Solute source depletion control of forward and back diffusion through low-permeability zones

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Solute diffusion between low-permeability aquitards and high-permeability aquifers significantly influences long-term contaminant fate.
• Contaminant back diffusion from aquitards occurs as depleted aquifer sources lead to declining concentrations.

Purpose of the Study:

• To experimentally evaluate archetypical transient source depletion models.
• To accurately account for dynamic aquitard-aquifer diffusive transfer using novel analytical solutions.

Main Methods:

• Laboratory diffusion experiments using a flow chamber to study solute exchange between sand aquifer and kaolinite aquitard layers.
• In situ measurement of aquitard solute concentration profiles using electrical conductivity.
• Application of analytical solutions to model source dissolution and back diffusion dynamics.

Main Results:

• Back diffusion initiation time and mass flux are dependent on the contaminant source depletion rate.
• Rapidly depleting sources lead to earlier back diffusion and larger mass flux.
• Analytical models demonstrated strong agreement with experimental aquifer breakthrough curves and aquitard concentration profiles.

Conclusions:

• The study links contaminant source dissolution and back diffusion, providing insights into contaminant transport dynamics.
• The developed modeling approach enables assessment of human exposure risk.
• Key parameters like back diffusion initiation time and plume persistence can be calculated, aiding environmental remediation strategies.