Forward and back diffusion of reactive contaminants through multi-layer low permeability sediments

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Contaminants in low permeability sediments (aquitards) pose a persistent threat to shallow groundwater systems post-aquifer remediation.
• Understanding contaminant storage and discharge in aquitards is crucial for effective site remediation, yet most studies focus on non-reactive solutes in homogeneous media.
• Existing models often oversimplify aquitard heterogeneity and boundary conditions, leading to inaccurate predictions of contaminant behavior.

Purpose of the Study:

• To develop novel analytical solutions for contaminant forward and back diffusion in multi-layer low permeability sediments.
• To incorporate abiotic and biotic environmental degradation into contaminant transport models.
• To realistically simulate dense non-aqueous phase liquid (DNAPL) source depletion using instantaneous, linear, and exponential patterns.

Main Methods:

• Analytical modeling of contaminant diffusion and degradation through multi-layer aquitard systems.
• Comparison of a proposed multi-layer model with equivalent homogeneous models and semi-infinite boundary assumptions.
• Evaluation of contaminant back-diffusion under varying degradation rates and source depletion scenarios.

Main Results:

• Contaminant mass storage in aquitards varies significantly with source depletion patterns (instantaneous > linear > exponential).
• Equivalent homogeneous models underestimate back-diffusion onset and overestimate plume persistence compared to the multi-layer model.
• The semi-infinite boundary assumption leads to substantial long-term prediction errors (>200% for thin aquitards).
• Effective mitigation of back-diffusion occurs when degradation half-life is <16 years, with <10% diffusing into the aquifer.

Conclusions:

• The proposed diffusion-reaction coupled model for multi-layer media accurately predicts back-diffusion behavior in heterogeneous aquitards.
• Simpler models and assumptions significantly underestimate risks associated with contaminant back-diffusion.
• Biostimulation or bioaugmentation strategies are effective in mitigating back-diffusion risks from aquitards.