USING NOBLE GAS ISOTOPES TO CONSTRAIN A 3-D FINITE ELEMENT MODEL OF GROUNDWATER FLOW IN MOUNTAINOUS TERRAIN

A significant fraction of recharge to intermountain valley-fill aquifers may consist of subsurface inflow from the mountains adjacent to the basin ('subsurface inflow'). Because head data are typically scarce within the mountains, groundwater flow models that include the mountain block are seldom constructed. As a result, computed subsurface inflow rates are poorly constrained, and fundamental questions regarding the geometry of flow within the mountain block remain unanswered. While environmental tracer data by itself can place some important limits on subsurface inflow, a well-constrained model that includes both the basin and the mountain block is required to fully characterize this recharge pathway. Noble gas data from valley wells are being used to constrain a 3-D coupled mountain-basin finite element model of groundwater flow within the Wasatch Mountains and the adjacent Salt Lake Valley in northern Utah. These data consist of recharge temperatures derived from noble gas concentrations, and ages determined using the 3H/3He and 4He methods. Recharge temperatures serve as a proxy for recharge elevation through the application of a recharge temperature versus elevation curve derived from Wasatch Mountain springs. Preliminary results indicate that the noble gas data place useful constraints on the bulk permeability structure of the mountain block, the flow geometry within the mountain block, and rates of subsurface inflow.