Why Black Colour Water Comes From Fire Hydrant Pipe Lines?

Dear sir,
I observed black colour water from fire hydrant pipelines especially in some points, but not in all points. What was the exact reason why water becomes black in colour.
Initial filling water was good. During checking water in next day they became black in colour

Dear Mr. Shrikant ,
Two reasons : 1. If water is Stagnant 2. due to oxidation of MS - water became black in colour . Don't worry . You need to take weekly trials of hydrant points so that this problem will be resolved .
Thanks
Abhay

Dear SRIKANTH,
As rightly said by Mr. Abhay,Corrosion can be one of the reasons for this. Also make sure there is no other impurity present near by which entering into hydrant line due to leakage like crude oil or any slug or solution which is making water black.
Regards,
Hansa Vyas