Theme: 01. Remediation Technology Innovations

Topic Area: 01m. In Situ Technologies: Lessons Learned

Formatted Title:

Lessons Learned from Treating over 60,000 Pounds of TCE with 475 Metric Tons of ZVI in Dense Clay Soils

Background/Objectives:

A Phase I site assessment and resulting Phase II subsurface investigation revealed a significant amount of trichloroethene (TCE) in soils near a former above ground storage tank at a previous manufacturing operation. Soil sampling results indicated TCE concentrations in soil up to 49,000 mg/kg, with the majority of impacts located in the upper 20 feet of soil in a dense clay till of glacial origin. The remediation objective for TCE in soil was 15 mg/kg.

Approach/Activities:

Initially, a Membrane Interface Probe and Hydraulic Profiling Tool (MiHPT) investigation was performed to determine the magnitude and extent of the contamination. Then, using the results from the MiHPT investigation, over 100 soil samples were collected to precisely define the location of the impacts. Using Earth Volumetric Studio (EVS) software, a statistically based 3D conceptual site model was created to quantify the volume of contaminated soil (3,700 cubic yards) and mass of the TCE in the subsurface (62,500 pounds).

Once calculated, a bench-scale study was performed on a sample of impacted soils using several different chemical amendments to determine the optimal remedial chemistry. Zero-valent iron (ZVI) outperformed all other amendments. Next, using the data from the 3D model, the remediation area was divided into 44 treatment cells and ZVI dosages were calculated. Weight percent of ZVI of each cell ranged from about 1% up to 8% (averaging 6%) with respect to the soil mass.

The impacted soils were mixed with 475 metric tons of ZVI and magnetic separation tests were performed throughout the mixing process to ensure sufficient homogenization. A drone equipped with a LiDAR sensor collected surface elevation data during the remediation to help quantify fluffed soil for disposal and assist with mass-balancing to meet the proposed grade. The first round of confirmation samples will be collected in February 2024, approximately six months after treatment. These results will be available during the presentation.

Results/Lessons Learned:

Several lessons were learned during the site investigation and remediation activities:

Utilize HRSC tools whenever possible - The MiHPT investigation defined the extent of the contamination both laterally and vertically and described the geologic setting of the Site in high resolution. Additionally, there was a strong correlation of the PID data with the TCE concentrations found in the soils, and as a result, the MiHPT investigation helped to fill data gaps where soil analytical data did not exist.
3D models are more than just visualization tools - Not only did the 3D visual calculate the volume of impacted soil and estimate the mass of the contamination, but it was also used to design a remediation plan that was accepted by the state regulatory agency.
Always perform a pilot study before going full scale - The pilot study demonstrated that several common remediation amendments (including potassium permanganate and persulfate) performed quite poorly due to the high natural oxidant demand of the native soils.
Statistical modeling assists with amendment dosing - The remediation contractor utilized the 3D statistical model to optimize the remedial design and accurately distribute the ZVI within the treatment cells.
Utilize LiDAR for earth moving activities - Collecting LiDAR more accurately estimated the amount of fluffed soil for disposal and provided significant cost savings over traditional survey techniques during site regrading.

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David Kulczycki