As drought and climate change ravage California’s once-reliable supply of drinking water, officials in Los Angeles are setting their sights on a relatively new, almost untapped resource for the city’s 4 million residents: the Superfund site in their own backyard. Nearly 70% of the city’s 115 wells in the San Fernando Valley groundwater basin — the largest such basin under the purview of the Los Angeles Department of Water and Power — have been sitting unused for decades after dangerous contaminants seeped into the aquifer.