Locating and quantifying spatially distributed groundwater/surface water interactions using temperature signals with paired fiber-optic cables

Diurnal temperature fluctuations of surface water, as result of solar heating, function as a tracer that continuously exchanges energy between streams, streambed sediments, and discharging groundwater. Analytical solutions exist to estimate discharge by extracting the amplitude ratio between pairs of subsurface temperature time series measurements. The research presented here adds to the expanding body of heat tracing literature by applying the amplitude-shift time series discharge estimation method to pairs of distributed temperature sensor (DTS) fiber-optic cables. A pair of DTS fiber-optic cables is placed in an experimental streambed, one over the other, with a small vertical separation to measure continues heat-based vertical streambed fluxes along the entire length of cable, thus eliminating a long series of point measurements. This study utilized time series data from synthetic data sets, modeled numerically using COMSOL Multiphysics, and physical data sets, modeled in a 10 m long sandbox model to assess the viability of this new distributed flux quantification method. Discharge estimated with spatially averaged temperature data are accurately approximated where groundwater flow is uniform and the temperature signal is constant at the streambed surface. Error is introduced where focused groundwater discharge exists, resulting in temperature profiles that vary laterally throughout the streambed. Spatial averaging inherent to DTS data results in dampening of flux measurements over focused discharge zones, as temperature is averaged as a result of the measurement technique. This leads to underestimating peak flux at localized discharge zones and overestimating discharge measurements away from these locations. The spatial integration of the DTS as well as the sampling interval and cable position can lead to error in calculated groundwater fluxes. Results demonstrate the potential advantages and disadvantages of using paired fiber-optic cables to quantify high-resolution groundwater discharge to streams at the reach scale.