Yikes!
Duckweed is amazingly resilient for such a tiny little plant.

It's spring-fed with a stand pipe for a drain that goes into the city's
storm drain system.

Since your water can get into the city's storm drain system
I would not do anything unless I have written permission from the
authorities. We usually recommend contacting your county extension
agent for help and permission.

Good luck and let us know what happens.

k :-)