Ibn Uyaynah (may Allah have mercy on him) says: I have been in Makkah for seventy years; I have not seen anyone, young or old, who knew the hadith of the Zanji, which they narrated, that he fell into Zamzam. I have not heard anyone say that Zamzam was drained. Abu Ubayd (may Allah have mercy on him) says that regarding Zamzam there are reports that its water will not be drained nor will it be dishonoured. I do not know whether Abu Qudamah narrated this from Abu Ubayd, from Abu al-Walid al-Faqih (may Allah have mercy on him). Imam al-Shafi'i (may Allah have mercy on him) said to those who disagreed with him: You yourselves narrate the marfu' hadith of Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Nothing makes water impure." Do you then think that Sayyiduna Ibn Abbas (may Allah be pleased with them both) would narrate a report from the Prophet (peace and blessings be upon him) and then abandon it, if this is indeed his narration? You also narrate from him that he performed ablution from a pool in which a carcass was floating, and you narrate from him that water does not become impure. If any of this is authentic, it shows that he did not drain Zamzam because of impurity but for cleaning, if he did so at all; Zamzam is for drinking, and blood may have appeared on the water until it could be seen in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1263