GEOTHERMAL SYSTEMS IN THE PASO ROBLES GROUNDWATER BASIN, CENTRAL COAST RANGES, CALIFORNIA

Sources of water in the Paso Robles area of the central California Coast Ranges are a major concern because of increased demand associated with population growth. Aquifers within the Paso Robles groundwater basin are the primary water source and occur within the middle to lower Pleistocene Paso Robles Formation, and Quaternary younger and older alluvial deposits. The basin is bounded on the west by the Rinconada Fault and on the east by the Red Hills and San Juan faults. On the west side of the Paso Robles Basin a shallow, low temperature (18-48 degrees C) geothermal system along the Rinconada Fault extends eastward into the basin. The geothermal waters are characterized by elevated concentrations of B, Cl, and bicarbonate. After the December 2003 San Simeon M 6.5 earthquake, numerous hot springs developed along the Rinconada fault including a hot spring within the central commercial district of Paso Robles. Because of the chemistry of the hot spring effluent (high B, Cl, sulfide and bicarbonate), the water is diverted into the Salinas River. The ratio of B/Cl in the geothermal fluid is a constant and increases in B and Cl concentration correlate with increasing temperature of the geothermal fluid. The geothermal fluids are mixtures of meteoric and connate fluid, and B and Cl concentrations can be used to determine the relative proportion of each fluid source. On the east side of the basin, similar composition connate waters with high B, Cl and carbonate are present. The connate fluids are localized along the Red Hills Faults and extend westward into the basin. Silica-carbonate type mercury deposits are localized along the Rinconada and adjacent faults and formed along the margin of the basin in the past. The present day geothermal system has similar fluid chemistry and alteration to that associated with the mercury deposits and has the potential to develop similar mineralization. Geothermal waters have been an important in the evolution of the Paso Robles groundwater basin. Both past hydrothermal alteration and mineralization and present day geothermal systems potentially contribute to degradation of the quality of the groundwater resource.