If you're referring to around T-7, that's the sound suppression system, which is a water blanket designed to protect the shuttle stack from damage due to noise reflecting from the concrete.
No, I wasn't referring to the blast of water below the engines used to dampen engine noise. Take a look at the engine nozzle on the left in this image. The light colored plume just right of the nozzle centerline is propellants dumping out of the combustion chamber that have not yet ignited.
