HYDROCHEMICAL INTERPRETATIONS OF SATURATED-ZONE FLOW NEAR YUCCA MOUNTAIN, NEVADA

Identification of general flow direction and mixing relations of ground water near Yucca Mountain, Nevada, a proposed nuclear waste repository, is facilitated by the delineation of five relatively distinct hydrochemical facies by Oliver and Patterson in 2002. Limited data indicate that water flows beneath Yucca Mountain from the north and northwest. Recharge to the water table through Yucca Mountain increases the fluoride concentrations and ²³⁴U/²³⁸U isotopic ratios (reported by Paces and others in 2002) of the ground water. These parameters are characteristic of the Yucca Mountain facies. From Yucca Mountain, some of the flow is to the southeast, where the water mixes with water in the Fortymile Wash facies, which is characterized by elevated calcium and magnesium concentrations reflecting recharge through alluvium. The ²³⁴U/²³⁸U ratios decrease to the south along Fortymile Wash, and fluoride concentrations remain slightly elevated until diluted by mixing with water from the eastern Amargosa River facies.

Flow paths directly south from Yucca Mountain would shorten the distance to the accessible environment but are more difficult to identify. Isopleths of several ions, including calcium, magnesium, chloride, sulfate, and fluoride, outline plumes of water with similar chemical characteristics south of Yucca Mountain between the Fortymile Wash facies to the east and the Bare Mountain facies to the west. Water from the Bare Mountain facies has a carbonate aquifer signature derived from carbonate rocks at Bare Mountain and does not appear to mix with the water south of Yucca Mountain. Because water from the Fortymile Wash facies is a mixture of water derived from volcanic rocks and alluvium, the somewhat similar water directly south of Yucca Mountain may be water that has moved from the volcanic rocks beneath Yucca Mountain through the alluvium directly to the south without flowing eastward to Fortymile Wash.