The number was, and is, eye-opening: $10.8 billion. That’s an estimate issued by city leaders in San Bernardino County for how much their taxpayers might have to pay, over the next two decades, to meet possible new standards for cleaning the water that flows out of their streets and yards and farms and into the culverts, creeks and tributaries connected to the Santa Ana River Watershed, a stretch that includes much of San Bernardino, Orange and Riverside counties. Leaders from 17 cities and agencies in San Bernardino County made that $10.8 billion claim during a public hearing in September, in Cypress, that involved representatives from all three counties. Their estimate was part of a broader negotiation over the details of the region’s next MS4 permit, a federally mandated document that will set limits on how much pollution can legally flow into local waters and, by extension, the ocean.