Evaluating the impact of back diffusion on groundwater cleanup time

Area of Science:

• Hydrogeology
• Environmental Science
• Geochemistry

Background:

• Back diffusion of groundwater contaminants from low permeability (K) zones is a critical factor affecting cleanup timelines.
• Understanding contaminant release from these zones is essential for effective remediation strategies.

Purpose of the Study:

• To identify key aquifer and contaminant characteristics influencing cleanup time (T_OoM) after source removal.
• To develop predictive relationships for contaminant concentration decline by 1, 2, and 3 orders of magnitude (T₁, T₂, T₃).

Main Methods:

• A semi-analytical modeling approach was used to simulate contaminant concentration decline.
• Approximately 21,000 simulations were performed for layered and boundary aquifer geometries.
• Linear regression analysis was applied to interpret simulation data and develop estimation relationships.

Main Results:

• Mass residence time (T_M) was the most significant predictor of T₁, T₂, and T₃ for both geometries.
• Diffusion time (T_D) strongly influenced T₃ in layered geometries, while the low K to high K mass storage ratio (γ) was influential in boundary geometries.
• Contaminant decay in low K zones can reduce cleanup times, particularly when λ_LT_D > 0.01.

Conclusions:

• The 1st Damköhler number (Da = T_M/T_D) indicates the relative importance of back diffusion.
• Back diffusion has the greatest impact on T₃ at Da values between 0.01 and 0.1.
• A conceptual model was developed to evaluate back diffusion impacts on contaminant concentration decline.