MIXING OF DEEP GEOTHERMAL AND SHALLOW ALLUVIAL GROUNDWATER IN DIXIE VALLEY, NEVADA

Dixie Valley, in north-central Nevada, is a hydrologically-closed groundwater flow system that terminates in the Humboldt Salt Marsh, a moist playa. The basin is a fault-bounded graben structure bordered with steep mountains to the east and west. Fresh groundwater occurs in the alluvial aquifer system around the margins of the basin and becomes hypersaline towards the playa center. Since 1988, a 62 megawatt double-flash geothermal power plant has operated northwest of the playa. Due to reduced pressure from pumping of the geothermal water, cold alluvial groundwater has been used to maintain pressure heads within the geothermal field. Currently, alluvial groundwater is being considered for drinking water export; however, several studies conducted in the 1990’s indicate that there is the potential for interaction between the geothermal and alluvial aquifer systems. Therefore, understanding the degree of mixing between geothermal and alluvial aquifer systems is necessary to evaluate the potential groundwater resource. Geothermal and alluvial aquifer chemical compositions and chemical evolution were evaluated using data from geothermal wells, alluvial wells, springs, and streams. Geothermal indicators such as elevated temperature, lithium, boron, chloride, and silica indicate that mixing occurs in many wells tapping the alluvial aquifer, particularly in the south and west sides of the basin. Geothermal waters have high lithium and boron concentrations and low magnesium, whereas alluvial aquifer water derived from volcanic rocks in the basin have high magnesium and low lithium concentrations. The hypersaline playa water has low lithium concentrations, probably due to exchange with smectite clays in the basin center. Silica concentrations in alluvial groundwater is typically greater than 30 mg/L, which is often indicative of a geothermal source; however, silica might also be derived from weathering of volcanic tuffs and diatomite that occur in the basin. Mixing ratios of end member geothermal and alluvial aquifer water indicate that approximately 15 percent of the elemental concentrations found in the alluvial groundwater might be derived from geothermal water.