East County’s biggest infrastructure project, Advanced Water Purification, is officially costing more than $1 billion after the Joint Powers Authority that oversees it approved about $80 million in changes to its construction contract. AWP, planned since 2015 to provide water reliability to the region, was initially estimated to cost about $500 million to build, but by 2021, its former CEO Alan Carlisle told the Santee City Council the new estimate for the program was “north of $600 million.”