DATING BASE FLOW IN STREAMS USING DISSOLVED GASES AND SF6

The effective residence time of groundwater in a basin or watershed is a vital parameter in estimating groundwater availability and resource sustainability. Age tracers that have been used most effectively to help estimate residence times in young (<50-year-old) groundwater rely on concentrations of dissolved gases (e.g., ³He, SF₆, and chlorofluorocarbons (CFCs)). These gases can be measured effectively at wells, but when groundwater discharges to streams the gases quickly reequilibrate with the atmosphere. Streamflow and gas-exchange parameters must be accurately measured if concentrations of the gas tracers in the groundwater inflow to streams are to be estimated accurately. The rapid gas reequilibration and high variability of stream parameters have proven to be an impediment to dating base flow in streams. If a technique could be developed to estimate accurately and simultaneously the stream parameters and tracer concentrations of groundwater inflow, it would promise to be a powerful tool in the estimation of mean groundwater residence times in watersheds.

Such a method is under development, and uses dissolved gases (argon and nitrogen) and SF₆ or CFC concentrations in streams to estimate streamflow and gas-exchange parameters and the mean age of base flow. The method is based on the incentive to minimize costs by collecting samples at a single site during one visit, and the desire for a result that yields one mean age of base flow upstream from the sampling location. The method uses the fact that diurnal temperature changes in most streams result in a sinusoidal variation in the equilibrium concentration of most gases, with the exact form of the transient concentration signal being a function of three parameters: the two mean residence times of the gas and the stream water, and the concentration of the gas in the groundwater inflow. Argon, nitrogen, and SF₆ were measured over a 14-hour period at Little Difficult Run, Fairfax County, Virginia on April 3, 2012. Results indicate the mean residence time of the argon, SF₆, and stream water were 4, 5, and 28 hours, respectively, and that the mean age of the base flow was 11 years (piston-flow assumption) or 14 years (distributed-recharge assumption). Future tests at other sites are planned and should help determine under what field conditions the method is viable.