A colossal water service pipeline in the south Las Vegas Valley will be drilled under the Sloan Canyon National Conservation Area, with the blessing of both political parties and some conservation advocates. Water managers’ route for the so-called Horizon Valley Lateral Pipeline involves tunneling pipes large enough to drive a truck through. If not for this $2 billion pipeline , up to a million customers could be left without service if the existing and aging South Valley Lateral needed to come down for repairs, officials said. “What we see around the country is, a lot of times, infrastructure costs come last,” said Clark County Commissioner Marilyn Kirkpatrick, who chairs the Southern Nevada Water Authority board.…