The mean transit time of groundwater is a fundamental and robust characteristic of a subsurface flow system. In unconfined aquifers, the mean groundwater transit time is related to (1) the volume of water stored in the aquifer and (2) the flux of water into or out of the aquifer. Environmental tracers such as tritium have been used to estimate the mean transit time, but generally require a time series of measurements from the early 1960s to approximately 1980 and such data sets are very rare. Precise groundwater dating using dissolved gases such as chlorofluorocarbons (CFCs) has become relatively common. If CFC groundwater ages were preserved in the base flow of streams it might be possible to obtain flow-weighted mean transit time estimates even when long-term time series are not available. Although one might think that CFCs in streams would equilibrate with the atmosphere, samples collected from a variety of streams during base flow conditions are NOT in equilibrium with atmospheric CFCs (at the temperature and elevation of the stream) suggesting that exchange with the atmosphere is sluggish. The central hypothesis of the proposed research is that gaining streams will not equilibrate with CFCs in the atmosphere provided that the flux of non-modern groundwater can offset the rate of exchange with the atmosphere. It is further hypothesized that by quantifying the rate of gas exchange in a particular stream, the mean concentration of CFCs in groundwater can be estimated. If these hypotheses can be validated, a new tool will exist for obtaining integrated hydrologic properties in the subsurface. Such integrated values can then form the basis for transforming properties measured at the scale of boreholes to the scale of a groundwatershed (i.e. upscaling.) This research will be conducted primarily in the Red Butte watershed in the Wasatch Mountains, Utah, along with an application of the results to regional streams draining the Colorado Plateau. Gas exchange mechanisms, including the effects of bubble formation, will be evaluated by conducting gas tracer tests in streams. The spatial variability of aquifer properties (thickness, specific yield, etc.) and preferential (macropore) flow will be considered by comparing groundwater ages in short-screen piezometers with integrated values obtained in stream baseflow. The development of a tool to evaluate the mean transit time of a subsurface watershed will provide the basis for developing new paradigms for the regulation and protection the Nation's water resources that are based on measurable values for the average transit time of water and solutes in the subsurface.
