Experimental investigation of thermal retardation and local thermal non-equilibrium effects on heat transport in highly permeable, porous aquifers

Abstract

Actively stimulated temperature changes are common in many groundwater applications. A widely used concept to determine water fluxes, mean transit times, and other parameters from heat tracer tests is the use of the thermal retardation factor. Different methods are available to determine the thermal velocity from thermal breakthrough curves (BTCs) depending on the input signal. This study systematically compares these methods and investigates possible local thermal non-equilibrium effects (LTNE) in coupled solute and heat tracer experiments for highly permeable, porous aquifers. One-dimensional column experiments with saturated gravel of grain size 7–15 mm are conducted to compare the measured, effective thermal retardation with the computed thermal retardation predicted by the apparent thermal retardation factor. The results demonstrate that for scenarios with a step input of the heat tracer, the effective thermal retardation for thermal velocities derived by an analytical model, and the mean between injection temperature and initial temperature, can be predicted by the apparent thermal retardation factor. This indicates that possible LTNE effects do not significantly influence the derived velocities within the investigated range of seepage velocity between 5 and 50 md−1. Other methods showed constant deviation from the apparent retardation factor at higher seepage velocities. In scenarios with a finite duration pulse input of the heat tracer, the effective retardation derived by the peak velocity showed deviations from the apparent retardation up to 35% at seepage velocities lower than 10 m d−1. At higher seepage velocities, the peak velocity could be predicted by the apparent retardation factor.