A Practical Modeling Framework for Non-Fickian Transport and Multi-Species Sequential First-Order Reaction

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Standard advection-dispersion equation (ADE) fails to accurately model non-Fickian contaminant transport due to heterogeneity and mobile-immobile mass exchange.
• Non-Fickian transport, especially with simultaneous reactions, requires advanced modeling for accurate environmental risk assessment.

Purpose of the Study:

• To present an extended ADE modeling framework based on continuous time random walk theory.
• To develop and derive new transient and steady-state analytical solutions for non-Fickian transport with sequential first-order reactions.
• To evaluate the performance of the extended ADE model against the standard ADE for contaminant plume behavior and remediation assessment.

Main Methods:

• Developed an extended ADE framework using continuous time random walk theory.
• Derived new transient analytical solutions in Laplace space and closed-form steady-state solutions for multi-species transport.
• Applied the model to simulate contaminant plumes, including a 30-year groundwater cleanup case study.

Main Results:

• The extended ADE framework accurately characterizes non-Fickian transport coupled with sequential first-order reactions for various contaminants.
• Derived analytical solutions are applicable to heterogeneous aquifers with sorption-desorption and matrix diffusion, allowing differential degradation rates.
• Simulations showed that the standard ADE can overestimate cleanup rates and underestimate cleanup timeframes compared to the extended model.
• The extended ADE model successfully replicated observed long concentration tails at a real-world remediation site.

Conclusions:

• The extended ADE model provides a more accurate representation of contaminant transport in heterogeneous aquifers than the standard ADE.
• The developed analytical solutions offer practical tools for assessing groundwater remediation effectiveness and predicting long-term contaminant behavior.
• This framework is crucial for effective management of contaminated sites involving complex hydrogeological conditions and multiple degrading contaminants.