other replies, I have already tried all the normal stuff. Now
I'm ready to try the less normal stuff.