CHARACTERIZING FLUXES AT THE GROUNDWATER-SURFACE WATER INTERFACE USING DYE TRACERS

Determining discharge across the groundwater—surface water interface remains a challenge due to streambed heterogeneity and integrating measurements over diverse spatial and temporal scales. In this study, mini-piezometers are first used to identify areas of groundwater discharge in lakes and streams, and a micro-pulse of fluorescein dye is released at a precisely known shallow depth as a semi-conservative tracer to measure shallow groundwater fluxes and average discharge velocities at various transport length scales. The semi-permanent installation of “SedPoints” (MHEproducts.com) – constructed of 1⁄4” polyethylene tubing attached to a polypropylene screen, and inserted to a user-defined depth using a hollow stainless-steel installation tool – allow for tracer release from the same precise location to assess reproducibility in dye breakthroughs and quantify seasonal variations in groundwater discharge. The use of fluorescein dye allows for visual dye detection regardless of light conditions and the additional use of a fluorometer allows for accurate determination of groundwater velocity, dispersivity, and other transport parameters through the analysis of breakthrough curves, which are captured using a Seapoint Fluorescein Fluorometer and the data subsequently fit using a GRG nonlinear solver to the advection-dispersion equation. Two identical tracer tests performed in the same SedPoint exhibit adequate reproducibility with velocity and dispersivity estimates varying approximately 10% and 20%, respectively. Visual dye breakthroughs occurred approximately 45% earlier than average (bulk) breakthroughs, and thus, these initial data sets indicate that estimates based on time to visual breakthrough of the dye tend to overestimate groundwater discharge velocity. The development of this dye tracer method is advantageous over traditional, physically (hydraulically) based measurements that utilize Darcy’s Law to provide indirect estimates of velocity, which are subject to a high degree of parametric uncertainty and subsequent error. These precise dye injections are effective for investigating small-scale variations in groundwater discharge and velocity due to small-scale sediment heterogeneity and temporal variability.