If Zamzam water is mixed with other water, the mixture will have the barakah of Zamzam as much as the ratio of Zamzam in the water

 

Are you allowed to mix Zamzam (Hajj water) with ordinary water from a tap and does the water become Hajj water after mixing it with normal water?.

Praise be to Allaah.

If Zamzam water is mixed with other water, the mixture will
have the barakah of Zamzam as much as the ratio of Zamzam in the water.
That is because the person who drinks the mixture will inevitably drink
some of the Zamzam water that has been mixed with other water, so it is
true to say that he is drinking it. 

Al-Kaasaani (may Allah have mercy on him) said: If a person
swears that he will not drink from Zamzam water, and a lot of other water is
poured over it until the Zamzam water becomes a smaller quantity, and he
drinks it, he has broken his oath. 

End quote from Badaa‘i’ al-Sanaa‘i’, 3/63 

Shaykh Ibn Jibreen (may Allah have mercy on him) was asked: 

If Zamzam water is mixed with ordinary water, does it still
comes under the rulings on Zamzam water? 

He replied: 

It is better to drink Zamzam water on its own; if it is mixed
with other water, the rulings on its virtues still apply and it is
permissible to treat sickness with it, even though that detracts from how it
would be if it was purely Zamzam water. 

End quote from the website of Shaykh Ibn Jibreen. 

And Allah knows best.