Estimating Mobile-Immobile Mass Transfer Parameters Using Direct Push Tools

Dr. Robert Borden | Draper Aden Associates

ER-2529

Objective

The slow, yet persistent, release of contaminants from low-permeability zones represents a significant challenge for the groundwater site remediation community. This release (including back diffusion and slow advection) can continue for years, or even decades, after the contaminant source is controlled. This project will develop methods to better characterize and model the mass transfer of contaminants between higher and lower mobility zones and its impact on the long-term release of contaminants in groundwater. Specific technical objectives include:

  1. Develop low-cost direct-push (DP) methods for high-resolution characterization of spatial variations in hydraulic conductivity (K) and porosity (θ) over the range commonly found in unconsolidated settings.
  2. Determine if high-resolution solute transport models, primarily calibrated using DP tools, can provide satisfactory predictions of contaminant mass transfer between high and low K zones.
  3. Develop methods to calibrate existing and new mobile-immobile zone models using high resolution K and θ distributions obtained with DP tools.

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Technical Approach

This project will develop a complementary set of field and modeling methods for quantifying mobile-immobile mass transfer in shallow, unconsolidated settings.

ER-2529_project graphic

Field: The field methods will be based on DP technology, which uses hydraulic rams supplemented with vehicle weight and, in some variants, high-frequency hammers to rapidly advance small-diameter tools into the subsurface. The hydraulic profiling tool (HPT), which consists of a water injection port with a transducer positioned behind it and an electrical conductivity (EC) sensor array, will be used for high-resolution (1.5 cm) characterization of K. The power of this tool has been demonstrated for units of moderate to high K, but significant modifications will be required to extend its applicability to lower K settings. A WCP-CPT tool (water content probe integrated into a standard cone penetrometer system) will be utilized to provide high-resolution information about spatial variations in θ.

Modeling: High-resolution solute transport simulations will be performed at two test sites using the MODFLOW/MT3DMS software and the high-resolution field data from DP methods. Three upscaled mobile-immobile zone mass transfer models (dual-porosity, matrix diffusion tool kit, and MINC) will be evaluated by comparing them to the high-resolution simulation results. This will allow the team to systematically assess the performance of different models and develop techniques that can utilize the high-resolution DP data to estimate the parameters required for each model.

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Benefits

This project will improve understanding of mobile-immobile mass transfer and its impacts on the fate and transport of contaminants in shallow groundwater systems. It will provide site managers with a simple and easy-to-use comprehensive set of tools to better characterize sites with significant low-K zones; estimate the rate, timing and duration of contaminant release from those zones; and evaluate the impact of contaminant mass stored in low K zones on long-term plume behavior. (Anticipated Project Completion - 2020)

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Publications

Falta, R.W. and W. Wang. 2017. A Semi-Analytical Method for Simulating Matrix Diffusion in Numerical Transport Models. Journal of Contaminant Hydrology, 197:39-49.

Liu, G., R.C. Borden, and J.J. Butler. 2018. Simulation Assessment of Direct Push Injection Logging for High Resolution Aquifer Characterization. Groundwater. doi: 10.1111/gwat.12826

Muskus, N. and R.W. Falta. 2018. Semi-Analytical Method for Matrix Diffusion in Heterogeneous and Fractured Systems with Parent-Daughter Reactions. Journal of Contaminant Hydrology, 218:94-109.

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Points of Contact

Principal Investigator

Dr. Robert Borden

Solutions-IES

Phone: 919-873-1060

Program Manager

Environmental Restoration

SERDP and ESTCP

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