Analytical solutions for reactive transport under an infiltration-redistribution cycle

Area of Science:

• Environmental Science
• Soil Science
• Hydrology
• Geochemistry

Background:

• Investigates reactive solute transport in unsaturated soils, a critical process for contaminant hydrogeology.
• Builds upon existing models of vertical flow and transport by incorporating linear nonequilibrium kinetics.
• Addresses the complexities of infiltration-redistribution cycles, which are common in natural soil environments.

Purpose of the Study:

• To generalize existing models by accounting for linear nonequilibrium kinetics during infiltration-redistribution cycles.
• To develop and apply the partial equilibrium infiltration-redistribution model (PEIRM) for solute transport analysis.
• To investigate the influence of soil heterogeneity on solute plume behavior and penetration depth.

Main Methods:

• Derived an exact analytical solution for irreversible desorption reactions.
• Modeled solute transport using linear kinetics, assuming equilibrium during redistribution.
• Applied approximate closed-form solutions to field-scale concentrations using the Dagan and Bresler column model.
• Investigated soil heterogeneity by treating hydraulic conductivity as a random function.

Main Results:

• The PEIRM provides approximate closed-form solutions for solute transport in homogeneous soils.
• Solute penetration depth is finite and depends on water applied and residual water content for irreversible desorption.
• For sorption-desorption kinetics, penetration depth is bounded by equilibrium and irreversible desorption limits, influenced by application rate and conductivity.
• Non-equilibrium conditions lead to solute spreading behind the front, unlike equilibrium transport.
• Soil heterogeneity causes additional solute plume spreading.

Conclusions:

• The PEIRM accurately estimates solute mass in the mobile phase, with accuracy depending on the Damköhler number.
• The infiltration-redistribution cycle leads to finite solute penetration, explaining plume contraction phenomena.
• Non-equilibrium kinetics and soil heterogeneity are crucial factors influencing solute transport dynamics and spatial distribution in soils.