Hoping to create another water source in south Orange County, five water districts have partnered to fund and build a $107 million treatment plant that will provide drinking water for most of the region beginning next year. The Baker Water Treatment Plant, on Biscayne Bay Drive in Lake Forest, is expected to begin operating in January. The five-acre plant will provide an estimated 28.1 million gallons of drinking water per day to about 63,300 homes, Irvine Ranch Water District officials said.