The effects of temperature on flow hydraulics and mass transport kinetics in unsaturated gravelly soil

Extended Abstract

Introduction

The shape of breakthrough curves (BTCs) is governed by two simultaneous and often competing sets of processes: hydraulic parameters dictating the flow regime and kinetic parameters controlling interaction rates (e.g., Ka and Kd). Temperature, acting as a key thermodynamic variable, exerts profound and complex effects on these parameters and, consequently, on the overall shape of the BTCs. Elevated temperatures alter unsaturated hydraulic functions by reducing viscosity and surface tension; concurrently, they influence the equilibrium between mobile and immobile colloid phases during attachment and detachment processes by modifying energy barriers.

A fundamental challenge in prior studies lies in the failure to decouple these interacting mechanisms, which can lead to ambiguous interpretations and compromise the generalizability of predictive models under non-isothermal conditions. The majority of existing research has evaluated the lumped effect of temperature without isolating the specific contributions of the hydraulic and kinetic components.

Accordingly, the primary objective of the present study is to introduce a sequential modeling approach designed to decouple the hydraulic and kinetic effects of temperature on colloid transport under unsaturated conditions. To this end, employing inverse modeling in HYDRUS-1D and maintaining baseline kinetic coefficients constant, the temperature effect on hydraulic parameters is first evaluated independently; subsequently, BTCs are reproduced at varying temperatures. This approach not only provides a deeper mechanistic understanding but also enhances the reliability of numerical models for simulating colloid transport.

Materials and Methods

To decouple the thermal effects on the hydraulic and kinetic processes governing colloid transport, unsaturated column experiments were conducted utilizing a custom-designed automated temperature-control system (Abadis). Washed gravelly porous media (d50=3.6 mm) and commercial SiO2 colloids (d50=240 nm) were employed. Transport experiments were executed under three isothermal conditions (15, 30, and 45 °C). Initially, a conservative tracer (KNO3) was applied to evaluate hydrodynamic dispersion, followed by a colloid pulse to derive the breakthrough curves (BTCs). The baseline Soil Water Retention Curve (SWRC) was experimentally determined at 30 °C, from which the van Genuchten-Mualem parameters were fitted.

Numerical modeling was performed via HYDRUS-1D, coupling the advection-dispersion equation with a one-site kinetic attachment model. To isolate temperature effects, a sequential optimization approach was adopted. Baseline kinetic coefficients were extracted from the 30 °C BTCs. Assuming constant kinetic properties and boundary water contents, the van Genuchten shape parameters (α and n) for 15 °C and 45 °C were inversely estimated. Utilizing these adjusted parameters alongside experimentally measured saturated hydraulic conductivities, temperature-specific SWRC and unsaturated hydraulic conductivity functions were reconstructed. Ultimately, forward simulations incorporating the modified hydraulic parameters and fixed baseline kinetic coefficients were executed. The deviations between the simulated and observed BTCs systematically delineate the isolated contribution of temperature-induced hydraulic variations to the overall colloid transport behavior.





Results and Discussion

Investigation into the effect of temperature on flow hydraulics revealed that an increase in heat from 15 to 45 °C enhances the saturated hydraulic conductivity (Ks) by 20% following a linear trend (Ks=0.8965T+123.27), which is directly attributed to the reduction in both the dynamic and kinematic viscosity of the fluid. The inverse modeling results of the soil water retention curve (SWRC) indicated a downward and leftward shift of the curve at higher temperatures; such that parameter α experienced a 52% increase (from 0.023 to 0.035 cm−1) and parameter n encountered a 26% decrease (from 5.73 to 4.24). Examination of the unsaturated hydraulic conductivity (K(h)) revealed a crossover point, indicating a shift in the dominant mechanism from viscosity (at near-saturation moisture levels) to surface tension and capillary forces (at higher suctions).

Evaluation of the breakthrough curves demonstrated that elevated temperatures significantly enhance colloid retention. This phenomenon was accompanied by a 78% increase in the attachment coefficient (Ka) and a 93% drop in the detachment coefficient (Kd). Despite the increased flow velocity at higher temperatures, the persistence of curve tailing signifies the definitive dominance of kinetic mechanisms over the hydrodynamic flushing process.

Ultimately, sequential simulation with the decoupling of hydraulic and kinetic effects proved that thermal fluctuations (in addition to direct kinetic impacts) influence the transport and retention patterns of colloids in the porous medium solely by modifying the soil water retention curve and altering the flow regime. Neglecting this temperature dependence in unsaturated modeling leads to the error of parameter compensation; a phenomenon in which the model falsely attributes structural (hydraulic) inadequacies to kinetic mechanisms. Therefore, incorporating the thermal dynamics of hydraulics is an essential prerequisite for the reliable modeling of colloid transport.

Conclusion

This study demonstrated that disregarding the temperature dependence of hydraulic and kinetic parameters may introduce structural model errors. Such errors are often artifactually compensated for through the calibration of kinetic parameters, thereby constraining the predictive capability of the model under varying thermal conditions.

The findings suggest that the effects of temperature on kinetic and hydraulic processes operate in a competitive manner. Temperature-induced alterations in the fluid’s hydraulic properties led to flow acceleration and reduced residence times for both the fluid and the contaminant, as corroborated by the behavior of the conservative tracer (nitrate). However, the pronounced decline in the concentration peak and the diminished recovery observed in the colloidal breakthrough curves at elevated temperatures indicate the predominance of kinetic effects over hydraulic processes. Recognizing this distinction is a crucial prerequisite for the accurate modeling of colloid transport under non-isothermal conditions within coarse-grained gravelly media.

Although the current findings provide key insights into these competitive thermal mechanisms, they are predicated on a specific particle size distribution and a steady moisture regime. Since medium texture (governing specific surface area and pore distribution) and varying moisture levels can significantly alter hydraulic-kinetic interactions, future research should evaluate this decoupling approach across diverse soil gradations and dynamic moisture conditions. Such investigations will ultimately facilitate the development of more robust and generalizable models for predicting contaminant transport.