Development Of A Paired Heat-Pulse And High-Resolution Fiber-Optic Temperature Sensing Technique To Quantify Groundwater Upwelling In Strongly Gaining Streams

Strong groundwater upwelling to streams creates thermal refugia, which are particularly important to aquatic ecosystems in a warming climate, and may be point sources of pollution when groundwater is contaminated. Passive thermal tracer methods along vertical profiles often rely on the surface diurnal temperature signal propagating a sufficient depth into the streambed to facilitate modeling of vertical fluid flux rates. High rates of groundwater discharge (\u3e -2.0 md-1) restrict propagation of diurnal temperature signals to less than 0.10 m into the streambed, which limits the applicability of passive thermal methods in zones of strong upwelling. Active streambed heating combined with high-resolution fiber-optic temperature sensors (Active HRTS) has the potential to define both vertical and horizontal fluid flux below the extinction depth of the passive diurnal signal. This method was demonstrated at a site with strong upward vertical flux and minimal horizontal flow on Cape Cod, Massachusetts, USA. Nine HRTS were emplaced into the streambed in a grid with 0.40 m spacing. Long-term heating events were performed with the goal of quantifying the horizontal component of groundwater flow by detecting a perturbation from background temperature at “receiving” HRTS in the grid. Absence of a temperature perturbation suggested a lack of horizontal flow, combined with strong upward vertical flux. To determine upwelling flux, short-term heating event decay was numerically modeled in 2-D using VS2DH; modeled flux values of -1.8 md-1 and -2.5 md-1 are similar to those obtained with conventional seepage meters and Darcy methods. We quantify flux values to depths of 0.8 m, revealing previously unattainable advective transport information well below the depth of diurnal signal extinction. The Active HRTS technique provides researchers with a method to quantify strong groundwater upwelling rates to streams and resolve deeper streambed hydrodynamics including the vertical component of fluid flux