Where exactly would that come from?

Well we could start by nationalizing water services so the inefficiencies that are inherent to monopolistic capitalist entities could be eliminated, drastically reducing costs. And we could pay for it over time with some sort of fee... one that might target those that have hoarded over 98% of all wealth produced over the last 50 years which should have gone into infrastructure maintenance and upgrades. We might even, if we're really clever, directly seize the wealth of the companies that sold these solutions to these water companies given they've compromised national security and public health.

That all would drastically reduce the inflated $2.1 Trillion number which isn't the actual cost, it is the price for private contractors to do it for the government, to something you could reasonably pay for with the wealth already accumulated or that could be accumulated by actually fairly taxing those that have actively harmed society.