A MICRO-PULSE DYE TRACER APPROACH FOR QUANTIFYING FLUID AND SOLUTE FLUX ACROSS THE GROUND WATER – SURFACE WATER INTERFACE

We propose a dye tracer method to characterize fluid and solute fluxes across the sediment-surface water interface. Zones of groundwater discharge within the streambed are first identified, and micro-slugs of fluorescein dye are released at precisely known subsurface depths. Fluorescein dye allows for visual identification of breakthrough locations and travel times, and dye concentrations at the point of discharge are recorded by a fluorometer to generate high resolution breakthrough curves. Groundwater velocity and dispersivity are estimated by numerically fitting dye breakthrough curves to the classical advection-dispersion equation, although the methodology is not limited to a specific transport model. Breakthroughs across the stream-sediment interface at the study site were found to be non-linear with tracer release depth, and velocity estimates from breakthrough analysis are significantly more reliable than both visual dye and Darcy methods which tended to overestimate and underestimate the mean groundwater velocity, respectively. The use of semi-permanent injection points within the streambed and demonstrated reproducibility of breakthroughs allow for the study of fluid and solute fluxes under seasonally varying hydrologic conditions. The proposed approach also provides a quantitative framework for implementation of non-conservative, reactive solutes and allows for the determination of characteristic residence times at various depths to better understand chemical and nutrient transformations within the hyporheic zone.