For decades, California’s coastal aquifers have been plagued by invading seawater, turning pristine wells into salty ruins. But the state’s coastal water agencies now plan to get more aggressive in holding back the invasion by injecting millions of gallons of treated sewage and other purified wastewater deep underground. The additional groundwater will both enhance potable water supplies and help prevent saltwater from seeping further into coastal California’s massive subterranean reservoirs. A decade ago, Orange County was the first in California to successfully employ this tactic — mocked by critics as a “toilet to tap” solution.