In October, Santa Rosa, Calif., came one step closer to pumping its treated wastewater into the ground. The city settled the fifth lawsuit threatening to block construction of a pipeline to carry the water east to the Geysers geothermal steamfield in Napa Valley. Injected into the ground there, the wastewater will replenish the steam that provides energy for cities in the Bay Area. Unfortunately, it will also create an undesirable by-product: many very small earthquakes.
Image: F. C. WHITMORE, USGS
Although some residents are concerned, the plan is attractive to power companies because the geothermal field offers steam for free¿without the need for an energy source to convert liquid to vapor. The field is like an open-face sandwich, with hot rocks at the bottom, a sandstone layer in the middle that holds steam in its pores like fat in bologna, and a thin layer of a caprock above that. The caprock keeps pressure on the reservoir of steam below, such that the steam continuously seeps up through the rocks on top. (In fact, there are no geysers in the Geysers; William Bell Elliott misnamed them in 1847 when he stumbled across hot springs.)
The first geothermal power plant in the U.S. called the Geysers home, and in 1922 it produced 30 kilowatts for the local resort and tourist trade that sprang up around the springs. Not until the 1950s did major power companies, such as Unocal and PG&E, start to pump steam from the Geysers. At its peak in 1987, the field produced almost 2,000 megawatts (MW), enough electricity to supply an estimated two million homes and businesses with power.
That rate of production soon dropped, though, says Mitch Stark, a seismologist at the Calpine Corporation, "because the steam supply was no longer sufficient to keep up with all the new plants and wells that had been drilled." By the late 1980s, companies had drilled about 600 wells. Today the field produces about 1,000 MW from only 350 wells. In 1999 Calpine bought out most of the other producers, "unifying the field" under one management strategy. Calpine runs 19 of the 21 powerhouses in the Geysers field; the Northern California Power Association, a group of towns that includes Palo Alto and Healdsburg, runs the other two. Calpine sends 850 MW of electricity into the grid, supplying power to Santa Rosa, San Francisco and other communities in the northern Bay Area.
To keep the Geysers producing that much electricity, the power companies started pumping lake water and treated wastewater into the field from nearby Lake County in 1997. "By injecting the water, they¿re keeping the pressure in the reservoir up and thereby stabilizing the rate of decline for the steam generation," says Cathy Janik, a geochemist with the Volcano Hazards Group of the U.S. Geological Survey (USGS).
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The water is pumped 5,000 to 10,000 feet into the ground, where it is heated by hot rocks and gases. "What they¿re doing," Janik explains, "is injecting the water deep enough so that it¿s flashing in the reservoirs and then flowing back to the wells, so it¿s mining the heat from the rock." The liquid water heats enough to vaporize, or "flash," as Janik says, "because the pressure in the reservoir is less than it needs to be for this water to boil almost immediately." The Geysers is one of about five vapor-dominated geothermal fields in the world (Lardarello, Italy, has a similar field) and is the largest steam producer, pumping out 15 tons a day.