New heat tracer transport models to improve understanding of Groundwater-surface water interactions

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 water and heat transport model to investigate how LTNE effects affect the estimation of heat tracer-based streambed fluxes in heterogeneous streambed sediments, characterized by variations in both hydraulic and thermal properties. Results show that, in a heterogeneous streambed containing both clay and sand, increasing LTNE effects in the sand portion can result in a substantially lower effective thermal diffusivity inside the clay region. The use of a local thermal equilibrium (LTE) model to interpret temperature measurements taken on sand portion can lead to an overestimation of Darcy fluxes and an underestimation of effective thermal diffusivity. Increasing particle size enhances LTNE effects and yields a greater effective thermal diffusivity inside the zone of high-flow preferential flows, and using an LTE model in the presence of heterogeneous streambed sediments could result in a 30-fold underestimate of Darcy fluxes in the saturated clay at Darcy fluxes ranging from 0.009 to 5.6 . In a homogeneous streambed, both the VFLUX 2 and LPMLEn can estimate streambed fluxes at low hydrodynamic flow conditions, even when LTNE effects are present. When there are considerable LTNE effects, however, both models can underestimate the effective thermal diffusivity.