It was bath time and Rosalba Moralez heard a cry. She rushed to the bathroom and found her 7-year-old daughter, Alexxa, being doused with brown, putrid water. “We kept running the tub, we turned on the sink, we flushed the toilet. All the water was coming out dirty,” Ms. Moralez said. For more than a year, discolored water has regularly gushed from faucets in the family’s bathroom and kitchen, as in hundreds of other households here in Willowbrook, Calif., an unincorporated community near Compton in South Los Angeles.