A stormwater treatment system includes water treatment media that defines a helical flow path within a conduit. The water treatment media may include a plurality of helical water treatment media components that are constructed and arranged to seal against an inside surface of the conduit. The individual helical water treatment media components may be installed in situ within the conduit when the conduit is positioned within a chamber having an access opening, and periodically removed and replaced by passing the components to the access opening. The components further may have a flexible frame, which allows the water treatment media to dynamically adjust to swelling of the filtration media and other forces that may occur during use. The performance of the system may be adjusted by adjusting the helical pitch of the helical water treatment media components.