The lease is the most important thing. It’s not illegal to have tenants split a bill, despite what some have offered. Now armed with all the information I’d proceed to the landlord, you have no problem paying for water, but paying for a leak that’s not in your unit is not your responsibility, and the timeliness of the repair matters as well. Email the landlord, don’t call, and if it’s handled in person or via a call, email them anyway highlighting the points you talked about. Ex- I was just reaching out after our conversation yesterday and just wanted to confirm…

Edit: the key legal term here is foreign load, the extra amount added to the bill by the leak, which should be pretty easy to show with records