The Nevada Assembly on Monday approved a wide-ranging water conservation bill that would allow the Southern Nevada Water Authority to limit residential water use in Las Vegas if Lake Mead’s levels plummet. But it was a provision in the bill that would require some septic tank users in Las Vegas to convert to the municipal sewer system within the next 30 years that has drawn the most criticism thus far. … The water authority has said the legislation is needed in case the federal government imposes steep cuts to Nevada’s share of the Colorado River in the next few years.