You might want to consider doing nothing. You pass all these costs along to the tenants anyway right, and if the city maintains most of the lines that will save you tons of money if you ever have to replace the water or sewer lines.
Why is this not a better option?
If you risk removing the base charges you also risk assuming ownership / maintenance for those lines. I would argue that the city should allow direct billing to the tenants, and not fight the base charges.