Theme: 5. Fractured Rock and Complex Geology

Topic Area: 5c2. Challenges and Lessons Learned in Remediating Sites with Complex Geology

Formatted Title:

Remediation of DNAPL in Competent Mudstone: Challenges and Importance of Post Monitoring to Evaluate Performance

Background/Objectives:

The site is a former dry cleaning facility in northern Virginia where shallow competent bedrock is impacted with DNAPL. The primary contaminants are TCE and daughter compounds. As part of a Voluntary Remediation Program (VRP), in situ remediation was evaluated to address source mass, mitigate indoor vapor intrusion issues and also prevent the bedrock plume migrating offsite.

DNAPL remediation in competent rock is always challenging with respect to three main criteria:

Changes in the effective DNAPL-water interfacial area in bedrock fractures due to dissolution of the DNAPL source will determine the right remedial approach.
Accessing the source mass trapped in low permeability zones using the most appropriate reagent delivery method that will enable contact between the contaminant mass and the reactive amendment
Evaluating performance through an extensive monitoring program

Approach/Activities:

A controlled jet injection approach was selected to deliver in situ chemical reduction (ISCR) reagents to address chlorinated solvent impacts. The creation and connectivity of new fractures overcomes the transport limitations inherent at the Site due to its geology and enables direct contact of the amendment with the source mass trapped in portions of the bedrock that would conventionally be inaccessible. Jet injection also allows the solid regent to act as a proppant ensuring the fractures don’t collapse and achieving the expected radius of influence (ROI). Multiple fracturing intervals were implemented to inject ISCR reagents in eight open locations in the hotspot with each interval separated vertically by at least 2 ft. Borehole geophysics was an integral component used to (a) refine the potential injection zones/intervals, and (b) understand the fate and transport of ISCR reagents.

Results/Lessons Learned:

Results from a comprehensive post-monitoring program (> 24 months) will be presented including discussion of some key components essential for successful optimization of the remedial strategy and key takeaways from challenges encountered:

Importance of fluid conductivity and temperature measurements to identify the preferred zone of groundwater flow.
Optimizing the jet injection approach in competent mudstone.
Advantages in combining solid and liquid ISCR amendments for long-term reduction in source mass (>90%).
Tracking geochemical signature to verify distribution and demonstrate the performance of ISCR reagents.
Managing gas production post ISCR at sites that have shallow bedrock.

Menu

Eric Magdar