Evaluating the Ability of Tracer Tests to Quantify Reactive Solute Transport in Stream-Aquifer Systems

Tracer experiments are valuable tools for characterizing the fate and transport of solutes carried in stream waters; however, the results can have high uncertainty when the technique is not properly imple-mented. The goal of this study was to identify the limitations that apply when we couple conservative-tracer injection with reactive solute sampling to identify the transport and reaction processes active in a stream. The conservative-tracer injection is used to characterize the physical transport processes of advection, dis-persion, ground-water inflow, and stream-storage exchange (the movement of stream water and solute between the active stream channel and storage zones in the channel margins and in the subsurface). The reactive-solute sampling is used to characterize the removal of reactive solute due to geochemical/biotic processes occurring in the storage zones. We apply the methodology of Wagner and Harvey (1997) to eval-uate the tracer approach for the wide range of transport and reaction conditions likely encountered in high-gradient streams. The methodology couples solute transport simulation with parameter uncertainty analysis in a Monte Carlo framework to identify those combinations of stream transport and reaction properties that pose limitations to the tracer approach. Our results show that the uncertainty associated with determining the reaction rate constant is strongly related to the reactive loss factor, which is a dimensionless combina-tion of the reaction rate constant, the average solute residence time in the storage zone, the experimental reach length, and the average distance travelled by a stream solute before entering a storage zone. As the reactive loss factor increases, the effect of reactive loss in the storage zones along the study reach increases and the uncertainty in the reaction rate estimate decreases