It appears when sand-lurkers digest the river-weeds of the upper Baranduin, it gives the plants a mild healing ability.
'I examined the weeds you found, and I am glad you brought them to me. Normally, the river-weeds are of little consequence. One can eat them if hungry enough, but there was nothing special about them of themselves.
'It seems something happens when the sand-lurkers begin to digest the weeds. Somehow, that brings out a mild healing ability from the plants. I have tried them on several scrapes and cuts around camp here, and the results are undeniable.
'There is no sense in going swimming to get more, since they are undigested. I need you to go out and get more from the sand-lurkers themselves. A slightly foul proposition, I realize, taking dinner right after it has been devoured, but I would like to have more of these weeds to make some much-needed poultices.'
Sand-lurkers can be found in the Barandalf, south of the colossus.
Herthdúr asked you to search sand-lurkers in the Barandalf for piles of soggy weeds.
Herthdúr is at the colossus, north of the Barandalf.
Herthdúr will be eager to begin making poultices from the weeds you collected.