https://www.al-feqh.com/en/purification-of-water-in-islam" rel="nofollow - purification in Islam

1-We assume water is pure; unless proven otherwise. Therefore if there is water whose purity/impurity is not certain, we assume it to be pure. Being overly concerned about its purity or impurity in such a case is discouraged.

2-It is permissible to perform ablution with Zamzam Water. This is shown by the report where the Prophet ﷺ called for a bucket of Zamzam water, and then drank and performed ablution from it.

source: https://www.al-feqh.com/en/purification-of-water-in-islam