Heat and Solute Tracers: How Do They Compare in Heterogeneous Aquifers?

Abstract

A comparison of groundwater velocity in heterogeneous aquifers estimated from hydraulic methods, heat and solute tracers was made using numerical simulations. Aquifer heterogeneity was described by geostatistical properties of the Borden, Cape Cod, North Bay, and MADE aquifers. Both heat and solute tracers displayed little systematic under‐ or over‐estimation in velocity relative to a hydraulic control. The worst cases were under‐estimates of 6.63% for solute and 2.13% for the heat tracer. Both under‐ and over‐estimation of velocity from the heat tracer relative to the solute tracer occurred. Differences between the estimates from the tracer methods increased as the mean velocity decreased, owing to differences in rates of molecular diffusion and thermal conduction. The variance in estimated velocity using all methods increased as the variance in log‐hydraulic conductivity (K) and correlation length scales increased. The variance in velocity for each scenario was remarkably small when compared to σ2ln(K) for all methods tested. The largest variability identified was for the solute tracer where 95% of velocity estimates ranged by a factor of 19 in simulations where 95% of the K values varied by almost four orders of magnitude. For the same K‐fields, this range was a factor of 11 for the heat tracer. The variance in estimated velocity was always lowest when using heat as a tracer. The study results suggest that a solute tracer will provide more understanding about the variance in velocity caused by aquifer heterogeneity and a heat tracer provides a better approximation of the mean velocity.

Field validation of thermal tracer tomography for reconstruction of aquifer heterogeneity

“…In recent years, heat as an actively injected tracer has raised increasing attention [Anderson, 2005;Saar, 2011;Rau et al, 2014]. Irvine et al [2013] pointed out that using heat over solute tracers provides better approximation of groundwater velocity at the expense of detecting heterogeneities. In comparison to the transport of solute tracers, conductive heat transport into the aquifer matrix can play a substantial role.…”

“…This slows down the tracer transport [Shook, 2001] and the tracer signals are more attenuated since a large part of the injected energy is used to heat up the aquifer matrix. Due to the high diffusivity in comparison to solute tracers and the potential influence of background temperature perturbations, heat is not considered an ideal tracer [Xue et al, 1990;Irvine et al, 2013;Xie et al, 2015]. Aside from this, physical parameters such as hydraulic conductivity, K, are dependent on temperature.…”

“…This technique has been used for fracture detection and characterization by Klepikova et al [2014]. Third, despite the strong influence of heat diffusion and density effects, thermal tracer testing in heterogeneous formations allows for the delineation of preferential pathways, where heat transport is dominated by advection [Irvine et al, 2013;Wagner et al, 2014;Djibrilla Saley et al, 2016]. In our earlier theoretical study [Somogyv ari et al, 2016], we have shown that by using a travel time-based inversion technique, the influence of density changes, viscosity variations, and diffusion effects is minimized and a wide application window for thermal tracer tomography is available.…”

“…In this case, the phase shift between the injected and observed signal can be investigated (Figure 1c) [Keys and Brown, 1978]. A more common approach is to use an on-site heating device to produce warm water continuously [Molz et al, 1981;Palmer et al, 1992;Vandenbohede et al, 2011;Irvine et al, 2013;Wildemeersch et al, 2014;Colombani et al, 2015;Doro et al, 2015;Cherubini et al, 2017], because temperature changes can be self-defined and higher (Figure 1a). Although observations from step injection signals are easier to detect, their technical implementation with active heating devices is difficult.…”

High-Resolution Wellbore Temperature Logging Combined with a Borehole-Scale Heat Budget: Conceptual and Analytical Approaches to Characterize Hydraulically Active Fractures and Groundwater Origin

“…At the borehole scale, high-resolution temperature profiling is of particular interest in hydrogeology, and its use has become more frequent over the last decade, coinciding with temperature sensor resolution improvement to 0.001 ∘ C. Hydrogeological information obtained from recent passive and/or active temperature measurement techniques [24,25] are now capable of competing with other, more conventional investigation techniques (e.g., involving hydraulic packer tests or solute tracing) to provide valuable information about aquifer hydraulic and fracturing structure, used to infer groundwater flow paths.…”