A Dye Tracer Approach for Quantifying Fluid and Solute Flux Across the Sediment–Water Interface

We propose a dye tracer method to characterize fluid and solute fluxes across the sediment‐water interface. Zones of groundwater discharge within the streambed are first identified, and small volume slugs of 0.5 to 1 mL fluorescein dye are released at known subsurface depths. Fluorescein dye allows for visual identification of interface breakthrough locations and times, and dye concentrations at the point of discharge are recorded over time by a fluorometer to generate high resolution breakthrough curves. Groundwater velocities and dispersivities at the demonstration site are estimated by numerically fitting dye breakthroughs 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 demonstration site are nonlinear with tracer release depth, and velocity estimates from breakthrough analysis are significantly more reliable than visual dye (time to first dye expression) and Darcy methods which tend to overestimate and underestimate groundwater velocity, respectively. The use of permanent injection points within the streambed and demonstrated reproducibility of dye breakthroughs allow for study of fluid and solute fluxes under seasonally varying hydrologic conditions. The proposed approach also provides a framework for field study of nonconservative, reactive solutes and allows for the determination of characteristic residence times at various depths in the streambed to better understand chemical and nutrient transformations. (a) Experimental setup used to inject a small volume of fluorescein dye (1 mL syringe with red fluid) followed by an injection of pore water (50 mL syringe) into streambed sediment, and (b) visual expression of fluorescein dye breakthrough at the sediment‐–water interface.