Effects of Local Thermal Nonequilibrium and Sediment Heterogeneity on Heat Tracer-Based Downwelling Flux Quantification in Streambeds

Local thermal nonequilibrium (LTNE) effects in heterogeneous media can affect subsurface temperature distributions, as well as the capacity of the heat transport model to solve the inverse problem of estimating groundwater fluxes. We present a synthetic coupled flow and heat transport numerical model with five scenarios to analyze the influence of subsurface hydraulic and thermal property variations on heat transport in heterogeneous streambed sediments, while also evaluating the role of LTNE effects in heat transport processes within heterogeneous streambed sediments and their impact on streambed fluxes estimation. Heterogeneous streambed sediments with varying sand-gravel-clay fractions are stochastically generated using a Markov Chain model. Synthetic streambed temperature-time series are produced to estimate effective thermal diffusivity and thermal front velocity using a heat transport model based on homogeneous and local thermal equilibrium assumptions, and these estimates were compared to known values from numerical models of flow fields analogous to losing streams. Results show that neglecting thermal heterogeneity in streambed sediments leads to significant errors in streambed fluxes estimation, where the effective thermal diffusivity can be underestimated by about 40%, while the thermal front velocity can be overestimated by more than two times. In addition to the effects of streambed heterogeneity, LTNE effects further amplify these errors. Furthermore, the influences of streambed heterogeneity on LTNE effects are primarily influenced by flow velocity, with higher clay content reducing Darcian velocity and weakening LTNE effects.