Hydrogeologic Framework of the Southeastern Funeral Mountains, California-Nevada, and Implications for the Major Water-Supply Springs in Death Valley National Park

We are using a combination of geologic mapping, geophysical surveys, hydrologic computer modeling, and a drilling-and-testing program to evaluate the hydrologic framework of the southeastern Funeral Mountains. Our work addresses: (1) the hydrologic connection of the Furnace Creek springs on the south side of the Funeral Mountains to the regional aquifer system on the north side, and (2) potential impacts on these springs from human activities, including possible leakage from the proposed radioactive waste repository under Yucca Mountain, ~50 km to the northeast, and ongoing agricultural overdrafting of groundwater in the southern Amargosa Desert, ~25 km to the northeast. Discharge from the springs at Furnace Creek provides the major water supply for Death Valley National Park and, at 5000 acre-ft/yr, is at least 10 times larger than that attributable to recharge in the adjacent, arid Funeral Mountains. Moreover, hydrochemical data indicate that the spring water is derived mainly from interbasin groundwater flow through the regional carbonate aquifer. This aquifer extends northeastward across much of southeastern Nevada. Our geologic map data indicate that the carbonate aquifer is continuous under the southeastern Funeral Mountains. The base of this aquifer is, however, structurally uplifted under the axis of the range, to an elevation that is much higher than most of the springs at Furnace Creek, but that is locally lower than the water table on the opposite (northeast) side of the range. Rather than forming a barrier that blocks groundwater flow under the Funeral Mountains, as previously interpreted, this uplift evidently forms a spillway. The ~700 m drop in the water-table elevation across this range, into Death Valley, thus does not indicate the presence of any feature that would divert or slow groundwater flow. Because of the spillway mechanism, flow from the springs at Furnace Creek may be sensitive to the water-mining activities that have been progressively lowering the hydraulic head on the northeast side of the Funeral Mountains. DOE proposes to emplace nuclear waste in Yucca Mountain, above a volcanic aquifer, which is separated from the underlying regional carbonate aquifer by a leaky volcanic confining unit. An upward hydraulic gradient, from the carbonate aquifer to the overlying volcanic aquifer, has been measured in a single well. This upward gradient is inferred to act as a natural barrier that will keep radionuclides from leaking into the regional carbonate aquifer. There are reasons to question the efficacy and durability of this mechanism. First, with data from only one well penetration into the carbonates, it is unknowable whether the observed upward gradient is a regional feature, or a relatively local one. Second, if recent trends in water mining in this region are projected into the future, it is plausible that the current upward gradient, even if it is regionally effective today, could be reversed. We hope to shed some light on these issues by: (1) testing the chemistry of water in the carbonate aquifer under the Funeral Mountains to see if volcanic-aquifer water from nearby areas such as Yucca Mountain is entering the carbonate aquifer, and (2) developing better computer models of this hydrologic system that could be used to forecast potential long-term impacts of water mining.