Doing surgery on San Francisco’s water system is no simple task. Replacing one mile of distribution main costs about $3.8 million dollars. That’s just the direct cost of installing a section of drinking water pipe. There are also side effects: disruptions to traffic, sidewalks, and businesses when streets are pried open. In one of the nation’s densest and highest-cost cities the expense amounts to an incentive for well-informed decisions about what to dig up and when.