The specific part you need to poison is the top of the waterfall that discharges into the small pool on the low ground south of the plantation, labeled Savrethi Water Source.
The following Journal ID and Index codes can be used with the Journalconsole command to manually update the quest to a certain point.
Poisoning the Well (TR_m4_Ob_Poisoning_The_Well)
Index
Finishes Quest
Journal Entry
10
I have met Hibdun Senatmiran at the Obainat Tribe's camp. He expressed his anger towards a disrespectful Hlaalu plantation owner named Mandaras Savrethi and mentioned that he has plans to poison the water that supplies the plantation.
12
I tried to inform Musa Ashadallit, the wise woman of the Obainat tribe, of Hibdun Senatmiran's plot to poison the water supplying Mandaras Savrethi's distillery. However, my claim was dismissed.
15
Finishes quest
I have refused to help Hibdun Senatmiran poison Mandaras Savrethi's water supply.
20
I have agreed to poison the water that is supplying Mandaras Savrethi's distillery. The source, a small waterfall flowing into a pool, is on the southern side of the Savrethi Distillery. I am to go there and pour the poison inside.
40
I drank the poison that Hibdun Senatmiran gave me.
45
Finishes quest
I informed Hibdun Senatmiran that I drank the poison he gave me. He wants nothing more to do with me.
50
The water supply has been poisoned. I should return to Hibdun Senatmiran.
100
Finishes quest
Hibdun Senatmiran was delighted that I completed the task for him and rewarded me with several potions.
110
Finishes quest
Hibdun Senatmiran is dead.
150
Finishes quest
I informed the wise woman, Musa Ashadallit, that I helped Hibdun poison the Savrethi water supply. She was furious.
200
Finishes quest
I informed the wise woman, Musa Ashadallit, of Hibdun's plot. She rewarded me with a handful of ingredients.