Praise be to Allah.
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-Kasani (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.” (Bada‘i’ al-Sanai’, 3/63)
Shaykh Ibn Jibrin (may Allah have mercy on him) was asked:
If Zamzam water is mixed with ordinary water, does it still come under the rulings on Zamzam water?
“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.” (From the website of Shaykh Ibn Jibrin)
And Allah knows best.