Re: suggestions to agitate stream flow?

Yes, but the only head available is the hydraulic gradient of the stream bed. 
And because the grade is not very steep, the entry to a narrow vertical channel
would become quite long.

Me, I'd try to build the stream bed up, with concrete or plywood, so the stream
becomes wide and shallow and the narrow channel is horizontal.

That would also allow the sun to interact with the VOCs, by increasing the
water temperature and raising the evaporation rate, and by exposing them to UV
radiation and the possibility of reacting chemically to become something else.

The plywood is also degradable, so the apparatus will eventually decompose and
flush itself away.

-Mike-