Dissolution of nonuniformly distributed immiscible liquid: intermediate-scale experiments and mathematical modeling

Area of Science:

• Environmental Science
• Geochemistry
• Chemical Engineering

Background:

• Non-aqueous phase liquids (NAPLs) like trichloroethene (TCE) and 1,2-dichloroethane (DCA) pose environmental contamination risks.
• Understanding NAPL dissolution and transport is crucial for effective remediation strategies.

Purpose of the Study:

• To investigate how nonuniform NAPL distribution affects dissolution rate coefficients.
• To evaluate the performance of mathematical models with varying complexity in predicting dissolution behavior.

Main Methods:

• Experiments using intermediate-scale flow cells packed with sand containing emplaced TCE and DCA saturation zones.
• In-situ NAPL saturation measurement using a dual-energy gamma radiation system.
• Quantitative analysis of flow and transport using a 3D mathematical model considering NAPL distribution and permeability variability.

Main Results:

• Aqueous concentrations of TCE and DCA were below solubility due to dilution and flow bypass.
• A 3D model explicitly accounting for heterogeneity accurately predicted NAPL dissolution and mass removal.
• Simpler models required dissolution rate coefficients three orders of magnitude smaller than those from column experiments.

Conclusions:

• Explicitly modeling larger-scale factors influencing dissolution allows the use of local-scale mass transfer coefficients.
• Simpler models yield lumped coefficients that incorporate larger-scale processes not explicitly represented.
• Local-scale dissolution coefficients can predict larger-scale contaminant behavior when heterogeneity is properly modeled.