For decades, energy companies have generated electricity from bubbling hot springs near the Northern California city of Santa Rosa. But the underground reservoirs are slowly being depleted. Only 20 percent of the steam captured and sent through geothermal generators can be returned to the ground as water. Now highly treated city wastewater is being injected into the steam fields at the rate of 12 million gallons daily. The Santa Rosa Geysers Recharge Project — a $200 million engineering feat that took three years to complete — launched in 2003, helping to refill the steam reservoir and rid the city of excess wastewater.