Is urban water distribution a natural monopoly?

 Arguably, for water, it's a natural monopoly. Public streets typically provide easements for both water and sewer. Both leak, especially with age. So there is a clear need to separate them, so cities put water lines on one side, and sewer lines on the other. Water lines have to be buried below the local frost line, so a second set would need to be even deeper, and provide structural support for the first set previously provided by undisturbed soil. The same is true of sewer, which must be low enough to drain into.