Periodic forced-gradient heat and solute tracer tests to characterize the hydraulic, thermodynamic and transport properties of unconfined weakly permeable aquifers

Abstract

Shallow unconfined weakly permeable aquifers (UWPAs) often serve as contaminant accumulation zones. Understanding the hydraulic (e.g., hydraulic conductivity), thermodynamic (e.g., thermal conductivity) and solute transport (e.g., dispersivity) parameters of UWPAs is crucial for studying contaminant transport and contamination control. Simultaneous in-situ acquisition of these multi-physical properties is often achieved through heat and solute tracer tests. However, due to the poor permeability of UWPAs, the traditional finite-time pulse injection method generates weak signals that decay rapidly. This was demonstrated by the failure to observe a breakthrough signal in the preliminary single-pulse test attempts. Step injection requires a continuous constant amount of injected signal, demanding large amounts of energy and reliable operation of the heating system. These limitations make it challenging to conduct pulse and step injection tracer experiments at a field scale to capture breakthrough signals. To address this problem, this study proposed periodic forced-gradient heat and solute tracer tests (PFHST). Unlike single pulse and step injections, PFHST established cumulative effects through repeated pulse stimulation, thus resolving both the weak signal issue of single pulse tests and the continuous energy supply challenges of step tests. Field experiments coupled with numerical modeling confirmed the feasibility of PFHST for estimating the multi-physical parameters of UWPAs. Additionally, the stability and diffusion behavior of heat and solute tracers were systematically evaluated under the given conditions, revealing that the heat tracer was more reliable than the solute tracer within UWPAs. Furthermore, key factors affecting groundwater flow, heat and solute transport in UWPA were identified through orthogonal-test sensitivity analysis, providing insights for the development of streamlined contaminant transport models. The proposed framework offers a promising way for rapid and simultaneous acquisition of multi-physical parameters of UWPAs when conventional methods cannot be carried out.