'Moutain springs are always the purest of water, especially in these parts. Something must be causing the water to run foul.
'Will you explore the source of the River? Perhaps there will be evidence as to what caused the vile plague, because now we don't even know what it is that's making my poor dwarves sick.
'If we can make the river run clean again, I have hope for the animals and the snows here as well. Maybe Thorin's Hall won't be lost to sickness.'
Putrid water flows from the Blue Mountains' spring from above Thorin's Hall, poisoning all creatures unfortunate enough to drink from it.
The mountain-path is north-west of Frerin's Court in Ered Luin.
You should climb the mountain path and find the source of the befouled waters and fend off any guards of the corruption along the way.
Elrohir of Rivendell, the other son o Elrond, stands outside the entrance to a cave on the mountain path.
You should speak with Elrohir of your errrand for Dwalin and Elladan.